A polyacrylate binder and its application, an electrode sheet and a lithium-ion battery

Abstract

The present invention discloses a polyacrylate binder and its application, an electrode sheet and a lithium-ion battery, which relate to the technical field of secondary battery materials. The polyacrylate binder provided by the present invention realizes good adhesion performance by regulating the molecular weight of the binder and the contents of cyano groups, ester groups and carboxylate groups in the molecular chain, and can also realize the regulation of the electrolyte absorption rate of the binder; moreover, the binder is harmless to humans, environmentally friendly and inexpensive. By introducing the above binder as a raw material for the aqueous safety coating into the electrode sheet, based on its excellent adhesion performance, the aqueous safety coating can be firmly adhered to the surface of the current collector of the electrode sheet, reducing the surface contact resistance and improving the safety of the battery cell. Not only that, by limiting the contents of cyano groups, ester groups and carboxylate groups in the binder, the rate performance and high and low temperature discharge performance of the battery cell can be significantly improved, and the safety performance and cycle performance of the battery can be improved.

Description

Technical Field

[0001] The present invention relates to the technical field of secondary battery materials, and particularly to a polyacrylate binder and its application, an electrode sheet, and a lithium ion battery. Background Art

[0002] Lithium ion batteries are widely used in products such as 3C, electric vehicles, and power tools due to their advantages of high energy density, no memory effect, long cycle life, environmental friendliness, and adaptability to various environments. In practical applications, in order to improve the safety performance of the battery, introducing a safety coating into the electrode sheet of the battery is one of the commonly used measures in the industry.

[0003] In related technologies, most safety coatings contain a certain proportion of inorganic fillers, conductive agents, and binders. Currently, the commonly used binder is mainly polyvinylidene fluoride (PVDF). However, PVDF binders have the following deficiencies: (1) Performance: The adhesion provided by PVDF mainly depends on the van der Waals force between molecules, resulting in insufficient adhesion of the safety coating using PVDF; (2) Price and source: Currently, high-performance PVDF is expensive and has limited sources, greatly increasing the manufacturing cost of the electrode sheet; (3) Environmental friendliness: When using PVDF, volatile organic solvents such as NMP (N-methylpyrrolidone) need to be used, which not only pollutes the environment but also is harmful to human health. At the same time, the use of NMP further increases the material cost and the corresponding recycling and treatment cost.

[0004] Therefore, it is of great significance to solve the problems of insufficient adhesion, high cost, limited sources, and environmental unfriendliness of the existing binders, and to provide a binder with good adhesion performance, green safety, cost advantages, environmental friendliness, and capable of effectively improving the safety performance and cycle performance of the battery. Summary of the Invention

[0005] The present invention aims to solve at least one of the technical problems existing in the prior art. For this purpose, the present invention provides a polyacrylate binder and its application, an electrode sheet, and a lithium ion battery, aiming to solve the problems of insufficient adhesion, high cost, limited sources, and environmental unfriendliness of the existing binders.

[0006] In the first aspect of the present invention, a polyacrylate binder is provided. The structural formula of the polyacrylate binder is:

[0007] ;

[0008] Wherein, each occurrence of R1 is independently selected from H, Li, or Na;

[0009] Each occurrence of R2 is independently selected from substituted or unsubstituted C1~C10 alkyl groups;

[0010] and 5 ≤ (x + z) / y ≤ 20, 1.2 ≤ z / x ≤ 2, where x, y, and z are all non-zero.

[0011] The polyacrylate binder according to the embodiments of the present invention has at least the following beneficial effects: The polyacrylate binder provided by the present invention realizes good bonding performance by limiting the contents of cyano (-CN), ester group (-COOR2), and carboxylate group (-COOR1) in the molecular chain of the binder; and the binder is water-soluble, the solvent is water, and harmful organic solvents such as NMP do not need to be used, which is harmless to the human body and environmentally friendly; it is rich in sources and cheap in price, and can be widely used in industrial production. Introducing the above binder as a raw material of the water-based safety coating into the electrode sheet, based on its excellent bonding performance, the water-based safety coating can be firmly adhered to the surface of the current collector of the electrode sheet, which can not only reduce the surface contact resistance, but also reduce the generation of current collector burrs during the safety test of the battery cell, and reduce the contact short circuit between the current collector burrs and the unstable active substances in the charged state, thereby improving the safety of the battery cell. Moreover, by limiting the molecular weight of the binder and the contents of cyano, ester group, and carboxylate group in the molecular chain, the electrolyte absorption rate of the binder can be adjusted, which can significantly improve the rate performance and high and low temperature discharge performance of the battery cell, and improve the safety performance and cycle performance of the battery.

[0012] The binder provided by the present invention has a molecular chain containing cyano (-CN, the proportion of its number in all side chain groups is n1, n1 = z / (x + y + z)), carboxylate group (-COOR1, R1 is independently selected from H, Li, Na each time it appears, and the proportion of its number in all side chain groups is n2, n2 = x / (x + y + z)), and ester group (-COOR2, R2 is independently selected from substituted or unsubstituted C1~C 10 alkyl groups, and the proportion of its number in all side chain groups is n3, n3 = y / (x + y + z)), and n1 + n2 + n3 = 1.

[0013] Among them, -CN is a strongly polar group with good electrolyte affinity and electrochemical stability, and can provide a large bonding force. However, this group has a large rigidity and cannot cope with the huge volume change that occurs when lithium ions are inserted into / extracted from the active material.

[0014] Among them, -COOR1 carries a negative charge and repels each other, which helps the stretching of the binder molecules, promotes the uniform dispersion of the conductive agent, and at the same time can also regulate the swelling degree of the binder in the electrolyte solvent, weakening the erosion of the binder by the electrolyte dissolution, and enabling it to better maintain the bonding ability and bonding effect.

[0015] Among them, -COOR2 is close to the polarity and solubility parameter of the carbonate solvent in the electrolyte, which is beneficial for the binder to moderately absorb the electrolyte. The absorption of the electrolyte will plasticize the binder, making it elastic and flexible to adapt to the periodic volume change of the electrode active material during charge and discharge with the insertion / extraction of lithium ions. At the same time, the absorption of the electrolyte will also increase the ionic conductivity of the aqueous safety coating and promote the improvement of the low-temperature discharge performance of the battery cell.

[0016] The binder is an essential part to maintain the integrity of the electrode sheet and is very important for improving the performance of the battery such as specific capacity and cycle stability. Polyacrylic acid is used as the binder for the positive and negative electrodes of lithium batteries because it contains many polar functional groups, is soluble in water, and has good adhesion. However, the polar groups in polyacrylic acid cause the formation of hydrogen bonds between molecular chains, resulting in relatively large chain rigidity, which is not conducive to maintaining the integrity of the electrode sheet during charge and discharge. Therefore, it is imperative to control the number of functional groups of polyacrylic acid, change the type of functional groups, and the molecular chain structure of polyacrylic acid to improve the electrical performance of lithium batteries. The present invention limits the contents of cyano groups, ester groups, and carboxylate groups in the molecular chain of the binder: (1) It is limited that 1.2 ≤ z / x ≤ 2, that is, n1 and n2 satisfy 1.2 ≤ n1 / n2 ≤ 2; when n1 / n2 < 1.2, it is easy to cause insufficient adhesion of the binder; when n1 / n2 > 2, the molecular chain is in a curled state, which is not conducive to the dispersion of the conductive agent, and the binder is more easily affected by electrolyte swelling; (2) It is limited that 5 ≤ (x + z) / y ≤ 20, that is, n1, n2, and n3 satisfy 5 ≤ (n1 + n2) / n3 ≤ 20; when (n1 + n2) / n3 < 5, the binder contains an excessive amount of -COOR2. At this time, the binder is easy to absorb an excessive amount of electrolyte, the adhesion force rapidly decreases, the elasticity deteriorates, and the performance of the battery cell deteriorates; when (n1 + n2) / n3 > 20, the content of -COOR2 in the binder is too low. At this time, the elasticity of the binder is insufficient, the ionic conductivity of the aqueous safety coating is low, and the low-temperature discharge performance and rate discharge performance of the battery cell are significantly reduced. Finally, the present invention realizes good adhesion of the binder and the adjustment of the electrolyte absorption rate of the binder by regulating the contents of cyano groups, ester groups, and carboxylate groups in the molecular chain of the binder, and improves the rate and high- and low-temperature discharge performance of the battery cell.

[0017] In some embodiments of the present invention, each occurrence of R2 is independently selected from a straight-chain alkyl group, a branched-chain alkyl group, or a cycloalkyl group having C1 to C 10 and is preferably a straight-chain alkyl group having C1 to C 10 and more preferably an alkyl group having C1 to C4.

[0018] In some embodiments of the present invention, the weight-average molecular weight M of the polyacrylate binder wThe range is from 200,000 to 500,000, and for example, it can be 200,000, 250,000, 300,000, 350,000, 400,000, 450,000, 500,000.

[0019] In some embodiments of the present invention, the number-average molecular weight M of the polyacrylate binder n The range is from 100,000 to 400,000, and for example, it can be 100,000, 150,000, 200,000, 250,000, 300,000, 350,000, 400,000.

[0020] In some embodiments of the present invention, the weight-average molecular weight M of the polyacrylate binder w and the number-average molecular weight M n The ratio satisfies M w / M n ≤3, preferably M w / M n The range is from 1.5 to 1.9.

[0021] If the molecular weight of the binder is too low, not only the bonding force is too low, but also the amount of the binder used will increase; if the molecular weight is too high, it will lead to too high viscosity and difficult processing. And when M w / M n >3, the molecular weight distribution of the binder is too dispersed, deteriorating the performance of the water-based safety coating.

[0022] In some embodiments of the present invention, the electrolyte absorption rate c of the polyacrylate binder at 80 °C is 10% - 50%, and for example, it can be 10% - 20%, 20% - 30%, 30% - 40%, 40% - 50%.

[0023] The present invention realizes the adjustment of the electrolyte absorption rate of the binder by limiting the contents of cyano groups, ester groups and carboxylate groups in the molecular chain of the binder. The electrolyte absorption rate c of the binder at 80 °C satisfies 10% ≤ c ≤ 50%. When c < 10%, the binder absorbs insufficient electrolyte, has poor elasticity, cannot buffer the volume periodic changes generated when lithium ions are inserted into / extracted from the active material, and the ionic conductivity of the water-based safety coating is low, deteriorating the low-temperature discharge performance and rate discharge performance of the battery cell; when c > 50%, the binder absorbs excessive electrolyte, the water-based safety coating expands, the bonding force decreases, and the internal resistance increases, also leading to the deterioration of the battery cell performance.

[0024] The preparation method of the above polyacrylate binder can be obtained by conventional preparation methods in the art. For example, it can be prepared by any one of emulsion polymerization, solution polymerization, bulk polymerization and suspension polymerization.

[0025] In a specific embodiment, the present invention further provides a method for preparing the above-mentioned polyacrylate binder, comprising the steps:

[0026] S1. Add a dispersion medium into a reaction vessel and deoxygenate it;

[0027] S2. Add a certain amount of acrylonitrile, monomer A providing -COOR1, and monomer B providing -COOR2 into the reaction vessel according to the ratio of x, y, and z;

[0028] S3. Add an initiator and heat to initiate the reaction;

[0029] S4. After the reaction is completed, the product is filtered, dried, pulverized, and sieved to obtain the polyacrylate binder.

[0030] In some embodiments of the present invention, the monomer A includes at least one of acrylic acid, lithium acrylate, and sodium acrylate, but is not limited thereto. The purpose of adding monomer A is to provide -COOR1, and R1 is selected from H, Li, or Na. Those skilled in the art can independently select a suitable monomer A according to needs, and all should be regarded as reasonable transformations within the scope of the present invention.

[0031] In some embodiments of the present invention, the monomer B includes at least one of methyl acrylate, ethyl acrylate, and butyl acrylate, but is not limited thereto, and may also be at least one of C1-C 10 acrylates. The purpose of adding monomer B is to provide -COOR2, and R2 is selected from substituted or unsubstituted C1-C 10 alkyl groups. Those skilled in the art can independently select a suitable monomer B according to needs, and all should be regarded as reasonable transformations within the scope of the present invention.

[0032] In some embodiments of the present invention, in step S1, distilled water is added into the reaction vessel, stirring is started, and high-purity nitrogen is introduced for deoxygenation.

[0033] In some embodiments of the present invention, in step S2, after adding acrylonitrile, monomer A providing -COOR1, and monomer B providing -COOR2, it is heated to 65 °C and kept at a constant temperature under an inert atmosphere.

[0034] In some embodiments of the present invention, the initiator includes 20% ammonium persulfate, but is not limited thereto.

[0035] In the second aspect of the present invention, an aqueous safety coating is proposed. The raw materials of the aqueous safety coating include the above-mentioned polyacrylate binder, a first conductive agent, and inorganic fillers.

[0036] The water-based safety coating according to the embodiments of the present invention has at least the following beneficial effects: The water-based safety coating provided by the present invention can obtain a water-based safety coating with good safety performance and cycle performance, and at the same time can improve the high and low temperature discharge performance and cycle performance by limiting the related parameters such as the molecular weight of the binder in the coating and the contents of cyano group, ester group and carboxylate group in the molecular chain. The above water-based safety coating can be introduced into the electrode sheet, which can not only improve the safety performance and cycle performance of the battery, but also significantly improve the high and low temperature discharge performance and rate discharge performance of the battery.

[0037] In some embodiments of the present invention, the mass percentage of the polyacrylate binder in the water-based safety coating is 1 wt% to 24 wt%, preferably 10 wt% to 15 wt%. When the amount of the binder is <1 wt%, the adhesion of the water-based safety coating is insufficient, and it is easy to fall off during later use; when the amount of the binder is >24 wt%, it will cause difficulties in the processing of the water-based safety coating during gravure printing, and at the same time will deteriorate the internal resistance of the battery core.

[0038] In some embodiments of the present invention, the water-based safety coating includes at least one of the following:

[0039] A1) The mass ratio of the first conductive agent, polyacrylate binder, and inorganic filler is (0.5 to 5):(1 to 24):(71 to 98.5);

[0040] A2) The mass percentage of the first conductive agent in the water-based safety coating is 0.5% to 5%, preferably 1.5 to 3;

[0041] A3) The median particle size range of the inorganic filler is 0.2 to 4 μm, and the specific surface area is 5 to 50 m 2 / g;

[0042] A4) The raw materials of the water-based safety coating further include a thermally stable active substance;

[0043] A5) When including A4), the mass ratio of the first conductive agent, polyacrylate binder, inorganic filler, and thermally stable active substance is (0.5 to 5):(1 to 24):(1 to 97.5):(1 to 97.5);

[0044] A6) When including A4), the thermally stable active substance includes one or both of lithium iron phosphate and lithium manganese iron phosphate.

[0045] In some embodiments of the present invention, the mass ratio of the first conductive agent, the polyacrylate binder, and the inorganic filler is (0.5~5):(1~24):(71~98.5). For example, by weight, the first conductive agent can be 0.5~5 parts, preferably 1.5~3 parts, the binder can be 1~25 parts, preferably 10 - 15 parts, and the inorganic filler can be 71~98.5 parts.

[0046] In some embodiments of the present invention, the mass percentage of the first conductive agent in the aqueous safety coating is 0.5%~5%, preferably 1.5%~3%. If the content of the conductive agent is too low, the film resistance of the aqueous safety coating will be too large, and the cycling performance of the battery cell will be poor; if the content is too high, the film resistance of the aqueous safety coating will be too low, and the safety of the battery cell will be poor. When the content of the conductive agent is between 0.5wt%~5wt%, both safety and cycling can be taken into account.

[0047] In some embodiments of the present invention, the first conductive agent includes at least one of acetylene black, graphene, graphdiyne, carbon nanotubes, carbon fibers, and conductive carbon black. There are no special requirements for the first conductive agent in the present invention, and conventional conductive agents in the art can be used. For example, it can be conductive carbon black or carbon nanotubes, and more specifically, it can be Timcal Super P Li.

[0048] In some embodiments of the present invention, the inorganic filler is selected from at least one of alumina, boehmite, aluminum hydroxide, magnesium hydroxide, titanium dioxide, silicon dioxide, zirconium oxide, barium titanate, lithium niobate, yttrium-doped zirconia, gadolinium-doped ceria, montmorillonite, aluminosilicate, lithium lanthanum zirconium oxide, lithium lanthanum zirconium tantalum oxide, lithium lanthanum zirconium niobate oxide, lithium titanium aluminum phosphate, lithium titanium germanium phosphate, and lithium lanthanum titanate. There are no special requirements for the inorganic filler in the present invention, and conventional inert inorganic fillers in the art can be used. Considering cost and processability, the inorganic filler can include one or two of alumina, boehmite, aluminum hydroxide, and magnesium hydroxide, and preferably boehmite.

[0049] In some embodiments of the present invention, the median particle size D v50 of the inorganic filler ranges from 0.2~4μm, and the specific surface area is 5~50m 2 / g. When the D v50 of the inorganic filler is less than 0.2μm and the specific surface area is greater than 50m 2 / g, the inorganic filler is prone to agglomeration and difficult to disperse; when the D v50 of the inorganic filler is greater than 4μm and the specific surface area is less than 5m 2When the median particle size is 100 µm or more, it is difficult to obtain a thin safety coating due to the large particle size, which affects the energy density of the battery cell. When the median particle size and specific surface area of the inorganic filler are within the above ranges, a good balance can be achieved between the processability of the aqueous safety coating and the energy density of the battery cell.

[0050] In some embodiments of the present invention, the raw materials of the aqueous safety coating further include a thermally stable active substance.

[0051] In some embodiments of the present invention, the mass ratio of the first conductive agent, polyacrylate binder, inorganic filler, and thermally stable active substance is (0.5~5):(1~24):(1~97.5):(1~97.5).

[0052] In some embodiments of the present invention, the raw materials of the aqueous safety coating include the above-mentioned binder, first conductive agent, inorganic filler, and thermally stable active substance, and their mass ratio satisfies: first conductive agent: binder: inorganic filler: thermally stable active substance = (0.5~5):(1~24):(1~97.5):(1~97.5). For example, by weight, the first conductive agent can be 0.5~5 parts, the binder can be 1~24 parts, the inorganic filler can be 1~97.5 parts, and the thermally stable active substance can be 1~97.5 parts. In some embodiments of the present invention, the thermally stable active substance includes one or both of lithium iron phosphate and lithium manganese iron phosphate.

[0053] In a third aspect of the present invention, an electrode sheet is proposed, including: a current collector, an active material layer, and an aqueous safety coating, and the raw materials of the aqueous safety coating include the above-mentioned polyacrylate binder, first conductive agent, and inorganic filler.

[0054] The electrode sheet according to the embodiment of the present invention has at least the following beneficial effects: The present invention provides an electrode sheet including an aqueous safety coating. By limiting the relevant parameters such as the molecular weight of the binder in the aqueous safety coating and the contents of cyano groups, ester groups, and carboxylate groups in the molecular chain, an aqueous safety coating with good safety performance and cycle performance, and capable of improving the high and low temperature discharge performance and cycle performance at the same time is obtained. Introducing the above aqueous safety coating into the electrode sheet can not only improve the safety performance and cycle performance of the battery, but also significantly improve the high and low temperature discharge performance and rate discharge performance of the battery.

[0055] In some embodiments of the present invention, the aqueous safety coating is coated on at least one surface of the current collector, and the active material layer is coated on the surface of the aqueous safety coating away from the current collector and / or coated on the surface of the current collector away from the aqueous safety coating.

[0056] Specifically, the electrode sheet is a positive electrode sheet, the aqueous safety coating is coated on at least one surface of the current collector, and the active material layer is coated on the surface of the aqueous safety coating away from the current collector and / or coated on the surface of the current collector away from the aqueous safety coating.

[0057] More specifically, the electrode sheet is a positive electrode sheet, the aqueous safety coating is coated on both surfaces of the current collector, and the active material layer is coated on the surface of the aqueous safety coating away from the current collector.

[0058] In some other embodiments of the present invention, the active material layer is coated on at least one surface of the current collector, and the aqueous safety coating is coated on the surface of the active material layer away from the current collector and / or coated on the surface of the current collector away from the active material layer.

[0059] Specifically, the electrode sheet is a negative electrode sheet, the active material layer is coated on at least one surface of the current collector, and the aqueous safety coating is coated on the surface of the active material layer away from the current collector and / or coated on the surface of the current collector away from the active material layer.

[0060] More specifically, the electrode sheet is a negative electrode sheet, the active material layer is coated on both surfaces of the current collector, and the aqueous safety coating is coated on the surface of the active material layer away from the current collector.

[0061] The electrode sheet provided by the present invention can be used as a positive electrode sheet or a negative electrode sheet, preferably a positive electrode sheet. When used as a positive electrode sheet, the aqueous safety coating is closer to the current collector, and its structure is "current collector - aqueous safety coating - active material layer", as Figure 1 shown. Among them, the positive electrode sheet 01 includes the following structure: a current collector 022; an aqueous safety coating 023 provided on at least one surface of the current collector 022; an active material layer 011 provided on the surface of the aqueous safety coating 023 away from the current collector 022. When used as a negative electrode sheet, the active material layer is closer to the current collector, and its structure is "current collector - active material layer - aqueous safety coating".

[0062] Internal short circuits of lithium-ion batteries can generally be divided into several types: 1) short circuits between the positive and negative current collectors; 2) short circuits between the positive and negative active materials; 3) short circuits between the negative active material and the positive current collector; 4) short circuits between the positive active material and the negative current collector. Among them, the impedance of the short circuit point between the negative active material and the positive current collector is small, and the triggering temperature of the exothermic reaction of the negative electrode is low. Therefore, the short circuit between the negative active material and the Al foil is the most dangerous. In the present invention, the above-mentioned binder is introduced as a raw material for the aqueous safety coating into the electrode sheet. By introducing an aqueous safety coating with high adhesion force onto the surface of the positive current collector, based on its excellent adhesion performance, the aqueous safety coating can be firmly adhered to the surface of the current collector of the electrode sheet, which can not only reduce the surface contact resistance, but also reduce the generation of burrs on the positive current collector during the safety test of the battery cell, and reduce the contact short circuit between the burrs on the positive current collector and the unstable negative active material in the charged state, thereby effectively improving the safety of the battery cell.

[0063] In some embodiments of the present invention, the thickness of the aqueous safety coating is 1-20 μm, for example, it can be 1-7 μm, 7-14 μm, 14-20 μm.

[0064] In some embodiments of the present invention, the coating surface density of the aqueous safety coating is 2-35 mg / 1540.25mm 2 . For example, it can be 2-8 mg / 1540.25mm 2 , 8-15 mg / 1540.25mm 2 , 14-21 mg / 1540.25mm 2 , 20-28 mg / 1540.25mm 2 , 28-35 mg / 1540.25mm 2 .

[0065] There is a positive correlation between the coating surface density and the thickness of the aqueous safety coating. If the thickness is too low, the safety performance of the battery cell is poor; if the thickness is too large, the energy density of the battery cell will be reduced.

[0066] In some embodiments of the present invention, the film resistance of the aqueous safety coating under a test pressure of 0.4 t is 0.5-5 Ω, for example, it can be 0.5-2 Ω, 2-3.5 Ω, 3.5-5 Ω. The film resistance of the aqueous safety coating is the primary factor affecting the safety performance of the battery cell. When this value is too small, the safety performance of the battery cell will deteriorate, while when it is too large, the cycle performance of the battery cell will become poor. When this value is between 0.5-5 Ω @ 0.4T, the safety performance and cycle performance can be taken into account.

[0067] In some embodiments of the present invention, the adhesion between the aqueous safety coating and the current collector is not less than 100 N / m. If the adhesion is less than 100 N / m, there is a risk of the aqueous safety coating peeling off during subsequent use.

[0068] In some embodiments of the present invention, the raw materials of the active material layer include an active material, a second conductive agent, and a second binder.

[0069] In some embodiments of the present invention, the mass percentage of the active material in the active material layer is 90% - 98%.

[0070] In some embodiments of the present invention, the mass percentage of the second conductive agent in the active material layer is 0.5% - 5%, preferably 0.5% - 2%. If the content of the second conductive agent is too low, the cycling performance of the battery cell is poor; if the content is too high, it is beneficial to improve the cycling performance, but it will lead to poor safety performance of the battery cell.

[0071] In some embodiments of the present invention, the second conductive agent includes at least one of acetylene black, graphene, graphyne, carbon nanotubes, carbon fibers, and conductive carbon black. There are no special requirements for the second conductive agent in the present invention, and conventional conductive agents in the art can be applied. For example, it can be conductive carbon black or carbon nanotubes, and more specifically, it can be Super P Li, a highly conductive carbon black.

[0072] In some embodiments of the present invention, the mass percentage of the second binder in the active material layer is 0.5% - 5%. If the amount of the second binder is too small, the adhesion of the active material layer is insufficient, and it is easy to peel off during later use; if the amount of the second binder is too large, it will make the processing of the active material layer difficult and also deteriorate the internal resistance of the battery cell.

[0073] In some embodiments of the present invention, the second binder includes at least one of polyvinylidene fluoride (PVDF), sodium carboxymethyl cellulose (CMC), styrene - butadiene rubber (SBR), and PAA - type binders. There are no special requirements for the second binder in the present invention, and conventional binders in the art can be applied.

[0074] In a specific embodiment, the active material layer can be a positive electrode active material layer, and the raw materials of the positive electrode active material layer include a positive electrode active material, a second conductive agent, and a second binder.

[0075] The positive electrode active material is not limited and can be one or more of common lithium cobaltate, lithium iron phosphate, and ternary materials. The dosage is 90wt% - 98wt% of the total weight of the active material layer. Under the condition of ensuring that the active material has a certain conductivity and adhesion, the higher the content of the active material, the better, so as to make the battery cell have a higher energy density.

[0076] The second binder is not limited and can be a common cathode binder, including polyvinylidene fluoride (PVDF), sodium carboxymethyl cellulose (CMC), and PAA - type binders. The dosage is 0.5wt% - 5wt% of the total weight of the active material layer. If the dosage of the second binder is too small, the binding force of the active material layer is insufficient, and it is likely to fall off during later use. If the dosage of the second binder is too large, it will cause difficulties in processing the active material layer and also deteriorate the internal resistance of the battery cell. Considering that it does not directly contact the current collector, the upper limit of its dosage can be appropriately reduced to ensure that the positive active material does not fall off during the use of the battery cell. At the same time, a lower dosage can also improve the cycle performance of the battery cell, increase the dosage of the positive active material, and enhance the energy density of the battery cell.

[0077] In a specific embodiment, the active material layer can be a negative active material layer, and the raw materials of the negative active material layer include a negative active material, a second conductive agent, and a second binder.

[0078] The negative active material is not limited and can be one or several of common artificial graphite, natural graphite, modified graphite, and silicon - based negative materials such as silicon oxide compounds and silicon - carbon composites. The dosage is 90wt% - 98wt% of the total weight of the active material layer.

[0079] In some embodiments of the present invention, the electrode sheet further includes a current collector. The current collector is not limited and can be a common current collector. For example, when the electrode sheet is a positive electrode sheet, the current collector can be aluminum foil or composite aluminum foil; when the electrode sheet is a negative electrode sheet, the current collector can be copper foil or composite copper foil.

[0080] In some preferred embodiments of the present invention, the dyne value of the current collector is ≥30 dyn / cm, preferably ≥38 dyn / cm.

[0081] In some embodiments of the present invention, the water - based safety coating is coated on both side surfaces of the current collector, and the water - based safety coating satisfies at least one of the following:

[0082] B1) The double - sided thickness of the water - based safety coating is 1 - 20 μm;

[0083] B2) The double - sided coating surface density of the water - based safety coating is 2 - 35 mg / 1540.25mm 2 ;

[0084] B3) The double - sided film resistance of the water - based safety coating tested under a pressure of 0.4 t is 0.5 - 5Ω;

[0085] B4) The numerical values of the double - sided thickness d of the water - based safety coating and the double - sided film resistance R under a pressure of 0.4 t satisfy the condition: 2 ≤ R×d ≤ 50;

[0086] B5) The water-based safety coating is coated on both side surfaces of the current collector. Along the direction perpendicular to the unwinding direction of the current collector, the distance W between the left edge of the water-based safety coating on the A side of the current collector and the left edge of the current collector A1 ranges from 2 mm to 30 mm, and the distance W between the right edge of the water-based safety coating on the A side of the current collector and the right edge of the current collector A2 ranges from 2 mm to 30 mm; the distance W between the left edge of the water-based safety coating on the B side of the current collector and the left edge of the current collector B1 ranges from 3 mm to 30 mm, and the distance W between the right edge of the water-based safety coating on the B side of the current collector and the right edge of the current collector B2 ranges from 3 mm to 30 mm; and W B1 -W A1 ≥1 mm, and W B2 -W A2 ≥1 mm.

[0087] In some embodiments of the present invention, the double-sided thickness d of the water-based safety coating (i.e., the sum of the thicknesses of the water-based safety coatings on both sides of the current collector surface) is 1 to 20 μm, for example, it can be 1 to 7 μm, 7 to 14 μm, 14 to 20 μm.

[0088] In some embodiments of the present invention, the double-sided coating surface density of the water-based safety coating (i.e., the sum of the coating surface densities of the water-based safety coatings on both sides of the current collector surface) is 2 to 35 mg / 1540.25 mm 2 . For example, it can be 2 to 8 mg / 1540.25 mm 2 , 8 to 15 mg / 1540.25 mm 2 , 14 to 21 mg / 1540.25 mm 2 , 20 to 28 mg / 1540.25 mm 2 , 28 to 35 mg / 1540.25 mm 2 .

[0089] There is a positive correlation between the coating surface density and the thickness of the water-based safety coating. If the thickness is too low, the safety performance of the battery cell is poor; if the thickness is too large, the energy density of the battery cell will be reduced.

[0090] In some embodiments of the present invention, the double-sided film resistance R of the aqueous safety coating under a pressure of 0.4 t (i.e., the sum of the film resistances of the aqueous safety coating films on both sides coated on the surface of the current collector) is 0.5 - 5 Ω, and for example, it can be 0.5 - 2 Ω, 2 - 3.5 Ω, 3.5 - 5 Ω. The film resistance of the aqueous safety coating is the primary factor affecting the safety performance of the battery cell. When this value is too small, the safety performance of the battery cell will deteriorate, while when it is too large, the cycling performance of the battery cell will become poor. When this value is between 0.5 - 5 Ω @ 0.4 T, the safety performance and cycling performance can be balanced.

[0091] In some embodiments of the present invention, the double-sided thickness d (unit: μm) of the aqueous safety coating and the value of the double-sided film resistance R (unit: Ω) under a pressure of 0.4 t satisfy the condition: 2 ≤ R × d ≤ 50. Generally speaking, both parameters R and d are positively correlated with the safety of the battery cell, and negatively correlated with the cycling performance of the battery cell. When the relationship 2 ≤ R × d ≤ 50 is satisfied, a good balance can be achieved between the safety performance and electrochemical performance of the battery cell.

[0092] In some embodiments of the present invention, the adhesion force between the aqueous safety coating and the current collector is not less than 100 N / m. If the adhesion force is lower than 100 N / m, there is a risk of the aqueous safety coating peeling off during subsequent use.

[0093] In a specific embodiment, the present invention provides a positive electrode sheet, which includes a positive electrode active material layer, an aqueous safety coating, and a positive electrode current collector. Among them, the aqueous safety coating contains the aforementioned water-soluble binder, and the structural formula of the binder is:

[0094] ;

[0095] Among them, each occurrence of R1 is independently selected from H, Li, Na; each occurrence of R2 is independently selected from substituted or unsubstituted C1 - C 10 alkyl; and 5 ≤ (x + z) / y ≤ 20, 1.2 ≤ z / x ≤ 2, and x, y, z are all non-zero.

[0096] The electrolyte absorption rate of the binder at 80 °C is 10% - 50%.

[0097] The aqueous safety coating is coated on both side surfaces of the current collector.

[0098] The double-sided thickness d of the aqueous safety coating is 1 - 20 μm.

[0099] The double-sided film resistance R of the aqueous safety coating under a pressure of 0.4 t is 0.5 - 5 Ω.

[0100] The double-sided thickness d (unit: μm) of the aqueous safety coating and the value of the double-sided film resistance R (unit: Ω) under a pressure of 0.4 t satisfy the condition: 2 ≤ R×d ≤ 50.

[0101] By limiting the content relationship of various groups contained in the side chain of the binder and selecting the electrolyte absorption rate c (10% ≤ c ≤ 50%), the present invention overcomes the problems of poor low-temperature discharge performance and rate discharge performance of conventional aqueous coatings. At the same time, the aqueous safety coating also retains the advantages of conventional aqueous safety coatings, can firmly adhere to the surface of the current collector, not only can reduce the surface contact resistance, but also can reduce the generation of burrs on the positive current collector during the safety test of the battery cell, and reduce the contact short circuit between the burrs on the positive current collector and the unstable negative active material in the charged state, thereby improving the safety of the battery cell. In addition, the double-sided film resistance and thickness after coating the aqueous safety coating are also limited, so as to achieve a balance between the safety performance and cycle performance of the battery cell.

[0102] In the fourth aspect of the present invention, a method for preparing the above-mentioned electrode sheet is proposed, including the steps:

[0103] S100. Mix the polyacrylate binder with water and then size to obtain a sizing solution;

[0104] S200. Mix the first conductive agent and the inorganic filler uniformly with the sizing solution, or mix the first conductive agent, the thermally stable active material and the inorganic filler uniformly with the sizing solution to obtain an aqueous safety coating slurry;

[0105] S300. Coat the aqueous safety coating slurry on one or both surfaces of the current collector, or on the surface of the active material layer away from the current collector side to obtain the electrode sheet.

[0106] The method for preparing the electrode sheet according to the embodiment of the present invention has at least the following beneficial effects: By introducing an aqueous safety coating into the electrode sheet and limiting the parameters of the binder and the coating, the safety performance and cycle performance of the battery cell can be taken into account without changing the original process, and the low-temperature discharge performance and rate discharge performance can be improved. This method not only has a simple process, but also has mild reaction conditions and is compatible with the original preparation process, and has the potential for large-scale application.

[0107] In some embodiments of the present invention, the preparation method includes at least one of the following:

[0108] C1) The mass ratio of the first conductive agent, the polyacrylate binder, and the inorganic filler is (0.5~5):(1~24):(71~98.5);

[0109] C2) The solid content of the sizing solution is 10%~30%;

[0110] C3) The viscosity of the adhesive solution is 1×10 3 ~3×10 4 mPa·s;

[0111] C4) The solid content of the aqueous safety coating slurry is ≥10%;

[0112] C5) The viscosity of the aqueous safety coating slurry is ≥50 mPa·s;

[0113] C6) The dyne value of the current collector is ≥30 dyn / cm;

[0114] C7) Coating the aqueous safety coating slurry on one or both surfaces of the current collector, or on the surface of the active material layer away from the current collector by gravure printing;

[0115] C8) When including C7), the temperature of the oven during gravure printing is 90~110°C, and the printing speed is 10~50 m / min;

[0116] C9) The mass ratio of the first conductive agent, polyacrylate binder, inorganic filler, and thermally stable active material is (0.5~5):(1~24):(1~97.5):(1~97.5).

[0117] In some embodiments of the present invention, the mass ratio of the first conductive agent, polyacrylate binder, and inorganic filler is (0.5~5):(1~24):(71~98.5). For example, by weight, the first conductive agent can be 0.5~5 parts, the binder can be 1~24 parts, and the inorganic filler can be 71~98.5 parts.

[0118] In some embodiments of the present invention, the solid content of the adhesive solution is 10%~30%, for example, it can be 10%~20%, 20%~25%, 25%~30%.

[0119] In some embodiments of the present invention, the viscosity of the adhesive solution is 1×10 3 ~3×10 4 mPa·s, preferably 1.5×10 4 ~2.5×10 4 mPa·s, more preferably 1.6×10 4 ~2.2×10 4 mPa·s, for example, it can be 2×10 4 mPa·s.

[0120] If the solid content and viscosity of the adhesive-made glue solution are too low, it will not only reduce the processing efficiency, but also may lead to insufficient adhesion of the water-based safety coating; while if the solid content and viscosity are too high, it may cause difficulties in processing the water-based safety coating.

[0121] In some embodiments of the present invention, the raw materials of the water-based safety coating further include a heat-stable active substance, and its mass ratio satisfies: the first conductive agent: the binder: the inorganic filler: the heat-stable active substance = (0.5~5):(1~24):(1~97.5):(1~97.5). For example, by weight, the first conductive agent can be 0.5~5 parts, the binder can be 1~24 parts, the inorganic filler can be 1~97.5 parts, and the heat-stable active substance can be 1~97.5 parts.

[0122] In some embodiments of the present invention, the solid content of the water-based safety coating slurry ≥ 10%, preferably 10%~21%.

[0123] In some embodiments of the present invention, the viscosity of the water-based safety coating slurry ≥ 50 mPa·s.

[0124] The solid content of the water-based safety coating slurry should not be less than 10%, and the viscosity should not be less than 50 mPa·s. Too low solid content will not only reduce the production efficiency, but also it is difficult to obtain a water-based safety coating with a specified thickness and surface density, while too low viscosity is not conducive to the gravure printing process.

[0125] In some embodiments of the present invention, the dyne value of the current collector ≥ 30 dyn / cm, preferably ≥ 38 dyn / cm. When the dyne value of the current collector is lower than 30 dyn / cm, on the one hand, it is not conducive to the water-based safety coating slurry to wet the current collector, resulting in missed coating of the undercoat, and on the other hand, it will reduce the adhesion between the water-based safety coating and the current collector, leading to easy peeling of the water-based safety coating during subsequent use, which has an adverse impact on the safety of the battery cell. A dyne value of the current collector not less than 38 dyn / cm can ensure good appearance morphology of the undercoat and good adhesion between the water-based safety coating and the current collector.

[0126] In some embodiments of the present invention, the water-based safety coating slurry is coated on one or both surfaces of the current collector, or on the surface of the active material layer away from the current collector by gravure printing.

[0127] In some embodiments of the present invention, when performing gravure printing, the temperature of the oven is 90~110°C; the printing speed is not less than 10 m / min, preferably 10~50 m / min.

[0128] When performing intaglio printing, the temperature of the oven should be 90-110°C, and the printing speed should be 10-50 m / min. If the temperature of the oven is lower than 90°C, the production efficiency will be reduced. If it is higher than 110°C, there may be problems such as cracking and migration of the conductive agent to the surface. If the printing speed is too low, the production efficiency will also be reduced. If it is too high, it will cause the problem of missing coating of the water-based safety coating, thus having a negative impact on the safety performance of the battery cell.

[0129] In a specific embodiment of the present invention, a method for preparing the above-mentioned electrode sheet is provided, including the following steps:

[0130] (1) Using deionized water as a solvent, adding a binder to make a glue solution with appropriate viscosity and solid content, and then adding the first conductive agent and inorganic filler to the glue solution and mixing evenly to obtain a water-based safety coating slurry;

[0131] (2) Transferring the water-based safety coating slurry obtained in step (1) to one or both surfaces of the current collector, or the surface of the active material layer away from the current collector, by intaglio printing, to obtain an electrode sheet in which the water-based safety coating area and the non-water-based safety coating area are arranged at intervals along the unwinding direction of the electrode sheet.

[0132] In some embodiments of the present invention, the total length of the electrode sheet containing the water-based safety coating is L, the length of the water-based safety coating area is L1, the length of the non-water-based safety coating area is L2, and L1 > L2, L1 + L2 = L.

[0133] In some embodiments of the present invention, the water-based safety coating can be coated on both sides (A side and B side) of the electrode sheet. Among them, the length of the water-based safety coating area on the A side is L A1 , the length of the non-water-based safety coating area is L A2 , and L A1 > L A2 , L A1 + L A2 = L A ; the length of the water-based safety coating area on the B side is L B1 , the length of the non-water-based safety coating area is L B2 , and L B1 > L B2 , L B1 + L B2 = L B .

[0134] In some preferred embodiments of the present invention, to improve production efficiency and reduce costs at the same time, LA2 and LB2 can both be set to 0, that is, L A1 = L B1 = L A = L B .

[0135] In some embodiments of the present invention, when applying the aqueous safety coating, N1 slots for tab welding need to be reserved in the aqueous safety coating area or the non-aqueous safety coating area along the unwinding direction of the electrode sheet, where N1≥0.

[0136] In some embodiments of the present invention, when applying the aqueous safety coating, N2 slots for tab welding need to be reserved in the aqueous safety coating area or the non-aqueous safety coating area along the direction perpendicular to the unwinding direction of the electrode sheet, where N2≥0.

[0137] In some embodiments of the present invention, when applying the aqueous safety coating, N3 aqueous safety coatings can also be applied simultaneously along the unwinding direction of the electrode sheet, where N3≥1.

[0138] In some embodiments of the present invention, if safety coatings are applied on both sides of the electrode sheet, when reserving the slots for tab welding, the slots on the A / B surfaces need to be opposite to each other, and the misalignment distances in the horizontal and vertical directions shall not exceed 2 mm.

[0139] In some embodiments of the present invention, if safety coatings are applied on both sides of the electrode sheet, along the direction perpendicular to the unwinding direction of the current collector, the distance between the left edge of the aqueous safety on the A surface and the left edge of the current collector is W A1 (i.e., the left blank on the A surface is W A1 ), the right blank on the A surface is W A2 , the left blank on the B surface is W B1 , the right blank on the B surface is W B2 , satisfying 2 mm≤W A1 , W A2 ,W B1 , W B2 ≤30 mm, and W B1 -W A1 ≥1 mm, W B2 -W A2 ≥1 mm.

[0140] In some embodiments of the present invention, the electrode sheet is a positive electrode sheet, and the preparation method of the positive electrode sheet includes the steps:

[0141] (10) Using deionized water as a solvent, adding a binder to make a glue solution with appropriate viscosity and solid content, and then adding a first conductive agent and an inorganic filler to the glue solution and mixing evenly, or mixing the first conductive agent, a thermally stable active substance, and the inorganic filler with the glue solution to obtain an aqueous safety coating slurry;

[0142] (20) Transfer the aqueous safety coating slurry obtained in step (10) to one or both surfaces of the current collector by gravure printing, to obtain a positive electrode sheet in which the aqueous safety coating regions and the non-aqueous safety coating regions are arranged at intervals along the unwinding direction of the current collector.

[0143] Among them, the non-aqueous safety coating region corresponds to the head and / or tail of the positive electrode sheet of the battery cell. Usually, one side thereof is the active material, the other side is the empty foil, or a separately provided ceramic layer. The ceramic layer usually only contains inorganic fillers and binders, which can reduce the generation of burrs on the positive current collector during the battery cell test, and reduce the probability of contact between the positive current collector and the negative active material, thereby improving the safety performance of the battery cell. However, after the safety coating is set, an additional process is required to coat the ceramic layer, which not only reduces the production efficiency but also increases the production cost.

[0144] In some embodiments of the present invention, the total length of the positive electrode sheet containing the aqueous safety coating is L, the length of the aqueous safety coating region is L1, the length of the non-aqueous safety coating region is L2, and L1 > L2, L1 + L2 = L.

[0145] In some embodiments of the present invention, the aqueous safety coating can be coated on both sides (side A and side B) of the positive electrode sheet. Among them, the length of the aqueous safety coating region on side A is L A1 , the length of the non-aqueous safety coating region is L A2 , and L A1 > L A2 , L A1 + L A2 = L A ; the length of the aqueous safety coating region on side B is L B1 , the length of the non-aqueous safety coating region is L B2 , and L B1 > L B2 , L B1 + L B2 = L B .

[0146] In some preferred embodiments of the present invention, to improve the production efficiency and reduce the cost at the same time, the function of the ceramic layer can be borne by the aqueous safety coating, that is, the non-aqueous safety coating region is also set with the aqueous safety coating, and L A2 and L B2 are both set to 0, that is, L A1 = L B1 = L A = L B .

[0147] Figure 2Shown is a sectional view of the current collector coated with the aqueous safety coating along the unwinding direction in one specific embodiment of the present invention. In the figure, 02 - positive current collector with safety coating; 021 - reserved slot; 022 - current collector; 023 - aqueous safety coating; 024 - non - safety coating area. Among them, the aqueous safety coating areas and non - aqueous safety coating areas on the A side and B side of the positive electrode sheet can be arranged relatively, such as Figure 2 in (a); it can also be arranged that one side has a non - aqueous safety coating area and the other side has an aqueous safety coating area, such as Figure 2 shown in (b). The total length of the current collector containing the aqueous safety coating is L, the length of the aqueous safety coating area on the B side is L B1 , the length of the non - aqueous safety coating area is L B2 , and L B1 >L B2 , L B1 +L B2 =L B =L A1 =L A =L; it is also possible to set the non - aqueous safety coating area with an aqueous safety coating as well, and both L A2 and L B2 are set to 0, that is, L A1 =L B1 =L A =L B , such as Figure 2 in (c).

[0148] In some embodiments of the present invention, when applying the aqueous safety coating, N1 slots for tab welding need to be reserved in the aqueous safety coating area or non - aqueous safety coating area along the unwinding direction of the current collector, where N1≥0.

[0149] In some embodiments of the present invention, when applying the aqueous safety coating, N2 slots for tab welding need to be reserved in the aqueous safety coating area or non - aqueous safety coating area along the direction perpendicular to the unwinding direction of the current collector, where N2≥0.

[0150] In some embodiments of the present invention, when applying the aqueous safety coating, N3 aqueous safety coatings can be applied simultaneously along the unwinding direction of the current collector, where N3≥1.

[0151] When applying the aqueous safety coating in the present invention, slots for tab welding can be reserved or not reserved as needed, such as Figure 3 shown.

[0152] In some embodiments of the present invention, if safety coatings are applied on both sides of the current collector, when reserving slots for tab welding, the slots on the A / B sides need to be kept opposite, and the lateral and longitudinal misalignments shall not exceed 2 mm.

[0153] In some embodiments of the present invention, the aqueous safety coating is coated on both side surfaces of the current collector. Along the direction perpendicular to the unwinding direction of the current collector, the distance W between the left edge of the aqueous safety coating on the A side of the current collector and the left edge of the current collector A1 ranges from 2 mm to 30 mm, and the distance W between the right edge of the aqueous safety coating on the A side of the current collector and the right edge of the current collector A2 ranges from 2 mm to 30 mm; the distance W between the left edge of the aqueous safety coating on the B side of the current collector and the left edge of the current collector B1 ranges from 3 mm to 30 mm, and the distance W between the right edge of the aqueous safety coating on the B side of the current collector and the right edge of the current collector B2 ranges from 3 mm to 30 mm; and W B1 -W A1 ≥1 mm, W B2 -W A2 ≥1 mm. That is to say, if safety coatings are coated on both sides of the current collector, along the direction perpendicular to the unwinding direction of the current collector, the distance between the left edge of the aqueous safety coating on the A side and the left edge of the current collector is W A1 (i.e., the left blank of the A side is W A1 ), the right blank of the A side is W A2 , the left blank of the B side is W B1 , the right blank of the B side is W B2 , satisfying 2 mm ≤ W A1 , W A2 ≤30 mm, 3 mm ≤ W B1 , W B2 ≤30 mm, and W B1 -W A1 ≥1 mm, W B2 -W A2 ≥1 mm. Figure 4 The figure shows a cross-sectional view of the aqueous safety coating perpendicular to the unwinding direction in a specific embodiment of the present invention. In the figure, 021 - reserved slot; 022 - current collector; 023 - aqueous safety coating; such a setting can make the width of the aqueous safety coating on the A side 2 mm wider than that on the B side, thereby reducing the edge bulging during the winding of the undercoat.

[0154] In a fifth aspect of the present invention, a lithium-ion battery is proposed, which includes a positive electrode sheet, a negative electrode sheet, and a separator spaced between the positive electrode sheet and the negative electrode sheet; the positive electrode sheet and / or the negative electrode sheet is the electrode sheet as described above or the electrode sheet obtained by the preparation method as described above.

[0155] Since the lithium-ion battery adopts all the technical solutions of the electrode sheet of the above embodiment, it has at least all the beneficial effects brought by the technical solutions of the above embodiment. That is, by introducing an aqueous safety coating into the electrode sheet and defining relevant parameters such as the molecular weight of the binder and the contents of cyano groups, ester groups, and carboxylate groups in the molecular chain in the aqueous safety coating, the lithium-ion battery obtains an aqueous safety coating with good safety performance and cycling performance, and at the same time can improve the high and low temperature discharge performance and cycling performance. It can not only improve the safety performance and cycling performance of the battery, but also significantly improve the high and low temperature discharge performance and rate discharge performance of the battery.

[0156] In some embodiments of the present invention, the positive electrode sheet is the electrode sheet as described above. The positive electrode sheet includes the above positive electrode current collector containing an aqueous safety coating and a positive electrode active material layer coated on at least one surface of the positive electrode current collector. The positive electrode current collector is a commonly used positive electrode current collector in the art, such as aluminum foil, but is not limited thereto. The positive electrode active material layer includes a positive electrode active material, and the positive electrode active material is a commonly used positive electrode active material in current lithium-ion batteries, including but not limited to compounds shown by the chemical formula such as Li x Ni h Co y M z O 2-d N d (where 0.95 ≤ x ≤ 1.2, h > 0, y ≥ 0, z ≥ 0, and h + y + z = 1, 0 ≤ d ≤ 1, M is selected from one or more combinations of Mn and Al, and N is selected from one or more combinations of F, P, and S), and one or more combinations of the compounds shown. The positive electrode active material can also be, for example, one or more combinations of LiCoO2, LiNiO2, LiVO2, LiCrO2, LiMn2O4, LiCoMnO4, Li2NiMn3O8, LiNi 0.5 Mn 1.5 O4, LiCoPO4, LiMnPO4, LiFePO4, LiNiPO4, LiCoFSO4, CuS2, FeS2, MoS2, NiS, TiS2, etc. The positive electrode active material can also be subjected to modification treatment, and the methods for modifying the positive electrode active material should be known to those skilled in the art. For example, the positive electrode active material can be modified by methods such as coating and doping, and the materials used for the modification treatment can be one or more combinations of, for example, Al, B, P, Zr, Si, Ti, Ge, Sn, Mg, Ce, W, etc.

[0157] In some embodiments of the present invention, the negative electrode sheet is the electrode sheet as described above. The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer coated on at least one surface of the negative electrode current collector. There are no special requirements for the negative electrode current collector, and it can be a copper foil commonly used in the industry. The negative electrode active material layer includes a negative electrode active material, and the negative electrode active material includes a silicon-based material, which can be selected from one or more of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon alloys, or a mixture of a silicon negative electrode material and other commonly used negative electrode active materials at present. The other negative electrode active materials include, but are not limited to, one or more of graphite, soft carbon, hard carbon, carbon fiber, mesophase carbon microspheres, tin-based materials, lithium titanate, or other metals that can form alloys with lithium, etc. Among them, the graphite can be selected from one or more of artificial graphite, natural graphite, and modified graphite; the tin-based materials can be selected from one or more of elemental tin, tin oxide compounds, and tin alloys.

[0158] In some embodiments of the present invention, the separator can be various materials suitable for lithium-ion battery separators in the art. For example, it can be a combination of one or more of polyethylene, polypropylene, polyvinylidene fluoride, aramid, polyethylene terephthalate, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, polyester, and natural fibers, etc. In actual production, the material and structure of the separator are not strictly limited. For example, it can be a multi-layer structure formed by laminating the above materials, or a single-layer structure formed by mixing the above materials, or a single-layer structure formed by a single material; as long as it can play the basic role of the separator.

[0159] In some embodiments of the present invention, the lithium battery further includes an electrolyte, and the electrolyte infiltrates the positive electrode, negative electrode, and separator. The electrolyte includes an organic solvent, an electrolyte lithium salt, and an additive. Among them, the electrolyte lithium salt can be LiPF6 and / or LiBOB used in high-temperature electrolytes; it can also be at least one of LiBF4, LiBOB, and LiPF6 used in low-temperature electrolytes; it can also be at least one of LiBF4, LiBOB, LiPF6, and LiTFSI used in overcharge-preventing electrolytes; it can also be at least one of LiClO4, LiAsF6, LiCF3SO3, and LiN(CF3SO2)2. The organic solvent can be a cyclic carbonate, including PC and EC; it can also be a chain carbonate, including DEC, DMC, or EMC; it can also be a carboxylic acid ester, including PP, MA, EA, EP, etc. The additives include, but are not limited to, at least one of a film-forming additive, a conductive additive, a flame-retardant additive, an overcharge-preventing additive, an additive for controlling the content of H2O and HF in the electrolyte, an additive for improving low-temperature performance, and an aqueous safety additive.

[0160] The present invention also provides a secondary battery, which includes a positive electrode sheet, a negative electrode sheet, and a separator disposed between the positive electrode sheet and the negative electrode sheet; the positive electrode sheet and / or the negative electrode sheet is the electrode sheet as described above or the electrode sheet obtained by the preparation method as described above. The electrode sheet proposed by the present invention can be applied not only to the above-mentioned lithium-ion battery, but also to other secondary batteries, including sodium-ion batteries, potassium-ion batteries, etc., without any limitation herein.

[0161] In the sixth aspect of the present invention, an application of the above-mentioned lithium-ion battery in an energy storage device, an electrical device or an electronic device is provided. Description of the Drawings

[0162] The present invention will be further described below in conjunction with the drawings and embodiments, where:

[0163] Figure 1 It is a schematic structural diagram of a positive electrode sheet including an aqueous safety coating provided by the present invention, where 01 - positive electrode sheet; 011 - active material layer; 022 - current collector; 023 - aqueous safety coating;

[0164] Figure 2 It is a schematic cross-sectional structural diagram of a current collector including an aqueous safety coating provided by the present invention along the unwinding direction, where 02 - positive current collector with safety coating; 021 - reserved slot; 022 - current collector; 023 - aqueous safety coating; 024 - non-safety coating area;

[0165] Figure 3 It is a top view of a current collector including an aqueous safety coating (in an unslit state) provided by the present invention;

[0166] Figure 4 It is a schematic cross-sectional structural diagram of a current collector including an aqueous safety coating provided by the present invention along the unwinding direction, where 021 - reserved slot; 022 - current collector; 023 - aqueous safety coating. Detailed Embodiments

[0167] The concept of the present invention and the technical effects generated will be clearly and completely described below in conjunction with the embodiments to fully understand the purpose, features and effects of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative efforts shall fall within the scope of protection of the present invention.

[0168] In the description of the present invention, the description with reference to terms such as "one embodiment", "some embodiments", "schematic embodiments", "examples", "specific examples", or "some examples" means that the specific features, structures, materials, or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic expressions of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described can be combined in a suitable manner in any one or more embodiments or examples.

[0169] In the description of the present invention, unless otherwise specified, the numerical range "a~b" represents an abbreviated representation of any real number combination between a and b, where both a and b are real numbers. Unless otherwise specified, each reaction or operation step can be carried out in sequence or not in sequence. Preferably, the reaction method in the present invention is carried out in sequence.

[0170] In the following examples, where specific technologies or conditions are not indicated, they are carried out according to the technologies or conditions described in the literature in the field or according to the product specifications. All reagents or instruments without indicating the manufacturer can be obtained as conventional products through commercial purchase.

[0171] Example 1

[0172] This example provides a lithium-ion battery, the positive electrode sheet of which contains an aqueous safety coating. The aqueous safety coating contains a binder, a first conductive agent Super P Li, and an inorganic filler boehmite. Among them, the structural formula of the binder is:

[0173] 。

[0174] (1) Preparation of the binder:

[0175] a) Add 1000 parts by weight of distilled water to the reaction vessel, start stirring, introduce high-purity nitrogen to remove oxygen for 1 hour, and then add 50 parts by weight of acrylonitrile, 35 parts by weight of lithium acrylate, and 15 parts by weight of methyl acrylate, and heat to 65 °C and keep it constant under an inert atmosphere;

[0176] b) Then add 5 parts by weight of a 20% ammonium persulfate solution as an initiator to initiate the reaction, and react for 5 hours;

[0177] c) After the reaction is completed, filter, dry, crush, and sieve to obtain the binder. The proportion of -CN in the binder is n1 = 50%, the proportion of -COOLi is n2 = 35%, the proportion of -COOCH3 is n3 = 15%, and the weight-average molecular weight M w is 3.5×10 5 , and the number-average molecular weight M w is 1.8×10 5, M w / M n = 1.94, the electrolyte absorption rate c = 23%.

[0178] (2) Preparation of the positive current collector with a water-based safety coating:

[0179] Using deionized water as the solvent, 10 parts of the above binder were added for sizing, and the amount of deionized water was adjusted until a sizing solution with a solid content of 15% and a viscosity of 2.0×10 4 mPa·s was obtained; then 88 parts of the inorganic filler boehmite (median particle size D v50 is 0.4 μm, specific surface area is 9.5 m 2 / g) and 2 parts of the first conductive agent Super P Li were added to the sizing solution and mixed evenly to obtain a water-based safety coating slurry with a solid content of 20% and a viscosity of 750 mPa·s. The water-based safety coating slurry was coated on both sides of the aluminum foil by gravure printing. When coating, both L A2 and L B2 were set to 0, that is, L A1 = L B1 = L A = L B = L, N1 = N3 = 1, N2 = 4. In addition, the temperature of the oven during printing needs to be set to 95 °C, and the printing speed is 30 m / min. Finally, a positive current collector with a double-sided bottom-coated water-based safety coating is obtained. The double-sided coating surface density of the water-based safety coating is 10 mg / 1540.25 mm 2 , the double-sided thickness is 5.1 μm, the double-sided film sheet resistance is 2.8 Ω @0.4T, and the adhesion force between the water-based safety coating and the current collector is 1318 N / m.

[0180] (3) Preparation of the positive electrode sheet:

[0181] The positive electrode active material, conductive agent (a mixture of conductive carbon black and carbon nanotubes, mass ratio 6:5), PVDF binder, and NMP were mixed evenly according to a mass ratio of 97.6:1.1:1.3:35 to prepare a positive electrode slurry with a solid content of 75% and a viscosity of 7000 mPa·s. The positive electrode slurry was coated on one side of the above aluminum foil current collector with a water-based safety coating. After drying and winding at 85 °C, the positive electrode slurry was coated and dried on the other side of the aluminum foil current collector with a water-based safety coating according to the above method. Then, the positive electrode sheet with a double-sided positive electrode active material layer was cold-pressed; then trimming and slitting were carried out to make a lithium-ion battery positive electrode sheet.

[0182] (4) Preparation of the negative electrode sheet:

[0183] Using water as the solvent, graphite, thickening agent, and SBR binder are mixed evenly in a mass ratio of 97.7:1.1:1.2 to prepare a negative electrode slurry for lithium-ion batteries with a solid content of 50% and a viscosity of 5000 mPa·s. The slurry is coated on one side surface of the current collector copper foil and dried and wound at 80 °C. Then, the negative electrode slurry is coated and dried on the other side of the copper foil according to the above method to obtain a negative electrode sheet with active substances coated on both sides.

[0184] (5)Preparation of electrolyte:

[0185] Lithium hexafluorophosphate (LiPF6) is dissolved in a mixed solvent of dimethyl carbonate (DMC), ethylene carbonate (EC), and ethyl methyl carbonate (EMC) (the mass ratio of DMC, EC, and EMC is 3:5:2) to obtain the electrolyte.

[0186] (6)Preparation of battery:

[0187] The positive electrode sheet, negative electrode sheet, and separator prepared above are wound into an electric core with a capacity of about 5 Ah. The separator is located between adjacent positive and negative electrode sheets. The positive electrode is led out by spot welding with an aluminum tab, and the negative electrode is led out by spot welding with a nickel tab. Then, the electric core is placed in an aluminum-plastic packaging bag, baked, and the above electrolyte is injected. After processes such as encapsulation, formation, and grading, a lithium-ion battery is finally made.

[0188] Example 2

[0189] The difference from Example 1 is that 35 parts of lithium acrylate in the synthetic binder are replaced with an equal amount of sodium acrylate, and the structural formula of the obtained binder is:

[0190] .

[0191] The rest is the same as in Example 1 and will not be elaborated here.

[0192] Example 3

[0193] The difference from Example 1 is that 15 parts of methyl acrylate in the synthetic binder are replaced with an equal amount of butyl acrylate, and the structural formula of the binder is:

[0194] .

[0195] The rest is the same as in Example 1 and will not be elaborated here.

[0196] Example 4

[0197] The difference from Example 1 is as follows: the proportion n1 of -CN in the binder is 56.7%, the proportion n2 of -COOR1 is 28.3%, and the proportion n3 of -COOR2 is 15%. Correspondingly, when synthesizing the binder, the weight portion of acrylonitrile is changed to 56.7 parts, the weight portion of lithium acrylate is changed to 28.3 parts, and the weight portion of methyl acrylate remains 15 parts.

[0198] The rest is the same as in Example 1 and will not be elaborated here.

[0199] Example 5

[0200] The difference from Example 1 is as follows: the proportion n1 of -CN in the binder is 46.4%, the proportion n2 of -COOR1 is 38.6%, and the proportion n3 of -COOR2 is 15%. Correspondingly, when synthesizing the binder, the weight portion of acrylonitrile is changed to 46.4 parts, the weight portion of lithium acrylate is changed to 38.6 parts, and the weight portion of methyl acrylate remains 15 parts.

[0201] The rest is the same as in Example 1 and will not be elaborated here.

[0202] Example 6

[0203] The difference from Example 1 is as follows: the proportion n1 of -CN in the binder is 56%, the proportion n2 of -COOR1 is 39.2%, and the proportion n3 of -COOR2 is 4.8%. Correspondingly, when synthesizing the binder, the weight portion of acrylonitrile is changed to 56 parts, the weight portion of lithium acrylate is changed to 39.2 parts, and the weight portion of methyl acrylate is changed to 4.8 parts.

[0204] The rest is the same as in Example 1 and will not be elaborated here.

[0205] Example 7

[0206] The difference from Example 1 is as follows: the proportion n1 of -CN in the binder is 49.1%, the proportion n2 of -COOR1 is 34.3%, and the proportion n3 of -COOR2 is 16.6%. Correspondingly, when synthesizing the binder, the weight portion of acrylonitrile is changed to 49.1 parts, the weight portion of lithium acrylate is changed to 34.3 parts, and the weight portion of methyl acrylate is changed to 16.6 parts.

[0207] The rest is the same as in Example 1 and will not be elaborated here.

[0208] Example 8

[0209] The difference from Example 1 is as follows: the proportion n1 of -CN in the binder is 55%, the proportion n2 of -COOR1 is 30%, and the proportion n3 of -COOR2 is 15%. Correspondingly, when synthesizing the binder, the weight portion of acrylonitrile is changed to 55 parts, the weight portion of lithium acrylate is changed to 30 parts, and the weight portion of methyl acrylate remains 15 parts.

[0210] The rest is the same as in Example 1 and will not be elaborated here.

[0211] Example 9

[0212] The difference from Example 1 is that: in the binder, the proportion of -CN is n1 = 53%, the proportion of -COOR1 is n2 = 37%, and the proportion of -COOR2 is n3 = 10%. Correspondingly, when synthesizing the binder, the weight parts of acrylonitrile are changed to 53 parts, the weight parts of lithium acrylate are changed to 37 parts, and the weight parts of methyl acrylate are changed to 10 parts.

[0213] The rest is the same as in Example 1 and will not be elaborated here.

[0214] Example 10

[0215] The difference from Example 1 is that: in the aqueous safety coating, the dosage of the first conductive agent Super P Li is 3 parts, the dosage of the inorganic filler boehmite is 87 parts, and the dosage of the binder is 10 parts.

[0216] The rest is the same as in Example 1 and will not be elaborated here.

[0217] Example 11

[0218] The difference from Example 1 is that: in the aqueous safety coating, the dosage of the first conductive agent Super P Li is 1.5 parts, the dosage of the inorganic filler boehmite is 88.5 parts, and the dosage of the binder is 10 parts.

[0219] The rest is the same as in Example 1 and will not be elaborated here.

[0220] Example 12

[0221] The difference from Example 1 is that: in the aqueous safety coating, the dosage of the first conductive agent Super P Li is 2 parts, the dosage of the inorganic filler boehmite is 83 parts, and the dosage of the binder is 15 parts.

[0222] The rest is the same as in Example 1 and will not be elaborated here.

[0223] Example 13

[0224] The difference from Example 1 is that: during coating, a ceramic layer is separately provided in the non-aqueous safety coating area on side B, that is, L B2 ≠0, as Figure 2 shown, the total length of the current collector containing the aqueous safety coating is L, the length of the aqueous safety coating area on side B is L B1 , the length of the non-aqueous safety coating area is L B2 , and L B1 >L B2 , LB1 +L B2 =L B =L A1 =L A =L.

[0225] The rest is the same as in Example 1 and will not be elaborated here.

[0226] Comparative Example 1

[0227] The difference from Example 1 is that: the proportion of -CN in the binder is n1 = 60%, the proportion of -COOR1 is n2 = 40%, the proportion of -COOR2 is n3 = 0%, and (n1 + n2) / n3 does not exist. Correspondingly, when synthesizing the binder, the weight portion of acrylonitrile is changed to 60 parts, the weight portion of lithium acrylate is changed to 40 parts, and methyl acrylate is not contained.

[0228] The rest is the same as in Example 1 and will not be elaborated here.

[0229] Comparative Example 2

[0230] The difference from Example 1 is that: the proportion of -CN in the binder is n1 = 0%, the proportion of -COOR1 is n2 = 70%, the proportion of -COOR2 is n3 = 30%, and n1 / n2 = 0. Correspondingly, when synthesizing the binder, the weight portion of lithium acrylate is changed to 70 parts, the weight portion of methyl acrylate is changed to 30 parts, and acrylonitrile is not contained.

[0231] The rest is the same as in Example 1 and will not be elaborated here.

[0232] Comparative Example 3

[0233] The difference from Example 1 is that: the proportion of -CN in the binder is n1 = 75%, the proportion of -COOR1 is n2 = 0%, the proportion of -COOR2 is n3 = 25%, and n1 / n2 does not exist. Correspondingly, when synthesizing the binder, the weight portion of acrylonitrile is changed to 75 parts, the weight portion of methyl acrylate is changed to 25 parts, and lithium acrylate is not contained.

[0234] Comparative Example 4

[0235] The difference from Example 1 is that: the proportion of -CN in the binder is n1 = 42.5%, the proportion of -COOR1 is n2 = 42.5%, the proportion of -COOR2 is n3 = 15%. Correspondingly, when synthesizing the binder, the weight portion of acrylonitrile is changed to 42.5 parts, the weight portion of lithium acrylate is changed to 42.5 parts, and the weight portion of methyl acrylate remains 15 parts.

[0236] The rest is the same as in Example 1 and will not be elaborated here.

[0237] Comparative Example 5

[0238] The difference from Example 1 is that: in the binder, the proportion of -CN is n1 = 65%, the proportion of -COOR1 is n2 = 20%, and the proportion of -COOR2 is n3 = 15%. Correspondingly, when synthesizing the binder, the weight parts of acrylonitrile are changed to 65 parts, the weight parts of lithium acrylate are changed to 20 parts, and the weight parts of methyl acrylate remain 15 parts.

[0239] The rest is the same as in Example 1 and will not be elaborated here.

[0240] Comparative Example 6

[0241] The difference from Example 1 is that: in the binder, the proportion of -CN is n1 = 56.5%, the proportion of -COOR1 is n2 = 39.5%, and the proportion of -COOR2 is n3 = 4%. Correspondingly, when synthesizing the binder, the weight parts of acrylonitrile are changed to 56.5 parts, the weight parts of lithium acrylate are changed to 39.5 parts, and the weight parts of methyl acrylate are changed to 4 parts.

[0242] The rest is the same as in Example 1 and will not be elaborated here.

[0243] Comparative Example 7

[0244] The difference from Example 1 is that: in the binder, the proportion of -CN is n1 = 29.4%, the proportion of -COOR1 is n2 = 20.6%, and the proportion of -COOR2 is n3 = 50%. Correspondingly, when synthesizing the binder, the weight parts of acrylonitrile are changed to 29.4 parts, the weight parts of lithium acrylate are changed to 20.6 parts, and the weight parts of methyl acrylate are changed to 50 parts.

[0245] The rest is the same as in Example 1 and will not be elaborated here.

[0246] Comparative Example 8

[0247] The difference from Example 1 is that: by adjusting the weight parts of the initiator (ammonium persulfate solution with a mass fraction of 20%) to 2 parts, the weight-average molecular weight M w of the obtained adhesive is 5×10 5 , the number-average molecular weight M w is 1×10 5 , and M w / M n = 5.

[0248] The rest is the same as in Example 1 and will not be elaborated here.

[0249] Comparative Example 9

[0250] The difference from Example 1 is that: the dosage of the first conductive agent in the aqueous safety coating is increased to 6.0 parts, the dosage of boehmite is reduced to 84 parts, the dosage of the binder remains unchanged at 10 parts, and the diaphragm resistance is rapidly reduced to 0.3 Ω @ 0.4 T.

[0251] The rest is the same as in Example 1 and will not be elaborated here.

[0252] Comparative Example 10

[0253] The difference from Example 1 is that: the dosage of the first conductive agent in the aqueous safety coating is reduced to 0.2 parts, the dosage of boehmite is increased to 89.8 parts, the dosage of the binder remains unchanged at 10 parts, and the diaphragm resistance is rapidly increased to 12.1 Ω @ 0.4 T.

[0254] The rest is the same as in Example 1 and will not be elaborated here.

[0255] Comparative Example 11

[0256] The difference from Example 1 is that: the solid content of the prepared aqueous safety coating slurry is increased to 28%, and the thickness of the obtained aqueous safety coating is increased to 15.9 μm.

[0257] The rest is the same as in Example 1 and will not be elaborated here.

[0258] Comparative Example 12

[0259] The difference from Example 1 is that: the dosage of the binder is reduced to 0.53 parts, and the dosage of boehmite is increased to 97.47 parts.

[0260] The rest is the same as in Example 1 and will not be elaborated here.

[0261] Comparative Example 13

[0262] The difference from Example 1 is that: the dosage of the binder is increased to 25 parts, and the dosage of boehmite is reduced to 73 parts.

[0263] The rest is the same as in Example 1 and will not be elaborated here.

[0264] Comparative Example 14

[0265] The difference from Example 1 is that: the binder is replaced with a PVDF binder, and the solvent is changed from deionized water to NMP (N-methylpyrrolidone).

[0266] The rest is the same as in Example 1 and will not be elaborated here.

[0267] Comparative Example 15

[0268] The difference from Example 1 is that: the surface of the positive current collector is not coated with the aqueous safety coating.

[0269] The rest is the same as in Example 1 and will not be elaborated here.

[0270] The parameter settings of Examples 1-13 and Comparative Examples 1-15, the solid content, viscosity (unit: mPa·s) of the obtained aqueous safety coating slurry, and the adhesion force (unit: N / m) between the aqueous safety coating and the current collector are shown in Table 1 (Examples) and Table 2 (Comparative Examples) as follows:

[0271] Test method for the adhesion force of the aqueous safety coating: Cut the aluminum foil coated with the aqueous safety coating into rectangular strips with a width of 15 mm and a length of 250 mm. Then cut a transparent tape with the same width and stick it to the non-test surface of the primer-coated aluminum foil to prevent the sample from breaking during tearing. Try to make the transparent tape smooth when sticking to reduce the generation of bubbles and prevent jitter during testing. Take a steel plate with a length * width * thickness of 150 mm * 50 mm * 2 mm and stick double-sided tape on one side of it (the double-sided tape is parallel to the edge of the steel plate and its length is 60 mm), and tear off the release paper. Align one end of the aforementioned strip with the exposed double-sided tape, and the overlapping length of the strip and the double-sided tape is the length of the tape. Then roll it back and forth three times with a 2 kg pressure roller to complete the preparation of the test sample. Conduct the adhesion force test on a universal tensile testing machine. Place the lower part of the steel plate into the lower test fixture, and vertically align the part of the sample without sticking the double-sided tape into the upper test fixture so that the strip can rotate 180° for testing. Tighten the fixture, set the test speed to 300 mm / min, the test gauge length to 50 mm, start the test and read the indication.

[0272] Table 1

[0273]

[0274] Table 2

[0275]

[0276] Test Example

[0277] To verify the influence of the introduction of the aqueous safety coating of the present invention on the performance of the battery cell, the electrolyte absorption rate c of the binder, the double-sided film resistance R of the aqueous safety coating, and the puncture resistance, low-temperature discharge performance, rate performance, and cycle performance of the battery cell were tested.

[0278] Electrolyte absorption rate test method: Pour the binder solution into a polytetrafluoroethylene dish and bake it to volatilize deionized water to obtain a binder polymer film. The thickness of the film is controlled at 100 μm, and it is cut into a film piece with a length of 50 mm × a width of 50 mm. Then, the film piece is vacuum-dried for 24 h and weighed, and then the completely dried film piece is immersed in the electrolyte at 80 °C for 12 h and then taken out. After wiping off the electrolyte adhering to the surface of the film, the weight of the film piece after absorbing the electrolyte is weighed. The increase rate (%) of the weight of the film piece before and after immersion in the electrolyte is the electrolyte absorption rate of the binder at 80 o °C.

[0279] Film resistance test method: The instrument used for film resistance test is the ACCFILM film resistance test system (model TT-ACCF-G2A) of Hangzhou Chuanyuan Technology Co., Ltd. The pressure during the test is 0.4 t and the pressure holding time is 10 s.

[0280] Pinprick test method: Charge at a constant current and constant voltage of 1.0 C to 4.45 V at room temperature, with a cut-off rate of 0.05 C. Then, perform a pinprick test on the fully charged battery cell. During the test, the deep pit surface of the battery cell faces up, use a steel nail with a diameter of 4.0 mm, and completely pierce the battery cell at a speed of 40 mm / s at one time. The piercing position is at the left, middle, and right positions of the largest surface of the battery cell (5 battery cells are tested at each position), and keep it for 1 h. If the battery cell does not catch fire or explode, it passes the test.

[0281] Low-temperature discharge performance test method: Discharge at a constant current of 1 C to 3 V and set aside for 5 min; Set the temperature chamber to 25 °C and set aside for 60 min; Charge at a constant current and constant voltage of 1.0 C to 4.45 V, with a cut-off rate of 0.02 C; Set aside for 5 min and discharge at a constant current of 0.2 C to 3 V, and record the capacity C0 when discharging to 3 V at 0.2 C; Set the temperature chamber to 25 °C and set aside for 60 min; Charge at a constant current and constant voltage of 1.0 C to 4.45 V, with a cut-off rate of 0.02 C; Set aside for 5 min, set the temperature chamber to -10 °C, and set aside for 120 min. Then, discharge the battery cell at 0.2 C to 3.0 V, and record the capacities C1 and C2 when discharging to 3.4 V and 3.0 V. The ratio of C1 to C0 is the capacity retention rate when discharging to 3.4 V at -10 °C.

[0282] Method for testing rate discharge performance: Discharge at a constant current of 1 C until 3 V, then rest for 5 min; Charge at a constant current and constant voltage of 1.0 C until 4.45 V, with a cut-off rate of 0.02 C, then rest for 5 min; Discharge at a constant current of 0.2 C until 3 V, and record the capacity when discharging to 3 V at 0.2 C as the initial capacity C3; Charge at a constant current and constant voltage of 1.0 C until 4.45 V, with a cut-off rate of 0.02 C, then rest for 5 min; Then discharge the battery cell at 0.2 C until 3.0 V, and record the capacities C4 and C5 when discharging to 3.4 V and 3.0 V. The ratio of C4 to C3 is the capacity retention rate when discharging to 3.4 V at 2C.

[0283] Method for testing cycle performance: Under the condition that the ambient temperature is 25±2°C, discharge the battery cell at a constant current of 0.2 until 3.0V, then charge at a constant current and constant voltage of 3C until 4.45 V, with a cut-off rate of 0.05 C, and record the voltage, internal resistance, capacity, and thickness (600 gPPG is used for thickness measurement) of the battery cell during the first full charge. The cycle process follows the following HFC format: Discharge at a constant current of 0.2 C until 3 V; Charge at a constant current of 3.0 C until 4.25 V; Charge at a constant current of 2.5 C until 4.25 V; Charge at a constant current of 2.0 C until 4.45 V; Charge at a constant current and constant voltage of 1.4 C until 4.50 V, with a cut-off rate of 0.3 C; Charge at a constant current and constant voltage of 2 A until 4.45 V, with a cut-off rate of 0.05 C; Discharge at a constant current of 1.0 C until 3 V. Completing the above steps is one cycle. After 49 cycles, perform small current recovery according to the following format: Charge at a constant current of 3.0 C until 4.25 V; Charge at a constant current of 2.5 C until 4.25 V; Charge at a constant current of 2.0 C until 4.45 V; Charge at a constant current and constant voltage of 1.4 C until 4.50 V, with a cut-off rate of 0.3 C; Charge at a constant current and constant voltage of 2 A until 4.45 V, with a cut-off rate of 0.05 C; Discharge at a constant current of 0.2 C until 3 V; Charge at a constant current and constant voltage of 3.0 C until 4.45 V, with a cut-off rate of 0.05 C. Record the voltage, internal resistance, and thickness (600 gPPG is used for thickness measurement) of the fully charged battery cell every 100 weeks.

[0284] The secondary batteries obtained in Examples 1-13 and Comparative Examples 1-15 were tested, and the test results are shown in Table 3 (Examples) and Table 4 (Comparative Examples) below.

[0285] Table 3

[0286]

[0287] Table 4

[0288]

[0289] From the comparison of the test results in the above table, it can be seen that compared with Comparative Examples 1-15, the aqueous safety coatings in Examples 1-13 satisfy 2 ≤ R×d ≤ 50, 10% ≤ c ≤ 50%, 5 ≤ (x + z) / y ≤ 20, and 1.2 ≤ z / x ≤ 2. The passing rate of the needle penetration test for the corresponding battery cells is significantly improved, increasing from 0% to over 90%. At the same time, the low-temperature discharge performance, rate discharge performance, and cycle performance are also improved, and they are comparable to the performance of battery cells with conventional PVDF binders used for the oily primer coat and battery cells with a ceramic layer separately provided in the non-aqueous safety coating area.

[0290] Examples 1-13 and Comparative Examples 1-3 show that cyano group, ester group, and carboxylate group in the binder are all essential groups. The lack of any one of these groups will lead to the deterioration of the battery cell performance.

[0291] Examples 1-13 and Comparative Examples 4-8 show that the values of x, y, z and their mutual relationship, and the ratio of M w / M n in the binder will all affect the aqueous safety coating. When they are not within the scope defined by the present invention, the battery cell performance will deteriorate.

[0292] From Examples 1-13 and Comparative Examples 9-11, it can be known that if the content of the conductive agent and the solid content of the slurry are not reasonably set when preparing the aqueous coating slurry, resulting in R×d not being within the range given by the present invention, it will also lead to the deterioration of the battery cell performance. In Comparative Examples 12-13, the amount of the binder is too low or too high, resulting in poor battery cell performance.

[0293] In summary, the binder provided by the present invention, by limiting the proportion of -CN, -COOR1, and -COOR2 contained in the side chain of the PAA-based binder among all groups (the proportion of -CN among all groups is n1, the proportion of -COOR1 among all groups is n2, and the proportion of -COOR2 among all groups is n3), and the relative relationship among n1, n2, and n3, at the same time limiting the electrolyte absorption rate c of the binder at 80 °C, and limiting the resistance R and thickness d of the double-sided membrane with the aqueous safety coating, thus achieving the purpose that the safety performance and cycle performance of the battery cell do not deteriorate, and at the same time improving the low-temperature discharge performance and rate discharge performance of the battery. For the battery cell provided by the present invention, when 2 ≤ R×d ≤ 50, 10% ≤ c ≤ 50%, 5 ≤ (x + z) / y ≤ 20, and 1.2 ≤ z / x ≤ 2, the needle penetration passing rate can be greater than 90%. At the same time, the capacity retention rate after 800 cycles at 25 °C is not less than 89%, the discharge capacity at -10 °C is not less than 70% @3.4V, and the discharge capacity at 2C is not less than 70% @3.4V.

[0294] The above-described embodiments merely represent several implementation manners of the present invention. The description thereof is relatively specific and detailed, but it should not be construed as a limitation to the scope of the patent of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can still be made, and these all fall within the protection scope of the present invention.

Claims

1. A polyacrylate binder, characterized in that, The structural formula of the polyacrylate binder is as follows: ； Wherein, each occurrence of R1 is independently selected from H, Li or Na; Each occurrence of R2 is independently selected from substituted or unsubstituted C1-C 10 alkyl; And 5 ≤ (x + z) / y ≤ 20, 1.2 ≤ z / x ≤ 2, and x, y, z are all non-zero; The weight-average molecular weight M of the polyacrylate binder w ranges from 200,000 to 500,000, and the number-average molecular weight M n ranges from 100,000 to 400,000; The electrolyte absorption rate of the polyacrylate binder at 80 °C is 10% - 50%.

2. The polyacrylate binder according to claim 1, characterized in that, The weight-average molecular weight M of the polyacrylate binder w and the number-average molecular weight M n have a ratio M w / M n ≤ 3.

3. An aqueous safety coating, characterized in that, The raw materials of the aqueous safety coating include the polyacrylate binder as described in any one of claims 1 - 2, a first conductive agent, and an inorganic filler.

4. The aqueous safety coating according to claim 3, wherein The mass percentage of the polyacrylate binder in the aqueous safety coating is 1% - 24%.

5. The waterborne safety coating according to claim 3, wherein Including at least one of the following: A1) The mass ratio of the first conductive agent, the polyacrylate binder, and the inorganic filler is (0.5 - 5):(1 - 24):(71 - 98.5); A2) The mass percentage of the first conductive agent in the aqueous safety coating is 0.5% - 5%; A3) The median particle size range of the inorganic filler is 0.2 to 4 μm, and the specific surface area is 5 to 50 m 2 / g; A4) The raw materials of the aqueous safety coating further include a thermally stable active substance; A5) When including A4), the mass ratio of the first conductive agent, the polyacrylate binder, the inorganic filler, and the thermally stable active substance is (0.5 - 5):(1 - 24):(1 - 97.5):(1 - 97.5); A6) When including A4), the thermally stable active substance includes one or both of lithium iron phosphate and lithium manganese iron phosphate.

6. An electrode sheet, characterized in that, Including: A current collector; An active material layer; The aqueous safety coating as described in any one of claims 3 - 5.

7. The electrode sheet according to claim 6, characterized in that, The aqueous safety coating is coated on at least one side surface of the current collector, and the active material layer is coated on the side surface of the aqueous safety coating away from the current collector and / or coated on the side surface of the current collector away from the aqueous safety coating; Or, the active material layer is coated on at least one side surface of the current collector, and the aqueous safety coating is coated on the side surface of the active material layer away from the current collector and / or coated on the side surface of the current collector away from the active material layer.

8. The electrode sheet according to claim 7, wherein, The aqueous safety coating is coated on both side surfaces of the current collector; the aqueous safety coating satisfies at least one of the following: B1) The double-sided thickness of the aqueous safety coating is 1 - 20 μm; The double-sided coating surface density of the water-based safety coating is 2 to 35 mg / 1540.25 mm 2 ; B3) The double-sided film resistance of the aqueous safety coating tested under a pressure of 0.4 t is 0.5 - 5 Ω; B4) The numerical values of the double-sided thickness d of the aqueous safety coating and the double-sided film resistance R under a pressure of 0.4 t satisfy the condition: 2 ≤ R×d ≤ 50; B5) Along the direction perpendicular to the unwinding direction of the current collector, the distance W between the left edge of the water-based safety coating on the A side of the current collector and the left edge of the current collector A1 ranges from 2 mm to 30 mm, and the distance W between the right edge of the water-based safety coating on the A side of the current collector and the right edge of the current collector A2 ranges from 2 mm to 30 mm; the distance W between the left edge of the water-based safety coating on the B side of the current collector and the left edge of the current collector B1 ranges from 3 mm to 30 mm, and the distance W between the right edge of the water-based safety coating on the B side of the current collector and the right edge of the current collector B2 ranges from 3 mm to 30 mm; and W B1 -W A1 ≥ 1 mm, W B2 -W A2 ≥ 1 mm.

9. A method for preparing an electrode sheet according to any one of claims 6-8, characterized in that, Including the steps of: Mix the polyacrylate binder as described in claim 1 with water and then perform sizing to obtain a sizing solution; Mix the first conductive agent and the inorganic filler uniformly with the sizing solution, or mix the first conductive agent, the thermally stable active substance, and the inorganic filler uniformly with the sizing solution to obtain an aqueous safety coating slurry; Coat the aqueous safety coating slurry on one or both side surfaces of the current collector, or on the side surface of the active material layer away from the current collector to obtain the electrode sheet.

10. The method for preparing an electrode sheet according to claim 9, characterized in that, Including at least one of the following: C1) The mass ratio of the first conductive agent, polyacrylate binder, and inorganic filler is (0.5 to 5):(1 to 24):(71 to 98.5); C2) The solid content of the glue solution is 10% to 30%; C3) The viscosity of the adhesive solution is 1×10 3 ~3×10 4 mPa·s; C4) The solid content of the aqueous safety coating slurry is ≥10%; C5) The viscosity of the aqueous safety coating slurry is ≥50 mPa·s; C6) The dyne value of the current collector is ≥30 dyn / cm; C7) The aqueous safety coating slurry is coated on one or both surfaces of the current collector, or on the surface of the active material layer away from the current collector, by gravure printing; C8) When including C7), the temperature of the oven during gravure printing is 90 to 110°C, and the printing speed is 10 to 50 m / min; C9) The mass ratio of the first conductive agent, polyacrylate binder, inorganic filler, and thermally stable active material is (0.5 to 5):(1 to 24):(1 to 97.5):(1 to 97.5).

11. A lithium-ion battery, characterized in that, It includes a positive electrode sheet, a negative electrode sheet, and a separator spaced between the positive electrode sheet and the negative electrode sheet; the positive electrode sheet and / or the negative electrode sheet is the electrode sheet according to any one of claims 6 - 8 or the electrode sheet obtained by the preparation method according to any one of claims 9 - 10.

12. Use of a lithium-ion battery according to claim 11 in an energy storage device, an electrical device, or an electronic device.

Images