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

Abstract

Provided is a binder composition for negative electrodes, which contains a silicon-based active material, has little change in thickness during charging and discharging, and does not increase in thickness even if repeated charging and discharging are performed. This binder composition for negative electrodes contains a fluorine-containing polymer, a silicon-based negative electrode active material, a tackifier, and water, and is characterized in that: the fluorine-containing polymer has a unit derived from tetrafluoroethylene but does not have a unit derived from vinylidene fluoride; or has units based on tetrafluoroethylene and units based on vinylidene fluoride, the ratio of the units based on vinylidene fluoride to all the units in the fluorine-containing polymer is less than 40 mol%, and the amount of the tackifier is greater than the amount of the fluorine-containing polymer. Also provided are a negative electrode produced using the binder composition for negative electrodes, and a secondary battery provided with the negative electrode.

Description

Technical Field

[0001] The present invention relates to the field of batteries, and particularly to an adhesive composition for a negative electrode, a negative electrode for a secondary battery prepared from the adhesive composition for a negative electrode, and a secondary battery including the negative electrode for a secondary battery. Background Art

[0002] An adhesive composition for an electrode is prepared by dissolving or dispersing a polymer as an adhesive, an electrode active material, a conductive material, etc. in water or an organic solvent (for example, Patent Document 1). As an electrode active material, a negative electrode active material containing a silicon atom is known. Using a silicon-based negative electrode can effectively improve the energy density of a lithium-ion battery. However, when using a negative electrode active material containing a silicon atom, the negative electrode material obtained from the adhesive composition sometimes expands in volume and significantly changes in thickness during charge and discharge. As a result, the silicon negative electrode material is easily pulverized and peeled off from the current collector, ultimately affecting the capacity of the battery.

[0003] Therefore, there is a need for a negative electrode material with a small thickness change during charge and discharge and whose thickness does not increase even after repeated charge and discharge.

[0004] Prior Art Documents

[0005] Patent Document 1: International Publication No. WO2023007933A1 Summary of the Invention

[0006] In view of the problems existing in the prior art, the purpose of the present invention is to provide an adhesive composition for a negative electrode that includes a silicon-based active material, has a small thickness change during charge and discharge, and whose thickness does not increase even after repeated charge and discharge.

[0007] The present invention also aims to provide a negative electrode for a secondary battery prepared using the adhesive composition for a negative electrode of the present invention, and a secondary battery including the negative electrode for a secondary battery.

[0008] To achieve the above object, the present inventors conducted intensive research and found that when manufacturing an adhesive composition for a negative electrode using a negative electrode active material containing a silicon atom, by using a specific fluoropolymer having a unit based on tetrafluoroethylene as an adhesive, the thickness change of the negative electrode material obtained from the adhesive composition for a negative electrode during charge and discharge can be suppressed to a low level, and its thickness does not increase even after repeated charge and discharge, thereby completing the present invention.

[0009] The present invention provides the following technical content.

[0010] [1] An adhesive composition for a negative electrode, which is an adhesive composition for a negative electrode containing a fluoropolymer, a negative electrode active material, a tackifier, and water, wherein the fluoropolymer has units based on tetrafluoroethylene but does not have units based on vinylidene fluoride, the amount of the tackifier is greater than the amount of the fluoropolymer, and the negative electrode active material contains a silicon atom.

[0011] [2] The adhesive composition for a negative electrode according to [1], wherein the tackifier is carboxymethyl cellulose.

[0012] [3] The adhesive composition for a negative electrode according to [1], wherein the weight content ratio of the tackifier to the fluoropolymer is 1.1 to 10.

[0013] [4] The adhesive composition for a negative electrode according to [1], wherein the fluoropolymer further has units based on propylene.

[0014] [5] The adhesive composition for a negative electrode according to [1], wherein the maximum thickness change rate during charge and discharge of the negative electrode sheet made from the adhesive composition for a negative electrode is within 10%.

[0015] [6] An adhesive composition for a negative electrode, which is an adhesive composition for a negative electrode containing a fluoropolymer, a negative electrode active material, and water, wherein the fluoropolymer has units based on tetrafluoroethylene and units based on vinylidene fluoride, and the proportion of the units based on vinylidene fluoride relative to all the units in the fluoropolymer is less than 40 mol%, and the negative electrode active material contains a silicon atom.

[0016] [7] The adhesive composition for a negative electrode according to [6], wherein the adhesive composition for a negative electrode further contains a tackifier, and the tackifier is carboxymethyl cellulose.

[0017] [8] The adhesive composition for a negative electrode according to [7], wherein the weight content ratio of the tackifier to the fluoropolymer is 1.1 to 10.

[0018] [9] The adhesive composition for a negative electrode according to [6], wherein the fluoropolymer further has units based on propylene.

[0019]

[10] The adhesive composition for a negative electrode according to [6], wherein the maximum thickness change rate during charge and discharge of the negative electrode sheet made from the adhesive composition for a negative electrode is within 10%.

[0020]

[11] A negative electrode for a secondary battery, which is obtained by coating the negative electrode current collector with the adhesive composition for a negative electrode according to any one of [1] to

[10] above, and then removing the dispersion medium.

[0021]

[12] A secondary battery includes the negative electrode for a secondary battery described in the above

[11] .

[0022] Advantages of the Invention

[0023] According to the binder composition for a negative electrode of the present invention, even when a silicon-based active material is used, the binder composition for a negative electrode functions as a binder, and when formed into an electrode for use, it can suppress thickness change, and its thickness does not increase even after repeated charge and discharge.

[0024] According to the present invention, it is also possible to provide a negative electrode for a secondary battery obtained by using the binder composition for a negative electrode of the present invention, and a secondary battery including the negative electrode for a secondary battery.

[0025] Other aspects, features, and advantages of the present invention will become apparent in the following detailed description. Detailed Description of Embodiments

[0026] A "unit" in a polymer may be a group of atoms directly formed from a monomer by a polymerization reaction, or may be a group of atoms formed by treating a polymer obtained by a polymerization reaction by a specified method to convert a part of the structure. In addition, a unit based on monomer A is also denoted as monomer A unit.

[0027] The "average particle diameter (D50) of particles" is the volume-based cumulative 50% diameter of particles obtained by the laser diffraction scattering method. That is, the particle size distribution of particles is measured by the laser diffraction scattering method, and a cumulative curve is obtained with the total volume of the particles being 100%, and the particle diameter at the point where the cumulative volume reaches 50% on the cumulative curve.

[0028] In this specification, unless otherwise specified, the addition amount, content, concentration, and ratio described in the specification refer to the addition amount, content, concentration, and ratio based on mass.

[0029] In addition, "~" indicating a numerical range includes the upper limit and the lower limit of the numerical value.

[0030] 〔First Embodiment〕

[0031] The binder composition for a negative electrode (also referred to as "binder composition 1") according to the first embodiment of the present invention includes a fluorine-containing polymer, a negative electrode active material, a tackifier, and water. The fluorine-containing polymer has a unit based on tetrafluoroethylene but does not have a unit based on vinylidene fluoride. The amount of the tackifier is greater than the amount of the fluorine-containing polymer, and the negative electrode active material contains a silicon atom.

[0032] In this embodiment, the adhesive composition contains a fluoropolymer used as an adhesive. The above-mentioned fluoropolymer is a polymer having units composed of tetrafluoroethylene (TFE units) and not having units based on vinylidene fluoride, and is preferably a copolymer of a polymer having units composed of tetrafluoroethylene and units composed of propylene (TFE units / P units) and not having units based on vinylidene fluoride.

[0033] The fluorine-containing copolymer in this embodiment may also contain repeating units based on fluorine-containing monomers other than tetrafluoroethylene and vinylidene fluoride or repeating units based on hydrocarbon monomers other than propylene in a proportion of 10 mol% or less, specifically, to such an extent that the functions of the present invention are not impaired.

[0034] As fluorine-containing monomers other than tetrafluoroethylene and vinylidene fluoride, fluorine-containing olefins such as hexafluoropropylene, chlorotrifluoroethylene, and (perfluorobutyl)ethylene, and fluorine-containing vinyl ethers such as perfluoropropyl vinyl ether and perfluoromethyl vinyl ether can be used.

[0035] As hydrocarbon monomers other than propylene, 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 can be exemplified.

[0036] In this embodiment, the adhesive composition uses a silicon-based negative electrode active material, and thickness changes are likely to occur during charge and discharge, causing the adhesive composition to peel off from the current collector, ultimately resulting in a decrease in the capacity of the battery. When vinylidene fluoride is used as the adhesive, the thickness change of the adhesive composition tends to increase. Therefore, it is preferred that the adhesive does not contain vinylidene fluoride. In other words, it is preferred that the adhesive polymer does not contain repeating units based on vinylidene fluoride.

[0037] In the fluorine-containing copolymer in this embodiment, when it does not have repeating units based on vinylidene fluoride, the ideal composition of the fluorine-containing copolymer preferably has a ratio of TFE units / P units of 80 / 20 to 30 / 70 (mol%), more preferably 70 / 30 to 40 / 60 (mol%), and most preferably 60 / 40 to 50 / 50 (mol%).

[0038] If within this composition ratio range, the swelling with respect to the solvent of the electrolyte at high temperature is small, and when integrating the current collector and the electrode, the adhesion of the adhesive composition to the current collector is good.

[0039] When using the above-mentioned fluorine-containing copolymer as an adhesive, one copolymer can be used, or two or more copolymers having different polymerization unit compositions can be combined and used. In addition, other high molecular compounds can be used in combination as needed.

[0040] The above-mentioned fluorine-containing copolymer can be produced by known polymerization methods, and among them, radical copolymerization is preferred. There is no particular limitation on the radical polymerization method, and various radical polymerization methods can be used, but a method initiated by an organic or inorganic radical polymerization initiator, light, heat, ionizing radiation, etc. is preferred. As the polymerization method, it can be produced by currently known polymerization methods such as bulk polymerization, suspension polymerization, emulsion polymerization, solution polymerization, etc., and emulsion polymerization is preferred.

[0041] In the adhesive composition of the present embodiment, the weight-average molecular weight of the above-mentioned 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 lower than the lower limit value, it is likely to swell in the electrolyte, and if the molecular weight exceeds the upper limit value, the adhesiveness decreases. The weight-average molecular weight can be adjusted by known methods, such as the addition of a chain transfer agent, control of the polymerization temperature, polymerization pressure, etc.

[0042] After the adhesive composition containing the silicon-based active material of the present embodiment is coated on the current collector to form an electrode, the thickness of the adhesive composition changes greatly during charge and discharge. By using the above-mentioned fluorine-containing copolymer as the adhesive, it is possible to suppress the thickness change of the adhesive composition during charge and discharge, and its thickness will not increase even after repeated charge and discharge. Moreover, the adhesiveness between the current collector of the electrode and the electrode active material is high, and the charge and discharge characteristics are excellent.

[0043] Although the mechanism by which the use of the adhesive composition of the present embodiment can suppress the thickness change during charge and discharge is not very clear, it is considered that the above-mentioned fluorine-containing copolymer has excellent alkali resistance. Even if the adhesive composition contains a silicon-based active material with high alkalinity, the adhesive composition can still function as an adhesive, resulting in high adhesiveness between the current collector of the electrode and the electrode active material, and its thickness will not increase even after repeated charge and discharge.

[0044] In the present embodiment, the fluorine-containing copolymer used in the adhesive composition is preferably emulsified or dispersed in an aqueous medium, and more preferably emulsified or dispersed in the form of particles. The latex state is particularly preferred.

[0045] When the fluorine-containing copolymer is emulsified or dispersed as particles, the average particle diameter of the fluorine-containing copolymer is preferably 10 to 500 nm, more preferably 20 to 200 nm, further preferably 30 to 150 nm, still more preferably 50 to 150 nm, and particularly preferably 50 to 100 nm. If the average particle diameter is less than the lower limit value, the entire surface of the electrode active material will be densely covered, so the internal resistance is likely to increase. In addition, if the average particle diameter is greater than the upper limit value, the adhesive force of the binder is likely to decrease. The average particle diameter of the fine particles of the fluorine-containing copolymer can be adjusted by known methods such as the type and amount of the emulsifier. In addition, the average particle diameter of the fine particles of the binder can be measured by the dynamic light scattering method using a laser zeta potential meter ELS-8000 manufactured by Otsuka Electronics Co., Ltd.

[0046] In the binder composition of the present embodiment, the solid content concentration when using the fluorine-containing copolymer in a latex state is 5% to 95%. More preferably 20% to 60%, and further preferably 30% to 40%. If the concentration is 5% or more, it is preferred because of the good effect as a binder, and if it is 50% or less, it is preferred because of the good dispersibility.

[0047] In the present embodiment, the binder composition may contain a tackifier. The tackifier can increase the viscosity of the binder composition and can also act as a binder in the same way as the fluorine-containing copolymer. As the tackifier, a water-soluble adhesive resin can be used, such as carboxymethyl cellulose (CMC), methyl cellulose, etc., and carboxymethyl cellulose is preferred. Carboxymethyl cellulose has good wettability to particles such as electrode active materials and conductive fillers, but has weak suspension ability. By combining with the fluorine-containing copolymer, it can further improve the dispersion and suspension ability of particles such as electrode active materials and conductive fillers, which helps to obtain a dispersion-stable binder composition, so it is preferred.

[0048] In the present embodiment, the amount of the tackifier in the binder composition is greater than the amount of the above-mentioned fluorine-containing polymer. By making the amount of the tackifier greater than the amount of the above-mentioned fluorine-containing polymer, the adhesiveness between the current collector of the electrode and the electrode active material can be improved, and the thickness change of the binder composition during charge and discharge can be suppressed to a low level. The content ratio of the tackifier to the above-mentioned fluorine-containing polymer is preferably 1.1 or more, more preferably 2 or more, and particularly preferably 3 or more. In addition, the content ratio is preferably 10 or less, more preferably 9 or less, and particularly preferably 8 or less. By making the above content ratio within the range of 1.1 to 10, the binder effect of the tackifier can be further exerted, the adhesiveness between the current collector of the electrode and the electrode active material can be improved, and at the same time, the physical properties of the fluorine-containing polymer can be further exerted, and the thickness change of the binder composition during charge and discharge can be suppressed to a low level, for example, the maximum thickness change rate can be suppressed to 10% or less, so it is preferred.

[0049] The negative electrode binder composition of the present embodiment contains a negative electrode active material containing a silicon atom. The negative electrode active material containing a silicon atom at least contains a graphite-based carbon material and an active material mainly composed of silicon (also referred to as a silicon-based active material) capable of alloying with lithium. Examples of the graphite-based carbon material include graphite (natural graphite, artificial graphite), graphitized pitch-based carbon fiber, and the like. The silicon-based active material is not particularly limited, and examples thereof include Si, SiO, SiO2, SiO x and the like. In addition, from the viewpoint of suppressing the expansion of the negative electrode active material itself, as the silicon-based active material, it is preferable to use SiO formed by at least one of SiO and SiO2 and Si x (0.01 ≤ x < 2). Here, the SiO x refers to a general term for non-stoichiometric silicon monoxide having a structure in which nano-level Si crystals are precipitated in an amorphous SiO2 phase. The production method and characteristics of this compound have been disclosed in, for example, Japanese Patent Laid-Open No. 2002-47404, Journal of Power Sources 170 (2007) 456-459, and the like. As the production method of this compound, there is no particular limitation, and examples thereof include a method of cooling / precipitating silicon monoxide gas generated by heating a mixture of SiO2 and Si, and a method of obtaining this compound by heating SiO to promote the disproportionation reaction between Si and SiO2. As the negative electrode active material containing a silicon atom, a mixture of graphite and a silicon-based active material is preferably used, and a mixture of graphite and the above-mentioned SiO x active material is more preferably used. As an example of the negative electrode active material containing a silicon atom, SL500A-SOC nano-silicon carbon (manufactured by Liyang Tianmu Pilot Battery Materials Technology Co., Ltd.) can be cited.

[0050] The binder composition of the present embodiment contains water as a dispersion medium. However, within the range that does not affect the effects of the present invention, 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, dimethyl sulfoxide, can also be added to water. These organic solvents can be used alone or in combination of two or more. As the water-soluble organic solvent, N-methyl-2-pyrrolidone or N-ethyl-2-pyrrolidone is preferable, and N-methyl-2-pyrrolidone is more preferable. By appropriately adding a water-soluble organic solvent, the uniform mixing of each component can be promoted or the viscosity of the binder composition can be adjusted. The content of the water-soluble organic solvent in the dispersion medium is preferably small, preferably 1% to 5%, more preferably 1% to 3%. The binder composition of the present invention is most preferably substantially free of a water-soluble organic solvent.

[0051] The adhesive composition of the present embodiment may contain a conductive material. By including a conductive material, the electrical contact between electrode active materials can be improved and the resistance within the active material layer can be reduced. As the conductive material, at least one selected from carbon black, carbon fiber, carbon nanotubes, conductive graphite, and graphene is preferred. By adding a small amount of the conductive material, the effect of reducing the resistance is large and good.

[0052] In the adhesive composition of the present embodiment, surfactants, emulsifiers, dispersants, defoamers, thickeners, etc. commonly used in the art may also be added within the range that does not affect the effects of the present invention.

[0053] The solid content in the adhesive composition of the present embodiment may be 30% or more, preferably 35% or more. In addition, in order to obtain a dispersion-stable adhesive composition and avoid sedimentation caused by too high a solid content, the solid content is preferably 60% or less, more preferably 50% or less.

[0054] In the solid components of the adhesive composition of the present embodiment, relative to 100 parts by mass of the total content of all solid components, the content of the negative electrode active material is preferably 80 to 99 parts by mass, and the content of the adhesive (fluoropolymer and tackifier) is preferably 1 to 20 parts by mass. Specifically, when the adhesive is composed of a fluoropolymer and a tackifier, the content of the fluoropolymer is preferably 0.3 to 5 parts by mass, the content of the tackifier is preferably 0.5 to 15 parts by mass, and the content of the tackifier is greater than the content of the fluoropolymer. When the adhesive composition contains a conductive material, the conductive material is preferably used within the range where the contents of the negative electrode active material and the adhesive satisfy the above 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 fluoropolymer is 0.3 to 5 parts by mass, the content of the tackifier is 0.5 to 10 parts by mass, and the usage amount of the conductive material is 0 to 5 parts by mass.

[0055] The maximum thickness change rate of the negative electrode sheet prepared from the negative electrode adhesive composition 1 of the present embodiment during charge and discharge is within 10%, preferably within 8%, and more preferably within 5%.

[0056] According to the negative electrode adhesive composition 1 of the present embodiment, even when a silicon-based active material is used, the negative electrode adhesive composition functions as an adhesive, and when it is made into an electrode for use, it can suppress thickness changes and its thickness will not increase even after repeated charge and discharge.

[0057] 〔Second Embodiment〕

[0058] The negative electrode binder composition of the second embodiment of the present invention (also referred to as "binder composition 2") contains a fluoropolymer, a negative electrode active material, and water. The above-mentioned fluoropolymer has units based on tetrafluoroethylene and units based on vinylidene fluoride. The proportion of the above-mentioned units based on vinylidene fluoride relative to all units in the fluoropolymer is less than 40 mol%, and the above-mentioned negative electrode active material contains silicon atoms.

[0059] In this embodiment, the binder composition contains a fluoropolymer used as a binder. The above-mentioned fluoropolymer has units based on tetrafluoroethylene (TFE units) and units based on vinylidene fluoride (VdF units), and the proportion of the above-mentioned units based on vinylidene fluoride relative to all units in the fluoropolymer is less than 40 mol%. The above-mentioned fluoropolymer preferably further has units composed of propylene (P units).

[0060] The fluorine-containing copolymer in this embodiment may also contain repeating units based on fluorine-containing monomers other than tetrafluoroethylene and vinylidene fluoride or repeating units based on hydrocarbon monomers other than propylene in a proportion of 10 mol% or less without impairing the functions of the present invention.

[0061] As fluorine-containing monomers other than tetrafluoroethylene and vinylidene fluoride, fluorine-containing olefins such as hexafluoropropylene, chlorotrifluoroethylene, and (perfluorobutyl)ethylene, and fluorine-containing vinyl ethers such as perfluoropropyl vinyl ether and perfluoromethyl vinyl ether can be used.

[0062] 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.

[0063] In this embodiment, a silicon-based negative electrode active material is used in the binder composition, and thickness changes are likely to occur during charge and discharge, causing the binder composition to peel off from the current collector, ultimately resulting in a decrease in the capacity of the battery. When vinylidene fluoride is used as the binder, the thickness change of the binder composition tends to increase. Therefore, it is preferred that the amount of vinylidene fluoride or repeating units based on vinylidene fluoride in the binder is small. Specifically, the proportion of units based on vinylidene fluoride relative to all units in the fluoropolymer is less than 40 mol%, more preferably less than 35 mol%, further preferably less than 30 mol%, further preferably less than 20 mol%, and particularly preferably less than 10 mol%.

[0064] When the fluorine-containing copolymer used in this embodiment contains TFE units, P units, and PVDF units, its composition preferably has a ratio of TFE units / P units / PVDF units in the range of 30 - 85 / 15 - 70 / 1 - 40 (mol%), more preferably 30 - 70 / 20 - 60 / 1 - 35 (mol%).

[0065] If it is within the range of this composition ratio, the swelling at high temperature with respect to the solvent of the electrolyte is small, and when integrating the current collector and the electrode, the adhesiveness of the adhesive composition to the current collector is good.

[0066] When using the above fluorine-containing copolymer as an adhesive, one kind of copolymer can be used, or two or more kinds of copolymers having different polymerization unit compositions can also be used in combination. In addition, other polymer compounds can be used in combination as needed.

[0067] The fluorine-containing copolymer in this embodiment can be produced by the same polymerization method as the fluorine-containing copolymer in the first embodiment, and the preferred form is also the same.

[0068] After coating the current collector with the adhesive composition containing the silicon-based active material of this embodiment to form an electrode, the thickness of the adhesive composition changes greatly during charge and discharge. By using the above fluorine-containing copolymer as an adhesive, the thickness change of the adhesive composition during charge and discharge can be suppressed, and its thickness will not increase even after repeated charge and discharge. Moreover, the mutual adhesiveness between the current collector of the electrode and the electrode active material is high, and the charge and discharge characteristics are excellent.

[0069] Although the mechanism by which the use of the adhesive composition of this embodiment can suppress the thickness change during charge and discharge is not very clear, it is considered that the above fluorine-containing copolymer has excellent alkali resistance. Even if the adhesive composition contains a silicon-based active material with high alkalinity, the adhesive composition can still play the role of an adhesive, making the mutual adhesiveness between the current collector of the electrode and the electrode active material high, and its thickness will not increase even after repeated charge and discharge.

[0070] In this embodiment, the adhesive composition preferably further contains a tackifier. The tackifier can increase the viscosity of the adhesive composition and can play the role of an adhesive in the same way as the fluorine-containing copolymer. As the tackifier, a water-soluble adhesive resin can be used, such as carboxymethyl cellulose (CMC), methyl cellulose, etc., and carboxymethyl cellulose is preferred. The amount of the tackifier in the adhesive composition is preferably greater than the amount of the above fluorine-containing polymer. By making the amount of the tackifier greater than the amount of the above fluorine-containing polymer, the mutual adhesiveness between the current collector of the electrode and the electrode active material can be improved, and the thickness change of the adhesive composition during charge and discharge can be suppressed to a low level. The content ratio of the tackifier to the above fluorine-containing polymer is preferably 1.1 to 10, more preferably 2 to 9, and particularly preferably 3 to 8. By making the content ratio of the tackifier to the fluorine-containing polymer within the above range, the adhesive effect of the tackifier can be further exerted, the mutual adhesiveness between the current collector of the electrode and the electrode active material can be improved, and at the same time, the physical properties of the fluorine-containing polymer can be further exerted, and the thickness change of the adhesive composition during charge and discharge can be suppressed to a low level, for example, the maximum thickness change rate can be suppressed to less than 10%.

[0071] Except that the composition of the fluoropolymer contained in the negative electrode binder composition 2 of this embodiment is different from that of the first embodiment, other configurations and preferred forms are the same as those of the first embodiment, and the description of the same configurations is omitted.

[0072] The maximum thickness change rate of the negative electrode sheet prepared from the negative electrode binder composition 2 of this embodiment during charge and discharge is within 10%, preferably within 8%, and more preferably within 5%.

[0073] According to the negative electrode binder composition 2 of this embodiment, even when a silicon-based active material is used, the negative electrode binder composition functions as a binder, and when used to make an electrode, it can suppress thickness change, and its thickness will not increase even after repeated charge and discharge.

[0074] 〔Manufacturing method of binder composition〕

[0075] The binder composition of the present invention can be obtained by mixing a negative electrode active material and a conductive material as required, adding an aqueous solution of a tackifier and water, stirring evenly, and then adding a fluoropolymer as a binder and stirring evenly. Among them, the preferred forms and contents of the negative electrode active material, conductive material, fluoropolymer, tackifier, and dispersion medium are as described above.

[0076] In the manufacturing method of the above binder composition, when mixing the negative electrode active material and the conductive material, it is preferred to grind the negative electrode active material and the conductive material until there is no obvious particle feeling, and then use a blender for mixing. As the blender, there is no particular limitation, and conventional blenders, mixers, homogenizers, etc. in the art can be used. From the perspective of being able to stir evenly and being able to defoam in subsequent steps, it is preferred to use a planetary homogenizer. The stirring rate is 1800 - 2500 rpm, preferably 2000 - 2200 rpm, and the stirring time is 1 - 20 minutes, preferably 2 - 10 minutes.

[0077] When adding the tackifier, it is preferred to add it in the form of an aqueous solution of the tackifier. As the concentration of the tackifier, there is no particular limitation, and it can be 0.5 - 30% by mass, preferably 1 - 10% by mass.

[0078] The aqueous solution of the tackifier can be added at once or in several portions. From the perspective of obtaining a more stable dispersion of the binder composition, it is preferably added in more than 2 portions, for example, 4, 5, 6, 8, or 10 portions. On the other hand, from the perspective of improving the addition efficiency or simplifying the operation process, it is preferably added in 5 portions or less. After adding the aqueous solution of the tackifier each time, it is preferably stirred thoroughly and uniformly using a planetary homogenizer at a stirring rate of 1800 - 2500 rpm, preferably 2000 - 2200 rpm, and the stirring time is 5 - 30 minutes, preferably 10 - 15 minutes.

[0079] After adding the fluoropolymer, homogenization mixing can be carried out using a ball mill, preferably a planetary ball mill. When using a planetary ball mill, the stirring rate is 300 - 800 rpm, preferably 400 - 500 rpm, and the stirring time is 15 - 60 seconds, preferably 20 - 30 seconds. By stirring at a low stirring rate using a ball mill, the materials can be homogenized and mixed sufficiently to obtain a dispersion with high dispersion stability and few foams.

[0080] In some preferred embodiments, the binder composition for the electrode comprises a fluoropolymer, CMC, a silicon-based anode active material, a conductive material, and water. Among them, in the solid components of the binder composition, the content ratio of the silicon-based anode active material, fluoropolymer, CMC, and conductive material is 90 - 98:0.3 - 5:0.5 - 10:1 - 5. Its manufacturing method includes: after mixing the silicon-based anode active material and the conductive material and grinding them until there is no obvious particle feeling, stirring and mixing using a planetary homogenizer; then adding the CMC aqueous solution with a concentration of 1 - 10% by mass for the first time to make the solid content of the dispersion reach 50 - 60% by mass, and stirring and mixing using a planetary homogenizer, then adding the CMC aqueous solution with a concentration of 1 - 10% by mass and water to make the solid content of the dispersion reach 30 - 40% by mass, and stirring and mixing using a planetary homogenizer; finally, adding the latex of the fluoropolymer and carrying out homogenization mixing using a ball mill.

[0081] 〔Negative electrode for secondary battery and secondary battery〕

[0082] A secondary battery generally includes a positive electrode, a negative electrode, a separator, and an electrolyte. In the secondary battery of the present invention, the electrode prepared using the binder composition for the negative electrode of the present invention (binder composition for the negative electrode 1 or 2) can be used for the negative electrode (hereinafter also referred to as the negative electrode for the secondary battery). As the counter electrode, a lithium alloy such as lithium metal or lithium aluminum alloy can be used.

[0083] The manufacturing method of the negative electrode for the secondary battery of the present invention is a method of coating the binder composition for the negative electrode of the present invention on a current collector and then removing the dispersion medium. In addition, after removing the dispersion medium, it is preferably formed into a desired thickness by pressure as needed.

[0084] As a method for coating the negative electrode binder composition of the present invention on a current collector, various coating methods can be exemplified. For example, a method of coating using a coating tool such as a doctor blade can be exemplified. There is no particular limitation on the coating temperature, but a temperature near room temperature is generally preferred.

[0085] Removal of the dispersion medium is generally preferably carried out by drying at room temperature or by heating. Drying can be carried out using various dryers, and examples include a heating type vacuum dryer. There is no particular limitation on the drying temperature, but a temperature of room temperature to 150 °C is generally preferred.

[0086] As an extrusion method, a die press or a roll press can be used. The thickness of the coating layer of the binder composition, in terms of the thickness after drying, or in the case of further extrusion, in terms of the thickness after extrusion, is preferably 0.5 to 2000 μm, more preferably 1 to 1000 μm, and particularly preferably 10 to 500 μm.

[0087] As the current collector of the present invention, any current collector made of a conductive material can be used without particular limitation. Generally, examples include metal foils such as aluminum, nickel, stainless steel, and copper, metal meshes, metal porous bodies, etc. 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 it is less than 1 μm, the durability of the battery is insufficient, and the reliability of the battery may be low. In addition, if it exceeds 100 μm, the mass of the battery increases.

[0088] The secondary battery electrode of the present invention can be used in batteries of any shape such as cylindrical, sheet-shaped, and square-shaped.

[0089] Moreover, a secondary battery in which the negative electrode uses the electrode of the present invention, and the positive electrode and the negative electrode are accommodated in a housing together with a separator and an electrolytic solution has high reliability even at high temperatures.

[0090] As the separator, a microporous polymer membrane can be used, and examples of its material include polypropylene resin, polyethylene resin, polypropylene - polyethylene - polypropylene, etc.

[0091] Examples of the solvent of the electrolytic solution include aprotic organic solvents such as isopropyl carbonate, ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, γ-butyrolactone, and diethoxyethane. In addition, examples of the electrolyte include lithium salts such as LiClO4, LiBF4, LiPF6, LiAsF5, CF3SO3Li, and (CF3SO2)2NLi.

[0092] In some preferred embodiments, a negative electrode prepared from the binder composition for a negative electrode of the present invention is used, and a secondary battery is assembled through the following steps: (1) Place the prepared positive electrode sheet in the center of the positive electrode case, drop an electrolyte solution thereon, and completely soak the positive electrode sheet; (2) Place the separator flat on the positive electrode sheet, then drop an electrolyte solution thereon, and completely soak the separator; (3) Place a lithium sheet as a counter electrode on the separator; (4) Successively place a gasket and a spring piece on the lithium sheet to make them in the central position of the battery, and then fasten the negative electrode case; (5) Use a packaging machine to perform pressure packaging to obtain a button-type half cell.

[0093] The negative electrode for a secondary battery according to the present invention has a small thickness change during charge and discharge, and the thickness will not increase even after repeated charge and discharge. Specifically, the maximum thickness change rate of the negative electrode material prepared from the binder composition for a negative electrode of the present invention during charge and discharge is within 10%.

[0094] Examples

[0095] The constitution and advantages of the present invention will be further described below through examples. However, it should be understood that the following examples are only illustrative of the implementation of the present invention and are not intended to limit the protection scope of the present invention.

[0096] First, the preparation method of the fluoropolymer used in the examples and comparative examples will be described.

[0097] (Production Example 1)

[0098] After degassing the inside of a 3200 mL stainless steel pressure-resistant reactor equipped with an anchor-type blade 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-butanol were added to the reactor. Further, an aqueous solution prepared by dissolving 0.27 g of disodium ethylenediaminetetraacetate dihydrate and 0.36 g of ferrous sulfate heptahydrate in 200 g of ion-exchanged water was added. Then, a monomer mixed gas of TFE / P = 88 / 12 (molar ratio) was pressed in at 40°C so that the internal pressure of the reactor was 2.50 MPaG. The anchor-type blade was rotated at 300 rpm, and an aqueous solution of 2.5 mass% sodium hydroxymethanesulfinate dihydrate (hereinafter also referred to as Rongalite) adjusted to pH 10.0 with sodium hydroxide was added to initiate the polymerization reaction. Hereinafter, an aqueous solution of 2.5 mass% Rongalite was continuously added using a high-pressure pump.

[0099] As the polymerization proceeds, the pressure inside the reactor decreases. Therefore, at the time point when the internal pressure of the reactor drops to 2.49 MPaG, a monomer mixed gas of TFE / P = 56 / 44 (molar ratio) is pressured into the reactor under its own pressure until the internal pressure of the reactor rises to 2.51 MPaG. This operation is repeated to keep the internal pressure of the reactor at 2.49 - 2.51 MPaG, and the polymerization reaction is continued. At the time point when the total amount of the pressured monomer mixed gas of TFE / P reaches 700 g, the internal temperature of the reactor is cooled to 10 °C, the polymerization reaction is stopped, and a latex containing fluorinated copolymer 1 is obtained. The content of fluorinated copolymer 1 in the latex is 33% by mass, and the copolymer unit composition is TFE unit / P unit = 56 / 44 (molar ratio).

[0100] (Production Example 2)

[0101] After degassing the inside of a 3200 mL stainless steel pressure-resistant reactor equipped with an anchor-type blade 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-butanol are added to the reactor. Further, an aqueous solution prepared by dissolving 0.27 g of disodium ethylenediaminetetraacetate dihydrate and 0.36 g of ferrous sulfate heptahydrate in 200 g of ion-exchanged water is added. Then, a monomer mixed gas of TFE / P / VDF = 25 / 5 / 70 (molar ratio) is pressured into the reactor at 25 °C to make the internal pressure of the reactor 2.30 MPaG. The anchor-type blade is rotated at 300 rpm, and an aqueous solution of 2.5% by mass of sodium hydroxymethanesulfinate dihydrate (hereinafter also referred to as Rongalite) adjusted to pH 10.0 with sodium hydroxide is added to initiate the polymerization reaction. Hereinafter, an aqueous solution of 2.5% by mass of Rongalite is continuously added using a high-pressure pump.

[0102] As the polymerization proceeds, the pressure inside the reactor decreases. Therefore, at the time point when the internal pressure of the reactor drops to 2.29 MPaG, a monomer mixed gas of TFE / P / VdF = 40 / 25 / 35 (molar ratio) is pressured into the reactor under its own pressure until the internal pressure of the reactor rises to 2.31 MPaG. This operation is repeated to keep the internal pressure of the reactor at 2.29 - 2.31 MPaG, and the polymerization reaction is continued. At the time point when the total amount of the pressured monomer mixed gas of TFE / P / VdF reaches 770 g, the internal temperature of the reactor is cooled to 10 °C, the polymerization reaction is stopped, and a latex containing fluorinated copolymer 2 is obtained. The content of fluorinated copolymer 2 in the latex is 33% by mass, and the copolymer unit composition is TFE unit / P unit / VdF unit = 40 / 25 / 35 (molar ratio).

[0103] (Production Example 3)

[0104] An aqueous solution of 3300ppm F(CF2)5COONH4 and 200ppm CH2=CFCF2OCF(CF3)CF2OCF(CF3)COONH4 was added to a stainless steel pressure-resistant reactor with an internal volume of 3000mL and an anchor blade for stirring, and the polymerization tank was sealed. The tank was evacuated after replacing the vacuum with nitrogen, and 400cc of isopropyl alcohol as a chain transfer agent was added by syringe while evacuating. Thereafter, a mixed gas monomer of 80 / 20 mol% of vinylidene fluoride (VdF) / tetrafluoroethylene (TFE) was added to the tank while stirring at 70°C to 0.8MPaG. Thereafter, an aqueous solution of ammonium persulfate dissolved in an amount equivalent to 100ppm was pressed into it with nitrogen to initiate the reaction. In order to maintain the pressure in the tank, a mixed monomer of 48 / 52 mol% of VdF / TFE was added. When the amount of the additional monomer reached 346 g, the internal temperature of the reactor was cooled to 10°C to terminate the polymerization reaction, thereby obtaining a latex containing the fluorinated copolymer 3. The content of the fluorinated copolymer 3 in the latex was 20% by mass, and the copolymer unit composition was VdF unit / TFE unit = 48 / 52 (molar ratio).

[0105] The latex of fluorinated copolymer 1, the latex of fluorinated copolymer 2 and the latex of fluorinated copolymer 3 were prepared by the above method and set aside for use.

[0106] [Example 1]

[0107] 15 g of silicon-carbon negative electrode material (Lianyang Tianmu Pioneer Battery Material Technology Co., Ltd., SL500A-SOC, D50 is 10.9 μm) and 0.173 g of carbon black Super P (Guangdong Candlelight New Energy Technology Co., Ltd., MA-EN-CO-01) as a conductive material were put into a mortar and ground until there was no obvious granularity. Then, a planetary homogenizer (KURABO Mazerustar planetary stirring degassing machine) was used to dry mix at 2000 rpm for 2 minutes.

[0108] 13.4 g of a 2 mass % CMC aqueous solution was added so that the solid content after the addition was 54 mass %, and the mixture was homogenized at 2000 rpm for 10 minutes using a planetary homogenizer (KURABO Mazerustar planetary stirring deaerator).

[0109] Next, 11.06 g of a 2% by mass CMC (Daicel Co., Ltd., CMC2200) aqueous solution and 1.6 g of water were added to give a solid content of 38% by mass, and the mixture was homogenized at 2000 rpm for 10 minutes using a planetary homogenizer (KURABO Mazerustar planetary stirring deaerator).

[0110] Finally, 0.185 g of the latex of fluorine-containing copolymer 1 (TFE / P copolymer latex, TFE unit / P unit = 56 / 44 (molar ratio), solid content 33% by mass) as an adhesive was added, and after ball milling for 0.5 minutes at 500 revolutions per minute using a ball mill (QM-3SP04 planetary ball mill), a uniformly mixed aqueous negative electrode slurry was obtained (denoted as the adhesive composition A1 for the negative electrode).

[0111] [Example 2]

[0112] 15 g of the silicon-carbon negative electrode material and 0.173 g of the conductive material Super P were placed in a mortar and ground until there was no obvious sense of particles, and then dry-mixed for 2 minutes at 2000 revolutions per minute using a planetary homogenizer (KURABO Mazerustar planetary stirring and defoaming machine).

[0113] 13.4 g of a 2% by mass aqueous CMC solution was added, and the solid content after addition reached 54% by mass. Then, it was homogenized for 10 minutes at 2000 revolutions per minute using a planetary homogenizer (KURABO Mazerustar planetary stirring and defoaming machine).

[0114] Next, 6.25 g of a 2% by mass aqueous CMC solution and 6.10 g of water were added, and the solid content after addition reached 38% by mass. Then, it was homogenized for 10 minutes at 2000 revolutions per minute using a planetary homogenizer (KURABO Mazerustar planetary stirring and defoaming machine).

[0115] Finally, 0.476 g of the latex of fluorine-containing copolymer 1 (TFE / P copolymer latex, TFE unit / P unit = 56 / 44 (molar ratio), solid content 33% by mass) as an adhesive was added, and after ball milling for 0.5 minutes at 500 revolutions per minute using a ball mill (QM-3SP04 planetary ball mill), a uniformly mixed aqueous negative electrode slurry was obtained (denoted as the adhesive composition A2 for the negative electrode).

[0116] [Example 3]

[0117] 15 g of the silicon-carbon negative electrode material and 0.173 g of the conductive material Super P were placed in a mortar and ground until there was no obvious sense of particles, and then dry-mixed for 2 minutes at 2000 revolutions per minute using a planetary homogenizer (KURABO Mazerustar planetary stirring and defoaming machine).

[0118] 13.4 g of a 2% by mass aqueous CMC solution was added, and the solid content after addition reached 54% by mass. Then, it was homogenized for 10 minutes at 2000 revolutions per minute using a planetary homogenizer (KURABO Mazerustar planetary stirring and defoaming machine).

[0119] Next, 2.32 g of a 2 mass% aqueous CMC solution and 9.80 g of water were added. After the addition, the solid content reached 38 mass%. It was homogenized for 10 minutes at 2000 revolutions per minute using a planetary homogenizer (KURABO Mazerustar planetary stirring and defoaming machine).

[0120] Finally, 0.715 g of the latex of fluorine-containing copolymer 1 as an adhesive (TFE / P copolymer latex, TFE unit / P unit = 56 / 44 (molar ratio), solid content 33 mass%) was added. After ball-milling for 0.5 minutes at 500 revolutions per minute using a ball mill (QM-3SP04 planetary ball mill), a uniformly mixed aqueous negative electrode slurry was obtained (denoted as negative electrode binder composition A3).

[0121] [Example 4]

[0122] 15 g of a silicon-carbon negative electrode material and 0.173 g of conductive material Super P were placed in a mortar and ground until there was no obvious sense of granularity, and then dry-mixed for 2 minutes at 2000 revolutions per minute using a planetary homogenizer (KURABO Mazerustar planetary stirring and defoaming machine).

[0123] 13.4 g of a 2 mass% aqueous CMC solution was added. After the addition, the solid content reached 54 mass%. It was homogenized for 10 minutes at 2000 revolutions per minute using a planetary homogenizer (KURABO Mazerustar planetary stirring and defoaming machine).

[0124] Next, 11.06 g of a 2 mass% aqueous CMC solution and 1.6 g of water were added. After the addition, the solid content reached 38 mass%. It was homogenized for 10 minutes at 2000 revolutions per minute using a planetary homogenizer (KURABO Mazerustar planetary stirring and defoaming machine).

[0125] Finally, 0.185 g of the latex of fluorine-containing copolymer 2 as an adhesive (TFE / P / VdF copolymer latex, TFE unit / P unit / VdF unit = 40 / 25 / 35 (molar ratio), solid content 33 mass%) was added. After ball-milling for 0.5 minutes at 500 revolutions per minute using a ball mill (QM-3SP04 planetary ball mill), a uniformly mixed aqueous negative electrode slurry was obtained (denoted as negative electrode binder composition A4).

[0126] [Comparative Example 1]

[0127] 15 g of a silicon-carbon negative electrode material and 0.173 g of conductive material Super P were placed in a mortar and ground until there was no obvious sense of granularity, and then dry-mixed for 2 minutes at 2000 revolutions per minute using a planetary homogenizer (KURABO Mazerustar planetary stirring and defoaming machine).

[0128] Add 7.86 g of an aqueous CMC solution with a concentration of 2% by mass and 5.5 g of water. After the addition, the solid content reaches 54% by mass. Homogenize the mixture for 10 minutes at 2000 revolutions per minute using a planetary homogenizer (KURABO Mazerustar planetary stirring and defoaming machine).

[0129] Then add 11.8 g of water. After the addition, the solid content reaches 38% by mass. Homogenize the mixture for 10 minutes at 2000 revolutions per minute using a planetary homogenizer (KURABO Mazerustar planetary stirring and defoaming machine).

[0130] Finally, add 1.191 g of the latex of fluorine-containing copolymer 1 (TFE / P copolymer latex, TFE unit / P unit = 56 / 44 (molar ratio), solid content 33% by mass) as the binder. After ball-milling for 0.5 minutes at 500 revolutions per minute using a ball mill (QM-3SP04 planetary ball mill), a uniformly mixed aqueous anode slurry is obtained (denoted as anode binder composition B1).

[0131] [Comparative Example 2]

[0132] Put 15 g of the silicon-carbon anode material and 0.173 g of the conductive material Super P into a mortar and grind until there is no obvious sense of particles. Then, perform dry mixing for 2 minutes at 2000 revolutions per minute using a planetary homogenizer (KURABO Mazerustar planetary stirring and defoaming machine).

[0133] Add 11.01 g of an aqueous CMC solution with a concentration of 2% by mass and 2.3 g of water. After the addition, the solid content reaches 54% by mass. Homogenize the mixture for 10 minutes at 2000 revolutions per minute using a planetary homogenizer (KURABO Mazerustar planetary stirring and defoaming machine).

[0134] Then add 12.0 g of water. After the addition, the solid content reaches 38% by mass. Homogenize the mixture for 10 minutes at 2000 revolutions per minute using a planetary homogenizer (KURABO Mazerustar planetary stirring and defoaming machine).

[0135] Finally, add 0.726 g of styrene-butadiene latex resin (SBR) latex (JSR Corporation, TRD104A, solid content 45.5% by mass) as the binder. After ball-milling for 0.5 minutes at 500 revolutions per minute using a ball mill (QM-3SP04 planetary ball mill), a uniformly mixed aqueous anode slurry is obtained (denoted as anode binder composition B2).

[0136] [Comparative Example 3]

[0137] Put 15 g of the silicon-carbon anode material and 0.173 g of the conductive material Super P into a mortar and grind until there is no obvious sense of particles, then use a planetary homogenizer (KURABO Mazerustar planetary stirring and degassing machine) to dry mix for 2 minutes at 2000 revolutions per minute.

[0138] Add 13.4 g of an aqueous CMC solution with a concentration of 2% by mass. After the addition, the solid content reaches 54% by mass. Use a planetary homogenizer (KURABO Mazerustar planetary stirring and degassing machine) to homogenize for 10 minutes at 2000 revolutions per minute.

[0139] Then add 2.32 g of an aqueous CMC solution with a concentration of 2% by mass and 9.8 g of water. After the addition, the solid content reaches 38% by mass. Use a planetary homogenizer (KURABO Mazerustar planetary stirring and degassing machine) to homogenize for 10 minutes at 2000 revolutions per minute.

[0140] Finally, add 1.18 g of a dispersion (solid content 20% by mass) of PVDF (Arkema, HSV900) as an adhesive, and use a ball mill (QM-3SP04 planetary ball mill) to ball mill for 0.5 minutes at 500 revolutions per minute to obtain a uniformly mixed aqueous anode slurry (denoted as the adhesive composition B3 for the anode).

[0141] [Comparative Example 4]

[0142] Put 15 g of the silicon-carbon anode material and 0.173 g of the conductive material Super P into a mortar and grind until there is no obvious sense of particles, then use a planetary homogenizer (KURABO Mazerustar planetary stirring and degassing machine) to dry mix for 2 minutes at 2000 revolutions per minute.

[0143] Add 13.4 g of an aqueous CMC solution with a concentration of 2% by mass. After the addition, the solid content reaches 54% by mass. Use a planetary homogenizer (KURABO Mazerustar planetary stirring and degassing machine) to homogenize for 10 minutes at 2000 revolutions per minute.

[0144] Then add 2.32 g of an aqueous CMC solution with a concentration of 2% by mass and 9.8 g of water. After the addition, the solid content reaches 38% by mass. Use a planetary homogenizer (KURABO Mazerustar planetary stirring and degassing machine) to homogenize for 10 minutes at 2000 revolutions per minute.

[0145] Finally, 1.18 g of the latex of the fluorine-containing copolymer 3 (VdF / TFE copolymer latex, VdF unit / TFE unit = 48 / 52 (molar ratio), solid content 20% by mass) as an adhesive was added, and after ball-milling for 0.5 minutes at 500 revolutions per minute using a ball mill (QM-3SP04 planetary ball mill), a uniformly mixed aqueous negative electrode slurry was obtained (denoted as the adhesive composition B4 for the negative electrode).

[0146] The compositions of the adhesive compositions for the negative electrodes of Examples 1 to 4 and Comparative Examples 1 to 4 are summarized in Table 2.

[0147] [Table 2]

[0148]

[0149] In Table 2, in the adhesive composition A1 for the negative electrode of Example 1, water was used as the dispersion medium, and the content ratio of the negative electrode active material, the conductive material, and the adhesive in the solid component was 95.4:1.1:3.5, and the content ratio of the fluorine-containing copolymer 1 to CMC as the adhesive was 1 / 8. The compositions of the adhesive compositions for the negative electrodes of Examples 2 to 4 and Comparative Examples 1 to 4 are interpreted in the same way.

[0150] <Manufacture of negative electrode sheet>

[0151] For the adhesive composition A1 for the negative electrode obtained in Example 1, it was uniformly coated on a copper foil current collector with a thickness of 10 μm using an automatic coating machine (brand: BEVS; model: 1811), the coating thickness was 200 μm, and then it was placed in a vacuum drying oven at 80 °C for 8 hours to remove water. After drying, it was calendered at room temperature under the condition that the coating layer became 49 μm using a roll press to obtain an electrode sheet. The above electrode sheet was cut using a manual punching machine to obtain a negative electrode sheet specimen with a diameter of 12 mm.

[0152] For the adhesive compositions A2 to A4 for the negative electrodes of Examples 2 to 4 and the adhesive compositions B1 to B4 for the negative electrodes of Comparative Examples 1 to 4, except for changing the coating thickness and the thickness of the coating layer after roll pressing as shown in Table 3 below, negative electrode sheet specimens were manufactured in the same manner as in Example 1.

[0153]

[0154] <Assembly of coin-type half cell>

[0155] The negative electrode sheet specimens prepared in each example were transferred to a glove box filled with argon and assembled into a 2032 coin-type half cell, in which a pure lithium sheet was used as the counter electrode, and The 2325 polypropylene-polyethylene-polypropylene (PP-PE-PP) membrane is used as the separator, and the electrolyte is a mixed solution of ethylene carbonate (EC) and diethyl carbonate (DEC) (volume ratio 1:1) containing 1 M lithium hexafluorophosphate (LiPF6), with 10% by volume of fluoroethylene carbonate (FEC) added.

[0156] The specific assembly steps of the above button-type half-cell are as follows: (1) Place the negative electrode sheet in the center of the negative electrode case, and drop the electrolyte on it to completely wet the negative electrode sheet; (2) Place the separator flat on the negative electrode sheet, and then drop the electrolyte to completely wet the separator; (3) Place the lithium sheet as the counter electrode on the separator; (4) Place the gasket and spring piece on the lithium sheet in turn to make it in the center position of the battery, and then fasten the positive electrode case; (5) Use a packaging machine for pressure packaging to obtain a button-type half-cell.

[0157] <Electrochemical Test>

[0158] The prepared 2032 button-type half-cell is left to stand for 6 h and then starts to be tested. Use a Blue-Electric battery test system to perform a constant current charge-discharge cycle test on the battery in the voltage range of 2.5 - 4.0 V.

[0159] <Peel Strength Test>

[0160] The peel strength of the electrode sheet is tested according to GB / T 2792—1998. The specific method is as follows:

[0161] 1. After rolling the electrode sheet, cut it into a long strip with a width of 19 mm.

[0162] 2. Paste the side coated with the negative electrode adhesive composition on the single-sided adhesive tape, and roll it back and forth 5 times with a rolling wheel to remove air bubbles.

[0163] 3. Use double-sided adhesive tape to bond the side not coated with the negative electrode adhesive composition to the steel plate.

[0164] 4. Fix one end of the electrode sheet and the tape on the tensile machine through clamps respectively, and use a universal tensile machine to peel the tape from the electrode sheet at a speed of 10 mm·min -1 at an angle of 180°.

[0165] <Flexibility Test>

[0166] The flexibility of the electrode sheet is tested by winding the electrode sheet. Use steel cores with diameters of 5 mm, 2.5 mm, 1 mm, and 0.5 mm to wind the rolled electrode sheet on the surface of the steel core, and observe whether there are cracks or peeling on the surface coating of the electrode sheet. If there are no such phenomena, it is judged to pass the test and recorded as "〇"; if there are such phenomena, it is judged to fail the test and recorded as "×".

[0167] <In-situ thickness test>

[0168] The change of the negative electrode thickness during the charge and discharge process was monitored using an in-situ thickness tester (Xi'an Arize New Energy Technology Co., Ltd.), and the battery was subjected to constant current charge and discharge tests (discharge current 0.05-0.1C, charge current 0.1C, 3 cycles) at a working pressure of 0.3MPa and a voltage range of 0-1.5V. The initial thickness of the negative electrode (μm) and the maximum thickness increase during charge and discharge (μm) were measured, and the maximum thickness change rate was calculated from this.

[0169] Maximum thickness change rate = maximum thickness increase during charge and discharge / initial thickness of negative electrode × 100%

[0170] Table 4 shows the test results of parameters (areal density, areal capacity, compression density) and peel strength, flexibility, thickness change, and electrical properties of the electrode sheets prepared using the negative electrode binder compositions of Examples 1 to 4 and Comparative Examples 1 to 4.

[0171] [Table 4]

[0172]

[0173] According to the test results in Table 4, the adhesive compositions of Examples 1 to 3 use TFE / P copolymer as an adhesive, and the content ratio of the thickener CMC to the adhesive is greater than 1.1. The peel strength between the negative electrode material and the current collector prepared by the adhesive composition is high, and the maximum thickness change rate of the electrode sheet during charge and discharge is less than 10%, which effectively suppresses the thickness change of the electrode sheet during charge and discharge, and suppresses the continuous increase in the thickness of the electrode sheet during charge and discharge.

[0174] In addition, it can be seen from Examples 1 to 3 that as the content ratio of thickener CMC to binder increases, the peel strength between the negative electrode material and the current collector increases, and the maximum thickness change rate of the electrode sheet during charge and discharge is suppressed to a smaller level.

[0175] The adhesive composition of Example 4 uses a TFE / P / VdF copolymer with a VdF unit content of less than 40 mol% as an adhesive, and the content ratio of the thickener CMC to the adhesive is greater than 1.1. The negative electrode material prepared from the adhesive composition has a high peel strength with the current collector, and the maximum thickness change rate of the electrode sheet during charge and discharge is 5%, which effectively suppresses the thickness change of the electrode sheet during charge and discharge, and suppresses the continuous increase in the thickness of the electrode sheet during charge and discharge.

[0176] On the other hand, the binder composition of Comparative Example 1 uses the same binder as Examples 1 to 3, but the content ratio of the thickener CMC to the binder is 0.4. The negative electrode material prepared from the binder composition has low peel strength and unqualified flexibility test results, and is therefore not suitable for use in manufacturing negative electrodes.

[0177] In Comparative Example 2, SBR was used as the binder in the binder composition, and the content ratio of CMC to SBR was 0.67. The stripping strength of the negative electrode material prepared from this binder composition was high, but the thickness change of the electrode sheet during charge and discharge was 5.9 → 7.2 → 7.3 μm, showing a continuous increasing trend. Moreover, the maximum thickness change rate of the electrode sheet during charge and discharge was 15%, and the maximum thickness change rate of the electrode sheet during charge and discharge could not be suppressed to a lower level, nor could the increase in the thickness of the electrode sheet be suppressed.

[0178] In Comparative Example 3, PVdF was used as the binder in the binder composition. The thickness change of the electrode sheet prepared from this binder composition during charge and discharge was 5.5 → 6.8 → 7.4 μm, showing a continuous increasing trend. Moreover, the maximum thickness change rate of the electrode sheet during charge and discharge was 15%, and the maximum thickness change rate of the electrode sheet during charge and discharge could not be suppressed to a lower level, nor could the increase in the thickness of the electrode sheet be suppressed.

[0179] In Comparative Example 4, a VdF / TFE copolymer with a VdF unit content of 48 mol% was used as the binder in the binder composition. The thickness change of the electrode sheet prepared from this binder composition during charge and discharge was 4.9 → 5.5 → 6.8 μm, showing a continuous increasing trend. Moreover, the maximum thickness change rate of the electrode sheet during charge and discharge was 14%, and the maximum thickness change rate of the electrode sheet during charge and discharge could not be suppressed to a lower level, nor could the increase in the thickness of the electrode sheet be suppressed.

[0180] Industrial applicability

[0181] The binder composition for a negative electrode of the present invention can be used to manufacture a negative electrode material containing a silicon-based active material, and can suppress the thickness change when used as an electrode, and its thickness will not increase even after repeated charge and discharge. Therefore, it can be used to manufacture a negative electrode with stable performance and a secondary battery equipped with this negative electrode.

[0182] Finally, it should be understood that the above descriptions of the embodiments and examples are illustrative in all aspects and do not constitute a limitation to the present invention. Those of ordinary skill in the art can make various improvements without creative labor within the scope of the spirit of the present invention. The scope of the present invention is represented by the claims, rather than by the above embodiments or examples. In addition, the scope of the present invention includes all changes within the meaning and scope equivalent to the claims.

Claims

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

2. The binder composition for a negative electrode according to claim 1, wherein, The tackifier is carboxymethyl cellulose.

3. The binder composition for a negative electrode according to claim 1, wherein, The weight content ratio of the tackifier to the fluoropolymer is 1.1 to 10.

4. The binder composition for a negative electrode according to claim 1, wherein, The fluoropolymer further has units based on propylene.

5. The binder composition for a negative electrode according to claim 1, wherein, The maximum thickness change rate of the negative electrode material prepared from the negative electrode binder composition during charge and discharge is within 10%.

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

7. The binder composition for a negative electrode according to claim 6, wherein, The negative electrode binder composition further contains a tackifier, and the tackifier is carboxymethyl cellulose.

8. The binder composition for a negative electrode according to claim 7, wherein, The weight content ratio of the tackifier to the fluoropolymer is 1.1 to 10.

9. The binder composition for a negative electrode according to claim 6, wherein, The fluoropolymer further has units based on propylene.

10. The binder composition for a negative electrode according to claim 6, wherein, The maximum thickness change rate of the negative electrode material prepared from the negative electrode binder composition during charge and discharge is within 10%.

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

12. A secondary battery, which includes the negative electrode for a secondary battery according to claim 11.

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