Polyphenylene sulfide powder for lithium ion battery binder, binder for lithium ion battery negative electrode, slurry for forming lithium ion battery negative electrode composite layer, lithium ion battery negative electrode and lithium ion battery

Abstract

To enable provision of a binder for lithium ion battery negative electrode enabling creation of a negative electrode with an excellent cycle characteristic using high capacity active material and to provide slurry for forming a lithium ion battery negative electrode mixture layer using the binder, and further to provide a lithium ion battery negative electrode and a lithium ion battery each of which has excellent charge-discharge behavior and an extended cycle life by using the binder.SOLUTION: Lithium ion battery binder polyphenylene sulfide powder having a median size of 1 to 10 μm and a particle size distribution width of 1 to 4 μm is used.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a polyphenylene sulfide powder for a lithium ion battery binder, a binder for a lithium ion battery negative electrode, a slurry for forming a lithium ion battery negative electrode composite layer, a lithium ion battery negative electrode, and a lithium ion battery. [Background technology]

[0002] Lithium-ion batteries have been widely used in recent years as power sources for electrical devices, etc. Furthermore, their use has recently expanded to include power sources for electric vehicles, and there is a demand for improved performance, such as higher capacity, higher output, and improved cycle life, as well as high safety.

[0003] The negative electrode of a lithium-ion battery has a structure in which a porous body consisting mainly of powdered negative electrode active material, a conductive additive, and a binder is layered and bound onto a current collector, and it is known that its performance is greatly affected not only by the properties of the negative electrode active material but also by the type of binder.

[0004] The negative electrode active material for lithium-ion batteries is typically a multilayer carbon material. However, with the recent development of high-capacity active materials, silicon, tin, and their alloys and oxides are being considered.

[0005] Conventionally, polyvinylidene fluoride (PVDF) and styrene butadiene rubber (SBR) have been the mainstream binders for negative electrodes. However, in electrodes using high-capacity active materials, the volume change associated with the absorption and release of lithium ions during charging and discharging is extremely large, and conventional binders are unable to suppress this volume change. This can lead to peeling at the interface between the current collector and the active material layer, or the high-capacity active material itself can become pulverized and easily fall off or peel away from the current collector, causing the electrode structure to collapse and resulting in a short charge-discharge cycle life for the battery.

[0006] One method proposed to address these problems is to use polyimide resin or polyamide-imide resin as a binder (see, for example, Patent Documents 1, 2, 3, and 4). By using a polyimide resin or polyamide-imide resin with specific mechanical properties as a binder for the negative electrode, it appears possible to absorb and mitigate the expansion and contraction of the negative electrode active material, thereby suppressing a decline in battery performance. However, there is a problem in that the active material tends to be completely covered by the binder, which can easily inhibit the formation of a stable interface (SEI) on the negative electrode surface. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] International Publication No. 2004 / 004031 [Patent Document 2] Japanese Patent Application Publication No. 11-158277 [Patent Document 3] Japanese Patent Application Laid-Open No. 2008-34352 [Patent Document 4] Japanese Patent Application Laid-Open No. 2000-200608 Summary of the Invention [Problem to be solved by the invention]

[0008] In view of the above problems, the present invention aims to provide a polyphenylene sulfide powder for a lithium ion battery binder, which enables the production of a negative electrode having excellent cycle characteristics using a high-capacity active material, a slurry for forming a lithium ion battery negative electrode composite layer, a binder for a lithium ion battery negative electrode, a lithium ion battery negative electrode, and a lithium ion battery. [Means for solving the problem]

[0009] As a result of intensive research to solve the above problems, the present inventors have found that the above object can be achieved by using polyphenylene sulfide controlled to a specific particle size as a negative electrode binder, and have thus completed the present invention.

[0010] The embodiments of the present invention are [1] to [7] shown below.

[0011] [1] Polyphenylene sulfide powder for lithium-ion battery binders, having a median diameter of 1 μm to 10 μm and a particle size distribution width of 1 to 4 μm.

[0012] [2] The polyphenylene sulfide powder for lithium ion battery binders according to [1] above, characterized in that the melt viscosity is 500 to 5,000 poise.

[0013] [3] A binder for a lithium ion battery negative electrode, containing the polyphenylene sulfide powder according to [1] or [2] above.

[0014] [4] A slurry for forming a negative electrode mixture layer for a lithium ion battery, comprising a negative electrode active material, a conductive additive, the binder according to [3] above, and water.

[0015] [5] The slurry for forming a lithium ion battery negative electrode mixture layer according to the above [4], wherein the solid content of the polyphenylene sulfide powder is 5 to 20 mass %.

[0016] [6] A negative electrode for a lithium ion battery, wherein the binder contains polyphenylene sulfide, and the component ratio of polyphenylene sulfide in the negative electrode is 5 to 20 mass %.

[0017] [7] A lithium ion battery comprising the negative electrode for a lithium ion battery according to [6] above.

[0018] The present inventors speculate as follows as to why the above object can be achieved by the binder for a lithium ion battery negative electrode of the present invention.

[0019] That is, in the present invention, by using polyphenylene sulfide powder controlled to a specific median diameter and particle size distribution width as a negative electrode binder and optimizing the polyphenylene sulfide composition ratio in the negative electrode composite layer-forming slurry, the following effects are achieved: the adhesion between the negative electrode active material or conductive additive and the current collector is improved; and the high mechanical strength of polyphenylene sulfide can suppress volumetric changes in the high-capacity active material. As a result, excellent charge-discharge characteristics and a long cycle life can be achieved.

[0020] Furthermore, by controlling the particle number of the polyphenylene sulfide powder by adjusting the content of the polyphenylene sulfide powder controlled to a specific median diameter in the slurry for forming the negative electrode composite layer and by using polyphenylene sulfide powder with an appropriate melt viscosity, the molten polyphenylene sulfide and the negative electrode active material form a funicular or pendular state, which allows the negative electrode active material to be adequately coated while ensuring a lithium ion conduction channel and enabling excellent charge / discharge characteristics to be maintained. [Effects of the Invention]

[0021] According to the present invention, by using polyphenylene sulfide powder, it is possible to provide a binder for lithium ion battery negative electrodes that enables the production of negative electrodes that use high-capacity active materials and have excellent cycle characteristics, and a slurry for forming a lithium ion battery negative electrode composite layer using the binder.Furthermore, by using the binder, it is possible to provide a lithium ion battery negative electrode and a lithium ion battery that have excellent charge / discharge characteristics and an extended cycle life. [Brief explanation of the drawings]

[0022] [Figure 1] FIG. 2 is a schematic diagram showing a mixed state of a negative electrode active material and polyphenylene sulfide in an electrode mixture layer. [Figure 2] 1 is a backscattered electron image by a scanning electron microscope of the negative electrode described in Example 4 after 50 cycles of charge and discharge. [Figure 3]10 is an energy dispersive X-ray spectrometer sulfur element mapping image of the negative electrode described in Example 4 after 50 charge-discharge cycles. DETAILED DESCRIPTION OF THE INVENTION

[0023] The present invention will be described in detail below based on preferred embodiments thereof.

[0024] The polyphenylene sulfide powder for lithium ion battery binders according to one embodiment of the present invention is a polyphenylene sulfide powder having a median diameter of 1 μm to 10 μm and a particle size distribution width of 1 to 4 μm.

[0025] Polyphenylene sulfide has a simple linear structure in which benzene rings and sulfur atoms are alternately bonded, and as a crystalline thermoplastic resin, it has high mechanical strength. In addition, its high fluidity when molten means that it has an anchoring effect that makes it excellent for bonding to metals.

[0026] The polyphenylene sulfide may have any of a linear structure, a branched structure, and a crosslinked structure.

[0027] The median diameter of the polyphenylene sulfide powder is preferably 1 to 10 μm, more preferably 4 to 7 μm. Because the polyphenylene sulfide powder acts as a resistive component, it is preferable to have a small median diameter or a low ratio within the composite layer. If the median diameter of the polyphenylene sulfide powder is less than 1 μm, the fibers formed when the polyphenylene sulfide melts become too thin to withstand the expansion of the active material, resulting in ineffective performance. Furthermore, if the median diameter is greater than 10 μm, the polyphenylene sulfide distribution within the composite layer becomes sparse, necessitating a higher ratio within the composite layer. However, this results in a lower active material ratio and therefore lower electrode capacity. After extensive investigation into a median diameter that can solve the above-mentioned problem, the median diameter of the polyphenylene sulfide powder is preferably 1 to 10 μm, more preferably 4 to 7 μm.

[0028] Here, the term "median diameter" refers to the diameter at which, when a powder is divided into two parts based on particle size, the larger particle size and the smaller particle size account for 50% each. Specifically, the term refers to the particle diameter at the point where the cumulative curve reaches 50% when the total volume of particles obtained by analyzing scattered laser light using the Microtrack method is set to 100%.

[0029] The particle size distribution width of the polyphenylene sulfide powder is preferably 1 to 4 μm to improve dispersibility within the electrode mixture layer. Here, the "particle size distribution width" refers to the value obtained by analyzing scattered laser light using the Microtrack method, calculating a cumulative curve with the total volume of particles as 100%, subtracting the particle diameter at the point where the cumulative curve is 84% ​​from the particle diameter at the point where the cumulative curve is 16% and dividing the result by 2.

[0030] Generally, the median diameter of industrially produced polyphenylene sulfide powder is several tens of μm to several hundred μm, and polyphenylene sulfide powder having the above median diameter and particle size distribution width can be obtained by pulverizing industrially produced polyphenylene sulfide powder. There are no particular limitations on the pulverization method, and examples include wet pulverization and dry pulverization.

[0031] Polyphenylene sulfide at 300℃ and a shear rate of 500 seconds -1 The melt viscosity measured by is preferably 500 to 5,000 poise, more preferably 2,000 to 4,000 poise, from the viewpoint of the strength and ease of flow of the molten polyphenylene sulfide into the gaps between the active materials.

[0032] To increase the melt viscosity, a common method such as oxygen curing can be used.

[0033] A binder for a lithium ion battery negative electrode, which is one embodiment of the present invention, contains the polyphenylene sulfide powder described above.

[0034] A lithium ion battery negative electrode mixture layer forming slurry according to one embodiment of the present invention contains a negative electrode active material, a conductive additive, the binder described above, and water.

[0035] A negative electrode mixture layer forming slurry containing polyphenylene sulfide is applied to a current collector, dried, and then the negative electrode is heated to 200 to 300° C., whereby the polyphenylene sulfide powder melts and flows.

[0036] The solid content ratio of the polyphenylene sulfide powder for lithium ion battery binders in the slurry for forming an electrode mixture layer is preferably 5 to 20 mass %, where the solid content ratio refers to the mass ratio of the corresponding component when the mass of the components other than the solvent in the slurry for forming an electrode mixture layer is taken as 100 mass %.

[0037] The solid content ratio of the polyphenylene sulfide powder for lithium ion battery binders in the slurry for forming an electrode mixture layer is preferably 5 to 20 mass % because a funicular or pendular state can be formed. The solid content ratio refers to the mass ratio of the corresponding component when the mass of the components other than the solvent in the slurry for forming an electrode mixture layer is taken as 100 mass %.

[0038] Polyphenylene sulfide is insulating and does not have lithium ion conductivity. However, by controlling the median diameter and amount of polyphenylene sulfide, a funicular or pendular state is formed between the molten polyphenylene sulfide and the negative electrode active material, allowing the electrolyte to penetrate into the electrode mixture layer, resulting in lithium ion conductivity.

[0039] Here, the pendular state refers to a state in which liquid (molten polyphenylene sulfide) is present but in small amounts, and the entire surface of the powder (negative electrode active material) is not coated with the liquid; the funicular state refers to a state in which the surface of the powder (negative electrode active material) is coated with liquid (molten polyphenylene sulfide), but gas is present in the gaps between the powder particles; and the capillary state refers to a state in which groups of powder (negative electrode active material) particles are immersed in liquid (molten polyphenylene sulfide), and a free liquid surface exists.

[0040] The slurry for forming a lithium-ion battery negative electrode composite layer according to one embodiment of the present invention contains the binder, a negative electrode active material, a conductive additive, and water. The slurry may contain a viscosity modifier such as carboxymethyl cellulose or a pH modifier such as an acid or alkali, as needed. The solids concentration of the slurry is not particularly limited, but is preferably 20 to 80 mass % in consideration of the viscosity of the slurry, the dispersibility of the solids, the load on the drying process, and other factors. Furthermore, the solids ratio in the slurry is preferably negative electrode active material:conductive additive:binder=70 to 94:1 to 10:5 to 20 by mass. The method for producing the slurry is also not particularly limited, and examples include a method in which the binder, negative electrode active material, and conductive additive are mixed and dispersed in water all at once to produce a slurry; a method in which the binder is first dispersed in water, and then the negative electrode active material and conductive additive are added to the binder aqueous solution and mixed to produce a slurry; and a method in which the negative electrode active material and conductive additive are first mixed and then mixed with the binder dispersion. There are also no particular restrictions on the mixer used to prepare the slurry, and a mortar, roll mill, ball mill, screw mill, vibration mill, homogenizer, planetary mixer, etc. may be used.

[0041] The negative electrode active material that can be blended into the electrode mixture layer-forming slurry is not particularly limited, but examples include natural graphite, artificial graphite, graphite, mesocarbon microbeads (MCMB), tin and / or tin alloys, tin oxide, silicon and / or silicon alloys, and silicon oxide. When the negative electrode active material is an alloy, the negative electrode active material may contain a material that alloys with lithium. Examples of materials that alloy with lithium include germanium, tin, lead, zinc, magnesium, sodium, aluminum, gallium, indium, and alloys thereof. However, to increase the battery capacity of the negative electrode, the negative electrode active material is preferably silicon and / or silicon alloys or silicon oxide, and particularly preferably silicon. Furthermore, multiple negative electrode active materials may be mixed.

[0042] When the negative electrode active material is in a particulate form, the average particle size is not particularly limited, but is preferably 5 μm or more and 20 μm or less from the viewpoint of particle dispersibility within the negative electrode mixture layer forming slurry.

[0043] The conductive additive contained in the negative electrode is not particularly limited, and any electron-conductive material that does not adversely affect battery characteristics can be used. Specific examples include conductive carbons such as ketjen black and acetylene black, carbon materials such as natural graphite, artificial graphite, carbon whiskers, carbon nanotubes, and carbon fiber powder, metal powders or fibers such as Cu, Fe, Ag, Ni, Pd, Au, Pt, In, and W, and conductive metal oxides such as indium oxide and tin oxide. The amount of the conductive additive to be added is preferably 1 to 30% by mass relative to the negative electrode active material.

[0044] The negative electrode for a lithium ion battery according to one embodiment of the present invention contains the binder. Such a negative electrode for a lithium ion battery preferably comprises a negative electrode current collector and a negative electrode composite layer obtained by applying the slurry onto the negative electrode current collector and drying it. The thickness of the negative electrode composite layer, which is made of the negative electrode active material, binder, and conductive additive, is preferably 10 to 200 μm.

[0045] The component ratio of polyphenylene sulfide contained in the negative electrode for a lithium ion battery is preferably 5 to 20 mass % in the negative electrode, because it can form a funicular or pendular state, can withstand the expansion of the active material, and can be designed to have a high active material ratio in the electrode. The component ratio in the negative electrode refers to the mass ratio of the corresponding component when the component mass of the negative electrode mixture layer is taken as 100 mass %.

[0046] The negative electrode current collector preferably has a surface roughness Ra of 0.1 μm or more. When a negative electrode layer is formed on a negative electrode current collector having a surface roughness Ra of 0.1 μm or more, the binder has a significant anchoring effect in the negative electrode layer, significantly improving the adhesion between the negative electrode current collector and the negative electrode layer.

[0047] The material of the negative electrode current collector may be any conductor as long as the surface in contact with the negative electrode mixture layer exhibits electrical conductivity, and examples thereof include metals such as copper, nickel, iron, titanium, and cobalt, and alloys thereof. In particular, a metal foil containing copper is preferred, and copper foil or a copper alloy foil is more preferred. Furthermore, the metal foil containing copper may be a metal foil made of a metal element other than copper, on the surface of which a layer containing copper is formed.

[0048] The thickness of the negative electrode current collector is not particularly limited, but is usually in the range of 10 μm to 100 μm.

[0049] There are no particular limitations on the method for producing a negative electrode for a lithium ion battery, and the electrode can be produced by applying the above-mentioned slurry to a negative electrode current collector and drying it. There are also no particular limitations on the method for applying the slurry, and methods such as slit coating, die coating, roll coating, dip coating, blade coating, knife coating, and wire bar coating can be used. There are also no particular limitations on the drying method and conditions, and a typical hot air circulation dryer, reduced pressure dryer, infrared dryer, or microwave heating dryer can be used. There are also no particular limitations on the heating temperature, and the electrode can be heated and dried at 50 to 150°C.

[0050] Furthermore, by heating the negative electrode to 200 to 300°C after drying, the polyphenylene sulfide melts and flows, and the anchor effect strengthens the bond between the mixture layer and the current collector. In addition, a funicular or pendular state is formed between the negative electrode active material.

[0051] By applying pressure and pressing, the porous structure can be made uniform.

[0052] A lithium-ion battery according to one embodiment of the present invention includes the above-described lithium-ion battery negative electrode. Using the above-described lithium-ion battery negative electrode allows for the provision of a high-performance lithium-ion battery with excellent charge / discharge characteristics and a long cycle life. A lithium-ion battery generally comprises a positive electrode, a negative electrode, a separator, a nonaqueous electrolyte, and other components. The negative electrode is constructed by binding the above-described negative electrode active material together with the above-described conductive additive to a negative electrode current collector using a binder, with a negative electrode composite layer comprising the negative electrode active material, binder, and conductive additive formed on the current collector. The positive electrode has a similar structure to the negative electrode, consisting of the following positive electrode active material and the above-described conductive additive bound to the positive electrode current collector using a binder. The separator is typically a porous film such as polyolefin, sandwiched between the positive and negative electrodes to provide a shutdown function in the event of thermal runaway. The nonaqueous electrolyte is constructed by dissolving an electrolyte salt such as LiPF6 in an organic solvent such as cyclic carbonate. The inside of the battery is filled with a non-aqueous electrolyte, and lithium ions move from the positive electrode to the negative electrode during charging, and from the negative electrode to the positive electrode during discharging.

[0053] The positive electrode used in the lithium-ion battery is not particularly limited and can be made of known materials. The positive electrode is composed of a positive electrode mixture layer made of a positive electrode active material, a conductive additive, and a binder, and a current collector.

[0054] Any material that can insert and extract lithium ions can be used as the positive electrode active material. For example, transition metal oxides such as CuO, Cu2O, MnO2, MoO3, V2O5, CrO3, Fe2O3, Ni2O3, and CoO3, and Li X CoO2, Li X NiO2, Li X MnO2, Li X Mn2O4, LiNi X Co (1-X) O2, LiNi X Mn (2-X) O4, LiMn a Ni b Co cAmong these, a composite oxide of lithium and at least one transition metal selected from transition metals such as Co, Ni, and Mn is preferred, and specific examples include LiCoO2, LiNiO2, LiMnO2, LiMn2O4, and LiNi X Co (1-X) O2, LiNi X Mn (2-X) O4, LiMn a Ni b Co c O2 (a+b+c=1). These lithium composite oxides may be doped with a small amount of an element such as fluorine, boron, Al, Cr, Zr, Mo, or Fe, or the particle surfaces of the lithium composite oxide may be surface-treated with carbon, MgO, Al2O3, SiO2, or the like.

[0055] As the conductive additive for the positive electrode, the same conductive additives as those exemplified for the negative electrode can be used.

[0056] As the binder for the positive electrode, a known binder may be used, and examples thereof include fluorine-based resins such as polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, perfluoromethyl vinyl ether-tetrafluoroethylene copolymer, polytetrafluoroethylene, and fluororubber; hydrocarbon elastomers such as styrene-butadiene copolymer and ethylene-propylene copolymer; polysaccharides such as carboxymethyl cellulose, alginic acid, and sodium alginate; and polyimides.

[0057] The positive electrode current collector may be any conductor as long as the surface in contact with the positive electrode composite layer exhibits electrical conductivity, and examples thereof include conductors formed from metals such as copper, gold, aluminum, titanium, nickel, stainless steel, or alloys thereof, conductive metal oxides such as indium oxide or tin oxide, and conductive materials such as conductive carbon. There are no particular restrictions on the shape of the positive electrode current collector, and shapes such as foil, film, sheet, net, expanded metal, punched metal, and foam can be used. There are also no particular restrictions on the thickness of the positive electrode current collector, and it is preferably about 1 to 100 μm.

[0058] There are no particular restrictions on the non-aqueous electrolyte, and known materials can be used. The non-aqueous electrolyte is obtained by dissolving an electrolyte salt in an organic solvent. Examples of the electrolyte salt include CF3SO3Li, (CF3SO2)2NLi, (CF3SO2)2CLi, LiBF4, LiB(C6H8)4, LiPF4, LiClO4, LiAsF6, LiCl, and LiBr. Examples of organic solvents that dissolve the electrolyte salt include ethylene carbonate, vinylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, 1,2-dimethoxyethane, 1,2-diethoxyethane, γ-butyrolactone, tetrahydrofuran, 1,4-dioxane, anisole, diethyl ether, sulfolane, methyl sulfolane, acetonitrile, propionitrile, butyronitrile, valeronitrile, benzonitrile, dimethylformamide, dimethyl sulfoxide, trimethyl phosphate, triethyl phosphate, etc. The concentration of the electrolyte salt in the nonaqueous electrolyte can be selected from the range of 0.1 to 5 mol / L, preferably 0.5 to 3 mol / L.

[0059] There are no particular restrictions on the separator, and any known separator can be used. Examples of the separator include a polyethylene microporous membrane, a polypropylene microporous membrane, a laminated membrane of a polyethylene microporous membrane and a polypropylene microporous membrane, and a nonwoven fabric made of polyester fiber, aramid fiber, glass fiber, or the like. [Explanation of symbols]

[0060] 1 Pendular State 2 Funicular state 3 Capillary state 4 Immersion state 5 Negative electrode active material 6. Fused polyphenylene sulfide [Example]

[0061] The present invention will be described in more detail below based on examples, but the present invention is not limited to these examples. <Production of polyphenylene sulfide powder> (Reference example 1) Polyphenylene sulfide (hereinafter referred to as PPS) (A) with a median diameter of 30 μm and a melt viscosity of 550 poise was dry-ground, and the resulting powder was placed in an oven with an air flow rate of 15 L / min and heated at 245°C for 1 hour to obtain PPS powder (A-1). The PPS powder (A-1) had a median diameter of 4.5 μm, a particle size distribution width of 1.3 μm, and a melt viscosity of 2,070 poise.

[0062] (Reference example 2) PPS (A) with a median diameter of 30 μm and a melt viscosity of 550 poise was dry-ground, and the resulting powder was placed in an oven with an air flow rate of 15 L / min and heated at 245°C for 1 hour to obtain PPS powder (A-2). PPS powder (A-2) had a median diameter of 5.7 μm, a particle size distribution width of 2.9 μm, and a melt viscosity of 2,040 poise.

[0063] (Reference example 3) PPS (A) with a median diameter of 30 μm and a melt viscosity of 550 poise was dry-ground, and the resulting powder was placed in an oven with an air flow rate of 15 L / min and heated at 245°C for 1 hour to obtain PPS powder (A-3). The median diameter of PPS powder (A-3) was 11.3 μm, the particle size distribution width was 4.6 μm, and the melt viscosity was 2,130 poise.

[0064] (Reference example 4) PPS (B) with a median diameter of 30 μm and a melt viscosity of 230 poise was dry-pulverized to obtain PPS powder (B-1) with a median diameter of 4.4 μm, a particle size distribution width of 1.2 μm, and a melt viscosity of 230 poise. <Coin cell manufacturing> The coin cell was designed and fabricated using 20 parts by mass or more of SiO to achieve a high capacity of 930 mAh / cc or more.

[0065] Example 1 First, a negative electrode mixture was prepared from the following materials to fabricate a negative electrode. Negative electrode active material: 60.4 parts by mass of graphite (Gr), 22.6 parts by mass of silicon monoxide (SiO) Conductive additive: 1 part by mass of acetylene black (AB) Thickener: 1 part by weight of carboxymethyl cellulose (CMC) Binder: 15 parts by weight of polyphenylene sulfide (PPS).

[0066] Gr, SiO, AB, and PPS powder (A-1) were mixed and then mixed with a CMC aqueous solution to prepare a slurry for forming a negative electrode composite layer. The slurry was then applied to a copper foil using a bar coater and dried at 80°C for 10 minutes. After heat treatment at 300°C for 10 minutes, the mixture was pressed at 100°C under a load of 10 tons to prepare a negative electrode.

[0067] Next, a coin cell was fabricated using the obtained negative electrode as follows. That is, a lithium metal foil was used as the counter electrode, and lithium hexafluorophosphate was dissolved in a mixed solvent of ethylene carbonate / dimethyl carbonate (1 / 1) to a concentration of 1 mol / L to prepare an electrolyte. A polyolefin microporous film was used as the separator. The negative electrode and counter electrode were then placed on either side of the separator, stacked, and the electrolyte was poured into the cells to fabricate a 2032-type coin cell.

[0068] Example 2 A coin cell was produced in the same manner as in Example 1, except that Gr was changed to 66.5 parts by mass, SiO was changed to 21.5 parts by mass, and PPS powder (A-1) was changed to 10 parts by mass.

[0069] Example 3 A coin cell was produced in the same manner as in Example 1, except that Gr was 61.9 parts by mass, SiO was 20.0 parts by mass, and PPS powder (A-2) was used as the PPS powder, and the PPS powder (A-2) was changed to 16.3 parts by mass.

[0070] (Comparative Example 1) A coin cell was produced in the same manner as in Example 1, except that Gr was 42.2 parts by mass, SiO was 25.8 parts by mass, and PPS powder (A-3) was used as the PPS powder, and the amount of PPS powder (A-3) was changed to 30.0 parts by mass.

[0071] (Comparative Example 2) A coin cell was produced in the same manner as in Example 1, except that Gr was changed to 42.2 parts by mass, SiO was changed to 25.8 parts by mass, and PPS powder (A-1) was changed to 30.0 parts by mass.

[0072] (Comparative Example 3) A coin cell was produced in the same manner as in Example 1, except that the PPS powder was changed to PPS powder (B-1).

[0073] <Coin cell charge / discharge characteristic evaluation> The coin cells obtained in Examples 1 to 3 and Comparative Examples 1 to 3 were used to evaluate the charge / discharge characteristics under the following conditions. Charging: Constant current, constant voltage (CCCV mode) Discharge: Constant current (CC mode) Potential range: 0.05 to 2.0 V C rate: 0.2C

[0074] [Table 1]

[0075] <Laminated cell manufacturing> Example 4 First, a negative electrode mixture was prepared from the following materials to fabricate a negative electrode. Negative electrode active material: 60.4 parts by mass of graphite (Gr), 22.6 parts by mass of silicon monoxide (SiO) Conductive additive: 1 part by mass of acetylene black (AB) Thickener: 1 part by weight of carboxymethyl cellulose (CMC) Binder: 15 parts by weight of polyphenylene sulfide (PPS).

[0076] Gr, SiO, AB, and PPS powder (A-1) were mixed and then mixed with a CMC aqueous solution to prepare a negative electrode composite slurry. The slurry was then applied to copper foil using a bar coater and dried at 80°C for 10 minutes. After heat treatment at 300°C for 10 minutes, the mixture was pressed at 100°C under a load of 10 tons to prepare a negative electrode.

[0077] The positive electrode was fabricated by dry-mixing lithium nickel oxide (LiNiO) and acetylene black (AB) as the positive electrode active material, and then uniformly dispersing the mixture in N-methyl-2-pyrrolidone (NMP) containing polyvinylidene fluoride (PVDF) as a binder to prepare a slurry. This slurry was then applied to a 20 μm-thick aluminum foil, and the NMP was evaporated to form a positive electrode active material layer. The solids ratio (mass ratio) in the positive electrode active material layer was LiNiO:AB:PVDF = 94:3:3.

[0078] Next, a laminate cell was fabricated using the obtained negative electrode and positive electrode as follows. That is, lithium hexafluorophosphate was dissolved in a mixed solvent of ethylene carbonate / dimethyl carbonate (1 / 1) to a concentration of 1 mol / L to prepare an electrolyte solution. A polyolefin microporous membrane was used as the separator. The negative electrode and positive electrode were then placed on both sides of the separator, stacked, and the electrolyte solution was poured into the laminate cell to fabricate the laminate cell.

[0079] Example 5 A laminate cell was produced in the same manner as in Example 4, except that Gr was changed to 66.5 parts by mass, SiO was changed to 21.5 parts by mass, and PPS powder (A-1) was changed to 10 parts by mass.

[0080] <Charge-discharge characteristic evaluation of laminated cell> The laminate cells obtained in Examples 4 and 5 were used to evaluate the charge / discharge characteristics under the following conditions. Charging: Constant current, constant voltage (CCCV mode) Discharge: Constant current (CC mode) Potential range: 3.0 to 4.2 V C rate: Charge 0.2C, Discharge 0.4C

[0081] [Table 2]

[0082] From the backscattered electron image taken by a scanning electron microscope and the sulfur element mapping image taken by an energy dispersive X-ray spectrometer of the negative electrode described in Example 4 after 50 charge-discharge cycles, it was confirmed that the polyphenylene sulfide powder had melted and flowed and spread throughout the inside of the electrode mixture layer, forming a funicular state.

[0083] The negative electrode active material and polyphenylene sulfide form a funicular state, which allows the capacity retention rate to be maintained at a high level. [Industrial Applicability]

[0084] According to the present invention, it is possible to provide a binder for a lithium ion battery negative electrode that enables the production of a negative electrode using a high-capacity active material with excellent cycle characteristics, and a slurry for forming a lithium ion battery negative electrode composite layer using the binder. Furthermore, by using the binder, it is possible to provide a lithium ion battery negative electrode and a lithium ion battery that have excellent charge / discharge characteristics and an extended cycle life.

Claims

1. The polyphenylene sulfide powder has a median diameter of 1 to 10 μm, a particle size distribution width of 1 to 4 μm, and a melt viscosity of 500 to 5,000 poise; The slurry for forming a lithium ion battery negative electrode mixture layer has a solids content of the polyphenylene sulfide powder of 5 to 20 mass %.

2. A slurry for forming a lithium ion battery negative electrode composite layer as described in claim 1, characterized in that the median diameter of the polyphenylene sulfide powder is 4 to 7 μm.

3. A slurry for forming a lithium ion battery negative electrode composite layer as described in claim 1 or 2, further comprising a negative electrode active material, a conductive additive, and water.

4. A slurry for forming a lithium ion battery negative electrode composite layer as described in claim 3, wherein the negative electrode active material includes at least one selected from silicon, silicon alloys, and silicon oxides.

5. A slurry for forming a lithium ion battery negative electrode composite layer as described in claim 3, wherein the average particle diameter of the negative electrode active material is 5 μm or more and 20 μm or less.

6. A negative electrode active material comprising at least one selected from silicon, a silicon alloy, and a silicon oxide; a binder; The binder contains polyphenylene sulfide having a melt viscosity of 500 to 5,000 poise, and the proportion of polyphenylene sulfide in the negative electrode is 5 to 20 mass %.

7. A lithium ion battery comprising the negative electrode for a lithium ion battery according to claim 6.

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