JP7477290B2 - Anode resin composition, anode and secondary battery - Google Patents

Description

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

In response to growing environmental and energy issues, there has been active development of technologies aimed at realizing a low-carbon society that reduces dependency on fossil fuels. Examples of such technological development are diverse and include the development of low-emission vehicles such as hybrid electric vehicles and electric vehicles, the development of natural energy generation and storage systems such as solar and wind power generation, and the development of next-generation power transmission networks that supply electricity efficiently and reduce transmission losses.
Batteries are one of the key devices commonly required for these technologies, and high energy density is required for such batteries to miniaturize the systems. In addition, high output characteristics are required to enable stable power supply regardless of the temperature of the operating environment. Furthermore, good cycle characteristics that can withstand long-term use are also required. For this reason, conventional lead-acid batteries, nickel-cadmium batteries, and nickel-metal hydride batteries are being rapidly replaced by lithium-ion secondary batteries, which have higher energy density, output characteristics, and cycle characteristics.

Conventionally, the negative electrode of a lithium-ion secondary battery is manufactured by coating a current collector with an electrode paste containing a negative electrode active material and a binder. As the negative electrode active material, natural graphite, artificial graphite, difficult-to-graphitize carbon, easy-to-graphitize carbon, silicon compounds, lithium titanate, etc. have been used. Recently, in order to improve the lithium ion acceptance and shorten the charge/discharge time, and to prevent the repeated expansion and contraction of the negative electrode active material during charging and discharging, which would impair conductivity, the addition of conductive materials such as carbon black with developed aggregates (a structure in which multiple primary particles are fused together: primary agglomerates) and cylindrical carbon nanotubes with developed crystals has been considered.

The conductive material in the negative electrode has two basic roles. The first is to form a uniform conductive path between the negative electrode active materials, preventing the negative electrode active materials from repeatedly expanding and contracting during charging and discharging, which would impair conductivity. The second is to hold the electrolyte in which lithium ions are dissolved, and to supply sufficient lithium ions to all the negative electrode active materials, preventing a shortage of lithium ions during charging and discharging, which would impair battery performance. For this reason, it is important that the average primary particle size and specific surface area of the carbon black used as the conductive material in the negative electrode production be controlled within a certain range. If the control is insufficient or the dispersion between the negative electrode active materials is poor, the negative electrode active material and the carbon black cannot be in sufficient contact, the conductive path cannot be secured, and sufficient lithium ions can only be supplied to some active materials. As a result, parts of the electrode that are poor in conductivity and electrolyte retention appear locally, which causes the negative electrode active material to be used ineffectively, resulting in a decrease in discharge capacity and a shortened battery life.

Patent Document 1 discloses a technique for a lithium-ion secondary battery electrode mixture that uses a negative electrode active material, a conductive material, a binder made of a copolymer of vinyl alcohol and an alkali metal neutralized ethylenically unsaturated carboxylic acid, and a dispersion aid. Patent Document 1 describes that this configuration increases the storage stability of the lithium-ion secondary battery electrode mixture, and that a battery using a negative electrode made from this mixture has a good retention rate after 50 cycles.

Patent Document 2 also discloses technology for a negative electrode for a non-aqueous electrolyte secondary battery, in which a negative electrode active material, a conductive material, and a binder that is a polyvinyl alcohol resin with a saponification degree of 87 to 99.9 mol % are used in the electrode mixture layer. Patent Document 2 describes that a battery using this negative electrode has a good capacity retention rate after 100 cycles.

JP 2015-201267 A JP 2016-181414 A

As described above, techniques using polyvinyl alcohol or its copolymers as binders for the negative electrodes of lithium-ion secondary batteries have been proposed in the past, but there has been no technique that combines sufficient dispersibility and battery performance for practical use. For example, the technique disclosed in Patent Document 1 improves the dispersibility of conductive materials, but when a negative electrode using this technique is used in a lithium-ion secondary battery, there is a problem that the discharge capacity decreases during discharge at a high current load. However, in the current lithium-ion secondary battery market, performance that does not reduce capacity even when charging and discharging at a high current load is desired, and a negative electrode resin composition that combines dispersibility and battery performance is essential.

In view of the above problems and current circumstances, one of the objects of the present invention is to provide a negative electrode resin composition that has excellent dispersibility and battery performance. In addition, another object of the present invention is to provide a negative electrode having low plate resistance manufactured using this negative electrode resin composition, and further a secondary battery having excellent discharge rate characteristics and cycle characteristics manufactured using this negative electrode.

As a result of intensive research conducted by the present inventors to achieve the above object, they have found that the above problem can be solved by using a negative electrode resin composition containing polyvinyl alcohol having a specific saponification degree as a dispersant and carbon black having a specific average primary particle size as a conductive material.
Specifically, the present inventors have found that a negative electrode manufactured using a negative electrode resin composition containing carbon black as a conductive material, a binder, and polyvinyl alcohol having a specific saponification degree as a dispersant has low plate resistance, and further, a secondary battery manufactured using this negative electrode has excellent discharge rate characteristics and cycle characteristics. The present invention has been completed based on this finding.

That is, the present invention which solves the above problems is exemplified as follows.
[1]
The negative electrode resin composition contains a conductive material, a binder, and a dispersant, wherein 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 to 96.5 mol %, and the carbon black has an average primary particle size of 16 to 50 nm.
[2]
The negative electrode resin composition according to [1], wherein the average polymerization degree of the polyvinyl alcohol is 250 to 1,800.
[3]
The negative electrode resin composition according to [1] or [2], wherein the carbon black has a specific surface area of 35 to 400 m ² /g.
[4]
[4] The negative electrode resin composition according to any one of [1] to [3], wherein the mass ratio of the dispersant to the conductive material in terms of solid content {mass of dispersant/mass of conductive material} is 0.03 to 0.15.
[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 comprising the negative electrode resin composition according to any one of [1] to [5].
[7]
A secondary battery comprising the negative electrode according to [6].
In this specification, unless otherwise specified, the symbol "to" means a range of values "greater than or equal to" and "less than or equal to" the values at both ends. For example, "A to B" means A or greater 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, a negative electrode having low plate resistance can be provided.
According to one embodiment of the present invention, a secondary battery having excellent discharge rate characteristics and cycle characteristics can be provided.
Furthermore, according to a preferred embodiment of the present invention, it is possible to provide a negative electrode from which a secondary battery having high energy density and excellent discharge rate characteristics and cycle characteristics can be easily obtained.

FIG. 1 is a schematic diagram of a lithium ion secondary battery used in the present invention.

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

The constituent materials of the present invention are described in detail below.

<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. Carbon black alone can be used as the conductive material. The carbon black is selected from acetylene black, furnace black, channel black, etc., like carbon black used as a general conductive material for batteries. Among them, acetylene black, which has excellent crystallinity and purity, is preferred.

In the present invention, the average primary particle diameter of the carbon black is preferably 16 to 50 nm. By setting the average primary particle diameter 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 good conductivity imparting effect and liquid retention are obtained. By setting the average primary particle diameter to 16 nm or more, more preferably 18 nm or more, the interaction between the particles is suppressed, and the particles are uniformly dispersed between the negative electrode active material, and good conductive paths and liquid retention are obtained. From this viewpoint, the average primary particle diameter of the carbon black is more preferably 18 to 40 nm. In the present invention, the average primary particle diameter of the carbon black is the average value of the particle diameters measured based on photographs taken with a transmission electron microscope or the like. Specifically, five images at 100,000 times magnification were taken using a transmission electron microscope JEM-2000FX (manufactured by JEOL Ltd.), and the particle diameters of 200 or more randomly selected primary particles were obtained by image analysis, and the number average was calculated to measure. The particle diameter is the circle equivalent diameter of the primary particle.

The specific surface area of the carbon black in the present invention is preferably 35 to 400 m ² /g. By making the specific surface area 400 m ² /g or less, the interaction between particles is suppressed, so that the carbon black is uniformly dispersed among the negative electrode active material, and good conductive paths and liquid retention are easily obtained. In addition, by making the specific surface area 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 increases, and good conductivity-imparting effect and liquid retention are easily obtained. In the present invention, the specific surface area refers to the single point nitrogen adsorption specific surface area measured in accordance with JIS K6217-2:2017.

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

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

<Binding 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 used as the binder, and random copolymers, alternating copolymers, graft copolymers, block copolymers, and the like can also be used. Among these, styrene-butadiene copolymers are preferred in terms of battery performance.

<Dispersant>
The dispersant used in the present invention contains at least polyvinyl alcohol (hereinafter, sometimes abbreviated as PVA). The content 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, such as vinyl acetate.

Examples of the fatty acid vinyl esters 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. Of these, vinyl acetate is preferred.

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

Polyvinyl alcohol can be produced by a polymerization method known per se, such as a method of producing polyvinyl acetate by solution polymerization in an alcohol-based organic solvent, and then saponifying the polyvinyl acetate, but is not limited to this, and may be produced by, for example, bulk polymerization, emulsion polymerization, suspension polymerization, etc. When performing solution polymerization, it may be continuous polymerization or batch polymerization, and the monomers may be charged all at once or in portions, or may be added continuously or intermittently.

The polymerization initiator used in the solution polymerization is not particularly limited, but may be azo compounds such as azobisisobutyronitrile, azobis-2,4-dimethylvaleronitrile, and azobis(4-methoxy-2,4-dimethylvaleronitrile); peroxides such as acetyl peroxide, benzoyl peroxide, lauroyl peroxide, acetylcyclohexylsulfonyl peroxide, and 2,4,4-trimethylpentyl-2-peroxyphenoxyacetate; percarbonate compounds such as diisopropyl peroxydicarbonate, di-2-ethylhexyl peroxydicarbonate, and diethoxyethyl peroxydicarbonate; perester compounds such as t-butylperoxyneodecanate, α-cumylperoxyneodecanate, and t-butylperoxyneodecanate; and known radical polymerization initiators such as azobisdimethylvaleronitrile and azobismethoxyvaleronitrile.

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. In general, saponification can be performed by hydrolyzing the ester moiety in the molecule in an alcohol solution such as methanol in the presence of an alkali catalyst or an acid catalyst. Examples of alkali catalysts that can be used include alkali metal hydroxides such as sodium hydroxide, potassium hydroxide, sodium methylate, sodium ethylate, and potassium methylate, and alcoholates. Examples of acid catalysts that can be used include aqueous solutions of inorganic acids such as hydrochloric acid and sulfuric acid, and organic acids such as p-toluenesulfonic acid, but it is preferable 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 is preferably in the range of 30 minutes to 3 hours.

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

In the present invention, the average degree of polymerization of the polyvinyl alcohol 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, improving the dispersibility of the conductive material, and making it easier to obtain a slurry containing a uniform and low-viscosity negative electrode resin composition. 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 increased, making it easier to obtain a good conductive path. The average degree of polymerization here is a value measured by a method conforming 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 materials such as non-graphitizable carbon and graphitizable carbon polyacene, silicon atom-containing materials, tin atom-containing materials, and lithium titanate. These may be used alone or in combination of two or more. Among these, at least one selected from carbon materials and silicon atom-containing materials is preferred, and at least one selected from graphite and silicon atom-containing materials is more preferred.

Examples of silicon atom-containing substances include (i) silicon microparticles, (ii) alloys or compounds of silicon and magnesium, calcium, lithium, tin, nickel, copper, iron, cobalt, molybdenum, manganese, zinc, indium, silver, titanium, germanium, tantalum, bismuth, vanadium, tungsten, niobium, antimony or chromium, and (iii) compounds of silicon and boron, nitrogen, oxygen or carbon. Examples of silicon alloys or compounds include _SiB4 , _SiB6 , _Mg2Si , _Ni2Si , _TiSi2 , _MoSi2 , CoSi2, _NiSi2 , _CaSi2 , CrSi2, _{Cu5Si , FeSi2} , _MnSi2 , _NbSi2 , TaSi2, _VSi2 , _WSi2 , _{ZnSi2 ,} SiC, _Si3N4 , _{Si2N2O , SiOx} (0<x≦2) or _LiSiO , _etc.
Examples of tin-atom-containing substances include (i) alloys or compounds of tin with silicon, nickel, copper, iron, cobalt, manganese, zinc, indium, silver, titanium, germanium, bismuth, antimony or chromium, and (ii) compounds of tin with oxygen or carbon.

<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 these, the thickener is preferably polyethylene glycol, cellulose, or a derivative thereof, and more preferably carboxymethylcellulose (CMC).

<Negative Electrode Resin Composition>
A known method can be used to manufacture the negative electrode resin composition used in the present invention. For example, the negative electrode resin composition is obtained by mixing a solvent dispersion solution of the negative electrode active material, conductive material, binder, and dispersant with a ball mill, a sand mill, a two-axis kneader, a rotation-revolution type mixer, a planetary mixer, a dispersant mixer, or the like, and is generally 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, conductive material, binder, and dispersant, those already described 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, and methyl isobutyl ketone. When a styrene-butadiene copolymer is used as the polymer binder, water is preferred, and when polyvinylidene fluoride is used, N-methyl-2-pyrrolidone is preferred in terms of solubility.

The mass ratio (mass of dispersant/mass of conductive material) of the solid content of the dispersant in the negative electrode resin composition used in the present invention is preferably 0.03 to 0.15, more preferably 0.04 to 0.12, and most preferably 0.05 to 0.1. 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 to the conductive material, making it easier to obtain a higher dispersion effect, and by setting it to 0.05 to 0.1, in addition to the higher dispersion effect, the effect of excess dispersant covering the conductive material surface and interfering with the charge transfer reaction is suppressed, thereby suppressing the increase in battery resistance.

<Negative Electrode>
The negative electrode used in the present invention can be produced by the following procedure. First, a slurry containing the above-mentioned 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 to form a negative electrode composite layer, which is a porous body in which the negative electrode active material is bound to the current collector surface via a binder. Next, the current collector and the negative electrode composite layer are pressed together by a roll press or the like to be in close contact with each other, thereby obtaining the desired negative electrode.

<Lithium-ion secondary battery>
The method for producing the lithium ion secondary battery used in the present invention is not particularly limited, and may be any conventionally known method for producing a battery, but for example, the battery may be produced by the following method using the structure diagrammatically shown in Fig. 1. That is, an aluminum tab 5 is welded to a positive electrode 1, a nickel tab 6 is welded to a negative electrode 2 using the above-mentioned negative electrode, a polyolefin microporous membrane 3 serving as an insulating layer is disposed between the positive electrode 1 and the negative electrode 2, a nonaqueous electrolyte is poured into the voids of the positive electrode 1, the negative electrode 2, and the polyolefin microporous membrane 3 until the nonaqueous electrolyte is sufficiently impregnated, and the battery is sealed with an exterior 4.

The applications of the lithium ion secondary battery of the present invention are not particularly limited, but it can be used in a wide range of fields, such as portable AV devices such as digital cameras, video cameras, portable audio players, and portable LCD televisions, portable information terminals such as notebook computers, smartphones, and mobile PCs, as well as portable game devices, power tools, electric bicycles, hybrid cars, electric cars, and power storage systems.

The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to the examples shown below as long as the gist of the invention is not lost. In addition, the negative electrodes used in both the examples and comparative examples were vacuum dried at 100°C for 4 hours to volatilize the adsorbed moisture.

Example 1
(Preparation of slurry containing negative electrode resin composition)
Pure water (manufactured by Kanto Chemical Co., Ltd.) was used as a solvent, carbon black (manufactured by Denka Co., Ltd., "Li-435", hereinafter referred to as Li-435) was used as a conductive material, polyvinyl alcohol (referred to as polyvinyl alcohol A) was used as a dispersant, styrene-butadiene copolymer (hereinafter referred to as SBR) was used as a binder, carboxymethyl cellulose (hereinafter referred to as CMC) was used as a thickener, and artificial graphite (manufactured by Shenzhen BTR Co., Ltd., "AGP-2A") was used as an active material. Li-435 was weighed and mixed to a solid content of 1 mass%, polyvinyl alcohol A was weighed and mixed to a solid content of 0.1 mass% (mass of dispersant/mass of conductive material = 0.1), CMC was weighed and mixed to a solid content of 1 mass%, and artificial graphite was weighed and mixed to a solid content of 96 mass%, and pure water was added to this mixture, and the mixture was mixed until uniform using a rotation-revolution type mixer (manufactured by Thinky Corporation, "Awatori Rentaro ARV-310"). Furthermore, SBR was weighed out so as to have a solid content of 1.9 mass%, added to the above mixture, and mixed until homogenous using a planetary centrifugal mixer (manufactured by Thinky Corporation, "Awatori Rentaro ARV-310") to obtain a slurry containing a negative electrode resin composition with a solid content concentration of 50 mass%.

[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 a method using a rotational rheometer described in JIS K7244-10. Specifically, using a rotational rheometer (manufactured by Anton Paar, "MCR300"), 1 g of a slurry containing a negative electrode resin composition with a solid content of 50 mass% was applied onto a disk, and the shear rate was changed from 100 s ^-1 to 0.01 s ^-1 to perform measurements, and the viscosity at a shear rate of 1 s ^-1 was evaluated. The lower the viscosity value, the better the dispersibility. The viscosity in this example was 3.5 Pa·s.

(Preparation of negative electrode)
The slurry containing the negative electrode resin composition was applied to a copper foil (manufactured by UACJ) having a thickness of 10 μm by an applicator, and was pre-dried at 60° C. for one hour by placing it in a dryer. Next, the film was pressed with a linear pressure of 100 kg/cm using a roll press machine to prepare a coating film containing the copper foil having a thickness of 10 μm to a thickness of 50 μm. In order to completely remove residual moisture, the film was vacuum dried at 120° C. for 3 hours to obtain a negative electrode.

[Evaluation of negative electrode plate resistance]
The prepared negative electrode was cut into a disk shape with a diameter of 14 mm, and the front and back were sandwiched between SUS304 flat plate electrodes. Using an electrochemical measurement system (manufactured by Solartron, "Function Generator 1260" and "Potentiogalvanostat 1287"), 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 area of the cut-out disk was taken as the plate resistance. The plate resistance of the negative electrode in this example was 1.2 Ω cm ² .

(Preparation of Positive Electrode)
N-methyl-2-pyrrolidone (manufactured by Kanto Chemical Co., Ltd., hereafter referred to as NMP) was prepared as a solvent, polyvinylidene fluoride (manufactured by Arkema, "HSV900", hereafter referred to as PVdF) was prepared as a binder, carbon _black (manufactured by Denka, "Li-435", hereafter referred to as Li-435) was prepared as a conductive material, polyvinyl alcohol (referred to as polyvinyl alcohol A) was prepared as a dispersant, _{and LiNi0.5Mn0.3Co0.2O2} (manufactured by Umicore, "TX10" _, average primary particle size (D50) 10 μm, hereafter referred to as "NMC532") was prepared as an active material. PVdF was 1.9% by mass in solids, Li-435 was 1% by mass in solids, polyvinyl alcohol A was 0.1% by mass in solids (mass of dispersant / mass of conductive material = 0.1), and NMC532 was weighed and mixed to be 97% by mass in solids, and NMP was added to this mixture so that the solid content was 68% by mass, and a rotating and revolving mixer (manufactured by Thinky Corporation, "Awatori Rentaro ARV-310") was used to mix until uniform to obtain a slurry containing a positive electrode resin composition. The slurry containing the prepared positive electrode resin composition was formed into a film on one side of an aluminum foil (manufactured by UACJ Corporation) having a thickness of 15 μm with an applicator, and was left to stand in a dryer and pre-dried at 105 ° C. for one hour, and the NMP solvent was completely removed. Next, it was pressed with a linear pressure of 200 kg / cm with a roll press machine, and the thickness of the coating film containing the aluminum foil having a thickness of 15 μm was adjusted to be 80 μm. Next, in order to completely remove residual moisture, the resultant was dried in a vacuum at 170° C. for 3 hours to obtain a positive electrode.

(Fabrication of lithium ion secondary battery)
In a dry room controlled to a dew point of -50°C or less, the positive electrode was processed to 40mm x 40mm, and the negative electrode was processed to 44mm x 44mm, and then an aluminum tab was welded to the positive electrode and a nickel tab was welded to the negative electrode. The composite coated surfaces of the positive electrode and the negative electrode were arranged to face each other in the center, and a polyolefin microporous membrane processed to 45mm x 45mm was placed between the positive electrode and the negative electrode. Next, a sheet-like exterior cut and processed to a 70mm x 140mm 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 laminate of the positive electrode-polyolefin microporous membrane-negative electrode was sandwiched between the folded exterior. Next, using a heat sealer, two sides of the exterior including the side where the aluminum tab for the positive electrode and the nickel tab for the negative electrode were exposed were heat-sealed, and then 2 g of electrolyte (ethylene carbonate/diethyl carbonate=1/2 (volume ratio)+1M _LiPF6 solution, manufactured by Kishida Chemical Co., Ltd., hereinafter referred to as electrolyte) was poured from the other side that was not heat-sealed and allowed to fully soak into the positive electrode, negative electrode, and polyolefin microporous membrane. Then, using a vacuum heat sealer, the remaining side of the exterior was heat-sealed while reducing the pressure inside the battery, to obtain a lithium-ion secondary battery.

The battery performance of the lithium-ion secondary batteries was evaluated using the following method.

(Evaluation of lithium-ion secondary batteries)
[Discharge rate characteristics (discharge capacity maintenance rate at 3C discharge)]
The lithium ion secondary battery thus prepared was charged at a constant current and 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. Next, after recovery charging again at a constant current and constant voltage of 4.3 V and 0.2 C limit, the discharge current was set to 0.2 C and discharged to 3.0 V, and the discharge capacity at this time was measured. Subsequently, the above recovery charge conditions were maintained each time for charging, while the discharge current was changed stepwise to 0.5 C, 1 C, 2 C, and 3 C, and recovery charging and discharging were repeated, and the discharge capacity for each discharge current was measured. As an index of the discharge rate characteristics of the battery, the discharge capacity retention rate at 3 C discharge relative to 0.2 C discharge was calculated. The discharge capacity retention rate at 3 C discharge of the lithium ion secondary battery of this embodiment was 83.5%.

[Cycle characteristics (discharge capacity retention rate after cycling)]
The lithium ion secondary battery thus prepared was charged at a constant current and constant voltage of 4.3 V and limited to 1 C at 25° C., and then discharged to 3.0 V at a constant current of 1 C. The above charge and discharge were repeated 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 relative to that after 1 cycle was calculated. The discharge capacity retention rate after cycling of the lithium ion secondary battery of this example was 91%.

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

Example 2
A negative electrode, a positive electrode, and a lithium ion secondary battery were produced and evaluated in the same manner as in Example 1, except that the negative electrode dispersant in Example 1 was changed to polyvinyl alcohol B. The results are shown in Table 3.

Example 3
A negative electrode, a positive electrode, and a lithium ion secondary battery were produced and evaluated in the same manner as in Example 1, except that the negative electrode dispersant in Example 1 was changed to polyvinyl alcohol C. The results are shown in Table 3.

Example 4
A negative electrode, a positive electrode, and a lithium ion secondary battery were produced and evaluated in the same manner as in Example 1, except that the negative electrode dispersant in Example 1 was changed to polyvinyl alcohol D. The results are shown in Table 3.

Example 5
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 negative electrode conductive material in Example 1 was changed to SAB (manufactured by Denka Co., Ltd.), and each evaluation was performed. The results are shown in Table 3.

Example 6
The negative electrode conductive material 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 changed so that Li-250 was 1 mass% in solid content, polyvinyl alcohol A was 0.05 mass% in solid content (mass of dispersant/mass of conductive material = 0.05), CMC was 1 mass% in solid content, artificial graphite was 96 mass% in solid content, and SBR was 1.95 mass% in solid content. Except for this, a negative electrode, a positive electrode, and a lithium ion battery were produced in the same manner as in Example 1, 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 prepared in the same manner as in Example 1, except that the conductive material for the negative electrode in Example 1 was changed to Li-400 (manufactured by Denka Co., Ltd.), and each evaluation was performed. The results are shown in Table 3.

Example 8
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 in Example 1 was changed to ECP (manufactured by Lion Corporation), and the solid content composition of the negative electrode resin composition was changed so that ECP was 1 mass% in solid content, polyvinyl alcohol A was 0.16 mass% in solid content (dispersant mass/conductive material mass=0.16), CMC was 1 mass% in solid content, artificial graphite was 96 mass% in solid content, and SBR was 1.84 mass% in solid content, and each evaluation was performed. The results are shown in Table 3.

<Example 9>
The negative electrode conductive material 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 changed so that Li-250 was 1 mass% in solid content, polyvinyl alcohol A was 0.02 mass% in solid content (mass of dispersant/mass of conductive material = 0.02), CMC was 1 mass% in solid content, artificial graphite was 96 mass% in solid content, and SBR was 1.98 mass% in solid content. Except for this, a negative electrode, a positive electrode, and a lithium ion battery were produced in the same manner as in Example 1, and each evaluation was performed. The results are shown in Table 3.

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

<Comparative Example 2>
A negative electrode, a positive electrode, and a lithium ion battery were produced and evaluated in the same manner as in Example 1, except that the dispersant in Example 1 was changed to polyvinyl alcohol F. 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 changed so that #3030B was 1 mass% in solid content, polyvinyl alcohol A was 0.03 mass% in solid content (mass of dispersant/mass of conductive material = 0.03), CMC was 1 mass% in solid content, artificial graphite was 96 mass% in solid content, and SBR was 1.97 mass% in solid content. Except for this, a negative electrode, a positive electrode, and a lithium ion battery were produced in the same manner as in Example 1, and each evaluation was performed. The results are shown in Table 3.

<Comparative Example 4>
The conductive material of Example 1 was changed to BlackPearls 2000 (manufactured by Cabot Corporation), and the solid content composition of the negative electrode resin composition was changed to BlackPearls 2000 at 1% by mass in solid content, polyvinyl alcohol A at 0.15% by mass in solid content (dispersant mass/conductive material mass=0.15), CMC at 1% by mass in solid content, artificial graphite at 96% by mass in solid content, and SBR at 1.85% by mass in solid content. Except for this, a negative electrode, a positive electrode, and a lithium ion battery were produced in the same manner as in Example 1, and each evaluation was performed. The results are shown in Table 3.

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

<Comparative Example 6>
A negative electrode, a positive electrode, and a lithium ion battery were produced in the same manner as in Example 1, except that the dispersant of Example 1 was not added and the solid content composition of the negative electrode resin composition was changed to 1 mass% Li-435, 1 mass% CMC, 96 mass% artificial graphite, and 2 mass% SBR, and each evaluation was performed. The results are shown in Table 3.

It was found 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. This resulted in the negative electrodes of the examples of the present invention having lower plate resistance and suppressing voltage drop during discharge.

Furthermore, it was found that the lithium ion secondary batteries of Examples 1 to 9 had higher discharge rate characteristics and higher cycle characteristics than the lithium ion secondary batteries of Comparative Examples 1 to 6. This shows that the lithium ion secondary batteries using the negative electrode resin composition of the present invention are able to suppress the decrease in discharge rate characteristics that accompanies an increase in discharge current, and also have a long life.

1 Lithium ion secondary battery positive electrode 2 Lithium ion secondary battery negative electrode 3 Polyolefin microporous membrane 4 Outer packaging 5 Aluminum tab 6 Nickel tab

Claims (8)

A negative electrode resin composition containing a conductive material, a binder, and a dispersant, the dispersant containing at least polyvinyl alcohol, the conductive material containing at least carbon black, the polyvinyl alcohol having a degree of saponification of 85.5 to 96.5 mol %, an average degree of polymerization of 600 to 1700 , and an average primary particle size of the carbon black being 16 nm to 50 nm (excluding those containing both an anionic vinyl-based polymer having a carboxyl group and a polyfunctional amine as binders). 2. The negative electrode resin composition according to claim 1, wherein the binder is a styrene-butadiene copolymer. 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, the polyvinyl alcohol has a degree of saponification of 85.5 to 96.5 mol % and an average degree of polymerization of 600 to 1800, the binder is a styrene-butadiene copolymer, and the carbon black has an average primary particle size of 16 nm to 50 nm (however, the negative electrode resin composition does not include a binder containing both an anionic vinyl-based polymer having a carboxyl group and a polyfunctional amine). 4. The negative electrode resin composition according to claim 1 , wherein the polyvinyl alcohol has an average degree of polymerization of 600 to 1,300. 5. The negative electrode resin composition according to claim 1 , wherein the carbon black has a specific surface area of 35 m ² /g to 400 m ² /g. The negative electrode resin composition according to any one of claims 1 to 5 , wherein the mass ratio of the dispersant to the conductive material in terms of solid content {mass of dispersant/mass of conductive material} is 0.03 to 0.15. A negative electrode comprising the negative electrode resin composition according to any one of claims 1 to 6 . A secondary battery comprising the negative electrode according to claim 7 .

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