Binder composition for lithium-ion secondary battery negative electrode, negative electrode for lithium-ion secondary battery, and lithium-ion secondary battery

Abstract

[Problem] To provide a binder composition for negative electrodes, the binder composition enabling a lithium ion secondary battery to achieve a good balance between capacity enhancement and good capacity retention rate. [Solution] A binder composition for negative electrodes of lithium ion secondary batteries according to the present invention is characterized by containing a cellulose-based water-soluble polymer and a vinyl phosphorus-based polymer that is derived from a vinylphosphonic acid or a vinylphosphonic acid ester.

Description

Technical Field

[0001] The present invention relates to a binder composition for a negative electrode of a lithium-ion secondary battery. The present invention also relates to a negative electrode for a lithium-ion secondary battery using the binder composition. Furthermore, the present invention relates to a lithium-ion secondary battery including the negative electrode.

Background Art

[0002] Lithium-ion secondary batteries are expected to develop greatly because of their high voltage and high capacity. Recently, in particular, improvements have been made to various materials used in batteries, such as positive electrode active materials, negative electrode active materials, non-aqueous electrolytes, and binders used in positive and negative electrodes, which are involved in battery reactions.

[0003] In particular, for the field of environmental vehicles, the number of vehicles equipped with lithium-ion secondary batteries for driving is increasing, and the demand is booming. In order to increase the capacity of lithium-ion secondary batteries, an improvement in energy density is required. Although research on various material systems is progressing for both the positive and negative electrodes, there are also resource problems such as Co on the positive electrode side, and improvement on the negative electrode side is desired. Currently, graphite is widely used as a negative electrode active material for lithium-ion secondary batteries, but as a negative electrode active material for the purpose of increasing capacity, materials of the Si system (such as Si and SiO) are attracting attention from the viewpoints of weight energy density and volume energy density.

[0004] However, while Si-based negative electrodes offer the potential for higher capacity, a significant problem lies in their large volume changes during charging and discharging. Graphite expands by approximately 110% during full charge, while pure Si expands by approximately 400%. During charging and discharging in lithium-ion secondary batteries, a non-conductive solid electrolyte interface (Solid Electrolyte Interphase) is formed on the active material surface by reaction products with the electrolyte. When the volume change during charging and discharging is large, the hard and brittle oxide-based solid electrolyte interface cannot keep up with the volume change, leading to repeated desorption and reprecipitation. To address this problem, a lithium-ion secondary battery negative electrode incorporating polyacrylic acid as a binder has been proposed (see Patent Document 1). Furthermore, a lithium-ion secondary battery negative electrode incorporating polyvinylidene fluoride as a binder has also been proposed (see Patent Document 2). [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] International Publication No. 2015 / 159935 [Patent Document 2] Patent No. 6198687 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] However, even when using the negative electrode binders described in Patent Documents 1 and 2, it was not possible to achieve both high capacity and good capacity retention in lithium-ion secondary batteries. [Means for solving the problem]

[0007] The inventors of the present invention conducted diligent research to solve the above problems and found that the above problems can be solved by using a combination of a vinylphosphorus polymer and a cellulose-based water-soluble polymer as a binder composition for the negative electrode, thereby completing the present invention.

[0008] In other words, the present invention provides the following invention. [1] A binder composition for the negative electrode of a lithium-ion secondary battery, characterized by containing a vinylphosphorus polymer derived from vinylphosphonic acid or vinylphosphonic acid ester and a cellulose-based water-soluble polymer. [2] The vinylphosphorus polymer is defined by the following general formula (1): [ka] (In general formula (1), R1 and R2 each independently represent a hydrogen atom or a substituted or unsubstituted C1-C6 alkyl group. However, if both R1 and R2 are alkyl groups, they may form a ring together with the oxygen and phosphorus atoms in formula (1).) A binder composition for the negative electrode of a lithium-ion secondary battery, as described in [1], represented by [1]. [3] The binder composition for lithium-ion secondary battery negative electrode according to [1] or [2], wherein the vinylphosphorus polymer is polyvinylphosphonic acid or dimethyl polyvinylphosphonate. [4] The binder composition for the negative electrode of a lithium-ion secondary battery according to any one of [1] to [3], wherein the vinylphosphorus polymer is polyvinylphosphonic acid. [5] The binder composition for the negative electrode of a lithium-ion secondary battery according to any one of [1] to [4], wherein the cellulose-based water-soluble polymer is carboxymethylcellulose. [6] A negative electrode for a lithium-ion secondary battery comprising a negative electrode active material, a binder, and a conductive additive, The negative electrode active material is composed of a carbon material, a silicon compound, or a mixture of a carbon material and a silicon compound. The binder is composed of a lithium-ion secondary battery negative electrode binder composition according to any one of [1] to [5], The negative electrode for a lithium-ion secondary battery is wherein the conductive additive is carbonaceous fine particles. [7] The negative electrode for a lithium-ion secondary battery according to [6], wherein the conductive additive is acetylene black. [8] A lithium-ion secondary battery comprising a positive electrode and a negative electrode as described in [6] or [7]. [Effects of the Invention]

[0009] According to the present invention, it is possible to provide a binder composition for a negative electrode that can achieve both high capacity and good capacity retention rate (charge / discharge cycle characteristics) in a lithium-ion secondary battery. Furthermore, it is possible to provide a negative electrode for a lithium-ion secondary battery using the negative electrode binder composition. Moreover, it is possible to provide a lithium-ion secondary battery equipped with the negative electrode for a lithium-ion secondary battery. [Brief explanation of the drawing]

[0010] [Figure 1] This is a schematic diagram of the half-cell used in the charge-discharge test of the example. [Modes for carrying out the invention]

[0011] [Negative electrode binder composition for lithium-ion secondary batteries] The present invention relates to a negative electrode binder composition for lithium-ion secondary batteries, characterized by containing a vinylphosphorus polymer and a cellulose-based water-soluble polymer. By using the vinylphosphorus polymer and the cellulose-based water-soluble polymer in combination, it is possible to achieve both high capacity and good capacity retention (charge-discharge cycle characteristics) when used as a negative electrode for lithium-ion secondary batteries. The components constituting the binder composition will be described in detail below.

[0012] (Vinylphosphorus polymer) In this invention, the vinylphosphorus polymer is a polymer derived from vinylphosphonic acid or vinylphosphonic acid ester. As such a vinylphosphorus polymer, a polymer represented by the following general formula (1) can be used. [ka] In the general formula (1), in (1), R1 and R2 each independently represent a hydrogen atom, or a substituted or unsubstituted alkyl group having 1 to 6 carbon atoms. However, when both R1 and R2 are alkyl groups, they may form a ring together with the oxygen atom and the phosphorus atom in the formula (1). Examples of the alkyl group include a methyl group, an ethyl group, a propyl group, and a butyl group.

[0013] Specifically, examples of the polymer represented by the general formula (1) include polyvinylphosphonic acid and polyvinylphosphonic acid ester. Examples of the polyvinylphosphonic acid ester include dialkyl polyvinylphosphonates such as dimethyl polyvinylphosphonate. Among these, it is preferable to use polyvinylphosphonic acid. These vinyl phosphorus-based polymers may be used alone or in combination of two or more.

[0014] The weight average molecular weight (Mw) of the vinyl phosphorus-based polymer is not particularly limited, but is, for example, in the range of 3,000 to 300,000, preferably 5,000 to 200,000, and particularly preferably 7,500 to 100,000. The weight average molecular weight (Mw) can be measured by GPC (gel permeation chromatography).

[0015] (Method for producing vinyl phosphorus-based polymer) The method for producing the vinyl phosphorus-based polymer is not particularly limited and can be produced by a conventionally known method. An embodiment of the method for producing the vinyl phosphorus-based polymer will be described.

[0016] First, an embodiment of the method for producing a polymer derived from vinyl phosphonic acid ester (polyvinyl phosphonic acid ester) will be described. For example, as a method for producing a dialkyl polyvinylphosphonate, a dialkyl polyvinylphosphonate can be produced by anionic polymerization from a monomer component containing a dialkyl vinylphosphonate as a main component in the presence of an anionic polymerization initiator. The polymerization solvent is not particularly limited, but an aliphatic ether can be used.

[0017] Examples of anionic polymerization initiators include organolithium compounds such as methyllithium, ethyllithium, n-butyllithium, sec-butyllithium, t-butyllithium, and hexyllithium; and basic organometallic compounds such as organomagnesium compounds (Grignard reagents) such as MeMgBr, EtMgBr, t-BuMgBr, t-BuMgCl, and PhMgBr.

[0018] Examples of aliphatic ethers used as polymerization solvents in anionic polymerization include diethyl ether, dipropyl ether, methyl-tert-butyl ether (MTBE), ethyl-tert-butyl ether (ETBE), dibutyl ether, diisoamyl ether, hexyl methyl ether, octyl methyl ether, cyclopentyl methyl ether (CPME), and dicyclopentyl ether, which have approximately 2 to 10 carbon atoms.

[0019] The form of anionic polymerization in the present invention is not particularly limited, but a dropwise polymerization method is preferred, in which a monomer component solution containing dimethyl vinylphosphonate dissolved in the polymerization solvent is maintained at a predetermined temperature and a polymerization initiator is added dropwise thereto. Furthermore, it is preferable to carry out the polymerization under high vacuum or under an inert gas atmosphere such as nitrogen, argon, or helium.

[0020] The polymerization conditions in anionic polymerization are not particularly limited, but the polymerization temperature is usually -80 to 100°C, preferably -20 to 60°C, and more preferably 0 to 50°C. The polymerization time is usually 0.5 to 24 hours, preferably 1 to 12 hours, and more preferably 1.5 to 6 hours.

[0021] In anionic polymerization, the polymerization reaction can be stopped by adding a polymerization inhibitor to the reaction mixture once a polymer of the desired molecular weight has been formed. Examples of polymerization inhibitors that can be used include protic compounds such as water, methanol, isopropanol, acetic acid, and methanol solutions of hydrochloric acid. After stopping the polymerization reaction, the desired polyvinylphosphonic acid diester can be separated and obtained from the reaction mixture.

[0022] Furthermore, polyvinylphosphonic acid can be obtained by hydrolyzing the polyvinylphosphonic acid diester obtained by the above method in the presence of an acid. As the polyvinylphosphonic acid diester, the polymer recovered as a solid from the polymerization solution may be used, or the polymerization solution may be brought into contact with water to extract the polyvinylphosphonic acid diester into the aqueous phase, and the resulting aqueous polymer solution may be used.

[0023] The acid used for hydrolysis can be any acid commonly used in the hydrolysis of phosphate esters. Inorganic acids, organic acids, and solid acids can all be used, but in terms of reactivity, inorganic acids such as sulfuric acid, hydrochloric acid, and phosphoric acid are preferred.

[0024] Hydrolysis is usually carried out in a solvent consisting of water, a hydrophilic solvent, or a mixture thereof. Examples of hydrophilic solvents include polyhydric alcohols such as ethylene glycol, propylene glycol, and glycerin; and glycol ether solvents such as cellosolve, methyl cellosolve, ethyl cellosolve, carbitol, methyl carbitol, ethyl carbitol, butyl carbitol, propylene glycol monomethyl ether, propylene glycol monoethyl ether, dipropylene glycol monomethyl ether, dipropylene glycol monoethyl ether, triethylene glycol monomethyl ether, and triethylene glycol monoethyl ether.

[0025] The reaction temperature in hydrolysis is not particularly limited, but is usually selected from the range of 80-100°C, preferably 90-100°C, and more preferably 95-100°C, which is typical for water. The reaction time can be appropriately selected in conjunction with the reaction temperature, and is usually in the range of 2-24 hours, preferably 4-16 hours, and more preferably 6-8 hours.

[0026] Next, one embodiment of a method for producing a polymer derived from vinylphosphonic acid (polyvinylphosphonic acid) will be described. For example, as a method for producing polyvinylphosphonic acid, polyvinylphosphonic acid can be produced by radical polymerization from monomer components mainly containing vinylphosphonic acid in the presence of a radical polymerization initiator. In the case of radical polymerization, the polymerization solvent is not particularly limited, but water or a mixed solvent of water and an aqueous medium can be used.

[0027] While there are no particular limitations on the radical polymerization initiator used in the radical polymerization of polyvinylphosphonic acid, a water-soluble polymerization initiator that generates radicals upon heating is preferred. Examples of radical polymerization initiators include azo polymerization initiators such as 2,2'-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride, 2,2'-azobis(2-methylpropionamidine)dihydrochloride, 2,2'-azobis[N-(2-carboxyethyl)-2-methylpropionamidine]tetrahydrate, 2,2'-azobis[2-(2-imidazolin-2-yl)propane], 2,2'-azobis[2-methyl-N-(2-hydroxyethyl)propionamide], and 4,4'-azobis(4-cyanovaleric acid); and organic peroxides such as cumene hydroperoxide, di-t-butyl peroxide, t-butyl hydroperoxide, and t-butyl peroxyacetate. Radical polymerization initiators may be used individually or in combination of two or more.

[0028] Examples of aqueous media used as polymerization solvents in radical polymerization include water; monohydric alcohol solvents such as methanol, ethanol, and isopropanol; polyhydric alcohol solvents such as ethylene glycol; and amide solvents such as N,N-dimethylformamide. One of these may be used alone, or two or more may be used in combination. In the case of mixed solvents, it is preferable that water make up 50% (v / v) or more of the total volume of the aqueous media. Among these aqueous media, water, a mixed solvent of water and one or more selected from monohydric alcohol solvents, polyhydric alcohol solvents, and amide solvents is preferred, with water being more preferred.

[0029] The form of radical polymerization in the present invention is not particularly limited, but a dropwise polymerization method is preferred, in which a monomer component solution containing vinylphosphonic acid dissolved in the polymerization solvent is maintained at a predetermined temperature and a polymerization initiator is added dropwise thereto. Furthermore, it is preferable to carry out the polymerization under high vacuum or under an inert gas atmosphere such as nitrogen, argon, or helium.

[0030] The polymerization conditions in radical polymerization are not particularly limited, but the polymerization temperature is usually 40 to 100°C, preferably 50 to 90°C, and more preferably 60 to 80°C. The polymerization time is usually 1 to 24 hours, preferably 2 to 18 hours, and more preferably 4 to 12 hours.

[0031] (Cellulose-based water-soluble polymer) Examples of cellulose-based water-soluble polymers include carboxymethylcellulose (CMC), methylcellulose (MC), ethylcellulose (EC), hydroxyethylcellulose (HEC), and hydroxypropylcellulose (HPC). Among these, carboxymethylcellulose is preferred. These cellulose-based water-soluble polymers may be used individually or in combination of two or more.

[0032] The content of vinylphosphorus polymer in the binder composition is preferably 50% to 95% by mass, more preferably 60% to 90% by mass, and even more preferably 70% to 85% by mass, relative to the total amount of vinylphosphorus polymer and cellulose-based water-soluble polymer. If the content of vinylphosphorus polymer is within the above numerical range, the capacity and capacity retention rate when used as a negative electrode for lithium-ion secondary batteries can be further improved.

[0033] [Negative electrode for lithium-ion secondary batteries] The negative electrode for lithium-ion secondary batteries of the present invention comprises a negative electrode active material, a binder composition, and a conductive additive. Each component is described in detail below.

[0034] (Binder composition) The above-mentioned binder composition for lithium-ion secondary battery negative electrodes can be used as the binder composition. Details are as described above. The content of the binder composition in the negative electrode is preferably 10% to 50% by mass, more preferably 15% to 45% by mass, and even more preferably 20% to 40% by mass, based on the total amount of the negative electrode active material, binder composition, and conductive additive. If the content of the binder composition in the negative electrode is within the above numerical range, the capacity and capacity retention rate when used as a negative electrode for lithium-ion secondary batteries can be further improved.

[0035] (Negative electrode active material) The negative electrode active material is composed of a carbon material, a silicon compound, or a mixture of a carbon material and a silicon compound. Preferably, the silicon compound is one represented by SiOx. In SiOx, x is preferably a number between 0.5 and 1.5, and more preferably 1. Since the compound represented by SiOx exhibits less expansion and contraction than other silicon-based materials, the content of the carbon material added to the negative electrode active material to mitigate the expansion and contraction of the silicon compound can be reduced. This allows for a higher capacity of the lithium-ion secondary battery. Alternatively, the silicon compound may be a particle of the above compound represented by the general formula SiOx coated with carbon such as nanocarbon.

[0036] The silicon compound is preferably in powder form, more preferably in particulate form, for example, with an average particle diameter of 30 μm or less, more preferably 20 μm or less, even more preferably 10 μm or less, and even more preferably 5 μm or less. Using such a fine powder silicon compound suppresses pulverization due to expansion and contraction of the silicon compound. The lower limit of the range of average particle diameter of the silicon compound is not particularly limited, but is 0.5 μm. The average particle diameter refers to the particle size (D50) at which the volume integration is 50% in the particle size distribution of the silicon compound determined by laser diffraction scattering. Furthermore, the average particle diameter of the silicon compound can be adjusted to a desired value by grinding it using known methods such as a ball mill.

[0037] As the carbon material, graphite is preferred. Graphite is one of the allotropes of carbon and is a thermodynamically stable phase under normal pressure. Examples of graphite include natural graphite and artificial graphite. Natural graphite is graphite that occurs naturally. Examples of natural graphite include flaky graphite, massive graphite, and earthy graphite. On the other hand, artificial graphite is a material in which a graphite structure is developed by further heat-treating a carbon material produced by the thermal decomposition and carbonization of organic compounds to a high temperature of 2500°C or higher.

[0038] The average particle size of graphite is preferably 1 to 30 μm. When the average particle size of graphite is 1 to 30 μm, graphite particles can be interposed between silicon compound particles, and the expansion and contraction of the silicon compound can be sufficiently mitigated. From this viewpoint, the average particle size of graphite is more preferably 2 to 20 μm, and even more preferably 3 to 10 μm. Note that the average particle size refers to the particle size (D50) at which the volume integration is 50% in the particle size distribution of graphite determined by laser diffraction scattering method. Furthermore, the average particle size of graphite can be adjusted to a desired value by grinding it using known methods such as a ball mill.

[0039] When the negative electrode active material is a mixture of carbon material and silicon compound, the content of silicon compound in the negative electrode active material is preferably 20% to 80% by mass, more preferably 25% to 75% by mass, and even more preferably 30% to 70% by mass, relative to the total amount of silicon compound and carbon material. Furthermore, the content of carbon material in the negative electrode active material is preferably 20% to 80% by mass, more preferably 25% to 75% by mass, and even more preferably 30% to 70% by mass, relative to the total amount of silicon compound and carbon material. If the content of silicon compound and carbon material in the negative electrode active material is within the above numerical range, the capacity and capacity retention rate when used as a negative electrode for lithium-ion secondary batteries can be further improved.

[0040] The content of the negative electrode active material in the negative electrode is preferably 30% to 80% by mass, more preferably 35% to 75% by mass, and even more preferably 40% to 70% by mass, relative to the total amount of the negative electrode active material, binder composition, and conductive additive. If the content of the negative electrode active material in the negative electrode is within the above numerical range, the capacity and capacity retention rate when used as a negative electrode for lithium-ion secondary batteries can be further improved.

[0041] (Conductive additive) As conductive additives, materials with higher conductivity than silicon compounds are used. Specifically, examples of conductive additives include acetylene black, Ketjenblack, carbon nanotubes, and carbon rods. These conductive additives may be used individually or in combination of two or more.

[0042] The content of the conductive additive in the negative electrode is preferably 1% to 30% by mass, more preferably 5% to 25% by mass, and even more preferably 10% to 20% by mass, relative to the total amount of the negative electrode active material, binder composition, and conductive additive. If the content of the conductive additive in the negative electrode is within the above numerical range, the capacity and capacity retention rate when used as a negative electrode for lithium-ion secondary batteries can be further improved.

[0043] (Current collector) The negative electrode for a lithium-ion secondary battery may further include a current collector. If the negative electrode for a lithium-ion secondary battery further includes a current collector, a mixture of the negative electrode active material, a binder composition, and a conductive additive (negative electrode composition) may be laminated on at least one surface of the current collector.

[0044] Examples of materials that make up the current collector (electrode current collector) include conductive metals such as copper, aluminum, titanium, nickel, and stainless steel. Among these, copper or aluminum is preferred, and copper is more preferred. The current collector is generally made of metal foil. The thickness of the current collector is not particularly limited, but 1 to 50 μm is preferred.

[0045] [Manufacturing method for negative electrodes in lithium-ion secondary batteries] The method for manufacturing a negative electrode for a lithium-ion secondary battery is not particularly limited and can be manufactured by conventionally known methods. One embodiment of a method for manufacturing a negative electrode for a lithium-ion secondary battery will be described. A negative electrode for a lithium-ion secondary battery can be manufactured, for example, by coating a negative electrode composition onto the surface of a current collector and drying the coated negative electrode layer composition.

[0046] The negative electrode composition comprises a negative electrode active material, a binder, and a conductive additive, and may optionally contain a solvent and other components. Water is preferably used as the solvent in the negative electrode composition. Using water allows the binder to be easily dissolved in the negative electrode composition. The solid content concentration of the negative electrode composition is preferably 5 to 75% by mass, more preferably 20 to 65% by mass.

[0047] The negative electrode can be formed by applying a negative electrode composition onto a current collector and drying it. The method for applying the negative electrode composition to the surface of the current collector is not particularly limited and includes, for example, dip coating, spray coating, roll coating, doctor blade coating, bar coating, gravure coating, and screen printing. Among these, bar coating or gravure coating is preferred from the viewpoint of uniformly applying the negative electrode composition.

[0048] Furthermore, the drying temperature is not particularly limited as long as the solvent is removed, but is for example 40 to 120°C, preferably 50 to 90°C. Also, the drying time is not particularly limited, but is for example 30 seconds to 10 minutes.

[0049] [Lithium-ion rechargeable battery] The lithium-ion secondary battery of the present invention comprises the above-described negative electrode for lithium-ion secondary batteries. Specifically, the lithium-ion secondary battery of the present invention comprises a positive electrode and a negative electrode arranged to face each other, and preferably further comprises a separator disposed between the positive electrode and the negative electrode.

[0050] (positive electrode) The positive electrode for lithium-ion secondary batteries is not particularly limited, and conventionally known positive electrodes can be used. The positive electrode includes, for example, a positive electrode active material, a positive electrode binder, and a current collector.

[0051] Examples of lithium metal oxide compounds can be used as positive electrode active materials. Examples of lithium metal oxide compounds include lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), and lithium manganese oxide (LiMn2O4). Alternatively, olivine-type lithium iron phosphate (LiFePO4) may also be used. Furthermore, materials using multiple metals other than lithium may also be used, such as ternary oxides like NCM (nickel-cobalt-manganese) oxides and NCA (nickel-cobalt-aluminum) oxides. These positive electrode active materials may be used individually or in combination of two or more.

[0052] Examples of positive electrode binders include fluorine-containing resins such as polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), and polytetrafluoroethylene (PTFE); acrylic resins such as polymethyl acrylate (PMA) and polymethyl methacrylate (PMMA); polyvinyl acetate, polyimide (PI), polyamide (PA), polyvinyl chloride (PVC), polyethernitrile (PEN), polyethylene (PE), polypropylene (PP), polyacrylonitrile (PAN), acrylonitrile-butadiene rubber, styrene-butadiene rubber, poly(meth)acrylic acid, carboxymethylcellulose (CMC), hydroxyethylcellulose, and polyvinyl alcohol. These positive electrode binders may be used individually or in combination of two or more types.

[0053] The material used for the current collector is the same as the compound used for the negative electrode current collector, but preferably aluminum or copper, more preferably aluminum.

[0054] (Separator) In lithium-ion secondary batteries, a separator is provided to effectively prevent short circuits between the positive and negative electrodes. The separator is not particularly limited, and conventionally known separators for lithium-ion secondary batteries can be used. Examples of separators include porous polymer films, nonwoven fabrics, and glass fibers. Among these, porous polymer films are preferred. Examples of porous polymer films include olefin-based porous films.

[0055] A lithium-ion secondary battery may have a multilayer structure in which multiple negative electrodes and positive electrodes are stacked. In this case, the negative electrodes and positive electrodes may be arranged alternately along the stacking direction. If a separator is used, the separator may be placed between each negative electrode and each positive electrode.

[0056] In lithium-ion secondary batteries, the negative electrode and positive electrode, or the negative electrode, positive electrode, and separator described above, are housed within a battery cell. The battery cell may be prismatic, cylindrical, laminated, or any other type.

[0057] (electrolyte) A lithium-ion secondary battery includes an electrolyte. The electrolyte is not particularly limited, and any conventionally known electrolyte used in lithium-ion secondary batteries may be used. For example, an electrolyte solution may be used.

[0058] As the electrolyte, an electrolyte containing an organic solvent and an electrolyte salt can be used. Examples of organic solvents include polar solvents such as ethylene carbonate, propylene carbonate, dimethyl carbonate, γ-butyrolactone, sulfolane, dimethyl sulfoxide, acetonitrile, dimethylformamide, dimethylacetamide, 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrohydrafuran, 2-methyltetrahydrofuran, dioxolane, and methyl acetate, or mixtures of two or more of these solvents. Examples of electrolyte salts include lithium-containing salts such as LiClO4, LiPF6, LiBF4, LiAsF6, LiSbF6, LiCF3CO2, LiPF6SO3, LiN(SO2CF3)2, LiN(SO2CF2CF3)2, LiN(COCF3)2 and LiN(COCF2CF3)2, and lithium bisoxalate borate (LiB(C2O4)2). Other examples include complexes such as lithium organic acid salt-boron trifluoride complexes and complex hydrides such as LiBH4. These salts or complexes may be used individually or in combination of two or more.

[0059] Furthermore, the electrolyte may be a gel-like electrolyte containing a polymer compound in addition to the electrolyte solution. Examples of polymer compounds include fluorine-based polymers such as polyvinylidene fluoride and polyacrylic polymers such as poly(meth)acrylate. The gel-like electrolyte may also be used as a separator.

[0060] The electrolyte may be placed between the negative and positive electrodes. For example, the electrolyte may be filled into a battery cell containing the negative and positive electrodes, or the negative electrode, positive electrode, and separator. Alternatively, the electrolyte may be coated onto the negative or positive electrode and placed between them. [Examples]

[0061] The present invention will be specifically described below with reference to examples and comparative examples, but the present invention is not limited to these examples.

[0062] [Synthesis of polyvinylphosphonic acid] 120.0 g of vinylphosphonic acid (manufactured by Maruzen Petrochemical) as a monomer and 72.3 g of deionized water as a polymerization solvent were added to a 500 mL flask and mixed thoroughly. Then, 4.5 g of 2,2-azobis(2-methylpropionamidine) dihydrochloride (manufactured by Fujifilm Wako Pure Chemical Industries, hereinafter referred to as "AIBA") and 27.8 g of deionized water were added as polymerization initiators and stirred until the AIBA dissolved. Next, the flask containing the monomer was placed in a preheated water bath, and when the internal temperature reached 69°C, the polymerization initiator solution was added dropwise to start the polymerization reaction. The polymerization initiator solution was added dropwise over 6 hours, and after the addition was complete, the mixture was allowed to mature for 2 hours. During polymerization, the temperature of the water bath was adjusted as needed to maintain the reaction solution temperature at 70°C ± 2°C. After polymerization was complete, 169.2 g of deionized water was added and mixed thoroughly, then the flask was cooled in an ice bath to stop the reaction, yielding a slightly yellowish, transparent liquid polymer. The conversion rate of vinylphosphonic acid was 94.6%, and the Mw was 10,200.

[0063] [Manufacturing of negative electrodes for lithium-ion secondary batteries] [Example 1] A binder composition was prepared by mixing 80% by mass of polyvinylphosphonic acid (P0P) and 20% by mass of carboxymethylcellulose (CMC).

[0064] Next, 25% by mass of the above binder composition, 15% by mass of acetylene black (AB), 30% by mass of silicon (Si), and 30% by mass of graphite (G) were added and stirred for 12 hours to produce the negative electrode composition.

[0065] The obtained negative electrode composition was applied to a copper foil, which served as the negative electrode current collector, to a thickness of 10 μm, and the coated film was dried in a vacuum dryer at 80°C for 6 hours. Subsequently, the coated film was subjected to pressure pressing with a roller at 80°C for 3 hours, and then punched out into a circular shape with a diameter of 15 mm to form a negative electrode containing the negative electrode active material.

[0066] [Example 2] A binder composition and a negative electrode for a lithium-ion secondary battery were manufactured in the same manner as in Example 1, except that 30% by mass of silicon monoxide (SiO) was added instead of silicon.

[0067] [Comparative Example 1] A binder composition and a negative electrode for a lithium-ion secondary battery were manufactured in the same manner as in Example 1, except that 20% by mass of polyacrylic acid (PAA) was added instead of polyvinylphosphonic acid.

[0068] [Comparative Example 2] A binder composition and a negative electrode for a lithium-ion secondary battery were manufactured in the same manner as in Example 2, except that 20% by mass of polyacrylic acid was added instead of polyvinylphosphonic acid.

[0069] [Comparative Example 3] A binder composition and a negative electrode for a lithium-ion secondary battery were prepared in the same manner as in Example 1, except that 20% by mass of polyvinylidene fluoride (PVdF) was added instead of polyvinylphosphonic acid.

[0070] [Comparative Example 4] A binder composition and a negative electrode for a lithium-ion secondary battery were prepared in the same manner as in Example 2, except that 20% by mass of polyvinylidene fluoride was added instead of polyvinylphosphonic acid.

[0071] [Comparative Example 5] A binder composition and a negative electrode for a lithium-ion secondary battery (composition: Si: 37.5% by mass, G: 37.5% by mass, AB: 18.75%, CMC: 6.25%) were manufactured in the same manner as in Example 1, except that polyvinylphosphonic acid was not included.

[0072] [Comparative Example 6] A binder composition and a negative electrode for a lithium-ion secondary battery (composition: SiO: 37.5% by mass, G: 37.5% by mass, AB: 18.75%, CMC: 6.25%) were manufactured in the same manner as in Example 2, except that polyvinylphosphonic acid was not included.

[0073] In Comparative Examples 5 and 6, the binder composition contained only CMC as a polymer compound, resulting in insufficient adhesive strength, which prevented the creation of a stable negative electrode.

[0074] Table 1 shows a list of the formulations of the negative electrodes for lithium-ion secondary batteries produced in Examples 1-2 and Comparative Examples 1-6 described above. [Table 1]

[0075] [Evaluation of negative electrodes for lithium-ion secondary batteries] (Half-cell manufacturing) Next, the upper lid 1, wave washer 2, stainless steel plate 3, negative electrodes 4 from Examples 1-2 and Comparative Examples 1-4 manufactured above, O-ring 5, separator 6 (Celgard Monolayer PP2500 (manufactured by CELGARD)), counter electrode Li 7, and lower lid 8 were stacked in that order to assemble the half-cell shown in Figure 1.

[0076] Attempts were made to manufacture half-cells using the negative electrodes of Comparative Examples 5 and 6, but the adhesive strength was insufficient with CMC alone, making it difficult to fabricate test electrodes, and thus half-cell formation was not possible.

[0077] (Preparation of electrolyte solution) An electrolyte solution was prepared by dissolving LiPF6 as the electrolyte salt at a concentration of 1 mol / L in a solvent prepared by mixing ethylene carbonate (EC) and diethylene carbonate (DEC) in a 1:1 volume ratio. The following charge-discharge tests were performed using the prepared electrolyte solution.

[0078] (Charge / Discharge Test) After stabilizing the half-cells manufactured as described above by letting them stand for 6 hours, a charge-discharge test was performed at a charge-discharge rate of 1C. This charge-discharge operation was performed 200 times, with each cycle counting as one cycle, and the discharge capacity was measured after 100 and 200 cycles. The capacity retention rate (%) from 100 to 200 cycles was calculated using the following formula. The charge-discharge test results for Examples 1-2 and Comparative Examples 1-4 are shown in Table 2. • Capacity retention rate (%) = Discharge capacity after 200 cycles (mAh / g) / Discharge capacity after 100 cycles (mAh / g) × 100

[0079] [Table 2]

[0080] The results of the above charge-discharge tests showed that the half-cells using binder compositions containing polyvinylphosphonic acid (P0P) and carboxymethylcellulose (CMC) in Examples 1 and 2 degraded more slowly and had superior capacity retention (cycle characteristics) compared to the half-cells using binder compositions without polyvinylphosphonic acid in Comparative Examples 1 to 4. Furthermore, regardless of the type of binder, half-cells using SiO-C as the negative electrode active material obtained a higher discharge capacity than half-cells using Si-C as the negative electrode active material. This is because SiO is amorphous, which increases the contact opportunities between electrodes, and Li + One possible reason is that interpolation has become more likely to occur. Furthermore, when equal amounts of graphite and Si compounds are blended, the theoretical capacity is approximately 2,200 mAh / g for Si-C and approximately 1,200 mAh / g for SiO-C. Therefore, a half-cell containing PP and CMC as binders and using SiO-C as the negative electrode active material exhibited a discharge capacity that was roughly equal to the theoretical capacity. [Explanation of Symbols]

[0081] 1: Upper lid 2: Wave Washer 3: Stainless steel plate 4: Negative electrode 5: O-ring 6: Separator 7: Antipolar Li 8: Lower lid

Claims

1. A binder composition for the negative electrode of a lithium-ion secondary battery, comprising a vinylphosphorus polymer derived from vinylphosphonic acid or vinylphosphonic acid ester and a cellulose-based water-soluble polymer, The aforementioned vinylphosphorus polymer is defined by the following general formula (1): 【Chemistry 1】 (In general formula (1), R1 and R2 each independently represent a hydrogen atom or a substituted or unsubstituted C1-C6 alkyl group. However, if both R1 and R2 are alkyl groups, they may form a ring together with the oxygen and phosphorus atoms in formula (1).) A binder composition for the negative electrode of a lithium-ion secondary battery, represented by [formula].

2. The binder composition for the negative electrode of a lithium-ion secondary battery according to claim 1, wherein the vinylphosphorus polymer is polyvinylphosphonic acid or dimethyl polyvinylphosphonic acid.

3. The binder composition for the negative electrode of a lithium-ion secondary battery according to claim 1 or 2, wherein the vinylphosphorus polymer is polyvinylphosphonic acid.

4. The binder composition for the negative electrode of a lithium-ion secondary battery according to any one of claims 1 to 3, wherein the cellulose-based water-soluble polymer is carboxymethylcellulose.

5. A negative electrode for a lithium-ion secondary battery comprising a negative electrode active material, a binder, and a conductive additive, The negative electrode active material is composed of a carbon material, a silicon compound, or a mixture of a carbon material and a silicon compound. The binder is composed of the lithium-ion secondary battery negative electrode binder composition described in any one of claims 1 to 4. The negative electrode for a lithium-ion secondary battery is wherein the conductive additive is carbonaceous fine particles.

6. The negative electrode for a lithium-ion secondary battery according to claim 5, wherein the conductive additive is acetylene black.

7. A lithium-ion secondary battery comprising a positive electrode and a negative electrode according to claim 5 or 6.

Images