Binder for lithium ion battery and preparation method thereof

Abstract

The present invention belongs to the field of electrochemical technology, and specifically relates to a binder for lithium ion batteries. The binder contains polyvinyl alcohol, which contains chemically cross-linked carboxyl and hydroxyl structures. The binder of the present invention has excellent bonding performance and can improve the slurry stability during the preparation of electrodes.

Description

Technical Field

[0001] Lithium-ion batteries have the advantages of high specific energy, low self-discharge, good cycle performance, no memory effect, and green environmental protection. At present, lithium-ion batteries are widely used in aerospace, power grid energy storage, electronic communications, national defense, electric vehicles, hybrid vehicles, large and medium-sized power tools and other fields ("Analysis of the Development Status and Prospects of Lithium-ion Batteries", Yan Jinding, Acta Aeronautica Sinica, Vol. 35, No. 10, 2014, p. 2767, left column, paragraph 2, lines 1-2 and p. 2772, right column, paragraph 6, lines 1-7, published on October 25, 2014).

[0002] At present, the negative electrode materials of lithium-ion batteries mainly use natural graphite and artificial graphite. Among them, the discharge capacity of artificial graphite is generally 300-330mAh / g, and the discharge capacity of natural graphite is relatively high, close to the theoretical value of 372mAh / g (CN201810196557.1). However, its battery capacity is far from meeting people's demand for high energy density batteries. In this context, silicon negative electrode materials with a theoretical capacity of 4200mAh / g have become the ideal next-generation negative electrode materials for lithium-ion batteries.

[0003] At present, silicon negative electrode materials generally use CMC / SBR binder. However, the binding force of this binder is weak, and after the active material is coated on the electrode, it is easy to cause material falling, cracking, curling and other undesirable phenomena during the baking process. Background Art

[0004] In view of the above, an object of the present invention is to provide a binder for lithium ion batteries having excellent bonding performance.

[0005] During the research, the inventors also found that in the process of preparing electrodes using existing binders, the slurry has poor stability. After being coated on the negative electrode sheet, the electrode sheet is prone to powder loss, which ultimately leads to poor battery performance.

[0006] To achieve the above object, the technical solution of the present invention is:

[0007] A binder for lithium ion batteries, comprising polyvinyl alcohol, wherein the polyvinyl alcohol comprises a carboxyl group -COO of a chemically cross-linked structure - , -OH hydroxyl structure.

[0008] The lithium-ion battery refers to a battery that mainly relies on the movement of lithium ions between a positive electrode and a negative electrode to work.

[0009] The polyvinyl alcohol refers to a water-soluble resin obtained by alcoholysis of polyvinyl acetate ("Encyclopedia of Chemistry and Chemical Engineering", Editorial Committee of Encyclopedia of Chemistry and Chemical Engineering, Dictionary Editing Department of Chemical Industry Press, Beijing: Chemical Industry Press, first printing in January 2003, page 1285, published on January 31, 2003).

[0010] The chemical crosslinking refers to the process of connecting linear polymer chains into network or body polymers by covalent bonds through condensation polymerization and addition polymerization ("Chemical and Chemical Engineering Dictionary", Chemical and Chemical Engineering Dictionary Editorial Committee, Chemical Industry Press Dictionary Editing Department, Beijing: Chemical Industry Press, first printing in January 2003, page 1154, published on January 31, 2003).

[0011] The binder refers to a substance that can bind adhered substances together through adhesion ("Chemical and Chemical Engineering Dictionary", Chemical and Chemical Engineering Dictionary Editorial Committee, Chemical Industry Press Dictionary Editing Department, Beijing: Chemical Industry Press, first printing in January 2003, page 1161, published on January 31, 2003).

[0012] The carboxyl group refers to a monovalent functional group composed of a carbonyl group and a hydroxyl group, and has the structural formula -COOH ("Dictionary of Chemistry and Chemical Engineering", Editorial Committee of Dictionary of Chemistry and Chemical Engineering, Dictionary Editing Department of Chemical Industry Press, Beijing: Chemical Industry Press, first printing in January 2003, page 2204, published on January 31, 2003).

[0013] The hydroxyl group refers to a monovalent atomic group composed of an oxygen atom and a hydrogen atom connected together, with the structural formula -OH ("Dictionary of Chemistry and Chemical Engineering", Editorial Committee of the Dictionary of Chemistry and Chemical Engineering, Dictionary Editing Department of Chemical Industry Press, Beijing: Chemical Industry Press, first printing in January 2003, page 1824, published on January 31, 2003).

[0014] The binder of the present invention has strong binding force and can improve the slurry stability during the electrode preparation process.

[0015] The present invention also aims to protect a method for preparing the adhesive, which includes polymerization, distillation, alcoholysis, washing and drying steps, wherein the monomers used in the polymerization step include vinyl acetate, unsaturated carboxylic acid monomers or acrylate monomers (such as methyl methacrylate, methyl acrylate, methyl methacrylate, butyl methacrylate, ethyl acrylate, propyl acrylate, isopropyl acrylate, n-butyl acrylate, isobutyl acrylate, lauryl acrylate, etc.) and triallyl ether monomers or dienyl amide monomers.

[0016] Preferably, the unsaturated carboxylic acid monomer includes methacrylic acid, acrylic acid, maleic acid, crotonic acid or itaconic acid.

[0017] Preferably, the triallyl ether monomer includes pentaerythritol triallyl ether or pentaerythritol triallyl ether.

[0018] Preferably, the diene amide monomers include N,N-diallylacrylamide or N,N-methylenebisacrylamide.

[0019] Preferably, the mass ratio of vinyl acetate, unsaturated carboxylic acid monomer or acrylate monomer and crosslinking agent monomer triallyl ether monomer or bisvinyl amide monomer is: 86.3%-93.2%: 6.7%-13.1%: 0.1%-0.6%.

[0020] Preferably, the solvent used in the polymerization includes 1 to 3 of methanol, ethanol and tert-butanol.

[0021] Preferably, the mass ratio of the sum of vinyl acetate, unsaturated carboxylic acid monomers or acrylate monomers and crosslinking agent monomers triallyl ether monomers or dienyl amide monomers to the solvent used for polymerization is 70wt%-80wt%:20wt%-30wt%.

[0022] Preferably, the initiator used for the polymerization includes azobisisobutyl cyanide, azobisisoheptyl cyanide or a peroxide (such as benzoyl peroxide, tert-butyl peroxyneodecanoate).

[0023] Preferably, the polymerization temperature is 55-70° C. and the polymerization time is 2-8 hours.

[0024] Preferably, the distillation temperature of the rectification is 65-80°C.

[0025] Preferably, during the alcoholysis, the polymer solution containing 10% to 30% copolymer by mass after distillation is subjected to alcoholysis reaction under the catalysis of a catalyst.

[0026] Preferably, the catalyst comprises an alcohol solution of an inorganic base or an organic base.

[0027] Preferably, the inorganic base includes sodium hydroxide and lithium hydroxide.

[0028] Preferably, the organic base comprises sodium methoxide or sodium ethoxide.

[0029] Preferably, the catalyst is used in an amount with a molar ratio of 5:100 to 30:100 to the vinyl acetate copolymer obtained by the polymerization reaction.

[0030] Preferably, the mass concentration of the catalyst is 30%-50%.

[0031] Preferably, the alcoholysis reaction temperature is 40-50° C., and the reaction time is 1-2 hours.

[0032] Preferably, the washing refers to washing with an alcohol substance.

[0033] Preferably, the drying temperature is 60-110° C., and the drying time is 3-8 hours.

[0034] The present invention also aims to protect a negative electrode for a lithium electronic battery, comprising:

[0035] A binder as described above;

[0036] Silicon-based / graphite composite negative electrode material pole piece or pure silicon negative electrode material pole piece;

[0037] Conductive carbon black;

[0038] Copper foil.

[0039] The beneficial effects of the present invention are:

[0040] The adhesive of the present invention has excellent bonding performance.

[0041] The binder of the present invention can further improve the slurry stability during the electrode preparation process.

[0042] The preparation method of the invention is simple and is conducive to industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 is the infrared spectrum detection result, where the horizontal axis is the wave number and the vertical axis is the transmittance;

[0044] Figure 2 It is a thermogravimetric spectrum, where the abscissa is temperature and the ordinate is weight;

[0045] Figure 3 It is a cross-linked structure. DETAILED DESCRIPTION

[0046] The examples are provided to better illustrate the content of the present invention, but the content of the present invention is not limited to the examples. Therefore, those skilled in the art can make non-essential improvements and adjustments to the implementation scheme according to the above content of the invention, which still fall within the protection scope of the present invention.

[0047] Example 1

[0048] The binder is prepared by using the following raw materials according to the following steps:

[0049] In a three-necked glass flask, vinyl acetate, acrylic acid, pentaerythritol triallyl ether and ethanol were added in a mass ratio of 68.6:9:0.4:22, and then an initiator azobisisobutyl cyanide was added in an amount of 0.02% by mass of vinyl acetate, and the mixture was reacted at a reaction temperature of 61° C. for 6 hours to obtain a vinyl acetate copolymer;

[0050] The polymer solution was distilled and purified at 70°C to obtain a vinyl acetate copolymer with a purity of 99.0%;

[0051] A copolymer of vinyl acetate with a purity of 99.0% is prepared into an ethanol solution with a mass concentration of 22.0%, and mixed with a sodium ethoxide-ethanol solution with a mass concentration of 30% according to a molar ratio of sodium ethoxide contained in the sodium ethoxide-ethanol solution to the vinyl acetate copolymer contained in the ethanol solution of 30:100, and subjected to alcoholysis reaction at 40°C for 1 hour;

[0052] The alcoholysis reaction product was washed with ethanol several times and then dried in a vacuum oven at 75°C for 6 hours.

[0053] Example 2

[0054] The difference between this embodiment and embodiment 1 is that the unsaturated carboxylic acid monomer is methacrylic acid, the solvent used for polymerization is methanol, the initiator is benzoyl peroxide, and the polymerization reaction time is 4 hours. The alcoholysis reaction uses a sodium methoxide-methanol solution with a mass concentration of 30%, and the amount of the solution is a molar ratio of sodium methoxide contained in the sodium methoxide-methanol solution to the vinyl acetate copolymer contained in the methanol solution of 30:100, and the alcoholysis reaction is carried out at 45°C for 1 hour.

[0055] Example 3

[0056] The difference between this embodiment and embodiment 1 is that maleic acid is used as the unsaturated carboxylic acid monomer, methanol is used as the polymerization solvent, tert-butyl peroxydecanoate is used as the initiator, and the polymerization reaction time is 5 hours.

[0057] The alcoholysis reaction uses a sodium hydroxide-methanol solution with a mass concentration of 40%. The amount of the solution used is a molar ratio of 30:100 between the sodium hydroxide contained in the sodium hydroxide-methanol solution and the vinyl acetate copolymer contained in the methanol solution of the polymer. The alcoholysis reaction is carried out at 45° C. for 1 hour.

[0058] Example 4

[0059] The difference between this embodiment and embodiment 1 is that itaconic acid is used as the unsaturated carboxylic acid monomer, methanol is used as the polymerization solvent, a lithium hydroxide-methanol solution with a mass concentration of 30% is used for the alcoholysis reaction, the amount of the solution is 30:100 in molar ratio of lithium hydroxide contained in the lithium hydroxide-methanol solution to vinyl acetate copolymer contained in the methanol solution of the polymer, and the alcoholysis reaction is carried out at 45° C. for 1 hour.

[0060] Example 5

[0061] The difference between this embodiment and embodiment 1 is that crotonic acid is used as the unsaturated carboxylic acid monomer, methanol is used as the polymerization solvent, a sodium methoxide-methanol solution with a mass concentration of 30% is used for the alcoholysis reaction, and the molar ratio of the sodium methoxide contained in the sodium methoxide-methanol solution to the vinyl acetate copolymer contained in the methanol solution of the polymer is 30:100, and the alcoholysis reaction is carried out at 45°C for 1 hour.

[0062] Example 6

[0063] The difference between this embodiment and embodiment 1 is that: the acrylic acid ester monomer is methyl methacrylate, and the solvent used for polymerization is methanol. The initiator is azobisisobutyl cyanide, the reaction temperature is 65°C, and the reaction time is 5 hours. The alcoholysis reaction uses a sodium hydroxide-methanol solution with a mass concentration of 40%, and the amount of the solution is a molar ratio of 30:100 between the sodium hydroxide contained in the sodium hydroxide-methanol solution and the vinyl acetate copolymer contained in the methanol solution of the polymer, and the alcoholysis reaction is carried out at 40°C for 1 hour.

[0064] Example 7

[0065] The difference between this embodiment and embodiment 1 is that: the acrylic acid ester monomer is methyl acrylate, and the solvent used for polymerization is methanol. The initiator is tert-butyl peroxydecanoate, the reaction temperature is 65°C, and the reaction time is 4 hours. The alcoholysis reaction uses a sodium methoxide-methanol solution with a mass concentration of 30%, and the amount of the solution is a molar ratio of sodium methoxide contained in the sodium methoxide-methanol solution to the vinyl acetate copolymer contained in the methanol solution of the polymer of 30:100, and the alcoholysis reaction is carried out at 45°C for 1 hour.

[0066] Example 8

[0067] The difference between this embodiment and embodiment 1 is that the acrylic acid ester monomer is n-butyl acrylate, and the solvent used for polymerization is methanol. The initiator is benzamide peroxide, the reaction temperature is 61°C, and the reaction time is 5 hours. The alcoholysis reaction uses a lithium hydroxide-methanol solution with a mass concentration of 40%, and the amount of the solution is a molar ratio of lithium hydroxide contained in the lithium hydroxide-methanol solution to vinyl acetate copolymer contained in the methanol solution of the polymer of 30:100, and the alcoholysis reaction is carried out at 45°C for 1 hour.

[0068] Example 9

[0069] The difference between this embodiment and embodiment 1 is that the acrylic acid ester monomer is n-butyl acrylate, and the solvent used for polymerization is methanol. The initiator is benzamide peroxide, the reaction temperature is 61°C, and the reaction time is 5 hours. The alcoholysis reaction uses a lithium hydroxide-methanol solution with a mass concentration of 40%, and the amount of the solution is a molar ratio of lithium hydroxide contained in the lithium hydroxide-methanol solution to vinyl acetate copolymer contained in the methanol solution of the polymer of 30:100, and the alcoholysis reaction is carried out at 45°C for 1 hour.

[0070] Example 10

[0071] The difference between this embodiment and embodiment 1 is that the triallyl ether monomer is pentaerythritol triallyl ether, and the solvent used for polymerization is methanol. The initiator is tert-butyl peroxyneodecanoate, the reaction temperature is 65°C, and the reaction time is 4 hours. The alcoholysis reaction uses a sodium methoxide-methanol solution with a mass concentration of 30%, and the amount of the solution is a molar ratio of 30:100 between the sodium methoxide contained in the sodium methoxide-methanol solution and the vinyl acetate copolymer contained in the methanol solution of the polymer, and the alcoholysis reaction is carried out at 45°C for 1 hour.

[0072] Embodiment 11

[0073] The difference between this embodiment and embodiment 1 is that the divinylamide monomer uses N,N-methylenebisacrylamide. The solvent used for polymerization is methanol. The initiator uses tert-butyl peroxyneodecanoate, the reaction temperature is 65°C, and the reaction time is 4 hours. The alcoholysis reaction uses a sodium methoxide-methanol solution with a mass concentration of 30%, and the amount of the solution is a molar ratio of 30:100 between the sodium methoxide contained in the sodium methoxide-methanol solution and the vinyl acetate copolymer contained in the methanol solution of the polymer, and the alcoholysis reaction is carried out at 45°C for 1 hour.

[0074] Example 12

[0075] The difference between this embodiment and embodiment 1 is that the diene amide monomer uses N,N-diallylacrylamide. The solvent used for polymerization is methanol. The initiator uses tert-butyl peroxyneodecanoate, the reaction temperature is 65°C, and the reaction time is 4 hours. The alcoholysis reaction uses a sodium methoxide-methanol solution with a mass concentration of 30%, and the amount of the solution is a molar ratio of 30:100 between the sodium methoxide contained in the sodium methoxide-methanol solution and the vinyl acetate copolymer contained in the methanol solution of the polymer, and the alcoholysis reaction is carried out at 45°C for 1 hour.

[0076] Comparative Example 1

[0077] Preparation process of styrene-butadiene rubber (SBR): 70 parts by mass of butadiene and 30 parts by mass of styrene are uniformly mixed to prepare a hydrocarbon phase. Emulsifier rosin soap, rongalite, EDTA, and ferrous sulfate are prepared as activators according to 5 parts, 0.2 parts, 0.08 parts, and 0.03 parts, and then 0.3 parts of diffusant sodium dodecyl sulfonate and 1.0 parts of potassium chloride are added, and finally 250 parts of soft water are added to obtain an aqueous phase. The above two phases are added to a multi-mouth flask respectively, and finally 0.4 parts of initiator hydrogen peroxide-ferrous sulfate-rongalite and 0.2 parts of regulator sodium hydroxide are added, and polymerized at 8°C for 10 hours. The emulsion obtained by polymerization is filtered, and the unreacted butadiene is removed in a flash bottle, and then the unreacted styrene is removed by vacuum degassing to obtain a styrene-butadiene rubber emulsion. The prepared SBR emulsion is mixed with sodium carboxymethyl cellulose (CMC) at a mass ratio of 60:40 to obtain a binder with dispersibility.

[0078] Example 13

[0079] Battery electrode preparation:

[0080] The binders prepared in the above examples and comparative examples were dissolved in water to prepare a binder aqueous solution with a mass concentration of 10%;

[0081] Any one of natural graphite, artificial graphite, and silicon-carbon composite material is mixed with binder electrode slurry and conductive carbon black in a mass ratio of 82:8:10, and then an appropriate amount of deionized water is added in a ratio of 50% of the mixed material to make a binder slurry. The slurry is stirred at high speed in a ball mill for 30 minutes. After the evenly dispersed slurry is passed through a 100-mesh screen, the slurry is added to the slurry buffer tank of the coating machine, and then evenly coated on both sides of a 15μm thick copper foil to form a wet film electrode, which is placed in an oven and dried at a temperature of 120°C. Then, at room temperature, 10×10 4 The electrode sheet is obtained by rolling with a unit length load of N / m.

[0082] Binder dispersion test

[0083] The binder obtained in the above embodiments and comparative examples was mixed with the silicon-carbon composite material, the conductive agent carbon black and water in a mass ratio of 3:47:50 to prepare a slurry with a solid content of 50%, and the slurry was mixed by high-speed stirring in a ball mill to obtain a uniform slurry.

[0084] Test the viscosity of the slurry (hereinafter referred to as initial viscosity);

[0085] After standing at room temperature for 24 hours, the viscosity of the slurry was tested again and compared with the initial viscosity;

[0086] If the viscosity change is less than ±100Mpa.s, the binder has good dispersibility; if the viscosity change is less than ±200Mpa.s and greater than ±100Mpa.s, the binder has good dispersibility. The results are shown in Table 1.

[0087] Adhesive performance testing

[0088] The adhesive prepared in Example 1 was subjected to infrared and thermogravimetric tests, and the results were as follows: Figure 1-2 As shown in Table 1;

[0089] Among them, the infrared detection method is: first, add binder powder and distilled water in a beaker according to a mass ratio of 3:97, heat to 70-80°C with a magnetic stirrer, stir to dissolve, and make a dilute solution with a concentration of 3%, then pour it on a glass plate and scrape it with a glass rod to form a thick film with a gradient, wait for the water to evaporate to form a thin film (the thickness is preferably 0.01-0.05mm), and peel it with a blade; measure the MIR-TR spectrum of the formed film in the mid-infrared transmission mode;

[0090] The thermal weight loss detection method is: use an analytical balance to weigh an appropriate amount of sample and place it in a platinum crucible. In a nitrogen atmosphere, heat the temperature from room temperature to 800°C at a rate of 20°C / min to obtain a mass-temperature curve.

[0091] Depend on Figure 1 It can be seen that the binder prepared in Example 1 has a -1 A broad and strong peak appears at 3650-3580cm, which is the stretching vibration peak of OH. -1 The stretching vibration absorption peak of OH group appears at 1567cm, and the peak shape is generally sharp, which is the stretching vibration absorption peak of free OH group; due to the association, it shifts to the low wave number direction; 1567cm -1 and 1408cm -1 The characteristic peak of -COO- appears at

[0092] Depend on Figure 2 It can be seen that the mass change curve of the binder prepared in Example 1 with temperature has two obvious weight loss "steps". The first mass change step corresponds to the loss of -COO - The second mass change step corresponds to the process of losing -CC- and cross-linked skeleton structure. The curve shows the mass loss change curve of two block copolymers, indicating that it is a copolymer structure of at least two monomers.

[0093] In summary, the binder prepared in Example 1 has a chemically cross-linked carboxylate-COO - , hydroxyl-OH structure (such as Figure 3 shown).

[0094] Battery pole performance test

[0095] In the battery manufacturing process, the peeling force of the battery pole piece is a very important indicator, which directly reflects the bonding effect between the active material on the battery pole piece and the copper / aluminum current collector. If the peeling force of the battery pole piece is too low, the active material will expand in cycles and detach from the current collector and become inactive, so the battery decay will accelerate;

[0096] When testing, first observe the surface flatness of the battery electrode, and then test;

[0097] The method for testing the peeling force of battery electrodes is as follows: (1) Cutting the negative electrode after rolling. Use a customized knife die to cut the electrode and cut the electrode into strips of 20cm×2.5cm; (2) Pasting and fixing the electrode Take a flat thin steel plate with a length of about 20-30cm and a width of about 2-6cm. First, stick a double-sided tape in the center of the steel plate (the length can be greater than the sample test length and the same width as the electrode), and smooth it with force to ensure that the double-sided tape is tightly attached to the center of the steel plate. Peel off the double-sided tape and attach the electrode to the tape. It is necessary to ensure that the electrode and the tape are matched and attached, otherwise the width of the test peeling force will change, the test value will be inaccurate, and the peeling curve will have jump points or waves. (3) After the test sample is fixed, first calibrate and reset, set the test width, the electrode peeling length is 5-25cm, and peel it in the 180° direction at a speed of 100m / min to obtain the peeling force. Set five samples in each group for repeated testing, and take the average value as the test result. The results are shown in Table 1.

[0098] Table 1 Performance test results

[0099]

[0100]

[0101] In addition, it should be understood that although the present specification is described according to implementation modes, not every implementation mode contains only one independent technical solution. This description of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment may also be appropriately combined to form other implementation modes that can be understood by those skilled in the art.

Claims

1. A method for preparing a binder for lithium-ion batteries, comprising polymerization, distillation, alcoholysis, washing and drying steps, characterized in that: The monomers used for polymerization include vinyl acetate, unsaturated carboxylic acid monomers or acrylate monomers and crosslinking agent monomers such as triallyl ether monomers or dienyl amide monomers; the binder contains polyvinyl alcohol, which contains chemically crosslinked carboxyl and hydroxyl structures.

2. The preparation method according to claim 1, characterized in that: The unsaturated carboxylic acid monomer includes methacrylic acid, acrylic acid, maleic acid, crotonic acid or itaconic acid.

3. The preparation method according to claim 1, characterized in that: The triallyl ether monomers include pentaerythritol triallyl ethers or pentaerythritol triallyl ethers.

4. The preparation method according to claim 1, characterized in that: The diene amide monomers include N,N-bisvinyl amides or N,N-diallylacrylamide.

5. The preparation method according to any one of claims 1 to 4, characterized in that: The mass ratio of vinyl acetate, unsaturated carboxylic acid monomer or acrylate monomer and crosslinking agent monomer triallyl ether monomer or divinyl amide monomer is: 86.3%-93.2%: 6.7%-13.1%: 0.1%-0.6%.

6. The preparation method according to claim 5, characterized in that: The solvents used in the polymerization include 1 to 3 of methanol, ethanol and tert-butanol.

7. The preparation method according to any one of claims 1 to 4 or 6, characterized in that: The drying temperature is 60-110° C., and the drying time is 3-8 hours.

Images