An adhesive, a negative electrode sheet, and a battery

Abstract

The present invention provides an adhesive, a negative electrode sheet and a battery. The adhesive comprises an aromatic-acrylate copolymer, and the aromatic-acrylate copolymer comprises acrylate structural units, aromatic hydrocarbon structural units, organic salt structural units, and functional structural units containing polar functional groups. The adhesive has high adhesive force, can improve the peel strength between the negative electrode active material layer and the negative electrode current collector, and reduce the rebound of the electrode sheet and the cell.

Description

Technical Field

[0001] The present invention relates to the field of batteries, and particularly to an adhesive, a negative electrode sheet and a battery. Background Art

[0002] Lithium-ion batteries have the advantages of high energy density, high output power, long service life, wide operating temperature range, low self-discharge rate, environmental friendliness, etc., and have been widely used in the fields of 3C, energy storage, power batteries, etc. Due to the theoretical specific capacity of silicon negative electrode materials being as high as 3572 mAh / g, rich reserves in the earth's crust, low cost, and environmental friendliness, they have been widely used in high energy density battery cell systems. Although silicon has significant specific capacity advantages, the volume expansion during the lithium insertion / extraction process is very serious, resulting in the self-crushing of silicon particles and detachment from the current collector, thereby accelerating the performance decay of the battery cell. To solve the problem of silicon negative electrode expansion, the size of silicon can be nano-sized, but currently, silicon-based negative electrodes have problems such as low peel strength, electrode sheet rebound, large full charge rebound of the battery cell, and short cycle life of the battery cell.

[0003] The mainstream binder in the conventional silicon-based negative electrode system is a combination of styrene-butadiene rubber (SBR) emulsion with point connection and linear polyacrylic acid (PAA) solution. However, due to the relatively large particle size of SBR, it is impossible to ensure effective contact with nano-scale silicon-based active materials and ensure high adhesion. In addition, the SBR and PAA combination system usually uses a high proportion of the combined binder to improve the peel strength between the negative electrode active material layer and the negative electrode current collector, which will sacrifice the energy density and comprehensive electrical performance of the battery cell.

[0004] Therefore, there is an urgent need to develop an adhesive with high adhesion, which can improve the peel strength between the negative electrode active material layer and the negative electrode current collector, and reduce the electrode sheet rebound and battery cell rebound. Summary of the Invention

[0005] The present application provides an adhesive, a negative electrode sheet and a battery. The adhesive has high adhesion, can improve the peel strength between the negative electrode active material layer and the negative electrode current collector, and reduce the electrode sheet rebound and battery cell rebound, solving the defects existing in the prior art.

[0006] One aspect of the present invention provides an adhesive, comprising an aromatic-acrylate copolymer, and the aromatic-acrylate copolymer comprises acrylate structural units, aromatic hydrocarbon structural units, organic salt structural units, and functional structural units containing polar functional groups.

[0007] According to an embodiment of the present invention, the aromatic-acrylate copolymer comprises a first aromatic-acrylate copolymer and a second aromatic-acrylate copolymer, and the average particle size D of the first aromatic-acrylate copolymer 50 is not greater than the average particle size D of the second aromatic-acrylate copolymer 50 .

[0008] According to an embodiment of the present invention, the mass ratio of the first aromatic-acrylate copolymer to the second aromatic-acrylate copolymer is 5:95 to 40:60.

[0009] According to an embodiment of the present invention, the D of the first aromatic-acrylate copolymer 50 = 80 nm to 300 nm; and / or, the D of the second aromatic-acrylate copolymer 50 = 300 nm to 600 nm.

[0010] According to an embodiment of the present invention, the acrylate structural unit includes one or more of a methyl acrylate structural unit, an ethyl acrylate structural unit, an isobutyl acrylate structural unit, a tert-butyl acrylate structural unit, a butyl acrylate structural unit, a 2-ethylhexyl acrylate structural unit, a lauryl acrylate structural unit, a stearyl acrylate structural unit, a methyl methacrylate structural unit, a butyl methacrylate structural unit, a phenyl methacrylate structural unit, a n-hexyl acrylate structural unit; and / or, the aromatic hydrocarbon structural unit includes a styrene structural unit; and / or, the organic salt structural unit includes an organic ammonium salt structural unit, and the organic ammonium salt structural unit includes one or more of a methacryloyloxyethyl dimethyl dodecyl ammonium bromide structural unit, a methacryloyloxyethyl dimethyl hexadecyl ammonium bromide structural unit, a methacryloyloxyethyl trimethyl ammonium bromide structural unit, a methacryloyloxyethyl dimethyl butyl ammonium bromide structural unit, a methacryloyloxyethyl dimethyl pentyl ammonium bromide structural unit, a methacryloyloxyethyl trimethyl ammonium chloride structural unit, an N,N-dimethylaminoethyl methacrylate structural unit, an acryloyloxyethyl trimethyl ammonium chloride structural unit, a dimethyldiallyl ammonium chloride structural unit; and / or, the functional structural unit containing a polar functional group includes an acrylamide structural unit, and the acrylamide structural unit specifically includes one or more of a hydroxyethyl acrylamide structural unit, an isobutoxymethyl acrylamide structural unit, a diacetone acrylamide structural unit.

[0011] According to an embodiment of the present invention, the viscosity of the binder is 35 to 85 mPa s.

[0012] On the other hand, the present invention provides a negative electrode sheet, including a negative electrode current collector and a negative electrode active material layer located on at least one side of the negative electrode current collector, and the negative electrode active material layer includes the above-mentioned binder or a binder prepared by the above-mentioned preparation method of the binder.

[0013] According to an embodiment of the present invention, the negative electrode active material layer further includes polyacrylic acid.

[0014] According to an embodiment of the present invention, the peel strength between the negative electrode active material layer and the negative electrode current collector is 12 to 16 N / m.

[0015] Another aspect of the present invention provides a battery, comprising a positive electrode sheet, a separator, and the above-mentioned negative electrode sheet.

[0016] The implementation of the present invention has at least the following beneficial effects: By using a binder including an aromatic-acrylate copolymer, which includes acrylate structural units, aromatic hydrocarbon structural units, organic salt structural units, and functional structural units containing polar functional groups, the adhesion of the binder is improved, which is beneficial to increasing the peel strength between the negative electrode active material layer and the negative electrode current collector, and reducing the pole piece rebound and the cell rebound. Detailed implementation manners

[0017] To make the objectives, technical solutions, and advantages of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0018] In the prior art, the mainstream binders for conventional silicon-based negative electrode systems are a combination of point-connected styrene-butadiene rubber (SBR) emulsion and linear polyacrylic acid (PAA) solution. SBR is a high polymer copolymerized from 1,3-butadiene and styrene. The SBR emulsion itself is a product with a balance of hydrophilicity and hydrophobicity. On the one hand, it combines with the main material (negative electrode active material) through point connection by its hydrophobic characteristics. On the other hand, it undergoes a condensation reaction with the oxide or hydroxyl groups on the surface of the copper foil through hydrophilic groups to form chemical bonds, providing the adhesion between the negative electrode active material particles and between the active material layer and the current collector through point connection; in addition, due to the huge expansion caused by the intrinsic material characteristics of the silicon-based negative electrode particles, linear or branched PAA is needed to effectively inhibit the volume expansion and contraction of the silicon-based negative electrode material. After the silicon-based negative electrode material undergoes a huge volume change, its structural stability can still be maintained. And PAA bonding with some functional groups on the surface of silicon (Si) can greatly improve the cycling performance of the silicon-based negative electrode, providing sufficient adhesion, thereby reducing the full charge rebound of the cell. By adjusting the ratio of the two and increasing the dosage of PAA, although the adhesion can be ensured to a certain extent and the full charge rebound can be reduced, due to the large particle size of SBR, effective contact with nano-scale silicon-based active materials cannot be ensured, and high adhesion cannot be ensured; and the SBR and PAA combination system usually uses a high combined binder ratio to increase the peel strength between the negative electrode active material layer and the negative electrode current collector, which will sacrifice the energy density and comprehensive electrical performance of the cell.

[0019] Based on this, an embodiment of the present invention provides an adhesive, which includes an aromatic-acrylate copolymer. The aromatic-acrylate copolymer includes acrylate structural units, aromatic hydrocarbon structural units, organic salt structural units, and functional structural units containing polar functional groups.

[0020] In some embodiments, the aromatic-acrylate copolymer includes a first aromatic-acrylate copolymer and a second aromatic-acrylate copolymer. The average particle size D of the first aromatic-acrylate copolymer 50 is not greater than the average particle size D of the second aromatic-acrylate copolymer 50 , that is, the average particle size D of the first aromatic-acrylate copolymer 50 is less than or equal to the average particle size D of the second aromatic-acrylate copolymer 50 .

[0021] As is well known to those skilled in the art, the above average particle size D 50 is the volume average particle size Dv 50 , which refers to the particle size corresponding to when the cumulative particle size distribution percentage of a sample reaches 50%. Its physical meaning is that 50% of the particles are larger than it and 50% of the particles are smaller than it.

[0022] In some embodiments, the mass ratio of the first aromatic-acrylate copolymer to the second aromatic-acrylate copolymer is 5:95 to 40:60, such as 5:95, 6:90, 1:10, 1:9, 1:5, 1:3, 3:7, 1:2, 40:60, or any range composed of any two of them, which is beneficial to improving the peeling strength between the negative electrode active material and the negative electrode current collector.

[0023] In some embodiments, the average particle size D of the first aromatic-acrylate copolymer 50 = 80 nm to 300 nm, such as 80 nm, 100 nm, 150 nm, 200 nm, 250 nm, 300 nm, or any range composed of any two of them, which is beneficial to bonding the silicon-carbon material with smaller particles in the negative electrode active material.

[0024] Specifically, the average particle size D of the second aromatic-acrylate copolymer 50 = 300 nm to 600 nm, such as 300 nm, 350 nm, 400 nm, 450 nm, 500 nm, 550 nm, 600 nm, or any range composed of any two of them, which is beneficial to bonding the graphite material with larger particles in the negative electrode active material.

[0025] In some embodiments, the acrylate structural units include one or more of methyl acrylate structural units, ethyl acrylate structural units, isobutyl acrylate structural units, tert-butyl acrylate structural units, butyl acrylate structural units, 2-ethylhexyl acrylate structural units, lauryl acrylate structural units, stearyl acrylate structural units, methyl methacrylate structural units, butyl methacrylate structural units, phenyl methacrylate structural units, and n-hexyl acrylate structural units.

[0026] In addition, the aromatic hydrocarbon structural units include styrene structural units.

[0027] In some embodiments, the organic salt structural units include organic ammonium salt structural units, and the organic ammonium salt structural units include one or more of methacryloyloxyethyl dimethyl dodecyl ammonium bromide structural units, methacryloyloxyethyl dimethyl hexadecyl ammonium bromide structural units, methacryloyloxyethyl trimethyl ammonium bromide structural units, methacryloyloxyethyl dimethyl butyl ammonium bromide structural units, methacryloyloxyethyl dimethyl pentyl ammonium bromide structural units, methacryloyloxyethyl trimethyl ammonium chloride structural units, N,N-dimethylaminoethyl methacrylate structural units, acryloyloxyethyl trimethyl ammonium chloride structural units, and dimethyldiallyl ammonium chloride structural units.

[0028] In addition, the functional structural units containing polar functional groups include acrylamide structural units, and the acrylamide structural units include one or more of hydroxyethyl acrylamide structural units, isobutoxymethyl acrylamide structural units, and diacetone acrylamide structural units.

[0029] In some embodiments, the viscosity of the binder is 35~85 mPa s, such as 35 mPa s, 40 mPa s, 45 mPa s, 50 mPa s, 55 mPa s, 60 mPa s, 65 mPa s, 70 mPa s, 75 mPa s, 80 mPa s, 85 mPa s or the range composed of any two of them.

[0030] In the present invention, as is well known to those skilled in the field of chemical synthesis, the above-mentioned "certain" structural unit refers to the structural unit corresponding to the monomer in the copolymer obtained by copolymerization of the "certain" monomer. For example, the acrylate structural unit refers to the corresponding structural unit present in the copolymer after the copolymerization of acrylate monomers.

[0031] An embodiment of the present invention further provides a method for preparing the above-mentioned binder, comprising the following steps: mixing a raw material system including acrylate monomers, aromatic hydrocarbon monomers, organic salt monomers, and functional monomers containing polar functional groups, and carrying out copolymerization reaction to obtain an aromatic-acrylate copolymer.

[0032] Specifically, the acrylate monomers include one or more of methyl acrylate, ethyl acrylate, isobutyl acrylate, tert-butyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, lauryl acrylate, stearyl acrylate, methyl methacrylate, butyl methacrylate, phenyl methacrylate, and n-hexyl acrylate.

[0033] In addition, the aromatic hydrocarbon monomers (or unsaturated double bond aromatic monomers) include styrene.

[0034] Specifically, the organic salt monomers (or cationic monomers) include organic ammonium salts, and the organic ammonium salts include organic ammonium salts containing unsaturated double bonds. The organic ammonium salts containing unsaturated double bonds include one or more of methacryloyloxyethyl dimethyl dodecyl ammonium bromide, methacryloyloxyethyl dimethyl hexadecyl ammonium bromide, methacryloyloxyethyl trimethyl ammonium bromide, methacryloyloxyethyl dimethyl butyl ammonium bromide, methacryloyloxyethyl dimethyl pentyl ammonium bromide, methacryloyloxyethyl trimethyl ammonium chloride, N,N-dimethylaminoethyl methacrylate, acryloyloxyethyl trimethyl ammonium chloride, and dimethyldiallyl ammonium chloride.

[0035] Specifically, the functional monomers containing polar functional groups include acrylamide functional monomers, and the acrylamide functional monomers include one or more of hydroxyethyl acrylamide, isobutoxymethyl acrylamide, and diacetone acrylamide.

[0036] Generally, the above-mentioned raw material system further includes a cationic emulsifier, a polymeric emulsifier, a buffer, an initiator, and a solvent.

[0037] Among them, the cationic emulsifier includes one or more of dodecyl ammonium chloride, cetyl trimethyl ammonium chloride, octadecyl trimethyl ammonium chloride, cetyl trimethyl ammonium bromide, cetyl pyridinium bromide, and octadecylamine polyoxyethylene ether bisquaternary ammonium salt.

[0038] Specifically, the polymeric emulsifier includes one or more of allyl polyoxyethylene ether, ammonium allyl polyoxyethylene ether sulfonate, sodium salt of allyl-containing ether sulfonic acid, sodium vinyl sulfonate, and double-bond alkyl polyoxyethylene phosphate.

[0039] Specifically, the buffer includes one or more of sodium bicarbonate, sodium dihydrogen phosphate, disodium hydrogen phosphate, and N-(2-hydroxyethyl)iminodiacetic acid.

[0040] Specifically, the initiator includes one or more of azobisisobutyronitrile, azobis(2-methylpropionamidine) dihydrochloride, 2,2'-azobis(2-isopropylimidazoline) dihydrochloride, dimethyl 2,2'-azobis(2-methylpropionate), ammonium persulfate, sodium persulfate, and potassium persulfate.

[0041] In addition, the solvent includes water.

[0042] In specific implementation, after mixing the polymeric emulsifier, cationic emulsifier and solvent, they are mixed with acrylate monomers, aromatic hydrocarbon monomers, organic salt monomers, and functional monomers containing polar functional groups to obtain a pre-emulsion. Then, the pre-emulsion is mixed with the buffer, solvent, and initiator for copolymerization reaction to obtain an aromatic-acrylate copolymer.

[0043] Among them, the reaction temperature of the copolymerization reaction is 60 °C to 90 °C, such as 60 °C, 65 °C, 70 °C, 75 °C, 80 °C, 85 °C, 90 °C, or the range composed of any two of them, which is beneficial to controlling the particle size of the aromatic-acrylate copolymer.

[0044] Specifically, the reaction time of the copolymerization reaction is 1 h to 10 h, such as 1 h, 2 h, 3 h, 4 h, 5 h, 6 h, 7 h, 8 h, 9 h, 10 h, or the range composed of any two of them.

[0045] In specific implementation, the mass ratio of acrylate monomers, aromatic hydrocarbon monomers, organic salt monomers, functional monomers containing polar functional groups, cationic emulsifier, polymeric emulsifier, buffer, and initiator is 20-50:5-30:5-30:10-20:0.05-0.4:0.1-3.0:0.01-10:0.1-3, preferably 30-40:10-20:5-30:12-15:0.15-0.25:0.5-1.0:0.01-10:0.5-1.5, which is beneficial to controlling the particle size of the aromatic-acrylate copolymer.

[0046] Among them, the mass ratio of acrylate monomers to aromatic hydrocarbon monomers is 0.6-10, such as 0.6, 0.7, 1, 1.5, 2, 2.5, 3, 3.5, 4, 5, 6, 7, 8, 9, 10, or the range composed of any two of them.

[0047] Specifically, the mass ratio of the acrylate monomer to the organic salt monomer is 0.6 to 10, such as 0.6, 0.7, 1, 1.5, 2, 2.5, 3, 3.5, 4, 5, 6, 7, 8, 9, 10, or the range composed of any two of them.

[0048] Specifically, the mass ratio of the acrylate monomer to the functional monomer containing a polar functional group is 1 to 5, such as 1, 2, 2.3, 2.5, 3, 3.3, 3.5, 4, 4.5, 5, or the range composed of any two of them.

[0049] Specifically, the mass ratio of the acrylate monomer to the cationic emulsifier is 50 to 1000, such as 50, 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000, or the range composed of any two of them.

[0050] Specifically, the mass ratio of the acrylate monomer to the polymeric emulsifier is 6 to 500, such as 6, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 120, 150, 200, 300, 400, 500, or the range composed of any two of them.

[0051] Specifically, the mass ratio of the acrylate monomer to the buffer is 2 - 5000, such as 2, 3, 5, 10, 50, 100, 500, 1000, 2000, 3000, 4000, 5000, or the range composed of any two of them.

[0052] Specifically, the mass ratio of the acrylate monomer to the initiator is 6 to 500, such as 6, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 120, 150, 200, 300, 400, 500, or the range composed of any two of them.

[0053] In the present invention, the above-mentioned first aromatic - acrylate copolymer and the second aromatic - acrylate copolymer can be prepared using the same or different raw material monomers, and there are no special restrictions in the present invention. As is well known to those skilled in the chemical synthesis field, the particle size of the polymer can be adjusted by adjusting the reaction temperature and stirring speed during emulsion polymerization. In the present invention, as those skilled in the art, according to the different combinations of raw material monomers provided by the present invention, the stirring speed, reaction temperature, and reaction time during emulsion polymerization can be correspondingly adjusted, and the average particle size D 50 can be adjusted to the aforementioned range.

[0054] An embodiment of the present invention also provides a negative electrode sheet, which includes a negative electrode current collector and a negative electrode active material layer located on at least one side of the negative electrode current collector. The negative electrode active material layer includes the above-mentioned binder or a binder prepared according to the preparation method of the above-mentioned binder.

[0055] Specifically, the negative electrode active material layer includes a binding material, and the binding material includes the above-mentioned binder or a binder prepared according to the preparation method of the above-mentioned binder.

[0056] In some embodiments, the binding material further includes polyacrylic acid, which is beneficial to inhibiting the volume expansion and contraction of the silicon-based negative electrode material and reducing the rebound of the electrode sheet and the battery cell.

[0057] According to the research of the inventor, by controlling the particle size of the aromatic-acrylate copolymer and using it in combination with polyacrylic acid, the bonding strength can be improved and the electrode sheet rebound can be reduced, thereby reducing the full-charge rebound of the battery cell and extending the cycle life of the battery.

[0058] Specifically, the negative electrode active material layer further includes a negative electrode active material, a negative electrode conductive agent, and a thickening agent. The negative electrode active material may include one or more of hard carbon and silicon-carbon materials; the negative electrode conductive agent may include one or more of conductive carbon black, carbon nanotubes (CNT), acetylene black, graphene, Ketjen black, and carbon fiber; the thickening agent may include sodium carboxymethyl cellulose, and the negative electrode current collector includes copper foil.

[0059] In the embodiment of the present invention, the negative electrode sheet can be prepared by a conventional method in the art, for example, by a coating method. Specifically, components for forming the negative electrode active material layer such as the negative electrode active material, the conductive agent, the thickening agent, and the negative electrode binder can be dispersed in a negative electrode solvent. The negative electrode solvent includes, for example, water, to prepare a negative electrode slurry, and then it is coated on the surface of the negative electrode current collector. After processes such as drying and rolling, the negative electrode sheet is obtained. Among them, the processes such as coating, drying, and rolling involved are conventional operations for preparing the negative electrode sheet by the coating method, and no special limitation is made thereto.

[0060] In some embodiments, the peel strength between the negative electrode active material layer and the negative electrode current collector is 12-16 N / m, such as 12 N / m, 13 N / m, 14 N / m, 15 N / m, 16 N / m, or a range composed of any two of them.

[0061] An embodiment of the present invention also provides a battery, which includes a positive electrode sheet, a separator, and the above-mentioned negative electrode sheet.

[0062] In some embodiments, the above-mentioned battery can be a lithium-ion battery, which has the corresponding advantages as the above-mentioned negative electrode binder and will not be elaborated herein.

[0063] Generally, a battery includes a battery cell, an electrolyte, and a housing for encapsulating the battery cell. The electrolyte is injected into the battery cell within the housing. The battery cell includes a positive electrode sheet, a negative electrode sheet, and a separator located between the positive electrode sheet and the negative electrode sheet. Among them, the battery cell can be a wound battery cell and / or a stacked battery cell.

[0064] In the embodiments of the present invention, a battery can be prepared by conventional methods in the art. For example, the positive electrode sheet, the separator, and the negative electrode sheet can be stacked alternately to obtain a stacked battery cell; or the positive electrode sheet, the separator, and the negative electrode sheet can be wound in sequence to obtain a wound battery cell. Then, the battery cell is placed in a housing, and after conventional processes such as liquid injection (i.e., injecting the electrolyte), encapsulation, standing, formation, and grading, the battery is obtained.

[0065] Specifically, the positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer located on at least one surface of the positive electrode current collector. Specifically, a positive electrode active material layer can be provided on one surface of the positive electrode current collector, or positive electrode active material layers can be respectively provided on opposite surfaces in the thickness direction of the positive electrode current collector.

[0066] Specifically, the positive electrode active material layer can include a positive electrode active material, a conductive agent, and a positive electrode binder, all of which can be conventional materials in the art. For example, the positive electrode active material can include one or more of lithium iron phosphate, lithium cobaltate, lithium manganate, and positive electrode ternary materials. The positive electrode ternary materials can include nickel cobalt manganese ternary materials and / or nickel cobalt aluminum ternary materials. The conductive agent can include one or more of conductive carbon black (Super. P), conductive graphite, carbon nanotubes (CNT), acetylene black, graphene, Ketjen black, and carbon fiber. The positive electrode binder can include one or more of polyvinylidene fluoride (PVDF), polyvinyl fluoride, polyethylene, polypropylene, polyvinyl alcohol, polyvinyl chloride, carboxylated polyvinyl chloride, polymers containing ethylene oxide, polyvinylpyrrolidone, and polyurethane.

[0067] In the embodiments of the present invention, a conventional positive electrode current collector in the art can be used. For example, the positive electrode current collector includes aluminum foil.

[0068] In the embodiments of the present invention, the positive electrode sheet can be obtained by conventional methods in the art. For example, it can be obtained by a coating method. Specifically, components for forming the positive electrode active material layer such as the positive electrode active material, the conductive agent, and the positive electrode binder can be dispersed in a positive electrode solvent. The positive electrode solvent includes, for example, N-methylpyrrolidone (NMP) to prepare a positive electrode slurry, and then it is coated on the surface of the positive electrode current collector. After processes such as drying and rolling, the positive electrode sheet is obtained. Among them, the processes such as coating, drying, and rolling involved are conventional operations for preparing the positive electrode sheet by the coating method, and no special limitation is made thereto.

[0069] In the embodiments of the present invention, the separator is used to separate the positive electrode sheet and the negative electrode sheet to prevent the positive electrode sheet and the negative electrode sheet from contacting and short - circuiting. Conventional separators in the art can be used in the embodiments of the present invention. For example, the separator includes a polypropylene film (PP film), polyethylene (PE), but is not limited thereto.

[0070] In the embodiments of the present invention, the electrolyte can be a non - aqueous electrolyte. The non - aqueous electrolyte generally includes a lithium salt, a carbonate solvent, and an additive. Among them, the carbonate solvent can include one or more of a cyclic carbonate solvent and a chain - like carbonate solvent, specifically including one or more of ethylene carbonate (EC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), propylene carbonate (PC), and ethyl propionate (EP). The lithium salt can include one or more of lithium hexafluorophosphate (LiPF 6 ) and lithium perchlorate. The additive can include one or more of fluoroethylene carbonate, difluoroethylene carbonate, fluorinated cyclotriphosphazene, hexafluorocyclotriphosphazene, and vinylene carbonate (VC).

[0071] In the embodiments of the present invention, conventional housing materials in the art can be used to encapsulate the battery cell. The housing includes, for example, soft packaging materials such as aluminum - plastic films (in this case, the battery is a soft - packaged battery), but is not limited thereto.

[0072] Hereinafter, the present invention will be further introduced through specific embodiments.

[0073] Example 1

[0074] 1. Preparation of the first aromatic - acrylate copolymer: By mass, the polymeric emulsifier, acrylate monomer, aromatic hydrocarbon monomer, organic salt monomer, functional monomer containing polar functional groups, initiator, cationic emulsifier, buffer, and solvent are 0.8 parts of allyl polyoxyethylene ether, 30 parts of methyl acrylate, 10 parts of styrene, 10 parts of methacryloyloxyethyl dimethyl dodecyl ammonium bromide, 12 parts of hydroxyethyl acrylamide, 1 part of azobisisobutyronitrile, 0.05 parts of dodecyl ammonium chloride, 1 part of sodium bicarbonate, and 70 parts of water.

[0075] Mix the polymeric emulsifier, cationic emulsifier with 30 parts of water, and then mix with the acrylate monomer, aromatic hydrocarbon monomer, organic salt monomer, and functional monomer containing polar functional groups to obtain a pre - emulsion. Then, mix 40 parts of water, buffer, initiator, and the pre - emulsion for copolymerization reaction. The reaction temperature is 80 °C and the time is 8 h to obtain an emulsion of the first aromatic - acrylate copolymer. Among them, the average particle size D 50 = 207 nm and its solid content is 45 wt%.

[0076] 2. Preparation of the second aromatic - acrylate copolymer

[0077] Using the same raw materials as in the preparation method of the first aromatic-acrylate copolymer, adjusting the temperature and stirring speed of the copolymerization reaction, an emulsion of the second aromatic-acrylate copolymer is obtained, where the average particle size D 50 of the second aromatic-acrylate copolymer is 416 nm and its solid content is 45 wt%.

[0078] Mix the emulsion of the first aromatic-acrylate copolymer and the emulsion of the second aromatic-acrylate copolymer according to the mass ratio of the polymers of 30:70 (i.e., the mass ratio of the first aromatic-acrylate copolymer to the second aromatic-acrylate copolymer is 30:70) to obtain an emulsion (binder) of the aromatic-acrylate copolymer with a viscosity of 51 mPa s.

[0079] 3. Preparation of the negative electrode sheet

[0080] By mass, 86 parts of graphite, 10 parts of silicon carbide, 0.5 part of thickening agent sodium carboxymethylcellulose, 2.0 parts of PAA binder, and 1 part of conductive carbon black are stirred at a low speed of 15 rpm for 15 min to obtain a mixed solution. Water is added to make the solid content of the mixed solution 55 wt%. After pre-stirring at 40 rpm for 15 min, kneading is carried out. After stirring at 40 rpm for 40 min, dispersion is carried out at a high speed of 15 m / s for 40 min. Then, 0.5 part by mass of the binder in step 2 (i.e., 0.5 part by mass of the aromatic-acrylate copolymer) is added. After dispersion at 5 m / s for 20 min, vacuum defoaming and discharging are carried out. Then, the negative electrode slurry is coated on both sides of the negative electrode current collector copper foil, and the single-sided coating surface density is 90 g / m 2 After drying, cold pressing, and compaction density of 1.6 g / cm 3 and other processes, a negative electrode sheet is obtained.

[0081] 4. Preparation of the positive electrode sheet

[0082] Mix the nickel-cobalt-manganese ternary positive electrode active material (NCM622), conductive carbon black, and PVDF according to the mass ratio of 97.3:1.5:1.2, and add NMP, stir evenly to prepare a positive electrode slurry;

[0083] Coat the positive electrode slurry on both the front and back surfaces of the aluminum foil. After drying and rolling, a positive electrode coating is formed on both the front and back surfaces of the aluminum foil to obtain a positive electrode sheet.

[0084] 5. Preparation of the battery

[0085] The negative electrode sheet, separator (PE film), and positive electrode sheet are wound in sequence, with the separator placed between the positive and negative electrodes to play an insulating role, forming an electrode core. The electrode core is placed in an outer package, filled with a commercially available electrolyte and sealed. After that, through processes such as liquid injection, formation, gas exhaust, and grading, a secondary battery is obtained. Among them, the electrolyte is composed of EC, DEC, PC, EP, VC, and LIPF 6 and the mass ratio of EC, DEC, PC, EP, and VC is 25:30:10:30:5. The concentration of LiPF 6 in the electrolyte is 15 wt%.

[0086] Example 2

[0087] The binder, negative electrode sheet, and battery are prepared according to the method of Example 1, with the differences being as follows:

[0088] 1. Preparation of the first aromatic-acrylate copolymer

[0089] By mass, the polymeric emulsifier, acrylate monomer, aromatic hydrocarbon monomer, organic salt monomer, functional monomer containing polar functional groups, initiator, cationic emulsifier, buffer, and solvent are 0.8 parts of allyl polyoxyethylene ether, 40 parts of butyl acrylate, 15 parts of styrene, 15 parts of N,N-dimethylaminoethyl methacrylate, 18 parts of isobutoxymethylacrylamide, 1 part of azobisisobutyronitrile, 0.05 parts of dodecylammonium chloride, 1 part of sodium bicarbonate, and 70 parts of water respectively.

[0090] The polymeric emulsifier and cationic emulsifier are mixed with 30 parts of water, and then mixed with the acrylate monomer, aromatic hydrocarbon monomer, organic salt monomer, and functional monomer containing polar functional groups to obtain a pre-emulsion. Then, 40 parts of water, buffer, initiator, and the pre-emulsion are mixed for copolymerization reaction. The reaction temperature is 80°C and the time is 10 h to obtain an emulsion of the first aromatic-acrylate copolymer. Among them, the average particle size D 50 of the first aromatic-acrylate copolymer is 288 nm, and its solid content is 45 wt%.

[0091] 2. Preparation of the second aromatic-acrylate copolymer

[0092] The same raw materials as those in the preparation method of the first aromatic-acrylate copolymer are used, and the temperature and stirring speed of the copolymerization reaction are adjusted to obtain an emulsion of the second aromatic-acrylate copolymer. Among them, the average particle size D 50 of the second aromatic-acrylate copolymer is 506 nm, and its solid content is 45 wt%.

[0093] Mix the emulsion of the above-mentioned first aromatic-acrylate copolymer and the emulsion of the second aromatic-acrylate copolymer at a polymer mass ratio of 40:60 (i.e., the mass ratio of the first aromatic-acrylate copolymer to the second aromatic-acrylate copolymer is 40:60) to obtain an emulsion (binder) of the aromatic-acrylate copolymer with a viscosity of 64 mPa s.

[0094] Example 3

[0095] Prepare the binder, negative electrode sheet and battery according to the method of Example 1, with the difference that:

[0096] 1. Preparation of the first aromatic-acrylate copolymer

[0097] By mass, the polymeric emulsifier, acrylate monomer, aromatic hydrocarbon monomer, organic salt monomer, functional monomer containing polar functional groups, initiator, cationic emulsifier, buffer, and solvent are 0.8 parts of allyl polyoxyethylene ether, 40 parts of ethyl acrylate, 20 parts of styrene, 10 parts of methacryloyloxyethyl trimethyl ammonium bromide, 20 parts of hydroxyethyl acrylamide, 1 part of azobisisobutyronitrile, 0.05 part of dodecyl ammonium chloride, 1 part of sodium bicarbonate, and 70 parts of water.

[0098] Mix the polymeric emulsifier, cationic emulsifier with 30 parts of water, and then mix with the acrylate monomer, aromatic hydrocarbon monomer, organic salt monomer, functional monomer containing polar functional groups to obtain a pre-emulsion; mix 40 parts of water, buffer, initiator and the pre-emulsion for copolymerization reaction. The reaction temperature is 70 °C and the time is 6 h to obtain an emulsion of the first aromatic-acrylate copolymer. Among them, the average particle size D 50 = 97 nm, and its solid content is 45 wt%.

[0099] 2. Preparation of the second aromatic-acrylate copolymer

[0100] Use the same raw materials as in the preparation method of the first aromatic-acrylate copolymer, adjust the temperature and stirring speed of the copolymerization reaction to obtain an emulsion of the second aromatic-acrylate copolymer. Among them, the average particle size D 50 = 591 nm, and its solid content is 45 wt%.

[0101] Mix the emulsion of the above-mentioned first aromatic-acrylate copolymer and the emulsion of the second aromatic-acrylate copolymer at a polymer mass ratio of 10:90 (i.e., the mass ratio of the first aromatic-acrylate copolymer to the second aromatic-acrylate copolymer is 10:90) to obtain an emulsion (binder) of the aromatic-acrylate copolymer with a viscosity of 69 mPa s.

[0102] Example 4

[0103] The binder, negative electrode sheet and battery were prepared according to the method of Example 1, except that: there was no second aromatic-acrylate copolymer.

[0104] Example 5

[0105] The binder, negative electrode sheet and battery were prepared according to the method of Example 1, except that: there was no first aromatic-acrylate copolymer.

[0106] Comparative Example 1

[0107] The difference from Example 1 was that 1.5 parts of SBR (solid content 40%, viscosity 45 mPa s) was used to replace the aromatic-acrylate copolymer.

[0108] The binders, negative electrode sheets, and batteries in the examples and comparative examples were tested through the following processes respectively, and the results are shown in Table 1:

[0109] (1) Solid content test: For the negative electrode slurry (the solid content test was carried out using a solid content meter. A small amount of the negative electrode slurry was spread on the surface of the copper foil of the cut round piece, and heated and baked with the solid content meter until the solid content no longer changed.

[0110] (2) Viscosity test: After the negative electrode slurry was prepared, the viscosity measured under a 3# rotor at 12 revolutions was the viscosity of the negative electrode slurry.

[0111] (3) Peel strength test: The negative electrode slurry was coated on one side of the negative electrode current collector copper foil, and the single-sided coating surface density was 90 g / m 2 , dried, and after compaction at 1.6 g / cm 3 , a single-sided negative electrode sheet was obtained. A tensile machine with a range of 20 N was used. The negative electrode sheet was cut into a length of 20 cm × a width of 3 cm. The 3M double-sided tape was pasted on the steel plate, and the negative electrode sheet was fixed on the tape on the steel plate with the coated surface facing down. After rolling back and forth 6 times with a 2.5 kg roller, then the coating and the copper foil were torn apart. The upper plate clamped the copper foil side, the speed was 50 mm / min, and the tension was measured at 180 °C. The data in the stable tension section was recorded as the peel strength (N / m).

[0112] (4) Electrode sheet rebound: The negative electrode slurry was coated on one side of the negative electrode current collector copper foil, and the single-sided coating surface density was 90 g / m 2, dried to ensure a compaction density of 1.7 g / cm3 to obtain a negative electrode sheet. The thickness of the negative electrode sheet at this time is measured as the initial thickness. After the negative electrode sheet is placed in a moisture-proof cabinet in a constant-temperature room for 24 hours, the thickness measured with a micrometer is the thickness of the negative electrode sheet after 24 hours. The electrode sheet rebound rate = (thickness of the negative electrode sheet after 24 hours - initial thickness) / initial thickness × 100%. Take 10 points for each electrode sheet and calculate the average value of the 10 points.

[0113] (5) Cell full-charge rebound: The battery after capacitance grading is charged at a constant current and constant voltage to full charge at 0.5C, then the battery is disassembled to obtain a cell. The cell is then placed in a drying room (relative humidity 0.5%) for full-charge disassembly. The thickness of the negative electrode sheet after disassembly is immediately measured and recorded as the thickness of the negative electrode sheet after full-charge disassembly. The cell full-charge rebound rate = (thickness of the negative electrode sheet after full-charge disassembly - thickness of the negative electrode sheet after 24 h) / thickness of the negative electrode sheet after 24 h × 100%. Take 10 points for each negative electrode sheet and calculate the average value of the 10 points.

[0114] (6) High-temperature cycle performance test: After the battery is placed in a constant-temperature test chamber at 45 °C ± 2 °C for 1 h, it is charged at a constant current and constant voltage of 1C to 3.65 V with a cut-off current of 0.05 C; discharged at a constant current of 1 C to 2.5 V, and the discharge capacity Q is recorded 1 ; Repeat the above steps 500 times and record the discharge capacity Q 500 , with Q 500 / Q 1 Calculate the capacity retention rate.

[0115] Table 1

[0116]

[0117] As can be seen from Table 1, compared with Comparative Example 1, the binders in Examples 1 to 5 include aromatic-acrylate copolymers. The aromatic-acrylate copolymers include acrylate structural units, aromatic hydrocarbon structural units, organic salt structural units, and functional structural units containing polar functional groups, which are beneficial to improving the peel strength between the negative active material layer and the negative current collector, reducing the electrode sheet rebound and the cell full-charge rebound, and improving the high-temperature cycle performance of the battery. In addition, compared with Comparative Example 1 (1.5 parts by mass of SBR), Examples 1 to 5 reduced the addition amount of the binder (0.5 parts by mass of aromatic-acrylate copolymer), improved the peel strength between the negative active material layer and the negative current collector, reduced the electrode sheet rebound and the cell full-charge rebound, and improved the high-temperature cycle performance of the battery.

[0118] Compared with Example 4 and Example 5, the binder in Example 1 includes a first aromatic-acrylate copolymer and a second aromatic-acrylate copolymer. The average particle size D 50 of the first aromatic-acrylate copolymer is not greater than the average particle size D50 , further improving the peeling strength between the negative electrode active material layer and the negative electrode current collector, reducing the pole piece rebound and the full charge rebound of the battery cell, and improving the high-temperature cycle performance of the battery.

[0119] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements for some or all of the technical features; and these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A binder, characterized in that: The invention comprises an aromatic-acrylate copolymer, wherein the aromatic-acrylate copolymer comprises an acrylate structural unit, an aromatic hydrocarbon structural unit, an organic salt structural unit, and a functional structural unit containing a polar functional group, wherein the aromatic hydrocarbon structural unit is selected from a styrene structural unit, the organic salt structural unit is selected from an organic ammonium salt structural unit, and the functional structural unit containing a polar functional group is selected from an acrylamide structural unit, wherein the aromatic-acrylate copolymer comprises a first aromatic-acrylate copolymer and a second aromatic-acrylate copolymer, and wherein the average particle size D of the first aromatic-acrylate copolymer is 50 Not greater than the average particle size D of the second aromatic-acrylate copolymer 50 ; The aromatic-acrylate copolymer can be prepared by a process comprising the following steps: mixing a raw material system comprising an acrylate monomer, an aromatic hydrocarbon monomer, an organic salt monomer, and a functional monomer containing a polar functional group, and performing a copolymerization reaction to obtain the aromatic-acrylate copolymer, wherein the mass ratio of the acrylate monomer, the aromatic hydrocarbon monomer, the organic salt monomer, and the functional monomer containing a polar functional group is 20-50:5-30:5-30:10-20, the organic salt monomer is selected from an organic ammonium salt containing an unsaturated double bond, the aromatic hydrocarbon monomer is selected from styrene, and the functional monomer containing a polar functional group is selected from an acrylamide functional monomer.

2. The adhesive according to claim 1, characterized in that The mass ratio of the first aromatic-acrylate copolymer to the second aromatic-acrylate copolymer is 5:95 to 40:

60.

3. The adhesive according to claim 1 or 2, characterized in that: The D of the first aromatic-acrylate copolymer 50 = 80 nm ~ 300 nm; and / or, D of the second aromatic-acrylate copolymer 50 =300 nm~600 nm.

4. The adhesive according to claim 1, characterized in that The acrylic acid ester structural unit includes one or more of a methyl acrylate structural unit, an ethyl acrylate structural unit, and a butyl acrylate structural unit; And / or, the organic ammonium salt structural unit includes one or more of a methacryloyloxyethyl dimethyl dodecyl ammonium bromide structural unit, a methacryloyloxyethyl dimethyl hexadecyl ammonium bromide structural unit, a methacryloyloxyethyl trimethyl ammonium bromide structural unit, a methacryloyloxyethyl dimethyl butyl ammonium bromide structural unit, a methacryloyloxyethyl dimethyl pentyl ammonium bromide structural unit, a methacryloyloxyethyl trimethyl ammonium chloride structural unit, an acryloyloxyethyl trimethyl ammonium chloride structural unit, and a dimethyl diallyl ammonium chloride structural unit; And / or, the acrylamide structural unit specifically includes one or more of a hydroxyethyl acrylamide structural unit, an isobutyloxymethyl acrylamide structural unit, and a diacetone acrylamide structural unit.

5. The adhesive according to claim 1, characterized in that The viscosity of the binder is 35-85 mPa s.

6. A negative electrode sheet, characterized in that: The invention comprises a negative electrode current collector and a negative electrode active material layer located on at least one side of the negative electrode current collector, wherein the negative electrode active material layer comprises the binder according to any one of claims 1 to 5.

7. The negative electrode sheet according to claim 6, characterized in that: The negative electrode active material layer further includes polyacrylic acid.

8. The negative electrode sheet according to claim 6, characterized in that: The peel strength between the negative electrode active material layer and the negative electrode current collector is 12-16 N / m.

9. A battery, characterized in that: It comprises a positive electrode sheet, a separator, and the negative electrode sheet according to any one of claims 6 to 8.

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