WO2012008559A1

WO2012008559A1 - Separator for lithium ion secondary battery and lithium ion secondary battery using same

Info

Publication number: WO2012008559A1
Application number: PCT/JP2011/066172
Authority: WO
Inventors: 松岡　昌伸; 友洋佐藤; 加寿美加藤; 誉子笠井; 緑川　正敏; 伯志松田; 裕夫鍛冶; 山本　浩和; 宏明渡邉; 佃　貴裕; 郁夫藤田; 兵頭　建二
Original assignee: 三菱製紙株式会社
Priority date: 2010-07-14
Filing date: 2011-07-08
Publication date: 2012-01-19
Also published as: JP5767222B2; CN102986060A; CN102986060B; JPWO2012008559A1

Abstract

Disclosed is a separator that is for a lithium ion secondary battery and that has low water content, strong mechanical strength, and an excellent internal resistance and rate of internal short-circuit failure, in particular, excellent cycle characteristics and excellent discharge characteristics and variation thereof at high rates. The separator comprises a porous sheet containing 10-90 mass% of a synthetic fiber and 10-90 mass% of a solvent-spun cellulose fiber having a modified freeness of 0-250 ml measured in compliance with JIS P8121 when the sample concentration is not 0.1%, and uses an 80 mesh wire screen having 0.18 mm apertures and a wire diameter of 0.14 mm as a plate sieve. Further disclosed is a lithium ion secondary battery using same.

Description

Lithium ion secondary battery separator and lithium ion secondary battery using the same

The present invention relates to a separator for a lithium ion secondary battery and a lithium ion secondary battery using the separator.

With the recent spread of portable electronic devices and higher performance, secondary batteries having high energy density are desired. As this type of battery, a lithium ion secondary battery using an organic electrolyte (non-aqueous electrolyte) has attracted attention. The average voltage of the lithium ion secondary battery is 3.7 V, which is about three times that of the alkaline secondary battery, and the energy density is high. However, an aqueous electrolyte solution cannot be used unlike the alkaline secondary battery. A nonaqueous electrolytic solution having sufficient oxidation-reduction resistance is used.

As a separator for a lithium ion secondary battery, a film-like porous film made of polyolefin is often used (see, for example, Patent Document 1), but has low ionic conductivity due to low electrolyte retention. There was a problem that the internal resistance increased.

Further, as a separator for a lithium ion secondary battery, a paper separator (for example, see Patent Document 2) mainly composed of regenerated cellulose fiber beats has been proposed. Lithium ion secondary batteries have a negative effect on battery characteristics if even a small amount of water is mixed in. Therefore, if a paper separator with a high moisture content is used for the separator, it will dry for a long time when producing lithium ion secondary batteries. Processing is required. Moreover, since the separator strength was weak, there was a problem that the separator could not be thinned.

Further, as separators for lithium ion secondary batteries, nonwoven fabric separators made of synthetic fibers (see, for example, Patent Documents 3 to 5) have also been proposed. However, these separators have low electrolyte retention and internal resistance. There is a problem that the high short-circuiting rate is high, the internal short-circuit defect rate is high, and the high rate characteristics and discharge characteristics are inferior.

Further, as a separator for a lithium ion secondary battery, a separator made of fibrillated heat-resistant fiber, fibrillated cellulose, or non-fibrillated fiber has been proposed (see, for example, Patent Documents 6 to 8). There was still room for improvement in strength.

JP 2002-105235 A Japanese Patent No. 3661104 JP 2003-123728 A JP 2007-317675 A JP 2006-19191 A International Publication No. 2005/101432 Pamphlet International Publication No. 1996/030954 Pamphlet JP 2004-146137 A

The present invention has been made in view of the above circumstances, and has a low moisture content, high mechanical strength, and excellent internal resistance and internal short-circuit failure rate, in particular, high-rate discharge characteristics and variations, and cycle characteristics. The object is to provide a lithium ion secondary battery separator and a lithium ion secondary battery using the same.

As a result of diligent research to solve the above problems, the following batteries were found in the following lithium ion secondary battery separator and lithium ion.

(1) The modified freeness measured in accordance with JIS P8121 is 0 ~ except that an 80 mesh wire net having a wire diameter of 0.14 mm and an aperture of 0.18 mm is used as the sieve plate, and the sample concentration is 0.1%. A separator for a lithium ion secondary battery comprising a porous sheet containing 250-ml solvent-spun cellulose fiber of 10 to 90% by mass and synthetic fiber of 10 to 90% by mass.
(2) The separator for a lithium ion secondary battery according to (1), wherein the solvent-spun cellulose fiber has a length weighted average fiber length of 0.20 to 2.00 mm.
(3) The solvent-spun cellulose fiber has a maximum frequency peak between 0.00 and 1.00 mm in the length-weighted fiber length distribution histogram, and a fiber having a length-weighted fiber length of 1.00 mm or more. The separator for lithium ion secondary batteries according to (1) or (2), wherein the ratio is 10% or more.
(4) In the length-weighted fiber length distribution histogram of solvent-spun cellulose fibers, the slope of the ratio of fibers having a length-weighted fiber length of 0.05 mm between 1.00 and 2.00 mm is −3.0 or more The separator for lithium ion secondary batteries according to (3), which is −0.5 or less.
(5) The solvent-spun cellulose fiber has a maximum frequency peak between 0.00 and 1.00 mm in the length-weighted fiber length distribution histogram, and a fiber having a length-weighted fiber length of 1.00 mm or more. The separator for lithium ion secondary batteries according to (1) or (2), wherein the ratio is 50% or more.
(6) The separator for a lithium ion secondary battery according to (5), which has a peak between 1.50 and 3.50 mm in addition to the maximum frequency peak in a length-weighted fiber length distribution histogram of solvent-spun cellulose fibers.
(7) The separator for a lithium ion secondary battery according to (1) or (2), wherein the solvent-spun cellulose fiber has a length weighted average fiber length of 0.50 to 1.25 mm and an average curl degree of 25 or less.
(8) Furthermore, the porous sheet was measured according to JIS P8121, except that an 80 mesh wire net having a wire diameter of 0.14 mm and an aperture of 0.18 mm was used as the sieve plate, and the sample concentration was 0.1%. The separator for a lithium ion secondary battery according to any one of (1) to (7), comprising 20% by mass or less of fibrillated natural cellulose fiber having a freeness of 0 to 400 ml.
(9) The separator for a lithium ion secondary battery according to (1), wherein the porous sheet further contains carboxymethylcellulose.
(10) The lithium ion secondary battery according to (1), wherein the porous sheet contains a core-sheath type heat-sealing fiber composed of a heat-seal component and a non-heat-seal component as at least one synthetic fiber. Separator.
(11) The separator for a lithium ion secondary battery according to (10), wherein the core of the core-sheath type heat-sealing fiber is polyethylene terephthalate and the sheath is a polyester copolymer.
(12) The separator for a lithium ion secondary battery according to (10) or (11), wherein the porous sheet is heat-treated.
(13) The separator for a lithium ion secondary battery according to any one of (1) to (12), wherein the porous sheet has an average pore diameter of 0.10 μm or more and a maximum pore diameter of 6.0 μm or less.
(14) In the porous sheet, the value measured by the method defined in JIS B7502 (the thickness of the sheet measured by an outer micrometer at 5 N load) is 6 to 50 μm. A separator for a lithium ion secondary battery according to claim 1.
(15) The separator for a lithium ion secondary battery according to any one of (1) to (12), wherein the basis weight of the porous sheet is 5 to 40 g / m ² .
(16) The lithium ion secondary battery according to any one of (1) to (15), wherein the synthetic resin constituting the synthetic fiber is at least one selected from polyester resins, acrylic resins, and polyolefin resins. Separator.
(17) The separator for a lithium ion secondary battery according to any one of (1) to (16), wherein the synthetic fiber has an average fiber diameter of 0.1 to 20 μm.
(18) The lithium ion secondary battery according to (1), wherein the porous sheet has a multilayer structure, and at least two layers are layers containing solvent-spun cellulose fibers having a modified freeness of 0 to 250 ml as essential components. Separator for use.
(19) A lithium ion secondary battery using the lithium ion secondary battery separator according to any one of (1) to (18).

The separator for a lithium ion secondary battery of the present invention comprises a nonwoven fabric containing solvent-spun cellulose fibers having a modified freeness of 0 to 250 ml and synthetic fibers. The solvent-spun cellulose fiber having a modified freeness of 0 to 250 ml and the synthetic fiber are entangled with each other, so that the liquid retaining property of the lithium ion secondary battery separator can be improved. The resistance can be lowered, and the discharge characteristics at a particularly high rate can be made excellent. Furthermore, since the lithium ion secondary battery separator can be made dense, variations in internal short-circuit failure rate and discharge characteristics can be suppressed. Moreover, the moisture content of a separator can be restrained low by containing a synthetic fiber, and the drying process time at the time of battery manufacture can be shortened more.

Furthermore, the synthetic fiber is easily entangled with synthetic fibers and solvent-spun cellulose fibers having a modified freeness of 0 to 250 ml, and a fiber network is formed, whereby the strength of the separator can be increased, and the separator can be more dense and The internal resistance can be lowered, the internal short-circuit failure rate and the variation in discharge characteristics can be suppressed, and the cycle characteristics can be improved.

It is a length weighted fiber length distribution histogram of solvent-spun cellulose fiber [I]. It is a length weighted fiber length distribution histogram of solvent-spun cellulose fiber [II]. In the length-weighted fiber length distribution histogram of solvent-spun cellulose fibers [I] and [II], approximate to a graph of the proportion of fibers having a length-weighted fiber length of every 0.05 mm between 1.00 and 2.00 mm It is the figure which showed the straight line. It is a length weighted fiber length distribution histogram of solvent-spun cellulose fiber [i]. It is a length weighted fiber length distribution histogram of solvent-spun cellulose fiber [ii].

Hereinafter, the lithium ion secondary battery separator of the present invention (hereinafter also referred to as “separator”) and a lithium ion secondary battery using the same will be described in detail.

The solvent-spun cellulose fiber in the present invention is different from the so-called regenerated cellulose fiber in which cellulose is once chemically converted into a cellulose derivative and then returned to cellulose like conventional viscose rayon or copper ammonia rayon. This refers to a fiber in which cellulose is precipitated by dry and wet spinning of a spinning stock solution dissolved in amine oxide in water without chemical change, and is also referred to as “lyocell fiber”. Solvent-spun cellulose fibers have a higher molecular arrangement in the fiber long axis direction than natural cellulose fibers, bacterial cellulose fibers, and rayon fibers, so when mechanical forces such as friction are applied in a wet state, It is easy to form, and fine and long fine fibers are formed. In order to firmly hold the electrolyte solution between the fine fibers, the solvent-spun cellulose fibers that are refined are superior in liquid retention of the electrolyte solution compared to the refined products of natural cellulose fibers, bacterial cellulose fibers, and rayon fibers.

In the present invention, solvent-spun cellulose fibers having a modified freeness of 0 to 250 ml are used. The modified drainage degree of the solvent-spun cellulose fiber is more preferably 0 to 200 ml, and further preferably 0 to 160 ml. When the modified freeness is more than 250 ml, the density of the separator becomes insufficient and the internal short circuit defect rate becomes high.

The modified freeness in the present invention was measured in accordance with JIS P811, except that an 80 mesh wire net having a wire diameter of 0.14 mm and an aperture of 0.18 mm was used as a sieve plate, and the sample concentration was 0.1%. It is a value.

In the case of solvent-spun cellulose fibers, the fiber length becomes shorter as the microfabrication progresses. In particular, when the sample concentration is low, the entanglement between fibers decreases and it becomes difficult to form a fiber network. Will slip through the holes in the sieve plate. In other words, in the case of finely solvent-spun cellulose, an accurate freeness cannot be measured by the measuring method of JIS P8121. In more detail, natural cellulose fibers are in a state where many fine fibrils are torn apart from the trunk of the fiber as the degree of refinement progresses. Therefore, the fibers are easily entangled with each other through the fibrils, and a fiber network is easily formed. On the other hand, the solvent-spun cellulose fiber is easily finely divided in parallel to the long axis of the fiber by the refining treatment, and since the uniformity of the fiber diameter in each fiber after division is high, the shorter the average fiber length, It is considered that the fibers do not easily entangle with each other and it is difficult to form a fiber network. Therefore, in the present invention, in order to measure the exact freeness of the solvent-spun cellulose fiber, an 80-mesh wire mesh having a wire diameter of 0.14 mm and an opening of 0.18 mm is used as a sieve plate, and the sample concentration is 0.1%. A modified freeness measured in accordance with JIS P8121 was used.

The length weighted average fiber length of solvent-spun cellulose fibers having a modified freeness of 0 to 250 ml is preferably 0.20 to 3.00 mm, more preferably 0.20 to 2.00 mm, and 0.20 to 1. 60 mm is more preferable. If the length-weighted average fiber length is shorter than 0.20 mm, the separator may fall off, and if it is longer than 3.00 mm, the fibers may be tangled and become lumpy, resulting in uneven thickness.

1 and 2 are length-weighted fiber length distribution histograms of solvent-spun cellulose fibers. As shown in FIGS. 1 and 2, in the length-weighted fiber length distribution histogram of solvent-spun cellulose fibers, the maximum frequency peak is between 0.00 and 1.00 mm, and the length-weighted fiber length is 1.00 mm or more. In the lithium ion secondary battery separator (3) in which the ratio of the fibers having 10% or more is easy to entangle the fibers and easily form a fiber network, the separator strength is increased and the internal short circuit rate is low. It is preferable. In the length-weighted fiber length distribution histogram, the maximum frequency peak is between 0.30 and 0.70 mm, and the fiber having a length-weighted fiber length of 1.00 mm or more in terms of a decrease in internal short-circuit failure rate. More preferably, the ratio is 12% or more. A higher ratio of fibers having a length-weighted fiber length of 1.00 mm or more is preferable, but about 50% is sufficient.

In the length-weighted fiber length distribution histogram of the solvent-spun cellulose fiber of the separator for lithium ion secondary battery (3), the proportion of fibers having a length-weighted fiber length of every 0.05 mm between 1.00 and 2.00 mm The lithium ion secondary battery separator (4) having a slope of −3.0 or more and −0.5 or less is more preferable because of high mechanical strength of the separator and less variation in discharge capacity. In the lithium ion secondary battery separator (4) of the present invention, the inclination of the ratio of fibers having a length-weighted fiber length of 0.05 mm between 1.00 and 2.00 mm is −2.5 or more and −0. Is more preferably 0.8 or less, and further preferably -2.0 or more and -1.0 or less. When the inclination is smaller than −3.0, the mechanical strength may be lowered. Further, when the inclination exceeds −0.5, the variation in discharge capacity may increase. As shown in FIG. 1 and FIG. 2, “large inclination” means that the length-weighted fiber length distribution of the solvent-spun cellulose fiber is wide, and “small inclination” means the length of the solvent-spun cellulose fiber. The weighted fiber length distribution is narrow and the length weighted fiber lengths are more uniform. The slope of the solvent-spun cellulose fiber [I] in FIG. 1 is −2.9, and the slope of the solvent-spun cellulose fiber [II] in FIG. 2 is −0.6.

The “inclination of the proportion of fibers having a fiber length of every 0.05 mm between 1.00 and 2.00 mm” is 0 between 1.00 and 2.00 mm, as shown in FIG. Approximate straight line is calculated by the method of least squares with respect to the value of the ratio of fibers having a length-weighted fiber length every .05 mm, and the inclination of the obtained approximate straight line is meant.

Like the lithium ion secondary battery separator (5) of the present invention, the length-weighted fiber length distribution histogram of solvent-spun cellulose fibers has a maximum frequency peak between 0.00 and 1.00 mm. The proportion of fibers having a length-weighted fiber length of 00 mm or more is preferably 50% or more. In the separator for lithium ion secondary batteries (5), the porous sheet has a dense structure, the separator strength is strong, the internal short circuit failure rate is low, and the variation in discharge capacity can be reduced.

FIG. 4 shows the length weight of solvent-spun cellulose fibers having a maximum frequency peak between 0.00 and 1.00 mm, and the proportion of fibers having a length-weighted fiber length of 1.00 mm or more is 50% or more. It is a fiber length distribution histogram. In terms of improving mechanical strength, in the length-weighted fiber length distribution histogram, the ratio of fibers having a maximum frequency peak between 0.30 and 0.70 mm and having a length-weighted fiber length of 1.00 mm or more is It is preferable that it is 55% or more. A higher proportion of fibers having a length-weighted fiber length of 1.00 mm or more is desirable, but about 70% is sufficient.

In the length-weighted fiber length distribution histogram of the solvent-spun cellulose fiber of the lithium ion secondary battery separator (5), as shown in FIG. 5, between 1.50 and 3.50 mm in addition to the above maximum frequency peak. The lithium ion secondary battery separator (6) having a peak at 5 is preferable because the mechanical strength is increased and the variation in discharge capacity is reduced. Further, it preferably has a peak between 1.75 and 3.25 mm, and more preferably has a peak between 2.00 and 3.00 mm. When the length-weighted fiber length of peaks other than the maximum frequency peak is shorter than 1.50 mm, the mechanical strength may decrease. Moreover, when it exceeds 3.50 mm, the variation in discharge capacity may increase.

Like the lithium ion secondary battery separator (7) of the present invention, the solvent-spun cellulose fiber preferably has a length weighted average fiber length of 0.50 to 1.25 mm and an average curl degree of 25 or less. . Good solvent retention of lithium-ion secondary battery separator by entanglement of solvent-spun cellulose fiber and synthetic fiber with length-weighted average fiber length of 0.50 to 1.25 mm and average curl of 25 or less As a result, the internal resistance can be lowered, and in particular, the discharge characteristics at a high rate can be made excellent. Furthermore, since the lithium ion secondary battery separator can be made dense, variations in internal short-circuit failure rate and discharge characteristics can be suppressed. In addition, the solvent-spun cellulose fiber having a weight-weighted average fiber length of 0.50 to 1.25 mm and an average curl degree of 25 or less is easily entangled with the synthetic fiber, and a fiber network is formed. The separator can be made denser and thinner, the internal resistance can be lowered, the internal short-circuit failure rate and the variation in discharge characteristics can be suppressed, and the cycle characteristics can be improved. It can be excellent.

The average curl degree of the solvent-spun cellulose fiber is more preferably 20 or less, and further preferably 15 or less. If the average curl degree of the solvent-spun cellulose fiber is larger than 25, the uniformity of the separator is impaired, and the internal short circuit defect rate may be increased, or the strength of the separator may be decreased. In addition, the small numerical value of the average curl degree indicates that the degree of bending of the solvent-spun cellulose fiber is small, that is, closer to a straight line, and therefore there is no particular limitation on the lower limit of the average curl degree.

The length-weighted fiber length and length-weighted fiber length distribution histogram, length-weighted average fiber length, and average curl degree of the solvent-spun cellulose fiber of the present invention are as follows. According to 52 “Paper and pulp fiber length test method (optical automatic measurement method)”, it was measured using Kajaani Fiber Lab V3.5 (manufactured by Metso Automation).

In Kajaani Fiber Lab V3.5 (Metso Automation Co., Ltd.), for each fiber passing through the detection unit, the total true length (L) of the bent fiber and the shortest length (l) of both ends of the bent fiber are determined. Can be measured. The “length-weighted average fiber length” is an average fiber length obtained by measuring and calculating the shortest length (l) of both ends of the bent fiber. “Average curl degree” is calculated for each individual fiber by measuring the total true length (L) of the bent fiber and the shortest length (l) of both ends of the bent fiber, and calculating by the following formula: This is the average curl degree. As the degree of bending of the fiber increases, the “curl degree” increases.

Curling degree = (L / l) -1 (1) (1)

Solvent-spun cellulose fiber having a modified drainage of 0 to 250 ml, solvent-spun cellulose fiber having a modified drainage of 0 to 250 ml and a length-weighted fiber length of 0.20 to 2.00 mm, modified drainage As a method for producing solvent-spun cellulose fibers having a degree of 0 to 250 ml, a length-weighted fiber length of 0.50 to 1.25 mm, and an average curl degree of 25 or less, a refiner, a beater, a mill, or grinding Equipment, rotary blade homogenizer that applies shear force with a high-speed rotary blade, double-cylindrical high-speed homogenizer that generates shear force between a cylindrical inner blade that rotates at high speed and a fixed outer blade, by ultrasonic Ultrasonic crusher that is refined by impact, giving a pressure difference of at least 20 MPa to the fiber suspension, passing through a small-diameter orifice to a high speed, and colliding with this to rapidly decelerate the fiber Shear force, a method using a high-pressure homogenizer or the like to apply the cutting force and the like. Among these, a method using a refiner is particularly preferable. By adjusting the type of beating / dispersing equipment and processing conditions (fiber concentration, temperature, pressure, rotation speed, refiner blade shape, gap between refiner disks, number of treatments, etc.), the desired modified freeness, Solvent-spun cellulose length-weighted fiber length, length-weighted fiber length distribution, and average curl degree can be achieved.

As a method for preparing solvent-spun cellulose fibers having a modified freeness of 0 to 250 ml, a length-weighted fiber length of 0.50 to 1.25 mm, and an average curl of 25 or less, a refiner blade shape, a crusher rotor, It is an effective means to appropriately select the shape of the stator and perform beating and grinding with the above-mentioned apparatus in a low concentration state with a dispersion concentration of 2.0% by mass or less.

The separator for a lithium ion secondary battery of the present invention contains 10 to 90% by mass of solvent-spun cellulose fiber having a modified freeness of 0 to 250 ml. The content of the solvent-spun cellulose fiber is more preferably 20 to 70% by mass, and further preferably 30 to 60% by mass. When the content of the solvent-spun cellulose fiber is less than 10% by mass, the liquid retainability of the electrolytic solution is insufficient and the internal resistance is increased, or the separator is insufficiently dense and the internal short circuit defect rate is increased. To do. When the content of the solvent-spun cellulose fiber exceeds 90% by mass, the mechanical strength of the separator becomes weak and the moisture content of the separator increases.

The separator for a lithium ion secondary battery of the present invention contains 10 to 90% by mass of synthetic fiber. The content of the synthetic fiber is more preferably 30 to 80% by mass, and further preferably 40 to 70% by mass. When the synthetic fiber content is less than 10% by mass, the strength of the separator is weakened. When the synthetic fiber content exceeds 90% by mass, the electrolyte retainability is insufficient and the internal resistance is high, the separator is insufficiently dense, and the internal short circuit failure rate and the variation in discharge characteristics are high. It becomes.

As in the lithium ion secondary battery separator (8) of the present invention, the porous sheet preferably contains 20% by mass or less of fibrillated natural cellulose fibers having a modified freeness of 0 to 400 ml. The content of the fibrillated natural cellulose fiber is more preferably 10% by mass or less, and further preferably 5% by mass or less. Fibrilized natural cellulose fibers tend to be less uniform in thickness of one fiber than solvent-spun cellulose fibers, but are characterized by strong physical entanglement between fibers and hydrogen bonding strength. When the content of the fibrillated natural cellulose fiber exceeds 20% by mass, a film is formed on the separator surface, and the ionic conductivity is inhibited, so that the internal resistance may be increased or the discharge characteristics may be decreased. .

Fibrilization refers to a fiber that is not film-like but has a fiber portion that is mainly finely divided in a direction parallel to the fiber axis, and at least a portion of which has a fiber diameter of 1 μm or less. The aspect ratio of length to width is preferably in the range of about 20 to about 100,000. Further, the length-weighted plain fiber length is preferably in the range of 0.10 to 2.00 mm, more preferably 0.1 to 1.5 mm, and still more preferably 0.10 to 1.00 mm.

Natural cellulose fibers can be fibrillated by refiners, beaters, mills, milling devices, rotary blade homogenizers that apply shearing force with high-speed rotary blades, cylindrical inner blades that rotate at high speed, and outer blades that are fixed. Double-cylindrical high-speed homogenizer that generates a shearing force between the two, an ultrasonic crusher that is refined by ultrasonic shock, and a high pressure by passing a small-diameter orifice by applying a pressure difference of at least 20 MPa to the fiber suspension. A method using a high-pressure homogenizer or the like that applies a shearing force or a cutting force to the fiber by causing it to collide with it and rapidly decelerate it. Among these, a method using a high-pressure homogenizer is particularly preferable.

Synthetic fibers include polyester, acrylic, polyolefin, wholly aromatic polyester, wholly aromatic polyester amide, polyamide, semi-aromatic polyamide, wholly aromatic polyamide, wholly aromatic polyether, wholly aromatic polycarbonate, polyimide, polyamideimide ( PAI), polyetheretherketone (PEEK), polyphenylene sulfide (PPS), poly-p-phenylenebenzobisoxazole (PBO), polybenzimidazole (PBI), polytetrafluoroethylene (PTFE), ethylene-vinyl alcohol copolymer Examples thereof include single fibers and composite fibers made of a resin such as coalescence. These synthetic fibers may be used alone or in combination of two or more. Moreover, you may use what divided | segmented various split type composite fibers. Among these, polyester, acrylic, polyolefin, wholly aromatic polyester, wholly aromatic polyester amide, polyamide, semi-aromatic polyamide, and wholly aromatic polyamide are preferable, and polyester, acrylic, and polyolefin are more preferable. When polyester, acrylic, and polyolefin are used, each fiber and fibrillated solvent-spun cellulose fibers are more easily entangled with each other than other synthetic fibers to easily form a network structure. A separator for a lithium ion secondary battery having excellent mechanical strength can be obtained.

The average fiber diameter of the synthetic fiber is preferably 0.1 to 20 μm, more preferably 0.1 to 15 μm, and further preferably 0.1 to 10 μm. If the average fiber diameter is less than 0.1 μm, the fibers may be too thin and fall off from the separator. If the average fiber diameter is larger than 20 μm, it may be difficult to reduce the thickness of the separator. The average fiber diameter is an average value of 100 randomly selected fibers obtained by measuring the fiber diameter of the fibers forming the separator from a scanning electron micrograph of the separator.

The fiber length of the synthetic fiber is preferably 0.1 to 15 mm, more preferably 0.5 to 10 mm, and further preferably 2 to 5 mm. When the fiber length is shorter than 0.1 mm, the separator may fall off, and when the fiber length is longer than 15 mm, the fiber may be entangled, resulting in uneven thickness.

The separator for a lithium ion secondary battery of the present invention contains solvent-spun cellulose fibers having a modified freeness of 0 to 250 ml and synthetic fibers, and fibers other than fibrillated natural cellulose fibers having a modified freeness of 0 to 400 ml. Also good. For example, natural cellulose fibers, pulped natural cellulose fibers, fibrils made of synthetic resins, pulped products made of synthetic resins, fibrillated products made of synthetic resins, inorganic fibers, solvent-spun cellulose fibers having a modified freeness of more than 250 ml, Examples thereof include fibrillated natural cellulose fibers having a modified freeness of more than 400 ml.

As in the lithium ion secondary battery separator (9) of the present invention, the porous sheet preferably contains carboxymethyl cellulose. When forming a separator for a lithium ion secondary battery, solvent-spun cellulose fibers having a modified freeness of 0 to 250 ml and synthetic fibers are allowed to contain carboxymethyl cellulose so that the carboxymethyl cellulose is adsorbed on these fibers, particularly cellulose fibers. While dispersibility improves, fiber twist is suppressed, the formation at the time of using a separator improves, and mechanical strength becomes strong. Furthermore, since the dehydrating property of the fibers can be adjusted moderately, the pores of the separator sheet can be easily controlled, the pore diameter distribution can be brought close to the desired value, and the separator can be made dense and homogeneous, so that the internal short circuit failure rate, discharge Variations in characteristics can be suppressed.

In addition, solvent-spun cellulose fibers having a modified freeness of 0 to 250 ml and synthetic fibers are easily entangled with each other, and carboxymethylcellulose improves the strength of the separator by forming a more homogeneous fiber network. The separator can be made denser and thinner, the internal resistance can be lowered, the internal short-circuit failure rate and the variation in discharge characteristics can be suppressed, and the cycle characteristics can be improved. It has an excellent effect to make it better.

Carboxymethyl cellulose is a cellulose derivative synthesized by reacting monochloroacetic acid and the like using wood pulp, linter pulp and the like as raw materials, and is industrially obtained by a known production method such as a water medium method or a solvent method. is there. Carboxymethylcellulose can be produced in various degrees of polymerization depending on the properties and production methods of the pulp fibers used, but the viscosity of a 1% by weight aqueous solution is 5 to 16,000 mPa · s (0.1 N-NaCl solvent, 25 ° C, B-type viscometer), average degree of polymerization of 100 to 4,500, and average molecular weight of 20,000 to 1,000,000. Most of the carboxymethyl cellulose according to the present invention is a carboxylic acid sodium salt or potassium salt. To be precise, it is carboxymethyl cellulose sodium or carboxymethyl cellulose potassium, but the description of sodium or potassium is conventionally omitted, and carboxymethyl cellulose is simply used. Is displayed. In the present invention, it is preferable to use a sodium salt of carboxymethyl cellulose which is inexpensive and easily obtains the effects of the present invention.

In preparing a fiber slurry containing carboxymethyl cellulose, a method in which fibers are added or dispersed sequentially or simultaneously in a solution in which carboxymethyl cellulose is dissolved in a solvent, usually water in advance, 1 in a solution in which carboxymethyl cellulose is dissolved in advance. Examples thereof include a method in which more than one type of fiber is introduced and mixed with another type of fiber slurry prepared elsewhere, or a method in which carboxymethyl cellulose is added to a slurry containing at least one type of fiber. Among these, a method of preparing a fiber slurry in a solution in which carboxymethyl cellulose is dissolved is preferable. In the case where fibers are added and dispersed sequentially or simultaneously in a solution in which carboxymethyl cellulose is dissolved in advance, the fibers may be added after forming a high-concentration slurry.

In addition, when adding carboxymethyl cellulose to the fiber slurry, it is preferable to add a solution in which carboxymethyl cellulose is dissolved in advance. At that time, if the concentration of carboxymethyl cellulose is too high, the viscosity is high and the diffusion into the fiber slurry is poor. Therefore, the concentration of carboxymethyl cellulose is preferably 0.5 to 5% by mass, more preferably 0.5 to 3% by mass. preferable. Further, when dissolving carboxymethylcellulose, an organic solvent (for example, methanol, ethanol, etc.) that is compatible with water is added in an amount of 10 to 30% by mass with respect to water, as long as carboxymethylcellulose is stably dissolved. You may mix in the ratio. Furthermore, another substance such as an electrolyte such as mirabilite may be mixed within a range in which carboxymethylcellulose is stable.

Carboxymethylcellulose is particularly effective for the dispersion of cellulosic fibers such as solvent-spun cellulose fibers and fibrillated natural cellulose fibers, so even when carboxymethylcellulose is added in advance to the fiber slurry preparation liquid, Even when it is added during the preparation of the fiber slurry, it is preferably used in combination with at least a cellulosic fiber.

The addition rate of carboxymethylcellulose is preferably 0.5 to 2.0 mass%, more preferably 0.8 to 1.5 mass%, based on the total fiber mass used in the separator for lithium ion secondary batteries of the present invention. When the addition rate of carboxymethyl cellulose is less than 0.5% by mass, the effect of improving the formation may not be recognized. Conversely, when the addition rate of carboxymethyl cellulose exceeds 2.0% by mass, the water retention of carboxymethyl cellulose Therefore, there is a case where a longer drying process is required or the moisture content of the lithium ion secondary battery separator is increased, which may adversely affect the battery characteristics. Furthermore, since the drainage is reduced, the productivity during papermaking may be reduced.

As described above, carboxymethylcellulose is synthesized by reacting monochloroacetic acid or the like with a pulp raw material, but polar carboxyl groups solubilize cellulose and facilitate chemical reaction. At this time, the introduction ratio of monochloroacetic acid or the like to cellulose is represented by “degree of etherification”. The dispersibility of the fiber can be improved by adsorbing carboxymethylcellulose to the fiber according to the present invention, particularly the cellulosic fiber, but the higher the degree of etherification in carboxymethylcellulose is, the better the dispersibility of the fiber is. The degree of etherification of carboxymethylcellulose is preferably 0.5 or more, more preferably 0.7 or more.

As in the separator for lithium ion secondary battery (10) of the present invention, a core-sheath type heat-fusible fiber having a non-thermal adhesive component in the core and a thermal adhesive component in the sheath is used as at least one synthetic fiber. It is preferable to contain. By containing the core-sheath type heat-sealable fiber, the fibers can be bonded to each other with the core-sheath type heat-sealable fiber, and as a result, the self generated when stored for a long time in a charged state at a high voltage Since discharge can be suppressed, a separator having excellent voltage maintenance ratio characteristics can be obtained. Also. A core-sheath type heat-sealing fiber is used to create a porous fiber network formed by entanglement between synthetic fibers and solvent-spun cellulose fibers having a modified freeness of 0 to 250 ml and synthetic fibers, and hydrogen bonds between solvent-spun celluloses. It is possible to increase the separator strength by heat bonding without damaging the quality structure.

Examples of the heat-fusible fiber include a core-sheath type, an eccentric type, a split type, a side-by-side type, a sea-island type, an orange type, a multi-bimetal type, a single component fiber (single fiber), and the like. The core-sheath type heat-sealable fiber maintains the fiber shape of the core part, and softens, melts or wet-heat dissolves only the sheath part to thermally bond the fibers to each other, so that the porous structure of the separator is not impaired. It is suitable for adhering. Occurs when the sheath of the core-sheath type heat-bonded fiber is softened, melted or wet-heat-dissolved by heating or moist heat, and the fibers are heat-bonded to each other and stored for a long time in a state of being charged at a high voltage. Since the self-discharge can be suppressed, a separator having excellent voltage retention characteristics can be obtained.

The resin component constituting the core and sheath of the core-sheath type heat-sealing fiber is not particularly limited, and any resin having fiber-forming ability may be used. For example, the core / sheath combination includes polyethylene terephthalate / polyester copolymer, polyethylene terephthalate / polyethylene, polyethylene terephthalate / polypropylene, polyethylene terephthalate / ethylene-propylene copolymer, polyethylene terephthalate / ethylene-vinyl alcohol copolymer. , Polypropylene / polyethylene, and high melting point polylactic acid / low melting point polylactic acid. The melting point, softening point or wet heat melting temperature of the resin component in the core part is preferably 20 ° C. or more higher than the melting point or softening point of the resin component in the sheath part from the viewpoint of easy production of the nonwoven fabric.

As the core-sheath type heat-sealing fiber used for the lithium ion secondary battery separator of the present invention, a combination of core: polyethylene terephthalate / sheath: polyester copolymer is preferable because the separator strength becomes higher. The polyester copolymer used for the sheath is a copolymer of polyethylene terephthalate and one or more compounds selected from isophthalic acid, sebacic acid, adipic acid, diethyl glycol, 1,4-butadiol and the like. preferable.

The core-sheath type heat-sealing fiber preferably has a fineness of 0.007 to 1.7 dtex, more preferably 0.02 to 1.1 dtex, and even more preferably 0.05 to 0.5 dtex. If the fineness is less than 0.007 dtex, it may be too thin and fall off from the separator. If the fineness exceeds 1.7 dtex, it will be difficult to get entangled with the fibrillated solvent-spun cellulose fiber, ensuring the required denseness. It may not be possible.

The fiber length of the core-sheath type heat-sealing fiber is preferably 0.1 to 15 mm, more preferably 0.5 to 10 mm, and further preferably 2 to 5 mm. When the fiber length is shorter than 0.1 mm, the separator may fall off, and when the fiber length is longer than 15 mm, the fiber may be entangled, resulting in uneven thickness.

Core-sheath-type heat fusion fiber In the lithium ion secondary battery separator of the present invention, the content of the core-sheath-type heat fusion fiber is preferably 5 to 40% by mass, and more preferably 8 to 30% by mass. Is more preferably 10 to 20% by mass. If the content is less than 5% by mass, the voltage maintenance rate characteristics and mechanical strength of the separator may be insufficient. If it exceeds 40% by mass, a film is formed on the separator surface, and the ion conductivity is inhibited, so that the internal resistance may increase or the discharge characteristics may decrease.

The separator for the lithium ion secondary battery of the present invention is a circular paper machine, a long paper machine, a short paper machine, an inclined paper machine, a combination paper machine formed by combining the same or different types of paper machines from these, and the like Can be produced by a wet method of wet paper making. In addition to the fiber raw material, a dispersant, a thickener, an inorganic filler, an organic filler, an antifoaming agent, and the like are appropriately added to the raw material slurry as necessary, and a solid content concentration of about 5 to 0.001% by mass is added. A raw material slurry is prepared. This raw slurry is further diluted to a predetermined concentration to make paper. The separator for a lithium ion secondary battery obtained by papermaking is subjected to calendering, thermal calendering, heat treatment and the like as necessary.

Also, it is preferable that the separator for lithium ion secondary battery obtained by blending the core-sheath-type heat-sealing fiber and heat-treating is subjected to heat treatment to increase the mechanical strength. Examples of the heat treatment method include a heat treatment method using a heating device such as a hot air dryer, a heating roll, an infrared (IR) heater, or the like while continuously performing the heat treatment or pressurizing. The heat treatment temperature is equal to or higher than the temperature at which the sheath portion of the core-sheath fiber is melted or softened, and lower than the temperature at which the core portion of the core-sheath fiber and other contained fibers are melted, softened or decomposed. It is preferable.

The thickness of the lithium ion secondary battery separator of the present invention is preferably 6 to 50 μm, more preferably 8 to 45 μm, and even more preferably 10 to 40 μm. If the thickness is less than 6 μm, sufficient mechanical strength cannot be obtained, insulation between the positive electrode and the negative electrode is insufficient, internal short-circuit failure rate, variation in discharge characteristics, and capacity maintenance ratio and cycle characteristics are low. It may get worse. If it is thicker than 50 μm, the internal resistance of the lithium ion secondary battery may increase or the discharge characteristics may decrease. The thickness of the separator of the present invention means a value measured by a method defined in JIS B7502, that is, a value measured by an outer micrometer at a load of 5N.

In the separator for a lithium ion secondary battery of the present invention, it is preferable that the average pore diameter is 0.10 μm or more and the maximum pore diameter is 6.0 μm or less. This pore diameter can be achieved by entanglement of solvent-spun cellulose fibers with a modified freeness of 0-250 ml with synthetic fibers. If the average pore diameter is less than 0.10 μm, the internal resistance of the lithium ion secondary battery may be high, or the discharge characteristics may be low. When the maximum pore diameter is 6 μm or more, the internal short-circuit failure rate and the variation in discharge characteristics of the lithium ion secondary battery may increase. More preferably, the average pore diameter is 0.10 μm or more and the maximum pore diameter is 4.0 μm or less, more preferably the minimum pore diameter is 0.15 μm or more and the maximum pore diameter is 3.0 μm or less. .

The basis weight of the lithium ion secondary battery separator of the present invention is preferably 5 ~ 40g / ^{m 2,} more preferably 7 ~ 30g / ^{m 2,} more preferably 10 ~ 20g / ^{m 2.} If it is less than 5 g / m ² , sufficient mechanical strength may not be obtained, or insulation between the positive electrode and the negative electrode may be insufficient, resulting in increased internal short-circuit failure rate and variation in discharge characteristics. If it exceeds 40 g / m ² , the internal resistance of the lithium ion secondary battery may increase or the discharge characteristics may decrease.

The layer structure of the lithium ion secondary battery separator of the present invention is not particularly limited, and may be a single layer structure or a multilayer structure such as two layers or three layers. From the viewpoint of suppression of generation, a multilayer structure such as two layers or three layers is more preferable. In the case of a multilayer structure, there are no particular restrictions on the method of laminating each layer, but since there is no delamination between the layers, a weaving method can be suitably used. Wet-making method is a papermaking slurry in which fibers are dispersed in water to form a uniform papermaking slurry, and this papermaking slurry is made using a papermaking machine having at least two wires such as a circular net, a long net, and an inclined type. This is a method for obtaining a fibrous web. Further, for example, when a two-layer structure consisting of a surface layer and a back layer is used, each layer may have the same blending composition or may be different, but at least each layer is solvent-spun with a modified freeness of 0 to 250 ml. A layer containing cellulose fibers as an essential component is preferred. If there is a layer containing no solvent-spun cellulose fiber having a modified freeness of 0 to 250 ml, the peel strength between the layers is inferior, sufficient mechanical strength is not obtained, or the electrolyte retainability is insufficient. The internal resistance may be high, or the separator may be insufficiently dense, resulting in a high internal short-circuit failure rate.

Examples of the negative electrode active material of the lithium ion secondary battery include carbon materials such as graphite and coke, metallic lithium, aluminum, silica, tin, nickel, and an alloy of lithium and lithium, SiO, SnO, Metal oxides such as Fe ₂ O ₃ , WO ₂ , Nb ₂ O ₅ , Li _4/3 Ti _5/3 O ₄ , and nitrides such as Li _0.4 CoN are used. As the positive electrode active material, lithium cobaltate, lithium manganate, lithium nickelate, lithium titanate, lithium nickel manganese oxide, or lithium iron phosphate is used. The lithium iron phosphate may further be a composite with one or more metals selected from manganese, chromium, cobalt, copper, nickel, vanadium, molybdenum, titanium, zinc, aluminum, gallium, magnesium, boron, and niobium.

As an electrolytic solution for a lithium ion secondary battery, a solution obtained by dissolving a lithium salt in an organic solvent such as propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, dimethoxyethane, dimethoxymethane, or a mixed solvent thereof is used. Examples of the lithium salt include lithium hexafluorophosphate (LiPF ₆ ) and lithium tetrafluoroborate (LiBF ₄ ). As solid electrolyte, what melt | dissolved lithium salt in gel-like polymers, such as polyethyleneglycol, its derivative (s), polymethacrylic acid derivative, polysiloxane, its derivative (s), polyvinylidene fluoride, is used.

Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited to the examples. In the examples, all parts and percentages are by mass unless otherwise specified.

<< Examples 1 to 24, Comparative Examples 1 to 10 >>
<Fiber A1>
Using a refiner, solvent-spun cellulose fibers having an average fiber diameter of 10 μm and a fiber length of 4 mm were treated, and solvent-spun cellulose fibers having a modified freeness of 0 ml were designated as fiber A1.

<Fiber A2>
Using a refiner, solvent-spun cellulose fibers having an average fiber diameter of 10 μm and a fiber length of 4 mm were treated, and solvent-spun cellulose fibers having a modified freeness of 120 ml were designated as fiber A2.

<Fiber A3>
Using a refiner, solvent-spun cellulose fibers having an average fiber diameter of 10 μm and a fiber length of 4 mm were treated, and solvent-spun cellulose fibers having a modified freeness of 250 ml were designated as fiber A3.

<Fiber A4>
Using a refiner, solvent-spun cellulose fibers having an average fiber diameter of 10 μm and a fiber length of 4 mm were treated, and solvent-spun cellulose fibers having a modified freeness of 260 ml were designated as fiber A4.

<Fiber A5>
Using a refiner, solvent-spun cellulose fibers having an average fiber diameter of 10 μm and a fiber length of 4 mm were treated, and solvent-spun cellulose fibers having a modified freeness of 350 ml were designated as fiber A5.

<Synthetic fiber B1>
Polyethylene terephthalate fiber having an average fiber diameter of 3 μm and a fiber length of 3 mm was designated as synthetic fiber B1.

<Synthetic fiber B2>
An acrylic fiber having an average fiber diameter of 5 μm and a fiber length of 3 mm was designated as synthetic fiber B2.

<Synthetic fiber B3>
A polypropylene fiber having an average fiber diameter of 4 μm and a fiber length of 3 mm was defined as a synthetic fiber B3.

<Synthetic fiber B4>
Polyester core-sheath type heat-sealable fiber having an average fiber diameter of 10 μm, a fiber length of 5 mm, a polyethylene terephthalate (melting point: 253 ° C.) sheath and a polyethylene terephthalate-isophthalate copolymer (softening point: 75 ° C.) sheath B4.

<Synthetic fiber B5>
Polyethylene terephthalate fiber having an average fiber diameter of 20 μm and a fiber length of 5 mm was designated as synthetic fiber B5.

<Synthetic fiber B6>
A polyethylene terephthalate fiber having an average fiber diameter of 22 μm and a fiber length of 5 mm was defined as a synthetic fiber B6.

<Synthetic fiber B7>
A polyethylene terephthalate fiber having an average fiber diameter of 0.1 μm and a fiber length of 2 mm was defined as a synthetic fiber B7.

<Synthetic fiber B8>
A polyethylene terephthalate fiber having an average fiber diameter of 0.08 μm and a fiber length of 2 mm was designated as synthetic fiber B8.

<Fibrylated natural cellulose fiber C1>
The linter was treated using a high-pressure homogenizer to produce a fibrillated natural cellulose fiber C1 having a modified freeness of 0 ml.

<Fibrylated natural cellulose fiber C2>
The linter was treated with a high-pressure homogenizer to produce a fibrillated natural cellulose fiber C2 having a modified freeness of 270 ml.

<Fibrylated natural cellulose fiber C3>
The linter was treated with a high-pressure homogenizer to produce a fibrillated natural cellulose fiber C3 having a modified freeness of 400 ml.

<Fibrylated natural cellulose fiber C4>
The linter was treated using a high-pressure homogenizer to produce a fibrillated natural cellulose fiber C4 having a modified freeness of 500 ml.

<Fiber D1>
Using a refiner, para-type wholly aromatic polyamide having an average fiber diameter of 10 μm and fiber length of 3 mm was treated, and fibrillated para-type wholly aromatic polyamide fiber having a modified freeness of 500 ml was designated as fiber D1.

<Fiber E1>
The hemp fiber having an average fiber diameter of 7 μm was designated as fiber E1.

In Table 1, “kinds” of synthetic fibers and other fibers are as follows.
“PET”: Polyethylene terephthalate fiber “AA”: Acrylic fiber “PP”: Polypropylene fiber “PET / PEs-C”: Polyester core-sheath type heat-sealing fiber “PA”: fibrillated para-type wholly aromatic polyamide fiber

Examples 1 to 24 and Comparative Examples 1 to 10
<Separator>
According to the raw materials and contents shown in Table 1, a papermaking slurry was prepared, and wet papermaking was performed using a circular paper machine to prepare separators of Examples 1 to 24 and Comparative Examples 1 to 9. The thickness was adjusted by calendaring at room temperature. In addition, a porous polyethylene film (thickness 22 μm, porosity 40%) was used as the separator of Comparative Example 10.

<Lithium ion secondary battery A>
[Preparation of Negative Electrode 1]
A slurry in which 97% by mass of natural graphite and 3% by mass of polyvinylidene fluoride were mixed and dispersed in N-methyl-2-pyrrolidone was prepared, applied to both sides of a copper foil having a thickness of 15 μm, and rolled. A negative electrode for a lithium ion secondary battery having a thickness of 100 μm was produced by vacuum drying at a temperature of 2 ° C. for 2 hours.

[Preparation of Positive Electrode 1]
A slurry in which 95% by mass of LiMn ₂ O ₄ , ₂ % by mass of acetylene black and 3% by mass of polyvinylidene fluoride are mixed and dispersed in N-methyl-2-pyrrolidone is prepared, and both surfaces of an aluminum foil having a thickness of 20 μm are prepared. After being applied and rolled onto the substrate, it was vacuum-dried at 150 ° C. for 2 hours to produce a positive electrode for a lithium ion secondary battery having a thickness of 100 μm.

[Production of lithium ion secondary battery A]
The negative electrode 1 and the positive electrode 1 are wound so that the separators of Examples 1 to 24 and Comparative Examples 1 to 7 and 9 are interposed between the electrodes, respectively, and stored in a cylindrical container made of aluminum alloy, and the lead body is welded. Went. Next, the whole cylindrical container was vacuum-dried at 150 ° C. for 10 hours. This was allowed to cool to room temperature in a vacuum, and then an electrolyte solution was injected and sealed up, and lithium ion secondary batteries A of Examples 1 to 24 and Comparative Examples 1 to 7 and 9 were produced. As the electrolytic solution, a solution obtained by dissolving LiPF _{6 in a} mixed solvent composed of 30% by mass of ethylene carbonate and 70% by mass of diethyl carbonate so as to be 1.2M was used.

The negative electrode 1 and the positive electrode 1 were each wound so that the separator of Comparative Example 8 was interposed between the electrodes, housed in a cylindrical container made of aluminum alloy, and the lead body was welded. Next, the whole cylindrical container was vacuum-dried at 110 ° C. for 24 hours. This was allowed to cool to room temperature in a vacuum, and then an electrolytic solution was injected and sealed to prepare a lithium ion secondary battery A of Comparative Example 8. As the electrolytic solution, a solution obtained by dissolving LiPF _{6 in a} mixed solvent composed of 30% by mass of ethylene carbonate and 70% by mass of diethyl carbonate so as to be 1.2M was used.

Moreover, the negative electrode 1 and the positive electrode 1 were wound so that the separator of Comparative Example 10 was interposed between the electrodes, accommodated in a cylindrical container made of aluminum alloy, and the lead body was welded. Next, the whole cylindrical container was vacuum-dried at 80 ° C. for 10 hours. This was allowed to cool to room temperature in a vacuum, and then an electrolytic solution was injected and sealed to prepare a lithium ion secondary battery A of Comparative Example 10. As the electrolytic solution, a solution obtained by dissolving LiPF _{6 in a} mixed solvent composed of 30% by mass of ethylene carbonate and 70% by mass of diethyl carbonate so as to be 1.2M was used.

<Lithium ion secondary battery B>
[Preparation of Negative Electrode 2]
A slurry in which 81% by mass of mesocarbon microbeads, 14% by mass of acetylene black and 5% by mass of polytetrafluoroethylene were mixed and dispersed in N-methyl-2-pyrrolidone was prepared, and both sides of a copper foil having a thickness of 15 μm After being applied and rolled onto the substrate, it was vacuum-dried at 150 ° C. for 2 hours to produce a negative electrode for a lithium ion secondary battery having a thickness of 100 μm.

[Preparation of positive electrode 2]
A slurry in which 95% by mass of LiMn ₂ O ₄ , ₂ % by mass of acetylene black and 3% by mass of polytetrafluoroethylene were mixed and dispersed in N-methyl-2-pyrrolidone was prepared, and an aluminum foil having a thickness of 20 μm was prepared. After applying and rolling on both surfaces, vacuum drying was performed at 150 ° C. for 2 hours to prepare a positive electrode for a lithium ion secondary battery having a thickness of 100 μm.

[Preparation of lithium ion secondary battery B]
The separators of Examples 1 to 24 and Comparative Examples 1 to 7 and 9, the positive electrode 2 and the negative electrode 2 were bonded together in this order, and the lead wires were pulled out to produce a battery body. Next, the battery body was vacuum-dried at 140 ° C. for 10 hours, allowed to cool to room temperature in a vacuum, and then inserted into an aluminum laminate film. From 1M-LiPF ₆ / EC + DEC (3: 7 vol%) An appropriate amount of the resulting electrolyte was poured and vacuum impregnated, and then the excess electrolyte was removed and sealed to prepare a lithium ion secondary battery B (battery capacity = 30 mAh equivalent).

The separator of Comparative Example 8, the positive electrode 2, and the negative electrode 2 were bonded together in this order, and the lead wire was drawn out to produce a battery body. Next, the battery body was vacuum-dried at 110 ° C. for 24 hours, allowed to cool to room temperature in a vacuum, and then inserted into an aluminum laminate film. From 1M-LiPF ₆ / EC + DEC (3: 7 vol%) An appropriate amount of the resulting electrolyte was poured and vacuum impregnated, and then the excess electrolyte was removed and sealed to prepare a lithium ion secondary battery B (battery capacity = 30 mAh equivalent).

The separator of Comparative Example 10, the positive electrode 2 and the negative electrode 2 were bonded together in this order, and the lead wires were drawn out to produce a battery body. Next, the battery body was vacuum-dried at 80 ° C. for 15 hours, allowed to cool to room temperature in a vacuum, and then inserted into an aluminum laminate film, and from 1M-LiPF ₆ / EC + DEC (3: 7 vol%). An appropriate amount of the resulting electrolyte was poured and vacuum impregnated, and then the excess electrolyte was removed and sealed to prepare a lithium ion secondary battery B (battery capacity = 30 mAh equivalent).

The following evaluation was performed on the separators and lithium ion secondary batteries of Examples and Comparative Examples, and the results are shown in Tables 2 and 3.

[Basis weight]
The basis weight was measured in accordance with JIS P8124.

[Thickness]
The thickness was measured by the method defined in JIS B7502, that is, by an outer micrometer at 5N load.

[Pore diameter measurement]
About the produced separator, it measured according to JIS K3832, ASTM F316-86, and ASTM E1294-89 using the product name: Palm porometer CFP-1500A made from PMI, and the largest pore diameter of each base material and an average The pore diameter was measured.

[Separator moisture content]
After measuring the sample mass of the separator left for 2 days under the conditions of temperature 20 ° C. and humidity 55%, it is dried at 105 ° C. for 4 hours. Subsequently, after standing to cool in a desiccator, the weight loss was calculated | required by measuring mass, and the moisture content of the separator was computed by following Formula.
Separator moisture content (%) = (loss on drying (g) / sample mass (g)) × 100

[Separator strength]
A metal needle having a diameter of 1 mm with a curvature of 1.6 at the tip is attached to a desktop material testing machine (trade name: STA-1150, manufactured by Orientec Co., Ltd.), and 1 mm / s perpendicular to the sample surface. It was lowered until it penetrated at a constant speed. The maximum load (g) at this time was measured and used as the separator strength.

[Internal resistance]
After charging 100 lithium ion secondary batteries A of Examples and Comparative Examples for 30 minutes at 1 C, the internal resistance was measured at 1 kHz AC, and each average value was calculated.

[Internal short-circuit failure rate]
Internal short circuit failure when repeating 100 cycles of constant current charging / discharging in the charging / discharging voltage range of 2.8-4.2V, charging / discharging current 1C using 100 lithium ion secondary batteries A of Examples and Comparative Examples The rate was calculated.

[C rate discharge test (discharge capacity)]
Using 50 each of the lithium ion secondary batteries B of Examples and Comparative Examples, after performing 3 cycle aging at 1C, constant current and constant voltage charging (1 / 10C cut) at 1C, 4.2V, and then 0. The constant current discharge test (2.8V cut) was performed by changing the current values to 2C, 0.5C, 1C, 3C, and 5C, and the average values of the discharge capacities at 0.2C and 5C were calculated.

[C-rate discharge test (discharge capacity variation)]
Using 50 each of the lithium ion secondary batteries B of Examples and Comparative Examples, after performing 3 cycle aging at 1C, constant current and constant voltage charging (1 / 10C cut) at 1C, 4.2V, and then 0. The constant current discharge test (2.8V cut) was performed by changing the current value to 2C, 0.5C, 1C, 3C, and 5C, and the discharge capacity at 0.2C and 5C was evaluated according to the following criteria.

A: The difference in discharge capacity is 1.0% or less with respect to the average value.
◯: The difference in discharge capacity is more than 1.0% and 2.5% or less with respect to the average value.
(Triangle | delta): The difference of discharge capacity exceeds 2.5% with respect to an average value, and is 5.0% or less.
X: The difference of discharge capacity is over 5.0% with respect to the average value.

[Cycle characteristics (capacity retention rate after 100 cycles)]
Using 20 lithium ion secondary batteries B of Examples and Comparative Examples, constant current and constant voltage charge (1 / 10C cut) at 1C and 4.2V, followed by constant current discharge test (2.8V under the condition of 1C) The average capacity retention rate after 100 cycles (after 100 cycles / 1 cycle capacity) was calculated.

As shown in Table 2, the separators for lithium ion secondary batteries of Examples 1 to 24 were 10 to 90% by mass of solvent-spun cellulose fibers having a modified freeness of 0 to 250 ml and 10 to 90% by mass of synthetic fibers. Since it contained, the moisture content was low, the mechanical strength was strong, and it was excellent.

That is, since the separators for lithium ion secondary batteries of Examples 1 to 24 contain 10 to 90% by mass of synthetic fiber, they are contained in comparison with the paper separator made of solvent-spun cellulose and hemp fiber of Comparative Example 8. The moisture content could be kept low. Furthermore, the strength of the separator has increased because the fibers are easily entangled and a fiber network is easily formed. On the other hand, in the separator for the lithium ion secondary battery of Comparative Example 1, the content of the solvent-spun cellulose fiber having a modified freeness of 0 to 250 ml in the separator is more than 90% by mass, and the synthetic fiber contained in the separator is 10% by mass. Since it was less, the moisture content was high and the separator strength was weak. Since the separator for the lithium ion secondary battery of Comparative Example 4 had a synthetic fiber content of less than 10% by mass in the separator, the separator strength was weak. Although the separator for the lithium ion secondary battery of Comparative Example 9 contained fibrillated heat-resistant fibers, the strength of the separator was weak because the modified freeness of solvent-spun cellulose fibers exceeded 250 ml.

As shown in Table 3, the lithium ion secondary batteries of Examples 1 to 24 contain 10 to 90% by mass of solvent-spun cellulose fibers having a modified freeness of 0 to 250 ml and 10 to 90% by mass of synthetic fibers. Since a separator made of a porous sheet is used, the internal resistance and internal short-circuit failure rate, in particular, the discharge characteristics at high rates, the variations thereof, and the cycle characteristics were excellent.

That is, in the lithium ion secondary batteries of Examples 1 to 24, since the separator contains 10 to 90% by mass of solvent-spun cellulose fibers having a modified freeness of 0 to 250 ml, the liquid retention of the electrolyte is good. Since the ion conductivity can be improved, the internal resistance is low, and the discharge characteristics and the cycle characteristics are particularly excellent at a high rate. On the other hand, in the lithium ion secondary batteries of Comparative Examples 2 and 3, the content of the solvent-spun cellulose fiber having a modified freeness of 0 to 250 ml is less than 10% by mass. In the secondary battery, since the separator did not contain solvent-spun cellulose fibers having a modified freeness of 0 to 250 ml, the electrolyte solution was inferior in liquid retention and had a high internal resistance.

Since the lithium ion secondary batteries of Examples 1 to 24 contain 10 to 90% by mass of solvent-spun cellulose fibers having a modified freeness of 0 to 250 ml in the separator, the separator can be made dense. Therefore, the internal short circuit failure rate and the variation in discharge capacity were low and excellent. On the other hand, in the lithium ion secondary batteries of Comparative Examples 2 and 3, the content of the solvent-spun cellulose fibers having a modified freeness of 0 to 250 ml in the separator is less than 10% by mass, and the separator is insufficiently dense. , Internal short-circuit defect rate and discharge capacity variation increased. In the lithium ion secondary battery of Comparative Example 5, the modified drainage degree of the solvent-spun cellulose fiber in the separator was larger than 0 to 250 ml, and the density of the separator was insufficient, so the internal short circuit defect rate was slightly increased. . In the lithium ion secondary batteries of Comparative Examples 6 and 7, since the separator does not contain solvent-spun cellulose fibers having a modified freeness of 0 to 250 ml, the separator is insufficiently dense, and the internal short-circuit failure rate, Dispersion of discharge capacity became high. The lithium ion secondary battery of Comparative Example 10 using a porous polyethylene film had a high internal resistance and a high rate discharge capacity.

The lithium ion secondary battery of Example 8 contains 5% by mass of fibrillated natural cellulose fibers having a modified freeness of 0 to 400 ml in the separator. In the lithium ion secondary batteries of Examples 9 to 12, the separator contains 10% by mass of fibrillated natural cellulose fibers having a modified freeness of 0 to 400 ml. In the lithium ion secondary battery of Example 13, 20% by mass of fibrillated natural cellulose fiber having a modified freeness of 0 to 400 ml was contained in the separator. Therefore, the separator can be made denser and thinner, and the lithium ion secondary batteries of Examples 8 to 13 have lower internal resistance and higher rate than the lithium ion secondary batteries of Examples 1 to 7 and 15 to 18. The discharge capacity of was high.

The lithium ion secondary batteries of Examples 9 to 12 use separators that are blended with fibrillated natural cellulose fibers C1 to C4 having the same basis weight and the same thickness and different modified drainage degrees. The lithium ion secondary battery of Example 12 using the separator containing the fibrillated natural cellulose fiber C4 having a modified freeness greater than 400 ml was slightly lacking in denseness when the thickness of the separator was reduced. The internal short circuit defect rate was slightly higher than that of the lithium ion secondary batteries of Examples 9 to 11 using separators containing fibrillated natural cellulose fibers C1 to C3 having a freeness of 0 to 400 ml.

In the lithium ion secondary battery of Example 14, since the content of the fibrillated natural cellulose fiber having a modified freeness of 0 to 400 ml in the separator is more than 20% by mass, the separator becomes slightly dense and the ion conductivity is increased. The internal resistance was slightly higher than those of the lithium ion secondary batteries of Examples 1 to 13, 15 to 17, 19, and 20, and the high rate discharge capacity was slightly lower.

Since the lithium ion secondary battery of Example 18 has a slightly larger basis weight, a separator having a slightly larger thickness, and an average pore diameter slightly smaller, the lithium ion secondary batteries of Examples 1 to 13, 15 to 17, 19, and 20 The internal resistance was slightly higher than that of the battery, and the high-rate discharge capacity was slightly lower.

The lithium ion secondary battery of Example 20 has a slightly smaller basis weight, a thickness of the separator is slightly thinner, and the maximum pore diameter is slightly larger. Therefore, the internal short circuit is greater than that of the lithium ion secondary batteries of Examples 1 to 11 and 13 to 19. The variation in defective rate and discharge capacity was slightly increased.

The lithium ion secondary battery of Example 22 has a slightly larger average fiber diameter of the synthetic fibers used, so the separator strength is slightly weaker and the discharge capacity variation is slightly higher than that of the lithium ion secondary battery of Example 21. became.

In the lithium ion secondary battery of Example 24, since the average fiber diameter of the synthetic fibers used was slightly thinner, the separator strength was slightly weaker than the lithium ion secondary battery of Example 23, and the cycle characteristics were slightly inferior.

<< Examples 25 to 59, Comparative Examples 11 to 18 >>
<Solvent-spun cellulose fiber>
Using a refiner, solvent-spun cellulose fibers having an average fiber diameter of 10 μm and a fiber length of 4 mm were processed to produce solvent-spun cellulose fibers having the physical properties shown in Tables 4 and 5.

<Physical properties of solvent-spun cellulose fiber>
(1) Ratio of fibers having a length-weighted fiber length of 1.00 mm or more with respect to the solvent-spun cellulose fiber produced by the above method: “Fiber ratio of 1.00 mm or more”
(2) Length weighted fiber length of maximum frequency peak in length weighted fiber length distribution histogram: “fiber length of maximum frequency peak”
(3) In the length-weighted fiber length distribution histogram, the slope of the proportion of fibers having a length-weighted fiber length of every 0.05 mm between 1.00 and 2.00 mm: “ratio of the proportion”
(4) Length-weighted average fiber length: “Average fiber length”
(5) Freeness measured according to JIS P8121, except that an 80-mesh wire mesh having a wire diameter of 0.14 mm and an aperture of 0.18 mm was used as the sieve plate, and the sample concentration was 0.1% by mass. Freeness "
As shown in Table 4.

(1) Ratio of fibers having a length-weighted fiber length of 1.00 mm or more with respect to the solvent-spun cellulose fiber produced by the above method: “Fiber ratio of 1.00 mm or more”
(2) Length weighted fiber length of maximum frequency peak in length weighted fiber length distribution histogram: “fiber length of maximum frequency peak”
(3) Length of peak other than maximum frequency peak Weighted fiber length: “Fiber length of second peak”
(4) Length-weighted average fiber length: “Average fiber length”
(5) Freeness measured according to JIS P8121, except that an 80-mesh wire mesh having a wire diameter of 0.14 mm and an aperture of 0.18 mm was used as the sieve plate, and the sample concentration was 0.1% by mass. Freeness "
As shown in Table 5.

<Fibrylated natural cellulose fiber>
The linter was treated using a high-pressure homogenizer to produce fibrillated natural cellulose fibers having a modified freeness of 0 ml, 270 ml, 400 ml, and 500 ml.

<Separator>
A papermaking slurry was prepared according to the raw materials and contents shown in Tables 6 and 7, and wet papermaking was performed using a circular paper machine to produce separators of Examples and Comparative Examples. The thickness was adjusted by calendaring at room temperature.

In Tables 6 and 7, “kinds” of the synthetic fibers 1 to 3 are as follows.
“PET”: polyethylene terephthalate “AA”: acrylic fiber “PP”: polypropylene fiber “NY”: nylon 66 fiber “PET-B1”: unstretched polyethylene terephthalate fiber (melting point 130 ° C.)

<Lithium ion secondary battery C>
The negative electrode 1 and the positive electrode 1 were wound so that the separators of Examples and Comparative Examples were interposed between the electrodes, respectively, and housed in a cylindrical container made of aluminum alloy, and the lead body was welded. Next, the whole cylindrical container was vacuum-dried at 110 ° C. for 15 hours. This was allowed to cool to room temperature in a vacuum, and then an electrolytic solution was injected and sealed to prepare lithium ion secondary batteries C of Examples and Comparative Examples. As the electrolytic solution, a solution obtained by dissolving LiPF _{6 in a} mixed solvent composed of 30% by mass of ethylene carbonate and 70% by mass of diethyl carbonate so as to be 1.2M was used.

<Lithium ion secondary battery D>
The separator of Example and Comparative Example, positive electrode 2 and negative electrode 2 were bonded together in this order, and the lead wires were drawn out to produce a battery body. Next, the battery main body was vacuum-dried at 110 ° C. for 15 hours, allowed to cool to room temperature in a vacuum, and then inserted into an aluminum laminate film. From 1M-LiPF ₆ / EC + DEC (3: 7 vol%) An appropriate amount of the resulting electrolyte was poured and vacuum impregnated, and then the excess electrolyte was removed and sealed to prepare a lithium ion secondary battery D (corresponding to battery capacity = 30 mAh).

About the separator and lithium ion secondary battery of Example and Comparative Example, basis weight, thickness, maximum pore diameter, average pore diameter, separator strength, internal short-circuit failure rate, 5C discharge capacity, capacity maintenance ratio, evaluation of variation in discharge capacity The results are shown in Table 8.

[Liquid retention rate]
The separators of Examples and Comparative Examples cut to 10 cm square were immersed in propylene carbonate for 1 minute, then suspended vertically and held for 15 minutes.

Liquid retention (%) = (Separator mass before immersion in propylene carbonate / Separator mass after holding for 15 minutes) × 100
As a result, the liquid retention rate was calculated and evaluated according to the following criteria.

A: The liquid retention rate is 200% or more.
○: The liquid retention rate is 150% or more and less than 200%.
Δ: The liquid retention rate is 50% or more and less than 150%.
X: The liquid retention rate is less than 50%.

[Internal short-circuit failure rate]
Internal short-circuit failure rate when 100 cycles of constant-current charge / discharge were performed in each of 100 lithium ion secondary batteries C of Examples and Comparative Examples, with a charge / discharge voltage range of 2.8 to 4.2 V and a charge / discharge current of 1 C. Was calculated.

[C rate discharge test (discharge capacity) (discharge capacity variation)]
Using 50 lithium ion secondary batteries D of Examples and Comparative Examples, after performing 3 cycle aging at 1C, constant current and constant voltage charging (1 / 10C cut) at 1C and 4.2V, and constant at 5C A current discharge test (2.8 V cut) was performed. The average value of the discharge capacity was calculated. Moreover, the variation in discharge capacity was calculated and evaluated according to the following criteria.

(Discharge capacity variation)
A: The difference in discharge capacity is 1.0% or less with respect to the average value.
◯: The difference in discharge capacity is more than 1.0% and 2.5% or less with respect to the average value.
(Triangle | delta): The difference of discharge capacity exceeds 2.5% with respect to an average value, and is 5.0% or less.
X: The difference of discharge capacity is over 5.0% with respect to the average value.

[Cycle characteristics (capacity retention rate after 100 cycles)]
Using 10 each of the lithium ion secondary batteries C of Examples and Comparative Examples, constant current and constant voltage charge (1 / 10C cut) at 1C and 4.2V, and then a constant current discharge test (2.8V under the condition of 1C) Cut). The average value of the capacity retention rate after 100 cycles (after 100 cycles / 1 cycle capacity) was calculated.

As shown in Table 8, the separators for lithium ion secondary batteries of Examples 26 to 59 have a modified freeness of 0 to 250 ml and a length weighted average fiber length of 0.20 to 2.00 mm. Since 10 to 90% by mass of a certain solvent-spun cellulose fiber and 10 to 90% by mass of a synthetic fiber are contained, the fibers are easily entangled and a fiber network is easily formed. Became stronger. Further, in the lithium ion secondary batteries of Examples 26 to 59, 10 solvent-spun cellulose fibers having a modified freeness of 0 to 250 ml and a length weighted average fiber length of 0.20 to 2.00 mm were used. Since a separator containing 90 to 90% by mass and 10 to 90% by mass of synthetic fiber is used, the electrolyte solution of the separator has a high liquid retentivity and good ion conductivity. Excellent discharge characteristics and cycle characteristics. Further, since the separator can be made dense, the internal short-circuit defect rate and the variation in discharge capacity were low and excellent.

On the other hand, the separator for the lithium ion secondary battery of Example 25 had a slightly weaker separator strength because the average fiber length of solvent-spun cellulose fibers in the separator was shorter than 0.20 mm. In the separator for the lithium ion secondary battery of Example 25, since the weighted average fiber length of the solvent-spun cellulose fiber in the separator is shorter than 0.20 mm, the fibers are hardly entangled and the fiber network is not easily formed. The short-circuit defect rate and discharge capacity variation were slightly higher.

In the separators for lithium ion secondary batteries of Comparative Examples 11 and 12, the modified drainage of the solvent-spun cellulose fiber in the separator exceeds 250 ml and the average fiber length exceeds 2.00 mm. As a result, the lithium ion secondary batteries of Comparative Examples 11 and 12 had high internal short-circuit failure rates and variations in discharge capacity. Since the separators for lithium ion secondary batteries of Comparative Examples 13 to 16 did not contain solvent-spun cellulose fibers, the liquid retention rate was low. In addition, since the lithium ion secondary batteries of Comparative Examples 13 to 16 used separators that did not contain solvent-spun cellulose fibers, the denseness of the separators became insufficient and the internal short-circuit defect rate increased. Since the lithium ion secondary battery of Comparative Example 17 contained synthetic fibers in excess of 90% by mass, sufficient density was not obtained, the internal short circuit failure rate was high, and the liquid retention rate was low. Since the lithium ion secondary battery of Comparative Example 18 had a synthetic fiber content of less than 10% by mass, the separator strength was weak.

Comparing Examples 26 to 39, the lithium ion secondary batteries of Examples 36 and 39 have a maximum frequency peak longer than 0.00 to 1.00 mm in the fiber length distribution histogram of solvent-spun cellulose fibers. As a result, the internal short-circuit defect rate was slightly higher than in Examples 26 to 35, and the variation in discharge capacity increased. In the lithium ion secondary batteries of Examples 37 and 38, in the fiber length distribution histogram of the solvent-spun cellulose fiber, the ratio of fibers having a fiber length of 1.00 mm or more is less than 10%, so the separator strength is slightly weakened. As compared with Examples 26 to 35, the 5C discharge capacity was slightly lowered.

Comparing Examples 26 to 35, the lithium ion secondary batteries of Examples 26 to 29 and 31 to 34 were 0.05 mm between 1.00 and 2.00 mm in the fiber length distribution histogram of solvent-spun cellulose fibers. Since the inclination of the ratio of the fiber having each fiber length is −3.0 or more and −0.5 or less, the separator strength is high and the variation in the discharge capacity is small. In the lithium ion secondary battery of Example 30, in the fiber length distribution histogram of the solvent-spun cellulose fiber, the slope of the ratio of fibers having a fiber length of every 0.05 mm between 1.00 and 2.00 mm is −3. Since it was 0 or less, compared with Examples 26 to 29 and 31 to 34, the separator strength was slightly low, and the variation in discharge capacity was large. In the lithium ion secondary battery of Example 35 having an inclination exceeding −0.5, the variation in the discharge capacity was larger than those of Examples 26 to 29 and 31 to 34.

When comparing Examples 40 to 50, the lithium ion secondary batteries of Examples 40 to 48 had a maximum frequency peak between 0.00 and 1.00 mm in the fiber length distribution histogram of the solvent-spun cellulose fiber, Since the ratio of the fibers having a fiber length of 1.00 mm or more is 50% or more, it has a dense structure, a strong separator strength, a low internal short-circuit failure rate, and a small variation in discharge capacity. In the lithium ion secondary battery of Example 49, since the maximum frequency peak was longer than 0.00 to 1.00 mm in the fiber length distribution histogram of the solvent-spun cellulose fiber, the internal short circuit rate was slightly inferior. The lithium ion secondary battery of Example 50 has a maximum frequency peak between 0.00 and 1.00 mm in the fiber length distribution histogram of the solvent-spun cellulose fiber, but the fiber having a fiber length of 1.00 mm or more. Since the ratio was less than 50%, the separator strength was weaker than in Examples 40 to 48.

Comparing Examples 40 to 48, the lithium ion secondary batteries of Examples 40 to 42 and 44 to 47 are 1.50 to 3.50 mm in addition to the maximum frequency peak in the fiber length distribution histogram of solvent-spun cellulose fibers. Since there is a peak between them, a fiber network is more easily formed, so the strength of the separator is strong, the internal short-circuit failure rate is low, and the variation in discharge capacity is small. In the fiber length distribution histogram of the solvent-spun cellulose fiber, the lithium ion secondary battery of Example 43 has a peak other than the maximum frequency peak smaller than 1.50 mm, so compared with Examples 40 to 42 and 44 to 47, The separator strength was slightly weak and the discharge capacity variation was slightly large. In the fiber length distribution histogram of the solvent-spun cellulose fiber, since the peak other than the maximum frequency peak is larger than 3.50 mm, the lithium ion secondary battery of Example 48 is compared with Examples 40 to 42 and 44 to 47. The internal short circuit failure rate was slightly high, and the variation in discharge capacity was slightly increased.

Since the lithium ion secondary batteries of Examples 26 to 54 contain 20% by mass or less of fibrillated natural cellulose fibers having a modified freeness of 0 to 400 ml in the separator, the separator can be made denser. The internal short circuit defect rate was lower than that of the lithium ion secondary batteries of Examples 58 and 59. The lithium ion secondary battery of Example 55 uses a separator having a modified freeness of 0 to 400 ml of fibrillated natural cellulose fiber of more than 20% by mass. The discharge capacity at a high rate and the capacity retention rate after 100 cycles showed slightly low values. In the lithium ion secondary battery of Example 56, the modified drainage of the fibrillated natural cellulose fiber is larger than 400 ml, so that the density of the separator is slightly lowered, and the separator strength is higher than that of Examples 40 and 53. Declined.

<< Examples 60 to 88, Comparative Examples 19 to 21 >>

<Solvent-spun cellulose fiber>
After untreated solvent-spun cellulose fibers having an average fiber diameter of 10 μm and a fiber length of 4 mm were dispersed at the beating concentration shown in Table 9, beating treatment was performed by changing the treatment time with a double disc refiner manufactured by Aikawa Tekko. Table 9 shows the length weighted average fiber length and average curl degree of each treated solvent-spun cellulose fiber.

<Fibrylated natural cellulose fiber>
Cotton pulp manufactured by Toho Special Pulp Co., Ltd. was treated with a high-pressure homogenizer, and fibrillated natural cellulose fibers having a length-weighted average fiber length shown in Table 10 were prepared.

<Separator>
A papermaking slurry was prepared according to the raw materials and contents shown in Table 11, and wet papermaking was performed using a circular paper machine to produce separators of Examples 60 to 88 and Comparative Examples 19 to 21. The thickness was adjusted by calendaring at room temperature.

In Table 11, the “type” of the synthetic fiber is as follows.
“PET”: Polyethylene terephthalate fiber “AA”: Acrylic fiber “PP”: Polypropylene fiber “PET / PEs-C”: Polyester-based sheath-heat-bonded fiber

<Lithium ion secondary battery E>
The negative electrode 1 and the positive electrode 1 are wound so that the separators of Examples 60 to 88 and Comparative Examples 19 to 21 are interposed between the electrodes, respectively, and housed in a cylindrical container made of aluminum alloy, and the lead body is welded. It was. Next, the whole cylindrical container was vacuum-dried at 150 ° C. for 10 hours. This was allowed to cool to room temperature in a vacuum, and then an electrolyte solution was injected and sealed up, and lithium ion secondary batteries E of Examples 60 to 88 and Comparative Examples 19 to 21 were produced. As the electrolytic solution, a solution obtained by dissolving LiPF _{6 in a} mixed solvent composed of 30% by mass of ethylene carbonate and 70% by mass of diethyl carbonate so as to be 1.2M was used.

<Lithium ion secondary battery F>
The separator of Example and Comparative Example, positive electrode 2 and negative electrode 2 were bonded together in this order, and the lead wires were drawn out to produce a battery body. Next, the battery main body was vacuum-dried at 110 ° C. for 15 hours, allowed to cool to room temperature in a vacuum, and then inserted into an aluminum laminate film. From 1M-LiPF ₆ / EC + DEC (3: 7 vol%) An appropriate amount of the resulting electrolyte solution was poured and vacuum impregnated, and then the excess electrolyte solution was removed and sealed to prepare a lithium ion secondary battery F (battery capacity = 30 mAh equivalent).

About the separator and lithium ion secondary battery of Examples and Comparative Examples, basis weight, thickness, separator moisture content, separator strength, maximum pore diameter, average pore diameter, internal resistance, internal short-circuit failure rate, discharge capacity, discharge capacity Variations and capacity maintenance rates were evaluated, and the results are shown in Tables 11 and 12.

[Internal resistance]
After charging 100 lithium ion secondary batteries E of Examples and Comparative Examples at 1 C for 30 minutes, the internal resistance was measured at an alternating current of 1 kHz, and each average value was calculated.

[Internal short-circuit failure rate]
Internal short circuit failure when repeating 100 cycles of constant current charging / discharging in the charging / discharging voltage range of 2.8-4.2V, charging / discharging current 1C, using 100 lithium ion secondary batteries E of Examples and Comparative Examples The rate was calculated.

[C rate discharge test (discharge capacity)]
Using 50 lithium ion secondary batteries F of Examples and Comparative Examples, after performing 3 cycle aging at 1C, constant current and constant voltage charging (1 / 10C cut) at 1C, 4.2V, and then 0. The constant current discharge test (2.8V cut) was performed by changing the current values to 2C, 0.5C, 1C, 3C, and 5C, and the average values of the discharge capacities at 0.2C and 5C were calculated.

[C-rate discharge test (discharge capacity variation)]
Using 50 lithium ion secondary batteries F of Examples and Comparative Examples, after performing three-cycle aging at 1C, charging at constant current and constant voltage at 1C and 4.2V (1 / 10C cut), then 0. The constant current discharge test (2.8V cut) was performed by changing the current value to 2C, 0.5C, 1C, 3C, and 5C, and the discharge capacity at 0.2C and 5C was evaluated according to the following criteria.

[Cycle characteristics (capacity retention rate after 100 cycles)]
Using 20 each of the lithium ion secondary batteries F of Examples and Comparative Examples, constant current and constant voltage charge (1 / 10C cut) at 1C and 4.2V, and then a constant current discharge test (2.8V under the condition of 1C) The average capacity retention rate after 100 cycles (after 100 cycles / 1 cycle capacity) was calculated.

As shown in Table 11, the separators for lithium ion secondary batteries of Examples 60 to 86 are solvent-spun cellulose fibers having a length-weighted average fiber length of 0.50 to 1.25 mm and an average curl degree of 2.5 or less. Since 10 to 90% by mass and 10 to 90% by mass of synthetic fiber were contained, the moisture content was low, and the mechanical strength was strong and excellent.

By comparing Examples 61 and 69 to 71 with Comparative Examples 20 and 21, 10 to 90% by mass of solvent-spun cellulose fibers having a length weighted average fiber length of 0.50 to 1.25 mm and an average curl degree of 25 or less. By containing 10 to 90% by mass of synthetic fiber, the moisture content of the separator could be kept low. Further, the strength of the separator was increased because the fibers were easily entangled and a fiber network was easily formed.

By comparing Examples 60 to 65 and Examples 87 and 88, the solvent-spun cellulose fiber has a length-weighted average fiber length of 0.50 to 1.25 mm. It becomes easy to form and it turns out that separator strength becomes strong. When the solvent-spun cellulose fiber has a length-weighted average fiber length of less than 0.50 mm (Example 87), the fiber is likely to fall off the paper machine wire during the manufacture of the separator, and the strength of the separator is hardly exhibited. When the length weighted average fiber length of the solvent-spun cellulose fiber is more than 1.25 mm (Example 88), the uniformity of the mass of the separator tends to be impaired, the strength of the separator is slightly reduced, and the maximum pore size of the separator is reduced. The diameter becomes slightly larger.

By comparing Examples 61 and 66 to 68 with Comparative Example 19, the average curl degree of the solvent-spun cellulose fiber is 25 or less, so that the fiber network is easily entangled with each other, and the maximum pore diameter of the separator is increased. It can be seen that it is adjusted to be smaller.

As shown in Table 12, in the lithium ion secondary batteries of Examples 60 to 86, 10 to 90 mass of solvent-spun cellulose fibers having a length weighted average fiber length of 0.50 to 1.25 mm and an average curl degree of 25 or less. %, And a separator made of a porous sheet containing 10 to 90% by mass of a synthetic fiber was used, the internal resistance and internal short circuit failure rate, in particular, the discharge characteristics at high rates and their variations and cycle characteristics were excellent.

That is, the lithium ion secondary batteries of Examples 60 to 86 contain 10 to 90% by mass of solvent-spun cellulose fibers having a length weighted average fiber length of 0.50 to 1.25 mm and an average curl degree of 25 or less. Therefore, since the electrolyte solution has good liquid retention and good ion conductivity, it has a low internal resistance and is particularly excellent in discharge characteristics and cycle characteristics at a high rate. On the other hand, the lithium ion secondary battery of Comparative Example 20 has a length weighted average fiber length of 0.50 to 1.25 mm, and the content of solvent-spun cellulose fibers having an average curl degree of 25 or less is less than 10% by mass. The liquid retention was inferior and the internal resistance was high. In the lithium ion secondary battery of Comparative Example 19, the average curl degree of the solvent-spun cellulose fiber was greater than 25. As a result, the uniformity of the mass of the separator was impaired, and the internal short circuit failure rate and the variation in discharge capacity were increased.

In the lithium ion secondary batteries of Examples 80 to 83, 10% by mass of fibrillated natural cellulose fibers having a modified freeness of 0 to 400 ml are contained in the separator. In this way, by adding fibrillated natural cellulose having a modified freeness of 0 to 400 ml to the separator, the strength of the separator can be increased and the separator can be made thinner. Compared to the secondary battery, the lithium ion secondary batteries of Examples 80 to 83 had a low internal resistance and a high discharge capacity at a high rate.

In the lithium ion secondary battery of Example 86, since the content of fibrillated natural cellulose fibers having a length-weighted average fiber length of 0.20 to 1.00 mm is more than 20% by mass, the separator is slightly too dense. Thus, the average pore diameter is also reduced, the ion conductivity is slightly deteriorated, the internal resistance is slightly higher than those of the lithium ion secondary batteries of Examples 81 and 85, and the discharge rate at a high rate is slightly lower. The value is shown.

From the comparison of Examples 60 to 65 and 69 to 71, the lithium ion secondary batteries of Examples 64, 65, and 69 have a maximum pore diameter larger than 6.0 μm. Therefore, the lithium ions of Examples 60 to 63 and 70 to 71 were used. The internal short circuit failure rate and the variation in discharge capacity were slightly higher than those of the ion secondary battery.

The lithium ion secondary battery of Example 76 has a slightly larger average fiber diameter of the synthetic fibers used, so the separator strength is slightly weaker than that of the lithium ion secondary battery of Example 75, and the discharge capacity variation is slightly higher. became.

In the lithium ion secondary battery of Example 78, since the average fiber diameter of the synthetic fibers used is slightly thinner, the separator strength is slightly weaker and the cycle characteristics are slightly inferior to those of the lithium ion secondary battery of Example 77.

<< Examples 89 to 103, Comparative Examples 22 to 23 >>

<Carboxymethylcellulose (CMC1)>
Carboxymethylcellulose having a degree of etherification of 0.5 (Daiichi Kogyo Seiyaku Co., Ltd., trade name: Cellogen (registered trademark) PL-15) was designated as CMC1.

<Carboxymethylcellulose (CMC2)>
Carboxymethylcellulose having a degree of etherification of 0.7 (trade name: 1205, manufactured by Daicel Chemical Industries, Ltd.) was designated as CMC2.

<CDS (cationic starch-based paper strength enhancer)>
A cationic starch-based paper strength enhancer (trade name: DD4280, manufactured by Seiko PMC) was used as CDS.

<GGS (Guar gum paper strength enhancer)>
Guar gum paper strength enhancer (product name: MAYPROID 2066, manufactured by Mayhole Chemical Co., Ltd.) was designated as GGS.

In Table 13, the “type” of the synthetic fiber is as follows.
"PET": Polyethylene terephthalate fiber

<Separator>
A papermaking slurry was prepared according to the raw materials and blending amounts (based on the total fiber amount) shown in Table 13, and wet papermaking was performed using a circular paper machine to produce separators of Examples 89 to 103 and Comparative Examples 22 to 23. did. The thickness was adjusted by calendaring at room temperature.

<Lithium ion secondary battery G>
The negative electrode 1 and the positive electrode 1 were wound so that the separators of Examples 89 to 103 and Comparative Examples 22 to 23 were interposed between the electrodes, respectively, and stored in a cylindrical container made of aluminum alloy, and the lead body was welded. It was. Next, the whole cylindrical container was vacuum-dried at 150 ° C. for 10 hours. This was allowed to cool to room temperature in a vacuum, and then an electrolyte solution was injected and sealed up, and lithium ion secondary batteries G of Examples 89 to 103 and Comparative Examples 22 to 23 were produced. As the electrolytic solution, a solution obtained by dissolving LiPF ₆ to 1.2 M in a mixed solvent composed of 30% by mass of ethylene carbonate (EC) and 70% by mass of diethyl carbonate (DEC) was used.

<Lithium ion secondary battery H>
The separators of Examples 89 to 103 and Comparative Examples 22 to 23, the positive electrode 2, and the negative electrode 2 were bonded together in this order, and the lead wires were pulled out to produce a battery body. Next, the battery body was vacuum-dried at 140 ° C. for 10 hours, allowed to cool to room temperature in a vacuum, and then inserted into an aluminum laminate film. From 1M-LiPF ₆ / EC + DEC (3: 7 vol%) An appropriate amount of the resulting electrolyte solution was poured and vacuum impregnated, and then the excess electrolyte solution was removed and sealed to prepare a lithium ion secondary battery H (battery capacity = 30 mAh equivalent).

About separator and lithium ion secondary battery of Example and Comparative Example, basis weight, thickness, maximum pore diameter, average pore diameter, separator moisture content, separator strength, internal resistance, internal short circuit failure rate, discharge capacity, discharge capacity The variation and capacity retention rate were evaluated, and the results are shown in Table 14.

[Internal resistance]
After charging 100 lithium ion secondary batteries G of Examples and Comparative Examples at 1 C for 30 minutes, the internal resistance was measured at an alternating current of 1 kHz, and each average value was calculated.

[Internal short-circuit failure rate]
Internal short circuit failure when repeating 100 cycles of constant current charging / discharging in the charging / discharging voltage range of 2.8-4.2V, charging / discharging current 1C using 100 lithium ion secondary batteries G of Examples and Comparative Examples The rate was calculated.

[C rate discharge test (discharge capacity)]
Using 50 each of the lithium ion secondary batteries H of Examples and Comparative Examples, after performing three-cycle aging at 1C, charging at constant current and constant voltage at 1C and 4.2V (1 / 10C cut), then 0. The constant current discharge test (2.8V cut) was performed by changing the current values to 2C, 0.5C, 1C, 3C, and 5C, and the average values of the discharge capacities at 0.2C and 5C were calculated.

[C-rate discharge test (discharge capacity variation)]
Using 50 each of the lithium ion secondary batteries H of Examples and Comparative Examples, after performing three-cycle aging at 1C, charging at constant current and constant voltage at 1C and 4.2V (1 / 10C cut), then 0. The constant current discharge test (2.8V cut) was performed by changing the current value to 2C, 0.5C, 1C, 3C, and 5C, and the discharge capacity at 0.2C and 5C was evaluated according to the following criteria.

[Cycle characteristics (capacity retention rate after 100 cycles)]
Using 20 each of the lithium ion secondary batteries H of Examples and Comparative Examples, constant current and constant voltage charge (1 / 10C cut) at 1C and 4.2V, and then a constant current discharge test (2.8V under the condition of 1C) The average capacity retention rate after 100 cycles (after 100 cycles / 1 cycle capacity) was calculated.

As shown in Table 14, the separator for the lithium ion secondary battery of Example 89 is 10% by mass of solvent-spun cellulose fiber having a modified freeness of 0 ml per total fiber, 90% by mass of synthetic fiber per total fiber, and Since 1% by mass of carboxymethylcellulose having a degree of etherification of 0.5 was contained, the separator strength was superior to that of Example 1 in Table 1 that did not contain carboxymethylcellulose.

The separator for the lithium ion secondary battery of Example 90 has a modified freeness of 120 ml of solvent-spun cellulose fiber of 50% by mass per total fiber, a synthetic fiber of 50% by mass per total fiber, and a degree of etherification of 0.5. Since 1% by mass of carboxymethylcellulose is contained, the maximum pore diameter is reduced, the internal resistance and the internal short-circuit failure rate are reduced, the variation in discharge capacity is improved, and no carboxymethylcellulose is contained. Compared to Example 3 in Table 1, the separator strength was excellent.

The separator for the lithium ion secondary battery of Example 91 is 90 mass% of solvent-spun cellulose fibers having a modified freeness of 250 ml per total fiber, 10 mass% of synthetic fibers per total fiber, and a degree of etherification of 0.5. Since 1% by mass of carboxymethylcellulose is contained, the pore diameter and internal resistance are improved as in Example 90, and the separator strength is superior to Example 3 in Table 1 that does not contain carboxymethylcellulose. It was.

The separator for the lithium ion secondary battery of Example 92 has a modified freeness of 120 ml of solvent-spun cellulose fiber of 50 mass% per total fiber, a synthetic fiber of 50 mass% per total fiber, and a degree of etherification of 0.7. Since 1% by mass of carboxymethylcellulose was contained, the pore diameter and separator strength were further improved as compared with Example 90. Moreover, since the separator for lithium ion secondary batteries of Example 92 contains 1% by mass of carboxymethyl cellulose having a degree of etherification of 0.7, 1 each of cationic starch-based paper strength enhancer and guar gum paper strength enhancer. Compared to the lithium ion secondary battery separators of Example 102 and Example 103 containing mass%, it is understood that not only the separator strength is excellent, but also the discharge capacity at high rate and the variation in discharge capacity are excellent. .

In the separators for lithium ion secondary batteries of Examples 93 to 96, the solvent-spun cellulose fiber having a modified freeness of 120 ml was 50% by mass per total fiber, the synthetic fiber was 50% by mass per total fiber, and the degree of etherification was 0. No. 5 carboxymethyl cellulose was contained, and the moisture content of the separator, the strength of the separator, the battery characteristics and the like were satisfactory. However, as in Example 93, when the content of carboxymethyl cellulose is 0.2% by mass relative to the total fiber mass, the effect of improving formation during papermaking is small, the separator strength is slightly lower, and the pore diameter is also small. There was a tendency to increase slightly. Further, as in Example 96, when the content of carboxymethyl cellulose is 3.0% by mass with respect to the total fiber mass, each separator performance reaches its peak, which is disadvantageous in terms of cost. Since the dehydration property was insufficient, the production efficiency was not good.

In the separators for lithium ion secondary batteries of Examples 97 to 101, 45 to 25% by mass of solvent-spun cellulose fiber having a modified freeness of 120 ml is used as the separator, 50% by mass of synthetic fiber per total fiber, and carboxymethyl cellulose. Is contained in an amount of 1.0% by mass based on the total fiber mass, and further, 5-25% by mass of fibrillated natural cellulose fiber having a modified freeness of 0 to 400 ml is contained per total fiber. Therefore, a high separator strength can be obtained even at a low basis weight, and thus the separator can be made denser and thinner. Therefore, the embodiment 97 is more preferable than the lithium ion secondary battery separator of the embodiment 90. The ~ 101 lithium ion secondary battery separator had a low internal resistance and a high discharge capacity at a high rate. In the lithium ion secondary battery of Example 100, since the content of the fibrillated natural cellulose fiber having a modified freeness of 0 to 400 ml in the separator is more than 20% by mass per total fiber, the separator is slightly too dense, The ion conductivity was slightly deteriorated, the internal resistance was slightly higher than those of the lithium ion secondary batteries of Examples 97 to 99, and the high rate discharge capacity was slightly lower.

In the separator for the lithium ion secondary battery of Example 101, the solvent-spun cellulose fiber having a modified freeness of 120 ml is 40% by mass per total fiber, the synthetic fiber is 50% by mass per total fiber, and the degree of etherification is 0.00. 7 carboxymethyl cellulose is contained in an amount of 1.0% by mass relative to the total fiber mass, and further, fibrillated natural cellulose fibers having a modified freeness of 270 ml are contained in an amount of 10% by mass. Therefore, the maximum pore diameter was smaller than in Example 98, and the separator strength was also increased.

Although the separator for the lithium ion secondary battery of Comparative Example 22 contains carboxymethyl cellulose in the separator, the content of solvent-spun cellulose fibers having a modified freeness of 0 to 250 ml in the separator is more than 90% by mass per total fiber. Many synthetic fibers were less than 10% by mass per total fiber, so the moisture content was high and the separator strength was weak. Although the separator for the lithium ion secondary battery of Comparative Example 23 contains carboxymethyl cellulose in the separator, the content of solvent-spun cellulose fibers having a modified freeness of 0 to 250 ml in the separator is more than 10% by mass per total fiber. Since the amount of synthetic fibers is less than 90% by mass per total fiber, the maximum pore diameter is increased, and the internal short circuit defect rate is extremely increased.

<< Examples 104 to 126, Comparative Examples 24 to 31 >>

<Heat-bonding fiber M1>
A polyester core-sheath type heat-sealable fiber having a fineness of 0.5 dtex, a fiber length of 5 mm, a core of polyethylene terephthalate (melting point 253 ° C.), and a sheath of polyethylene terephthalate-isophthalate copolymer (softening point 75 ° C.) is heated. A fusion fiber M1 was obtained.

<Heat-bonding fiber M2>
A polyester core-sheath type heat-sealable fiber having a fineness of 1.1 dtex, a fiber length of 5 mm, a core part of polyethylene terephthalate (melting point 253 ° C.), and a sheath part of a polyethylene terephthalate-isophthalate copolymer (softening point 75 ° C.) is heated. A fusion fiber M2 was obtained.

<Heat-bonding fiber M3>
A polyolefin core-sheath type heat-seal fiber having a fineness of 0.8 dtex, a fiber length of 5 mm, a core part of polypropylene (melting point 165 ° C.), and a sheath part of high-density polyethylene (melting point 135 ° C.) was designated as heat-seal fiber M3.

<Heat-bonding fiber M4>
Polyester / ethylene-vinyl alcohol core / sheath with a fineness of 1.1 dtex, fiber length of 5 mm, core of polyethylene terephthalate (melting point 253 ° C.), and sheath of ethylene-vinyl alcohol copolymer (wet heat melting temperature 100 ° C.) The fusing fiber was designated as heat fusing fiber M4.

<Heat-bonding fiber M5>
An unstretched polyethylene terephthalate fiber (melting point: 130 ° C.) having a fineness of 1.1 detex and a fiber length of 5 mm was designated as a heat-sealing fiber M5.

<Heat-bonding fiber M6>
An unstretched polyethylene terephthalate fiber (hot-pressure melting temperature 200 ° C.) having a fineness of 0.5 dtex and a fiber length of 5 mm was used as a heat-sealing fiber M6.

<Heat-bonding fiber M7>
Polyvinyl alcohol fiber (wet heat melting temperature 100 ° C.) having a fineness of 0.8 dtex and a fiber length of 5 mm was used as the heat fusion fiber M7.

<Heat-bonding fiber M8>
A split type composite fiber (16 splits) made of polypropylene (melting point 165 ° C.) and an ethylene-vinyl alcohol copolymer (wet heat melting temperature 100 ° C.) having a fineness of 2.2 dtex and a fiber length of 5 mm was used as a heat sealing fiber M8.

According to the raw materials and contents shown in Table 15, a 0.1% concentration papermaking slurry was prepared.

In Table 15, “types” of synthetic fibers, heat-sealing fibers and other fibers are as follows.
“PET”: Polyethylene terephthalate fiber “AA”: Acrylic fiber “PP”: Polypropylene fiber “PET / PEs-C”: Polyester core-sheath type heat-sealable fiber “PP / PE”: Polyolefin-type core-sheath type heat-seal Fiber “PET / EvOH”: Polyester / ethylene-vinyl alcohol-based core-sheath type heat-sealing fiber “PET-B1”: Unstretched polyethylene terephthalate fiber (melting point: 130 ° C.)
“PET-B2”: unstretched polyethylene terephthalate fiber (hot-pressure melting temperature 200 ° C.)
“PVA”: Polyvinyl alcohol fiber “PP-EvOH”: Polypropylene / ethylene-vinyl alcohol-based heat-bonding fiber “PA”: fibrillated para-type wholly aromatic polyamide fiber

<Separator>
Examples 104-119
Papermaking slurries 1 to 16 were made up by a wet method using a circular paper machine, and heat-bonded fibers were thermally bonded by a cylinder dryer at 140 ° C. to prepare a nonwoven fabric. Next, supercalender treatment was performed to obtain lithium ion secondary battery separators of Examples 104 to 119.

Examples 120 and 121
The papermaking slurries 17 and 18 were made up by a wet method using a circular paper machine, and a heat-bonded fiber was thermally bonded by a cylinder dryer at 140 ° C. to produce a nonwoven fabric. Next, the nonwoven fabric was brought into contact with a heat roll having a diameter of 1.2 m heated to 200 ° C. at a speed of 20 m / min and heat-treated. Next, a super calendar process was performed to obtain lithium ion secondary battery separators of Examples 120 and 121.

(Examples 122 to 124, Comparative Examples 24 to 29)
The papermaking slurries 19 to 27 were made up by a wet method using a circular net paper machine, and heat-bonded fibers were thermally bonded by a cylinder dryer at 140 ° C. to prepare a nonwoven fabric. Next, supercalender treatment was performed to obtain lithium ion secondary battery separators of Examples 122 to 124 and Comparative Examples 24 to 29.

(Example 125)
The papermaking slurry 28 was made up using a wet method using a circular paper machine and dried with a cylinder dryer at 140 ° C. to produce a nonwoven fabric. Next, using a heat roll having a diameter of 1.2 m heated to 200 ° C., pressure heat treatment was performed at a pressure of 2 MPa and a speed of 10 m / min to thermally bond the heat-fusible fiber, and the lithium ion secondary battery of Example 125 A separator was used.

(Example 126, Comparative Examples 30 and 31)
The papermaking slurries 29 to 31 were made up using a wet method with a circular paper machine and dried with a cylinder dryer at 140 ° C. to produce a nonwoven fabric. Next, supercalender treatment was performed to obtain lithium ion secondary battery separators of Example 126 and Comparative Examples 30 and 31.

<Lithium ion secondary battery I>
The negative electrode 1 and the positive electrode 1 are wound so that the separators of Examples 104 to 126 and Comparative Examples 24 to 31 are interposed between the electrodes, respectively, and housed in a cylindrical container made of aluminum alloy, and the lead body is welded. It was. Next, the whole cylindrical container was vacuum-dried at 110 ° C. for 24 hours. This was allowed to cool to room temperature in a vacuum, and then an electrolytic solution was injected and sealed up, and lithium ion secondary batteries I of Examples 102 to 124 and Comparative Examples 24 to 31 were produced. As the electrolytic solution, a solution obtained by dissolving LiPF _{6 in a} mixed solvent composed of 30% by mass of ethylene carbonate and 70% by mass of diethyl carbonate so as to be 1.2M was used.

<Lithium ion secondary battery J>
The separator of Example and Comparative Example, positive electrode 2 and negative electrode 2 were bonded together in this order, and the lead wires were drawn out to produce a battery body. Next, the battery main body was vacuum-dried at 110 ° C. for 15 hours, allowed to cool to room temperature in a vacuum, and then inserted into an aluminum laminate film. From 1M-LiPF ₆ / EC + DEC (3: 7 vol%) An appropriate amount of the resulting electrolyte solution was poured and vacuum impregnated, and then the excess electrolyte solution was removed and sealed to prepare a lithium ion secondary battery J (battery capacity = 30 mAh equivalent).

About separator and lithium ion secondary battery of Example and Comparative Example, basis weight, thickness, maximum pore diameter, average pore diameter, separator moisture content, separator strength, internal resistance, internal short circuit failure rate, discharge capacity, discharge capacity The variation, capacity maintenance rate, and voltage maintenance rate were evaluated, and the results are shown in Table 16 and Table 17.

[Internal resistance]
After charging 100 lithium ion secondary batteries I of Examples and Comparative Examples at 1 C for 30 minutes, the internal resistance was measured at 1 kHz AC, and each average value was calculated.

[Internal short-circuit failure rate]
Internal short circuit failure when repeating 100 cycles of constant current charging / discharging in the charging / discharging voltage range of 2.8-4.2V, charging / discharging current 1C using 100 lithium ion secondary batteries I of Examples and Comparative Examples The rate was calculated.

[C rate discharge test (discharge capacity)]
Using 50 each of the lithium ion secondary batteries J of Examples and Comparative Examples, after performing 3 cycle aging at 1C, after performing constant current and constant voltage charging (1 / 10C cut) at 1C and 4.2V, 0.2C , 0.5C, 1C, 3C, and 5C, the constant current discharge test (2.8V cut) was performed, and the average values of the discharge capacities at 0.2C and 5C were calculated.

[C-rate discharge test (discharge capacity variation)]
Using 50 lithium ion secondary batteries J of Examples and Comparative Examples, after performing 3 cycle aging at 1C, constant current and constant voltage charging (1 / 10C cut) at 1C, 4.2V, and then 0. The constant current discharge test (2.8V cut) was performed by changing the current value to 2C, 0.5C, 1C, 3C, and 5C, and the discharge capacity at 0.2C and 5C was evaluated according to the following criteria.

[Cycle characteristics (capacity retention rate after 100 cycles)]
Using 20 lithium ion secondary batteries of Examples and Comparative Examples, constant current and constant voltage charge (1 / 10C cut) at 1C and 4.2V, and then constant current discharge test (2.8V cut) under the condition of 1C And the average value of the capacity retention rate after 100 cycles (after 100 cycles / 1 cycle capacity) was calculated.

[Voltage maintenance ratio]
Using 50 lithium ion secondary batteries of Examples and Comparative Examples, after performing 3 cycle aging at 1C, the lithium ion secondary was charged with constant current and constant voltage (1 / 10C cut) at 0.2C and 4.2V. The battery was stored in a thermostat set at 60 ° C. for 30 days, then the voltage of the lithium ion secondary battery was measured, and the average value of the voltage maintenance ratio before and after storage was calculated. The voltage maintenance rate was evaluated according to the following criteria.

○: The voltage maintenance rate is 95% or more.
Δ: The voltage maintenance ratio is less than 95% and 90% or more.
X: The voltage maintenance rate is less than 90%.

As shown in Table 16, the separators for lithium ion secondary batteries of Examples 104 to 121 were 10 to 90% by mass of solvent-spun cellulose fibers having a modified freeness of 0 to 250 ml and 10 to 90% by mass of synthetic fibers. Since it contains a low moisture content, it contains a core-sheath type heat-sealing fiber composed of a heat-sealing component and a non-heat-sealing component as at least one synthetic fiber, so the separator strength is strong. Was excellent.

That is, since the separators for lithium ion secondary batteries of Examples 104 to 121 contain 10 to 90% by mass of synthetic fiber, compared with the paper separator made of solvent-spun cellulose fiber and hemp fiber of Comparative Example 31, The moisture content could be kept low. Furthermore, the fibers are easily entangled with each other, and a fiber network is formed. The fiber is firmly heat-bonded with the core-sheath type heat-sealing fiber, so that the separator strength is increased.

In addition, from the comparison of Examples 110, 111, and 112, when using a core-sheath type heat-bonded fiber in which the core part is polyethylene terephthalate and the sheath part is a polyester copolymer, the separator strength tends to increase. The separators for lithium ion secondary batteries of Examples 120 and 121 subjected to the heat treatment tended to have higher strength. Since the separators for lithium ion secondary batteries of Examples 122 to 125 used heat fusion fibers other than the core-sheath type, the separators for lithium ion secondary batteries of Example 124 contained core-sheath type heat fusion fibers. As a result, the separator strength was slightly weakened.

On the other hand, in the separators for lithium ion secondary batteries of Comparative Examples 24 and 25, the content of solvent-spun cellulose fibers having a modified freeness of 0 to 250 ml in the separator is more than 90% by mass, and the separator contains 10 synthetic fibers. Since it is less than mass%, the moisture content is high and the separator strength is weak. Since the separator for the lithium ion secondary battery of Comparative Example 31 had a synthetic fiber content of less than 10% by mass in the separator, the separator strength was weak.

As shown in Table 17, the lithium ion secondary batteries of Examples 104 to 121 contain 10 to 90% by mass of solvent-spun cellulose fibers having a modified freeness of 0 to 250 ml and 10 to 90% by mass of synthetic fibers. Since a separator made of a porous sheet containing a core-sheath-type heat-sealing fiber composed of a heat-sealing component and a non-heat-sealing component is used as at least one kind of synthetic fiber, internal resistance, internal short-circuit failure rate In particular, the discharge characteristics at a high rate and its variation, cycle characteristics, and voltage maintenance ratio characteristics were excellent.

That is, in the lithium ion secondary batteries of Examples 104 to 121, since the separator contains 10 to 90% by mass of solvent-spun cellulose fibers having a modified freeness of 0 to 250 ml, the electrolyte solution retainability is good. Since the ion conductivity can be improved, the internal resistance is low, and the discharge characteristics and the cycle characteristics are particularly excellent at a high rate. On the other hand, the lithium ion secondary batteries of Comparative Examples 26 and 27 have a content of solvent-spun cellulose fibers having a modified freeness of 0 to 250 ml of less than 10% by mass. Therefore, the lithium ion secondary battery of Comparative Example 28 is Since the separator did not contain solvent-spun cellulose fibers having a modified freeness of 0 to 250 ml, the electrolyte solution was inferior in liquid retention and had a high internal resistance.

Since the lithium ion secondary batteries of Examples 104 to 121 contain 10 to 90% by mass of solvent-spun cellulose fibers having a modified freeness of 0 to 250 ml in the separator, the separator can be made dense. Therefore, the internal short circuit failure rate and the variation in discharge capacity were low and excellent. On the other hand, in the lithium ion secondary batteries of Comparative Examples 26 and 27, the content of the solvent-spun cellulose fiber having a modified freeness of 0 to 250 ml in the separator is less than 10% by mass, and the separator is insufficiently dense. , Internal short-circuit defect rate and discharge capacity variation increased. In the lithium ion secondary battery of Comparative Example 28, since the separator does not contain solvent-spun cellulose fibers having a modified freeness of 0 to 250 ml, the separator is insufficiently dense, the internal short circuit failure rate, the discharge capacity The variation of was high. In the lithium ion secondary battery of Comparative Example 29, the modified drainage degree of the solvent-spun cellulose fiber in the separator is larger than 0 to 250 ml, and the separator is insufficiently dense. Was slightly higher.

The lithium ion secondary batteries of Examples 104 to 121 contain 10 to 90% by mass of a synthetic fiber, and are core-sheath type heat fusion made of at least one synthetic fiber and comprising a heat fusion component and a non-heat fusion component. In order to heat-bond the fibers together without impairing the dense structure of the fiber network, the fibers had a low internal resistance and a high voltage retention rate. On the other hand, the lithium ion secondary batteries of Examples 122 to 125 use a heat-sealable fiber other than the core-sheath-type heat-sealable fiber for the separator, so that the shape of the heat-sealable fiber is lost during thermal bonding. As a result, the porosity of the separator was locally obstructed, so that the value of the internal resistance was slightly high and the discharge rate at a high rate was slightly low. Since the lithium ion secondary battery of Example 126 did not contain the core-sheath type fusion-bonded fiber in the separator, the voltage maintenance rate was slightly low.

<< Examples 127 to 132, Comparative Examples 32 to 34 >>

<Fiber A18>
Using a refiner, solvent-spun cellulose fibers having an average fiber diameter of 10 μm and a fiber length of 4 mm were treated, and solvent-spun cellulose fibers having a modified freeness of 125 ml were designated as fiber A18.

<Fiber A19>
Using a refiner, solvent-spun cellulose fibers having an average fiber diameter of 10 μm and a fiber length of 10 mm were treated, and solvent-spun cellulose fibers having a fiber diameter of 2.5 μm and a fiber length of 6 mm were designated as fiber A19.

<Synthetic fiber B9>
Polyethylene terephthalate fiber having a fiber diameter of 2.5 μm and a fiber length of 6 mm was designated as synthetic fiber B9.

<Separator>
Papermaking slurries were prepared according to the raw materials and contents shown in Table 18, and wet papermaking was performed using a circular paper machine to produce separators of Examples 127 to 132 and Comparative Examples 32 to 34. The thickness was adjusted by calendaring at room temperature.

<Lithium ion secondary battery K>
The negative electrode 1 and the positive electrode 1 were wound so that the separators of Examples and Comparative Examples were interposed between the electrodes, respectively, and housed in a cylindrical container made of aluminum alloy, and the lead body was welded. Next, the whole cylindrical container was vacuum-dried at 110 ° C. for 15 hours. This was allowed to cool to room temperature in a vacuum, and then an electrolytic solution was injected and sealed, to prepare lithium ion secondary batteries K of Examples and Comparative Examples. As the electrolytic solution, a solution obtained by dissolving LiPF _{6 in a} mixed solvent composed of 30% by mass of ethylene carbonate and 70% by mass of diethyl carbonate so as to be 1.2M was used.

<Lithium ion secondary battery L>
The separators of Examples 127 to 132 and Comparative Examples 33 to 34 were bonded together in order of the positive electrode 2 and the negative electrode 2 so that the A layer was on the negative electrode side and the B layer was on the positive electrode side. Part was produced. Next, the battery body was vacuum-dried at 140 ° C. for 10 hours, allowed to cool to room temperature in a vacuum, and then inserted into an aluminum laminate film to be composed of 1M-LiPF6 / EC + DEC (3: 7 vol%) An appropriate amount of electrolyte solution was poured and vacuum impregnated, and then the excess electrolyte solution was removed and sealed to produce a lithium ion secondary battery L (battery capacity = 30 mAh equivalent).

For the separators and lithium ion secondary batteries of Examples and Comparative Examples, the basis weight, thickness, maximum pore diameter, average pore diameter, separator moisture content, separator strength, internal resistance, internal short circuit failure rate, discharge capacity, discharge capacity The variation and capacity retention rate were evaluated, and the results are shown in Table 19.

[Internal resistance]
After charging 100 lithium ion secondary batteries K of Examples and Comparative Examples at 1 C for 30 minutes, the internal resistance was measured at AC 1 kHz, and each average value was calculated.

[Internal short-circuit failure rate]
Internal short circuit failure when 100 cycles of constant current charge / discharge were repeated with charge / discharge voltage range of 2.8-4.2V, charge / discharge current of 1C using 100 lithium ion secondary batteries K of Examples and Comparative Examples The rate was calculated.

[C rate discharge test (discharge capacity)]
Using 50 each of the lithium ion secondary batteries L of Examples and Comparative Examples, after performing 3 cycle aging at 1C, charging at constant current and constant voltage at 1C and 4.2V (1 / 10C cut), then 0. The constant current discharge test (2.8V cut) was performed by changing the current values to 2C, 0.5C, 1C, 3C, and 5C, and the average values of the discharge capacities at 0.2C and 5C were calculated.

[C-rate discharge test (discharge capacity variation)]
Using 50 each of the lithium ion secondary batteries L of Examples and Comparative Examples, after performing 3 cycle aging at 1C, charging at constant current and constant voltage at 1C and 4.2V (1 / 10C cut), then 0. The constant current discharge test (2.8V cut) was performed by changing the current value to 2C, 0.5C, 1C, 3C, and 5C, and the discharge capacity at 0.2C and 5C was evaluated according to the following criteria.

As shown in Table 19, when Example 127 and Example 132 are compared, the separator for the lithium ion secondary battery of Example 127 has a two-layer structure, and a solvent-spun cellulose fiber having a modified freeness of 125 ml in each layer. Is 60% by mass per total fiber and 40% by mass of synthetic fiber per total fiber, the 5C discharge capacity and capacity retention rate were excellent.

The separator for the lithium ion secondary battery of Example 128 has a two-layer structure, and the layer A is 6% by mass of solvent-spun cellulose fiber having a modified freeness of 125 ml and 6% by mass of synthetic fiber per total fiber. The layer B contains solvent-spun cellulose fibers having a modified freeness of 125 ml of 100% by mass per total fiber, so that the separator has a slightly lower separator strength than the example 125, but the maximum pore diameter The internal resistance was slightly improved.

The separator for the lithium ion secondary battery of Example 129 has a two-layer structure, and the layer A is a solvent-spun cellulose fiber having a modified freeness of 125 ml of 60% by mass per total fiber, and the synthetic fiber is 40% by mass per total fiber. The layer B contains 90 mass% of solvent-spun cellulose fibers having a modified freeness of 125 ml per total fiber, and 40 mass% of synthetic fibers per total fiber. Although the separator strength was slightly inferior, the maximum pore diameter was reduced and the internal resistance was slightly improved.

The separator for the lithium ion secondary battery of Example 130 has a two-layer structure, and the layer A is 60% by mass of solvent-spun cellulose fibers having a modified freeness of 125 ml and 60% by mass of synthetic fibers per total fiber. The layer B contains solvent-spun cellulose fibers having a modified freeness of 125 ml of 10% by mass per total fiber and 90% by mass of synthetic fibers per total fiber, compared to Example 125, Although the separator strength was slightly improved, the 5C discharge capacity and the capacity retention rate were slightly inferior.

The separator for the lithium ion secondary battery of Example 131 has a two-layer structure, and the layer A is 60% by mass of solvent-spun cellulose fibers having a modified freeness of 125 ml and 60% by mass of synthetic fibers per total fiber. Since the layer B contains 100% by mass of synthetic fiber per total fiber, the separator strength is slightly improved as compared with Example 125, but the 5C discharge capacity and capacity retention rate are slightly inferior. It was.

Although the separator for the lithium ion secondary battery of Comparative Example 32 has a two-layer structure, the content of the solvent-spun cellulose fiber having a modified freeness of 0 to 250 ml in the separator is more than 90% by mass, and the synthetic fiber Is less than 10% by mass per total fiber, the moisture content is high and the separator strength is weak. Although the separator for the lithium ion secondary battery of Comparative Example 33 has a two-layer structure, the content of the solvent-spun cellulose fiber having a modified freeness of 0 to 250 ml in the separator is less than 10% by mass, and the synthetic fiber Is more than 90% by mass per total fiber, the maximum pore diameter is increased, and the internal short circuit defect rate is extremely increased. Although the lithium ion secondary battery separator of Comparative Example 34 has a two-layer structure, it uses a solvent-spun cellulose fiber with a fiber diameter of 2.5 μm and a fiber length of 6 mm. Also, the discharge capacity at high rate was greatly inferior.

As a utilization example of the present invention, a lithium ion secondary battery separator and a lithium ion polymer secondary battery separator are suitable.

Claims

A solvent having a modified freeness of 0 to 250 ml measured according to JIS P8121, except that an 80-mesh wire mesh with a wire diameter of 0.14 mm and an aperture of 0.18 mm is used as the sieve plate, and the sample concentration is 0.1%. A separator for a lithium ion secondary battery comprising a porous sheet containing 10 to 90% by mass of a spun cellulose fiber and 10 to 90% by mass of a synthetic fiber.
2. The separator for a lithium ion secondary battery according to claim 1, wherein the solvent-spun cellulose fiber has a length weighted average fiber length of 0.20 to 2.00 mm.
The solvent-spun cellulose fiber has a maximum frequency peak between 0.00 and 1.00 mm in the length-weighted fiber length distribution histogram, and the ratio of fibers having a length-weighted fiber length of 1.00 mm or more is 10 The separator for a lithium ion secondary battery according to claim 1 or 2, wherein the separator is at least%.
In the length-weighted fiber length distribution histogram of solvent-spun cellulose fibers, the slope of the ratio of fibers having a length-weighted fiber length of 0.05 mm between 1.00 and 2.00 mm is −3.0 or more and −0. The separator for a lithium ion secondary battery according to claim 3, which is 5 or less.
The solvent-spun cellulose fiber has a maximum frequency peak between 0.00 and 1.00 mm in the length-weighted fiber length distribution histogram, and the proportion of fibers having a length-weighted fiber length of 1.00 mm or more is 50. The separator for a lithium ion secondary battery according to claim 1 or 2, wherein the separator is at least%.
6. The separator for a lithium ion secondary battery according to claim 5, wherein the length-weighted fiber length distribution histogram of the solvent-spun cellulose fiber has a peak between 1.50 and 3.50 mm in addition to the maximum frequency peak.
The separator for a lithium ion secondary battery according to claim 1 or 2, wherein the solvent-spun cellulose fiber has a length weighted average fiber length of 0.50 to 1.25 mm and an average curl degree of 25 or less.
Furthermore, the modified drainage measured according to JIS P8121 except that the porous sheet uses an 80 mesh wire net having a wire diameter of 0.14 mm and an aperture of 0.18 mm as a sieve plate, and the sample concentration is 0.1%. The separator for a lithium ion secondary battery according to any one of claims 1 to 7, comprising 20% by mass or less of fibrillated natural cellulose fibers having a degree of 0 to 400 ml.
Furthermore, the separator for lithium ion secondary batteries according to claim 1, wherein the porous sheet contains carboxymethyl cellulose.
The separator for a lithium ion secondary battery according to claim 1, wherein the porous sheet contains a core-sheath type heat fusion fiber comprising a heat fusion component and a non-heat fusion component as at least one synthetic fiber.
The separator for a lithium ion secondary battery according to claim 10, wherein the core portion of the core-sheath type heat-sealing fiber is polyethylene terephthalate, and the sheath portion is a polyester copolymer.
The separator for a lithium ion secondary battery according to claim 10 or 11, wherein the porous sheet is heat-treated.
The separator for a lithium ion secondary battery according to any one of claims 1 to 12, wherein the porous sheet has an average pore diameter of 0.10 µm or more and a maximum pore diameter of 6.0 µm or less.
The lithium sheet according to any one of claims 1 to 12, wherein the porous sheet has a value (the thickness of the sheet measured by an outer micrometer at a load of 5 N) measured by a method defined in JIS B7502 of 6 to 50 µm. Separator for ion secondary battery.
Lithium-ion secondary battery separator according to any one of claims 1 to 12 basis weight of the porous sheet is 5 ~ 40g / m 2.
The separator for a lithium ion secondary battery according to any one of claims 1 to 15, wherein the synthetic resin constituting the synthetic fiber is at least one selected from a polyester resin, an acrylic resin, and a polyolefin resin.
The separator for a lithium ion secondary battery according to any one of claims 1 to 16, wherein the synthetic fiber has an average fiber diameter of 0.1 to 20 µm.
The separator for a lithium ion secondary battery according to claim 1, wherein the porous sheet has a multilayer structure, and at least two layers are layers containing solvent-spun cellulose fibers having a modified freeness of 0 to 250 ml as essential components.
A lithium ion secondary battery using the lithium ion secondary battery separator according to any one of claims 1 to 18.