A battery cell, a preparation method thereof, and a lithium-ion battery

Abstract

The present invention discloses a battery cell, a preparation method thereof, and a lithium-ion battery, relating to the technical field of secondary battery materials. The battery cell provided by the present invention introduces a safety coating into the positive electrode sheet and limits the capacity exerted by the first active substance in the safety coating. The first active substance not only reduces the possibility of positive and negative electrode contact and improves the mechanical safety performance of the battery cell, but also can exert a certain capacity, thereby reducing the N / P ratio (the ratio of the negative electrode capacity to the positive electrode capacity) of the battery cell, reducing the potential of the positive electrode in the fully charged state, and improving the thermal abuse performance and cycle performance of the battery. Finally, by limiting the relevant parameters, the present invention can provide a battery cell that can balance the mechanical safety performance, thermal box performance and cycle performance of the battery cell, and improve the low-temperature discharge performance and rate discharge performance without changing the original process.

Description

Technical Field

[0001] The present invention relates to the technical field of secondary battery materials, and in particular to a battery cell, a preparation method thereof, and a lithium-ion battery. Background Art

[0002] Lithium-ion batteries are widely used in products such as 3C, electric vehicles, and power tools because of their advantages of high energy density, no memory effect, long cycle life, environmental friendliness, and ability to adapt to various environments.

[0003] In practical applications, the requirements for mechanical safety tests such as lithium battery pinprick and thermal abuse safety tests such as hot box are also getting higher and higher. In order to improve the mechanical safety performance of the battery cell, the industry mostly adopts the measure of coating a safety coating on the positive current collector. The safety coating usually contains a certain proportion of inorganic fillers, conductive agents, and binders. However, the conventional safety coating has poor improvement effect on the thermal abuse safety performance of the battery cell. In order to improve the thermal abuse safety performance of the battery cell, most methods are to dope and coat the positive and negative active materials, or develop electrolytes with high thermal stability, etc. However, these methods have deficiencies such as long development cycle and high development cost.

[0004] Therefore, it is of great significance to solve the problem that the existing conventional improvement measures cannot effectively improve the mechanical safety performance and thermal abuse safety performance of the battery cell at the same time, and to provide a battery cell that can take into account the mechanical safety performance, hot box performance, and cycle performance without changing the original process, and improve the low-temperature discharge performance and rate discharge performance. 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 battery cell, a preparation method thereof, and a lithium-ion battery, aiming to solve the problem that the existing conventional improvement measures cannot effectively improve the mechanical safety performance and thermal abuse safety performance of the battery cell at the same time.

[0006] In the first aspect of the present invention, a battery cell is provided, including: a positive electrode sheet and a negative electrode sheet; the positive electrode sheet includes a safety coating and a positive active material layer, the safety coating includes a first active material and a dispersant, and the positive active material layer includes a positive active material; the negative electrode sheet includes a negative active material layer, and the negative active material layer includes a negative active material;

[0007] The first capacity ratio NP1 of the battery cell satisfies 1.019 ≤ NP1 ≤ 1.089, and the relative capacity ratio satisfies 0.002 ≤ ≤ 0.04;

[0008] Wherein, the calculation formula of the first capacity ratio NP1 is: and the relative capacity ratio The calculation formula is as follows: ;

[0009] C1 is the capacity exerted by the first active substance in the safety coating per unit area. The calculation formula for C1 is: C1 = k × c w × a, where 0 < k < 1, 0 < a ≤ 300, and c w is the single-sided coating areal density of the safety coating, a is the theoretical specific capacity of the first active substance, and k is a constant;

[0010] C2 is the capacity exerted by the positive electrode active substance in the positive electrode active substance layer per unit area;

[0011] C0 is the capacity exerted by the negative electrode active substance in the negative electrode active substance layer per unit area.

[0012] The battery cell according to the embodiment of the present invention has at least the following beneficial effects: In the battery cell provided by the present invention, a safety coating is introduced into the positive electrode sheet, and the capacity exerted by the first active substance in the safety coating is limited. Without changing the original process, the mechanical safety performance, thermal box performance, and cycling performance of the battery cell can be taken into account, and the low-temperature discharge performance and rate discharge performance can be improved.

[0013] In the present invention, C1 is defined as the capacity exerted by the first active substance in the safety coating per unit area. The calculation formula for C1 is: C1 = k × c w × a, where 0 < k < 1, 0 < a ≤ 300, and c w is the single-sided coating areal density of the safety coating, a is the theoretical specific capacity of the first active substance, and k is a constant related to the battery cell system and depth of discharge. Generally speaking, the larger a is, the worse the stability of the active substance is. In the present invention, it is limited to 0 < a ≤ 300. At this time, the thermal stability of the first active substance is good, and it is not easy to collapse or deform in structure. It is also not easy to decompose during mechanical abuse tests such as pinprick of the battery cell, reducing the risk of thermal runaway of the battery cell and improving the safety of the battery. In addition, the first active substance not only reduces the possibility of contact between the positive and negative electrodes, improves the mechanical safety performance of the battery cell, but also can exert a certain capacity, thereby reducing the N / P ratio (the ratio of the negative electrode capacity to the positive electrode capacity) of the battery cell, reducing the potential of the positive electrode in the fully charged state, and improving the thermal abuse performance and cycling performance of the battery. Define C2 as the capacity exerted by the positive electrode active substance in the positive electrode active substance layer per unit area, and C0 as the capacity exerted by the negative electrode active substance in the negative electrode active substance layer per unit area. In the present invention, the ratio of C0 to C2 is defined as the N / P ratio (the ratio of the negative electrode capacity to the positive electrode capacity, N / P ratio = C0 / C2), and the change in the NP value caused by the introduction of the safety coating is defined as (relative capacity ratio), The calculation formula is as follows: , the capacity ratio of the corresponding battery cell is NP1, and the calculation formula of NP1 is: . The present invention defines to take values in the range of 0.002 to 0.04. When is lower than 0.002, the corresponding c w is relatively small. At this time, not only is the processing more difficult, the adjustment of the NP value is not obvious, but also the safety coating is thinner, which will have an adverse impact on the mechanical safety performance of the battery cell; when is greater than 0.04, the corresponding c w is also relatively large, which will have an adverse impact on the energy density of the battery cell. The present invention also limits the value range of NP1. When NP1 is less than 1.019, it indicates that the capacity of the negative electrode is lower than that of the positive electrode, and there is a risk of lithium plating in the battery cell; when this value is greater than 1.089, it indicates that the negative electrode has sufficient margin, which will cause greater irreversible lithium loss during formation, reduce the first efficiency of the battery cell, and at the same time the negative electrode is not fully utilized, which will lead to a decrease in the energy density of the battery cell. In addition, it will deepen the charging depth of the positive electrode, increase the potential of the fully charged positive electrode, and make the cycle performance and thermal box performance of the battery cell worse. Considering that a certain margin needs to be left to cope with process fluctuations, the present invention limits the value range of NP1 to 1.019 to 1.089.

[0014] Finally, through the limitation of relevant parameters, the present invention provides a battery cell with good mechanical safety performance, thermal box performance and cycle performance, and at the same time can improve the low-temperature discharge performance and cycle performance.

[0015] In the present invention, C1 is the capacity exerted by the first active substance in the safety coating per unit area, C2 is the capacity exerted by the positive electrode active substance in the positive electrode active substance layer per unit area, and C0 is the capacity exerted by the negative electrode active substance in the negative electrode active substance layer per unit area. "Per unit area" can refer to "per square meter", "per square centimeter", "per square millimeter" or other suitable units, as long as the units of C1, C2, and C0 are unified. Because the relative capacity ratio The calculation formula is: , and the calculation formula of the first capacity ratio NP1 is: , it can be seen that the units of C1, C2, and C0 will not affect or the value of NP1.

[0016] In some embodiments of the present invention, the first capacity ratio NP1 of the battery cell satisfies 1.028 ≤ NP1 ≤ 1.053.

[0017] In some embodiments of the present invention, the calculation formula of C1 is: C1 = k × c w × a, 0 < k < 1, 0 < a ≤ 300, c wis the single-sided coating areal density of the safety coating, a is the theoretical specific capacity (constant) of the first active material, and k is a constant related to the cell system and depth of discharge. Preferably, 100 ≤ a ≤ 200. When a is greater than 200, the stability of the active material is poor; when a is less than 100, additional amount of the first active material is required to reduce the same N / P ratio.

[0018] In some embodiments of the present invention, c w has a value range of 3 mg / 1540.25mm 2 ~18 mg / 1540.25mm 2 . It has been found in the present invention that when c w is lower than 3 mg / 1540.25mm 2 , it will not affect the NP value of the system, and the processing is relatively difficult, the safety coating is relatively thin, which will also have an adverse effect on the mechanical safety performance of the cell; while when c w is higher than 18 mg / 1540.25mm 2 , at this time, there is no improvement in the safety performance of the cell (the passing rate of the cell in the nail penetration test has reached 100% when it is lower than this value), on the contrary, it will have an adverse effect on the energy density of the cell. Based on the above considerations, the value range of c w in the present invention is 3 mg / 1540.25mm 2 ~18 mg / 1540.25mm 2 .

[0019] In some embodiments of the present invention, the first active material includes one or more of lithium cobaltate, lithium manganate, lithium iron phosphate, lithium manganese iron phosphate, and ternary materials, preferably one or more of active materials with better thermal stability such as lithium iron phosphate, lithium manganese phosphate, and lithium manganese iron phosphate. The crystal structure of the active material with better thermal stability is a very stable olivine structure, which is not prone to structural collapse or deformation, and is not prone to decomposition during mechanical abuse tests such as nail penetration of the cell, reducing the risk of thermal runaway of the cell and improving the safety of the battery. In addition, the first active material can also exert a certain capacity, thereby reducing the N / P ratio (the ratio of the negative electrode capacity to the positive electrode capacity) of the cell, reducing the potential of the positive electrode at full charge state, and improving the thermal abuse performance and cycle performance of the battery.

[0020] In some embodiments of the present invention, the median particle size D v50 of the first active material has a range of 0.1~15μm, and the specific surface area is 2~50 m 2 / g. If the median particle size D v50 of the first active material is less than 0.1μm, or the specific surface area is greater than 50m 2 / g, it is very easy to agglomerate during slurry stirring, while when D v50Greater than 15 μm, or when the specific surface area is less than 2 m 2 / g, due to the relatively large particle size, it is difficult to obtain a relatively thin safety coating, which affects the energy density of the battery cell, and it is easy to have missed coating at the bottom coating during the coating of the safety coating, affecting the safety of the battery cell.

[0021] In some embodiments of the present invention, the battery cell includes at least one of the following:

[0022] A1) The first active material is a ternary material, and the ternary material includes at least one of nickel-cobalt-manganese ternary material and nickel-cobalt-aluminum ternary material;

[0023] A2) When including A1), the nickel-cobalt-manganese ternary material includes at least one of NCM333, NCM523, NCM613, NCM622, and NCM811;

[0024] A3) The surface of the first active material is coated with an inorganic ceramic material;

[0025] A4) When including A3), the inorganic ceramic material includes silicon dioxide, aluminum oxide, or titanium oxide.

[0026] In some embodiments of the present invention, the ternary material includes at least one of nickel-cobalt-manganese ternary material (NCM) and nickel-cobalt-aluminum (NCA) ternary material. The ternary material used in the present invention can be common ternary materials in the art and is not limited herein. For example, the nickel-cobalt-manganese ternary material can include at least one of NCM333, NCM523, NCM613, NCM622, and NCM811.

[0027] In some embodiments of the present invention, the surface of the first active material is coated with an inorganic ceramic material.

[0028] In some embodiments of the present invention, the inorganic ceramic material includes silicon dioxide, aluminum oxide, or titanium oxide.

[0029] The first active material can be coated, and an inorganic ceramic material such as silicon dioxide, aluminum oxide, or titanium oxide is coated on the surface of its particles. These ceramic materials can inhibit the migration of free lithium ions on the surface of the first active material into the solution, reduce the pH of the coating slurry, and reduce the corrosion of the current collector aluminum foil.

[0030] In some embodiments of the present invention, the dispersant includes one or more of an ionic dispersant, a non-ionic dispersant, an amphoteric dispersant, and a fluorosurfactant.

[0031] In some embodiments of the present invention, the ionic dispersant includes one or more of polyacrylic acid and its salts, sodium dodecylbenzenesulfonate, sodium dodecyl sulfate, and polyethyleneimine.

[0032] In some embodiments of the present invention, the non-ionic dispersant includes one or more of polyvinylpyrrolidone, polyethylene glycol, polyvinyl alcohol, polyoxyethylene, and carboxymethyl cellulose.

[0033] In some embodiments of the present invention, the amphoteric dispersant includes betaine.

[0034] Since the first active material is insoluble in water, has small particle size, and large specific surface area, it is extremely easy to agglomerate during stirring and dispersion. Therefore, a dispersant must be added. The dispersant can improve the dispersion of the first active material with poor dispersibility in the slurry, and can be one or more of the following dispersants: ionic dispersants, such as polyacrylic acid (PAA) and its salts, sodium dodecylbenzenesulfonate (SDBS), sodium dodecyl sulfate (SDS), polyethyleneimine (PEI), etc.; non-ionic dispersants, such as polyvinylpyrrolidone (PVP), polyethylene glycol (PEG), polyvinyl alcohol (PVA), polyoxyethylene (POE), carboxymethyl cellulose (CMC), etc.; amphoteric dispersants represented by betaine; and fluorosurfactants. The dispersant can reduce the mutual attraction between inorganic filler particles, thereby improving the dispersion of the particles in the sizing material. Good dispersion can ensure the uniform distribution of inorganic filler particles, reduce the agglomeration phenomenon, lower the viscosity of the slurry, improve the uniformity of coating, and contribute to improving the electrochemical performance of the battery cell. Preferably, the dispersant is selected from polyacrylic acid (PAA) and its salts, or polyvinylpyrrolidone (PVP).

[0035] In some embodiments of the present invention, the safety coating further includes a first conductive agent, a first binder, and an inorganic filler.

[0036] In some embodiments of the present invention, the safety coating satisfies at least one of the following:

[0037] B1) The mass ratio of the first conductive agent, the first binder, the inorganic filler, the first active material, and the dispersant is (0.5~5):(1~21.2):(1~50):(1~95):(0.01~1);

[0038] B2) The median particle size D v50 of the inorganic filler ranges from 0.1 to 5 μm, and the specific surface area is 8 to 15 m 2 / g;

[0039] B3) The inorganic filler includes one or more of alumina, boehmite, aluminum hydroxide, magnesium hydroxide, silica, or titanium dioxide;

[0040] B4) The first binder includes one or more of a polyvinylidene fluoride binder, a sodium carboxymethyl cellulose binder, or a polyacrylate binder.

[0041] In some embodiments of the present invention, the mass ratio of the first conductive agent, the first binder, the inorganic filler, the first active material, and the dispersant is (0.5~5):(1~21.2):(1~50):(1~95):(0.01~1). For example, by weight, the first conductive agent can be 0.5~5 parts, preferably 1.5~3 parts, the first binder can be 1~21.2 parts, preferably 5~15 parts, the inorganic filler can be 1~50 parts, the first active material can be 1~95 parts, and the dispersant can be 0.01~1 part.

[0042] In some embodiments of the present invention, the median particle size D of the inorganic filler v50 ranges from 0.1~5 μm, and the specific surface area is 8~15 m 2 / g. When the D of the inorganic filler v50 is less than 0.1 μm and the specific surface area is greater than 15 m 2 / g, the inorganic filler is prone to agglomeration and difficult to disperse; when the D of the inorganic filler v50 is greater than 5 μm and the specific surface area is less than 8 m 2 / g, due to the large particle size, it is difficult to obtain a thin safety coating, which affects the energy density of the battery cell, and it is easy to have missed bottom coating during the safety coating application, affecting the safety 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 safety coating and the energy density of the battery cell.

[0043] In some embodiments of the present invention, the inorganic filler includes one or more of alumina, boehmite, aluminum hydroxide, magnesium hydroxide, silicon dioxide, or titanium dioxide. Boehmite is preferably used. Compared with other inorganic fillers, boehmite not only has a lower price, but also has a lower Rockwell hardness and less wear on the gravure roll, thus reducing the manufacturing cost.

[0044] In some embodiments of the present invention, the mass fraction of the inorganic filler in the safety coating is 1%~50%.

[0045] In some embodiments of the present invention, the first binder includes one or more of a polyvinylidene fluoride (PVDF) binder, a sodium carboxymethyl cellulose (CMC) binder, or a polyacrylate (PAA) binder.

[0046] In some preferred embodiments of the present invention, the first binder is a polyacrylate binder, which can reduce the corrosion of the active material to the aluminum foil, has good bonding performance, and is inexpensive. The solvent is deionized water, which is environmentally friendly.

[0047] In some embodiments of the present invention, the structural formula of the polyacrylate binder is:

[0048] ;

[0049] wherein, each occurrence of R1 is independently selected from H, Li or Na;

[0050] each occurrence of R2 is independently selected from substituted or unsubstituted C1-C 10 alkyl;

[0051] and 5 ≤ (x + z) / y ≤ 20, 1.2 ≤ z / x ≤ 2, and x, y, z are all non-zero.

[0052] The polyacrylate binder provided by the present invention realizes good bonding performance by limiting the contents of cyano (-CN), ester group (-COOR2) and carboxylate (-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 humans and environmentally friendly; it is rich in sources and cheap, and can be widely used in industrial production. Introducing the above binder as a raw material for the aqueous safety coating into the electrode sheet, based on its excellent bonding performance, the 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 in the molecular chain, the electrolyte absorption rate of the binder can also 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.

[0053] 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 (-COOR1, each occurrence of R1 is independently selected from H, Li, Na, the proportion of its number in all side chain groups is n2, n2 = x / (x + y + z)) and ester group (-COOR2, each occurrence of R2 is independently selected from substituted or unsubstituted C1-C 10 alkyl, the proportion of its number in all side chain groups is n3, n3 = y / (x + y + z)), and n1 + n2 + n3 = 1.

[0054] Among them, -CN is a strong polar group, which has 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 / extracted from the active material.

[0055] Among them, -COOR1 carries a negative charge and repels each other, which helps the binder molecules to stretch, 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.

[0056] Among them, -COOR2 is close to the polarity and solubility parameter of the carbonate solvent in the electrolyte, which is beneficial to 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.

[0057] 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 of polyacrylic acid cause hydrogen bonds to form 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 necessary 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 group, ester group and carboxylate in the binder molecular chain: (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 bonding force 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 bonding force rapidly decreases, the elasticity becomes poor, 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 bonding force of the binder and the adjustment of the electrolyte absorption rate of the binder by regulating the contents of cyano group, ester group and carboxylate in the binder molecular chain, and improves the rate and low / high-temperature discharge performance of the battery cell.

[0058] 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 of C1~C 10 preferably C1~C 10The linear alkyl group is more preferably an alkyl group having 1 to 4 carbon atoms.

[0059] In some embodiments of the present invention, the electrolyte absorption rate c of the polyacrylate binder at 80 °C is 10% to 50%, for example, it can be 10% to 20%, 20% to 30%, 30% to 40%, 40% to 50%.

[0060] By limiting the contents of cyano groups, ester groups and carboxylate groups in the molecular chain of the binder, the present invention realizes the regulation of the electrolyte absorption rate of the binder, and the electrolyte absorption rate c of the binder at 80 °C satisfies 10% ≤ c ≤ 50%. When c < 10%, the electrolyte absorbed by the binder is insufficient, the elasticity is poor, and it cannot buffer the periodic volume change generated when lithium ions are inserted / extracted from the active material. Moreover, the ionic conductivity of the aqueous safety coating is low, and the low-temperature discharge performance and rate discharge performance of the battery cell deteriorate; when c > 50%, the binder absorbs an excessive amount of electrolyte, the aqueous safety coating swells, the bonding force decreases, and the internal resistance increases, which will also lead to the deterioration of the battery cell performance.

[0061] In some embodiments of the present invention, the weight-average molecular weight M of the polyacrylate binder w ranges from 200,000 to 500,000, for example, it can be 200,000, 250,000, 300,000, 350,000, 400,000, 450,000, 500,000.

[0062] In some embodiments of the present invention, the number-average molecular weight M of the polyacrylate binder n ranges from 100,000 to 400,000, for example, it can be 100,000, 150,000, 200,000, 250,000, 300,000, 350,000, 400,000.

[0063] 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 satisfy the ratio of M w / M n ≤ 3, preferably M w / M n ranges from 1.5 to 1.9.

[0064] 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, the viscosity will be too high and the processing will be difficult. When M w / M n > 3, the molecular weight distribution of the binder is too dispersed, deteriorating the performance of the aqueous safety coating.

[0065] In some embodiments of the present invention, the mass fraction of the polyacrylate binder in the safety coating is 1 wt% to 21.2 wt%, preferably 5 wt% to 15 wt%. When the amount of the binder in the safety coating is less than 1%, the adhesion of the safety coating is insufficient and it is likely to fall off during later use; when the amount of the binder is greater than 21.2%, it will cause difficulties in the processing of the safety coating during gravure printing, and at the same time, it will also deteriorate the internal resistance of the battery cell. In addition, when the amount of the binder is 1% to 21.2%, it can also adjust the pH of the slurry to below 8.5, thereby reducing the corrosion of the current collector aluminum foil by the aqueous solution of the alkaline active material.

[0066] 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.

[0067] In a specific embodiment, the present invention also provides a preparation method of the above polyacrylate binder, including the steps:

[0068] S1. Add a dispersion medium to the reaction vessel and deoxygenate it;

[0069] S2. Add a certain amount of acrylonitrile, monomer A providing -COOR1, and monomer B providing -COOR2 to the reaction vessel according to the ratio of x, y, and z;

[0070] S3. Add an initiator and heat to initiate the reaction;

[0071] S4. After the reaction is completed, the product is filtered, dried, crushed, and sieved to obtain the polyacrylate binder.

[0072] 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 variations within the scope of the present invention.

[0073] 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 can 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 variations within the scope of the present invention.

[0074] In some embodiments of the present invention, in step S1, distilled water is added to the reaction vessel, stirring is started, and high-purity nitrogen is introduced to remove oxygen.

[0075] 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.

[0076] In some embodiments of the present invention, the initiator includes 20% ammonium persulfate, but is not limited thereto.

[0077] In some embodiments of the present invention, the mass fraction of the first conductive agent in the safety coating is 0.5% - 5%. Preferably, it is 1% - 3.5%. If the content of the conductive agent is too low, the resistance of the water-based safety coating film will be too large, and the cycle performance of the battery cell will be poor; if the content is too high, the resistance of the water-based safety coating film 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 cycle performance can be taken into account.

[0078] 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. The present invention has no special requirements for the first conductive agent, 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.

[0079] In some embodiments of the present invention, the positive electrode sheet further includes a positive electrode current collector, the safety coating is coated on at least one surface of the positive electrode current collector, and the positive electrode active material layer is coated on the surface of the safety coating away from the positive electrode current collector and / or the surface of the positive electrode current collector away from the safety coating.

[0080] In some embodiments of the present invention, the negative electrode sheet further includes a negative electrode current collector, and the negative electrode active material layer is coated on at least one surface of the negative electrode current collector.

[0081] The positive electrode sheet provided by the present invention has the safety coating closer to the current collector, and its structure is "current collector - safety coating - active material layer", as Figure 1 shown, wherein, the positive electrode sheet 01 includes the following structure: a current collector 022; a safety coating 023 provided on at least one surface of the above current collector 022; a positive electrode active material layer 011 provided on the surface of the above safety coating 023 away from the current collector 022.

[0082] The internal short circuit of a lithium-ion battery can generally be divided into several types: 1) short circuit between the positive and negative current collectors; 2) short circuit between the positive and negative active materials; 3) short circuit between the negative active material and the positive current collector; 4) short circuit 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, by introducing a safety coating with high adhesion force on the surface of the positive current collector, the probability of contact between the positive current collector and the negative active material is reduced, and the safety performance of the battery cell can be effectively improved.

[0083] In some embodiments of the present invention, the single-sided coating surface density of the safety coating is 3 mg / 1540.25mm 2 ~18 mg / 1540.25mm 2 . For example, it can be 3~6mg / 1540.25mm 2 , 6~9mg / 1540.25mm 2 , 9~12mg / 1540.25mm 2 , 12~15mg / 1540.25mm 2 , 15~18mg / 1540.25mm 2 . When it is lower than 3 mg / 1540.25mm 2 , although it will not affect the NP value of the system, the processing is more difficult, and the safety coating is thinner, which will also have an adverse impact on the mechanical safety performance of the battery cell; when it is higher than 18 mg / 1540.25mm 2 , at this time, the safety performance of the battery cell has no improvement (the passing rate of the battery cell acupuncture test has reached 100% when it is lower than this value), on the contrary, it will have an adverse impact on the energy density of the battery cell. Based on the above considerations, the value range of the single-sided coating surface density of the safety coating in the present invention is 3 mg / 1540.25mm 2 ~18mg / 1540.25mm 2 .

[0084] In some embodiments of the present invention, the safety coating is coated on both surfaces of the current collector. The safety coating satisfies at least one of the following:

[0085] C1) The double-sided thickness of the safety coating is 1~20μm;

[0086] C2) The double-sided film resistance of the safety coating tested under a pressure of 0.4t is 0.5~5Ω;

[0087] C3) The numerical values of the double-sided thickness d of the safety coating and the double-sided film resistance R under a pressure of 0.4t satisfy the condition: 2≤R×d≤50;

[0088] C4) Along the direction perpendicular to the unwinding direction of the positive current collector, the distance W between the left edge of the A-side safety coating of the positive current collector and the left edge of the positive current collector A1 ranges from 2 mm to 30 mm, and the distance W between the right edge of the A-side safety coating of the positive current collector and the right edge of the positive current collector A2 ranges from 2 mm to 30 mm; the distance W between the left edge of the B-side safety coating of the positive current collector and the left edge of the positive current collector B1 ranges from 3 mm to 30 mm, and the distance W between the right edge of the B-side safety coating of the positive current collector and the right edge of the positive current collector B2 ranges from 3 mm to 30 mm; and W B1 -W A1 ≥1 mm, and W B2 -W A2 ≥1 mm.

[0089] In some embodiments of the present invention, the double-sided thickness d of the safety coating (i.e., the sum of the thicknesses of the 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, or 14 to 20 μm.

[0090] In some embodiments of the present invention, the double-sided film resistance R of the safety coating under a pressure of 0.4 t (i.e., the sum of the film resistances of the safety coatings on both sides of the current collector surface) is 0.5 to 5 Ω, for example, it can be 0.5 to 2 Ω, 2 to 3.5 Ω, or 3.5 to 5 Ω. The film resistance of the 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, and when it is too large, the cycle performance of the battery cell will become worse. When this value is between 0.5 and 5 Ω @ 0.4 t, it can balance the safety performance and cycle performance.

[0091] In some embodiments of the present invention, the numerical values of the double-sided thickness d (in μm) of the safety coating and the double-sided film resistance R (in 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 cycle performance of the battery cell. When the relationship 2 ≤ R × d ≤ 50 is satisfied, a better 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 safety coating and the current collector is not less than 100 N / m. If the adhesion force is less than 100 N / m, there is a risk of the safety coating peeling off during subsequent use.

[0093] In some embodiments of the present invention, the raw materials of the positive active material layer include positive active material, a second conductive agent, and a second binder.

[0094] In some embodiments of the present invention, the mass percentage of the positive electrode active material in the positive electrode active material layer is 90% to 98%.

[0095] The positive electrode active material is not limited and can be one or more of common lithium cobalt oxide, lithium iron phosphate, and ternary materials, and the dosage is 90 wt% to 98 wt% of the total weight of the active material layer. Under the condition of ensuring that the active material has a certain electrical conductivity and adhesion, the higher the content of the active material, the better, so as to enable the battery cell to have a higher energy density.

[0096] In some embodiments of the present invention, the mass percentage of the second conductive agent in the positive electrode active material layer is 0.5% to 5%, preferably 0.5% to 2%. If the content of the second conductive agent is too low, the cycle performance of the battery cell is not good; if the content is too high, it is beneficial to improve the cycle performance, but it will lead to poor safety performance of the battery cell.

[0097] 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. The present invention has no special requirements for the second conductive agent, 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 superfine high-conductive carbon black.

[0098] In some embodiments of the present invention, the mass percentage of the second binder in the active material layer is 0.5% to 5%. If the dosage of the second binder is too small, the adhesion of the active material layer is insufficient, and it is easy 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 will also deteriorate the internal resistance of the battery cell.

[0099] 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-based binders. The present invention has no special requirements for the second binder, and conventional binders in the art can be applied.

[0100] The second binder is not limited and can be common cathode binders, 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 bonding 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.

[0101] In some embodiments of the present invention, the positive electrode sheet further includes a positive current collector. The positive current collector is not limited and can be common positive current collectors, such as aluminum foil, composite aluminum foil.

[0102] In some embodiments of the present invention, the dyne value of the positive current collector is not less than 30 dyn / cm. Preferably, the dyne value of the positive current collector is not less than 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 wetting of the current collector by the safety coating slurry, resulting in missed coating of the primer coating. On the other hand, it will reduce the bonding force between the safety coating and the current collector, causing the safety coating to be easily peeled off during subsequent use, which has an adverse impact on the safety of the battery cell. When the dyne value of the current collector is not less than 38 dyn / cm, it can ensure that the primer coating has a good appearance and the bonding between the safety coating and the current collector is good.

[0103] In some embodiments of the present invention, the raw materials of the negative active material layer include negative active material, a third conductive agent, and a third binder.

[0104] In some embodiments of the present invention, the mass percentage of the negative active material in the negative active material layer is 90% - 98%.

[0105] 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 electrode materials such as silicon oxide compounds and silicon - carbon composites. The dosage is 90wt% - 98wt% of the total weight of the negative active material layer.

[0106] In some embodiments of the present invention, the mass percentage of the third conductive agent in the negative active material layer is 0.5% - 5%, preferably 0.5% - 2%. If the content of the third conductive agent is too low, the cycle performance of the battery cell is not good. If the content is too high, it is beneficial to improving the cycle performance, but it will cause poor safety performance of the battery cell.

[0107] In some embodiments of the present invention, the third conductive agent includes at least one of acetylene black, graphene, graphyne, carbon nanotubes, carbon fibers, and conductive carbon black. The present invention has no special requirements for the third conductive agent, 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 Super P Li, a highly conductive carbon black.

[0108] In some embodiments of the present invention, the mass percentage of the third binder in the negative electrode active material layer is 0.5% - 5%. If the amount of the third 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 amount of the third binder is too large, it will cause difficulties in processing the active material layer and also deteriorate the internal resistance of the battery cell.

[0109] In some embodiments of the present invention, the third binder includes at least one of polyvinylidene fluoride (PVDF), sodium carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), and PAA-based binders. The present invention has no special requirements for the third binder, and conventional binders in the art can be used.

[0110] In some embodiments of the present invention, the negative electrode sheet further includes a negative electrode current collector. The negative electrode current collector is not limited and can be a common negative electrode current collector, such as copper foil or composite copper foil.

[0111] In some embodiments of the present invention, the dyne value of the negative electrode current collector is not less than 30 dyn / cm, and preferably, the dyne value of the negative electrode current collector is not less than 38 dyn / cm. When the dyne value of the current collector is less than 30 dyn / cm, on the one hand, it is not conducive to the wetting of the current collector by the safety coating slurry, resulting in missing coating of the primer coating. On the other hand, it will reduce the adhesion between the safety coating and the current collector, causing the safety coating to be easily detached during subsequent use, which has an adverse impact on the safety of the battery cell. When the dyne value of the current collector is not less than 38 dyn / cm, it can ensure a good appearance of the primer coating and good adhesion between the safety coating and the current collector.

[0112] In some embodiments of the present invention, the battery cell further includes a separator disposed between the positive electrode sheet and the negative electrode sheet.

[0113] In the second aspect of the present invention, a method for preparing the above-mentioned battery cell is provided, including the steps:

[0114] S100. Mix the raw materials for preparing the safety coating evenly in a solvent to obtain a safety coating slurry;

[0115] S200. Coat the safety coating slurry on at least one surface of the positive electrode current collector to obtain a positive electrode current collector with a safety coating;

[0116] S300. Coating a positive electrode active material layer on the surface of the safety coating away from the positive electrode current collector or on the surface of the positive electrode current collector away from the safety coating to obtain a positive electrode sheet;

[0117] S400. Coating a negative electrode active material layer on the negative electrode current collector to obtain a negative electrode sheet;

[0118] S500. Assembling the positive electrode sheet and the negative electrode sheet to obtain an electric core.

[0119] According to the method for preparing an electric core of an embodiment of the present invention, it has at least the following beneficial effects: By introducing a safety coating into the positive electrode sheet and limiting the capacity of the active material in the safety coating, without changing the original process, the mechanical safety performance, thermal box performance and cycle performance of the electric core can be taken into account, 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, is compatible with the original preparation process, and has the potential for large-scale application.

[0120] In some embodiments of the present invention, the safety coating includes at least one of the following:

[0121] D1) The solid content of the safety coating slurry ≥ 10%;

[0122] D2) The viscosity of the safety coating slurry ≥ 50 mPa·s;

[0123] D3) The dyne value of the positive electrode current collector ≥ 30 dyn / cm;

[0124] D4) Coating the safety coating slurry on one or both surfaces of the positive electrode current collector by intaglio printing;

[0125] D5) When including D4), the temperature of the oven during intaglio printing is 90 - 110°C, and the printing speed is 10 - 50 m / min.

[0126] In some embodiments of the present invention, the raw materials of the safety coating include a first conductive agent, a first binder, an inorganic filler, a first active material, a dispersant and a co-solvent.

[0127] In some embodiments of the present invention, the mass ratio of the first conductive agent, the first binder, the inorganic filler, the first active material, the dispersant, and the co-solvent is (0.5~5):(1~21.2):(1~50):(1~95):(0.01~1):(1~20). For example, by weight, the first conductive agent can be 0.5~5 parts, preferably 1.5~3 parts, the first binder can be 1~21.2 parts, preferably 5~15 parts, the inorganic filler can be 1~50 parts, the first active material can be 1~95 parts, the dispersant can be 0.01~1 part, and the co-solvent can be 1~20 parts.

[0128] In some embodiments of the present invention, the co-solvent includes one or more of isopropyl alcohol, ethanol, ethylene glycol, acetone, and ethyl acetate. The co-solvent can optimize the slurry properties and can be one or more of isopropyl alcohol (IPA), ethanol, ethylene glycol, acetone, ethyl acetate, etc. The co-solvent can reduce the viscosity of the slurry, improve the wettability of the slurry on the current collector, ensure that the slurry can be evenly coated on the surface of the current collector, form a more uniform and dense safety coating, and thus improve the coating quality. In addition, during the production of the safety coating, the evaporation rate of the co-solvent is usually faster than that of the main solvent, which helps to quickly form a uniform coating and reduce film defects. Preferably, the co-solvent is selected from isopropyl alcohol (IPA). IPA is a commonly used co-solvent with good solubility and volatility, and can effectively improve the properties of the slurry.

[0129] In some embodiments of the present invention, the solid content of the safety coating slurry is ≥10%, preferably 10%~21%.

[0130] In some embodiments of the present invention, the viscosity of the safety coating slurry is ≥50 mPa·s.

[0131] The solid content of the safety coating slurry should not be lower than 10%, and the viscosity should not be lower than 50 mPa·s. Too low solid content will not only reduce the production efficiency, but also make it 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.

[0132] In some embodiments of the present invention, the dyne value of the positive current collector and / or the negative current collector is ≥ 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 wetting of the current collector by the safety coating slurry, resulting in missed coating of the primer coating. On the other hand, it will reduce the adhesion between the safety coating and the current collector, resulting in the easy peeling off of the 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 lower than 38 dyn / cm can ensure a good appearance of the primer coating and good adhesion between the safety coating and the current collector.

[0133] In some embodiments of the present invention, the safety coating slurry is applied to one or both surfaces of the positive current collector by gravure printing.

[0134] 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 lower than 10 m / min, preferably 10 - 50 m / min.

[0135] When performing gravure printing, the temperature of the oven should be 90 - 110°C, and the printing speed is 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 lead to the problem of missed coating of the safety coating, thus having a negative impact on the safety performance of the battery cell.

[0136] In some embodiments of the present invention, the method for preparing the positive electrode sheet includes the steps:

[0137] (10) Mix the first binder and the solvent and then perform sizing to obtain a sizing solution;

[0138] (20) Mix the sizing solution, the first conductive agent, the inorganic filler, the first active material, the dispersant, and the co-solvent uniformly to obtain a safety coating slurry;

[0139] (30) Transfer the safety coating slurry obtained in step (20) to one or both surfaces of the current collector by gravure printing to obtain a positive electrode sheet in which the safety coating area and the non-safety coating area are arranged at intervals along the unwinding direction of the current collector.

[0140] Among them, the non-safe coating area corresponds to the head and / or tail of the positive electrode sheet of the battery cell. Usually, one side of it is active material, the other side is 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, reduce the probability of contact between the positive current collector and the negative active material, and thus improve 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.

[0141] In some embodiments of the present invention, the first binder is a polyacrylate binder.

[0142] In some embodiments of the present invention, the solid content of the glue solution is 10% - 30%, for example, it can be 10% - 20%, 20% - 25%, 25% - 30%.

[0143] In some embodiments of the present invention, the viscosity of the glue 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.

[0144] If the solid content and viscosity of the glue solution made from the binder are too low, it will not only reduce the processing efficiency but also may lead to insufficient adhesion of the safety coating; while if the solid content and viscosity are too high, it may cause difficulties in processing the safety coating.

[0145] In some embodiments of the present invention, the total length of the positive current collector with the safety coating is L, the length of the safety coating area is L1, the length of the non-aqueous safety coating area is L2, and L1 > L2, L1 + L2 = L.

[0146] In some embodiments of the present invention, the safety coating can be coated on both sides (side A and side B) of the positive current collector. Among them, the length of the safety coating area on side A is L A1 , the length of the non-safe coating area is L A2 , and L A1 >L A2 , L A1 +L A2 =L A ; the length of the safety coating area on side B is L B1 , the length of the non-safe coating area is L B2 , and L B1 >LB2 , L B1 +L B2 =L B .

[0147] In some preferred embodiments of the present invention, to improve production efficiency and reduce costs simultaneously, a ceramic layer may be provided only in the non-safe coating area of the B side, while a safe coating is also provided in the non-safe coating area of the A side, that is, a single-sided continuous coating structure is provided, L A2 is 0, that is, L B1 +L B2 =L B =L A1 =L A =L.

[0148] Figure 2 The figure shows a sectional view of the current collector coated with a safe coating along the unwinding direction in one specific embodiment of the present invention. In the figure, 02 - positive current collector with a safe coating; 021 - reserved slot; 022 - current collector; 023 - safe coating; 024 - non-safe coating area. Among them, the safe coating areas and non-safe coating areas of the A side and B side of the positive electrode sheet can be arranged oppositely, as shown in Figure (a); or one side can be a non-safe coating area and the other side can be a safe coating area, as shown in Figure (b). The total length of the current collector containing the safe coating is L, the length of the safe coating area on the B side is L B1 , and the length of the non-safe coating area is L B2 , and L B1 >L B2 , L B1 +L B2 =L B =L A1 =L A =L; or the non-safe coating area can also be provided with a safe coating, L A2 and L B2 are both set to 0, that is, L A1 =L B1 =L A =L B , as shown in Figure (c).

[0149] In some embodiments of the present invention, when applying the safe coating, N1 slots for tab welding need to be reserved in the safe coating area or non-safe coating area along the unwinding direction of the current collector, where N1 ≥ 0.

[0150] In some embodiments of the present invention, when applying the safe coating, N2 slots for tab welding need to be reserved in the safe coating area or non-safe coating area along the direction perpendicular to the unwinding direction of the current collector, where N2 ≥ 0.

[0151] In some embodiments of the present invention, when applying the safety coating, N3 safety coatings can also be applied simultaneously along the unwinding direction of the current collector, where N3 ≥ 1.

[0152] When applying the safety coating in the present invention, a slot for tab welding can be reserved or not reserved as needed, as Figure 3 shown.

[0153] In some embodiments of the present invention, if safety coatings are applied on both sides of the current collector, when reserving the slot for tab welding, the slots on the A / B surfaces need to be kept opposite, and the lateral and longitudinal misalignments shall not exceed 2 mm.

[0154] In some embodiments of the present invention, the safety coating is applied on both side surfaces of the positive current collector. Along the direction perpendicular to the unwinding direction of the positive current collector, the distance W between the left edge of the safety coating on the A surface of the positive current collector and the left edge of the positive current collector A1 ranges from 2 mm to 30 mm, and the distance W between the right edge of the safety coating on the A surface and the right edge of the positive current collector A2 ranges from 2 mm to 30 mm; the distance W between the left edge of the safety coating on the B surface of the positive current collector and the left edge of the positive current collector B1 ranges from 3 mm to 30 mm, and the distance W between the right edge of the safety coating on the B surface and the right edge of the positive 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 applied on both sides of the positive current collector, along the direction perpendicular to the unwinding direction of the positive current collector, the distance between the left edge of the safety coating 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 ≤ 30 mm, 3 mm ≤ W B1 , W B2 ≤ 30 mm, and W B1 -W A1 ≥ 1 mm, W B2 -W A2 ≥ 1 mm. Figure 4The figure shows a cross-sectional view of the safety coating perpendicular to the unwinding direction in a specific embodiment of the present invention. In the figure, 021 is a reserved slot; 022 is a current collector; 023 is a safety coating. This arrangement enables the width of the safety coating on side A to be more than 2 mm wider than that on side B, thereby reducing the bulging of the edge of the primer when it is rolled up.

[0155] In a third aspect of the present invention, a lithium-ion battery is provided, comprising the battery cell as described above or the battery cell obtained by the preparation method as described above.

[0156] Since the lithium-ion battery adopts all the technical solutions of the battery cell of the above embodiment, it has at least all the beneficial effects brought by the technical solutions of the above embodiment. That is, the lithium-ion battery introduces a safety coating in the electrode sheet and limits the capacity of the first active material in the safety coating, so that the mechanical safety performance, hot box 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.

[0157] In some embodiments of the present invention, the battery cell includes: a positive electrode sheet, a negative electrode sheet, and a separator between the positive electrode sheet and the negative electrode sheet.

[0158] In some embodiments of the present invention, the positive electrode sheet is an electrode sheet as described above. The positive electrode sheet includes the positive electrode current collector containing the 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 common positive electrode current collector in the art, such as aluminum foil, but not limited thereto. The positive electrode active material layer includes a positive electrode active material, which is a common positive electrode active material currently used in lithium-ion batteries, including but not limited to a chemical formula such as Li x Ni h Co y M z O 2-d N d (wherein 0.95≤x≤1.2, h>0, y≥0, z≥0, and h+y+z=1, 0≤d≤1, M is selected from a combination of one or more of Mn and Al, and N is selected from a combination of one or more of F, P, and S) The positive electrode active material may also be, but is not limited to, LiCoO2, LiNiO2, LiVO2, LiCrO2, LiMn2O4, LiCoMnO4, Li2NiMn3O8, LiNi 0.5 Mn 1.5One or a combination of more than one of O4, LiCoPO4, LiMnPO4, LiFePO4, LiNiPO4, LiCoFSO4, CuS2, FeS2, MoS2, NiS, TiS2, etc. The positive electrode active material can also be subjected to a modification treatment, and the method for modifying the positive electrode active material should be known to those skilled in the art. For example, methods such as coating and doping can be used to modify the positive electrode active material, and the materials used for the modification treatment can be a combination of one or more of, including but not limited to, Al, B, P, Zr, Si, Ti, Ge, Sn, Mg, Ce, W, etc.

[0159] In some embodiments of the present invention, the negative electrode sheet is the electrode sheet as described above. The negative electrode sheet includes the above-mentioned negative electrode current collector containing a safety coating 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 several 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 several 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. Among them, the graphite can be selected from one or several of artificial graphite, natural graphite, and modified graphite; the tin-based materials can be selected from one or several of elemental tin, tin oxide compounds, and tin alloys.

[0160] 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, including but not limited to, polyethylene, polypropylene, polyvinylidene fluoride, aramid, polyethylene terephthalate, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, polyester, and natural fibers. 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 perform the basic function of the separator.

[0161] In some embodiments of the present invention, the lithium-ion battery further includes an electrolyte, which infiltrates the positive electrode, the negative electrode, and the 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-proof 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 additive includes at least one of, but not limited to, a film-forming additive, a conductive additive, a flame retardant additive, an overcharge-proof 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.

[0162] The present invention also provides a secondary battery, including the battery cell as described above or the battery cell obtained by the preparation method as described above. The battery cell provided 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 here.

[0163] In the fourth 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. BRIEF DESCRIPTION OF THE DRAWINGS

[0164] The present invention will be further described below with reference to the drawings and embodiments, where:

[0165] Figure 1 FIG. is a schematic structural diagram of a positive electrode sheet including a safety coating provided by the present invention, where 01 - positive electrode sheet; 011 - positive electrode active material layer; 022 - current collector; 023 - safety coating;

[0166] Figure 2 FIG. is a schematic cross-sectional structural diagram of a current collector including a 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 - safety coating; 024 - non-safety coating area;

[0167] Figure 3 FIG. is a top view of a current collector including a safety coating (in an unslit state) provided by the present invention;

[0168] Figure 4Schematic cross-sectional structure diagram of the current collector with a safety coating provided by the present invention along the unwinding direction, where 021 - reserved slot; 022 - current collector; 023 - safety coating. Detailed implementation manners

[0169] The following will clearly and completely describe the concept of the present invention and the technical effects generated in combination 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 embodiments. Based on the embodiments of the present invention, other embodiments obtained by those skilled in the art without creative efforts fall within the scope of protection of the present invention.

[0170] In the description of the present invention, the description referring 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 descriptions 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.

[0171] In the description of the present invention, unless otherwise specified, the numerical range "a~b" represents the abbreviated representation of any real number combination between a and b, where a and b are both 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.

[0172] For those not specifying specific techniques or conditions in the following embodiments, they are carried out according to the techniques or conditions described in the literature in this field or according to the product specifications. All reagents or instruments not indicating the manufacturer can be obtained as conventional products through commercial purchase.

[0173] Example 1

[0174] This example provides a lithium-ion battery, whose positive electrode sheet contains a safety coating, and the safety coating contains the first active material lithium iron phosphate (LFP), the first binder, the first conductive agent Super P Li, and the inorganic filler boehmite;

[0175] Among them, the structural formula of the first binder is:

[0176]

[0177] (1) Preparation of the first binder:

[0178] a) Add 1000 parts by weight of distilled water to the reaction vessel, start stirring, purge with high-purity nitrogen for 1 hour to remove oxygen, 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 the temperature constant under an inert atmosphere;

[0179] b) Then add 5 parts by weight of a 20% ammonium persulfate solution as an initiator to initiate the reaction for 5 hours;

[0180] 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%, the weight-average molecular weight M w is 3.5×10 5 , the number-average molecular weight M w is 1.8×10 5 , M w / M n = 1.94, and the electrolyte absorption rate c = 23%.

[0181] (2) Preparation of the positive current collector with a safety coating:

[0182] Using deionized water as a solvent, add 10 parts of the above binder to make glue, adjust the amount of deionized water until a glue solution with a solid content of 15% and a viscosity of 2.0×10 4 mPa·s is obtained; then mix 58 parts of the first active material lithium iron phosphate (median particle size D v50 is 0.9 μm, specific surface area is 10.9 m 2 / g), 30 parts of the inorganic filler boehmite (median particle size D v50 is 0.4 μm, specific surface area is 9.5 m 2 / g), 1.8 parts of the first conductive agent Super P Li, 0.2 part of the dispersant polyvinylpyrrolidone (PVP), and 10% of the total weight of the slurry of the co-solvent isopropyl alcohol (IPA) evenly to obtain a safety coating slurry with a solid content of 28% and a viscosity of 50 - 400 mPa·s. Coat the safety coating slurry on both sides of the aluminum foil by gravure printing. When coating, set L A2 to 0, that is, L A1 = L A = L, N1 = N3 = 1, N2 = 4. The single-sided coating surface density c w of the safety coating is 9 mg / 1540.25 mm 2 . 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 double-sided coating surface density of 18 mg / 1540.25 mm 2, with a double-sided thickness of 10 μm, a double-sided film resistance of 1.1 Ω @0.4 T, and a base coating with an adhesion force of 225 N / m between the safety coating and the current collector. The capacity C1 exerted by lithium iron phosphate in each square meter of the safety coating = k × c w × a, where k is 0.306, a is 170, and c w is 9, so C1 is 468 mAh.

[0183] (3) Preparation of the positive electrode sheet:

[0184] Mix the positive electrode active material lithium cobaltate, the second conductive agent (a mixture of conductive carbon black and carbon nanotubes with a mass ratio of 6:5), PVDF binder, and NMP in a mass ratio of 97.6:1.1:1.3:35 to make a positive electrode slurry with a solid content of 75% and a viscosity of 7000 mPa·s. Coat the positive electrode slurry on one side of the current collector aluminum foil containing the safety coating above, with a coating areal density of 214 mg / 1540.25 mm 2 , dry and wind it up at 85 °C, then coat and dry the positive electrode slurry on the other side of the current collector aluminum foil containing the safety coating according to the above method, and then cold-press the positive electrode sheet with the positive electrode active material layer coated on both sides; then perform edge trimming and slitting to make a lithium-ion battery positive electrode sheet, and the capacity C2 exerted by lithium cobaltate in each square meter of the positive electrode sheet is 24409 mAh.

[0185] (4) Preparation of the negative electrode sheet:

[0186] Using water as a solvent, mix graphite, thickening agent, and SBR binder in a mass ratio of 97.7:1.1:1.2 to make a lithium-ion battery negative electrode slurry with a solid content of 50% and a viscosity of 5000 mPa·s, coat it on one side surface of the current collector copper foil, with a coating areal density of 115 mg / 1540.25 mm 2 , dry and wind it up at 80 °C, and then coat and dry the negative electrode slurry 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, and the capacity C0 exerted by graphite in each square meter of the negative electrode sheet is 25750 mAh.

[0187] (5) Preparation of the electrolyte:

[0188] Dissolve lithium hexafluorophosphate (LiPF6) 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.

[0189] (6) Preparation of the battery:

[0190] The prepared positive electrode sheet, negative electrode sheet and separator 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 of an aluminum tab, and the negative electrode is led out by spot welding of a nickel tab. Then, the electric core is placed in an aluminum-plastic packaging bag, baked, and the above-mentioned electrolyte is injected. After processes such as encapsulation, formation, and grading, a lithium-ion battery is finally manufactured.

[0191] Example 2

[0192] The difference from Example 1 is that when preparing the positive electrode sheet, the coating areal density of the positive electrode paste is 218 mg / 1540.25 mm 2 , and the capacity C2 exerted by lithium cobaltate in each square meter of the positive electrode sheet is 24865 mAh.

[0193] The rest is the same as in Example 1 and will not be elaborated here.

[0194] Example 3

[0195] The difference from Example 1 is that when preparing the positive electrode sheet, the coating areal density of the positive electrode paste is 208.5 mg / 1540.25 mm 2 , and the capacity C2 exerted by lithium cobaltate in each square meter of the positive electrode sheet is 23781 mAh.

[0196] The rest is the same as in Example 1 and will not be elaborated here.

[0197] Example 4

[0198] The difference from Example 1 is that when preparing the positive electrode sheet, the single-sided coating areal density c of the safety coating w is 18 mg / 1540.25 mm 2 , and the capacity C1 exerted by lithium iron phosphate in each square meter of the safety coating is 936 mAh; the coating areal density of the positive electrode paste is 200 mg / 1540.25 mm 2 , and the capacity C2 exerted by lithium cobaltate in each square meter of the positive electrode sheet is 22812 mAh.

[0199] The rest is the same as in Example 1 and will not be elaborated here.

[0200] Example 5

[0201] The difference from Example 1 is that when preparing the positive electrode sheet, the single-sided coating areal density c of the safety coating w is 3 mg / 1540.25 mm 2 , and the capacity C1 exerted by lithium iron phosphate in each square meter of the safety coating is 156 mAh.

[0202] The rest is the same as in Example 1 and will not be elaborated here.

[0203] Example 6

[0204] The difference from Example 1 is that when preparing the positive electrode sheet, the single-sided coating surface density c of the safety coating w is 6 mg / 1540.25 mm 2 , and the capacity C1 exerted by lithium iron phosphate in the safety coating per square meter is 312 mAh.

[0205] The rest is the same as in Example 1 and will not be elaborated here.

[0206] Example 7

[0207] The difference from Example 1 is that when preparing the positive electrode sheet, the single-sided coating surface density c of the safety coating w is 12 mg / 1540.25 mm 2 , and the capacity C1 exerted by lithium iron phosphate in the safety coating per square meter is 624 mAh.

[0208] The rest is the same as in Example 1 and will not be elaborated here.

[0209] Example 8

[0210] The difference from Example 1 is that when preparing the positive electrode sheet, the single-sided coating surface density c of the safety coating w is 18 mg / 1540.25 mm 2 , and the capacity C1 exerted by lithium iron phosphate in the safety coating per square meter is 936 mAh.

[0211] The rest is the same as in Example 1 and will not be elaborated here.

[0212] Example 9

[0213] The difference from Example 1 is that the first active material is changed from lithium iron phosphate (LFP) to lithium manganese iron phosphate (LMFP), and the rest remains unchanged. The capacity C1 exerted by lithium manganese iron phosphate in the safety coating per square meter is C1 = k × c w × a, where k is 0.306, a is 170, and c w is 9 mg / 1540.25 mm 2 , then C1 is 468 mAh.

[0214] The rest is the same as in Example 1 and will not be elaborated here.

[0215] Example 10

[0216] The difference from Example 1 is that the first active material is changed from lithium iron phosphate (LFP) to lithium cobalt oxide (LCO), and the rest remains unchanged. The capacity C1 exerted by lithium cobalt oxide in the safety coating per square meter is C1 = k × c w×a, where k is 0.399, a is 274, and c w is 9 mg / 1540.25 mm 2 , then C1 is 984 mAh.

[0217] The rest is the same as in Example 1 and will not be elaborated here.

[0218] Example 11

[0219] The difference from Example 1 is that in the first binder, the proportion of -CN is n1 = 55%, the proportion of -COOLi is n2 = 30%, and the proportion of -COOCH3 is n3 = 15%. The weight-average molecular weight M w is 3.6×10 5 , and the number-average molecular weight M w is 2.1×10 5 , M w / M n = 1.7, and the electrolyte absorption rate c = 22%.

[0220] The rest is the same as in Example 1 and will not be elaborated here.

[0221] Example 12

[0222] The difference from Example 1 is that in the first binder, the proportion of -CN is n1 = 53%, the proportion of -COOLi is n2 = 37%, and the proportion of -COOCH3 is n3 = 10%. The weight-average molecular weight M w is 3.4×10 5 , and the number-average molecular weight M w is 2.0×10 5 , M w / M n = 1.7, and the electrolyte absorption rate c = 17%.

[0223] The rest is the same as in Example 1 and will not be elaborated here.

[0224] Example 13

[0225] The difference from Example 1 is that the median particle size D of the first active material, lithium iron phosphate (LFP), v50 is 2 μm.

[0226] The rest is the same as in Example 1 and will not be elaborated here.

[0227] Example 14

[0228] The difference from Example 1 is that the median particle size D of the first active material, lithium iron phosphate (LFP), v50 is 15 μm.

[0229] The rest is the same as in Example 1 and will not be elaborated here.

[0230] Example 15

[0231] The difference from Example 1 is that: the first binder is replaced with PVDF binder, and the solvent is changed from deionized water to NMP (N-methylpyrrolidone).

[0232] The rest is the same as Example 1 and will not be elaborated here.

[0233] Comparative Example 1

[0234] The difference from Example 1 is that: when preparing the positive electrode sheet, the single-sided coating surface density c of the safety coating w is 30 mg / 1540.25 mm 2 , and the capacity C1 exerted by lithium iron phosphate in the safety coating per square meter is 1561 mAh; the coating surface density of the positive electrode slurry is 200mg / 1540.25 mm 2 , and the capacity C2 exerted by lithium cobaltate in the positive electrode sheet per square meter is 22812 mAh.

[0235] The rest is the same as Example 1 and will not be elaborated here.

[0236] Comparative Example 2

[0237] The difference from Example 1 is that: when preparing the positive electrode sheet, the single-sided coating surface density c of the safety coating w is 2.5mg / 1540.25 mm 2 , and the capacity C1 exerted by lithium iron phosphate in the safety coating per square meter is 130 mAh.

[0238] The rest is the same as Example 1 and will not be elaborated here.

[0239] Comparative Example 3

[0240] The difference from Example 1 is that: when preparing the positive electrode sheet, the single-sided coating surface density c of the safety coating w is 18 mg / 1540.25 mm 2 , and the capacity C1 exerted by lithium iron phosphate in the safety coating per square meter is 936 mAh; the coating surface density of the positive electrode slurry is 218mg / 1540.25 mm 2 , and the capacity C2 exerted by lithium cobaltate in the positive electrode sheet per square meter is 24865 mAh.

[0241] The rest is the same as Example 1 and will not be elaborated here.

[0242] Comparative Example 4

[0243] The difference from Example 1 is that when preparing the positive electrode sheet, the surface density of the coating of the positive electrode slurry is 202.5 mg / 1540.25 mm 2 , and the capacity C2 exerted by lithium cobaltate in each square meter of the positive electrode sheet is 23097 mAh.

[0244] The rest is the same as in Example 1 and will not be elaborated here.

[0245] Comparative Example 5

[0246] The difference from Example 1 is that when preparing the positive electrode sheet, the safety coating does not contain the first active substance.

[0247] The rest is the same as in Example 1 and will not be elaborated here.

[0248] Comparative Example 6

[0249] The difference from Example 1 is that when preparing the positive electrode sheet, the safety coating does not contain a dispersant.

[0250] The rest is the same as in Example 1 and will not be elaborated here.

[0251] Comparative Example 7

[0252] The difference from Example 1 is that the surface of the positive electrode current collector is not coated with a safety coating.

[0253] The rest is the same as in Example 1 and will not be elaborated here.

[0254] Test Example

[0255] To verify the influence of the introduction of the safety coating of the present invention on the performance of the battery cell, the electrolyte absorption rate c of the first binder, as well as the needle puncture, 132°C / 136°C hot box, low-temperature discharge performance, rate performance, and cycle performance of the battery cell were tested.

[0256] Test method for electrolyte absorption rate c: Pour the binder solution into a polytetrafluoroethylene dish, bake to volatilize the deionized water to obtain a binder polymer film, control the thickness of the film at 100 μm, and cut it into a film piece with a length of 50 mm × a width of 50 mm. Then, weigh the film piece after vacuum drying for 24 h, and then immerse the completely dry film piece in the electrolyte at 80°C for 12 h and take it out. After wiping off the electrolyte attached to the surface of the film, weigh the weight of the film piece after absorbing the electrolyte. The weight increase rate (%) of the film piece before and after being immersed in the electrolyte is the electrolyte absorption rate of the binder at 80 o C.

[0257] Needle penetration test method: At room temperature, charge at a constant current and constant voltage of 1.0 C until 4.45 V, with a cut-off rate of 0.05 C. Then, conduct a needle penetration test on the fully charged battery cell. During the test, the deep pit surface of the battery cell faces upward. Use a steel nail with a diameter of 4.0 mm and pierce the battery cell completely at a speed of 40 mm / s at the left, middle, and right positions of the largest surface of the battery cell (test 5 battery cells at each position). Keep it for 1 h. If the battery cell does not catch fire or explode, it passes the test.

[0258] 132℃ / 136℃ hot box test method: At room temperature, discharge at a constant current of 0.2 C until 3 V and set aside 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 0.05 C. Then, conduct a hot box test on the fully charged battery cell. During the test, suspend the battery cell in the oven. The oven temperature rises to 132℃ or 136℃ at a speed of 5±2 ℃ / min and stops after maintaining for 60 min. During the test process, it is necessary to monitor the surface temperature, ambient temperature, and voltage of the battery cell. If the battery cell does not catch fire or explode, it passes the test.

[0259] Low-temperature discharge performance test method: Discharge at a constant current of 1 C until 3 V and set aside for 5 min; set the temperature chamber to 25℃ and set aside for 60 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; set aside 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 C0; set the temperature chamber to 25℃ and set aside for 60 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; set aside for 5 min, set the temperature chamber to -10℃ and set aside for 120 min. Then, discharge the battery cell at 0.2 C until 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℃.

[0260] Rate discharge performance test method: Discharge at a constant current of 1 C until 3 V and set aside 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, and set aside 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, and set aside 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.

[0261] Cyclic performance test method: Under the condition that the ambient temperature is 25±2°C, the battery cell is discharged at a constant current of 0.2 C to 3.0 V, and then charged at a constant current and constant voltage of 3 C to 4.45 V, with a cut-off rate of 0.05 C. Record the voltage, internal resistance, capacity, and thickness (600 g PPG is used for thickness measurement) of the battery cell during the first full charge. The cycling process follows the following HFC format: Discharge at a constant current of 0.2 C to 3 V; Charge at a constant current of 3.0 C to 4.25 V; Charge at a constant current of 2.5 C to 4.25 V; Charge at a constant current of 2.0 C to 4.45 V; Charge at a constant current and constant voltage of 1.4 C to 4.50 V, with a cut-off rate of 0.3 C; Charge at a constant current and constant voltage of 2 A to 4.45 V, with a cut-off rate of 0.05 C; Discharge at a constant current of 1.0 C to 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 to 4.25 V; Charge at a constant current of 2.5 C to 4.25 V; Charge at a constant current of 2.0 C to 4.45 V; Charge at a constant current and constant voltage of 1.4 C to 4.50 V, with a cut-off rate of 0.3 C; Charge at a constant current and constant voltage of 2 A to 4.45 V, with a cut-off rate of 0.05 C; Discharge at a constant current of 0.2 C to 3 V; Charge at a constant current and constant voltage of 3.0 C to 4.45 V, with a cut-off rate of 0.05 C. Record the voltage, internal resistance, and thickness (600 g PPG is used for thickness measurement) of the fully charged battery cell every 100 weeks.

[0262] The parameter settings of Examples 1-15 and Comparative Examples 1-7 are shown in Table 1 below:

[0263] Table 1

[0264]

[0265] The test results of Examples 1-15 and Comparative Examples 1-7 are shown in Table 2 below:

[0266] Table 2

[0267]

[0268] From the comparison of the parameter settings in Tables 1-2 above and the test results of the battery cells, it can be seen that compared with Comparative Examples 1-7, the safety coatings in Examples 1-15 satisfy 0.002≤ ≤0.04, and 1.019≤NP1≤1.089. The needle-punching performance of the corresponding battery cells is significantly improved. The passing rate of the needle-punching test has increased from 0% to over 80%, the hot box has increased by 4°C. At the same time, the low-temperature discharge performance, rate discharge performance, and cyclic performance have also been improved. The capacity retention rate after 800 cycles at 25°C is not less than 90%, the discharge capacity at -10°C is not less than 70% @3.4 V, and the discharge capacity at 2 C is not less than 70% @3.4 V.

[0269] As can be seen from Comparative Example 1, although the performance of the battery cell did not deteriorate significantly compared to Example 1, its was 0.067, c w was 30 mg / 1540.25 mm 2 , which would lead to a decrease in its energy density and poor practicability.

[0270] As can be seen from Comparative Example 2, was only 0.001, and the corresponding c w was 2.5 mg / 1540.25 mm 2 , which required high precision for the process during coating, and the needle-punching performance deteriorated.

[0271] As can be seen from Comparative Example 3, NP1 was only 1.003, and the negative electrode capacity margin was insufficient, and lithium deposition was likely to occur during the cycling process, resulting in the failure of the battery cell due to cycling drop.

[0272] As can be seen from Comparative Example 4, NP1 was 1.098, and the potential was relatively high during full charge. It was found that both its hot box and cycling performance deteriorated.

[0273] Comparative Examples 5-7 show that when the safety coating does not contain the first active substance or dispersant, or the safety coating is not applied, the performance of the battery cell will deteriorate.

[0274] In summary, the safety coating provided by the present invention contains the first active substance. By limiting the coating surface density and capacity performance of the first active substance, the mechanical safety performance, thermal abuse performance and cycling performance of the battery cell are improved, and at the same time, the low-temperature discharge performance and rate discharge performance of the battery are improved. For the battery cell provided by the present invention, when the safety coating satisfies 0.002 ≤ ≤ 0.04, and 1.019 ≤ NP1 ≤ 1.089, the needle-punching passing rate can be greater than 80%, the temperature of the hot box is increased by 4 °C, and at the same time, the capacity retention rate after 800 cycles at 25 °C is not less than 90%, the discharge capacity at -10 °C is not less than 70% @3.4V, and the discharge capacity at 2 C is not less than 70% @3.4 V.

[0275] The above-described embodiments only represent several implementation manners of the present invention. The description is relatively specific and detailed, but it should not be construed as a limitation on 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 deformations and improvements can be made, and these all belong to the protection scope of the present invention.

Claims

1. A battery cell, characterized in that, Comprising: Positive electrode sheet, negative electrode sheet; the positive electrode sheet includes a safety coating and a positive electrode active material layer, the safety coating includes a first active material and a dispersant, and the median particle size D of the first active material v50 ranges from 0.1 to 15 μm, and the specific surface area is 2 to 50 m 2 / g; The safety coating further includes a first binder, the first binder being a polyacrylate binder, and the structural formula of the polyacrylate binder is: ； 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 positive electrode active material layer includes a positive electrode active material; The negative electrode sheet includes a negative electrode active material layer, and the negative electrode active material layer includes a negative electrode active material; The first capacity ratio NP1 of the battery cell satisfies 1.019 ≤ NP1 ≤ 1.089, and the relative capacity ratio satisfies 0.002 ≤ ≤ 0.04; Among them, the calculation formula for the first capacity ratio NP1 is: , the relative capacity ratio 's calculation formula is: ; C1 is the value of the mAh capacity exerted by the first active substance in the safety coating per unit area per square meter. The calculation formula for C1 is: C1 = k × c w × a, where 0 < k < 1, 3 ≤ c w ≤ 18, and c w is the value of the mg weight of the single-sided coating of the safety coating for every 1540.25 mm 2 0 < a ≤ 300, a is the value of the mAh theoretical gram capacity per gram of the first active substance, and k is a constant; C2 is the value of the mAh capacity exerted by the positive electrode active material in the positive electrode active material layer per unit area per square meter; C0 is the value of the mAh capacity exerted by the negative electrode active material in the negative electrode active material layer per unit area per square meter.

2. The battery cell according to claim 1, wherein, The first active material includes one or more of lithium cobaltate, lithium manganate, lithium iron phosphate, lithium manganese iron phosphate, and ternary materials.

3. The battery cell according to claim 2, wherein Including at least one of the following: A1) The first active material is a ternary material, and the ternary material includes at least one of nickel cobalt manganese ternary materials and nickel cobalt aluminum ternary materials; A2) When including A1), the nickel cobalt manganese ternary material includes at least one of NCM333, NCM523, NCM613, NCM622, and NCM811; A3) The surface of the first active material is coated with an inorganic ceramic material; A4) When including A3), the inorganic ceramic material includes silicon dioxide, aluminum oxide, or titanium oxide.

4. The cell according to claim 1, characterized in that, The dispersant includes one or more of an ionic dispersant and a non-ionic dispersant.

5. The battery cell according to claim 4, characterized in that, The ionic dispersant includes one or more of polyacrylic acid and its salts, sodium dodecylbenzenesulfonate, sodium dodecyl sulfate, and polyethyleneimine; And / or, the non-ionic dispersant includes one or more of polyvinylpyrrolidone, polyethylene glycol, polyvinyl alcohol, polyoxyethylene, and carboxymethyl cellulose.

6. The cell according to claim 1, wherein The safety coating further includes a first conductive agent and an inorganic filler; the safety coating satisfies at least one of the following: B1) The mass ratio of the first conductive agent, the first binder, the inorganic filler, the first active material, and the dispersant is (0.5~5):(1~21.2):(1~50):(1~95):(0.01~1); B2) The median particle size D of the inorganic filler v50 ranges from 0.1 to 5 μm, and the specific surface area is 8 to 15 m 2 / g; B3) The inorganic filler includes one or more of aluminum oxide, boehmite, aluminum hydroxide, magnesium hydroxide, silicon dioxide, or titanium dioxide.

7. The battery cell according to claim 1, characterized in that, 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; and / or, the weight-average molecular weight M of the polyacrylate binder w and the number-average molecular weight M n with a ratio M w / M n ≤ 3; And / or, the electrolyte absorption rate of the polyacrylate binder at 80°C is 10%~50%; And / or, the mass fraction of the polyacrylate binder in the safety coating is 1%~21.2%.

8. The battery cell according to claim 1, wherein The positive electrode sheet further includes a positive electrode current collector, the safety coating is coated on at least one side surface of the positive electrode current collector, and the positive electrode active material layer is coated on the side surface of the safety coating away from the positive electrode current collector or the side surface of the positive electrode current collector away from the safety coating; the negative electrode sheet further includes a negative electrode current collector, and the negative electrode active material layer is coated on at least one side surface of the negative electrode current collector; And / or, the single-sided coating surface density of the safety coating is 3 mg / 1540.25 mm 2 ~18 mg / 1540.25 mm 2 .

9. The battery cell according to claim 8, characterized in that, The safety coating is coated on both side surfaces of the positive electrode current collector; the safety coating satisfies at least one of the following: C1) The double-sided thickness of the safety coating is 1 to 20 μm; C2) The double-sided film resistance of the safety coating tested under a pressure of 0.4 t is 0.5 to 5 Ω; C3) The values of the double-sided thickness d of the safety coating and the double-sided film resistance R under a pressure of 0.4 t satisfy the condition: 2 ≤ R×d ≤ 50; C4) Along the direction perpendicular to the unwinding direction of the positive current collector, the distance W between the left edge of the safety coating on the A side of the positive current collector and the left edge of the positive current collector A1 ranges from 2 mm to 30 mm, and the distance W between the right edge of the safety coating on the A side of the positive current collector and the right edge of the positive current collector A2 ranges from 2 mm to 30 mm; the distance W between the left edge of the safety coating on the B side of the positive current collector and the left edge of the positive current collector B1 ranges from 3 mm to 30 mm, and the distance W between the right edge of the safety coating on the B side of the positive current collector and the right edge of the positive current collector B2 ranges from 3 mm to 30 mm; and W B1 -W A1 ≥ 1 mm, W B2 -W A2 ≥ 1 mm.

10. A method for preparing an electric core according to any one of claims 1-9, characterized in that, It includes the steps: Mix the raw materials for preparing the safety coating evenly in a solvent to obtain a safety coating slurry; Coat the safety coating slurry on at least one side surface of the positive current collector to obtain a positive current collector with a safety coating; Coat a positive active material layer on the side surface of the safety coating away from the positive current collector or on the side surface of the positive current collector away from the safety coating to obtain a positive electrode sheet; Coat a negative active material layer on the negative current collector to obtain a negative electrode sheet; Assemble the positive electrode sheet and the negative electrode sheet to obtain an electric core.

11. The method for preparing an electric core according to claim 10, wherein It includes at least one of the following: D1) The solid content of the safety coating slurry is ≥ 10%; D2) The viscosity of the safety coating slurry is ≥ 50 mPa·s; D3) The dyne value of the positive current collector is ≥ 30 dyn / cm; D4) Coat the safety coating slurry on one or both side surfaces of the positive current collector by gravure printing; D5) When including D4), the temperature of the oven during gravure printing is 90 to 110 °C, and the printing speed is 10 to 50 m / min.

12. A lithium-ion battery, characterized in that, It includes the electric core as described in any one of claims 1-9 or the electric core obtained by the preparation method as described in any one of claims 10-11.

13. An application of the lithium-ion battery as described in claim 12 in an electrical device.

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