Binder composition for negative electrodes, negative electrode for secondary batteries, and secondary battery

Abstract

The present invention provides a binder composition for negative electrodes, which contains a silicon-based active material, has a small thickness change during charging and discharging, and is not increased in the thickness even if charging and discharging are repeated. This binder composition for negative electrodes contains a fluorine-containing polymer, a silicon-based negative electrode active material, a thickener, and water. The fluorine-containing polymer has a tetrafluoroethylene-based unit but does not have a vinylidene fluoride-based unit, or alternatively has a tetrafluoroethylene-based unit and a vinylidene fluoride-based unit, with the proportion of the vinylidene fluoride-based unit with respect to all units in the fluorine-containing polymer being less than 40 mol%. The amount of the thickener is greater than the amount of the fluorine-containing polymer. The present invention also provides: a negative electrode which is produced using the binder composition for negative electrodes; and a secondary battery which is provided with the negative electrode.

Description

Binder composition for negative electrode, negative electrode for secondary battery, and secondary battery

[0001] The present invention relates to the field of batteries, and more particularly to a binder composition for a negative electrode, a negative electrode for a secondary battery produced using the binder composition for a negative electrode, and a secondary battery including the negative electrode for a secondary battery.

[0002] An electrode binder composition is prepared by dissolving or dispersing a polymer as a binder, an electrode active material, a conductive material, and the like in water or an organic solvent (see, for example, Patent Document 1). Anode active materials containing silicon atoms are known as electrode active materials, and the use of silicon anodes can effectively increase the energy density of lithium-ion secondary batteries. However, when anode active materials containing silicon atoms are used, the anode material obtained from the binder composition may expand in volume during charge and discharge, resulting in a significant change in thickness. This makes the silicon anode material susceptible to chalking and peeling from the current collector, ultimately affecting the battery capacity.

[0003] For this reason, there is a demand for a negative electrode material that exhibits little change in thickness during charge and discharge and that does not increase in thickness even with repeated charge and discharge.

[0004] International Publication No. 2023 / 007933

[0005] In view of the problems present in the current technology, an object of the present invention is to provide a binder composition for a negative electrode that contains a silicon-based active material, that exhibits little change in thickness during charge and discharge, and that does not increase in thickness even with repeated charge and discharge.

[0006] Another object of the present invention is to provide a negative electrode for a secondary battery produced using the negative electrode binder composition of the present invention, and a secondary battery including the negative electrode for a secondary battery.

[0007] In order to achieve the above object, the present inventors have conducted extensive research and have found that when a binder composition for a negative electrode is produced using a negative electrode active material containing silicon atoms, by using a specific fluorine-containing polymer having tetrafluoroethylene-based units as a binder, the change in thickness of the negative electrode material produced from the binder composition for a negative electrode can be kept low during charge and discharge, and the thickness does not increase even with repeated charge and discharge, which has led to the completion of the present invention.

[0008] The present invention provides the following technical features. [1] A binder composition for a negative electrode comprising a fluoropolymer, a negative electrode active material, a thickener, and water, wherein the fluoropolymer has tetrafluoroethylene-based units but does not have vinylidene fluoride-based units, the amount of the thickener is greater than the amount of the fluoropolymer, and the negative electrode active material contains silicon atoms. [2] The binder composition for a negative electrode according to [1], wherein the thickener is carboxymethyl cellulose. [3] The binder composition for a negative electrode according to [1], wherein the weight ratio of the thickener to the fluoropolymer is 1.1 to 10. [4] The binder composition for a negative electrode according to [1], wherein the fluoropolymer further has propylene-based units. [5] The binder composition for a negative electrode according to [1], wherein a negative electrode sheet produced from the binder composition for a negative electrode has a maximum thickness change rate of 10% or less during charge and discharge. [6] A binder composition for a negative electrode comprising a fluorine-containing polymer, a negative electrode active material, and water, wherein the fluorine-containing polymer has tetrafluoroethylene-based units and vinylidene fluoride-based units, the proportion of the vinylidene fluoride-based units relative to all units in the fluorine-containing polymer is less than 40 mol %, and the negative electrode active material contains silicon atoms. [7] The binder composition for a negative electrode according to [6], wherein the binder composition for a negative electrode further comprises a thickener, and the thickener is carboxymethyl cellulose. [8] The binder composition for a negative electrode according to [7], wherein the weight ratio of the thickener to the fluorine-containing polymer is 1.1 to 10. [9] The binder composition for a negative electrode according to [6], wherein the fluorine-containing polymer further comprises propylene-based units.

[10] The binder composition for a negative electrode according to [6], wherein a negative electrode sheet produced from the binder composition for a negative electrode has a maximum thickness change rate of 10% or less during charge and discharge.

[11] A negative electrode for a secondary battery, which is prepared by applying the negative electrode binder composition according to any one of [1] to

[10] above to a negative electrode current collector, and then removing the dispersion medium.

[12] A secondary battery comprising the negative electrode for a secondary battery according to

[11] above.

[0009] According to the negative electrode binder composition of the present invention, even when a silicon-based active material is used, the negative electrode binder composition functions as a binder, and further, thickness changes during electrode production and use can be suppressed, and the thickness does not increase even after repeated charge and discharge.

[0010] Furthermore, according to the present invention, it is possible to provide a negative electrode for a secondary battery produced using the negative electrode binder composition of the present invention, and a secondary battery including the negative electrode for a secondary battery.

[0011] Other aspects, features and advantages of the present invention will become apparent from the following detailed description.

[0012] The "unit" in the polymer may be an atomic group formed directly by a monomer through a polymerization reaction, or may be an atomic group formed by treating a polymer obtained through a polymerization reaction in a predetermined manner to convert part of the structure. Furthermore, a unit based on monomer A is also referred to as a monomer A unit.

[0013] The term "average particle diameter (D50)" refers to the diameter at 50% cumulative volume of particles determined by a laser diffraction / scattering method. That is, the particle size distribution of particles is measured by a laser diffraction / scattering method, a cumulative curve is determined with the total volume of the particles set to 100%, and the particle diameter is determined at the point on the cumulative curve where the cumulative volume reaches 50%.

[0014] In this specification, unless otherwise specified, additives, contents, concentrations and ratios referred to in this specification refer to additives, contents, concentrations and ratios based on mass.

[0015] Furthermore, "~" indicating a range of numerical values ​​includes the upper and lower limits of the numerical values.

[0016] [First Embodiment] A binder composition for a negative electrode according to a first embodiment of the present invention (also referred to as "binder composition 1") contains a fluoropolymer, a negative electrode active material, a thickener, and water, wherein the fluoropolymer has tetrafluoroethylene-based units and does not have vinylidene fluoride-based units, the amount of the thickener is greater than the amount of the fluoropolymer, and the negative electrode active material contains silicon atoms.

[0017] In this embodiment, the binder composition contains a fluorine-containing polymer used as a binder. The fluorine-containing polymer is a polymer having tetrafluoroethylene-based structural units (TFE units) and not having vinylidene fluoride-based units, and is preferably a copolymer of a polymer having tetrafluoroethylene-based structural units and propylene-based structural units (TFE units / P units) and not having vinylidene fluoride-based units.

[0018] Furthermore, the fluorine-containing copolymer in the present embodiment may contain repeating units based on a fluorine-containing monomer other than tetrafluoroethylene and vinylidene fluoride, or repeating units based on a hydrocarbon monomer other than propylene, to the extent that the functions of the present invention are not impaired, specifically in a proportion of 10 mol % or less.

[0019] Examples of fluorine-containing monomers that can be used other than tetrafluoroethylene and vinylidene fluoride include fluorine-containing olefins such as hexafluoropropylene, trifluoroethylene chloride and (perfluorobutyl)ethylene, and fluorine-containing vinyl ethers such as perfluoropropyl vinyl ether and perfluoromethyl vinyl ether.

[0020] Examples of hydrocarbon monomers other than propylene include α-olefins such as ethylene and 1-butene, vinyl ethers such as ethyl vinyl ether, butyl vinyl ether and hydroxybutyl vinyl ether, and vinyl esters such as vinyl acetate and vinyl benzoate.

[0021] In this embodiment, a silicon-based negative electrode active material is used in the binder composition, but thickness changes tend to occur during charge and discharge, causing the binder composition to peel off from the current collector and ultimately resulting in a decrease in battery capacity. When vinylidene fluoride is used as the binder, thickness changes in the binder composition tend to be large, so it is preferable that the binder does not contain vinylidene fluoride, in other words, that the binder polymer does not contain a vinylidene fluoride-based repeating unit.

[0022] When the fluorocopolymer in this embodiment does not have any vinylidene fluoride-based repeating units, the ideal composition of the fluorocopolymer is one in which the ratio of TFE units / P units is preferably 80 / 20 to 30 / 70 (mol %), more preferably 70 / 30 to 40 / 60 (mol %), most preferably 60 / 40 to 50 / 50 (mol %).

[0023] Within this composition ratio range, the solvent expands little in response to the electrolyte at high temperatures, and the adhesion between the binder composition and the current collector is good when the current collector and electrode are integrated together.

[0024] When the above-mentioned fluorine-containing copolymer is used as a binder, one type of copolymer may be used, or two or more types of copolymers having different compositions of polymerized units may be used in combination. Furthermore, other polymer compounds may be used as needed.

[0025] The fluorine-containing copolymer can be produced by known polymerization methods, among which free radical copolymerization is preferred.There is no particular limitation on the free radical polymerization method, and various free radical polymerization methods can be used, but methods initiating with an organic or inorganic free radical polymerization initiator, light, heat, ionizing radiation, etc. are preferred.As the polymerization method, the copolymer can be produced by known polymerization methods such as bulk polymerization, suspension polymerization, emulsion polymerization, and solution polymerization, and emulsion polymerization is preferred.

[0026] In the binder composition of this embodiment, the weight-average molecular weight of the fluorine-containing copolymer is 10,000 to 300,000, preferably 20,000 to 250,000, more preferably 20,000 to 200,000, and particularly preferably 30,000 to 190,000. If the weight-average molecular weight is below the lower limit, the copolymer will tend to swell in the electrolyte solution, while if it exceeds the upper limit, the binding strength will decrease. The weight-average molecular weight can be adjusted by known methods, such as adding a chain transfer agent or controlling the polymerization temperature and polymerization pressure.

[0027] After an electrode is produced by applying the binder composition containing the silicon-based active material of this embodiment to a current collector, the thickness of the binder composition changes significantly during charge and discharge. However, by using the above-mentioned fluorine-containing copolymer as a binder, the change in thickness of the binder composition during charge and discharge can be suppressed, and the thickness does not increase even after repeated charge and discharge. Furthermore, the mutual binding between the current collector of the electrode and the electrode active material is strong, resulting in excellent charge and discharge characteristics.

[0028] Although the mechanism by which the use of the binder composition of this embodiment can suppress thickness changes during charge and discharge is not clear, it is thought that the above-mentioned fluorine-containing copolymer has excellent alkali resistance, and even if a highly alkaline silicon-based active material is contained in the binder composition, the binder composition still exhibits its binder function, resulting in high mutual adhesion between the electrode current collector and the electrode active material, and therefore no increase in thickness even with repeated charge and discharge.

[0029] In the present embodiment, the fluorine-containing copolymer used in the binder composition is preferably emulsified or dispersed in an aqueous medium, more preferably in the form of particles, and particularly preferably in the form of a latex.

[0030] When the fluorine-containing copolymer is emulsified or dispersed as particles, the average particle size of the fluorine-containing copolymer is preferably 10 to 500 nm, more preferably 20 to 200 nm, even more preferably 30 to 150 nm, more preferably 50 to 150 nm, and particularly preferably 50 to 100 nm. If the average particle size is smaller than the lower limit, the entire surface of the electrode active material is densely covered, tending to increase internal resistance. On the other hand, if the average particle size is larger than the upper limit, the adhesive strength of the binder tends to decrease. The average particle size of the fluorine-containing copolymer particles can be adjusted by known methods, such as by adjusting the type and amount of emulsifier. In addition, the average particle size of the binder particles can be measured by dynamic light scattering using a laser zeta potential meter "ELS-8000" manufactured by Otsuka Electronics Co., Ltd.

[0031] In the binder composition of the present embodiment, when a fluorine-containing copolymer in a latex state is used, the concentration of the solid component is 5% to 95%, more preferably 20% to 60%, and even more preferably 30% to 40%. A concentration of 5% or more is preferred because the effect as a binder is good, and a concentration of 50% or less is preferred because the dispersibility is good.

[0032] In this embodiment, the binder composition may contain a thickener. The thickener can increase the viscosity of the binder composition and can function as a binder similarly to the fluorine-containing copolymer. As the thickener, for example, a water-soluble adhesive resin such as carboxymethyl cellulose (CMC) or methyl cellulose, preferably carboxymethyl cellulose, can be used. Carboxymethyl cellulose has good wettability with respect to particles such as electrode active material and conductive filler, but low suspendability. By combining it with a fluorine-containing copolymer, the dispersion and suspension properties of particles such as electrode active material and conductive filler can be further improved, and a binder composition with stable dispersion can be obtained, which is preferable.

[0033] In this embodiment, the amount of thickener in the binder composition is greater than the amount of the fluoropolymer. By increasing the amount of thickener relative to the amount of the fluoropolymer, the mutual adhesion between the electrode current collector and the electrode active material can be enhanced, and thickness change of the binder composition during charge and discharge can be minimized. The content ratio of the thickener to the fluoropolymer is preferably 1.1 or more, more preferably 2 or more, and particularly preferably 3 or more, and further preferably 10 or less, more preferably 9 or less, and particularly preferably 8 or less. By setting the content ratio in the range of 1.1 to 10, the binder function of the thickener can be further enhanced, the mutual adhesion between the electrode current collector and the electrode active material can be improved, and the properties of the fluoropolymer can be further maximized. This is preferable because thickness change of the binder composition during charge and discharge is reduced, for example, the maximum thickness change rate is suppressed to 10% or less.

[0034] The negative electrode binder composition of this embodiment contains a negative electrode active material containing silicon atoms. The negative electrode active material containing silicon atoms includes at least a graphite-based carbon material and an active material (also referred to as a silicon-based active material) whose main component is silicon that can undergo an alloying reaction with lithium. Examples of the graphite-based carbon material include graphite (natural graphite and artificial graphite) and graphitized pitch-based carbon fiber. Examples of the silicon-based active material include, but are not limited to, Si, SiO, SiO ２ , SiO ｘ In addition, from the viewpoint of suppressing the expansion of the negative electrode active material itself, SiO and SiO are used as the silicon-based active material. ２ SiO formed from at least one of the above and Si ｘ (0.01≦x<2) is preferably used. ｘ means SiO ２ This is a general term for non-stoichiometric silicon monoxide having a structure in which nano-level Si crystals are precipitated in an amorphous phase. The preparation method and properties of this compound are disclosed, for example, in Japanese Patent Application Laid-Open No. 2002-47404 and Journal of Power Sources 170 (2007) 456-459. There are also no particular limitations on the method for producing this compound, and SiO ２ a method of heating a mixed gas of silicon and silicon to produce silicon monoxide gas and cooling / precipitating the gas; a method of heating SiO to produce silicon and SiO ２ As the negative electrode active material containing silicon atoms, a mixture of graphite and a silicon-based active material is preferred, and more preferably a mixture of graphite and the above-mentioned SiO ｘ An example of a negative electrode active material containing silicon atoms is SL500A-SOC nano silicon carbon (manufactured by Ritsuyo Tenmoku Advanced Battery Materials Technology Co., Ltd.).

[0035] The binder composition of this embodiment contains water as a dispersion medium. However, a small amount of a water-soluble organic solvent such as N-methyl-2-pyrrolidone, N-ethyl-2-pyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, or dimethyl sulfoxide may be added to the water, provided that the effects of the present invention are not impaired. These organic solvents may be used alone or in combination of two or more. N-methyl-2-pyrrolidone or N-ethyl-2-pyrrolidone is preferred as the water-soluble organic solvent, with N-methyl-2-pyrrolidone being more preferred. Adding a water-soluble organic solvent as appropriate can promote uniform mixing of the components and adjustment of the viscosity of the binder composition. The lower the content of the water-soluble organic solvent in the dispersion medium, the better, preferably 1% to 5%, more preferably 1% to 3%. Most preferably, the binder composition of the present invention is substantially free of a water-soluble organic solvent.

[0036] The binder composition of this embodiment may contain a conductive material. By including a conductive material, electrical contact between electrode active materials can be improved and resistance within the active material layer can be reduced. The conductive material is preferably at least one selected from carbon black, carbon fiber, carbon nanotubes, conductive graphite, and graphene. Adding a small amount of conductive material is advantageous because it significantly reduces resistance.

[0037] The binder composition of the present embodiment may further contain surfactants, emulsifiers, dispersants, antifoaming agents, thickeners, and the like that are commonly used in the relevant field, as long as the effects of the present invention are not affected.

[0038] The solid content in the binder composition of the present embodiment may be 30% or more, preferably 35% or more, and further, in order to obtain a binder composition with stable dispersion and to avoid precipitation due to an excessively high solid content, the solid content is preferably 60% or less, and more preferably 50% or less.

[0039] Of the solid components of the binder composition of this embodiment, the content of the negative electrode active material is preferably 80 to 99 parts by mass, and the content of the binder (fluorine-containing polymer and thickener) is preferably 1 to 20 parts by mass, relative to 100 parts by mass of the total content of all solid components. Specifically, when the binder is composed of a fluorine-containing polymer and a thickener, the content of the fluorine-containing polymer is preferably 0.3 to 5 parts by mass, and the content of the thickener is preferably 0.5 to 15 parts by mass, with the thickener content being greater than the content of the fluorine-containing polymer. When a conductive material is contained in the binder composition, the conductive material is preferably used in a range in which the contents of the negative electrode active material and the binder satisfy the above-mentioned preferred values. In some preferred embodiments, relative to 100 parts by mass of the total content of all solid components, the content of the negative electrode active material is 90 to 98 parts by mass, the content of the fluorine-containing polymer is 0.3 to 5 parts by mass, the content of the thickener is 0.5 to 10 parts by mass, and the amount of the conductive material used is 0 to 5 parts by mass.

[0040] The negative electrode sheet produced from the negative electrode binder composition 1 of this embodiment has a maximum thickness change rate of 10% or less, preferably 8% or less, and more preferably 5% or less during charge and discharge.

[0041] According to the negative electrode binder composition 1 of the present embodiment, even when a silicon-based active material is used, the negative electrode binder composition functions as a binder, and further, thickness changes during electrode production and use can be suppressed, and the thickness does not increase even after repeated charge and discharge.

[0042] [Second embodiment] A binder composition for a negative electrode according to a second embodiment of the present invention (also referred to as "binder composition 2") contains a fluoropolymer, a negative electrode active material, and water, wherein the fluoropolymer has tetrafluoroethylene-based units and vinylidene fluoride-based units, the proportion of the vinylidene fluoride-based units relative to all units in the fluoropolymer is less than 40 mol%, and the negative electrode active material contains silicon atoms.

[0043] In this embodiment, the binder composition contains a fluorine-containing polymer used as a binder. The fluorine-containing polymer has tetrafluoroethylene-based units (TFE units) and vinylidene fluoride-based units (VdF units), and the proportion of the vinylidene fluoride-based units to all units in the fluorine-containing polymer is less than 40 mol %. The fluorine-containing polymer preferably further has propylene-based structural units (P units).

[0044] Furthermore, the fluorine-containing copolymer in the present embodiment may contain repeating units based on a fluorine-containing monomer other than tetrafluoroethylene and vinylidene fluoride, or repeating units based on a hydrocarbon monomer other than propylene, to the extent that the functions of the present invention are not impaired, specifically in a proportion of 10 mol % or less.

[0045] Examples of fluorine-containing monomers that can be used other than tetrafluoroethylene and vinylidene fluoride include fluorine-containing olefins such as hexafluoropropylene, trifluoroethylene chloride and (perfluorobutyl)ethylene, and fluorine-containing vinyl ethers such as perfluoropropyl vinyl ether and perfluoromethyl vinyl ether.

[0046] Examples of hydrocarbon monomers other than propylene include α-olefins such as ethylene and 1-butene, vinyl ethers such as ethyl vinyl ether, butyl vinyl ether and hydroxybutyl vinyl ether, and vinyl esters such as vinyl acetate and vinyl benzoate.

[0047] In this embodiment, a silicon-based negative electrode active material is used in the binder composition, but thickness changes tend to occur during charge and discharge, causing the binder composition to peel off from the current collector and ultimately resulting in a decrease in battery capacity. When vinylidene fluoride is used as the binder, thickness changes in the binder composition tend to be large, so it is preferable that the amount of vinylidene fluoride or vinylidene fluoride-based repeating units in the binder is small. Specifically, the ratio of vinylidene fluoride-based units to all units in the fluorine-containing polymer is less than 40 mol%, more preferably less than 35 mol%, even more preferably less than 30 mol%, still more preferably less than 20 mol%, and particularly preferably less than 10 mol%.

[0048] When the fluorine-containing copolymer used in the present embodiment contains TFE units, P units and PVDF units, the composition thereof preferably has a ratio of TFE units / P units / PVDF units in the range of 30 to 85 / 15 to 70 / 1 to 40 (mol %), more preferably 30 to 70 / 20 to 60 / 1 to 35 (mol %).

[0049] Within this composition ratio range, the solvent expands little in response to the electrolyte at high temperatures, and the adhesion between the binder composition and the current collector is good when the current collector and electrode are integrated together.

[0050] When the above-mentioned fluorine-containing copolymer is used as a binder, one type of copolymer may be used, or two or more types of copolymers having different compositions of polymerized units may be used in combination. Furthermore, other polymer compounds may be used as needed.

[0051] The fluorine-containing copolymer of this embodiment can be produced by the same polymerization method as that for the fluorine-containing copolymer of the first embodiment, and the preferred embodiments are also the same.

[0052] After an electrode is produced by applying the binder composition containing the silicon-based active material of this embodiment to a current collector, the thickness of the binder composition changes significantly during charge and discharge. However, by using the above-mentioned fluorine-containing copolymer as a binder, the change in thickness of the binder composition during charge and discharge can be suppressed, and the thickness does not increase even after repeated charge and discharge. Furthermore, the mutual binding between the current collector of the electrode and the electrode active material is strong, resulting in excellent charge and discharge characteristics.

[0053] Although the mechanism by which the use of the binder composition of this embodiment can suppress thickness changes during charge and discharge is not clear, it is thought that the above-mentioned fluorine-containing copolymer has excellent alkali resistance, and even if a highly alkaline silicon-based active material is contained in the binder composition, the binder composition still exhibits its binder function, resulting in high mutual adhesion between the electrode current collector and the electrode active material, and therefore no increase in thickness even with repeated charge and discharge.

[0054] In this embodiment, the binder composition preferably further contains a thickener. The thickener can increase the viscosity of the binder composition and can function as a binder similar to the fluorocopolymer. Examples of thickeners that can be used include water-soluble adhesive resins such as carboxymethyl cellulose (CMC) and methyl cellulose, with carboxymethyl cellulose being preferred. The amount of thickener in the binder composition is preferably greater than the amount of the fluoropolymer. By increasing the amount of thickener relative to the amount of the fluoropolymer, the mutual adhesion between the electrode current collector and the electrode active material can be improved, and thickness changes in the binder composition during charge and discharge can be minimized. The content ratio of the thickener to the fluoropolymer is preferably 1.1 to 10, more preferably 2 to 9, and particularly preferably 3 to 8. By setting the content ratio of the thickener to the fluoropolymer within the above range, the binder function of the thickener can be further exerted, the mutual binding strength between the current collector of the electrode and the electrode active material can be improved, and the physical properties of the fluoropolymer can be further exerted, and the change in thickness of the binder composition during charge and discharge can be reduced, for example, the maximum rate of change in thickness can be suppressed to 10% or less.

[0055] The negative electrode binder composition 2 of the present embodiment has the same configuration and preferred form as those of the first embodiment, except that the composition of the fluorine-containing polymer contained therein is different from that of the first embodiment, and therefore a description of the same configuration will be omitted.

[0056] The negative electrode sheet produced from the negative electrode binder composition 2 of this embodiment has a maximum thickness change rate of 10% or less, preferably 8% or less, and more preferably 5% or less during charge and discharge.

[0057] According to the negative electrode binder composition 2 of the present embodiment, even when a silicon-based active material is used, the negative electrode binder composition functions as a binder, and further, thickness changes during electrode production and use can be suppressed, and the thickness does not increase even after repeated charge and discharge.

[0058] [Method for producing binder composition] The binder composition of the present invention can be obtained by mixing a negative electrode active material and an optionally added conductive material, adding an aqueous solution of a thickener and water and stirring uniformly, and then adding a fluorine-containing polymer as a binder and stirring uniformly. The preferred forms and contents of the negative electrode active material, conductive material, fluorine-containing polymer, thickener and dispersion medium are as described above.

[0059] In the method for producing the binder composition, when mixing the negative electrode active material and the conductive material, it is preferable to grind the negative electrode active material and the conductive material until they no longer have a granular texture, and then mix them using a stirrer. The stirrer is not particularly limited, and stirrers, mixers, homogenizers, and the like commonly used in the art can be used. From the viewpoint of achieving uniform stirring and enabling degassing in a subsequent process, a planetary homogenizer is preferred. The stirring speed is 1800 to 2500 rpm, preferably 2000 to 2200 rpm, and the stirring time is 1 to 20 minutes, preferably 2 to 10 minutes.

[0060] When a thickener is added, it is preferably added in the form of an aqueous solution of the thickener. The concentration of the thickener is not particularly limited, but can be 0.5 to 30% by mass, preferably 1 to 10% by mass.

[0061] The aqueous solution of thickener may be added in one stage or in multiple stages. From the viewpoint of obtaining a binder composition with a more stable dispersion state, it is preferable to add it in two or more stages, for example, four, five, six, eight, or ten stages. On the other hand, from the viewpoint of improving addition efficiency or simplifying the operation process, it is preferable to add it in five stages or less. Preferably, after adding the aqueous solution of thickener in each stage, it is thoroughly and uniformly stirred using a planetary homogenizer. The stirring speed is 1800 to 2500 rpm, preferably 2000 to 2200 rpm, and the stirring time is 5 to 30 minutes, preferably 10 to 15 minutes.

[0062] After the fluoropolymer is added, homogenization and mixing may be carried out using a ball mill, preferably a planetary ball mill. When a planetary ball mill is used, the stirring speed is 300 to 800 rpm, preferably 400 to 500 rpm, and the stirring time is 15 to 60 seconds, preferably 20 to 30 seconds. By using a ball mill with a low stirring speed, the materials can be sufficiently homogenized and mixed, and a dispersion with high dispersion stability and little foaming can be obtained.

[0063] In some preferred embodiments, the electrode binder composition comprises a fluorine-containing polymer, CMC, a silicon-based negative electrode active material, a conductive material, and water, and the content ratio of the silicon-based negative electrode active material, the fluorine-containing polymer, the CMC, and the conductive material in the solid component of the binder composition is 90 to 98:0.3 to 5:0.5 to 10:1 to 5. The preparation method includes the following: mixing the silicon-based negative electrode active material and the conductive material, grinding the mixture until no obvious graininess is observed, and then stirring and mixing using a planetary homogenizer; then adding a 1 to 10% by mass aqueous CMC solution in one step so that the solid content of the dispersion becomes 50 to 60% by mass, and stirring and mixing using a planetary homogenizer; then adding a 1 to 10% by mass aqueous CMC solution and water so that the solid content of the dispersion becomes 30 to 40% by mass, and stirring and mixing using a planetary homogenizer; and finally adding a fluorine-containing polymer latex and homogenizing and mixing using a ball mill.

[0064] [Negative electrode for secondary battery and secondary battery] A secondary battery usually comprises a positive electrode, a negative electrode, a separator, and an electrolyte. In the secondary battery of the present invention, an electrode prepared using the negative electrode binder composition of the present invention (negative electrode binder composition 1 or 2) can be used as the negative electrode (hereinafter also referred to as negative electrode for secondary battery). Lithium metal or a lithium alloy such as a lithium aluminum alloy can be used as the counter electrode.

[0065] The method for producing a negative electrode for a secondary battery of the present invention is a method in which the negative electrode binder composition of the present invention is applied to a current collector, and then the dispersion medium is removed. After the dispersion medium is removed, it is preferable to further apply pressure as necessary to form the negative electrode into a desired thickness.

[0066] The negative electrode binder composition of the present invention can be applied to a current collector by various application methods, such as by using an applicator such as a scraper. The application temperature is not particularly limited, but a temperature around room temperature is usually preferred.

[0067] The removal of the dispersion medium is usually carried out by drying at room temperature or by heating. Drying can be carried out using various dryers, such as a heated vacuum dryer. The drying temperature is not particularly limited, but is generally preferably from room temperature to 150°C.

[0068] The rolling method can be carried out using a die press, a roller press, etc. The thickness of the coating layer of the binder composition is preferably 0.5 to 2000 μm, more preferably 1 to 1000 μm, and particularly preferably 10 to 500 μm, as the thickness after drying, or as the thickness after rolling if further rolling is carried out.

[0069] The current collector of the present invention may be any conductive material, and is not particularly limited. Examples of such current collectors include metal foils, metal meshes, and porous metals, such as aluminum, nickel, stainless steel, and copper. Aluminum is preferably used as the positive electrode current collector, and copper is preferably used as the negative electrode current collector. The thickness of the current collector is preferably 1 to 100 μm. If the thickness is less than 1 μm, the battery may not have sufficient durability and may have reduced reliability. Furthermore, if the thickness exceeds 100 μm, the mass of the battery increases.

[0070] The secondary battery electrode of the present invention can be used in batteries of any shape, such as cylindrical, sheet, or prismatic.

[0071] Furthermore, a secondary battery formed by using the electrode of the present invention as the negative electrode, sandwiching a separator between the positive electrode and the negative electrode, and incorporating the electrode and the electrolyte solution in a casing has high reliability even at high temperatures.

[0072] As the separator, a microporous polymer membrane can be used, and examples of the material thereof include polypropylene resin, polyethylene resin, polypropylene-polyethylene-polypropylene, and the like.

[0073] Examples of the solvent for the electrolyte include aprotic organic solvents, such as isopropenyl carbonate, ethylene carbonate, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, γ-butyrolactone, and diethoxyethane. ４ , LiBF ４ , LiPF ６ , LiAsF ５ , C.F. ３ SO ３ Li, (CF ３ SO ２ ) ２ Lithium salts such as NLi can be mentioned.

[0074] In some preferred embodiments, a secondary battery is assembled using a negative electrode made from the negative electrode binder composition of the present invention by the following steps: (1) placing the prepared positive electrode sheet in the center of a positive electrode casing, and dripping an electrolyte onto the top surface so that the positive electrode sheet is completely impregnated; (2) placing a separator flat on the positive electrode sheet, and further dripping an electrolyte onto the separator so that the separator is completely impregnated; (3) placing a lithium sheet on the separator as a counter electrode; (4) placing a gasket and a spring on the lithium sheet and positioning it at the center of the battery, and placing the negative electrode casing on a button; and (5) pressure-sealing using a sealing machine to obtain a button-type half cell.

[0075] The negative electrode for a secondary battery according to the present invention exhibits small thickness change during charge and discharge and does not increase in thickness even with repeated charge and discharge. Specifically, the negative electrode material prepared from the negative electrode binder composition according to the present invention exhibits a maximum thickness change rate of 10% or less during charge and discharge.

[0076] The configuration and advantages of the present invention will be further explained through the following examples. However, it should be understood that the following examples are merely illustrative of the present invention and are not intended to limit the scope of protection of the present invention.

[0077] First, the methods for preparing the fluorine-containing polymers used in the examples and comparative examples will be described.

[0078] (Production Example 1) After degassing the interior of a 3200 mL stainless steel pressure reactor equipped with an anchor impeller for stirring, 1579 g of ion-exchanged water, 8.9 g of sodium lauryl sulfate, 2.0 g of sodium hydroxide, 58 g of disodium hydrogen phosphate dodecahydrate, 4.2 g of ammonium persulfate, and 100 g of tert-butyl alcohol were charged into the reactor. An aqueous solution prepared by dissolving 0.27 g of ethylenediaminetetraacetic acid disodium salt dihydrate and 0.36 g of ferrous sulfate heptahydrate in 200 g of ion-exchanged water was added. Next, a monomer mixed gas of TFE / P = 88 / 12 (molar ratio) was injected at 40°C, and the pressure inside the reactor was set to 2.50 MPaG. While rotating the anchor blade at 300 rpm, an aqueous solution of 2.5% by mass sodium hydroxymethanesulfonate dihydrate (hereinafter also referred to as Rongalit) whose pH had been adjusted to 10.0 with sodium hydroxide was added to initiate the polymerization reaction. Thereafter, an aqueous solution of 2.5% by mass Rongalit powder was continuously added using a high-pressure pump.

[0079] As the polymerization proceeds, the reactor internal pressure decreases, and when the reactor internal pressure drops to 2.49 MPaG, a monomer mixed gas of TFE / P=56 / 44 (molar ratio) is introduced under its own pressure, and the reactor internal pressure is increased to 2.51 MPaG. This operation is repeated, and the reactor internal pressure is maintained at 2.49 to 2.51 MPaG, and the polymerization reaction is continued. When the total amount of injected TFE / P monomer mixed gas reaches 700 g, the internal temperature of the reactor is cooled to 10 ° C., and the polymerization reaction is stopped, and a latex containing fluorine-containing copolymer 1 is obtained. The content of fluorine-containing copolymer 1 in the latex is 33% by mass, and the copolymer unit composition is TFE unit / P unit=56 / 44 (molar ratio).

[0080] (Production Example 2) After degassing the interior of a 3200 mL stainless steel pressure reactor equipped with an anchor impeller for stirring, 1184 g of ion-exchanged water, 8.6 g of sodium lauryl sulfate, 2.3 g of sodium hydroxide, 35 g of disodium hydrogen phosphate dodecahydrate, 4.2 g of ammonium persulfate, and 179 g of tert-butyl alcohol were charged into the reactor. An aqueous solution prepared by dissolving 0.27 g of ethylenediaminetetraacetic acid disodium salt dihydrate and 0.36 g of ferrous sulfate heptahydrate in 200 g of ion-exchanged water was added. Next, a monomer mixed gas of TFE / P / VDF = 25 / 5 / 70 (molar ratio) was injected at 25°C, and the pressure inside the reactor was adjusted to 2.30 MPaG. While rotating the anchor blade at 300 rpm, an aqueous solution of 2.5% by mass sodium hydroxymethanesulfonate dihydrate (hereinafter also referred to as Rongalit) whose pH had been adjusted to 10.0 with sodium hydroxide was added to initiate the polymerization reaction. Thereafter, an aqueous solution of 2.5% by mass Rongalit powder was continuously added using a high-pressure pump.

[0081] As the polymerization proceeds, the reactor internal pressure decreases, and when the reactor internal pressure drops to 2.29 MPaG, a monomer mixed gas of TFE / P / VdF = 40 / 25 / 35 (molar ratio) was introduced under its own pressure, and the reactor internal pressure was increased to 2.31 MPaG. This operation was repeated, and the reactor internal pressure was maintained at 2.29 to 2.31 MPaG, and the polymerization reaction was continued. When the total amount of injected TFE / P / VdF monomer mixed gas reached 770 g, the internal temperature of the reactor was cooled to 10 ° C., and the polymerization reaction was terminated, thereby obtaining a latex containing fluorine-containing copolymer 2. The content of fluorine-containing copolymer 2 in the latex was 33% by mass, and the copolymer unit composition was TFE unit / P unit / VdF unit = 40 / 25 / 35 (molar ratio).

[0082] (Production Example 3) F(CF) was placed in a 3000 mL stainless steel pressure reactor equipped with anchor blades for stirring. ２ ) ５ COONH ４ Aqueous solution of 3300 ppm, CH ２ =CFCF ２ OCF (CF ３ )CF ２ OCF (CF ３ ) COONH４ was charged as an aqueous solution with a concentration of 200 ppm, and the polymerization tank was sealed. The tank was evacuated by replacing the vacuum with nitrogen gas, and isopropyl alcohol, a chain transfer agent, was injected in an amount equivalent to 400 cc using a syringe at the same time as the evacuation. Thereafter, while stirring at 70 ° C., a mixed gas monomer having a composition ratio of 80 / 20 mol% of vinylidene fluoride (VdF) / tetrafluoroethylene (TFE) was introduced into the tank up to 0.8 MPaG. Thereafter, an aqueous solution in which ammonium persulfate was dissolved in an amount equivalent to 100 ppm was introduced under nitrogen pressure to start the reaction. In order to maintain the pressure in the tank, a mixed monomer having a composition ratio of 48 / 52 mol% of VdF / TFE was additionally injected. When 346 g of monomer was added, the temperature in the reactor was cooled to 10 ° C., the polymerization reaction was stopped, and a latex containing fluorine-containing copolymer 3 was obtained. The content of Fluorine-containing copolymer 3 in the latex was 20% by mass, and the copolymer unit composition was VdF units / TFE units=48 / 52 (molar ratio).

[0083] The latex of fluorine-containing copolymer 1, the latex of fluorine-containing copolymer 2 and the latex of fluorine-containing copolymer 3 are ready for use after being obtained by the above-mentioned method.

[0084] Example 1 15 g of silicon-carbon negative electrode material ("SL500A-SOC" particle size D50: 10.9 μm, manufactured by Ritsuyo Tenmoku Advanced Battery Materials Technology Co., Ltd.) and 0.173 g of carbon black Super P ("MA-EN-CO-01" manufactured by Guangdong Candlelight New Energy Technology Co., Ltd.) as a conductive material were placed in a mortar and pulverized until the particles were no longer noticeable, and then dry-mixed for 2 minutes at 2000 rpm using a planetary homogenizer (Mazerustar planetary stirring and degassing device manufactured by Kurabo Industries, Ltd.).

[0085] 13.4 g of a 2% by mass aqueous CMC solution was added so that the solid content after addition was 54% by mass, and the mixture was homogenized at 2000 rpm for 10 minutes using a planetary homogenizer (Mazerustar planetary stirring and deaerating device manufactured by Kurabo Industries, Ltd.).

[0086] Next, 11.06 g of a 2 mass % aqueous solution of CMC ("CMC2200" manufactured by Daicel Corporation) and 1.6 g of water were added so that the solid content after addition would be 38 mass %, and the mixture was homogenized at 2000 rpm for 10 minutes using a planetary homogenizer (Mazerustar planetary stirring and deaerating device manufactured by Kurabo Industries, Ltd.).

[0087] Finally, 0.185 g of a latex of fluorine-containing copolymer 1 (TFE / P copolymer latex, TFE unit / P unit=56 / 44 (molar ratio), solid content 33% by mass) was added as a binder, and the mixture was ball milled at 500 rpm for 0.5 minutes using a ball mill (planetary ball mill "QM-3SP04") to obtain a uniformly mixed aqueous negative electrode slurry (hereinafter referred to as negative electrode binder composition A1).

[0088] [Example 2] 15 g of silicon-carbon negative electrode material and 0.173 g of conductive material Super P were placed in a mortar and pulverized until the particles were no longer noticeable, and then dry-mixed for 2 minutes at 2000 rpm using a planetary homogenizer (Mazerustar planetary stirring and degassing device manufactured by Kurabo Industries, Ltd.).

[0089] 13.4 g of a 2% by mass aqueous CMC solution was added so that the solid content after addition was 54% by mass, and the mixture was homogenized at 2000 rpm for 10 minutes using a planetary homogenizer (Mazerustar planetary stirring and deaerating device manufactured by Kurabo Industries, Ltd.).

[0090] Next, 6.25 g of a 2% by mass aqueous CMC solution and 6.10 g of water were added so that the solid content after addition was 38% by mass, and the mixture was homogenized at 2000 rpm for 10 minutes using a planetary homogenizer (Mazerustar planetary stirring and degassing device manufactured by Kurabo Industries, Ltd.).

[0091] Finally, 0.476 g of a latex of fluorine-containing copolymer 1 (TFE / P copolymer latex, TFE unit / P unit=56 / 44 (molar ratio), solid content 33% by mass) was added as a binder, and the mixture was ball milled at 500 rpm for 0.5 minutes using a ball mill (planetary ball mill "QM-3SP04") to obtain a uniformly mixed aqueous negative electrode slurry (hereinafter referred to as negative electrode binder composition A2).

[0092] [Example 3] 15 g of silicon-carbon negative electrode material and 0.173 g of conductive material Super P were placed in a mortar and pulverized until the particles were no longer noticeable, and then dry-mixed for 2 minutes at 2000 rpm using a planetary homogenizer (Mazerustar planetary stirring and degassing device manufactured by Kurabo Industries, Ltd.).

[0093] 13.4 g of a 2% by mass aqueous CMC solution was added so that the solid content after addition was 54% by mass, and the mixture was homogenized at 2000 rpm for 10 minutes using a planetary homogenizer (Mazerustar planetary stirring and deaerating device manufactured by Kurabo Industries, Ltd.).

[0094] Next, 2.32 g of a 2% by mass aqueous CMC solution and 9.80 g of water were added so that the solid content after addition was 38% by mass, and the mixture was homogenized at 2000 rpm for 10 minutes using a planetary homogenizer (Mazerustar planetary stirring and degassing device manufactured by Kurabo Industries, Ltd.).

[0095] Finally, 0.715 g of a latex of fluorine-containing copolymer 1 (TFE / P copolymer latex, TFE unit / P unit=56 / 44 (molar ratio), solid content 33% by mass) was added as a binder, and the mixture was ball milled at 500 rpm for 0.5 minutes using a ball mill (planetary ball mill "QM-3SP04") to obtain a uniformly mixed aqueous negative electrode slurry (hereinafter referred to as negative electrode binder composition A3).

[0096] Example 4 15 g of silicon-carbon negative electrode material and 0.173 g of conductive material Super P were placed in a mortar and pulverized until the particles were no longer noticeable, and then dry-mixed for 2 minutes at 2000 rpm using a planetary homogenizer (Mazerustar planetary stirring and degassing device manufactured by Kurabo Industries, Ltd.).

[0097] 13.4 g of a 2% by mass aqueous CMC solution was added so that the solid content after addition was 54% by mass, and the mixture was homogenized at 2000 rpm for 10 minutes using a planetary homogenizer (Mazerustar planetary stirring and deaerating device manufactured by Kurabo Industries, Ltd.).

[0098] Next, 11.06 g of a 2% by mass aqueous CMC solution and 1.6 g of water were added so that the solid content after addition was 38% by mass, and the mixture was homogenized at 2000 rpm for 10 minutes using a planetary homogenizer (Mazerustar planetary stirring and degassing device manufactured by Kurabo Industries, Ltd.).

[0099] Finally, 0.185 g of a latex of fluorine-containing copolymer 2 (TFE / P / VdF copolymer latex, TFE units / P units / VdF units=40 / 25 / 35 (molar ratio), solid content 33% by mass) was added as a binder, and the mixture was ball milled at 500 rpm for 0.5 minutes using a ball mill (planetary ball mill "QM-3SP04") to obtain a uniformly mixed aqueous negative electrode slurry (hereinafter referred to as negative electrode binder composition A4).

[0100] Comparative Example 1 15 g of silicon-carbon negative electrode material and 0.173 g of conductive material Super P were placed in a mortar and pulverized until the particles were no longer noticeable, and then dry-mixed for 2 minutes at 2000 rpm using a planetary homogenizer (Mazerustar planetary stirring and degassing device manufactured by Kurabo Industries, Ltd.).

[0101] 7.86 g of a 2% by mass aqueous CMC solution and 5.5 g of water were added so that the solid content after addition was 54% by mass, and the mixture was homogenized at 2000 rpm for 10 minutes using a planetary homogenizer (Mazerustar planetary stirring and defoaming device manufactured by Kurabo Industries, Ltd.).

[0102] Next, 11.8 g of water was added so that the solid content after addition was 38% by mass, and the mixture was homogenized at 2000 rpm for 10 minutes using a planetary homogenizer (Mazerustar planetary stirring and deaerating device manufactured by Kurabo Industries, Ltd.).

[0103] Finally, 1.191 g of a latex of fluorine-containing copolymer 1 (TFE / P copolymer latex, TFE unit / P unit=56 / 44 (molar ratio), solid content 33% by mass) was added as a binder, and the mixture was ball milled at 500 rpm for 0.5 minutes using a ball mill (planetary ball mill "QM-3SP04") to obtain a uniformly mixed aqueous negative electrode slurry (hereinafter referred to as negative electrode binder composition B1).

[0104] Comparative Example 2 15 g of silicon-carbon negative electrode material and 0.173 g of conductive material Super P were placed in a mortar and pulverized until the particles were no longer noticeable, and then dry-mixed for 2 minutes at 2000 rpm using a planetary homogenizer (Mazerustar planetary stirring and degassing device manufactured by Kurabo Industries, Ltd.).

[0105] 11.01 g of a 2% by mass aqueous CMC solution and 2.3 g of water were added so that the solid content after addition was 54% by mass, and the mixture was homogenized at 2000 rpm for 10 minutes using a planetary homogenizer (Mazerustar planetary stirring / defoaming device manufactured by Kurabo Industries, Ltd.).

[0106] Next, 12.0 g of water was added so that the solid content after addition was 38% by mass, and the mixture was homogenized at 2000 rpm for 10 minutes using a planetary homogenizer (Mazerustar planetary stirring and deaerating device manufactured by Kurabo Industries, Ltd.).

[0107] Finally, 0.726 g of styrene butadiene latex resin (SBR) latex (TRD104A manufactured by JSR Corporation, solid content 45.5% by mass) was added as a binder, and the mixture was ball milled at 500 rpm for 0.5 minutes using a ball mill (planetary ball mill QM-3SP04) to obtain a uniformly mixed aqueous negative electrode slurry (hereinafter referred to as negative electrode binder composition B2).

[0108] Comparative Example 3 15 g of silicon-carbon negative electrode material and 0.173 g of conductive material Super P were placed in a mortar and pulverized until the particles were no longer noticeable, and then dry-mixed for 2 minutes at 2000 rpm using a planetary homogenizer (Mazerustar planetary stirring and degassing device manufactured by Kurabo Industries, Ltd.).

[0109] 13.4 g of a 2% by mass aqueous CMC solution was added so that the solid content after addition was 54% by mass, and the mixture was homogenized at 2000 rpm for 10 minutes using a planetary homogenizer (Mazerustar planetary stirring and deaerating device manufactured by Kurabo Industries, Ltd.).

[0110] Next, 2.32 g of a 2% by mass aqueous CMC solution and 9.8 g of water were added so that the solid content after addition was 38% by mass, and the mixture was homogenized at 2000 rpm for 10 minutes using a planetary homogenizer (Mazerustar planetary stirring and degassing device manufactured by Kurabo Industries, Ltd.).

[0111] Finally, 1.18 g of a dispersion (solid content 20% by mass) of PVDF ("HSV900" manufactured by Arkema K.K.) was added as a binder, and the mixture was ball milled at 500 rpm for 0.5 minutes using a ball mill (planetary ball mill "QM-3SP04") to obtain a uniformly mixed aqueous negative electrode slurry (hereinafter referred to as negative electrode binder composition B3).

[0112] Comparative Example 4 15 g of silicon-carbon negative electrode material and 0.173 g of conductive material Super P were placed in a mortar and pulverized until the particles were no longer noticeable, and then dry-mixed for 2 minutes at 2000 rpm using a planetary homogenizer (Mazerustar planetary stirring and degassing device manufactured by Kurabo Industries, Ltd.).

[0113] 13.4 g of a 2% by mass aqueous CMC solution was added so that the solid content after addition was 54% by mass, and the mixture was homogenized at 2000 rpm for 10 minutes using a planetary homogenizer (Mazerustar planetary stirring and deaerating device manufactured by Kurabo Industries, Ltd.).

[0114] Next, 2.32 g of a 2% by mass aqueous CMC solution and 9.8 g of water were added so that the solid content after addition was 38% by mass, and the mixture was homogenized at 2000 rpm for 10 minutes using a planetary homogenizer (Mazerustar planetary stirring and degassing device manufactured by Kurabo Industries, Ltd.).

[0115] Finally, 1.18 g of a latex of fluorine-containing copolymer 3 (VdF / TFE copolymer latex, VdF unit / TFE unit=48 / 52 (molar ratio), solid content 20% by mass) was added as a binder, and the mixture was ball milled at 500 rpm for 0.5 minutes using a ball mill (planetary ball mill "QM-3SP04") to obtain a uniformly mixed aqueous negative electrode slurry (hereinafter referred to as negative electrode binder composition B4).

[0116] Table 2 shows the outline of the negative electrode binder compositions of Examples 1 to 4 and Comparative Examples 1 to 4.

[0117]

[0118] In Table 2, in the negative electrode binder composition A1 of Example 1, water was used as the dispersion medium, the content ratio of the negative electrode active material, conductive material, and binder in the solid components was 95.4:1.1:3.5, and the content ratio of the fluorine-containing copolymer 1 and CMC as the binder was 1 / 8. The same interpretation is adopted for the configurations of the negative electrode binder compositions of Examples 2 to 4 and Comparative Examples 1 to 4.

[0119] <Production of Negative Electrode Sheet> The negative electrode binder composition A1 obtained in Example 1 was applied to a thickness of 200 μm on a copper foil current collector uniformly covered to a thickness of 10 μm using an automatic film applicator (brand: BEVS; model: 1811), and then placed in a vacuum drying oven at 80°C for 8 hours to remove moisture. After drying, the coated layer was subjected to a calendaring treatment using a roller press at room temperature under conditions such that the coated layer was 49 μm thick, thereby obtaining an electrode sheet. The electrode sheet was cut using a manual punching machine to obtain a negative electrode sheet sample with a diameter of 12 mm.

[0120] For the negative electrode binder compositions A2 to A4 of Examples 2 to 4 and the negative electrode binder compositions B1 to B4 of Comparative Examples 1 to 4, negative electrode sheet samples were prepared in the same manner as in Example 1, except that the coating thickness and the thickness of the coating layer after rolling were changed as shown in Table 3 below.

[0121]

[0122] <Assembly of Button Half Cells> The negative electrode sheet samples prepared in each example were transferred to a glove box filled with argon gas and assembled into 2032 button half cells. A pure lithium sheet was used as the counter electrode, a Celgard® 2325 polypropylene-polyethylene-polypropylene (PP-PE-PP) membrane was used as the separator, and the electrolyte was 1 M lithium hexafluorophosphate (LiPF ６ ) A mixed solution of ethylene carbonate (EC) and diethyl carbonate (DEC) (volume ratio 1:1) was added with 10% volume fraction of fluoroethylene carbonate (FEC).

[0123] The specific assembly steps for the button half cell are as follows: (1) place the negative electrode sheet in the center of the negative electrode casing, and drip the electrolyte onto the top surface so that the negative electrode sheet is completely impregnated; (2) place the separator flat on the negative electrode sheet, and drip the electrolyte onto the separator so that the separator is completely impregnated; (3) place the lithium sheet on the separator as the counter electrode; (4) place the gasket and spring on the lithium sheet and position it in the center of the battery, and then place the negative electrode casing on the button; (5) use a sealing machine to seal under pressure to obtain a button half cell.

[0124] <Electrochemical Test> After the sealed and prepared 2032-type button half-cell was left for 6 hours, the test was started and a constant current charge-discharge cycle test was performed in the voltage range of 2.5 to 4.0 V using an indigo battery test system.

[0125] <Peel Strength Test> The peel strength of the electrode sheet was tested in accordance with GB / T 2792-1998. Specifically, the test was performed as follows: 1. After rolling, the electrode sheet was cut into a long strip with a width of 19 mm. 2. The surface coated with the negative electrode binder composition was attached to single-sided adhesive tape, and the sheet was rolled back and forth with a roller five times to remove air bubbles. 3. The surface not coated with the negative electrode binder composition was adhered to a steel plate with double-sided adhesive tape. 4. One end of the electrode sheet and one end of the adhesive tape were fixed to a tension machine using fixtures, and the sheet was tensioned at 10 mm min using a universal tension machine. －１ The adhesive tape is peeled off from the electrode sheet at a speed of 180°.

[0126] <Flexibility test> Flexibility test of the electrode sheet by wrapping it around the electrode sheet. Steel cores with diameters of 5 mm, 2.5 mm, 1 mm, and 0.5 mm were used, and the rolled electrode sheet was wrapped around the surface of the steel core, and the coating on the surface of the electrode sheet was observed for cracks and peeling. If the above phenomenon did not occur, it was judged as passing and recorded as "○". If the above phenomenon occurred, it was judged as failing and recorded as "×".

[0127] <In-situ Thickness Test> Using an in-situ thickness tester (Xi'an Eruize New Energy Technology Co., Ltd.), the change in thickness of the negative electrode during the charge and discharge process was monitored, and a constant current charge and discharge test (discharge current 0.05-0.1 C, charge current 0.1 C, cycle 3) was performed on the battery at an operating pressure of 0.3 MPa and a voltage range of 0-1.5 V. The initial thickness (μm) of the negative electrode and the maximum increase in thickness (μm) during charge and discharge were measured, and the maximum thickness change rate was calculated from these. Maximum thickness change rate = Maximum thickness increase rate during charge and discharge / Initial thickness of negative electrode × 100%

[0128] Table 4 shows the test results of the parameters (surface density, surface capacity, compressed density) of the electrode sheets prepared using the negative electrode binder compositions of Examples 1 to 4 and Comparative Examples 1 to 4, as well as the peel strength, flexibility, thickness change, and electrical properties.

[0129]

[0130] As can be seen from the test results in Table 4, the binder compositions of Examples 1 to 3 use a TFE / P copolymer as the binder, have a content ratio of the thickener CMC to the binder of greater than 1.1, have a high peel strength between the negative electrode material produced from the binder composition and the current collector, and have a maximum thickness change rate of 10% or less during charge and discharge, effectively suppressing thickness changes in the electrode sheet during charge and discharge and suppressing a continuous increase in the thickness of the electrode sheet during charge and discharge.

[0131] Furthermore, as can be seen from Examples 1 to 3, the higher the content ratio of the thickener CMC to the binder, the higher the peel strength of the negative electrode material from the current collector, and the smaller the maximum rate of change in thickness of the electrode sheet during charge and discharge.

[0132] The binder composition of Example 4 uses a TFE / P / VdF copolymer with a VdF unit content of 40 mol% or less as the binder, and the content ratio of the thickener CMC to the binder is greater than 1.1. The binder composition has a high peel strength between the negative electrode material and the current collector, and the maximum change rate of the electrode sheet thickness during charge and discharge is 5%, effectively suppressing the change in thickness of the electrode sheet during charge and discharge and suppressing the continued increase in thickness of the electrode sheet during charge and discharge.

[0133] On the other hand, in the binder composition of Comparative Example 1, the same binder as in Examples 1 to 3 was used, but the content ratio of the thickener CMC to the binder was 0.4, and the negative electrode material produced using this binder composition had low peel strength and also failed the flexibility test, so it was unsuitable for use in manufacturing negative electrodes.

[0134] In the binder composition of Comparative Example 2, SBR was used as the binder, and the content ratio of CMC to SBR was 0.67. The negative electrode material produced using this binder composition had high peel strength, but the thickness of the electrode sheet changed from 5.9 to 7.2 to 7.3 μm during charge and discharge and tended to continue to increase. The maximum rate of change in the thickness of the electrode sheet during charge and discharge was 15%, and it was not possible to suppress the maximum rate of change in the thickness of the electrode sheet during charge and discharge to a low level, and it was not possible to suppress the increase in the thickness of the electrode sheet.

[0135] PVdF was used as the binder in the binder composition of Comparative Example 3. The thickness of the negative electrode material produced using this binder composition changed from 5.5 to 6.8 to 7.4 μm during charge and discharge, and tended to continue to increase, with the maximum change rate of the electrode sheet thickness during charge and discharge being 15%. The maximum change rate of the electrode sheet thickness during charge and discharge could not be suppressed to a low level, and the increase in the electrode sheet thickness could not be suppressed.

[0136] A VdF / TFE copolymer with a VdF unit content of 48 mol% was used as the binder composition of Comparative Example 4. The thickness of the negative electrode material produced using this binder composition changed from 4.9 μm to 5.5 μm to 6.8 μm during charge and discharge, and tended to continue increasing. The maximum change rate of the electrode sheet thickness during charge and discharge was 14%, and the maximum change rate of the electrode sheet thickness during charge and discharge could not be suppressed to a low level, and the increase in the electrode sheet thickness could not be suppressed.

[0137] The negative electrode binder composition of the present invention makes it possible to produce a negative electrode material containing a silicon-based active material, and can suppress changes in thickness when the produced electrode is used, so that the thickness does not increase even after repeated charge and discharge.As a result, it is possible to produce a negative electrode with stable performance, and a secondary battery including such a negative electrode.

[0138] Finally, the above-described embodiments and examples are illustrative in all respects and do not limit the present invention. It should be understood that a person skilled in the art could make various modifications without departing from the spirit of the present invention without any creative effort. The scope of the present invention is defined by the claims, not by the above-described embodiments and examples. Furthermore, the scope of the present invention includes all modifications that are equivalent to the scope of the claims and fall within the scope thereof.

[0139] The disclosure of Chinese Patent Application No. 202311720758.8, filed on December 14, 2023, is incorporated herein by reference in its entirety, and all documents, patent applications, and technical standards mentioned herein are incorporated herein by reference to the same extent as if each individual document, patent application, and technical standard was specifically and individually indicated to be incorporated by reference.

Claims

1. A binder composition for a negative electrode comprising a fluoropolymer, a negative electrode active material, a thickener and water, wherein the fluoropolymer has tetrafluoroethylene-based units but does not have vinylidene fluoride-based units, the amount of the thickener is greater than the amount of the fluoropolymer, and the negative electrode active material contains silicon atoms.

2. The negative electrode binder composition according to claim 1, wherein the thickener is carboxymethyl cellulose.

3. The binder composition for negative electrodes according to claim 1, wherein the weight ratio of said thickener to said fluorine-containing polymer is 1.1 to 10.

4. The negative electrode binder composition according to claim 1, wherein the fluorine-containing polymer further comprises propylene-based units.

5. The negative electrode binder composition according to claim 1, wherein a negative electrode material produced from said negative electrode binder composition has a maximum thickness change rate of 10% or less during charging and discharging.

6. A binder composition for a negative electrode comprising a fluorine-containing polymer, a negative electrode active material and water, wherein the fluorine-containing polymer has tetrafluoroethylene-based units and vinylidene fluoride-based units, the proportion of the vinylidene fluoride-based units to all units in the fluorine-containing polymer is less than 40 mol %, and the negative electrode active material contains silicon atoms.

7. The negative electrode binder composition according to claim 6, wherein said negative electrode binder composition further comprises a thickener, said thickener being carboxymethyl cellulose.

8. The negative electrode binder composition according to claim 7, wherein the weight ratio of said thickener to said fluorine-containing polymer is 1.1 to 10.

9. The negative electrode binder composition according to claim 6, wherein the fluorine-containing polymer further comprises propylene-based units.

10. The negative electrode binder composition according to claim 6, wherein a negative electrode material produced from said negative electrode binder composition has a maximum thickness change rate of 10% or less during charging and discharging.

11. A negative electrode for a secondary battery, produced by applying the negative electrode binder composition according to any one of claims 1 to 10 to a negative electrode current collector, and then removing water.

12. A secondary battery comprising the negative electrode for secondary batteries according to claim 11.

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