JP2003157830A - Lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery wherein all of a low- temperature characteristic, a cycle characteristic and a high rate discharge characteristic are remarkably improved as compared with a conventional one. SOLUTION: This lithium ion secondary battery includes at least a positive electrode plate, a negative electrode plate and an electrolyte. The secondary battery is characterized by that (A) the positive electrode plate is composed by forming, on a collector, a coat layer containing (1) an active material formed of a Li-Co based composite oxide, (2) a granular conductive material wherein the sum quantity of a large-diameter constituent having a particle diameter of 4-8 μm and a small-diameter constituent having a particle diameter of 0.1 μm or less is 70 wt.% or more in its total quantity and the weight ratio of the large-diameter constituent to the small-diameter constituent is 1:0.01 to 1: 1, and polyvinylidene fluoride having a melting point of 165 deg.C or below, (4) the void content of the coat layer is set in the range of 0.08-0.14 CC/g and (B) the viscosity of the electrolyte is 3 cPs or less.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a lithium ion secondary battery.

[0002]

2. Description of the Related Art Generally, a lithium ion secondary battery has a structure in which a separator impregnated with an electrolytic solution is sandwiched between a positive electrode plate and a negative electrode plate. For the positive electrode plate and the negative electrode plate, a slurry containing at least an active material and a binder (a conductive material is usually used together with the active material in the positive electrode) is applied on a current collector such as a metal foil and dried. It is formed by providing the applied coating layer. A Li—Co-based composite oxide is generally used as the positive electrode active material, and a carbon material is generally used as the negative electrode active material.

The lithium-ion secondary battery thus constructed can achieve higher energy density and higher voltage than nickel-cadmium batteries and the like. Therefore, in recent years, lithium ion secondary batteries have been rapidly adopted as a drive source for mobile devices such as mobile phones and notebook computers.
Furthermore, it is expected that the scope of application will be expanded in the future.

As a problem of the lithium ion secondary battery, there is a property that when discharged at a low temperature, the discharge capacity and the discharge voltage are greatly reduced as compared with the case of discharging at room temperature. For this reason, it is difficult to apply the lithium-ion secondary battery to observation equipment, communication equipment, and equipment that is supposed to be used at low temperatures, such as electric vehicles and power storage equipment. Therefore, in order to apply the lithium ion secondary battery to the above-mentioned equipment, it is necessary to further improve the property capable of suppressing the decrease in discharge capacity and discharge voltage at low temperature, that is, the low temperature property. In addition, even if the low temperature characteristics are good,
It cannot be said that it is a practical lithium-ion secondary battery unless it has sufficient cycle characteristics. Further, for application to various devices, further improvement in discharge characteristics during large current discharge (high rate discharge) is required.

[0005]

SUMMARY OF THE INVENTION In view of the above circumstances, the present invention aims to provide a lithium ion secondary battery in which all of the low temperature characteristics, the cycle characteristics and the high rate discharge characteristics are greatly improved as compared with conventional ones. And

[0006]

Means for Solving the Problems As a result of intensive studies to achieve the above object, the inventors of the present invention have found that the polymer binder in the positive electrode coating layer of the positive electrode plate contains polyvinylidene fluoride having a melting point of 165 ° C. or lower. By using, the porosity of the positive electrode coating layer within a specific range larger than the conventional one, a large particle size component of a specific particle size range and a small particle size component of a specific particle size or less in the conductive material. By using a conductive material as the main component and further sufficiently reducing the viscosity of the electrolytic solution, it has been found that all of the low temperature characteristics, cycle characteristics and high rate discharge characteristics can be greatly improved compared to conventional ones. Completed the invention.

That is, the present invention is as follows. (1) A lithium ion secondary battery including at least a positive electrode plate, a negative electrode plate, and an electrolytic solution, wherein the positive electrode plate is Li—Co.
The total amount of the active material composed of a system-based complex oxide, the large-diameter component having a particle diameter in the range of 4 to 8 μm, and the small-diameter component having a particle diameter of 0.1 μm or less, is 70% by weight or more, and A coating layer including a granular conductive material having a weight ratio of a large diameter component and a small diameter component of 1: 0.01 to 1: 1 and polyvinylidene fluoride having a melting point of 165 ° C. or less is formed on a current collector. The porosity of the coating layer was set in the range of 0.08 to 0.14 CC / g, and the viscosity of the electrolytic solution (23 ° C.) was 3.0 c.
A lithium-ion secondary battery characterized by being ps or less. (2) The average particle size of the granular Li-Co-based composite oxide is 10
The lithium ion secondary battery according to (1) above, which has a thickness of at least μm.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The lithium ion secondary battery of the present invention will be described in detail below. The lithium-ion secondary battery of the present invention includes at least a positive electrode plate, a negative electrode plate, and an electrolytic solution, and the positive electrode plate has an active material made of a granular Li—Co-based composite oxide and a particle size within a range of 4 to 8 μm. The total amount of the large-diameter component and the small-diameter component having a particle size of 0.1 μm or less is 70% by weight or more of the whole, and the weight ratio of the large-diameter component and the small-diameter component is 1: 0.01 to 1: 1. And a polyvinylidene fluoride having a melting point of 165 ° C. or less on the current collector, and the porosity of the coating layer is 0.0
It is set in the range of 8 to 0.14 CC / g, and the viscosity of the electrolytic solution (23 ° C.) is 3.0 cps or less.

The design of the positive electrode plate is the most important factor for obtaining good battery characteristics in the lithium ion secondary battery. In addition to selecting the constituent material of the positive electrode plate, the structure of the positive electrode plate is also important for obtaining good battery characteristics. Therefore, in general, the positive electrode plate is formed by applying a slurry containing an active material, a conductive material and a polymer binder on a current collector, and drying and rolling the slurry to form a porous film on the current collector. Coating layer is provided. The control of the porosity in such a porous coating layer of the slurry is easy to adjust by the drying temperature, the pressure during rolling, etc.,
It is done this way exclusively. The porosity of the coating layer is preferably higher from the viewpoint of electrolyte retention (impregnation). However, since the porosity is large, the bonding force between the constituent materials (active material, conductive material, polymer binder, etc.) is correspondingly small, so the temperature changes due to repeated charge / discharge cycles, etc. The porous structure of the layer is easily broken, which causes deterioration of discharge characteristics. From such a background, conventionally, the porosity of the coating layer has been at most about 0.02 to 0.06 cc / g even if it can be increased.

In the lithium ion secondary battery of the present invention, the porosity of the coating layer of the positive electrode plate is 0.08 to 0.14 CC / g.
However, by using polyvinylidene fluoride having a melting point of 165 ° C. or less, a coating layer having a stable porous structure is achieved while having such a high porosity. Polyvinylidene fluoride having a melting point of 165 ° C. or lower (hereinafter, also referred to as PVdF) has a conventional melting point of 170 to 1 when dissolved in a solvent to prepare a slurry.
Behaves differently in the slurry than when PVdF of about 80 ° C. is used (how the polymer is entangled with the surface of the active material), and it is easy to form pores in the coating layer obtained by drying the slurry. In addition, it is considered that during the drying process of the slurry, the crystallinity of PVdF becomes higher than that of the conventional PVdF having a high melting point, and a stable porous structure is formed (ie,
When drying the slurry coated on the current collector, there is a problem that the mechanical strength of the current collector decreases due to its thermal history, and the current collector is easily cut during rolling, winding work, etc. From the point of view, the drying of the slurry is usually performed at about 100 to 200 ° C. so that such a problem does not occur, but the polyvinylidene fluoride having a melting point of 165 ° C. or less is used for PVdF which has been conventionally used at the time of drying the slurry. Compared with (melting point of about 170 to 180 ° C.), crystallization progresses more, the binding force with active material, conductive material, etc. becomes higher, and the coating layer forms a more stable porous structure than before. it is conceivable that).

The polyvinylidene fluoride having a melting point of 165 ° C. or lower used in the present invention preferably has a melting point of 150 to 165.
C., more preferably 155 to 160.degree. C., and
It is particularly preferable that the melt viscosity measured at 232 ° C. is 29 to 33 kps (kilopoise).

In the present invention, when the porosity of the coating layer of the positive electrode plate is less than 0.08 CC / g, such a positive electrode plate does not sufficiently permeate the electrolytic solution and its conductivity decreases. Therefore, the low temperature characteristics and the cycle characteristics cannot be improved sufficiently. Further, when the porosity of the coating layer exceeds 0.14 CC / g, it becomes difficult to obtain a target capacity in such a positive electrode plate due to a decrease in the filling amount of the active material (depending on the battery size. The thickness of the coating layer is regulated). In the present invention, the porosity of the coating layer of the positive electrode plate is 0.09 to 0.12C.
A range of C / g is preferred, and particularly preferably 0.09-0.
It is 10 CC / g.

In the present invention, it is also important that the viscosity of the electrolytic solution at 23 ° C. is 3.0 cps (centipoise) or less. That is, since the viscosity of the electrolytic solution is a low viscosity of 3.0 cps or less, the electrolytic solution is sufficiently permeated and retained in the pores of the coating layer, and Li ions are inserted between the active material and the active material. Desorption is actively performed and a sufficiently high discharge capacity is obtained. If the viscosity of the electrolyte is greater than 3.0 cps,
The electrolytic solution may not be retained in a sufficient amount in the coating layer, lowering the low temperature characteristics, the high rate discharge characteristics and the cycle characteristics, which may make it difficult to discharge at low temperature or high rate. The viscosity (23 ° C.) of the electrolytic solution is preferably 2.0 cps or less, and the lower limit of the viscosity (23 ° C.) of the electrolytic solution is preferably 0.1 cps or more. This is because when the viscosity of the electrolytic solution is less than 0.1 cps, the volatility increases and the high temperature storage characteristics tend to deteriorate.

The viscosity (23 ° C.) used in the present invention is 3.0 c
Electrolyte below ps is diethyl carbonate (DEC)
And at least one selected from ethyl methyl carbonate (EMC) and ethylene carbonate (EC)
Is preferably achieved by a mixed solvent of propylene carbonate (PC) and dimethyl carbonate (DMC).

The specific composition is, for example, 25% by volume to 50% by volume (preferably 30% by volume to 35% by volume) of at least one selected from diethyl carbonate and ethylmethyl carbonate, and 4% by volume to 20% by volume of ethylene carbonate. Volume% (preferably 6% to 18% by volume), propylene carbonate 3% to 17% by volume
(Preferably 5% to 15% by volume), dimethyl carbonate 40% to 60% by volume (preferably 45% by volume)
˜55% by volume). At this time, the total amount of ethylene carbonate (EC) and propylene carbonate (PC) is preferably 25% by volume or less of the whole.

In at least one selected from diethyl carbonate and ethyl methyl carbonate,
If the mixing ratio is less than 25% by volume, the freezing point of the electrolytic solution rises, the internal resistance of the battery is increased, and the charge / discharge cycle characteristics and the low temperature characteristics are deteriorated, especially at an extremely low temperature of -20 ° C or lower. Sometimes it is not preferable. On the other hand, if the mixing ratio exceeds 50% by volume, the volatilization of the electrolytic solution may easily proceed and the high temperature storage characteristics may deteriorate, which is not preferable.

In the case of ethylene carbonate, if the mixing ratio is less than 4% by volume, it is difficult to form a stable film on the surface of the negative electrode plate, which may deteriorate cycle characteristics, which is not preferable. On the other hand, if the mixing ratio exceeds 20% by volume, the viscosity of the electrolytic solution may increase, increasing the internal resistance of the battery and deteriorating the charge / discharge cycle characteristics, which is not preferable.

When the mixing ratio of propylene carbonate is less than 3% by volume, the effect of suppressing an increase in impedance associated with charge / discharge cycles becomes small, which may deteriorate cycle characteristics, which is not preferable. If the mixing ratio exceeds 17% by volume, the viscosity of the electrolytic solution may increase to increase the internal resistance of the battery and deteriorate the charge / discharge cycle characteristics, which is not preferable.

When the mixing ratio of dimethyl carbonate is less than 40% by volume, the viscosity of the electrolytic solution increases, the internal resistance of the battery increases, and the charge / discharge cycle characteristics deteriorate, which is not preferable. If the mixing ratio exceeds 60% by volume, volatilization of the electrolytic solution may easily proceed and the high temperature storage characteristics may deteriorate, which is not preferable.

Lithium salt used in the electrolytic solution is Li
_{ClO 4, LiBF 4, LiPF 6} , LiAsF 6, LiA
One or more selected from lCl ₄ and Li (CF ₃ SO ₂ ) ₂ N are suitable, and the concentration thereof in the non-aqueous solvent is preferably 0.1 mol / L to 2 mol / L, more preferably Is preferably 0.5 mol / L to 1.8 mol / L. When the concentration of the lithium salt is less than 0.1 mol / L, the ionic conductivity as the electrolytic solution cannot be sufficiently obtained, and when the concentration of the lithium salt exceeds 2 mol / L, the viscosity of the electrolytic solution increases. 3.
It becomes difficult to achieve a low viscosity of 0 cps or less.

In the present invention, further, as the conductive material in the positive electrode coating layer, a large-diameter component having a particle diameter in the range of 4 to 8 μm and a small-diameter component having a particle diameter of 0.1 μm or less are used as main components, and , The weight ratio of the large-diameter component and the small-diameter component is 1: 0.01 to 1:
It is important to use a conductive material that is 1. Here, when the main component is a large-diameter component having a particle diameter in the range of 4 to 8 μm and a small-diameter component having a particle diameter of 0.1 μm or less, the total amount of these two components is 70% by weight or more of the entire conductive material. , Preferably 80% by weight or more, more preferably 90% by weight
It means that it is above. The conductive material may include particles between the large-diameter component and the small-diameter component, the particle size of which is large, and, in addition to such particles, the particles whose particle size is larger than that of the large-diameter component. When these particles are contained, the amount thereof is less than 30% by weight based on the whole, though they may be contained further. The lower limit of the small diameter component is not particularly limited, but is preferably 0.001 μm or more.

In the present invention, the main component of the positive electrode coating layer is a large-diameter component having a particle diameter of 4 to 8 μm and a small-diameter component having a particle diameter of 0.1 μm or less, and The presence of a specific granular conductive material having a weight ratio of the large-diameter component to the small-diameter component of 1: 0.01 to 1: 1 allows for a coating layer having a large porosity (a coating having a large number of gaps). Layers) between the active material (particles) and the active material (particles) are mainly filled with large-diameter component particles, and small-diameter component particles of 0.1 μm or less mainly cover the surface of the active material. The conductivity can be secured sufficiently high. In the conductive material, if the weight ratio of the large-diameter component and the small-diameter component is out of the above range and the amount of the large-diameter component is too large or the amount of the small-diameter component is too large, sufficiently high conductivity of the positive electrode can be obtained. If the amount of the large-diameter component is too large, it may promote a rapid discharge drop at the beginning of discharge.
Further, especially when the amount of the small diameter component is too large, the safety tends to decrease. The weight ratio of the large diameter component and the small diameter component in the conductive material is preferably 1: 0.1 to 1: 0.5.

When the granular conductive material contains particles other than the large-diameter component and the small-diameter component in an amount of 30% by weight or more of the whole, the conductivity of the positive electrode plate (coating layer) decreases. , Battery performance (especially high rate discharge characteristics, low temperature characteristics)
Will decrease.

The "granular" in the granular conductive material includes scales, spheres, pseudo spheres, lumps, whiskers, and the like, and two or more kinds of particles having different shapes may be mixed. A carbon material is usually used as the granular conductive material, and examples of the carbon material include artificial or natural graphites (graphitized carbon), Ketjen black, acetylene black, oil furnace black, and ixtra conductive furnace black. Carbon blacks of Any one of these carbon materials may be used alone or two or more of them may be mixed, and it is particularly preferred that the large diameter component is made of graphite and the small diameter component is made of carbon black. In addition, in the case of such a preferred embodiment, the graphite having a large diameter component has an interplanar distance (d002) of the crystal lattice of 0.
34 nm or less, the crystallite size (Lc) in the c-axis direction is 10 n
More preferably, it is m or more graphitized carbon, and the carbon black of the small diameter component is more preferably oil furnace black.

In the present invention, the positive electrode active material is Li
-Co-based composite oxide is used. The Li-Co based composite
As an example of the oxide, LiCoO 2₂Or Li_ACo_1-XM
e_XO ₂The items shown in are listed. The latter smell
And A is 0.05 to 1.5, particularly 0.1 to 1.1.
Is preferred. X is 0.01 to 0.5, especially 0.0
It is preferably 2 to 0.2. As the element Me, Z
Periodic table of r, V, Cr, Mo, Mn, Fe, Ni, etc.
Group 3 to 10 elements, B, Al, Ge, Pb, Sn, S
Examples of the elements include Group 13 to Group 15 elements such as b.

The Li-Co type composite oxide is usually granular and has an average particle size of from the viewpoints of preventing abnormal battery reaction (securing safety) and forming pores in the coating layer. 1
Those having an average particle diameter of 17 μm or more are used. The upper limit of the average particle size is preferably 25 so that the electric resistance of the active material does not become too high.
It is less than or equal to μm, more preferably less than or equal to 23 μm.

The Li-Co type composite oxide has an average particle size of 10 μm or more, and is obtained by dividing 20 by the product of the average particle size [μm] and the specific surface area [m ² / g]. Those having a value of 7 to 9, that is, those satisfying the following formula (I) are particularly preferable. 7 ≦ [20 / (specific surface area [m ² / g] × average particle size [μm])] ≦ 9 (I)

When the value of 20 / (specific surface area [m ² / g] × average particle size [μm]) is in the range of 7 to 9, the resistance component of the positive electrode active material itself is reduced, and the cycle characteristics are reduced. Further, the low temperature characteristic and the high rate discharge characteristic are further improved. 20 / (specific surface area [m ² / g] × average particle size [μ
The value of m]) is more preferably 7.5 to 8.5.

The value obtained by dividing 20 by the product of the average particle size [μm] and the specific surface area [m ² / g] is 7 to 9
The Li-Co-based composite oxide that becomes is produced by the following method.

For example, a lithium compound as a starting material and a cobalt compound are mixed so that the atomic ratio of cobalt and lithium is 1: 1 to 0.8: 1, and the mixture is heated at a temperature of 700 ° C to 1200 ° C. 3 hours in the atmosphere of
The reaction is carried out by heating for 50 hours, etc., and the resulting reaction product is pulverized into particles, and only those having an average particle size of 10 μm or more and satisfying the above formula (I) are collected. There is a method of doing.

Further, as another example, a method of further heat-treating the granules obtained by the above-mentioned pulverization, for example, the granules obtained by the pulverization are 400 ° C. to 750 ° C., particularly 450 ° C. Examples include a method of heating at a temperature of about 700 ° C. for 0.5 hours to 50 hours, particularly about 1 hour to 20 hours. At this time, it is preferable to use particles having an average particle diameter within the range of 10 μm to 25 μm as described above as the granular material. When the granular material is heat-treated in this manner, the specific surface area can be reduced without substantially changing the average particle diameter of the granular material. Therefore, for example, a Li—Co-based composite oxide satisfying the above formula (I) can be obtained. You can get things easily.

Further, the heat treatment of the granules obtained by the pulverization can be carried out, for example, in an air atmosphere or an atmosphere of an inert gas such as nitrogen or argon. However, if carbon dioxide is present in the atmosphere, lithium carbonate may be generated and the content of impurities may increase, so it is preferable to carry out the atmosphere in which the partial pressure of carbon dioxide is about 10 mmHg or less.

Examples of the lithium compound as the starting material include lithium oxide, lithium hydroxide, lithium halide, lithium nitrate, lithium oxalate, lithium carbonate and the like, and mixtures thereof. Examples of the cobalt compound include cobalt oxide, cobalt hydroxide, cobalt halide, cobalt nitrate, cobalt oxalate, cobalt carbonate and the like, and mixtures thereof. In addition, Li
_{In the case} of producing a Li—Co-based composite oxide represented by _A Co _1-X Me _X O ₂ , the compound of the substitution element may be added to the mixture of the lithium compound and the cobalt compound in the required amount.

In the present invention, the positive electrode coating layer contains at least the above-mentioned active material, conductive material and polyvinylidene fluoride, and the amount of the conductive material is 3 with respect to 100 parts by weight of the active material. ~ 15 parts by weight is preferred, 3.5 ~
12 parts by weight is more preferable, and particularly preferably 4 to 8
Parts by weight. The amount of polyvinylidene fluoride is preferably 1 part by weight to 10 parts by weight, more preferably 2 parts by weight to 7 parts by weight, and particularly preferably 3 to 6 parts by weight, based on 100 parts by weight of the active material.

When the amount of the conductive material is less than 3 parts by weight, the conductivity of the positive electrode is not sufficiently high, and when it exceeds 15 parts by weight, the filling amount of the active material is reduced to obtain the target capacity. It is difficult to get rid of, and it is not preferable. Further, when the amount of polyvinylidene fluoride is less than 1 part by weight, the bond between the materials constituting the coating layer becomes insufficient and the active material is easily peeled off, resulting in deterioration of cycle characteristics. . Further, when the amount of polyvinylidene fluoride exceeds 10 parts by weight, sufficiently high conductivity of the coating layer (positive electrode) cannot be obtained, and particularly high rate discharge characteristics and low temperature characteristics deteriorate.

In the present invention, the current collector used for the positive electrode plate may be the same as the conventional one such as foil or expanded metal formed of aluminum, aluminum alloy, titanium or the like. When the current collector is a foil or a perforated foil, its thickness is usually about 10 to 100 μm, preferably about 15 to 50 μm. If the current collector is expanded metal, its thickness is usually 25
Is about 300 μm, preferably about 30 to 150 μm.

The positive electrode plate in the present invention comprises the above active material,
Prepared by preparing a slurry containing at least polyvinylidene fluoride, which is a conductive material and a polymer binder, coating the slurry on a current collector, drying, and subjecting the obtained coating layer to a rolling treatment. It

The slurry is usually prepared by kneading the active material, the conductive material and polyvinylidene fluoride with a suitable solvent. The solvent is not particularly limited, but N-
Methylpyrrolidone is preferred. Also, kneading, for example,
It can be carried out using a conventionally known kneading device such as a planetary spa kneading device (manufactured by Asada Iron Works).

The coating of the slurry on the current collector is carried out by a conventionally known coating machine such as a comma roll type or die coat type coating machine, and the drying of the slurry is carried out on the current collector. The slurry is dried with a current collector using a drying device such as a hot air drying oven at a temperature range of 80 to 200 ° C, preferably 100 to 180 ° C for 5 to 20 minutes.

The coating amount of the slurry is expressed by the amount of the active material, which is the amount of the deposits on the current collector after drying.
The amount of the active material is preferably about 1 to 100 mg / cm ² .

The rolling treatment of the coating layer is carried out by rolling the entire positive electrode plate (current collector + coating layer) using a rolling press or the like. In this rolling treatment, the rolling temperature is preferably 20 ° C to 100 ° C, more preferably 25 ° C to 50 ° C.
It is particularly preferably 20 to 35 ° C., and the rolling ratio is preferably 10% to 40%, more preferably 20% to 40%.
%, And particularly preferably 25 to 35%. Here, the rolling temperature is the temperature of the coating layer, and the rolling rate is a scale representing the workability of rolling, which is also called the rolling reduction, and the positive electrode plate (current collector + coating layer) before rolling. Where h1 is the thickness of the positive electrode plate (current collector + coating layer) after rolling, and h2 is the thickness of the current collector, the following formula (II)
It is calculated by. Rolling rate (%) = (h1-h2) × 100 / (h1-h3) (II)

The negative electrode plate in the lithium ion secondary battery of the present invention is not particularly limited, and a known negative electrode in this type of battery can be used, but the following ones are preferable.

A carbon material is used as the negative electrode active material, and among them, the specific surface area is preferably 2.0 m ² /
g or less, more preferably 0.5m ² /g~1.5m ² / g
The interplanar distance (d002) of the crystal lattice is preferably 0.
3380 nm or less, more preferably 0.3355 nm-
The crystallite size (Lc) in the c-axis direction at 0.3370 nm is preferably 30 nm or more, more preferably 40 nm to 7
Graphitized carbon having a thickness of 0 nm is suitable, and a specific example of such graphitized carbon is mesophase-based graphitized carbon.

By having the above specific surface area, when the electrolytic solution contains propylene carbonate, it is possible to prevent a decrease in battery capacity due to a decomposition reaction of propylene carbonate during charging. In addition, the inter-plane distance (d00
By having 2) and the crystallite size (Lc) in the c-axis direction, the potential rise of the negative electrode plate can be suppressed, and the average discharge potential of the battery becomes more stable.

The above graphitized carbon is usually granular, but the particle shape is not particularly limited, and for example, scale-like, fibrous,
Examples thereof include spherical shape, pseudo spherical shape, lump shape, and whisker shape. However, the fibrous shape is preferable because it can be easily applied to the current collector and the orientation of the particles after application can be controlled. Therefore, in the present invention, fibrous mesophase-based graphitized carbon (that is, mesophase-based graphitized carbon fiber) is particularly suitable as the active material of the negative electrode. A preferred example of the method for producing the mesophase-based graphitized carbon fiber is shown below.

First, pitches such as petroleum pitch and coal tar pitch having a length of 200 μm
The fibers are spun into fibers of about 300 μm. As the pitches, it is particularly preferable to use mesophase pitch having a mesophase content of 70% by volume or more. Next, this fiber is carbonized at 800 ° C. to 1500 ° C., and then crushed to an appropriate size, for example, an average fiber length of about 1 μm to 100 μm and an average fiber diameter of about 1 μm to 15 μm. Subsequently, the crushed fiber is treated at 2500 ° C to 3200 ° C, preferably 2 ° C.
The mesophase-based graphitized carbon fiber is obtained by heating at 800 ° C to 3200 ° C to graphitize.

However, the average fiber length of the above-mentioned pulverization is preferably 1 μm to 100 μm, more preferably 2 μm to improve the coatability of the slurry described below to the current collector.
50 μm, particularly preferably 3 μm to 25 μm, and the average fiber diameter is preferably 0.5 μm to 15
μm, more preferably 1 μm to 15 μm, and particularly preferably 5 μm to 10 μm. At this time, the aspect ratio (ratio of average fiber length to average fiber diameter) is preferably 1 to 5.

The method for producing the negative electrode plate is not particularly limited,
Although a general method in this field can be applied, a slurry containing a negative electrode active material and a polymer binder is prepared, and the slurry is coated on a current collector and dried (to form a coating layer), A method in which rolling treatment is performed as necessary is preferable. The polymer binder here is not particularly limited, but polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, ethylene-propylene-diene-based polymer and the like are preferable.

In the present invention, the negative electrode plate may be mixed with a conductive material together with the active material. In this case, the conductive material is preferably natural graphite, artificial graphite, carbon black or the like having an average particle size of 5 μm or less. Further, as the current collector used for the negative electrode plate, the same one as the conventional one can be used,
Examples include foils and expanded metals formed of copper, nickel, silver, stainless steel and the like.

Usually, a separator is interposed between the positive electrode plate and the negative electrode plate. As the separator, a known separator such as a polyolefin separator which has been conventionally used in lithium ion secondary batteries is used. Here, the separator may be a porous separator or a solid separator in which substantially no pores are formed. The polyolefin separator may be a single polyethylene layer or a single polypropylene layer, but a type in which a polyethylene layer and a polypropylene layer are laminated is preferable, and a three-layer type of PP / PE / PP is particularly preferable from the viewpoint of safety.

In the present invention, the form of the battery is not particularly limited. Known materials that have been conventionally used in lithium ion secondary batteries can be used. For example, Fe, Fe (Ni
A cylindrical can, a square can, a button can, etc. made of a metal such as plating), SUS, aluminum, an aluminum alloy, etc., and a sheet-shaped exterior material such as a laminated film are used. As the laminate film, one having a thermoplastic resin laminate layer such as polyester or polypropylene formed on at least one surface of a metal foil such as copper or aluminum is preferable.

The characteristics (physical properties) in this specification are as follows.
Describe the measurement method of. The porosity of the coating layer of the positive electrode plate was measured by the porosimeter method using mercury. The melting point of polyvinylidene fluoride was measured by DSC (indicative scanning calorimeter). Temperature rising rate is 5 ° C
/ Min, the measurement was performed in the range from room temperature (20 ° C) to 300 ° C. Melt viscosity of polyvinylidene fluoride (232 ° C.) It was measured with a Capillograph manufactured by Toyo Seiki. Viscosity of electrolyte (23 ° C.) Measured with an Ubbelohde viscometer. Particle size (average particle size) of Li-Co composite oxide and conductive material for positive electrode plate Microtrac particle size analyzer (Shimadzu Corporation, SA
LD-3000J) was used. In the procedure, first, the granular material to be measured is put into an organic liquid such as water or ethanol, ultrasonic waves of about 35 kHz to 40 kHz are applied, and a dispersion treatment is performed for about 2 minutes. Here, the amount of the particulate matter to be measured is such that the laser transmittance (ratio of the output light amount to the incident light amount) of the dispersion liquid after the dispersion process is 70% to 95%. Next, this dispersion is applied to a Microtrac particle size analyzer to measure the particle size (D1, D2, D3 ...) Of the individual particles due to laser light scattering, and the number of existing particles (N1, N2, N3) for each particle size. ...) was measured. The measurement of this particle size distribution is calculated as the particle size distribution of the spherical particle group having the theoretical intensity that is the closest to the observed scattering intensity distribution (particles have a cross section of the same area as the projected image obtained by laser light irradiation). It is assumed to be a sphere with a circle, and the diameter of this cross-section circle (sphere equivalent diameter) is measured as the particle size). Average particle size (μm)
Is calculated from the particle size (D) of each particle and the number of existing particles (N) for each particle size according to the following formula. Average particle size (μm) = (ΣND ³ / ΣN) ^1/3 Particles having a particle size of 0.1 μm or less may aggregate in the dispersion liquid. If such aggregation occurs, an electron microscope may be used. Was measured using. That is, first, a magnification is set so that 20 or more particles are included in the field of view, an electron micrograph is taken, then the area of the image of each particle shown in the photograph is calculated, and the same area is calculated from the calculated area. Calculate the diameter of a circle with an area (assuming a sphere with a cross-section circle of this diameter) and use this diameter as the particle size. Gas phase adsorption method (single-point method) using a specific surface area meter, Monosorb (produced by Quantachrome Co., Ltd.) of the active material (graphitized carbon) for the Li-Co composite oxide and the negative electrode plate, using nitrogen as an adsorbent. It was measured by. Interplanar distance (d002) between crystal lattices of a conductive material (graphitized carbon) for the positive electrode plate and an active material (graphitized carbon) for the negative electrode plate
And the crystallite size (Lc) in the c-axis direction were measured by the following method according to the Japan Society for the Promotion of Science. First, high-purity silicon for X-ray standard is ground in an agate mortar to a size below 325 mesh standard sieve to prepare a standard substance, and this standard substance and graphitized carbon of the sample to be measured are mixed in an agate mortar (graphitized carbon. An X-ray sample is prepared with 100% by weight of a standard substance relative to 100% by weight, and this X-ray sample is then subjected to, for example, an X-ray diffraction apparatus RINT2000 (Rigaku Denki Co., Ltd., X-ray source: CuKα). Line) to the sample plate uniformly. Next, the applied voltage to the X-ray tube is set to 40 kV and the applied current is set to 50 mA, the scanning range is 2θ = 23.5 degrees to 29.5 degrees, and the scanning speed is 0.25 degrees / min.
As for, the 002 peak of carbon and the 111 peak of the standard substance are measured. Then, from the obtained peak position and the half width thereof, the interplanar distance (d002) of the crystal lattice was calculated using the graphitization degree calculation software attached to the X-ray diffraction apparatus.
And the crystallite size (Lc) in the c-axis direction is calculated.

[0053]

EXAMPLES The present invention will be specifically described below with reference to examples.

Example 1 [Production of Positive Electrode Plate] LiCoO ₂ as a positive electrode active material (average particle size: 20 μm, 20 / (average particle size × specific surface area):
8.3) 91 parts by weight, and spherical graphitized carbon as a conductive material (average particle size: 6 μm, interplanar distance of crystal lattice: 0.336)
0 nm, crystallite size in the c-axis direction: 60 nm) 5 parts by weight, oil furnace black (average particle size: 0.01 μm) 1 part by weight as a conductive material, and polyfluoride having a melting point of 160 ° C. as a polymer binder. Vinylidene chloride (PVdF) (Hailer 301F, manufactured by Ausimont)
A positive electrode active material composition obtained by uniformly dispersing 3 parts by weight in N-methylpyrrolidone was kneaded into a slurry.
Here, a large-diameter component (particle size is 4 to 8 μm) in the entire conductive material composed of spherical graphitized carbon and oil furnace black
The ratio of particles in the range is 72% by weight, and the small diameter component (0.
The ratio of particles having a size of 1 μm or less) was 18% by weight, and the ratio of particles having a particle size other than these was 10% by weight.

The above slurry was applied on both sides of an aluminum foil (thickness 20 μm) to be a current collector, and at 5 ° C. at 5 ° C.
Minute drying, then rolling temperature is 30 ℃, rolling rate is 30%
The coating layer is formed on the current collector by rolling under the rolling conditions of No. 20 and L of 20 mg / cm ² is applied to one side of the aluminum foil.
A positive electrode plate having iCoO ₂ was used. The viscosity of the slurry immediately before coating was 10,000 cps. The porosity of the coating layer was 0.11 CC / g.

[Preparation of Negative Electrode Plate] Graphitized carbon Melbronn Meld FM-14 (specific surface area: 1.3
2 m ² / g, interplanar distance of crystal lattice: 0.3364 nm,
95 parts by weight of crystallite size in the c-axis direction: 50 nm, 5 parts by weight of polyvinylidene fluoride (PVdF) serving as a binder, and 50 parts by weight of N-methylpyrrolidone are mixed to form a slurry, and the slurry is a current collector. Copper foil (thickness 1
4 μm) on both sides and dried. The interplanar distance of the crystal lattice of the negative electrode active material and the crystallite size in the c-axis direction were measured in the same manner as in the above-mentioned spherical graphitized carbon.
Next, this copper foil was subjected to a rolling treatment under rolling conditions (rolling temperature: 120 ° C., rolling rate: 20%) generally used by those skilled in the art to obtain a negative electrode plate.

[Preparation of Electrolyte Solution] Diethyl carbonate 4
In a mixed solvent of vol%, ethyl methyl carbonate 29 vol%, ethylene carbonate 11 vol%, propylene carbonate 9 vol%, and dimethyl carbonate 47 vol%, LiPF ₆ was added at a concentration of 1.0 mol / L.
An electrolytic solution was prepared by dissolving it so that (to the prepared electrolytic solution). The viscosity of the electrolytic solution (23 ° C.) is 1.9 c
It was ps.

[Assembly of Lithium Ion Secondary Battery] The positive electrode plate and the negative electrode plate prepared above were made of porous polyethylene.
It was wound through a polypropylene composite separator and housed in a cylindrical battery can (outer diameter 18 mm, height 650 mm). Further, the electrolytic solution obtained above was impregnated into a separator to obtain a lithium ion secondary battery of the present invention.

Example 2 As a conductive material, 5 parts by weight of flake graphite (average particle size: 6 μm)
m, interplanar distance of crystal lattice: 0.34 nm, crystallite size in c-axis direction: 25 nm, and 1 part by weight of oil furnace black (average particle size: 0.01 μm) The proportion of particles having a diameter in the range of 4 to 8 μm) is 72% by weight, the proportion of small-diameter components (particles having a diameter of 0.1 μm or less) is 18% by weight, and the proportion of particles having other particle diameters is 10%.
A positive electrode plate was produced in the same manner as in Example 1 except that (% by weight) was used. Porosity of positive electrode coating layer is 0.10 CC / g
Met. Thereafter, the positive electrode plate was used, and a lithium ion secondary battery was assembled in the same manner as in Example 1 except for the positive electrode plate.

Example 3 The same slurry as that used in Example 1 was used, and this slurry was used as an aluminum foil (thickness 20 μm) as a current collector.
On both sides, dried at 150 ° C. for 4 minutes, and then rolled under the rolling conditions of a rolling temperature of 35 ° C. and a rolling rate of 25% to form a coating layer on the current collector. The positive electrode plate had 20 mg / cm ² of LiCoO ₂ on each side of the foil. When the porosity of the positive electrode coating layer was measured,
It was 0.12 CC / g. Thereafter, the positive electrode plate was used, and a lithium ion secondary battery was assembled in the same manner as in Example 1 except for the positive electrode plate.

Example 4 A positive electrode was prepared in the same manner as in Example 1 except that 91 parts by weight of LiCoO ₂ having an average particle size of 18 μm and 20 / (average particle size × specific surface area) of 8.5 was used as the positive electrode active material. A plate was made. The porosity of the positive electrode coating layer was 0.14 CC / g. Thereafter, the positive electrode plate was used, and a lithium ion secondary battery was assembled in the same manner as in Example 1 except for the positive electrode plate.

Example 5 PVdF having a melting point of 162 ° C. as a polymer binder
A positive electrode plate was produced in the same manner as in Example 1 except that 3 parts by weight of (KF-1300 manufactured by Kureha Chemical Co., Ltd.) was used. The porosity of the positive electrode coating layer was 0.08 CC / g. Thereafter, the positive electrode plate was used, and a lithium ion secondary battery was assembled in the same manner as in Example 1 except for the positive electrode plate.

Example 6 An electrolytic solution having a viscosity (23 ° C.) of 2.4 cps (composition: ethylene carbonate (11 vol%), propylene carbonate (11 vol%), dimethyl carbonate (44 vol%), diethyl carbonate (5% by volume),
A lithium ion secondary battery was produced in the same manner as in Example 1 except that ethyl methyl carbonate (29% by volume) was used.

Example 7 An electrolytic solution having a viscosity (23 ° C.) of 2.8 cps (composition: ethylene carbonate (12 vol%), propylene carbonate (12 vol%), dimethyl carbonate (43 vol%), diethyl carbonate (4% by volume),
A lithium ion secondary battery was produced in the same manner as in Example 1 except that ethyl methyl carbonate (29% by volume) was used.

Comparative Example 1 A positive electrode plate was produced in the same manner as in Example 1 except that the rolling temperature of the coating layer was changed to 120 ° C. and the rolling ratio was changed to 45%. The porosity of the positive electrode coating layer was 0.06 CC / g. Thereafter, the positive electrode plate was used, and a lithium ion secondary battery was assembled in the same manner as in Example 1 except for the positive electrode plate.

Comparative Example 2 A positive electrode plate was produced in the same manner as in Example 1 except that the rolling temperature of the coating layer was changed to 30 ° C. and the rolling rate was changed to 12%. The porosity of the positive electrode coating layer was 0.16 CC / g. Thereafter, the positive electrode plate was used, and a lithium ion secondary battery was assembled in the same manner as in Example 1 except for the positive electrode plate.

Comparative Example 3 As a conductive material, 1 part by weight of spherical graphitized carbon (average particle size:
6 μm, inter-plane distance of crystal lattice: 0.3360 nm, crystallite size in c-axis direction: 60 nm) (ratio of large-diameter component (particles having particle size in the range of 4 to 8 μm) in the entire conductive material) is 90% by weight The proportion of the small-diameter component (particles of 0.1 μm or less) was 0% by weight, and the proportion of the particle diameter other than these was 10% by weight), and the other conditions were the same as in Example 1 to prepare a positive electrode plate. The porosity of the positive electrode coating layer was 0.14 CC / g. Thereafter, the positive electrode plate was used, and a lithium ion secondary battery was assembled in the same manner as in Example 1 except for the positive electrode plate.

Comparative Example 4 Only 3 parts by weight of oil furnace black (average particle diameter: 0.01 μm) as a conductive material (the proportion of the large-diameter component (particles having a particle diameter in the range of 4 to 8 μm) in the entire conductive material) was used. 0
% By weight, the proportion of small-diameter components (particles of 0.1 μm or less) is 9
5% by weight, and the proportion of particles having a particle size other than these is 5% by weight)
A positive electrode plate was produced in the same manner as in Example 1 except that. The porosity of the positive electrode coating layer was 0.11 CC / g. Thereafter, the positive electrode plate was used, and a lithium ion secondary battery was assembled in the same manner as in Example 1 except for the positive electrode plate.

Comparative Example 5 As a conductive material, 15 parts by weight of spherical graphitized carbon (average particle diameter: 6 μm, interplanar distance of crystal lattice: 0.3360 nm,
Crystallite size in the c-axis direction: 60 nm) and 0.08 part by weight of oil furnace black (average particle size: 0.01 μ)
A positive electrode plate was produced in the same manner as in Example 1 except that m) was used. Large-diameter component (particle size is 4-8 in the entire conductive material
The proportion of particles in the range of μm) is 99.4% by weight, the proportion of small-diameter components (particles of 0.1 μm or less) is 0.5% by weight, and the proportion of particles having other particle diameters is 0.1% by weight. Met.
The porosity of the positive electrode coating layer was 0.11 CC / g. Thereafter, the positive electrode plate was used, and a lithium ion secondary battery was assembled in the same manner as in Example 1 except for the positive electrode plate.

Comparative Example 6 As a conductive material, 4 parts by weight of spherical graphitized carbon (average particle size:
Example 1 except that 6 μm, interplanar distance of crystal lattice: 0.3360 nm, crystallite size in c-axis direction: 60 nm) and 6 parts by weight of oil furnace black (average particle size: 0.01 μm) were used. A positive electrode plate was produced in the same manner as in. The proportion of the large-diameter component (particles having a particle diameter in the range of 4 to 8 μm) in the entire conductive material is 34% by weight, the proportion of the small-diameter component (particles having a diameter of 0.1 μm or less) is 51% by weight, and particles having other diameters Was 15% by weight. The porosity of the positive electrode coating layer was 0.11 CC / g. Thereafter, the positive electrode plate was used, and a lithium ion secondary battery was assembled in the same manner as in Example 1 except for the positive electrode plate.

Comparative Example 7 An electrolytic solution having a viscosity (23 ° C.) of 3.3 cps (composition: ethylene carbonate (20 vol%), propylene carbonate (20 vol%), dimethyl carbonate (10 vol%), diethyl carbonate A lithium ion secondary battery was produced in the same manner as in Example 1 except that (10% by volume) and ethylmethyl carbonate (40% by volume) were used.

Comparative Example 8 A positive electrode plate was prepared in the same manner as in Example 1 except that the polymer binder was changed to polyvinylidene fluoride having a melting point of 175 ° C. The porosity of the positive electrode coating layer was 0.05 CC / g. Thereafter, the positive electrode plate was used, and a lithium ion secondary battery was assembled in the same manner as in Example 1 except for the positive electrode plate.

For each of the lithium ion secondary batteries of Examples 1 to 7 and Comparative Examples 1 to 8 produced as described above, a low temperature characteristic test, a high rate discharge test and a cycle characteristic test were conducted in the following procedure.

[Low Temperature Characteristic Test] The lithium ion secondary battery obtained above is charged at room temperature, and then left in an atmosphere of −20 ° C. for 24 hours. In addition,
Charged at a constant current of 1C (1600mA) and a voltage of 4.2V
After flowing the current until, the total charging time is 2.5
The current was passed at a constant voltage of 4.2 V until the time was reached. Next, 0.5 C (8
(00 mAh) /2.5 V cutoff is performed, and the discharge capacity [mA · H] at that time is obtained. At room temperature (20
Even at (° C.), charging and discharging are performed under the same conditions to obtain the discharge capacity [mA · H]. Further, the discharge capacity at −20 ° C. is divided by the discharge capacity at room temperature to change the discharge capacity [%].
I asked.

[High Rate Discharge Test] 2 C (constant current of 3600 mA) was discharged at room temperature (20 ° C.), and the ratio of the discharge capacity to the discharge capacity at 0.2 C (constant current of 360 mA) (capacity maintenance) Rate) was calculated.

[Cycling Characteristic Test] The lithium ion secondary battery obtained above was charged and discharged at 1 C / 1 C for 500 cycles at room temperature (20 ° C.), and at 1 cycle and 5 cycles.
For 00 cycles, the discharge capacity [mA · H] is calculated from the discharge current value and the discharge time. Then, the discharge capacity [mA · H] at 500 cycles was divided by the discharge capacity [mA · H] at the first cycle to obtain the discharge capacity change rate [%].
The test results are shown in Table 1.

[0077]

[Table 1]

[0078]

As is apparent from the above description, according to the present invention, there is provided a lithium ion secondary battery in which all of the low temperature characteristics, the cycle characteristics and the high rate discharge characteristics are greatly improved as compared with the prior art. You can Therefore, it can be suitably used for observation equipment, communication equipment, electric vehicles, power storage equipment, and other equipment that is expected to be used at low temperatures and that requires large-current discharge.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Kenichi Kizu             4-3 Ikejiri, Itami City, Hyogo Prefecture Mitsubishi Electric Cable             Industrial Co., Ltd. Itami Works (72) Inventor Ken Moriuchi             4-3 Ikejiri, Itami City, Hyogo Prefecture Mitsubishi Electric Cable             Industrial Co., Ltd. Itami Works F term (reference) 5H029 AJ02 AJ05 AK03 AL06 AL07                       AM03 AM05 AM07 DJ08 DJ16                       EJ04 EJ12 HJ01 HJ05 HJ09                       HJ14                 5H050 AA02 AA06 AA07 BA17 CA08                       CB07 CB08 DA10 DA11 EA09                       EA10 EA24 FA17 GA22 HA01                       HA05 HA09 HA10 HA14

Claims (2)

[Claims] 1. A lithium ion secondary battery comprising at least a positive electrode plate, a negative electrode plate and an electrolytic solution, wherein the positive electrode plate comprises an active material composed of a Li—Co based complex oxide, and a particle size of 4 to 8 μm. 70% by weight of the total amount of the large-diameter component and the small-diameter component having a particle diameter of 0.1 μm or less within the range of
Above, and the weight ratio of the large diameter component and the small diameter component is 1:
A coating layer containing a granular conductive material of 0.01 to 1: 1 and polyvinylidene fluoride having a melting point of 165 ° C. or lower is formed on the current collector, and the porosity of the coating layer is adjusted. 0.
The lithium ion secondary battery is set in a range of 08 to 0.14 CC / g, and the viscosity (23 ° C.) of the electrolytic solution is 3.0 cps or less. 2. The lithium ion secondary battery according to claim 1, wherein the granular Li—Co-based composite oxide has an average particle size of 10 μm or more.