JP2021068552A - Positive electrode resin composition, positive electrode and secondary battery - Google Patents

Abstract

To provide a positive electrode resin composition superior in dispersibility and voltage endurance and in addition, a positive electrode low in plate resistance and arranged by use of the positive electrode resin composition, and a secondary battery superior in voltage endurance, discharge rate characteristic and cycle characteristic.SOLUTION: A positive electrode resin composition comprises a conductive material, a binding material and a dispersant. In the positive electrode resin composition, the dispersant contains at least polyvinyl alcohol; the conductive material contains at least carbon black; the polyvinyl alcohol has a saponification degree of 85.5-96.5 mol%; and the carbon black has an average primary particle diameter of 18-40 nm.SELECTED DRAWING: Figure 1

Description

The present invention relates to a positive electrode resin composition, a positive 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 electric 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 rate 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 conventional lead-acid batteries, nickel-cadmium batteries, and nickel-hydrogen batteries are replaced with lithium ion secondary batteries (hereinafter, also referred to as "lithium ion batteries") having higher energy density, rate characteristics, and cycle characteristics. Is progressing rapidly.

Conventionally, a positive electrode of a lithium ion secondary battery is manufactured by applying a positive electrode paste containing a positive electrode active material, a conductive material, and a binder (binding material) to a current collector. Lithium-containing composite oxides such as lithium cobalt oxide and lithium manganate have been used as the positive electrode active material. Since the active material itself has poor conductivity, carbon black with developed aggregates (structure in which multiple primary particles are fused: primary agglomerates) and anisotropic crystals have developed for the purpose of imparting conductivity. It has been practiced to add a conductive material such as graphite (Patent Document 1).

The basic role of the conductive material is to impart stable conductivity to an active material having poor conductivity, which is not impaired even if the positive electrode active material repeatedly expands and contracts during charging and discharging. Therefore, it is important that the size of the aggregate is controlled within a certain range in the carbon black used as the conductive material in the production of the positive electrode. If the control is not sufficient or the dispersion between the active materials is poor, sufficient contact between the active material and carbon black cannot be obtained, a conductive path cannot be secured, and the performance of the lithium-containing composite oxide, which is the active material, is improved. There is a problem that it cannot be pulled out sufficiently. As a result, a portion having poor conductivity appears locally in the positive electrode, which causes the active material to not be effectively used, the discharge capacity to decrease, and the battery life to be shortened.

Therefore, in Patent Document 2, in order to improve the dispersibility of carbon black in the slurry, a method of dispersing carbon black in a solvent on the submicron order by a high-pressure jet mill in the presence of polyvinylpyrrolidone as a dispersant is performed. ing. According to Patent Document 2, it is described that a lithium ion secondary battery having a high capacity and excellent cycle characteristics can be obtained by using carbon black having a stable dispersed state as a positive electrode.

Patent Document 3 describes a conductive material that exhibits dispersion stability and excellent conductivity with a small amount of addition in a positive electrode for a lithium ion secondary battery. Specifically, N-methyl-2-pyrrolidone is used as a dispersion medium, and carbon black having an average particle size of 0.1 to 1 μm is suspended in a proportion of 3 to 30% by mass, and a vinylpyrrolidone-based polymer is 0. A carbon black slurry having been added in an amount of 1 to 10% by mass has been proposed. An example of Patent Document 3 describes carbon black having an average particle size of 0.3 μm determined by laser diffraction / scattering spectroscopy, and lithium ions produced by using the carbon black as a conductive material for a positive electrode. It is shown that the secondary battery had a high discharge capacity.

As a means for overcoming poor dispersion of the conductive material, there is also a method of adding a polyvinylpyrrolidone-based polymer and a nonionic surfactant as dispersants (Patent Document 4).

In Patent Document 5, a carbon black dispersion liquid containing carbon black, polyvinyl alcohol as a dispersant, and N-methyl-2-pyrrolidone as a solvent is used, and the surface resistance of the battery positive electrode mixture layer and 50 It is noted that the discharge capacity retention rate after the cycle is improved.

Japanese Unexamined Patent Publication No. 2008-227481 Japanese Unexamined Patent Publication No. 2004-281906 Japanese Unexamined Patent Publication No. 2003-157846 International Publication No. 2012/014616 International Publication No. 2014/132809

As described above, a method of using a polymer as a dispersant has been conventionally proposed, but it cannot be said that it has sufficient dispersibility yet. For example, the dispersant described in Patent Document 4 can improve the poor dispersion of the conductive material, but when the positive electrode containing this dispersant is used as a lithium ion battery, the discharge capacity is extremely reduced after a float charge of 4.35 V. There was a problem such as doing. However, in the current lithium ion secondary battery market, the withstand voltage of the battery is desired, and a conductive resin composition for a positive electrode having both dispersibility and withstand voltage is indispensable.

An object of the present invention is to provide a positive electrode resin composition having excellent dispersibility and withstand voltage in view of the above problems and circumstances. In addition, it is intended to provide a positive electrode having a low electrode plate resistance manufactured by using this positive electrode resin composition, and a secondary battery manufactured by using this positive electrode and having excellent withstand voltage, discharge rate characteristics and cycle characteristics. With the goal.

As a result of diligent research to achieve the above object, the present inventors have conducted a positive electrode resin containing polyvinyl alcohol having a specific degree of saponification as a dispersant and carbon black having a specific average primary particle size. It has been found that the above problems can be solved by using the composition.
Specifically, the present inventor has prepared a positive electrode using a positive 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 by using this positive electrode is excellent in withstand voltage, discharge rate characteristics and cycle characteristics. The present inventor has been completed based on the above findings.

That is, the present invention that solves the above problems is exemplified below.
[1]
A positive 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 positive electrode resin composition having an average primary particle size of ~ 96.5 mol% and the carbon black having an average primary particle size of 18 nm to 40 nm.
[2]
The positive electrode resin composition according to [1], wherein the polyvinyl alcohol has an average degree of polymerization of 500 to 1500.
[3]
The positive electrode resin composition according to [1] or [2], wherein the DBP absorption amount of the carbon black is 250 to 310 ml / 100 g.
[4]
The item according to any one of [1] to [3], wherein the mass ratio of the dispersant to the conductive material {mass of the dispersant / mass of the conductive material} is 0.03 to 0.15. Positive electrode resin composition.
[5]
The positive electrode resin composition according to any one of [1] to [4], wherein the binder is polyvinylidene fluoride.
[6]
A positive electrode containing the positive electrode resin composition according to any one of [1] to [5].
[7]
A secondary battery provided with the positive 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 positive electrode resin composition having excellent dispersibility and withstand voltage.
According to one embodiment of the present invention, it is possible to provide a positive 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 withstand voltage resistance, rate characteristics and cycle characteristics.
Further, according to a preferred embodiment of the present invention, it is possible to provide a positive electrode capable of easily obtaining a secondary battery having a high energy density and excellent withstand voltage resistance, rate characteristics and cycle characteristics.

It is a schematic diagram of the lithium ion 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 18 to 40 nm. By setting the average primary particle size to 40 nm or less, the number of electrical contacts with the active material and the current collector is increased, and a good conductivity-imparting effect can be obtained. By setting the average primary particle size to 18 nm or more, the interaction between the particles is suppressed, so that the particles are uniformly dispersed between the positive electrode active materials, and a good conductive path can be obtained. From this point of view, the average primary particle size of carbon black is more preferably 20 to 35 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 amount of DBP absorbed by carbon black in the present invention is preferably 250 to 310 ml / 100 g. By setting the DBP absorption amount to 250 ml / 100 g or more, the aggregate when used as a conductive material has a sufficient length and spread, and a good conductive path and liquid retention property of a non-aqueous electrolytic solution can be obtained. Further, by setting the amount to 310 ml / 100 g or less, aggregation due to entanglement between the aggregates is suppressed, so that the aggregates are uniformly dispersed between the positive electrode active materials, a good conductive path is formed, and a sufficient non-aqueous electrolyte solution is retained. It is possible to achieve both sex. In the present invention, the amount of DBP absorbed is a value measured in accordance with JIS K6217-4: 2008.

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. Of these, polyvinylidene fluoride is preferable in terms of withstand voltage.

<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; alkyl (meth) acrylate, 2-hydroxyethyl (meth) acrylate, glycidyl (meth) acrylate and the like ( Meta) 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 compounds; 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 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 positive 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 500 to 1500. By setting the average degree of polymerization to 1500 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 the slurry containing the uniform and low viscosity positive electrode resin composition is improved. Is easy to obtain. By setting the average degree of polymerization to 500 or more, the dispersibility of the active material and the conductive material is enhanced, and a good conductive path can be easily obtained.

<Active material>
The active material used in the present invention is a lithium-containing composite oxide or a lithium-containing polyanion compound, which is a positive electrode active material capable of reversibly occluding and releasing cations. For example, LiCoO ₂ , LiMn ₂ O ₄ , LiNiO ₂ , LiMPO ₄ , Li ₂ MSiO ₄ , LiNi _X Mn _(2-X) O ₄ , Li (Mn _X Ni _Y Co _Z ) O ₂ , Li (Al _X Ni _Y). Examples thereof include Co _Z ) O _{2 and xLi 2} MnO _3- (1-x) LiMO ₂ . However, _X in LiNi X Mn _(2-X) O ₄ satisfies the relationship of 0 <X <2, and is in Li (Mn _X Ni _Y Co _Z ) O ₂ or Li (Al _X Ni _Y Co _Z ) O ₂ X, Y and Z in the above satisfy the relationship of X + Y + Z = 1 and satisfy the relationship of 0 <X <1, 0 <Y <1, 0 <Z <1, and xLi ₂ MnO _3- (1-x). X in LiMO ₂ satisfies the relationship of 0 <x <1, and M in LiMPO ₄ , Li ₂ MSiO ₄ or xLi ₂ MnO _3- (1-x) LiMO ₂ is Fe, Co, Ni, Mn. It is preferable that it is one or more of the elements selected from.

Among the above active materials, the active material used in the present invention preferably has an average particle size (D50) of 20 μm or less, preferably 5 μm or less, which is obtained from the cumulative distribution of volume-based particle sizes measured by the laser light scattering method. With such a configuration, the effect of reducing the viscosity of the slurry containing the positive electrode resin composition is sufficiently exhibited, and it becomes easy to obtain a positive electrode having improved dispersibility of the conductive material and a secondary battery having high cycle characteristics.

<Positive electrode resin composition>
A known method can be used for producing the positive electrode resin composition used in the present invention. For example, it can be obtained by mixing a solvent dispersion solution of an active material, a conductive material, a binder and a dispersant with a ball mill, a sand mill, a twin-screw kneader, a rotating or revolving stirrer, a planetary mixer, a disperser mixer, or the like. Is used as a slurry. As the 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 positive electrode resin composition include water, N-methyl-2-pyrrolidone, cyclohexane, methyl ethyl ketone, methyl isobutyl ketone and the like. When polyvinylidene fluoride is used as the polymer binder, N-methyl-2-pyrrolidone is preferable in terms of solubility, and when a styrene-butadiene copolymer is used, water is preferable.

The mass ratio of the dispersant to the conductive material in the positive electrode resin composition used in the present invention {mass of the dispersant / mass of the conductive material} is 0.03 to 0.15, more preferably 0.04 to 0.12. , 0.05 to 0.1 is the most preferable. By setting the mass ratio of the dispersant to the conductive material in the positive 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 a higher dispersion effect, the effect of excess dispersant covering the surface of the conductive material and interfering with the charge transfer reaction can be suppressed, and high resistance of the battery can be suppressed.

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

<Lithium-ion battery>
The method for producing the lithium ion 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 following method has the configuration schematically shown in FIG. It can also be produced by. That is, after welding the aluminum tab 5 to the positive electrode 1 using the positive electrode and welding the nickel tab 6 to the negative electrode 2, a polyolefin microporous film 3 to be an insulating layer is arranged between the positive electrode and the negative electrode. , The positive electrode 1, the negative electrode 2, and the void portion of the polyolefin microporous film 3 are injected until the non-aqueous electrolytic solution is sufficiently impregnated, and sealed with the exterior 4.

The application of the lithium ion battery of the present invention is not particularly limited, but for example, 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, a mobile information terminal such as a mobile PC, and the like. In addition, it can be used in a wide range of fields such as 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 positive electrodes used in both Examples and Comparative Examples were vacuum dried at 170 ° C. for 3 hours in order to volatilize the adsorbed water.

<Example 1>
(Preparation of slurry containing positive electrode resin composition)
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, carbon black as a conductive material. (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 as an active material (manufactured by Yumicore, " TX10 ”average particle size (D50) 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., Awatori). Using Kentarou ARV-310), the mixture was mixed until uniform to obtain a slurry containing a positive electrode resin composition.

[Evaluation of dispersibility (viscosity of slurry containing positive electrode resin composition)]
The dispersibility of the slurry containing the positive electrode resin composition was evaluated by the method using a rotary rheometer described in JIS K7244-10. Specifically, using a rotary rheometer (MCR300 manufactured by Anton Pearl Co., Ltd.), 1 g of a slurry containing a positive electrode resin composition having a solid content of 68% by mass was applied onto a disk, and the measurement temperature was set to 25 ° C. The shear rate was set and the shear rate ^{was changed from 100 s -1} to 0.01 s ^-1 for measurement, and the viscosity at the shear rate of 10 s ^{-1 was evaluated.} The lower the viscosity value, the better the dispersibility. The viscosity of this example was 1.24 Pa · s.

(Preparation of positive electrode)
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, and allowed to stand in a dryer for pre-drying 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.

[Evaluation of electrode plate resistance of positive electrode]
The prepared positive electrode was cut out into a disk shape with a diameter of 14 mm, and the front and back sides were sandwiched between SUS304 flat plates, and the amplitude voltage was 10 mV using an electrochemical measurement system (Solartron, Function Generator 1260 and Potential Galvanostat 1287). The AC impedance was measured in the 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 positive electrode of this example was 120 Ω · cm ² .

(Preparation of negative electrode)
Pure water (manufactured by Kanto Chemical Co., Inc.) as a solvent, artificial graphite (manufactured by Hitachi Kasei Co., Ltd., "MAG-D") as a negative electrode active material, styrene-butadiene rubber (manufactured by Nippon Zeon Co., Ltd., "BM-400B") as a binder, and the following , SBR), and carboxymethyl cellulose (manufactured by Daicel Corporation, "D2200", hereinafter referred to as CMC) was prepared as a dispersant. Next, the CMC was weighed and mixed so that the solid content was 1% by mass and the artificial graphite was 97% by mass in the solid content, pure water was added to this mixture, and a rotation / revolution type mixer (manufactured by Shinky Co., Ltd., Awa) was added. It was mixed using Tori Rentaro ARV-310) until it became uniform. Further, the SBR is weighed so as to have a solid content of 2% by mass, added to the above mixture, and mixed using a rotation / revolution type mixer (Sinky Co., Ltd., Awatori Rentaro ARV-310) until uniform. Then, a negative electrode slurry was obtained. Next, the negative electrode slurry 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, and the thickness of the coating film containing the copper foil was adjusted to 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.

(Making a lithium-ion battery)
In a dry room controlled to have a dew point of −50 ° C. or lower, the positive electrode was processed to 40 × 40 mm, the negative electrode was processed to 44 × 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 × 45 mm was arranged between the positive electrode and the negative electrode. Next, the sheet-shaped exterior cut and processed into a 70 × 140 mm square was folded in half at the center of the long side. Next, the exterior was arranged so that the aluminum tab for the positive electrode and the nickel tab for the negative electrode were exposed to the outside of the exterior, and 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. Pour an ethylene carbonate / diethyl carbonate = 1/2 (volume ratio) + 1M LiPF ₆ solution (hereinafter referred to as an electrolytic solution) manufactured by Kishida Chemical Co., Ltd., and allow it to sufficiently soak into the positive electrode, negative electrode and polyolefin microporous film. A lithium ion 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 battery was evaluated by the following method.

(Evaluation of lithium-ion batteries)
[Discharge rate characteristics (discharge capacity retention rate during 3C discharge)]
The produced lithium ion battery was charged at a constant current constant voltage of 4.3 V and 0.2 C limit at 25 ° C., and then discharged to 3.0 V at 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 battery of this example during 3C discharge was 80.5%.

[Cycle characteristics (discharge capacity retention rate after cycle)]
The produced lithium ion 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 battery of this example was 90%.

[Withstand voltage (discharge capacity retention rate after float charging)]
The produced lithium-ion battery was float-charged at 25 ° C. at a constant current constant voltage limited to 4.35 V and 0.5 C for 2 hours, and then discharged to 3.0 V at a constant current of 0.5 C. The capacity was measured. Then, the float was charged again with a constant current constant voltage limited to 4.35 V and 0.5 C for 48 hours, and then discharged to 3.0 V with a constant current of 0.5 C in the same manner, and the discharge capacity at this time was measured. As an index of the withstand voltage of the battery, the capacity retention rate after float charging during 48 hours charging with respect to 2 hours charging was calculated. The capacity retention rate of the lithium-ion battery of this example after float charging was 94%.

Table 1 shows the saponification degree and the average degree of polymerization of the polyvinyl alcohols used in Examples 1 to 8 and Comparative Examples 1 to 4. Table 2 shows the average primary particle size and DBP oil absorption of carbon black used in Examples 1 to 8 and Comparative Examples 1 to 6.

<Example 2>
A positive electrode resin composition, 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 B, and each evaluation was carried out. The results are shown in Table 3.

<Example 3>
A positive electrode resin composition, 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 C, and each evaluation was carried out. The results are shown in Table 3.

<Example 4>
A positive electrode resin composition, 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 D, and each evaluation was carried out. The results are shown in Table 3.

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

<Example 6>
The conductive material of Example 1 was changed to Li-250 (manufactured by Denka Co., Ltd.), PVdF was 1.95% by mass in solid content, Li-250 was 1% by mass in solid content, and polyvinyl alcohol A was 0 in solid content. The positive electrode resin composition, the positive electrode, and the lithium ion battery were prepared in the same manner as in Example 1 except that they were weighed and mixed so as to be 0.05% by mass (mass of dispersant / mass of conductive material = 0.05). It was prepared and each evaluation was carried out. The results are shown in Table 3.

<Example 7>
The conductive material of Example 1 was changed to ECP (manufactured by Lion), PVdF was 1.84% by mass in solid content, ECP was 1% by mass in solid content, and polyvinyl alcohol A was 0.16% by mass in solid content. A positive electrode resin composition, a positive electrode, and a lithium ion battery were prepared in the same manner as in Example 1 except that they were weighed and mixed so as to be (mass of dispersant / mass of conductive material = 0.16). Evaluation was carried out. The results are shown in Table 3.

<Example 8>
The conductive material of Example 1 was changed to Li-250 (manufactured by Denka Co., Ltd.), PVdF was 1.98% by mass in solid content, Li-250 was 1% by mass in solid content, and polyvinyl alcohol A was 0 in solid content. The positive electrode resin composition, the positive electrode, and the lithium ion battery were prepared in the same manner as in Example 1 except that they were weighed and mixed so as to be 0.02% by mass (mass of dispersant / mass of conductive material = 0.02). It was prepared and each evaluation was carried out. The results are shown in Table 3.

<Comparative example 1>
A positive electrode resin composition, 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 positive electrode resin composition, 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 # 3040B (manufactured by Mitsubishi Chemical Corporation), PVdF was 1.97% by mass in solid content, # 3040B was 1% by mass in solid content, and polyvinyl alcohol A was 0. A positive electrode resin composition, a positive electrode, and a lithium ion battery were prepared in the same manner as in Example 1 except that they were weighed and mixed so as to be 03% by mass (mass of dispersant / mass of conductive material = 0.03). Then, 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), PVdF was 1.85% by mass in solid content, BlackPearls2000 was 1% by mass in solid content, and polyvinyl alcohol A was 0.15% by mass in solid content. A positive electrode resin composition, a positive electrode, and a lithium ion battery were prepared in the same manner as in Example 1 except that they were weighed and mixed so as to be (mass of dispersant / mass of conductive material = 0.15). Evaluation was carried out. The results are shown in Table 3.

<Comparative example 5>
A positive electrode resin composition, 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 polyvinylpyrrolidone (manufactured by Nippon Shokubai Co., Ltd., K-90), and each evaluation was performed. Was carried out. The results are shown in Table 3.

<Comparative Example 6>
The same method as in Example 1 except that PVdF was weighed and mixed so that the solid content was 2% by mass and Li-435 was 1% by mass in the solid content without adding the dispersant of Example 1. A positive electrode resin composition, a positive electrode, and a lithium ion battery were prepared in 1 and each evaluation was carried out. The results are shown in Table 3.

It was revealed that the positive electrode resin compositions of Examples 1 to 8 had higher dispersibility than the positive electrode resin compositions of Comparative Examples 1 to 6. As a result, it was found that the positive electrode of the embodiment of the present invention has a low electrode plate resistance and can suppress the voltage drop during discharge.

Further, it was revealed that the lithium ion batteries of Examples 1 to 8 have higher discharge rate characteristics, higher cycle characteristics, and higher withstand voltage resistance than the lithium ion batteries of Comparative Examples 1 to 6. As a result, it was found that the lithium ion battery using the positive electrode resin composition of the present invention can suppress the decrease in the discharge rate characteristic due to the increase in the discharge current, and also has a long life.

1 Lithium-ion battery positive electrode 2 Lithium-ion battery negative electrode 3 Polyolefin microporous film 4 Aluminum tab 5 Nickel tab 6 Exterior

Claims (7)

A positive 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 positive electrode resin composition having an average primary particle size of ~ 96.5 mol% and the carbon black having an average primary particle size of 18 nm to 40 nm. The positive electrode resin composition according to claim 1, wherein the polyvinyl alcohol has an average degree of polymerization of 500 to 1500. The positive electrode resin composition according to claim 1 or 2, wherein the DBP absorption amount of the carbon black is 250 to 310 ml / 100 g. The positive electrode resin according to any one of claims 1 to 3, wherein the mass ratio of the dispersant to the conductive material {mass of the dispersant / mass of the conductive material} is 0.03 to 0.15. Composition. The positive electrode resin composition according to any one of claims 1 to 4, wherein the binder is polyvinylidene fluoride. A positive electrode containing the positive electrode resin composition according to any one of claims 1 to 5. The secondary battery provided with the positive electrode according to claim 6.

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