JP2021103664A - Negative electrode resin composition, negative electrode, and secondary battery - Google Patents

Abstract

To provide a negative electrode resin composition having excellent dispersibility and battery performance, and further provide a negative electrode using the negative electrode resin composition and having low electrode plate resistance, and a secondary battery having excellent discharge rate characteristics and cycle characteristics.SOLUTION: A negative electrode resin composition includes a conductive material, a binder, and a dispersant. The dispersant includes at least polyvinyl alcohol. The conductive material contains at least carbon black. A saponification degree of the polyvinyl alcohol is 85.5 to 96.5 mol%. An average primary particle size of the carbon black is 16 nm to 50 nm.SELECTED DRAWING: Figure 1

Description

The present invention relates to a negative electrode resin composition, a negative electrode and a secondary battery.

Due to growing environmental and energy problems, technologies for the realization of a low-carbon society that reduces dependence on fossil fuels are being actively developed. Examples of such technological development include the development of low-emission vehicles such as hybrid electric vehicles and electric vehicles, the development of renewable energy power generation and power storage systems such as solar power generation and wind power generation, the efficient supply of power, and transmission loss. There is a wide range of activities such as the development of next-generation power grids that reduce the number of electricity.
One of the key devices commonly required for these technologies is a battery, and such a battery is required to have a high energy density for miniaturizing the system. In addition, high output characteristics are required to enable stable power supply regardless of the operating environment temperature. Further, good cycle characteristics and the like that can withstand long-term use are also required. Therefore, the replacement of conventional lead-acid batteries, nickel-cadmium batteries, and nickel-hydrogen batteries with lithium-ion secondary batteries having higher energy density, output characteristics, and cycle characteristics is rapidly progressing.

Conventionally, a negative electrode of a lithium ion secondary battery is manufactured by applying an electrode paste containing a negative electrode active material and a binder to a current collector. As the negative electrode active material, natural graphite, artificial graphite, non-graphitized carbon, easily graphitized carbon, silicon compound, lithium titanate and the like have been used. Recently, in order to improve the acceptability of lithium ions and shorten the charge / discharge time, and to prevent the negative electrode active material from repeatedly expanding and contracting during charging / discharging and impairing conductivity, aggregates (primary). It is being studied to add a conductive material such as carbon black in which a structure in which a plurality of particles are fused: a primary agglomerate) or carbon nanotubes in which a cylindrical shape and crystals are developed.

There are two basic roles of the conductive material in the negative electrode. The first is to form a uniform conductive path between the negative electrode active materials, thereby preventing the negative electrode active materials from repeatedly expanding and contracting during charging and discharging to impair conductivity. The second is to retain the electrolytic solution in which lithium ions are dissolved and supply sufficient lithium ions to all the negative electrode active materials, and the supply of lithium ions is insufficient during charging and discharging, resulting in poor battery performance. Prevent damage. Therefore, in the fabrication of the negative electrode, it is important that the carbon black used as the conductive material is controlled within a certain range of the average primary particle size and the specific surface area. If the control is not sufficient or the dispersion between the negative electrode active materials is poor, sufficient contact between the negative electrode active material and carbon black cannot be obtained, a conductive path cannot be secured, and only some active materials are sufficient. Lithium ion cannot be supplied. As a result, a portion having inferior conductivity and electrolyte retention property appears locally in the electrode, which causes the negative electrode active material to not be effectively used, the discharge capacity to be reduced, and the battery life to be shortened. It has become.

Therefore, Patent Document 1 describes a negative electrode active material, a conductive material, a binder made of a copolymer of vinyl alcohol and an ethylenically unsaturated carboxylic acid alkali metal neutralizer, and a lithium ion II using a dispersion aid. A technique for a mixture for a secondary battery electrode is disclosed. According to Patent Document 1, such a configuration increases the storage stability of the mixture for the lithium ion secondary battery electrode, and the battery using the negative electrode produced by the mixture can be retained after 50 cycles. It is noted that the rate will be good.

Further, Patent Document 2 describes a negative electrode for a non-aqueous electrolyte secondary battery using a negative electrode active material, a conductive material, and a binder which is a polyvinyl alcohol resin having a saponification degree of 87 to 99.9 mol% as an electrode mixture layer. Technology is disclosed. According to Patent Document 2, a battery using this negative electrode has a good capacity retention rate after 100 cycles.

Japanese Unexamined Patent Publication No. 2015-201267 Japanese Unexamined Patent Publication No. 2016-181414

As described above, a technique using polyvinyl alcohol or a copolymer thereof as a binder for the negative electrode of a lithium ion secondary battery has been conventionally proposed, but there is still a technique for achieving both practically sufficient dispersibility and battery performance. There wasn't. For example, according to the technique disclosed in Patent Document 1, the dispersibility of the conductive material is improved, but when the negative electrode using this technique is used for a lithium ion secondary battery, the discharge capacity is reduced when discharging under a high current load. There was a problem of doing it. However, in the current lithium-ion secondary battery market, performance is desired in which the capacity does not decrease even when charging / discharging under a high current load, and a negative electrode resin composition having both dispersibility and battery performance is indispensable.

One object of the present invention is to provide a negative electrode resin composition having excellent dispersibility and battery performance in view of the above problems and circumstances. In addition, the present invention provides a negative electrode having a low electrode plate resistance manufactured by using this negative electrode resin composition, and further providing a secondary battery manufactured by using this negative electrode and having excellent discharge rate characteristics and cycle characteristics. For another purpose.

As a result of diligent research to achieve the above object, the present inventors have contained polyvinyl alcohol having a specific degree of saponification as a dispersant and carbon black having a specific average primary particle size as a conductive material. It has been found that the above-mentioned problems can be solved by using the negative electrode resin composition.
Specifically, the present inventor has prepared a negative electrode using a negative electrode resin composition containing carbon black as a conductive material, a binder, and polyvinyl alcohol having a specific degree of saponification as a dispersant. In addition, it was found that the secondary battery manufactured using this negative electrode is excellent in discharge rate characteristics and cycle characteristics. The present invention has been completed based on this finding.

That is, the present invention that solves the above problems is exemplified below.
[1]
A negative electrode resin composition containing a conductive material, a binder and a dispersant, wherein the dispersant contains at least polyvinyl alcohol, the conductive material contains at least carbon black, and the degree of saponification of the polyvinyl alcohol is 85.5. A negative electrode resin composition having an average primary particle size of ~ 96.5 mol% and an average primary particle size of 16 to 50 nm of the carbon black.
[2]
The negative electrode resin composition according to [1], wherein the polyvinyl alcohol has an average degree of polymerization of 250 to 1800.
[3]
The negative electrode resin composition according to [1] or [2], wherein the specific surface area of the carbon black is 35 to 400 m ^{2 / g.}
[4]
In any one of [1] to [3], the mass ratio {mass of the dispersant / mass of the conductive material} when the solid content of the dispersant and the conductive material is compared is 0.03 to 0.15. The negative electrode resin composition according to the above.
[5]
The negative electrode resin composition according to any one of [1] to [4], wherein the binder is a styrene-butadiene copolymer.
[6]
A negative electrode containing the negative electrode resin composition according to any one of [1] to [5].
[7]
A secondary battery provided with the negative electrode according to [6].
In the present specification, unless otherwise specified, the symbol "~" means a range of values "greater than or equal to" and "less than or equal to" at both ends. For example, "A to B" means that it is A or more and B or less.

According to one embodiment of the present invention, it is possible to provide a negative electrode resin composition having excellent dispersibility and battery performance.
According to one embodiment of the present invention, it is possible to provide a negative electrode having a low electrode plate resistance.
According to one embodiment of the present invention, it is possible to provide a secondary battery having excellent discharge rate characteristics and cycle characteristics.
Further, according to a preferred embodiment of the present invention, it is possible to provide a negative electrode capable of easily obtaining a secondary battery having a high energy density and excellent discharge rate characteristics and cycle characteristics.

It is a schematic diagram of the lithium ion secondary battery used in this invention.

Hereinafter, the present invention will be described in detail. The present invention is not limited to the embodiments described below.

Hereinafter, the constituent materials of the present invention will be described in detail.

<Conductive material>
The conductive material in the present invention contains at least carbon black. The content concentration of carbon black in the conductive material can be, for example, 50% by mass or more, preferably 70% by mass or more, and more preferably 90% by mass or more. It is also possible to use only carbon black as the conductive material. Carbon black is selected from acetylene black, furnace black, channel black, and the like, like carbon black as a general conductive material for batteries. Of these, acetylene black, which has excellent crystallinity and purity, is preferable.

The average primary particle size of carbon black in the present invention is preferably 16 to 50 nm. By setting the average primary particle size to preferably 50 nm or less, more preferably 40 nm or less, the number of contacts with the negative electrode active material and the current collector is increased, and a good conductivity-imparting effect and liquid retention property can be obtained. By setting the average primary particle size to 16 nm or more, more preferably 18 nm or more, the interaction between the particles is suppressed, so that the particles are uniformly dispersed between the negative electrode active materials, and a good conductive path and liquid retention property are obtained. Be done. From this point of view, the average primary particle size of carbon black is more preferably 18 to 40 nm. In the present invention, the average primary particle size of carbon black is a value obtained by averaging the particle size measured based on a photograph taken with a transmission electron microscope or the like. Specifically, five images of 100,000 times were taken using a transmission electron microscope JEM-2000FX (manufactured by JEOL Ltd.), and the particle size of 200 or more primary particles randomly selected was determined by image analysis. It was calculated and measured by calculating the average of the number of them. The particle size is the equivalent circle diameter of the primary particles.

The specific surface area of carbon black in the present invention ^{is preferably 35 to 400 m 2} / g. By setting the specific surface area to 400 m ² / g or less, the interaction between the particles is suppressed, so that the particles are uniformly dispersed between the negative electrode active materials, and a good conductive path and liquid retention property can be easily obtained. Further, when it is 35 m ² / g or more, more preferably 40 m ² / g or more, the number of contacts with the negative electrode active material and the current collector is increased, and it becomes easy to obtain a good conductivity-imparting effect and liquid retention property. .. In the present invention, the specific surface area refers to the single point method nitrogen adsorption specific surface area measured in accordance with JIS K6217-2: 2017.

The volume resistivity of carbon black in the present invention is not particularly limited, but the lower the volume resistivity is, the more preferable it is from the viewpoint of conductivity. Specifically, the volume resistivity measured under 7.5 MPa compression is preferably 0.30 Ω · cm or less, more preferably 0.25 Ω · cm or less.

The ash content and water content of carbon black in the present invention are not particularly limited, but from the viewpoint of suppressing side reactions, the smaller the amount, the more preferable. Specifically, the ash content is preferably 0.04% by mass or less, and the water content is preferably 0.10% by mass or less.

<Bundling material>
Examples of the binder used in the present invention include polyvinylidene fluoride, polytetrafluoroethylene, styrene-butadiene copolymer, and (meth) acrylic acid ester copolymer. There are no restrictions on the structure of the polymer as a binder, and random copolymers, alternating copolymers, graft copolymers, block copolymers, and the like can also be used. Among these, a styrene-butadiene copolymer is preferable in terms of battery performance.

<Dispersant>
The dispersant used in the present invention contains at least polyvinyl alcohol (hereinafter, may be abbreviated as PVA). The concentration of PVA in the dispersant can be, for example, 50% by mass or more, preferably 70% by mass or more, and more preferably 90% by mass or more. It is also possible to use only PVA as the dispersant. PVA can be obtained by a polymerization method known per se, for example, by polymerizing and hydrolyzing a fatty acid vinyl ester typified by vinyl acetate.

Examples of the fatty acid vinyl ester include vinyl formate, vinyl acetate, vinyl propionate, vinyl butyrate, vinyl caprate, vinyl laurate, vinyl palmitate, vinyl stearate and other linear or branched saturated fatty acid vinyl esters. Can be mentioned. Of these, vinyl acetate is preferable.

The polyvinyl alcohol can also be obtained by copolymerizing with a polymerizable unsaturated monomer other than the fatty acid vinyl ester. Examples of the polymerizable unsaturated monomer copolymerizable with the fatty acid vinyl ester include olefins such as ethylene and propylene; (meth) such as alkyl (meth) acrylate, 2-hydroxyethyl (meth) acrylate, and glycidyl (meth) acrylate. ) Acryloyl group-containing monomer; allyl ether such as allyl glycidyl ether; vinyl halide compound such as vinyl chloride, vinylidene chloride and vinyl fluoride; vinyl ether such as alkyl vinyl ether and 4-hydroxy vinyl ether. These can be used alone or in combination of two or more.

The method for polymerizing polyvinyl alcohol can be produced by a polymerization method known per se, for example, by solution-polymerizing vinyl acetate in an alcohol-based organic solvent to produce polyvinyl acetate, and then saponifying the polyvinyl acetate. However, the present invention is not limited to this, and for example, bulk polymerization, emulsion polymerization, suspension polymerization and the like may be used. When solution polymerization is carried out, continuous polymerization or batch polymerization may be performed, the monomers may be charged all at once, may be charged separately, or may be added continuously or intermittently.

The polymerization initiator used in solution polymerization is not particularly limited, but is azobisisobutyronitrile, azobis-2,4-dimethylvaleronitrile, azobis (4-methoxy-2,4-dimethylvaleronitrile), etc. Azo compounds; peroxides such as acetyl peroxide, benzoyl peroxide, lauroyl peroxide, acetylcyclohexylsulfonyl peroxide, 2,4,4-trimethylpentyl-2-peroxyphenoxyacetate; diisopropylperoxydicarbonate, diisopropylperoxydicarbonate. Percarbonate compounds such as -2-ethylhexyl peroxydicarbonate and diethoxyethyl peroxydicarbonate; such as t-butylperoxyneodecanate, α-cumylperoxyneodecanate and t-butylperoxyneodecanate. Perester compound; known radical polymerization initiators such as azobisdimethylvaleronitrile and azobismethoxyvaleronitrile can be used.

The polymerization reaction temperature is not particularly limited, but can usually be set in the range of about 30 to 150 ° C.

The saponification conditions for producing polyvinyl alcohol are not particularly limited, and saponification can be performed by a known method. Generally, it can be carried out by hydrolyzing the ester portion in the molecule in the presence of an alkali catalyst or an acid catalyst in an alcohol solution such as methanol. As the alkali catalyst, for example, hydroxides of alkali metals such as sodium hydroxide, potassium hydroxide, sodium methylate, sodium ethylate, potassium methylate and the like, alcoholates and the like can be used. As the acid catalyst, for example, an aqueous inorganic acid solution such as hydrochloric acid or sulfuric acid or an organic acid such as p-toluenesulfonic acid can be used, but it is desirable to use sodium hydroxide. The temperature of the saponification reaction is not particularly limited, but is preferably in the range of 10 to 70 ° C, more preferably 30 to 40 ° C. The reaction time is not particularly limited, but it is desirable to carry out the reaction in the range of 30 minutes to 3 hours.

The degree of saponification of polyvinyl alcohol in the present invention is preferably 85.5 to 96.5 mol%. By setting the saponification degree to 96.5 mol% or less, the solubility in a solvent such as N-methyl-2-pyrrolidone is improved, so that the dispersibility of the conductive material is improved, and a uniform and low viscosity negative electrode resin composition is prepared. It becomes easy to obtain a slurry containing. By setting the saponification degree to 85.5 mol% or more, it becomes easy to obtain high withstand voltage resistance. The degree of saponification of polyvinyl alcohol referred to here is a value measured by a method according to JIS K 6726: 1994.

The average degree of polymerization of polyvinyl alcohol in the present invention is preferably 250 to 1800. By setting the average degree of polymerization to preferably 1800 or less, more preferably 1300 or less, the solubility in a solvent such as N-methyl-2-pyrrolidone is increased, so that the dispersibility of the conductive material is improved, and the uniform and low viscosity is improved. It becomes easy to obtain a slurry containing the negative electrode resin composition of. By setting the average degree of polymerization to 250 or more, more preferably 500 or more, the dispersibility of the negative electrode active material and the conductive material is enhanced, and a good conductive path can be easily obtained. The average degree of polymerization referred to here is a value measured by a method according to JIS K 6726: 1994.

<Negative electrode active material>
Examples of the negative electrode active material include graphite such as natural graphite and artificial graphite, carbon-resistant carbon, carbon materials such as easily graphitized carbon polyacene, silicon atom-containing substances, tin atom-containing substances, and lithium titanate. These may be used alone or in combination of two or more. Among these, at least one selected from a carbon material and a silicon atom-containing substance is preferable, and at least one selected from graphite and a silicon atom-containing substance is more preferable.

Examples of silicon atom-containing substances include (i) silicon fine particles, (ii) magnesium, calcium, lithium, tin, nickel, copper, iron, cobalt, molybdenum, manganese, zinc, indium, silver, titanium, germanium, and tantalum. Examples thereof include alloys or compounds of bismuth, vanadium, tungsten, niobium, antimony or chromium and silicon, and (iii) boron, nitrogen, oxygen or compounds of carbon and silicon. Examples of silicon alloys or compounds include SiB ₄ , SiB ₆ , Mg ₂ Si, Ni ₂ Si, TiSi ₂ , MoSi ₂ , CoSi ₂ , NiSi ₂ , CaSi ₂ , CrSi ₂ , Cu ₅ Si, FeSi ₂ , MnSi. ₂ , NbSi ₂ , _{TaSi 2} , VSi ₂ , WSi ₂ , ZnSi ₂ , SiC, Si ₃ N ₄ , Si ₂ N ₂ O, SiO _x (0 <x ≦ 2), LiSiO, and the like can be mentioned.
Examples of the tin atom-containing substance include (i) an alloy or compound of silicon, nickel, copper, iron, cobalt, manganese, zinc, indium, silver, titanium, germanium, bismuth, antimony or chromium and tin, (ii). ) Examples thereof include oxygen or a compound of carbon and tin.

<Thickener>
A thickener can be appropriately added to adjust the viscosity of the negative electrode resin composition. When the dispersion medium described later is water, examples of the thickener include polyethylene glycol, cellulose, polyacrylamide, poly (N-vinylamide), poly (N-vinylpyrrolidone), and derivatives thereof. Among them, the thickener is preferably polyethylene glycol, cellulose and derivatives thereof, and more preferably carboxymethyl cellulose (CMC).

<Negative electrode resin composition>
A known method can be used for producing the negative electrode resin composition used in the present invention. For example, it can be obtained by mixing a solvent dispersion solution of a negative electrode active material, a conductive material, a binder and a dispersant with a ball mill, a sand mill, a twin-screw kneader, a rotating / revolving stirrer, a planetary mixer, a disper mixer, etc. Specifically, it is used as a slurry. It is also possible to add a conventional component such as a thickener to the negative electrode resin composition. As the negative electrode active material, the conductive material, the binder, and the dispersant, those described above may be used. Examples of the dispersion medium of the slurry containing the negative electrode resin composition include water, N-methyl-2-pyrrolidone, cyclohexane, methyl ethyl ketone, methyl isobutyl ketone and the like. When a styrene-butadiene copolymer is used as the polymer binder, water is preferable, and when polyvinylidene fluoride is used, N-methyl-2-pyrrolidone is preferable in terms of solubility.

The mass ratio {mass of the dispersant / mass of the conductive material} when comparing the solid content of the dispersant and the conductive material in the negative electrode resin composition used in the present invention is preferably 0.03 to 0.15. 0.04 to 0.12 is more preferable, and 0.05 to 0.1 is most preferable. By setting the mass ratio of the dispersant to the conductive material in the negative electrode resin composition to 0.03 to 0.15, the dispersant is adsorbed on the conductive material, and a higher dispersion effect can be easily obtained, which is 0.05 to 0. By setting it to 1.1, in addition to the higher dispersion effect, the effect of the excess dispersant covering the surface of the conductive material and interfering with the charge transfer reaction is suppressed, so that the increase in resistance of the battery can be suppressed.

<Negative electrode>
The negative electrode used in the present invention can be produced by the following procedure. First, the slurry containing the above negative electrode resin composition is applied onto a current collector such as a copper foil, and then the solvent contained in the slurry is removed by heating, and the negative electrode active material is applied to the surface of the current collector via the binder. A negative electrode mixture layer, which is a porous body bound to the above, is formed. Next, the target negative electrode can be obtained by pressing the current collector and the negative electrode mixture layer with a roll press or the like to bring them into close contact with each other.

<Lithium-ion secondary battery>
The method for producing the lithium ion secondary battery used in the present invention is not particularly limited, and a conventionally known method for producing a battery may be used. For example, the configuration schematically shown in FIG. 1 is as follows. It can also be produced by the method of. That is, after welding the aluminum tab 5 to the positive electrode 1 and welding the nickel tab 6 to the negative electrode 2 using the negative electrode, a polyolefin microporous film 3 serving as an insulating layer is formed between the positive electrode 1 and the negative electrode 2. It can be produced by arranging the electrodes, injecting the non-aqueous electrolytic solution into the voids of the positive electrode 1, the negative electrode 2 and the polyolefin microporous film 3 until the non-aqueous electrolytic solution is sufficiently impregnated, and sealing with the exterior 4.

The application of the lithium ion secondary battery of the present invention is not particularly limited, but for example, mobile information such as a digital camera, a video camera, a portable audio player, a portable AV device such as a portable LCD TV, a notebook computer, a smartphone, and a mobile PC. It can be used in a wide range of fields such as terminals, other portable game devices, electric tools, electric bicycles, hybrid vehicles, electric vehicles, and power storage systems.

Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples, but the present invention is not limited to the Examples shown below as long as the gist thereof is not impaired. The negative electrode used in both Examples and Comparative Examples was vacuum dried at 100 ° C. for 4 hours in order to volatilize the adsorbed water.

<Example 1>
(Preparation of slurry containing negative electrode resin composition)
Pure water (manufactured by Kanto Chemical Co., Inc.) as a solvent, carbon black (manufactured by Denka, "Li-435", hereinafter referred to as Li-435) as a conductive material, polyvinyl alcohol (described as polyvinyl alcohol A) as a dispersant, and a conclusion. Styrene-butadiene copolymer (hereinafter referred to as SBR) as a coating material, carboxymethyl cellulose (hereinafter referred to as CMC) as a thickener, and artificial graphite (manufactured by Shenzhen BTR, "AGP-2A") as an active material, respectively. I prepared it. Li-435 has a solid content of 1% by mass, polyvinyl alcohol A has a solid content of 0.1% by mass (mass of dispersant / mass of conductive material = 0.1), CMC has a solid content of 1% by mass, and artificial graphite. Weighed and mixed so that the solid content was 96% by mass, pure water was added to this mixture, and a rotating and revolving mixer (manufactured by Shinky Co., Ltd., "Awatori Rentaro ARV-310") was used. Mix until uniform. Further, the SBR is weighed so as to have a solid content of 1.9% by mass, added to the above mixture, and homogenized using a rotation / revolution type mixer (manufactured by Shinky Co., Ltd., "Awatori Rentaro ARV-310"). The mixture was mixed until a solid content concentration of 50% by mass was obtained to obtain a slurry containing a negative electrode resin composition.

[Evaluation of dispersibility (viscosity of slurry containing negative electrode resin composition)]
The dispersibility of the slurry containing the negative electrode resin composition was evaluated by the method using a rotary rheometer described in JIS K7244-10. Specifically, using a rotary rheometer (manufactured by Anton Pearl Co., Ltd., "MCR300"), 1 g of a slurry containing a negative electrode resin composition having a solid content of 50% by mass is applied onto a disk, and a shear rate is set to 100 s. ^The measurement was carried out by changing from -1 to 0.01s ^-1 , and the viscosity at ^{a shear rate of 1s -1 was evaluated.} The lower the viscosity value, the better the dispersibility. The viscosity of this example was 3.5 Pa · s.

(Preparation of negative electrode)
A slurry containing the negative electrode resin composition was formed on a copper foil (manufactured by UACJ Corporation) having a thickness of 10 μm with an applicator, allowed to stand in a dryer, and pre-dried at 60 ° C. for 1 hour. Next, it was pressed with a roll press machine at a linear pressure of 100 kg / cm to prepare a coating film containing a copper foil having a thickness of 10 μm so that the thickness of the coating film was 50 μm. In order to completely remove the residual water, vacuum drying was performed at 120 ° C. for 3 hours to obtain a negative electrode.

[Evaluation of electrode plate resistance of negative electrode]
The prepared negative electrode was cut out into a disk shape with a diameter of 14 mm, and the front and back surfaces were sandwiched between SUS304 flat plates, and an electrochemical measurement system (Solartron, "Function Generator 1260" and "Potensho Galvanostat 1287") was used. The AC impedance was measured at an amplitude voltage of 10 mV and a frequency range of 1 Hz to 100 kHz. The resistance value obtained by multiplying the obtained resistance component value by the cut-out disk-shaped area was defined as the electrode plate resistance. The electrode plate resistance of the negative electrode of this example was 1.2 Ω · cm ² .

(Preparation of positive electrode)
N-methyl-2-pyrrolidone (manufactured by Kanto Chemical Co., Inc., hereinafter referred to as NMP) as a solvent, polyvinylidene fluoride (manufactured by Alchema, hereinafter referred to as PVdF) as a binder, and carbon as a conductive material. Black (manufactured by Denka, "Li-435", hereinafter referred to as Li-435), polyvinyl alcohol as a dispersant (described as polyvinyl alcohol A), LiNi _0.5 Mn _0.3 Co _0.2 O ₂ (manufactured by Yumicore) “TX10” average primary particle diameter (D50) of 10 μm, hereinafter referred to as “NMC532”) was prepared. PVdF is 1.9% by mass in solid content, Li-435 is 1% by mass in solid content, polyvinyl alcohol A is 0.1% by mass in solid content (mass of dispersant / mass of conductive material = 0.1), NMC532 is weighed and mixed so that the solid content is 97% by mass, NMP is added to this mixture so that the solid content is 68% by mass, and a rotating and revolving mixer (manufactured by Shinky Co., Ltd., "Awa" Using Tori-Kentaro ARV-310 ”), the mixture was mixed until uniform to obtain a slurry containing a positive resin composition. A slurry containing the prepared positive electrode resin composition was formed on one side of a 15 μm-thick aluminum foil (manufactured by UACJ Corporation) with an applicator, allowed to stand in a dryer, and pre-dried at 105 ° C. for 1 hour. The NMP solvent was completely removed. Next, it was pressed with a roll press machine at a linear pressure of 200 kg / cm, and the thickness of the coating film containing the aluminum foil having a thickness of 15 μm was adjusted to 80 μm. Then, in order to completely remove the residual water, vacuum drying was performed at 170 ° C. for 3 hours to obtain a positive electrode.

(Manufacturing of lithium ion secondary battery)
In a dry room controlled to have a dew point of −50 ° C. or lower, the positive electrode was processed to 40 mm × 40 mm, the negative electrode was processed to 44 mm × 44 mm, and then an aluminum tab was welded to the positive electrode and a nickel tab was welded to the negative electrode. The coated surfaces of the mixture of the positive electrode and the negative electrode were opposed to each other in the center, and a microporous polyolefin film processed to 45 mm × 45 mm was arranged between the positive electrode and the negative electrode. Next, the sheet-like exterior cut and processed into a 70 mm × 140 mm square was folded in half at the center of the long side. Next, while arranging the exterior so that the aluminum tab for the positive electrode and the nickel tab for the negative electrode are exposed to the outside of the exterior, the laminated body of the positive electrode-polyolefin microporous film-negative electrode was sandwiched between the two-folded exterior. .. Next, using a heat sealer, two sides including the exposed side of the aluminum tab for the positive electrode and the nickel tab for the negative electrode of the exterior are heat-sealed, and then 2 g of the electrolytic solution (2 g of the electrolytic solution) is applied from one side that is not heat-sealed. Kishida Chemical Co., Ltd., ethylene carbonate / diethyl carbonate = 1/2 (volume ratio) + 1M LiPF ₆ solution, hereinafter referred to as electrolyte solution) is injected and sufficiently soaked into the positive electrode, negative electrode and polyolefin microporous film. A lithium ion secondary battery was obtained by heating and fusing the remaining one side of the exterior while depressurizing the inside of the battery with a vacuum heat sealer.

The battery performance of the produced lithium ion secondary battery was evaluated by the following method.

(Evaluation of lithium-ion secondary battery)
[Discharge rate characteristics (discharge capacity retention rate during 3C discharge)]
The prepared lithium ion secondary battery was charged at 25 ° C. with a constant current and constant voltage limited to 4.3 V and 0.2 C, and then discharged to 3.0 V with a constant current of 0.2 C. Then, after recovering and charging again at a constant current constant voltage limited to 4.3 V and 0.2 C, the discharge current was set to 0.2 C and the battery was discharged to 3.0 V, and the discharge capacity at this time was measured. Subsequently, the recovery charging conditions are maintained and charged each time, while the recovery charging and discharging are repeated while the discharge current is gradually changed to 0.5C, 1C, 2C, and 3C, and the discharge for each discharge current is performed. The capacity was measured. As an index of the discharge rate characteristic of the battery, the discharge capacity retention rate at the time of 3C discharge with respect to the time of 0.2C discharge was calculated. The discharge capacity retention rate of the lithium ion secondary battery of this example during 3C discharge was 83.5%.

[Cycle characteristics (discharge capacity retention rate after cycle)]
The produced lithium ion secondary battery was charged at a constant current constant voltage of 4.3 V and 1 C limit at 25 ° C., and then discharged to 3.0 V at a constant current of 1 C. The above charge and discharge were repeated for 500 cycles, and the discharge capacity in each cycle was measured. As an index of the cycle characteristics of the battery, the discharge capacity retention rate after 500 cycles with respect to one cycle was calculated. The discharge capacity retention rate after the cycle of the lithium ion secondary battery of this example was 91%.

Table 1 shows the saponification degree and the average degree of polymerization of the polyvinyl alcohols used in Examples 1 to 9 and Comparative Examples 1 to 4. Table 2 shows the average primary particle size and specific surface area of carbon black used in Examples 1 to 9 and Comparative Examples 1 to 6.

<Example 2>
A negative electrode, a positive electrode, and a lithium ion secondary battery were prepared in the same manner as in Example 1 except that the dispersant for the negative electrode of Example 1 was changed to polyvinyl alcohol B, and each evaluation was carried out. The results are shown in Table 3.

<Example 3>
A negative electrode, a positive electrode, and a lithium ion secondary battery were prepared in the same manner as in Example 1 except that the dispersant for the negative electrode of Example 1 was changed to polyvinyl alcohol C, and each evaluation was carried out. The results are shown in Table 3.

<Example 4>
A negative electrode, a positive electrode, and a lithium ion secondary battery were prepared in the same manner as in Example 1 except that the dispersant for the negative electrode of Example 1 was changed to polyvinyl alcohol D, and each evaluation was carried out. The results are shown in Table 3.

<Example 5>
A negative electrode, a positive electrode, and a lithium ion secondary battery were produced in the same manner as in Example 1 except that the conductive material for the negative electrode of Example 1 was changed to SAB (manufactured by Denka), and each evaluation was carried out. The results are shown in Table 3.

<Example 6>
The conductive material for the negative electrode of Example 1 was changed to Li-250 (manufactured by Denka Co., Ltd.), and the solid content composition of the negative electrode resin composition was such that Li-250 had a solid content of 1% by mass and polyvinyl alcohol A had a solid content. 0.05% by mass (mass of dispersant / mass of conductive material = 0.05), CMC is 1% by mass in solid content, artificial graphite is 96% by mass in solid content, and SBR is 1.95% by mass in solid content. A negative electrode, a positive electrode, and a lithium ion battery were produced in the same manner as in Example 1 except that the values were changed to the above, and each evaluation was performed. The results are shown in Table 3.

<Example 7>
A negative electrode, a positive electrode, and a lithium ion battery were produced in the same manner as in Example 1 except that the conductive material for the negative electrode of Example 1 was changed to Li-400 (manufactured by Denka Co., Ltd.), and each evaluation was carried out. The results are shown in Table 3.

<Example 8>
The conductive material of Example 1 was changed to ECP (manufactured by Lion), and the solid content composition of the negative electrode resin composition was 1% by mass for ECP and 0.16% by mass for polyvinyl alcohol A. Dispersant mass / conductive material mass = 0.16), CMC was changed to 1% by mass in solid content, artificial graphite was changed to 96% by mass in solid content, and SBR was changed to 1.84% by mass in solid content. A negative electrode, a positive electrode, and a lithium ion battery were produced in the same manner as in Example 1 except for the above, and each evaluation was carried out. The results are shown in Table 3.

<Example 9>
The conductive material for the negative electrode of Example 1 was changed to Li-250 (manufactured by Denka Co., Ltd.), and the solid content composition of the negative electrode resin composition was such that Li-250 had a solid content of 1% by mass and polyvinyl alcohol A had a solid content. 0.02% by mass (mass of dispersant / mass of conductive material = 0.02), CMC is 1% by mass in solid content, artificial graphite is 96% by mass in solid content, and SBR is 1.98% by mass in solid content. A negative electrode, a positive electrode, and a lithium ion battery were produced in the same manner as in Example 1 except that the values were changed to the above, and each evaluation was carried out. The results are shown in Table 3.

<Comparative example 1>
A negative electrode, a positive electrode, and a lithium ion battery were prepared in the same manner as in Example 1 except that the dispersant of Example 1 was changed to polyvinyl alcohol E, and each evaluation was carried out. The results are shown in Table 3.

<Comparative example 2>
A negative electrode, a positive electrode, and a lithium ion battery were prepared in the same manner as in Example 1 except that the dispersant of Example 1 was changed to polyvinyl alcohol F, and each evaluation was carried out. The results are shown in Table 3.

<Comparative example 3>
The conductive material of Example 1 was changed to # 3030B (manufactured by Mitsubishi Chemical Corporation), and the solid content composition of the negative electrode resin composition was such that # 3030B had a solid content of 1% by mass and polyvinyl alcohol A had a solid content of 0.03. Mass% (mass of dispersant / mass of conductive material = 0.03), CMC is 1% by mass in solid content, artificial graphite is 96% by mass in solid content, and SBR is 1.97% by mass in solid content. A negative electrode, a positive electrode, and a lithium ion battery were produced in the same manner as in Example 1 except that the results were changed to, and each evaluation was carried out. The results are shown in Table 3.

<Comparative example 4>
The conductive material of Example 1 was changed to BlackPearls2000 (manufactured by Cabot), and the solid content composition of the negative electrode resin composition was 1% by mass for BlackPearls2000 and 0.15% by mass for polyvinyl alcohol A (solid content). Dispersant mass / conductive material mass = 0.15), CMC was changed to 1% by mass in solid content, artificial graphite was changed to 96% by mass in solid content, and SBR was changed to 1.85% by mass in solid content. A negative electrode, a positive electrode, and a lithium ion battery were produced in the same manner as in Example 1 except for the above, and each evaluation was carried out. The results are shown in Table 3.

<Comparative example 5>
A negative electrode, a positive electrode, and a lithium ion battery were produced in the same manner as in Example 1 except that the conductive material of Example 1 was changed to polyvinylpyrrolidone (manufactured by Nippon Shokubai Co., Ltd., “K-90”), and each evaluation was performed. Carried out. The results are shown in Table 3.

<Comparative Example 6>
Without adding the dispersant of Example 1, the solid content composition of the negative electrode resin composition was such that Li-435 had a solid content of 1% by mass, CMC had a solid content of 1% by mass, and artificial graphite had a solid content of 96% by mass. A negative electrode, a positive electrode, and a lithium ion battery were prepared in the same manner as in Example 1 except that% and SBR were changed to 2% by mass in terms of solid content, and each evaluation was carried out. The results are shown in Table 3.

It was clarified that the negative electrode resin compositions of Examples 1 to 9 had higher dispersibility than the negative electrode resin compositions of Comparative Examples 1 to 6. As a result, it was found that the negative electrode of the embodiment of the present invention has a low electrode plate resistance and can suppress the voltage drop during discharging.

Further, it was revealed that the lithium ion secondary batteries of Examples 1 to 9 have higher discharge rate characteristics and higher cycle characteristics than the lithium ion secondary batteries of Comparative Examples 1 to 6. As a result, it was found that the lithium ion secondary battery using the negative electrode resin composition of the present invention can suppress the deterioration of the discharge rate characteristic due to the increase in the discharge current and also has a long life.

1 Lithium-ion secondary battery Positive electrode 2 Lithium-ion secondary battery Negative electrode 3 Polyolefin microporous film 4 Exterior 5 Aluminum tab 6 Nickel tab

Claims (7)

A negative electrode resin composition containing a conductive material, a binder and a dispersant, wherein the dispersant contains at least polyvinyl alcohol, the conductive material contains at least carbon black, and the degree of saponification of the polyvinyl alcohol is 85.5. A negative electrode resin composition having an average primary particle size of ~ 96.5 mol% and an average primary particle size of 16 nm to 50 nm of the carbon black. The negative electrode resin composition according to claim 1, wherein the polyvinyl alcohol has an average degree of polymerization of 250 to 1800. Negative resin composition of the specific surface area of the carbon black according to claim 1 or 2 which is 35m ² / g~400m ² / g. The method according to any one of claims 1 to 3, wherein the mass ratio {mass of the dispersant / mass of the conductive material} when the solid content of the dispersant and the conductive material is compared is 0.03 to 0.15. Negative electrode resin composition. The negative electrode resin composition according to any one of claims 1 to 4, wherein the binder is a styrene-butadiene copolymer. A negative electrode containing the negative electrode resin composition according to any one of claims 1 to 5. The secondary battery provided with the negative electrode according to claim 6.

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