TW202146493A - Polyvinyl alcohol graft copolymer and use thereof - Google Patents

Abstract

The present invention provides a polyvinyl alcohol graft copolymer comprising a polyvinyl alcohol main chain. The polyvinyl alcohol graft copolymer comprises branched chains comprising structure units derived from the following monomers: (a) a fluorine-containing ethylenically unsaturated monomer, and (b) an ethylenically unsaturated carboxylic acid monomer. The present invention also provides an aqueous binder composition and an electrode slurry composition comprising the polyvinyl alcohol graft copolymer. The electrode sheet manufactured by using the polyvinyl alcohol graft copolymer of the present invention has the advantageous properties such as good adhesion or low water absorption, and is not prone to brittle fracture.

Description

Polyvinyl alcohol graft copolymer and its use

The present invention relates to a polyvinyl alcohol graft copolymer and its use, in particular to a polyvinyl alcohol graft copolymer suitable for use as an aqueous binder in electrochemical devices.

In recent years, in the field of chemical energy storage systems, lithium-ion batteries have gained widespread attention due to their relatively light weight and relatively high electrical capacity (ie, relatively high energy density), high operating voltage, and high properties such as cycle life.

Lithium-ion batteries are mainly composed of a positive electrode, a negative electrode, an electrolyte and a separator, and are stored or released by the migration and removal of lithium ions at the positive and negative electrodes. During charging, lithium ions move out from the positive electrode and into the negative electrode; during discharge, lithium ions move out from the negative electrode and into the positive electrode.

The electrode sheet in a lithium-ion battery is a key component that plays an important role in battery performance. Specifically, it affects the battery's energy density, discharge capacity, and cycle life. The electrode sheet is mainly composed of active materials (such as positive and negative electrodes), conductive materials (such as carbon black), binders and metal current collectors (such as copper foil, aluminum foil).

Commonly used anode materials are carbon-based materials such as graphite, soft carbon, and hard carbon. However, in pursuit of higher energy density, silicon-based materials (such as silicon or silicon oxide) have been tried as anode materials for lithium-ion batteries. Although silicon-based materials can greatly improve the energy density of lithium-ion batteries compared with traditional carbon-based anode materials, they will experience severe volume expansion (about 400%) during battery charging (lithium ions migrate into silicon-based materials). . During the repeated charging and discharging process, such severe volume expansion and contraction will damage the structure of the negative electrode sheet, such as the disintegration of the conductive network, and/or the peeling of the electrode sheet, so that the negative electrode completely fails and has a great impact on the performance of the battery. negative effect.

In order to maintain the structure of the electrode sheet, a binder is used to bond the active material, the conductive material and the metal current collector together to form a structurally stable electrode sheet. If the adhesive force of the adhesive is not good, the aforementioned materials cannot be firmly bonded to each other. With repeated charging and discharging, they will fall off from the surface of the metal current collector, resulting in powdering of the electrode sheet and poor battery cycle life.

The binders commonly used in lithium-ion batteries mainly include polyvinylidene fluoride (PVDF), carboxymethyl cellulose (CMC) polymers, acrylic polymers (PAA) and styrene-butadiene rubber (SBR). PVDF binder is a relatively mature binder in the current technology, with good bonding performance, but the use of this binder to prepare electrode sheets requires a large amount of high-boiling organic solvents (such as N-methylpyrrolidone (NMP)), so PVDF Binders have the disadvantages of high cost and easy environmental pollution; in addition, PVDF binders are easily swelled in common electrolytes, resulting in decreased electrode stability. CMC and SBR are usually used together. This type of binder often uses water as a solvent, which has the advantages of low cost and not easy to pollute the environment. However, it cannot overcome the problem of volume expansion and contraction of silicon-based materials. PAA has a carboxyl group, which can form hydrogen bonding force with the surface of silicon-based materials, thereby inhibiting the volume expansion of silicon-based materials, thus attracting attention. However, PAA has the problem of easy moisture absorption, so it must be baked at a higher temperature and for a longer time when making electrode sheets; in addition, PAA usually needs to be used together with CMC and/or SBR to avoid pure PAA. brittle problem.

At present, in response to the birth of high-capacity silicon-based anode materials, a suitable binder is still to be developed, which can overcome the volume expansion and contraction problem of silicon-based anode materials and avoid the above-mentioned shortcomings of conventional binders.

In view of this, the inventors of the present application found a novel polyvinyl alcohol graft copolymer after research, which comprises a polyvinyl alcohol main chain, and the branch chain of the polyvinyl alcohol graft copolymer comprises a structure derived from the following monomers unit: (a) fluorine-containing ethylenically unsaturated monomers; and (b) Ethylenically unsaturated carboxylic acid monomers.

Another object of the present invention is to provide an aqueous adhesive composition comprising the above-mentioned polyvinyl alcohol graft copolymer.

Another object of the present invention is to provide an electrode slurry composition comprising the above-mentioned polyvinyl alcohol graft copolymer.

The polyvinyl alcohol graft copolymer of the present invention can be used as a single agent in the manufacture of electrode sheets, that is, it can replace CMC/SBR, and does not need to be matched with other binders, which can simplify the manufacturing process and reduce costs; even so, the polymer of the present invention can be used. Vinyl alcohol graft copolymers can also be used with other binders such as CMC/SBR. In addition, the polyvinyl alcohol graft copolymer of the present invention can avoid the problem of easy brittleness of pure PAA, and the produced electrode sheet is flexible and not easy to be cracked after winding, so the operability is good; in addition, the polymer of the present invention is Vinyl alcohol graft copolymer can improve the problem that PAA is easy to absorb moisture, and the obtained electrode sheet is easy to dry, so it can reduce the energy consumption required for drying; in addition, the adhesion of the electrode sheet is improved, and it is not easy to remove powder, thereby improving the battery cycle. life.

To facilitate understanding of the disclosure set forth herein, several terms are defined below.

The term "about" means the acceptable error for a particular value as determined by one of ordinary skill in the art, the margin of error depending on how the value is measured or determined.

As used herein, unless otherwise specified, the singular forms "a" and "the" also include the plural forms. Any and all examples and illustrative terms (“such as” and “such as”) herein are intended only to further highlight the invention and are not intended to limit the scope of the invention, and the terms in the specification should not be construed to imply any The claimed methods and conditions may constitute essential features in the practice of the present invention.

The word "or" with respect to a list of two or more items covers all interpretations of: any of the items in the list, all items in the list, and any combination of items in the list.

In the present invention, the term "structural unit" refers to the smallest unit with the same chemical composition in the copolymer after the monomer is polymerized to form a copolymer, which is called a structural unit, also called a repeating unit.

In the present invention, the term "ethylenically unsaturated monomer" refers to a monomer having at least one -C=C- double bond.

In the present invention, the term "fluorine-containing ethylenically unsaturated monomer" refers to a monomer having at least one -C=C- double bond and at least one fluorine atom.

In the present invention, the term "alkyl" refers to a saturated straight or branched chain saturated hydrocarbon group, which may have 1 to 12 carbon atoms, preferably 1 to 8 carbon atoms, more preferably 1 to 6 carbon atoms , preferably having 1 to 4 carbon atoms; examples include, but are not limited to, methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, tert-butyl, pentyl, hexyl and similar groups.

In the present invention, the term "aryl" is preferably a monocyclic or polycyclic aromatic carbocyclic group having 6 to 20 carbon atoms, and may also contain condensed rings, preferably 1, 2, 3 or 4 condensed rings. Ring or non-fused ring. Examples include, but are not limited to: phenyl, indenyl, naphthyl, fluorenyl, anthracenyl, phenanthrenyl, and the like. According to some preferred embodiments of the present invention, the aryl group is phenyl, biphenyl or naphthyl.

In the present invention, the term "aralkyl" refers to an alkyl group substituted by an aryl group, wherein the aryl group and the alkyl group are as defined above.

In the present invention, the term "ethylenically unsaturated carboxylic acid monomer" refers to a monomer having at least one -C=C- double bond and at least one carboxylic acid group.

In the present invention, the term "(meth)acrylic acid" refers to "acrylic acid or methacrylic acid"; the term "(meth)acrylic monomer" refers to "acrylic monomer or methacrylic monomer"; The rest of the related terms can be deduced accordingly.

The content of the present invention will be described in detail below. [ Polyvinyl alcohol graft copolymer ]

The polyvinyl alcohol graft copolymer of the present invention is based on polyvinyl alcohol as the main chain, and is subjected to graft copolymerization with fluorine-containing ethylenically unsaturated monomers, ethylenically unsaturated carboxylic acid monomers and other monomers as needed. prepared.

The polyvinyl alcohol used in the present invention is not particularly limited, and preferably has a weight average molecular weight ranging from 10,000 to 400,000. The lower limit value of the weight average molecular weight of polyvinyl alcohol may be 10,000, preferably 12,000 or 15,000. The upper limit of the weight average molecular weight of polyvinyl alcohol may be 400,000, preferably 350,000, 300,000, 250,000 or 200,000.

Specific embodiments of the polyvinyl alcohol used in the present invention, such as but not limited to: PVA 088-20 (Sinopec), GOHSENOL GL-05 (Japan Synthetic Chemical).

The fluorine-containing ethylenically unsaturated monomer (component (a)) used in the present invention can be, for example, but not limited to: fluoroalkyl (meth)acrylate, fluoroaryl (meth)acrylate , fluoroaralkyl (meth)acrylate, perfluoroalkyl (meth)acrylate, alkyl (fluoroalkyl)acrylate, aryl (fluoroalkyl)acrylate, or (fluoroalkyl)acrylate Alkyl) aralkyl acrylate; preferably fluoroalkyl (meth)acrylate, or (fluoroalkyl)alkyl acrylate. Specific examples include, but are not limited to: trifluoromethyl (meth)acrylate, 2,2,2-trifluoroethyl (meth)acrylate, 2,2,3,3-tetrafluoropropyl (meth)acrylate , 2,2,3,4,4,4-hexafluorobutyl (meth)acrylate, perfluorooctyl (meth)acrylate, methyl (trifluoromethyl)acrylate.

， 其中R₁ 及R₂ 中至少一者為經氟原子取代的基團，且R₁ 及R₂ 具有下述的定義： R₁ 為H、C₁ -C₄ 烷基、或經一或多個氟原子取代的C₁ -C₄ 烷基；且 R₂ 為H、C₁ -C₁₂ 烷基、經一或多個氟原子取代的C₁ -C₁₂ 烷基、芳基、經一或多個氟原子取代的芳基、芳烷基、或經一或多個氟原子取代的芳烷基。According to a preferred embodiment of the present invention, the fluorine-containing ethylenically unsaturated monomer (component (a)) has the chemical formula of formula (1):

, wherein _{at least one of R 1} and R ₂ is a group substituted by a fluorine atom, and R ₁ and R ₂ have the following definitions: R ₁ is H, C ₁ -C ₄ alkyl, or by one or more fluorine atom-substituted C ₁ -C ₄ alkyl; and R ₂ is H, C ₁ -C ₁₂ alkyl, substituted with one or more fluorine atoms, C ₁ -C ₁₂ substituted alkyl, aryl, or a Aryl, aralkyl, or aralkyl substituted with one or more fluorine atoms.

According to some embodiments of the present invention, R _{1 is} preferably H, C ₁ -C _{3 alkyl, or C 1} -C ₃ alkyl substituted with one or more fluorine atoms, more preferably H, methyl or trifluoromethyl.

According to some aspects of embodiments of the present invention, R ₂ is preferably H, C ₁ -C ₈ alkyl, substituted with one or more fluorine atoms substituted with C ₁ -C ₈ alkyl, C ₆ -C ₂₀ aryl, _{C 6} -C ₂₀ aryl substituted with one or more fluorine atoms, C ₆ -C ₂₀ aryl-C ₁ -C _{12 alkyl, or C 6} -C ₂₀ aryl substituted with one or more fluorine atoms - C ₁ -C ₁₂ alkyl.

According to some embodiments of the present invention, the fluorine-containing ethylenically unsaturated monomer has the formula (1), wherein R ₁ is H or C ₁ -C ₄ alkyl and R ₂ is through one or more fluorine atoms Substituted C ₁ -C ₁₂ alkyl. According to a preferred embodiment of the present invention, R ₁ is H or C ₁ -C ₃ alkyl and R _{2 is C 1} -C ₈ alkyl substituted with one or more fluorine atoms.

According to some embodiments of the present invention, per 100 parts by weight of polyvinyl alcohol, the content of component (a) is 20 parts by weight or more, preferably 25 parts by weight or more, more preferably 30 parts by weight or more. If the content of component (a) is too low, the resulting electrode sheet will easily absorb moisture, resulting in too high moisture content and poor electrical properties. The upper limit of the content of component (a) is not particularly limited in theory. However, if the content of component (a) is too high, the polyvinyl alcohol graft copolymer may be precipitated due to poor water solubility, resulting in unstable glue. Therefore, , the dosage can be adjusted appropriately to avoid precipitation. According to some embodiments of the present invention, per 100 parts by weight of polyvinyl alcohol, the content of component (a) is preferably not more than 160 parts by weight, more preferably not more than 150 parts by weight, particularly preferably not more than 140 parts by weight.

The ethylenically unsaturated carboxylic acid monomer (component (b)) used in the present invention can be, for example but not limited to: (meth)acrylic acid, maleic acid, fumaric acid, itaconic acid, 2-Ethylacrylic acid, isocrotonic acid, α-acetoxyacrylic acid, or β-trans-aryloxyacrylic acid.

(2) 其中R₃ 為H或C₁ -C₄ 烷基，較佳為甲基；且R₄ 為-OH。According to some embodiments of the present invention, the ethylenically unsaturated carboxylic acid monomer (component (b)) used in the present invention has the chemical formula of formula (2):

(2) wherein R ₃ is H or C ₁ -C ₄ alkyl, preferably methyl; and R ₄ is -OH.

According to some embodiments of the present invention, per 100 parts by weight of polyvinyl alcohol, the content of component (b) is 500 parts by weight to 3000 parts by weight, preferably 520 parts by weight to 2500 parts by weight, more preferably 530 parts by weight Parts by weight to 1500 parts by weight, particularly preferably 550 parts by weight to 1200 parts by weight. If the content of component (b) is too low, the adhesive produced will not adhere well to the pole piece, and the active material and the pole piece will fall off easily; if the content of component (b) is too high, the produced adhesive will Too hard and brittle, it is easy to crack after winding, and the process operation is not easy.

According to some embodiments of the present invention, the branch of the polyvinyl alcohol graft copolymer of the present invention may further comprise a structural unit derived from an ethylenically unsaturated carboxylate monomer (component (c)).

According to some embodiments of the present invention, the ethylenically unsaturated carboxylate monomer (component (c)) can be, for example, but not limited to: methyl (meth)acrylate, ethyl (meth)acrylate, Propyl (meth)acrylate, butyl (meth)acrylate, hexyl (meth)acrylate, n-/iso-octyl (meth)acrylate, isodecyl (meth)acrylate, lauryl (meth)acrylate ester, isobornyl (meth)acrylate, 2-phenoxyethyl acrylate, cumylphenoxyethyl acrylate, phenylphenoxyethyl acrylate, or beta-carboxyethyl acrylate .

(3) 其中R₅ 為H或C₁ -C₄ 烷基，較佳為H或C₁ -C₃ 烷基，更佳為H或甲基；且R₆ 為C₁ -C₁₂ 烷氧基或芳氧基，較佳為C₁ -C₈ 烷氧基或苯氧基。According to some embodiments of the present invention, the ethylenically unsaturated carboxylate monomer (component (c)) used in the present invention has the chemical formula of formula (3):

(3) wherein R ₅ is H or C ₁ -C ₄ alkyl, preferably H or C ₁ -C ₃ alkyl, more preferably H or methyl; and R ₆ is C ₁ -C ₁₂ alkoxy or aryloxy, preferably C ₁ -C ₈ alkoxy or phenoxy.

According to some embodiments of the present invention, per 100 parts by weight of polyvinyl alcohol, the content of component (c) is 0 parts by weight to 80 parts by weight, 0 parts by weight to 70 parts by weight, and 0 parts by weight to 60 parts by weight , 0 to 50 parts by weight, 0 to 40 parts by weight, 0 to 30 parts by weight, 0 to 20 parts by weight, or 0 to 10 parts by weight, preferably 5 to 75 parts by weight parts by weight, more preferably 10 parts by weight to 70 parts by weight.

The inventors of the present invention have surprisingly found that a structural unit derived from a fluorine-containing ethylenically unsaturated monomer and a monomer derived from an ethylenically unsaturated carboxylic acid and optionally an olefin are introduced into the branched chain using polyvinyl alcohol as the main chain. It belongs to the structural unit of unsaturated carboxylate monomer. The obtained graft copolymer can have good adhesion to the surface of the material in contact. When used as a binder for electrode sheets, it can solve the problem of easy moisture absorption or brittle cracking in the prior art. Therefore, the performance and cycle life of the battery can be greatly improved.

In addition, compared with the prior art, the polyvinyl alcohol graft copolymer of the present invention is not easy to absorb water, so when used in the production of electrode sheets, the baking conditions can be reduced (for example, a lower baking temperature and/or less Baking time), so as to achieve the beneficial effect of cost saving; when the polyvinyl alcohol graft copolymer of the present invention is used as a binder, it can provide good adhesion and no problem of easy brittle cracking, and can be used alone in electrode sheets as a binder There is no need to add carboxymethyl cellulose (CMC) type polymer or styrene-butadiene rubber (SBR), so the manufacturing procedure of the electrode sheet can be simplified and the cost can be reduced. [ Preparation method of polyvinyl alcohol graft copolymer ]

The polyvinyl alcohol graft copolymer of the present invention can be prepared by any suitable method. In one embodiment of the present invention, the polyvinyl alcohol graft copolymer can be prepared by the following method: (1) dissolving polyvinyl alcohol in a solvent to obtain a polyvinyl alcohol solution; (2) Passing inert gas into the polyvinyl alcohol solution of step (1) as needed to remove oxygen in the solution; (3) Add fluorine-containing ethylenically unsaturated monomers, ethylenically unsaturated carboxylic acid monomers, ethylenically unsaturated carboxylic acid esters or other monomers as needed, and initiators into the solution obtained in step (2) ; (4) The temperature is raised to carry out the polymerization reaction, so that the fluorine-containing ethylenically unsaturated monomer, the ethylenically unsaturated carboxylic acid monomer and the ethylenically unsaturated carboxylic acid ester monomer as needed are covalently grafted onto the polyvinyl alcohol, Obtained polyvinyl alcohol graft copolymer.

The solvent used in the above step (1) is an aqueous solvent, such as but not limited to water or deionized water.

The initiator used in the above step (3) can be any suitable initiator known to those with ordinary knowledge in the technical field of the present invention, such as but not limited to: persulfate. Specific examples of persulfate, such as but not limited to: ammonium persulfate, potassium persulfate, sodium persulfate, lithium persulfate, potassium hydrogen persulfate or a combination thereof, preferably ammonium persulfate or sodium persulfate.

In the above step (4), the initiator is decomposed into free radicals at the reaction temperature, and free radicals are introduced into the polyvinyl alcohol molecular chain to initiate fluorine-containing ethylenically unsaturated monomers and ethylenically unsaturated carboxylic acid monomers. and optionally ethylenically unsaturated carboxylate monomers undergo radical polymerization and covalently graft to polyvinyl alcohol. The reaction temperature is usually between about 30 and 100°C, such as 70°C; the time for the completion of the polymerization reaction depends on the situation, and is usually between 0.5 and 12 hours, such as about 4 hours. [ Aqueous binder composition and its preparation method ]

The polyvinyl alcohol graft copolymer of the present invention has good compatibility with an aqueous solvent, and can be formulated with an aqueous solvent to form an aqueous adhesive composition. The solvent can be the solvent from the process of preparing the polyvinyl alcohol graft copolymer, or it can be added after the preparation to adjust the viscosity if necessary. The above-mentioned aqueous solvents are obvious to those skilled in the art, such as, but not limited to, water or deionized water.

Compared with the use of organic solvents, the use of aqueous solvents is less costly and less likely to pollute the environment, and the health risks to operators are also lower.

The aqueous adhesive composition of the present invention can be optionally added with any additives known to those with ordinary knowledge in the technical field of the present invention, such as but not limited to basic compounds (such as lithium hydroxide, sodium hydroxide, potassium hydroxide, ammonium hydroxide, etc.) or a mixture of them, preferably lithium hydroxide or sodium hydroxide) or a crosslinking agent. When the water-based adhesive is used for a silicon-based material, the pH value can be adjusted by adding an alkaline compound, and the binding force of the adhesive on the silicon-based material can be increased by adjusting the pH value.

In a preferred embodiment of the present invention, the pH can be adjusted to between 3 and 11 (eg 3, 4, 5, 6, 7, 8, 9, 10 or 11), preferably adjusted to between 4 and 7.

According to an embodiment of the present invention, the aqueous adhesive composition of the present invention contains 1 wt % to 10 wt % of the polyvinyl alcohol graft copolymer (based on the total weight of the aqueous adhesive composition), for example, 1 wt % %, 2 wt %, 3 wt %, 4 wt %, 5 wt %, 6 wt %, 7 wt %, 8 wt %, 9 wt % or 10 wt % of the polyvinyl alcohol graft copolymer, preferably 2 % to 8% by weight of the polyvinyl alcohol graft copolymer, more preferably 3% to 7% by weight of the polyvinyl alcohol graft copolymer.

The water-based adhesive composition of the present invention can be prepared by any suitable method, and the polyvinyl alcohol graft copolymer, the water-based solvent and optional additives can be prepared after fully stirring.

The water-based binder of the present invention uses a polyvinyl alcohol graft copolymer, which can be used in a single agent, and can provide the required softness without adding CMC/SBR. The manufactured electrode sheet is soft and not easy to crack after winding , good operability, can improve the problem of easy brittleness of pure PAA; in addition, the fluorine-containing ethylenically unsaturated monomer is grafted on the main chain, effectively reducing the problem of easy moisture absorption of electrodes made of PAA or CMC/SBR, At the same time, the electrode sheet is easy to dry, and the energy consumption required for drying is reduced, which greatly improves the performance of the battery; in addition, by grafting the ethylenically unsaturated carboxylic acid monomer, the obtained graft copolymer can interact with the materials in contact with it. The surface has good adhesion, which effectively improves the adhesion of the electrode sheet, and is not easy to remove powder, thereby improving the battery cycle life. [ Electrode paste composition and its preparation method ]

The present invention further provides an electrode slurry composition comprising: the polyvinyl alcohol graft copolymer of the present invention, an active material, a conductive material, an aqueous solvent and optional additives.

Relative to the total weight of the solid content of the electrode slurry composition, the polyvinyl alcohol graft copolymer of the present invention has the following contents: 0.1 to 20% by weight, 0.2 to 15% by weight, 0.5 to 10% by weight, 0.8 to 8 wt%, or 1 to 5 wt%.

The above-mentioned active materials are negative active materials, including (but not limited to) graphite, hard carbon, soft carbon, silicon, silicon oxide (SiO _x ), silicon carbon, or a combination of the foregoing materials. With respect to the total weight of the solid content of the electrode slurry composition, the above-mentioned negative electrode active material has the following contents: 50 to 99% by weight, 60 to 99% by weight, 70 to 99% by weight, 80 to 99% by weight, 85 to 99% by weight 98% by weight, or 90 to 97% by weight.

The above conductive materials include, but are not limited to, conductive graphite, carbon black, carbon fiber, carbon nanotubes, graphene, or a combination of the foregoing materials. With respect to the total weight of the solid content of the electrode paste composition, the conductive material has the following content: 1 to 30% by weight, 1 to 20% by weight, 1 to 10% by weight, or 1 to 5% by weight, preferably 1 to 20% by weight, 1 to 10% by weight, or 2 to 5% by weight.

According to an embodiment of the present invention, the above-mentioned optional additives include basic compounds, such as (but not limited to) lithium hydroxide, sodium hydroxide, potassium hydroxide, ammonium hydroxide or a mixture thereof, preferably Lithium hydroxide or sodium hydroxide. In a preferred embodiment of the present invention, the pH can be adjusted to between 3 and 11 (eg 3, 4, 5, 6, 7, 8, 9, 10 or 11), preferably adjusted to between 4 and 7.

According to an embodiment of the present invention, the above-mentioned optional additive comprises a dispersant.

The above-mentioned aqueous solvent includes, but is not limited to, water or deionized water, preferably deionized water. Those with ordinary knowledge in the technical field to which the present invention pertains can, according to application requirements, add an appropriate amount of solvent as appropriate to adjust the viscosity suitable for use.

According to an embodiment of the present invention, the electrode slurry composition of the present invention may not contain carboxymethyl cellulose or styrene-butadiene rubber.

The electrode slurry of the present invention can be prepared by any suitable method. The polyvinyl alcohol graft copolymer of the present invention, active material, conductive material, aqueous solvent and optional additives are prepared after fully stirring.

The present invention will be further described with respect to the following examples, but it should be understood that the following examples are only used for illustration and should not be construed as a limitation of the implementation of the present invention. Preparation Example Preparation of polyvinyl alcohol graft copolymer

Preparation Examples 1 to 10: Dissolve polyvinyl alcohol (hereinafter referred to as "PVA") GOHSENOL GL-05 (Synthetic Chemicals) in 3000 ml of water, and stir well to obtain a uniform, transparent and viscous polyvinyl alcohol aqueous solution. The nitrogen gas flow was continued for about 60 minutes to remove oxygen from the aqueous solution. 2,2,2-trifluoroethyl (meth)acrylate (Sigma-Aldrich; 99% purity), (meth)acrylic acid (Sigma-Aldrich; 99% purity), hexyl (meth)acrylate ( Sigma-Aldrich; purity 99%) and 1% sodium persulfate aqueous solution, stir and mix evenly, heat up to 70 °C and continue to react for 4 hours to prepare polyvinyl alcohol graft copolymer (hereinafter referred to as "grafted PVA"), and then add 5 % lithium hydroxide aqueous solution to adjust the pH to between 4 and 7 to obtain the aqueous binders of Preparation Examples 1 to 10 (solid content of 5.2 wt%). The reactant dosage of each grafted PVA is recorded in Table 1. Preparation of Electrode Slurry Composition

Example 1-6 and Comparative Example 1-4: 38.46 grams of grafted PVA solution (solid content: 5.2% by weight), 95 grams of silicon oxide/graphite negative active material ( Model: BTR-500; Beterui New Energy Materials Co., Ltd.) and 3 g of carbon black (Super P; Taiwan Bolu Co., Ltd.), mix well at 25 °C, and add an appropriate amount of deionized water to adjust the viscosity to prepare negative electrode slurry material.

Comparative Example 5: 2 grams of CMC (Ashland Inc.; BondwellTM, BVH-8) was dissolved in 98 ml of water to prepare a 2 wt % CMC aqueous solution. Take 50 grams of 2 wt% CMC aqueous solution, add 95 grams of silicon oxide/graphite negative active material (model: BTR-500; Beterui New Energy Materials Co., Ltd.) and 3 grams of carbon black (Super P; Taiwan Polly Co., Ltd.), After thorough stirring, 2.22 g of 45 wt % styrene-butadiene rubber (JSR; TRD104A) was finally added, and after thorough stirring, a negative electrode slurry containing carboxymethyl cellulose/styrene-butadiene rubber was obtained.

Comparative Example 6: Dissolve 2 grams of PAA (Sigma-Aldrich, PAA450000) in 98 ml of water, configure 2 wt % PAA aqueous solution, take 50 grams of 2 wt % PAA aqueous solution, add 95 grams of silicon oxide/graphite negative electrode active material ( Model: BTR-500; Beterui New Energy Materials Co., Ltd.) and 3 grams of carbon black (Super P; Taiwan Bolu Co., Ltd.), after thorough stirring, finally add 2.22 grams of 45 wt% styrene-butadiene rubber ( JSR; TRD104A), after fully stirring, a negative electrode slurry containing PAA/styrene-butadiene rubber was obtained.

The dosage of the relevant components and the viscosity of the slurry are recorded in Table 2. Preparation of electrode sheets

The electrode slurry composition prepared above was coated on copper foil (10 µm battery copper foil in Changchun) using a doctor blade [coating weight: 5~7 mg/cm ² ], dried at 100°C for 5 minutes and then After cold pressing, it was cut into a circular shape with a 12 mm diameter cutter, and placed in a vacuum oven for heating at 100° C. for 6 hours to obtain a negative electrode sheet. Preparation of Button Cells

The electrolyte composition used contains 2% ethylene carbonate (EC)/diethyl carbonate (DEC) - vinylene carbonate (VC), 8% fluoroethylene carbonate (FEC) and lithium hexafluorophosphate (Formosa Plastics: LE ); the separator is a polypropylene film with a thickness of about 20 μm.

The negative electrode sheet and other components were assembled into a standard button battery (CR2032) in an inert environment by a conventional method, and its performance was tested. The assembly process is as follows: battery lower cover, lithium metal sheet (as the positive electrode), separator, negative electrode sheet, metal gasket, spring sheet and battery upper cover.

The assembled battery was allowed to stand for about 2 to 3 hours to allow the electrolyte to fully penetrate into the electrodes to improve conductivity. The open circuit voltage of the resulting battery was about 2.5 to 3 V. Test method 1. Fine measurement

After stirring the water-based adhesive slurry, take an appropriate amount and drop it to the deepest part of the groove of the fineness meter (brand: PSIS-303-50), touch it vertically with the scraper, and pull the scraper from the largest scale part to the smallest part of the scale, Observe the prominent position of the particles in the groove, and write down the corresponding scale value, which is the fineness. 2. Moisture content test

Detected using a coulomb Karl-Fischer moisture titrator. Put the electrode sheet sample into a sealed sample bottle, heat it to 130 °C for 7 minutes, so that the water in the electrode sheet sample evaporates, and the dry gas is introduced to send the water vapor into the electrolytic cell to participate in the reaction, and then measure the electrolysis process. electricity to measure moisture content.

Taking the electrode sheet of Comparative Example 5 using CMC/SBR as the adhesive as the control group (comparison benchmark), "◎" indicates that the water content is reduced by more than 20% compared with the electrode sheet of Comparative Example 5; "X" indicates that the water content is relatively low. Compared with the electrode sheet of Comparative Example 5, the reduction degree is less than 20%. The results obtained are recorded in Table 2. 3. Adhesion test

Use 3M 610 tape to closely fit the pole piece after drying, and then perform a 180-degree tensile test with a tensile force meter (model: RX-100; Shengqun Technology). 4. Pole piece crack test

Take a pole piece with a coating weight greater than 7 mg/cm ² and wind it with a 3 mm cylindrical rod. After unwinding, observe whether there are cracks on the surface. 5. Capacitance retention rate test

The battery performance was measured using a charge-discharger (model: LBT21084) from Arbin Instruments.

Preliminary work: Charging: After the constant current section is charged with a constant current of 0.1C for 10 hours, the constant voltage section is charged with a constant voltage of 0.01 V for 1 hour; Discharge: Discharge at a current of 0.1C for 10 hours. The charging and discharging were repeated 3 times under the above conditions, and the first 3 cycles were used to form a solid-electrolyte interface (SEI).

The discharge capacity of the 1st and 50th laps: Charging: After charging with a constant current of 0.5C for 2 hours in the constant current section, the constant voltage section is charged with a constant voltage of 0.01 V for 1 hour; Discharge: Discharge at a current of 0.5C for 2 hours. The aforementioned 3 cycles for forming the solid electrolyte interface were included in the calculation, and the discharge capacity measured at the 4th cycle was regarded as the discharge capacity of the first cycle. After repeated charging and discharging 49 times under the above conditions, the discharge capacity measured at the 50th time is the discharge capacity at the 50th cycle.

Capacitance retention rate=(discharge capacitance at the 50th cycle/discharge capacitance at the first cycle)×100%. Test Results

As can be seen from Tables 1 and 2, the polyvinyl alcohol graft copolymer of the present invention is comparable to CMC/SBR (Comparative Example 5) and PVA (Comparative Example) not grafted with fluorine-containing ethylenically unsaturated monomers. 2) and PAA (Comparative Example 6), with significantly lower water content. This result shows that the polyvinyl alcohol graft copolymer of the present invention is relatively difficult to absorb water, so when used in the production of electrode sheets, the baking energy consumption can be reduced and the cost can be saved.

According to Examples 1-6, the battery using the polyvinyl alcohol graft copolymer of the present invention has a good capacity retention rate, and the capacity retention rate for 50 cycles is more than 80%, which is better than all the comparative examples. In addition, the polyvinyl alcohol graft copolymer of the present invention can provide good adhesion, and the obtained electrode still has flexibility even under high coating weight, and no crack occurs after winding. In addition, the electrode sheet prepared with the polyvinyl alcohol graft copolymer of the present invention has a lower water content.

The proportion of the main chain of polyvinyl alcohol used in Comparative Example 1 is too high, so the number of branch chains is relatively insufficient, and the effect cannot be exerted, so the water content cannot be effectively reduced. The reduction degree of the electrode sheet is less than 20%, the battery capacity retention rate is not good, and the adhesion between the adhesive and the electrode sheet is also poor.

Comparative example 2 does not use fluorine-containing ethylenically unsaturated monomers to form branches of polyvinyl alcohol copolymers, and the moisture content of the prepared electrode sheet is less than 20% compared with the electrode sheet made of CMC/SBR , and the battery capacity retention rate is not good.

Comparative Example 4 used too much ethylenically unsaturated carboxylic acid monomer, the obtained battery negative electrode had poor flexibility, and cracked after winding; Comparative Example 3 used too little ethylenically unsaturated carboxylic acid monomer, and the adhesion was poor, This results in poor battery capacity retention.

It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the inventions. In view of the foregoing, the present invention is intended to cover modifications and variations of this invention, with the proviso that they fall within the scope of the following claims and their equivalents. Table I Preparation Example 1 Preparation Example 2 Preparation Example 3 Preparation Example 4 Preparation Example 5 Preparation Example 6 Preparation Example 7 Preparation Example 8 Preparation Example 9 Preparation Example 10 Grafted PVA (d) Polyvinyl alcohol (g) 15 15 15 18 18 20 40 15 20 3 (a) 2,2,2-trifluoroethyl (meth)acrylate (g) 6 12 20 6 10 15 6 0 10 2 (b) (Meth)acrylic acid (g) 138 132 127 138 127 115 113 138 90 160 (c) Hexyl (meth)acrylate (g) 6 6 3 3 10 15 6 12 45 0 water 3000 3000 3000 3000 3000 3000 3000 3000 3000 3000 [(a)+(b)+(c)]/(d) 10 10 10 8.2 8.2 7.25 3.125 10 7.25 54 Viscosity (cps) 6000 5200 4500 6500 6200 7500 20000 7000 6500 7600 pH 5 5 5 5 5 5 5 5 5 5 Table II Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Comparative Example 1 Comparative Example 2 Comparative Example 3 Comparative Example 4 Comparative Example 5 Comparative Example 6 Preparation Example 1 Preparation Example 2 Preparation Example 3 Preparation Example 4 Preparation Example 5 Preparation Example 6 Preparation Example 7 Preparation Example 8 Preparation Example 9 Preparation Example 10 CMC/SBR PAA/SBR Grafted PVA (g) Solid content: 5.2 wt% 38.46 38.46 38.46 38.46 38.46 38.46 38.46 38.46 38.46 38.46 0 0 CMC (g) Solids: 2 wt% 0 0 0 0 0 0 0 0 0 0 50 0 SBR (g) Solids: 45 wt% 0 0 0 0 0 0 0 0 0 0 2.22 2.22 PAA (g) Solids: 2 wt% 0 0 0 0 0 0 0 0 0 0 0 50 Silicon oxide/graphite anode active material (g) 95 95 95 95 95 95 95 95 95 95 95 95 Super P (grams) 3 3 3 3 3 3 3 3 3 3 3 3 Viscosity (CPs) 5500 5100 3800 4700 3500 4100 10000 6000 7500 6500 6200 NA Fineness (μm) 35 35 35 35 35 35 30 35 35 35 25 NA Capacitance retention rate (%) 85 91 88 93 85 82 65 70 66 75 70 NA Adhesion (N) 7.0 6.2 5.5 8.5 8.2 7.7 3.0 6.5 3.7 3.0 8.2 NA High-coated heavy pole piece crack slight none none slight none none none slight none have none have moisture content ◎ ◎ ◎ ◎ ◎ ◎ X X ◎ X -- X

Claims (12)

A polyvinyl alcohol graft copolymer comprising a polyvinyl alcohol backbone, wherein the branches of the polyvinyl alcohol graft copolymer comprise structural units derived from the following monomers: (1) Fluorine-containing ethylenically unsaturated monomers; and (2) Ethylenically unsaturated carboxylic acid monomers. The polyvinyl alcohol graft copolymer of claim 1, wherein the fluorine-containing ethylenically unsaturated monomer has the structure of formula (1):

Wherein R ₁ is H, C ₁ -C ₄ alkyl, or with one or more fluorine atoms substituted with C ₁ -C ₄ alkyl; and R ₂ is H, C ₁ -C ₁₂ alkyl, or a _{C 1} -C ₁₂ alkyl substituted with multiple fluorine atoms, aryl, aryl substituted with one or more fluorine atoms, aralkyl, or aralkyl substituted with one or more fluorine atoms; At least one of R ₁ and R ₂ is substituted with a fluorine atom. The polyvinyl alcohol graft copolymer of claim 2, wherein R ₁ is H or C ₁ -C ₄ alkyl and R _{2 is C 1} -C ₁₂ alkyl substituted with one or more fluorine atoms. The polyvinyl alcohol graft copolymer of claim 1, wherein the ethylenically unsaturated carboxylic acid monomer has the structure of formula (2):

(2) wherein R ₃ is H or C ₁ -C ₄ alkyl; and R ₄ is -OH. The polyvinyl alcohol graft copolymer of claim 1, wherein the branch of the polyvinyl alcohol graft copolymer further comprises a structural unit derived from an ethylenically unsaturated carboxylate monomer. The polyvinyl alcohol graft copolymer of claim 5, wherein the ethylenically unsaturated carboxylate monomer has the structure of formula (3):

(3) wherein R ₅ is H or C ₁ -C ₄ alkyl, and R ₆ is C ₁ -C ₁₂ alkoxy or aryloxy. The polyvinyl alcohol graft copolymer according to any one of claims 1 to 6, wherein the content of the fluorine-containing ethylenically unsaturated monomer is more than 20 parts by weight per 100 parts by weight of the polyvinyl alcohol. The polyvinyl alcohol graft copolymer according to any one of claims 1 to 6, wherein the content of the ethylenically unsaturated carboxylic acid monomer is 500 to 3000 parts by weight per 100 parts by weight of the polyvinyl alcohol. The polyvinyl alcohol graft copolymer of any one of claims 1 to 6, wherein the monomer systems are grafted to the polyvinyl alcohol backbone by a free radical reaction. An aqueous binder composition comprising the polyvinyl alcohol graft copolymer according to any one of claims 1 to 9. The aqueous adhesive composition of claim 10, further comprising an alkaline compound, wherein the alkaline compound comprises lithium hydroxide, sodium hydroxide, potassium hydroxide, ammonium hydroxide or a mixture thereof. An electrode slurry composition comprising the polyvinyl alcohol graft copolymer according to any one of claims 1 to 9.