Battery cell and charging method therefor, battery device, and electrical device

Abstract

The present application relates to a battery cell and a charging method therefor, a battery device, and an electrical device. The battery cell comprises an electrolyte solution and an electrode assembly. The electrode assembly comprises a positive electrode sheet, a negative electrode sheet, and a separator. The positive electrode sheet comprises a positive electrode current collector and a positive electrode film layer containing a positive electrode active material, wherein the positive electrode active material comprises a lithium-containing phosphate salt having an olivine structure. The negative electrode sheet comprises a negative electrode current collector and a negative electrode film layer. The charging process of the battery cell from a 0% state of charge to a 100% state of charge comprises N charging steps, wherein the difference between the maximum state of charge in any one charging step among the N charging steps and the maximum state of charge in an adjacent charging step thereof is less than or equal to a 5% state of charge, and N is a positive integer greater than or equal to 2. In the N charging steps, a cut-off voltage in any one charging step among N-1 charging steps is less than a cut-off voltage in an N-th charging step, and the cut-off voltage of the N-th charging step does not exceed 4.4 V. The present application can improve the use reliability of the battery cell.

Description

Battery cells and their charging methods, battery devices and electrical devices Technical Field

[0001] This application relates to a battery cell and its charging method, a battery device, and an electrical device. Background Technology

[0002] Battery cells possess characteristics such as high capacity and long lifespan, making them widely used in electronic devices such as mobile phones, laptops, electric vehicles, electric cars, electric airplanes, electric ships, electric toy cars, electric toy ships, electric toy airplanes, and power tools. With the development of the battery industry, higher requirements are being placed on the reliability of battery cells.

[0003] Summary of the Invention

[0004] This application provides a battery cell and its charging method, battery device and power supply device, which can reduce the risk of lithium plating in the battery cell during fast charging and improve the reliability of the battery cell.

[0005] In a first aspect, this application proposes a battery cell, including an electrolyte and an electrode assembly. The electrode assembly includes a positive electrode, a negative electrode, and a separator between the positive and negative electrode. The positive electrode includes a positive current collector and a positive electrode film layer disposed on at least one side of the positive current collector and containing a positive active material. The positive active material includes a lithium phosphate with an olivine structure. The negative electrode includes a negative current collector and a negative electrode film layer disposed on at least one side of the negative current collector and containing a negative active material. The negative electrode film layer includes a carbon-based material. The battery cell is in a 0% state of charge. The charging process to 100% state of charge includes N charging steps. The difference between the maximum state of charge in any of the N charging steps and the maximum state of charge in its adjacent charging step is less than or equal to 5% of the state of charge. N is a positive integer greater than or equal to 2. In the N charging steps, the cutoff voltage of any of the N-1 charging steps is less than the cutoff voltage of the Nth charging step. The cutoff voltage of the Nth charging step does not exceed 4.4V. At room temperature, the charging time for a single battery cell from 10% state of charge to 80% state of charge is 5 min to 10.5 min.

[0006] Therefore, in the embodiments of this application, during the fast charging process, the difference in the maximum state of charge of a single battery cell in adjacent charging steps is less than or equal to 5%, resulting in less polarization during the charging process. Before the Nth charging step, that is, the cutoff voltage of any charging step from the 1st to the N-1th charging step is relatively small, so that the voltage will not rise during the charging process. This allows the lithium ions extracted from the positive electrode film to be basically embedded in the negative electrode film, making it less likely for lithium to be deposited on the surface of the negative electrode, thereby improving the reliability of the single battery cell.

[0007] In some implementations, the cutoff voltage of the Nth charging step is greater than the cutoff voltage of any of the N-1 charging steps, and the difference between the cutoff voltage of the Nth charging step and the cutoff voltage of any of the N-1 charging steps is greater than or equal to 0.02V.

[0008] Therefore, when the battery cell in the embodiments of this application meets the above conditions, lithium plating on the surface of the negative electrode sheet is less likely to occur, thereby improving the reliability of the battery cell.

[0009] In some implementations, the cutoff voltage of the Nth charging step is greater than the cutoff voltage of any of the N-1 charging steps, and the difference between the cutoff voltage of the Nth charging step and the cutoff voltage of any of the N-1 charging steps is greater than or equal to 0.05V.

[0010] Therefore, when the battery cell in the embodiments of this application meets the above conditions, lithium plating on the surface of the negative electrode sheet is less likely to occur, thereby improving the reliability of the battery cell.

[0011] In some implementations, the cutoff voltage of the Nth charging step is greater than the cutoff voltage of any of the N-1 charging steps, and the difference between the cutoff voltage of the Nth charging step and the cutoff voltage of any of the N-1 charging steps is 0.05V to 0.2V.

[0012] Therefore, when the battery cell in the embodiments of this application meets the above conditions, lithium plating on the surface of the negative electrode sheet is less likely to occur, thereby improving the reliability of the battery cell.

[0013] In some implementations, the cutoff voltage for the Nth charging step is 3.65V to 4.4V.

[0014] Therefore, when the cutoff voltage of the Nth charging step is within the above range, the risk of lithium plating in the battery cell is relatively small, and the energy density of the battery cell can be improved.

[0015] In some embodiments, the charging rate of the Nth charging step is 0.05C to 0.30C, and optionally 0.10C to 0.30C.

[0016] Therefore, the risk of lithium plating on the negative electrode increases towards the end of the charging process. Charging the Nth charging step at the above-mentioned rate can further reduce the risk of lithium plating on the negative electrode and improve the reliability of the battery cell.

[0017] In some embodiments, the charging rate of a single battery cell in the Mth charging step is 3.5C to 6C, and the state of charge of the battery cell in the Mth charging step includes 50% state of charge, where M is less than N and M is a positive integer greater than or equal to 1.

[0018] Therefore, the risk of lithium plating on the negative electrode is relatively small during the middle of charging, and the charging rate can be increased by charging at a higher rate. For example, when a battery cell is charged at the above-mentioned rate in the Mth charging step, the reliability of the battery cell and its fast charging performance can be improved.

[0019] In some implementations, the charging rate during the charging process from 0% to 40% state of charge of a single battery cell is 4C to 8C. Charging the battery cell at this rate during the initial charging phase can improve its reliability and fast-charging performance.

[0020] In some implementations, the constant current value of the Qth charging step in the N charging steps is less than the constant current value of the (Q-1)th charging step, Q is less than or equal to N, and Q is a positive integer greater than or equal to 2.

[0021] Therefore, the constant current value of any charging step in the N charging steps is less than the constant current value of the previous charging step. As the state of charge increases, the risk of lithium plating on the negative electrode is relatively small, which is beneficial to improving the reliability of the battery cell.

[0022] In some embodiments, the olivine-structured lithium phosphate comprises phosphate particles and a coating layer. The phosphate particles consist of phosphate grains, and the coating layer is coated on the surface of the phosphate particles. The coating layer contains one or more elements selected from C, Fe, Ti, Zr, Hf, Ge, and Sn. The surface coating of the phosphate particles enhances the conductivity of the olivine-structured lithium phosphate, reduces the powder resistivity of the material, and facilitates the migration rate of lithium ions, thereby reducing the heat generation of the battery cells.

[0023] In some embodiments, the phosphate particles comprise the general formula Li x1 A y1 Me a M b P 1-c X c Y z The compound, wherein 0.5≤x1≤1.3, 0≤y1≤1.3, and 0.9≤x1+y1≤1.3, 0.9≤a≤1.5, 0≤b≤0.5, and 0.9≤a+b≤1.5, 0≤c≤0.5, 3≤z≤5, A includes one or more of Na, K, and Mg, Me includes one or more of Mn, Fe, Co, and Ni, M includes one or more of B, Mg, Al, Si, P, S, Ca, Sc, Ti, V, Cr, Cu, Zn, Sr, Y, Zr, Nb, Mo, Cd, Sn, Sb, Te, Ba, Ta, W, Yb, La, and Ce, X includes one or more of S, Si, Cl, B, C, and N, and Y includes one or more of O and F.

[0024] In some embodiments, the coating layer includes a general formula Li 3-d Fe 2-d M2 d (PO x2 ) y2 A fast ion conductor, M2 includes one or more elements from Ti, Zr, Hf, Ge and Sn, 0≤d≤1, 0<x2<5, 0<y2<4.

[0025] Coating the surface of phosphate particles with fast ion conductors can significantly improve the transport rate of lithium ions during multiple lithium intercalation / deintercalation at the positive electrode, enhance the ionic conductivity of the positive electrode active material, thereby increasing the specific capacity and, further, improving the energy density of the corresponding battery cell.

[0026] In some embodiments, the degree of graphitization of the positive electrode active material is 0.15 to 0.32; optionally, it is 0.19 to 0.26. When the degree of graphitization of the positive electrode active material is within the above range, it is beneficial to improve the conductivity of the positive electrode active material, reduce the heat generation of the positive electrode sheet, and thus reduce the heat generation of the battery cell.

[0027] In some embodiments, the carbon content in the olivine-structured lithium phosphate is 1% to 2% by mass; the specific surface area of ​​the olivine-structured lithium phosphate is 5 m². 2 / g to 18m 2 / g; optional 7.5m 2 / g to 14m 2 / g.

[0028] Therefore, the carbon content of the above-mentioned mass content in the embodiments of this application, combined with the material with the above-mentioned specific surface area, is more conducive to the effective contact between the electrolyte and the phosphate particles, and is conducive to the transport of lithium ions at the phase interface.

[0029] In some embodiments, the volume distribution particle size of the positive electrode active material satisfies: 1μm≤Dv50≤2μm, 0.4μm≤Dv10≤0.7μm. The relatively small particle size of the positive electrode active material results in a shorter lithium-ion insertion / extraction path and less heat generation. Furthermore, the particle size of the aforementioned positive electrode active material is not too small, preventing agglomeration during processing and preparation, thus ensuring stable performance of the positive electrode active material.

[0030] In some embodiments, the olivine-structured lithium phosphate is particulate, comprising secondary particles formed by the agglomeration of primary particles, with the average particle size of the primary particles ranging from 200 nm to 500 nm. The relatively small average particle size of the primary particles results in a shorter lithium-ion insertion / extraction pathway in the positive electrode active material, leading to less heat generation.

[0031] In some embodiments, the positive electrode film layer further includes one or more of the following: ternary materials, lithium phosphate, lithium hydrogen phosphate, lithium sulfate, lithium sulfite, lithium molybdate, lithium oxalate, lithium titanate, lithium tetraborate, lithium metasilicate, lithium metamanganese oxide, lithium tartrate, lithium trilithium citrate, lithium nickel oxide, and lithium ferrite. These materials can replenish lithium ions to the positive electrode film layer, compensate for irreversible lithium ion loss within the system, increase capacity, and thereby improve the energy density of the battery cell.

[0032] In some embodiments, the single-sided coating weight of the positive electrode film is 200 mg / 1540.25 mm. 2 Up to 370mg / 1540.25mm 2 The option is 240mg / 1540.25mm. 2 Up to 330mg / 1540.25mm 2 When the single-sided coating weight of the positive electrode film is within the above range, the heat generation per unit area of ​​the positive electrode sheet will not be too large, and the energy density of the battery cell can be improved at the same time.

[0033] In some embodiments, the compaction density of the positive electrode film layer is 2.50 g / cm³ when the battery cell is 100% charged. 3 Up to 2.80 g / cm 3 2.55g / cm³ is an optional value. 3 Up to 2.70 g / cm 3 When the compaction density of the positive electrode film is within the above range, it is beneficial to improve the energy density of the battery cell; and because the positive electrode active material in the positive electrode film is packed more tightly, the contact resistance between particles is smaller, which can further reduce the resistance of the electrode sheet, thereby reducing heat generation.

[0034] In some embodiments, the single-sided coating weight of the negative electrode film is 90 mg / 1540.25 mm. 2 Up to 170mg / 1540.25mm 2 ; 110mg / 1540.25mm is optional. 2 Up to 150mg / 1540.25mm 2 When the single-sided coating weight of the negative electrode film is within the above range, the heat generation per unit area of ​​the negative electrode sheet will not be too large, and the energy density of the battery cell can be improved at the same time.

[0035] In some embodiments, the compaction density of the negative electrode film layer is 1.15 g / cm³ when the battery cell is 100% charged. 3 Up to 1.36 g / cm 3 ; 1.25g / cm³ is optional 3 Up to 1.36 g / cm 3When the compaction density of the negative electrode film is within the above range, it is beneficial to improve the energy density of the battery cell; and because the negative electrode active material in the negative electrode film is packed more tightly, the contact resistance between particles is smaller, which can further reduce the resistance of the electrode sheet, thereby reducing heat generation.

[0036] In some embodiments, the negative electrode active material includes a carbon-based material, which includes graphite particles with a graphitization degree of 92.0% to 94.5%. When the graphitization degree of the graphite particles is within the above range, the graphite particles exhibit superior electrical conductivity, which can reduce heat generation in the negative electrode sheet and the individual battery cell; and can also improve the fast charging performance of the individual battery cell.

[0037] In some embodiments, the graphite particles include artificial graphite and a carbon coating layer. The artificial graphite comprises secondary particles formed by the aggregation of multiple primary particles; the carbon coating layer coats the surface of the artificial graphite. The carbon coating layer has more end faces and defects, resulting in a greater number of sites for lithium ion insertion and extraction, thus improving the conductivity of the carbon coating layer, reducing the internal resistance of the negative electrode, and reducing the heat generation of the battery cell.

[0038] In some embodiments, the carbon coating content is 2% to 5% by mass, based on the mass of the graphite particles. When the carbon coating content is within the above range, the internal resistance of the negative electrode sheet can be further reduced, and the heat generation of the battery cell can be reduced.

[0039] In some embodiments, the negative electrode film layer includes a first negative electrode film layer and a second negative electrode film layer. The first negative electrode film layer is disposed on the surface of the negative electrode current collector, and the carbon-based material in the first negative electrode film layer includes graphite particles. The second negative electrode film layer is connected to the side of the first negative electrode film layer away from the negative electrode current collector, and the carbon-based material in the second negative electrode film layer includes graphite particles. The volume average particle size Dv50 of the graphite particles in the first negative electrode film layer is greater than or equal to the volume average particle size Dv50 of the graphite particles in the second negative electrode film layer.

[0040] Therefore, the particle size difference between the first and second negative electrode film layers in the embodiments of this application can improve the fast charging performance of the battery cell. Specifically, during fast charging, the overpotential of the second negative electrode film layer is usually higher, and the bottleneck of fast charging is mainly in the second negative electrode film layer. However, in the embodiments of this application, the particle size of the second negative electrode film layer is relatively small, which can shorten the solid-phase transport path of lithium ions, improve fast charging performance, and improve the problem of lithium deposition on the surface of the negative electrode sheet.

[0041] In some embodiments, the volume average particle size Dv50 of the graphite particles in the first negative electrode film layer is between 9.5 μm and 18.5 μm. When the volume average particle size Dv50 of the negative electrode active material in the first negative electrode film layer is within the above range, fast charging performance can be improved.

[0042] In some embodiments, the volume average particle size Dv50 of the graphite particles in the second negative electrode film is between 7.8 μm and 14.3 μm. When the volume average particle size Dv50 of the negative electrode active material in the second negative electrode film is within the above range, the lithium-ion transport tortuosity can be reduced, thereby improving the fast charging performance of the battery cell.

[0043] In some embodiments, the tap density of the carbon-based material in the first negative electrode film is less than or equal to the tap density of the carbon-based material in the second negative electrode film. When the tap density of the carbon-based material in the second negative electrode film is greater than that in the first negative electrode film, the second negative electrode film is more densely packed, thereby increasing the energy density of the battery cell; the first negative electrode film is relatively sparsely packed with more pores, which can improve the fast charging performance of the battery cell.

[0044] In some embodiments, the tap density of the carbon-based material in the first negative electrode film layer is 0.82 g / cm³. 3 Up to 1.21 g / cm 3 When the tap density of the carbon-based material in the first negative electrode film is within a suitable range, it can improve the fast charging performance of the battery cell.

[0045] In some embodiments, the tap density of the carbon-based material in the second negative electrode film is 0.90 g / cm³. 3 Up to 1.25 g / cm 3 When the tap density of the carbon-based material in the second negative electrode film is within a suitable range, it can improve the energy density of the battery cell.

[0046] In some embodiments, the thickness ratio of the second negative electrode film to the first negative electrode film is 3:7 to 7:3, and optionally 4:6 to 6:4. By adjusting the thickness ratio of the first and second negative electrode films, the gradient porosity difference between the upper and lower layers can be further increased, the lithium-ion transport tortuosity can be reduced, and the fast charging capability of the battery cell can be improved.

[0047] In some implementations, after 10 full-charge tests during the initial lifespan of the battery cell, the thickness of the first negative electrode film is between 15 μm and 65 μm. When the thickness of the first negative electrode film is within this range, the first and second negative electrode films can be modulated to increase the gradient porosity difference between the upper and lower layers, reduce the tortuosity of lithium-ion transport, and improve the fast-charging capability of the battery cell.

[0048] In some implementations, after 10 full-charge cycles during the initial lifespan of the battery cell, the thickness of the second negative electrode film is between 15 μm and 65 μm. When the thickness of the second negative electrode film is within this range, the first and second negative electrode films can be modulated to increase the gradient porosity difference between the upper and lower layers, reduce the tortuosity of lithium-ion transport, and improve the fast-charging capability of the battery cell.

[0049] In some embodiments, after the battery cell undergoes an end-of-life (EOL) full-charge test, the thickness of the first negative electrode film is between 15 μm and 70 μm. When the thickness of the first negative electrode film is within the above range, the first and second negative electrode films can be modulated to increase the gradient porosity difference between the upper and lower layers, reduce the tortuosity of lithium-ion transport, and improve the fast charging capability of the battery cell.

[0050] In some embodiments, after the battery cell undergoes an end-of-life (EOL) full-charge test, the thickness of the second negative electrode film is between 15 μm and 70 μm. When the thickness of the second negative electrode film is within the above range, the first and second negative electrode films can be modulated to increase the gradient porosity difference between the upper and lower layers, reduce the tortuosity of lithium-ion transport, and improve the fast charging capability of the battery cell.

[0051] In some embodiments, the first negative electrode film layer further includes a first lithium-containing binder, and the second negative electrode film layer further includes a second lithium-containing binder, wherein the mass content of the first lithium-containing binder relative to the mass of the first negative electrode film layer is less than or equal to the mass content of the second lithium-containing binder relative to the mass of the second negative electrode film layer.

[0052] Therefore, in the embodiments of this application, the mass content of the second lithium-containing binder in the second negative electrode film layer is relatively high, and the second lithium-containing binder provides a relatively larger number of freely movable lithium ions for the second negative electrode film layer, which can further improve the fast charging performance of the battery cell.

[0053] In some embodiments, the mass content of the first lithium-containing binder relative to the first negative electrode film layer is 0.1% to 1%. When the mass content of the first lithium-containing binder is within the above range, it can improve the lithium-ion insertion / extraction rate and enhance the fast charging performance of the battery cell.

[0054] In some embodiments, the lithium content in the first lithium-containing binder is 3% to 10% by mass, optionally 3% to 8%. When the lithium content is within the above range, the number of freely moving lithium ions in the negative electrode film layer is relatively large, which can further shorten the distance that lithium ions diffuse to the surface of the negative electrode film layer, improve the lithium ion insertion / extraction rate, and improve the fast charging performance of the battery cell.

[0055] In some embodiments, the mass content of the second lithium-containing binder relative to the second negative electrode film layer is 0.1% to 1%. When the mass content of lithium in the second lithium-containing binder is within the above range, the lithium-ion insertion / extraction rate is increased, thereby improving the fast charging performance of the battery cell.

[0056] In some embodiments, the lithium content in the second lithium-containing binder is 3% to 10% by mass, optionally 3% to 8%. When the lithium content is within the above range, the number of freely moving lithium ions in the negative electrode film layer is relatively large, which can further shorten the distance that lithium ions diffuse to the surface of the negative electrode film layer, improve the lithium ion insertion / extraction rate, and improve the fast charging performance of the battery cell.

[0057] In some embodiments, the first lithium-containing binder comprises a lithium acrylate-acrylonitrile-acrylamide-hydroxyethyl acrylate copolymer, which is derived from lithium acrylate monomer, acrylonitrile monomer, acrylamide monomer and hydroxyethyl acrylate monomer, wherein the molar ratio of lithium acrylate monomer, acrylonitrile monomer, acrylamide monomer and hydroxyethyl acrylate monomer is 30% to 50% : 15% to 45% : 5% to 20% : 20% to 35%.

[0058] Therefore, the lithium-containing binder of the above material can provide a certain number of lithium ions to the negative electrode film layer, thereby improving the fast charging performance of the battery cell; moreover, it is not prone to swelling during charging and discharging, and its structure is stable, which improves the cycle performance of the negative electrode film layer during fast charging and discharging.

[0059] In some embodiments, the second lithium-containing binder comprises a lithium acrylate-acrylonitrile-acrylamide-hydroxyethyl acrylate copolymer, which is derived from lithium acrylate monomer, acrylonitrile monomer, acrylamide monomer and hydroxyethyl acrylate monomer, wherein the molar ratio of lithium acrylate monomer, acrylonitrile monomer, acrylamide monomer and hydroxyethyl acrylate monomer is 30% to 50% : 15% to 45% : 5% to 20% : 20% to 35%.

[0060] Therefore, the lithium-containing binder of the above material can provide a certain number of lithium ions to the negative electrode film layer, thereby improving the fast charging performance of the battery cell; moreover, it is not prone to swelling during charging and discharging, and its structure is stable, which improves the cycle performance of the negative electrode film layer during fast charging and discharging.

[0061] In some embodiments, the negative electrode active material further includes a silicon-based material, wherein the silicon content in the silicon-based material is 0.3% to 10.0% by mass, based on the mass of the negative electrode active material. The introduction of the silicon-based material can increase the capacity of the negative electrode active material and improve the energy density of the battery cell.

[0062] In some embodiments, the separator includes a porous base film with a porosity of 20% to 70%. When the porosity of the separator in the embodiments of this application is within the above range, it can enhance the migration ability of lithium ions in the separator, further reduce the internal resistance of the battery cell, and thus reduce heat generation.

[0063] In some embodiments, the separator includes a porous base film with a porosity of 35% to 60%. When the porosity of the separator in the embodiments of this application is within the above range, it can improve the migration ability of lithium ions in the separator, further reduce the internal resistance of the battery cells, and thus reduce heat generation.

[0064] In some embodiments, the thickness of the base film does not exceed 12 μm. When the thickness of the base film is within the above range, the migration path of lithium ions in the base film is shorter, which can further reduce the internal resistance of the battery cell, thereby reducing heat generation.

[0065] In some embodiments, the thickness of the base film does not exceed 9 μm. When the thickness of the base film is within the above range, the migration path of lithium ions in the base film is shorter, which can further reduce the internal resistance of the battery cell, thereby reducing heat generation.

[0066] In some embodiments, the separator includes a base film and a functional layer disposed on at least one side of the base film. The functional layer includes a first functional layer and a second functional layer. The first functional layer is located on one side of the base film and includes first inorganic particles. The second functional layer is located on the other side of the base film and includes composite particles. The composite particles include second inorganic particles and a plurality of non-fluoropolymer particles. The second inorganic particles are attached to the surface of the non-fluoropolymer particles and / or dispersed inside the non-fluoropolymer particles. The first and second functional layers have good heat resistance, which can improve the heat resistance of the separator.

[0067] In some embodiments, the non-fluoropolymer particles include acrylate copolymers. Acrylate copolymers have excellent adhesion properties and high adhesion stability to the base film.

[0068] In some embodiments, the first inorganic particles include one or more of silicon oxide, aluminum oxide, boehmite, barium sulfate, calcium oxide, titanium oxide, zinc oxide, magnesium oxide, zirconium oxide, and tin oxide. These first inorganic particles can improve the heat resistance of the first functional layer.

[0069] In some embodiments, the second inorganic particles include one or more of silicon oxide, aluminum oxide, boehmite, barium sulfate, calcium oxide, titanium oxide, zinc oxide, magnesium oxide, zirconium oxide, and tin oxide. These second inorganic particles can improve the heat resistance of the first functional layer.

[0070] In some embodiments, the average particle size of the second inorganic particles is between 5 nm and 100 nm. When the average particle size of the second inorganic particles is within the above range, it is beneficial to improve the heat resistance and compressive modulus of the composite particles.

[0071] In some embodiments, the conductivity of the electrolyte is between 13 mS / cm and 20 mS / cm. When the conductivity of the electrolyte is within this range, the migration rate of lithium ions in the electrolyte is relatively high, which can further reduce the internal resistance of the battery cell, thereby reducing heat generation and improving the fast charging performance of the battery cell.

[0072] In some embodiments, the viscosity of the electrolyte at room temperature is between 2.3 mPa·s and 3.5 mPa·s. When the viscosity of the electrolyte is within this range, the migration rate of lithium ions in the electrolyte is relatively high, which can further reduce the internal resistance of the battery cells, thereby reducing heat generation and improving the fast charging performance of the battery cells.

[0073] In some embodiments, the electrolyte density at room temperature is between 1.05 g / mL and 1.35 g / mL. When the electrolyte density is within this range, the migration rate of lithium ions in the electrolyte is higher, which can further reduce the internal resistance of the battery cells, thereby reducing heat generation and improving the fast charging performance of the battery cells.

[0074] In some embodiments, the carboxylic acid ester solvent includes a chain carboxylic acid ester solvent, wherein the chain carboxylic acid ester solvent has a mass content of 5% or more and less than 75% in the organic solvent, optionally 10% or more, optionally 30% to 70%, or optionally 50% to 70%. When the mass content of the chain carboxylic acid ester solvent is within the above range, the viscosity of the electrolyte system is relatively low, which is beneficial to the migration of lithium ions.

[0075] In some embodiments, the chain carboxylic acid ester solvent includes compounds represented by Formula I.

[0076] In formula I,

[0077] R1 includes a hydrogen atom, a halogen atom, a C1 to C5 alkyl group, or a C1 to C5 haloalkyl group.

[0078] R2 includes C1 to C5 alkyl or C1 to C5 haloalkyl.

[0079] Therefore, the above-mentioned chain carboxylic acid ester solvents in the embodiments of this application have high conductivity, which is beneficial to improving the fast charging capability of battery cells.

[0080] In some embodiments, R1 includes a hydrogen atom, a halogen atom, a C1 to C3 alkyl group, or a C1 to C3 haloalkyl group.

[0081] In some embodiments, R2 includes C1 to C3 alkyl or C1 to C3 haloalkyl.

[0082] In some embodiments, the chain carboxylic acid ester solvent includes one or more compounds of formulas I-1 to I-8.

[0083] In some embodiments, the organic solvent further includes carbonate solvents, including one or more of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate. The combined use of these carbonate solvents and chain carboxylic acid ester solvents improves the conductivity of the electrolyte, which is beneficial for lithium ion migration.

[0084] In some embodiments, the carbonate solvent includes one or more of ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate.

[0085] In some embodiments, the carbonate solvent comprises 30% to 70% by mass in the organic solvent, optionally 30% to 50%. The carbonate solvent content described above can further improve the conductivity of the electrolyte, which is beneficial for lithium ion migration.

[0086] In some embodiments, the electrolyte further includes additives, including one or more of carbonate additives, sulfur-containing additives, and lithium salt additives. These additives can improve the interfacial film performance on the positive and / or negative electrode sides, which is beneficial for improving the fast-charging performance of individual battery cells and enhancing cycle performance.

[0087] In some embodiments, the carbonate additives include one or more of vinylene carbonate (VC) and fluoroethylene carbonate (FEC).

[0088] In some embodiments, the sulfur-containing additives include one or more of vinyl sulfate DTD, vinyl disulfate 2-DTD, butenyl sulfite BS, 1,3-propanesulfonate lactone PS, vinyl sulfite ES, and methylene disulfonate MMDS.

[0089] In some embodiments, the lithium salt additive includes one or more of lithium difluorophosphate (LiPO2F2), lithium difluorooxalate borate (LiDFOB), lithium tetrafluoroborate (LiBF4), and lithium dioxalate borate (LiBOB).

[0090] In some embodiments, the additive has a mass content of 1% to 10% in the electrolyte, optionally 2% to 8%. The above-mentioned mass content of the additive can effectively improve the interfacial film performance on the positive and / or negative electrode sides, which is beneficial to improving the fast charging performance of the battery cell and improving cycle performance.

[0091] In some embodiments, the electrolyte further includes a lithium salt, which includes one or more of fluorosulfonyl imide salts and lithium hexafluorophosphate (LiPF6). These lithium salts are readily dissociated, facilitating rapid lithium-ion migration; and the electrolyte system is relatively stable and not easily decomposed, thus improving the cycle performance of the battery cells.

[0092] In some embodiments, the fluorosulfonyl imide salt includes one or more of lithium bisfluorosulfonyl imide (LiFSI) and lithium bistrifluoromethanesulfonate (LiTFSI).

[0093] In some embodiments, the lithium salt includes lithium bis(fluorosulfonyl)imide (LiFSI) and lithium hexafluorophosphate (LiPF6), wherein the molar concentration of lithium bis(fluorosulfonyl)imide (LiFSI) is from 0.2 mol / L to 0.5 mol / L, and the molar concentration of lithium hexafluorophosphate (LiPF6) is from 0.5 mol / L to 1.0 mol / L.

[0094] In some embodiments, the molar concentration ratio of lithium bis(fluorosulfonyl)imide to lithium hexafluorophosphate (LiPF6) is 0.2 to 1.0.

[0095] In some embodiments, the casing is made of steel, and its thickness is 0.1 mm to 0.5 mm, optionally 0.2 mm to 0.35 mm. A casing thickness within this range results in higher mechanical strength, improving the reliability and cycle performance of the battery cell; furthermore, the casing occupies less space, allowing for more internal space, which is beneficial for increasing the energy density of the battery cell.

[0096] In some embodiments, the current-carrying area of ​​a single first electrode terminal is 25 mm². 2 Up to 315mm 2 The relatively large overcurrent area of ​​the first electrode terminal results in a smaller resistance, which helps to reduce the overall internal resistance of the battery cell.

[0097] In some embodiments, the current-carrying area of ​​a single second electrode terminal is 25 mm². 2 Up to 315mm 2 The relatively large current-carrying area of ​​the second electrode terminal results in lower resistance, which helps reduce the overall internal resistance of the battery cell.

[0098] In some embodiments, the battery cell includes a first electrode terminal, and the positive electrode includes a positive electrode tab; the first electrode terminal and the positive electrode tab are directly welded together. Direct welding can reduce the resistance at the connection point, which helps to reduce the overall internal resistance of the battery cell.

[0099] In some embodiments, the battery cell includes a second electrode terminal, and the negative electrode includes a negative electrode tab; the second electrode terminal and the negative electrode tab are directly welded together. Direct welding can reduce the resistance at the connection point, which is beneficial for reducing the overall internal resistance of the battery cell.

[0100] Secondly, this application proposes a battery device, which includes a battery cell according to any embodiment of the first aspect of this application.

[0101] In some implementations, at room temperature, the charging time for the battery device from 10% state of charge to 80% state of charge is 5 to 10.5 minutes. A faster charging speed is beneficial for improving fast-charging capabilities.

[0102] Thirdly, this application proposes an electrical device, which includes the battery device according to any embodiment of the second aspect of this application.

[0103] Fourthly, this application proposes a charging method for a single battery cell. The charging method includes charging a single battery cell in a first state of charge (SOC) such that the increase in SOC of the single battery cell is less than or equal to 5%; repeating the above steps at least once until the single battery cell is charged to a second SOC, wherein the second SOC is greater than the first SOC, and the second SOC is greater than or equal to 95% and less than 100%; and charging the single battery cell in the second SOC to 100% SOC, wherein the cutoff voltage of any step before charging to the second SOC is less than the cutoff voltage of the step charging to 100% SOC, and the cutoff voltage of the step charging to 100% SOC does not exceed the theoretical upper limit of the voltage of lithium phosphate with an olivine structure. Attached Figure Description

[0104] To more clearly illustrate the technical solutions of the embodiments of this application, the drawings used in the embodiments of this application will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on the drawings without creative effort.

[0105] Figure 1 is a schematic diagram of the structure of a battery cell provided in some embodiments of this application;

[0106] Figure 2 is an exploded schematic diagram of a battery cell provided in some embodiments of this application;

[0107] Figure 3 is a schematic diagram of the structure of a battery module provided in some embodiments of this application;

[0108] Figure 4 is a schematic diagram of the structure of a battery pack provided in some embodiments of this application;

[0109] Figure 5 is a schematic diagram of the structure of an electrical device provided in some embodiments of this application.

[0110] The accompanying drawings may not be drawn to scale.

[0111] The reference numerals in the attached drawings are explained as follows: 1. Electrical device; 2. Battery pack; 3. Controller; 4. Motor; 5. Housing; 5a. First housing section; 5b. Second housing section; 5c. Receiving space; 6. Battery module; 7. Battery cell; 10. Electrode assembly; 111. First electrode tab; 112. Second electrode tab; 12. Main body section; 20. Outer shell; 21. Housing; 22. End cap; 31. First electrode terminal; 32. Second electrode terminal. Detailed Implementation

[0112] The following detailed description, with appropriate reference to the accompanying drawings, discloses embodiments of the battery cell, its charging method, battery device, and power supply device of this application. However, unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of practically identical structures may be omitted. This is to avoid unnecessarily lengthy descriptions and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided for the purpose of enabling those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0113] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of the particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60 to 120 and 80 to 110 are listed for a specific parameter, it is also expected that ranges of 60 to 110 and 80 to 120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1 to 3, 1 to 4, 1 to 5, 2 to 3, 2 to 4, and 2 to 5. In this application, unless otherwise stated, the numerical range "a to b" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0 to 5" means that all real numbers between "0 and 5" have been listed in this article; "0 to 5" is just a shortened representation of these numerical combinations. In addition, when a parameter is stated as an integer ≥ 2, it is equivalent to disclosing that the parameter is, for example, an integer 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0114] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0115] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0116] Unless otherwise specified, all steps of this application may be performed sequentially or randomly, preferably sequentially. For example, if the method includes steps (a) and (b), it means that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, if the method may also include step (c), it means that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0117] A single battery cell includes a positive electrode and a negative electrode. During charging, lithium ions are released from the positive electrode film in the positive electrode and migrate to the negative electrode. After gaining electrons, they are embedded in the negative electrode film. The degree of lithium embedding may vary in different regions of the negative electrode film, especially during fast charging, resulting in differences in the state of charge (SOC) of different regions of the negative electrode. Consequently, some regions of the positive electrode film are lithium-rich, while others are lithium-poor.

[0118] The positive electrode film layer includes lithium phosphate with an olivine structure. This type of material has a significant voltage plateau during charging. The positive electrode voltage remains constant in the early stage of charging, but rises at the end of charging, causing lithium-rich lithium ions to be rapidly extracted. This increases the local lithium ion activity, and the lithium deposition rate is greater than the lithium insertion rate, leading to lithium deposition on the surface of the negative electrode and causing reliability issues in the battery cell.

[0119] In view of the above problems, the embodiments of this application design the battery cell system so that lithium plating will basically not occur in the battery cell during charging, thereby improving the reliability of the battery cell.

[0120] battery cell

[0121] In one aspect, this application proposes a battery cell.

[0122] A single battery cell includes an electrolyte and an electrode assembly. The electrode assembly includes a positive electrode, a negative electrode, and a separator between the positive and negative electrodes. The positive electrode includes a positive current collector and a positive electrode film layer disposed on at least one side of the positive current collector and containing a positive active material. The positive active material includes a lithium phosphate with an olivine structure. The negative electrode includes a negative current collector and a negative electrode film layer disposed on at least one side of the negative current collector. The negative electrode film layer includes a carbon-based material.

[0123] The charging process of a single battery cell from 0% state of charge to 100% state of charge includes N charging steps. The difference between the maximum state of charge in any charging step and the maximum state of charge in the adjacent charging step is less than or equal to 5% state of charge, where N is a positive integer greater than or equal to 2.

[0124] In N charging steps, the cutoff voltage of any charging step in N-1 charging steps is less than the cutoff voltage of the Nth charging step, and the cutoff voltage of the Nth charging step does not exceed 4.4V.

[0125] In other words, the charging methods for individual battery cells include:

[0126] The battery cells in the first state of charge are charged so that the increase in the state of charge of the battery cells is less than or equal to 5% of the state of charge; that is, in each charging step, the difference between the maximum state of charge and the minimum state of charge of the battery cells is less than or equal to 5% of the state of charge; optionally, the first state of charge is less than or equal to 5% of the state of charge.

[0127] Repeat the above steps at least once until the device is charged to a second state of charge, which is greater than or equal to 95% and less than 100% of the state of charge.

[0128] The battery cells in the second state of charge are charged to 100% state of charge. The cutoff voltage for the step of charging to 100% state of charge does not exceed the theoretical upper limit of the voltage of the lithium phosphate with olivine structure. The cutoff voltage of any step before charging to the third state of charge is less than the cutoff voltage of the step of charging to 100% state of charge. Accordingly, the step of charging to 100% state of charge corresponds to the Nth charging step, i.e., the last charging step; any step before charging to the second state of charge refers to N-1 charging steps.

[0129] In the embodiments of this application, N is a positive integer greater than or equal to 2, and can be selected from 20 to 30, such as 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30 or any range of two of the above values.

[0130] The N charging steps refer to charging steps 1 through N; the N-1 charging steps refer to charging steps 1 through N-1, and any one of the N-1 charging steps refers to any one of the charging steps 1 through N-1. The Nth charging step is the last charging step in the N charging steps.

[0131] During the charging process, the difference in the maximum state of charge between adjacent charging steps is less than or equal to 5%, resulting in minimal polarization during charging. Before the Nth charging step, i.e., the cutoff voltage of any charging step from the 1st to the (N-1th)th charging step is relatively small, preventing voltage spikes during charging. This ensures that lithium ions extracted from the positive electrode film can be embedded in the negative electrode film, reducing the likelihood of lithium plating on the surface of the negative electrode and improving the reliability of the battery cell. During the charging process of a single battery cell, the state of charge (SOC) of the cell can be used as the basis for jumping to the next charging step. The increase in SOC in each step will not exceed 5%. When the increase in SOC reaches 5%, the charging will jump to the next step. Alternatively, the charging will jump to the next step even if the increase in SOC is less than 5%. The specific settings can be configured according to the actual charging strategy. For example, if the SOC increase is set to jump at 4%, then the charging will jump to the next step when the SOC increases by 4%. However, it should be noted that the cutoff voltage of any charging step in N-1 charging steps is lower than the cutoff voltage of the Nth charging step.

[0132] In this embodiment, the cutoff voltage of any one of the N-1 charging steps does not exceed the cutoff voltage of the Nth charging step. In other words, the cutoff voltage of the Nth charging step is greater than or equal to the cutoff voltage of any one of the N-1 charging steps. Optionally, the cutoff voltage of the Nth charging step is greater than the cutoff voltage of any one of the N-1 charging steps. More preferably, the difference between the cutoff voltage of the Nth charging step and the cutoff voltage of any one of the N-1 charging steps is greater than or equal to 0.02V. Even more preferably, the difference between the cutoff voltage of the Nth charging step and the cutoff voltage of any one of the N-1 charging steps is greater than or equal to 0.05V. In some embodiments, the cutoff voltage of the Nth charging step is greater than the cutoff voltage of any one of the N-1 charging steps, and the difference between the cutoff voltage of the Nth charging step and the cutoff voltage of any one of the N-1 charging steps is greater than or equal to 0.05V and less than or equal to 0.2V.

[0133] The embodiments of this application employ lithium phosphate with an olivine structure, whose charging cutoff voltage is less than or equal to 4.4V. In the final step of charging the battery device, the cutoff voltage is less than or equal to 4.4V, and in all other steps, it is less than 4.4V. During the charging of a single battery cell, the cutoff voltage of each charging step can be used as the basis for jumping to the next charging step. For example, the cutoff voltage of each charging step in N-1 charging steps is set to be less than the cutoff voltage of the Nth charging step. Specifically, the cutoff voltage of the Nth charging step is 3.65V, and the cutoff voltage of each charging step in N-1 charging steps is 3.6V. When the voltage reaches 3.6V in any of the N-1 charging steps, the process jumps to the next charging step. It should be noted that the increase in state of charge in each charging step is less than or equal to 5%.

[0134] In this embodiment of the application, the difference between the maximum state of charge of any charging step in the N charging steps and the maximum state of charge in its adjacent charging step is less than or equal to 5% state of charge, such as 1% state of charge, 1.5% state of charge, 2% state of charge, 2.5% state of charge, 3% state of charge, 3.5% state of charge, 4% state of charge, 4.5% state of charge, or any range of two of the above values.

[0135] In the embodiments of this application, the lithium-containing phosphate with an olivine structure may include a lithium iron phosphate system or a lithium manganese iron phosphate system.

[0136] For example, the difference between the cutoff voltage of the Nth charging step and the cutoff voltage of any one of the N-1 charging steps is 0.02V, 0.025V, 0.03V, 0.035V, 0.04V, 0.045V, 0.05V, 0.055V, 0.06V, 0.065V, 0.07V, 0.075V, 0.08V, 0.085V, 0.09V, 0.095V, 0.1V, 0.12V, 0.15V, 0.18V, 0.20V, or a range of any two of the above values.

[0137] When the difference between the cutoff voltage of the Nth charging step and the cutoff voltage of any charging step in the N-1th charging step is within the above range, the risk of lithium plating on the negative electrode can be further reduced, and the reliability of the battery cell can be improved.

[0138] In this embodiment, the cutoff voltage of the Nth charging step does not exceed the theoretical upper limit of the lithium phosphate with an olivine structure; in other words, the cutoff voltage of the Nth charging step is less than or equal to the theoretical upper limit of the lithium phosphate with an olivine structure. In some embodiments, the cutoff voltage of the Nth charging step is between 3.65V and 4.4V. Exemplarily, the cutoff voltage of the Nth charging step is 3.65V, 3.7V, 3.75V, 3.8V, 3.85V, 3.9V, 3.95V, 4V, 4.05V, 4.1V, 4.15V, 4.2V, 4.25V, 4.3V, 4.35V, 4.4V, or a range consisting of any two of the above values.

[0139] When the cutoff voltage of the Nth charging step is within the above range, the risk of lithium plating in the battery cell is relatively small, and the energy density of the battery cell can be improved.

[0140] In some embodiments, the Nth charging step is charged at a charging rate between 0.05C and 0.30C; alternatively, the Nth charging step is charged at a charging rate between 0.10C and 0.30C. For example, the charging rate in the Nth charging step is 0.05C, 0.10C, 0.15C, 0.20C, 0.25C, 0.30C, or a range consisting of any two of the above values.

[0141] Towards the end of the charging process, the risk of lithium plating on the negative electrode increases. Charging at the above-mentioned rate in the Nth charging step can further reduce the risk of lithium plating on the negative electrode and improve the reliability of the battery cell.

[0142] In some embodiments, the battery cell is charged at a rate of 3.5C to 6C in the Mth charging step, and the state of charge of the battery cell in the Mth charging step includes 50% state of charge, where M is less than N and M is a positive integer greater than or equal to 1. For example, the charging rate of the battery cell in the Mth charging step is 3.5C, 4C, 4.5C, 5C, 5.5C, 6C, or any combination of two of the above values. For example, M is 8, 9, 10, 11, 12, 13, 14, or 15, etc. When M is 10, the battery cell is charged at any charging rate between 3.5C and 6C in the 10th charging step.

[0143] During the middle of charging, the risk of lithium plating on the negative electrode is relatively small, and the charging rate can be increased by charging at a higher rate. For example, when a battery cell is charged at the above-mentioned rate in the Mth charging step, the reliability of the battery cell and its fast charging performance can be improved.

[0144] In some embodiments, the battery cell is charged at a rate of 4C to 8C in any charging step from 0% state of charge to 40% state of charge. For example, if the interval between adjacent charging steps is 5% state of charge, in any charging step from 0% state of charge to 5%, from 5% state of charge to 10% state of charge, from 10% state of charge to 15% state of charge, ... from 35% state of charge to 40% state of charge, the charging rate is 4C, 4.5C, 5C, 5.5C, 6C, 6.5C, 7C, 7.5C, 8C, or any combination of two of the above values. The state of charge of the battery cell in the Pth charging step includes 40% state of charge, the charging rate from the 1st charging step to the Pth charging step is 4C to 8C, P is less than M, and P is a positive integer greater than or equal to 1. For example, P is 6, 7, 8 or 9, etc. When P is 8, in the 9 charging steps from the first charging step to the ninth charging step, any charging step charges at a rate of 4C to 8C.

[0145] In the early stages of charging, the risk of lithium plating on the negative electrode is relatively low, and the charging rate can be increased by charging at a higher rate. For example, when a battery cell is charged at the aforementioned rate in the early stages of charging, the reliability and fast charging performance of the battery cell can be improved.

[0146] Optionally, the battery cell is charged at a rate of 4C to 8C during the charging step from 10% to 40% state of charge. For example, the charging rate during the charging step from 10% to 40% state of charge is 4C, 4.5C, 5C, 5.5C, 6C, 6.5C, 7C, 7.5C, 8C, or any combination of two of the above values. The state of charge of the battery cell in the P2 charging step includes 40% state of charge, and the state of charge in the P1 charging step includes 10% state of charge. The charging rate from the P1 charging step to the P2 charging step is 4C to 8C, P2 is less than M, P1 is less than P2, and P1 and P2 are positive integers greater than or equal to 1.

[0147] In some implementations, the constant current value of the Qth charging step in the N charging steps is less than the constant current value of the (Q-1)th charging step, Q is less than or equal to N, and Q is a positive integer greater than or equal to 2. For example, Q can be any positive integer among 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, and N. The constant current value of any charging step in the N charging steps is less than the constant current value of the previous charging step. As the state of charge increases, the risk of lithium plating on the negative electrode is relatively small, which is beneficial to improving the reliability of the battery cell.

[0148] With the development of the battery cell field, the requirements for the fast charging performance of battery cells are gradually increasing. The embodiments of this application are particularly applicable to fast-charging battery cells. For example, in an environment of 30°C, the charging time for a battery cell from 10% state of charge to 80% state of charge is less than or equal to 10.5 minutes, optionally between 5 minutes and 10.5 minutes. Exemplarily, the charging time for a battery cell from 10% state of charge to 80% state of charge is 10.5 minutes, 10 minutes, 9.5 minutes, 9 minutes, 8.5 minutes, 8 minutes, 7.5 minutes, 7 minutes, 6.5 minutes, 6 minutes, 5.5 minutes, 5 minutes, or a range of any two of the above values.

[0149] In some embodiments, during the charging process of a battery cell from 10% to 80% state of charge, the difference between the maximum state of charge in any charging step and the maximum state of charge in the adjacent charging step is less than or equal to 5% state of charge, such as 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, or a range of any two of the above values. This charging process can be performed at room temperature, for example, 30°C.

[0150] In some embodiments, the volumetric energy density of the battery cell is from 390 Wh / L to 500 Wh / L, optionally from 410 Wh / L to 470 Wh / L. Exemplarily, the volumetric energy density of the battery cell is 390 Wh / L, 400 Wh / L, 410 Wh / L, 420 Wh / L, 430 Wh / L, 440 Wh / L, 450 Wh / L, 460 Wh / L, 470 Wh / L, 480 Wh / L, 490 Wh / L, 500 Wh / L, or a range of any two of the above values. The volumetric energy density of the battery cell is relatively high.

[0151] The upper limit voltage for charging and the lower limit voltage for discharging of a single battery cell vary depending on the positive electrode active material. For example, when using phosphate materials, including lithium iron phosphate, the upper limit voltage for charging (i.e., the theoretical upper limit voltage) is 3.65V, and the lower limit voltage for discharging is 2.0V, or the upper limit voltage for charging is 3.7V, or the upper limit voltage for charging is 3.8V; as another example, when using phosphate materials, including lithium manganese iron phosphate, the upper limit voltage for charging is 4.2V, and the lower limit voltage for discharging is 2.5V, or the upper limit voltage for charging is 4.25V, or the upper limit voltage for charging is 4.35V, or the upper limit voltage for charging is 4.4V.

[0152] In this application embodiment, the volumetric energy density of a battery cell has a meaning known in the art and can be detected using equipment and methods known in the art. Taking a battery cell with a charging upper limit voltage of 3.65V and a discharge cutoff voltage of 2.0V as an example, the battery cell is placed at 25°C and charged to 3.65V with a constant current of 0.33C, and then charged to 0.05C with a constant voltage; it is then discharged to 2.0V with a constant current of 0.33C, and the discharge capacity A0 is recorded at this time, in Ah; the length, width, and height of the battery cell are measured using calipers (generally calculated based on the outer casing dimensions of the battery device, excluding the height of the electrode terminals and the insulating film outside the casing), and the volume of the battery cell V0 is calculated, in L; the volumetric energy density of the battery cell VED = (A0 × discharge plateau voltage) / V0, in Wh / L.

[0153] [Positive electrode plate]

[0154] The positive electrode includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector and comprising a positive electrode active material. For example, the positive current collector has two surfaces opposite each other in its thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0155] The following explanation uses a charging upper limit voltage of 3.65V and a discharging cutoff voltage of 2.0V as an example to illustrate the state of a single battery cell. In this embodiment, 0% state of charge (SOC) and 100% state of charge (SOC) are defined as follows:

[0156] The battery cell is charged at a constant current charging rate of 0.33C to the upper limit voltage of the battery cell, and then charged at a constant voltage to 0.05C, which corresponds to the 100% SOC state of the battery cell; the battery cell is discharged at a constant current discharging rate of 0.33C to the cutoff voltage, which corresponds to the 0% SOC state of the battery cell.

[0157] In some embodiments, the compaction density of the positive electrode film layer is 2.50 g / cm³ when the battery cell is at 100% state of charge (SOC). 3 Up to 2.80 g / cm 3 ; 2.55g / cm³ is optional 3 Up to 2.70 g / cm 3 For example, when the battery cell is at 100% state of charge (SOC), the compaction density of the positive electrode film is 2.50 g / cm³. 3 2.52g / cm 3 2.55g / cm 3 2.56 g / cm 3 2.57g / cm 3 2.58g / cm 3 2.60g / cm3 2.62 g / cm 3 2.65g / cm 3 2.68g / cm 3 2.70 g / cm 3 2.72 g / cm 3 2.75g / cm 3 2.78g / cm 3 2.80g / cm 3 Or a range consisting of any two of the above values.

[0158] When the compaction density of the positive electrode film is within the above range, it is beneficial to improve the energy density of the battery cell; and because the positive electrode active material in the positive electrode film is packed more tightly, the contact resistance between particles is smaller, which can further reduce the resistance of the electrode sheet, thereby reducing heat generation.

[0159] In some embodiments, the single-sided coating weight of the positive electrode film is 200 mg / 1540.25 mm. 2 Up to 370mg / 1540.25mm 2 ; 240mg / 1540.25mm is optional. 2 Up to 330mg / 1540.25mm 2 For example, the single-sided coating weight of the positive electrode film is 200 mg / 1540.25 mm. 2 210mg / 1540.25mm 2 220mg / 1540.25mm 2 230mg / 1540.25mm 2 240mg / 1540.25mm 2 250mg / 1540.25mm 2 260mg / 1540.25mm 2 270mg / 1540.25mm 2 280mg / 1540.25mm 2 290mg / 1540.25mm 2 300mg / 1540.25mm 2 310mg / 1540.25mm 2 320mg / 1540.25mm 2 330mg / 1540.25mm 2 340mg / 1540.25mm 2 350mg / 1540.25mm 2 360mg / 1540.25mm 2370mg / 1540.25mm 2 Or a range consisting of any two of the above values.

[0160] When the single-sided coating weight of the positive electrode film is within the above range, the heat generation per unit area of ​​the positive electrode sheet will not be too large, and the energy density of the battery cell can be improved at the same time.

[0161] In this embodiment, the compaction density of the positive electrode film layer of a single battery cell at 100% State of Charge (SOC) can be detected by the following method: The positive electrode sheet of the single battery cell at 100% SOC is disassembled, and the compaction density of the positive electrode film layer is measured. For example, a single-sided coated positive electrode sheet (if double-sided coated, the positive electrode film layer on one side can be wiped off first) is taken, cut into small circular pieces with an area of ​​S1, weighed, and recorded as M1, and its thickness H1 is measured. Then, the positive electrode film layer of the weighed positive electrode sheet is wiped off, the weight of the positive current collector is weighed and recorded as M0, and its thickness H0 is measured. The single-sided coating weight of the positive electrode film layer = (weight of the positive electrode sheet M1 - weight of the positive current collector M0) / S1, the thickness of the positive electrode film layer = thickness of the positive electrode sheet H1 - thickness of the positive current collector H0, and the compaction density of the positive electrode film layer = single-sided coating weight of the positive electrode film layer / thickness of the positive electrode film layer.

[0162] In some embodiments, the resistivity of the positive electrode active material powder is from 1 Ω·cm to 27.5 Ω·cm, optionally less than or equal to 20 Ω·cm, and optionally less than or equal to 11 Ω·cm. Exemplarily, the resistivity of the positive electrode active material powder can be 27.5 Ω·cm, 20 Ω·cm, 19 Ω·cm, 18 Ω·cm, 17 Ω·cm, 16 Ω·cm, 15 Ω·cm, 14 Ω·cm, 13 Ω·cm, 12 Ω·cm, 11 Ω·cm, 10 Ω·cm, 9 Ω·cm, 8 Ω·cm, 7 Ω·cm, 6 Ω·cm, 5 Ω·cm, 4 Ω·cm, 3 Ω·cm, 2 Ω·cm, 1 Ω·cm, or a range consisting of any two of the above values.

[0163] The powder resistivity of the positive electrode active material is relatively low, which results in relatively low resistance of the positive electrode sheet and less heat generation in the battery cell.

[0164] In the embodiments of this application, the powder resistivity of the material has a well-known meaning in the art and can be tested using methods and equipment well-known in the art, such as using a PRCD1100 powder resistivity meter according to the test standard GB / T30835-2014.

[0165] In some embodiments, the powder compaction density of the positive electrode active material at 30000N is 2.46 g / cm³. 3 Up to 2.8 g / cm 3For example, the compacted density of the positive electrode active material at 30000 N is 2.46 g / cm³. 3 2.47 g / cm 3 2.48 g / cm 3 2.49 g / cm 3 2.5g / cm 3 2.51g / cm 3 2.55g / cm 3 2.58g / cm 3 2.60g / cm 3 2.65g / cm 3 2.68g / cm 3 2.70 g / cm 3 2.72 g / cm 3 2.75g / cm 3 2.78g / cm 3 2.80g / cm 3 Or a range consisting of any two of the above values.

[0166] When the powder compaction density of the positive electrode active material at 30000N is within the above range, it can improve the energy density of the battery cell. Furthermore, since the positive electrode active material in the positive electrode film can be stacked more tightly, the contact resistance between particles is smaller, which can further reduce the resistance of the electrode sheet, thereby reducing heat generation.

[0167] In the embodiments of this application, the powder compaction density of the material has a meaning known in the art and can be tested using methods and equipment known in the art, according to the testing standard GB / T24533-2009. For example, a certain amount of positive electrode active material is taken as a sample and added to a UTM7305 electronic pressure testing machine with a bottom area of ​​1.327 cm². 2 In the mold, the pressure is increased to 3000 kg (equivalent to 30000 N), held for 30 s, then depressurized and held for 10 s. The compaction density of the positive electrode active material under a force of 30000 N is then recorded and calculated.

[0168] In some embodiments, the specific charge capacity of the positive electrode active material at a 0.1C rate is 150 mAh / g to 170 mAh / g, optionally 157 mAh / g to 170 mAh / g. Exemplarily, the specific charge capacity of the positive electrode active material at a 0.1C rate is 150 mAh / g, 151 mAh / g, 152 mAh / g, 153 mAh / g, 154 mAh / g, 155 mAh / g, 156 mAh / g, 157 mAh / g, 158 mAh / g, 159 mAh / g, 160 mAh / g, 161 mAh / g, 162 mAh / g, 163 mAh / g, 164 mAh / g, 165 mAh / g, 166 mAh / g, 167 mAh / g, 168 mAh / g, 169 mAh / g, 170 mAh / g, or a range consisting of any two of the above values.

[0169] When the specific capacity of the positive electrode active material at a 0.1C rate is within the above range, the energy density of the battery cell is relatively high.

[0170] In the embodiments of this application, the specific capacity of the active material has a meaning known in the art and can be tested using equipment and methods known in the art. The test methods for the first coulombic efficiency and the first discharge specific capacity in Appendix G of the national standard GB / T 24533-2019 can be adopted. A half-coin cell is assembled with lithium metal as the negative electrode and a sample electrode containing the above-mentioned material as the positive electrode. The half-coin cell is tested on a battery tester or other test equipment with equivalent performance at 23℃±2℃ by charging and discharging at a rate of 0.1C to obtain the coin capacity. The capacity is then divided by the mass of the electrode active material to obtain the charging specific capacity parameter.

[0171] In some embodiments, the mass percentage of olivine-structured lithium phosphate in the positive electrode active material can be greater than or equal to 80% and less than or equal to 100%, and the positive electrode active material of this application can be considered as an olivine-structured lithium phosphate system. When the mass percentage of olivine-structured lithium phosphate is less than 100%, the positive electrode active material may also include commonly used positive electrode active materials, such as, but not limited to, at least one of lithium transition metal oxides. Examples of lithium transition metal oxides include, but are not limited to, at least one of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, and their respective modified compounds.

[0172] Optionally, the lithium phosphate with an olivine structure in the positive electrode active material accounts for 100% by mass.

[0173] In this application, the lithium phosphate with olivine structure can be phosphate particles or a material obtained by coating and modifying them. For example, the lithium phosphate with olivine structure includes phosphate particles and a coating layer. The coating layer is coated on the surface of the phosphate particles and contains one or more elements selected from C, Fe, Ti, Zr, Hf, Ge and Sn.

[0174] Phosphate particles, through surface coating, can improve the conductivity of lithium phosphates with olivine structure, reduce the powder resistivity of the material, facilitate the migration rate of lithium ions, improve the fast charging capability of the battery, and reduce the heat generation of the battery cell.

[0175] In some embodiments, the phosphate particles comprise the general formula Li x1 A y1 Me a M b P 1-c X c Y z The compound contains 0.5 ≤ x1 ≤ 1.3, 0 ≤ y1 ≤ 1.3, and 0.9 ≤ x1 + y1 ≤ 1.3, 0.9 ≤ a ≤ 1.5, 0 ≤ b ≤ 0.5, and 0.9 ≤ a + b ≤ 1.5, 0 ≤ c ≤ 0.5, 3 ≤ z ≤ 5, where A includes one or more of Na, K, and Mg; Me includes one or more of Mn, Fe, Co, and Ni; M includes one or more of B, Mg, Al, Si, P, S, Ca, Sc, Ti, V, Cr, Cu, Zn, Sr, Y, Zr, Nb, Mo, Cd, Sn, Sb, Te, Ba, Ta, W, Yb, La, and Ce; X includes one or more of S, Si, Cl, B, C, and N; and Y includes one or more of O and F. Phosphate particles exhibit excellent cycle stability, which is beneficial for improving the cycle performance of battery cells.

[0176] For example, phosphate particles include one or more of LiFePO4, LiMnPO4, LiNiPO4, and LiCoPO4. During the charging and discharging process, active ions such as Li are de-intercalated and consumed in a single battery cell, resulting in different molar contents of Li in different discharged states. In the examples of positive electrode active materials such as LiFePO4, LiMnPO4, LiNiPO4, and LiCoPO4, the molar contents of Li represent the initial state of the material, i.e., the state before feeding. When the positive electrode active material is applied to the battery system, the molar contents of Li may change after charge-discharge cycles. In the embodiments of this application, the molar contents of oxygen (O) in the examples of positive electrode active materials such as LiFePO4, LiMnPO4, LiNiPO4, and LiCoPO4 are only theoretical values. Lattice oxygen release can cause changes in the molar contents of oxygen (O). In reality, the molar contents of oxygen (O) may fluctuate, and all of the above situations are within the scope of protection of this application.

[0177] In some embodiments, the coating layer includes a general formula Li 3-d Fe 2-d M2 d (PO x2 ) y2 A fast ion conductor, M2 includes one or more elements from Ti, Zr, Hf, Ge and Sn, 0≤d≤1, 0<x2<5, 0<y2<4.

[0178] For example, the fast ion conductor is a material having a NASICON structure, such as one or more of lithium iron phosphate (Li2FeTi(PO4)3), lithium zirconium iron phosphate (Li2FeZr(PO4)3), and lithium iron tin phosphate (Li2FeSn(PO4)3).

[0179] Fast ion conductors with a NASICON structure are materials with ultrafast ion conduction capabilities, possessing abundant three-dimensional lithium-ion diffusion and transport channels. They exhibit advantages such as high ion conduction efficiency and strong structural stability during multiple lithium delithiation and lithium intercalation processes. Coating the surface of phosphate particles with fast ion conductors containing a NASICON structure can significantly improve the lithium-ion transport rate during multiple lithium delithiation / intercalation at the positive electrode, enhance the ionic conductivity of the positive electrode active material, improve the fast charging capability of the battery cell, and further increase the specific capacity and energy density of the corresponding battery cell.

[0180] In some embodiments, the coating layer also includes elemental carbon.

[0181] The carbon element and the fast ion conductor can be layered. For example, the carbon element can be an independent carbon coating layer, and the fast ion conductor can be an independent fast ion conductor layer. The carbon coating layer can be applied to the surface of the phosphate particles, and the fast ion conductor layer can be located on the surface of the carbon coating layer, i.e., the fast ion conductor layer is located on the side of the carbon coating layer away from the phosphate particles. Alternatively, the fast ion conductor layer can be applied to the surface of the phosphate particles, and the carbon coating layer can be located on the surface of the fast ion conductor layer, i.e., the carbon coating layer is located on the side of the fast ion conductor layer away from the phosphate particles. Of course, the carbon element and the fast ion conductor can also be layered together.

[0182] Optionally, the carbon coating layer can be formed by carbonizing an organic carbon source (e.g., glucose, polyethylene glycol, etc.) onto the surface of the fast ion conductor layer. The carbon coating layer can partially or completely cover the fast ion conductor layer. The carbon coating layer can significantly improve the electronic conductivity of phosphate particles, compensating for the poor electronic conductivity of phosphate particles and increasing the energy density of the battery cell.

[0183] Specifically, the carbon coating layer gives the positive electrode active material of this application the following advantages:

[0184] The carbon coating layer in the positive electrode active material of this application provides a suitable channel for electron transport, which can significantly improve the electron conduction rate during multiple lithium delithiation and lithium insertion processes, improve the electronic conductivity of lithium phosphate, improve the charging capacity of the corresponding battery cell, and also improve the energy density.

[0185] The carbon coating layer of the positive electrode active material in this application is porous, which allows the electrolyte to come into full and effective contact with the lithium phosphate, thereby improving the lithium ion transport rate at the phase interface and enhancing the charging capacity of the battery cell.

[0186] Coating the surface of lithium phosphate with a carbon coating layer can not only improve the conductivity of lithium phosphate, but also improve the structural stability of the positive electrode active material. This effectively alleviates the iron dissolution phenomenon of the positive electrode active material during long-term storage and cyclic use of battery cells, thereby improving the cycle life of battery cells.

[0187] The positive electrode active material of this application uses lithium phosphate as a substrate, fully leveraging the advantages of lithium phosphate such as low cost, high reliability, and good cycle stability. Simultaneously, it utilizes coating layers (a fast ion conductor layer and a carbon coating layer) to overcome the drawbacks of poor electronic and ionic conductivity. Battery cells prepared using the positive electrode active material of this application can improve the energy density of battery cells while maintaining excellent cycle performance.

[0188] In this embodiment, the elemental content in the positive electrode active material is defined in a way known in the art and can be detected using equipment and methods known in the art. For example, referring to EPA 6010D-2014, it can be tested by inductively coupled plasma atomic emission spectrometry (ICP-OES, instrument model: Thermo ICAP7400). After discharging the battery cell to 0% state of charge (SOC), the positive electrode sheet is disassembled, cleaned and dried with DMC, and then calcined at high temperature to remove impurities. 0.4g of the positive electrode active material is weighed and 10ml (50% concentration) of aqua regia is added. Then it is placed on a plate at 180°C for 30min. After digestion on the plate, the volume is adjusted to 100mL, and quantitative testing is performed using the standard curve method.

[0189] In some embodiments, the degree of graphitization of the positive electrode active material is 0.15 to 0.32, optionally 0.19 to 0.26. Exemplarily, the degree of graphitization of the positive electrode active material is 0.15, 0.16, 0.17, 0.18, 0.19, 0.20, 0.21, 0.22, 0.23, 0.24, 0.25, 0.26, 0.27, 0.28, 0.29, 0.30, 0.31, 0.32, or a range consisting of any two of the above values.

[0190] When the degree of graphitization of the positive electrode active material is within the above range, it is beneficial to improve the conductivity of the positive electrode active material, reduce the heat generation of the positive electrode sheet, and thus reduce the heat generation of the battery cell.

[0191] In the embodiments of this application, the higher the degree of graphitization of the material, the lower the degree of disorder, and the test can be carried out according to the general rules of X-ray diffraction analysis method JIS / K 0131-1996.

[0192] In some embodiments, the carbon content in the olivine-structured lithium phosphate is 1% to 2% by mass, and the specific surface area of ​​the olivine-structured lithium phosphate is 5 m². 2 / g to 18m 2 / g.

[0193] Optionally, the carbon content in the olivine-structured lithium phosphate is 1% to 2% by mass, and the specific surface area of ​​the olivine-structured lithium phosphate is 7.5 m². 2 / g to 14m 2 / g.

[0194] For example, the mass content of carbon in the lithium phosphate with olivine structure is 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2%, or any combination of two of the above values.

[0195] For example, the specific surface area of ​​lithium phosphate with an olivine structure is 5 m². 2 / g、6m 2 / g、7m 2 / g、8m 2 / g、9m 2 / g, 10m 2 / g、11m 2 / g、12m 2 / g、13m 2 / g、14m 2 / g, 15m 2 / g, 16m 2 / g、17m 2 / g、18m 2 / g or a range consisting of any two of the above values.

[0196] Carbon mainly exists in the form of a carbon coating layer. The carbon coating layer is loose and porous, which helps to increase the specific surface area of ​​the material, facilitates effective contact between the electrolyte and phosphate particles, and promotes the transport of lithium ions at the phase interface. In addition, when the mass content of carbon is within the above range, it can significantly improve the conductivity of lithium phosphate with olivine structure, which is beneficial to improving the ionic and electronic conductivity of lithium phosphate with olivine structure, and can improve the rapid charging capability and energy density of battery cells.

[0197] In the embodiments of this application, the specific surface area of ​​the material has a meaning known in the art and can be detected using equipment and methods known in the art. For example, it can be detected according to the testing standard GB / T 19587-2017, using the positive electrode active material as a sample, and the specific surface area is tested using a Tri-Star 3020 specific surface area and pore size analyzer from Micromeritics, USA.

[0198] In some embodiments, the volume distribution particle size of the positive electrode active material satisfies: 1μm≤Dv50≤2μm, 0.4μm≤Dv10≤0.7μm.

[0199] For example, the Dv50 of the positive electrode active material can be 1 μm, 1.1 μm, 1.15 μm, 1.2 μm, 1.25 μm, 1.3 μm, 1.35 μm, 1.4 μm, 1.45 μm, 1.5 μm, 1.55 μm, 1.6 μm, 1.65 μm, 1.7 μm, 1.75 μm, 1.8 μm, 1.85 μm, 1.9 μm, 1.95 μm, 2 μm, or any range of two of the above values.

[0200] For example, the Dv10 of the positive electrode active material can be 0.4 μm, 0.45 μm, 0.5 μm, 0.55 μm, 0.6 μm, 0.65 μm, 0.7 μm, or any combination of two of the above values.

[0201] The positive electrode active material has a relatively small particle size, resulting in a shorter lithium ion insertion / extraction path and less heat generation. Furthermore, the particle size of the positive electrode active material is not too small, so it will not agglomerate during the processing and preparation process, thus ensuring the stable performance of the positive electrode active material.

[0202] In the embodiments of this application, the volume average particle size Dv50 of the material refers to the particle size corresponding to 50% of the volume distribution, and the volume average particle size Dv10 of the material refers to the particle size corresponding to 10% of the volume distribution. They can be detected using equipment and methods known in the art. For example, the positive electrode active material can be used as a sample, and the Dv50 and Dv10 of the particles can be tested using a Mastersizer 2000E laser particle size analyzer according to the test standard GB / T 19077-2016.

[0203] When the positive electrode active material includes other materials besides lithium phosphate with olivine structure, the volume distribution particle size of the positive electrode active material refers to the volume distribution particle size of all positive electrode active materials.

[0204] In some embodiments, the olivine-structured lithium phosphate is particulate, comprising secondary particles, which in turn include a plurality of primary particles, the average particle size of which is 200 nm to 500 nm. Exemplarily, the average particle size of the primary particles is 200 nm, 220 nm, 250 nm, 280 nm, 300 nm, 320 nm, 350 nm, 380 nm, 400 nm, 420 nm, 450 nm, 480 nm, 500 nm, or a range consisting of any two of the above values.

[0205] The average particle size of primary particles is relatively small, the lithium ion insertion / extraction path in the positive electrode active material is shorter, and the heat generation is less.

[0206] In this embodiment, secondary particles refer to aggregated particles formed by the aggregation of two or more primary particles. Primary and secondary particles can be easily distinguished experimentally (e.g., by taking SEM images using a scanning electron microscope). The average particle size of primary particles can be obtained by testing the SEM images. The SEM test parameters can be set as follows: operating voltage (EHT) of 10.00 kV, using an InLens detector, operating distance of 4.6 mm, and magnification of 1000X.

[0207] In some embodiments, the positive electrode film layer further includes one or more of the following: ternary materials, lithium phosphate, lithium hydrogen phosphate, lithium sulfate, lithium sulfite, lithium molybdate, lithium oxalate, lithium titanate, lithium tetraborate, lithium metasilicate, lithium metamanganese oxide, lithium tartrate, lithium trilithium citrate, lithium nickel oxide, and lithium ferrite. These materials can act as lithium replenishing agents, which can replenish lithium ions to the positive electrode film layer, compensate for irreversible lithium ion losses within the system, increase capacity, and thereby improve the energy density of the battery cell.

[0208] Optionally, ternary materials include Li x3 A y3 Ni a3 Co b3 Mn c M3(1-a3-b3-c3)Y3 z3 Wherein, 0 < x3 ≤ 2.1, 0 < y3 ≤ 2.1, and 0.9 ≤ x3 + y3 ≤ 2.1, 0 ≤ a3 ≤ 1, 0 ≤ b3 ≤ 1, 0 ≤ c3 ≤ 1, and 0.1 ≤ a3 + b3 + c3 ≤ 1, 1.8 ≤ z3 ≤ 3.5, A includes one or more of Na, K, and Mg, M3 includes one or more of B, Mg, Al, Si, P, S, Ca, Sc, Ti, V, Cr, Fe, Cu, Zn, Sr, Y, Zr, Nb, Mo, Cd, Sn, Sb, Te, Ba, Ta, W, Yb, La, and Ce, and Y3 includes one or more of O and F.

[0209] For example, ternary materials include LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2(NCM333), LiNi 0.5 Co 0.2 Mn 0.3 O2(NCM523), LiNi 0.6 Co 0.2 Mn 0.2 O2(NCM622), LiNi 0.8 Co 0.1 Mn 0.1 O2(NCM811), LiNi 0.80 Co 0.15 Al 0.05 At least one of O2.

[0210] In some embodiments, the lithium replenishing agent has a mass content of 0.5% to 5% in the positive electrode film, for example, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, or any combination of two of the above values. When the mass content of the lithium replenishing agent is within the above range, it can replenish lithium ions to the positive electrode film, compensate for irreversible lithium ion loss in the system, increase capacity, and thereby increase the energy density of the battery cell.

[0211] The lithium replenishing agent can be located in the same layer as the positive electrode active material or in a different layer. When the lithium replenishing agent and the positive electrode active material are in different layers, the lithium replenishing agent can be located in the lithium replenishing layer, and the positive electrode active material can be located in the positive electrode active material layer. In other words, the positive electrode film layer includes a lithium replenishing layer and a positive electrode active material layer. The positive electrode active material layer can be disposed on at least one side of the positive electrode current collector, and the lithium replenishing layer can be located between the positive electrode active material layer and the positive electrode current collector. Alternatively, the lithium replenishing layer can be disposed on at least one side of the positive electrode current collector, and the positive electrode active material layer can be located between the lithium replenishing layer and the positive electrode current collector. Optionally, the lithium replenishing layer can be located between the positive electrode active material layer and the positive electrode current collector. During the cycle charging and discharging of the battery cell, the lithium replenishing agent in the lithium replenishing layer can be gradually released into the system to compensate for the lithium loss of the battery system.

[0212] In some embodiments, the positive electrode film layer may optionally include a positive electrode conductive agent. This application does not impose particular limitations on the type of positive electrode conductive agent. As an example, the positive electrode conductive agent includes at least one selected from superconducting carbon, conductive graphite, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. In some embodiments, the mass content of the positive electrode conductive agent is ≤5% based on the mass of the positive electrode film layer.

[0213] In some embodiments, the positive electrode film layer may optionally include a positive electrode binder. This application does not impose particular limitations on the type of positive electrode binder. As an example, the positive electrode binder may include at least one selected from polyvinylidene fluoride, polytetrafluoroethylene, a terpolymer of vinylidene fluoride-tetrafluoroethylene-propylene, a terpolymer of vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene, a tetrafluoroethylene-hexafluoropropylene copolymer, polyacrylic acid, and fluorinated acrylate resins. In some embodiments, the mass content of the positive electrode binder is ≤5% based on the mass of the positive electrode film layer.

[0214] In some embodiments, the positive current collector may be a metal foil or a composite current collector. Examples of metal foils include at least one foil selected from aluminum, aluminum alloys, nickel, nickel alloys, titanium, titanium alloys, silver, and silver alloys. The composite current collector may include a polymeric material substrate and a metal material layer formed on at least one surface of the polymeric material substrate. As an example, the metal material layer may include at least one selected from aluminum, aluminum alloys, nickel, nickel alloys, titanium, titanium alloys, silver, and silver alloys. As an example, the polymeric material substrate may include at least one selected from polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE).

[0215] In some embodiments, the ratio of the thickness of the positive current collector to the thickness of the positive electrode film on one side is 0.05 to 0.3. Exemplarily, the ratio of the thickness of the positive current collector to the thickness of the positive electrode film on one side is 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, 0.12, 0.15, 0.18, 0.2, 0.22, 0.25, 0.28, 0.3, or a range consisting of any two of the above values.

[0216] When the ratio of the thickness of the positive current collector to the thickness of the positive electrode film layer on one side is within the above range, the fast charging capability and energy density of the battery cell can be improved.

[0217] In some embodiments, the thickness of the positive current collector is 10 μm to 15 μm, optionally 12 μm to 15 μm. Exemplarily, the thickness of the positive current collector is 10 μm, 10.5 μm, 11 μm, 11.5 μm, 12 μm, 12.5 μm, 13 μm, 13.5 μm, 14 μm, 14.5 μm, 15 μm, or a range consisting of any two of the above values.

[0218] When the thickness of the positive electrode current collector is within the above range, the current-carrying capacity of the positive electrode current collector is excellent, and it can enable the battery cell to have a high energy density.

[0219] In the embodiments of this application, the thickness of the positive electrode film and the positive electrode current collector are known in the art and can be detected using equipment and methods known in the art. For example, the thickness of the positive electrode sheet can be measured with a micrometer, the film layer on the surface of the positive electrode current collector can be removed, and the thickness of the positive electrode current collector can be measured with a micrometer. When the positive electrode film is coated on one side, the thickness of the positive electrode film is the thickness of the positive electrode sheet minus the thickness of the positive electrode current collector. When the positive electrode film is coated on both sides, the thickness of the positive electrode film is (the thickness of the positive electrode sheet minus the thickness of the positive electrode current collector) / 2.

[0220] The positive electrode film is typically formed by coating a positive electrode slurry onto a positive electrode current collector, followed by drying and cold pressing. The positive electrode slurry is usually formed by dispersing the positive electrode active material, optional conductive agent, optional binder, and any other components in a solvent and stirring until homogeneous. The solvent can be N-methylpyrrolidone (NMP), but is not limited to it.

[0221] The positive electrode sheet does not exclude other additional functional layers besides the positive electrode film layer. For example, in some embodiments, the positive electrode sheet of this application further includes a positive conductive layer sandwiched between the positive current collector and the positive electrode film layer and disposed on the surface of the positive current collector. In other embodiments, the positive electrode sheet of this application further includes a protective layer covering the surface of the positive electrode film layer.

[0222] In some embodiments, the positive electrode further includes a positive conductive layer located between the positive electrode film and the positive current collector. The positive conductive layer can further improve the conductivity of the positive electrode and reduce the heat generation of the positive electrode, thereby reducing the heat generation of the battery cell.

[0223] In some embodiments, the thickness of the positive electrode conductive layer is from 0.5 μm to 2 μm. For example, the thickness of the positive electrode conductive layer can be 0.5 μm, 0.8 μm, 1 μm, 1.2 μm, 1.5 μm, 1.6 μm, 1.8 μm, 2 μm, or any combination of two of the above values.

[0224] When the thickness of the positive electrode conductive layer is within the above range, the conductivity of the positive electrode sheet can be further improved, the heat generation of the positive electrode sheet can be reduced, thereby reducing the heat generation of the battery cell, and the energy density of the battery cell can also be improved.

[0225] In the embodiments of this application, the thickness of the positive electrode conductive layer has a meaning known in the art and can be detected using equipment and methods known in the art, such as performing a tomographic scan on the positive electrode sheet to directly measure the thickness of the positive electrode conductive layer.

[0226] In some embodiments, the positive conductive layer includes one or more of a positive conductive agent and a positive binder.

[0227] Optionally, the mass content of the positive electrode conductive agent in the positive electrode conductive layer is 30% to 50%. For example, the mass content of the positive electrode conductive agent is 30%, 35%, 40%, 45%, 50%, or any combination of two of the above values.

[0228] For example, the positive electrode conductive agent includes one or more of superconducting carbon, conductive graphite, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. The positive electrode conductive agent in the positive electrode conductive layer can improve the conductivity of the positive electrode conductive layer, thereby improving the conductivity of the positive electrode sheet and reducing the heat generation of the battery cell.

[0229] Optionally, the positive electrode binder has a mass content of 50% to 70% in the positive electrode conductive layer. For example, 50%, 60%, 65%, 70%, or any combination of two of the above values.

[0230] For example, the positive electrode binder includes one or more of polyvinylidene fluoride, polytetrafluoroethylene, a terpolymer of vinylidene fluoride-tetrafluoroethylene-propylene, a terpolymer of vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene, a tetrafluoroethylene-hexafluoropropylene copolymer, polyacrylic acid, and fluorinated acrylate resins. The positive electrode binder in the positive electrode conductive layer can improve the adhesion between the positive electrode current collector and the positive electrode film layer, thereby enhancing the structural stability of the positive electrode sheet.

[0231] [Negative electrode plate]

[0232] The negative electrode sheet includes a negative current collector and a negative electrode film layer disposed on at least one surface of the negative current collector and comprising a negative electrode active material. For example, the negative current collector has two surfaces opposite each other in its thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative current collector.

[0233] In some embodiments, the compaction density of the negative electrode film layer is 1.15 g / cm³ when the battery cell is 100% charged. 3 Up to 1.36 g / cm 3 The option is 1.25g / cm³. 3 Up to 1.36 g / cm 3 For example, the compaction density of the negative electrode film layer of a single battery cell at 100% charge is 1.15 g / cm³. 3 1.18 g / cm 3 1.20g / cm 3 1.22g / cm 3 1.25g / cm 3 1.28g / cm 3 1.3g / cm 3 1.32g / cm 3 1.35g / cm 3 1.36 g / cm 3 Or a range consisting of any two of the above values.

[0234] When the compaction density of the negative electrode film is within the above range, it is beneficial to improve the energy density of the battery cell. Furthermore, since the negative electrode active material in the negative electrode film is packed more tightly, the contact resistance between particles is smaller, which can further reduce the resistance of the electrode sheet, thereby reducing heat generation.

[0235] In the embodiments of this application, the compaction density of the negative electrode film layer of a single battery cell under 100% charge state has a well-known meaning in the art and can be detected using well-known equipment and methods in the art. The detection method is as described above for the compaction density test method of the positive electrode film layer.

[0236] In some embodiments, the single-sided coating weight of the negative electrode film is 90 mg / 1540.25 mm. 2 Up to 170mg / 1540.25mm 2 The option is 110mg / 1540.25mm. 2 Up to 150mg / 1540.25mm 2 For example, the single-sided coating weight of the negative electrode film is 90 mg / 1540.25 mm. 2 92mg / 1540.25mm 2 95mg / 1540.25mm 2 96mg / 1540.25mm 2 100mg / 1540.25mm 2 102mg / 1540.25mm 2 104mg / 1540.25mm 2 105mg / 1540.25mm 2 108mg / 1540.25mm 2 110mg / 1540.25mm 2 112mg / 1540.25mm 2 114mg / 1540.25mm 2 115mg / 1540.25mm 2 116mg / 1540.25mm 2 118mg / 1540.25mm 2 120mg / 1540.25mm 2 122mg / 1540.25mm 2 125mg / 1540.25mm 2 128mg / 1540.25mm 2 130mg / 1540.25mm 2 132mg / 1540.25mm 2 135mg / 1540.25mm2 137mg / 1540.25mm 2 140mg / 1540.25mm 2 142mg / 1540.25mm 2 145mg / 1540.25mm 2 148mg / 1540.25mm 2 150mg / 1540.25mm 2 152mg / 1540.25mm 2 155mg / 1540.25mm 2 160mg / 1540.25mm 2 165mg / 1540.25mm 2 170mg / 1540.25mm 2 Or a range consisting of any two of the above values.

[0237] When the single-sided coating weight of the negative electrode film is within the above range, the heat generation per unit area of ​​the negative electrode sheet will not be too large, and the energy density of the battery cell can be improved at the same time.

[0238] In the embodiments of this application, the single-sided coating weight of the negative electrode film layer has a meaning known in the art and can be detected using equipment and methods known in the art, such as the single-sided coating weight test method of the film layer described above.

[0239] In some embodiments, the resistivity of the negative electrode active material powder is from 0.005 Ω·cm to 0.043 Ω·cm, optionally 0.04 Ω·cm. Exemplarily, the resistivity of the negative electrode active material powder can be 0.043 Ω·cm, 0.04 Ω·cm, 0.035 Ω·cm, 0.03 Ω·cm, 0.025 Ω·cm, 0.02 Ω·cm, 0.015 Ω·cm, 0.01 Ω·cm, 0.005 Ω·cm, or a range consisting of any two of the above values.

[0240] The powder resistivity of the negative electrode active material is relatively low, which results in relatively low resistance of the negative electrode sheet and less heat generation in the battery cell.

[0241] In the embodiments of this application, the powder resistivity of the negative electrode active material has a well-known meaning in the art and can be detected using equipment and methods well-known in the art, such as the powder resistivity test method for the positive electrode active material described above.

[0242] In some embodiments, the powder compaction density of the negative electrode active material under a pressure of 20,000 N is 1.5 g / cm³. 3 Up to 1.85 g / cm 31.55g / cm³ is an optional value. 3 Up to 1.65 g / cm 3 For example, the compacted density of the negative electrode active material powder under a pressure of 20000N is 1.5 g / cm³. 3 1.55g / cm 3 1.6g / cm 3 1.65g / cm 3 1.7g / cm 3 1.75g / cm 3 1.8g / cm 3 1.85g / cm 3 Or a range consisting of any two of the above values.

[0243] When the powder compaction density of the negative electrode active material at 20000N is within the above range, it can improve the energy density of the battery cell. Furthermore, since the negative electrode active material in the negative electrode film can be stacked more tightly, the contact resistance between particles is smaller, which can further reduce the resistance of the electrode sheet, thereby reducing heat generation.

[0244] In this application, the powder compaction density of the material has a meaning known in the art and can be tested using methods and equipment known in the art, according to the testing standard GB / T24533-2009. As an example, a certain amount of negative electrode active material is taken as a sample and added to a UTM7305 electronic pressure testing machine with a base area of ​​1.327 cm². 2 In the mold, the pressure is increased to 2000 kg (equivalent to 20000 N), held for 30 s, then depressurized and held for 10 s. The compaction density of the negative electrode active material under a force of 20000 N is then recorded and calculated.

[0245] In some embodiments, the specific charging capacity of the negative electrode active material at a 0.1C rate is greater than or equal to 350 mAh / g, and can be selected from 350 mAh / g to 480 mAh / g. Exemplarily, the specific charging capacity of the negative electrode active material at a 0.1C rate is 350 mAh / g, 355 mAh / g, 360 mAh / g, 365 mAh / g, 370 mAh / g, 375 mAh / g, 380 mAh / g, 385 mAh / g, 390 mAh / g, 395 mAh / g, 400 mAh / g, 410 mAh / g, 420 mAh / g, 430 mAh / g, 440 mAh / g, 450 mAh / g, 460 mAh / g, 470 mAh / g, 480 mAh / g, or a range of any two of the above values.

[0246] When the charge capacity of the negative electrode active material at a 0.1C rate is within the above range, the energy density of the battery cell is relatively high.

[0247] In the embodiments of this application, the charging capacity of the negative electrode active material at a rate of 0.1C is a well-known concept in the art and can be detected using well-known equipment and methods in the art. The detection method is as described above for the charging capacity test method of the positive electrode active material at a rate of 0.1C.

[0248] In some embodiments, the negative electrode active material includes a carbon-based material, which has high cycle stability and can improve the cycle performance of the battery cell. Optionally, the mass percentage of the carbon-based material in the negative electrode active material can be greater than or equal to 80% and less than or equal to 100%.

[0249] The positive electrode active material of this application is mainly a lithium phosphate system with an olivine structure, and the negative electrode active material is mainly a carbon-based material system. When used together, the cycle performance of the battery cell is excellent.

[0250] Optionally, the carbon-based material includes graphite particles with a graphitization degree of 92.0% to 94.5%. Exemplarily, the graphitization degree of the graphite particles is 92.0%, 92.5%, 93%, 93.5%, 94%, 94.5%, or a range consisting of any two of the above values.

[0251] When the degree of graphitization of graphite particles is within the above range, the graphite particles have excellent electrical conductivity, which can reduce the heat generation of the negative electrode sheet and the heat generation of the battery cell; and can also improve the fast charging performance of the battery cell.

[0252] In some embodiments, the graphite particles include artificial graphite and a carbon coating layer. The artificial graphite includes secondary particles, which in turn include multiple primary particles aggregated to form secondary particles. The carbon coating layer coats the surface of the artificial graphite. Amorphous carbon refers to transitional carbon materials with a very low degree of graphitization and crystallization, exhibiting an approximately amorphous morphology (or lacking a fixed shape and periodic structural regularity). In this application, amorphous carbon refers to the product of carbonization treatment of an organic carbon source.

[0253] Artificial graphite includes secondary particles. There are more migration paths for lithium ions in artificial graphite, and the migration paths in primary particles are shorter, which can improve the migration rate of lithium ions. The carbon coating layer has more end faces and defects, which increases the number of sites where lithium ions can be inserted or extracted. This makes the carbon coating layer more conductive, which can reduce the internal resistance of the negative electrode and reduce the heat generation of the battery cell.

[0254] Optionally, the carbon coating content is 2% to 5% by mass, based on the mass of the graphite particles. For example, the carbon coating content is 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, or any range of two of the above values.

[0255] When the mass content of the carbon coating is within the above range, it can further reduce the internal resistance of the negative electrode and reduce the heat generation of the battery cell.

[0256] In the embodiments of this application, the graphite particles can be prepared using methods known in the art, such as: providing artificial graphite and an organic carbon source, mixing the two, and then carbonizing them to form a carbon coating layer on at least a portion of the surface of the artificial graphite particles.

[0257] Optionally, the organic carbon source includes one or more of coal tar pitch, petroleum asphalt, phenolic resin, and coconut shell. More preferably, the organic carbon source includes petroleum asphalt. Optionally, the softening point of the coal tar pitch or petroleum asphalt is below 250°C.

[0258] Optionally, the carbonization temperature is between 700°C and 1800°C. Optionally, the carbonization temperature is between 1000°C and 1300°C. Within a suitable range, the carbonization temperature allows the organic carbon source to be carbonized, forming a coating layer containing amorphous carbon on at least a portion of the surface of the artificial graphite.

[0259] Optionally, the carbonization treatment time is 1 hour to 6 hours.

[0260] In this application, the morphology and mass content of amorphous carbon have meanings known in the art and can be detected using equipment and methods known in the art. For example, the amorphous carbon layer can be confirmed using conventional methods for testing the crystal morphology of carbon materials, such as Raman spectroscopy, which can be used to analyze the characteristic peak information of carbon components in the spectrum (such as the intensity ratio of the D peak to the G peak, I...). D / G The formation of amorphous carbon layers was analyzed. The D and G peaks are Raman characteristic peaks of carbon atom crystals. The D peak represents defects in the carbon atom crystal; the more defects, the greater the intensity of the D peak. The intensity of the D peak reflects the content of amorphous (randomly stacked) regions. The G peak represents the in-plane stretching vibration of carbon atom sp2 hybridization; the intensity of the G peak reflects the content of graphitized (layered structure) regions. As the degree of carbon atom disorder increases, the intensity ratio of the D peak to the G peak also increases. This can be determined based on the Ig of the particle surface composition before and after coating. D / G Whether the intensity ratio of the D peak to the G peak constitutes a statistically significant difference (e.g., p < 0.05, p < 0.01, etc.), that is, based on the layered structure introduced after coating and the I before coating... D / G The degree of deviation is used to determine whether an amorphous carbon layer has formed, which is easily judged by those skilled in the art. The Raman spectra of specific structural layers of the particles under test can also be compared. D / G Standard Raman spectrum of graphite I D / G The differences between them are used to determine whether it is an amorphous carbon layer.

[0261] The degree of graphitization of graphite particles can be detected using the same methods used for detecting the degree of graphitization of positive electrode active materials.

[0262] In this embodiment, the mass percentage of the "amorphous carbon layer" in the graphite particles can be obtained through transmission electron microscopy (TEM) morphology observation combined with data analysis. The negative electrode sheet is cut, and powder samples are scraped from the cross-section for TEM testing. By comparing graphite particles coated with and without the amorphous carbon layer, a clear interface can be observed on the surface of the graphite particles coated with the amorphous carbon layer. Therefore, the thickness of the amorphous carbon layer at that location can be estimated from the TEM image. Analysis can be performed at multiple different cross-sectional locations, and the average value is taken as the thickness of the amorphous carbon layer. Assuming the graphite particles are spherical, based on the thickness of the amorphous carbon layer and statistical data on the particle size of the graphite particles, the volume percentage of the amorphous carbon layer in the carbon-based particles can be estimated based on the volume of the carbon-based particles and the volume of the carbon-based core. Furthermore, based on the average mass of a single graphite particle, the volume of artificial graphite (which can be calculated or obtained statistically from TEM images), and the density of artificial graphite, the mass content of the amorphous carbon layer in the graphite particles can be estimated.

[0263] In some embodiments, the carbon-based material may further include natural graphite. Specifically, the carbon-based material may include graphite particles, or it may include both graphite particles and natural graphite. Optionally, the carbon-based material is graphite particles.

[0264] In some embodiments, the negative electrode active material may also include a silicon-based material. The introduction of silicon-based materials can increase the capacity of the negative electrode active material and improve the energy density of the battery cell.

[0265] Optionally, based on the mass of the negative electrode active material, the mass content of silicon element in the silicon-based material is 0.3% to 10.0%, optionally 1% to 6%. Exemplarily, the mass content of silicon element in the silicon-based material is 0.3%, 0.5%, 0.8%, 1%, 1.2%, 1.5%, 1.8%, 2%, 2.2%, 2.5%, 2.8%, 3%, 3.2%, 3.5%, 3.8%, 4%, 4.2%, 4.5%, 4.8%, 5%, 5.2%, 5.5%, 5.8%, 6%, 6.2%, 6.5%, 6.8%, 7%, 7.2%, 7.5%, 7.8%, 8%, 8.2%, 8.5%, 8.8%, 9%, 9.2%, 9.5%, 9.8%, 10%, or a range consisting of any two of the above values.

[0266] When the mass content of silicon in silicon-based materials is within the above range, it can improve the capacity of the negative electrode active material and make the cycle stability of the negative electrode active material better, thereby improving the energy density and cycle performance of the battery cell.

[0267] Optionally, the silicon-based material may include at least one of elemental silicon, silicon oxide, silicon-carbon composite, silicon-nitrogen composite, and silicon alloy.

[0268] In some embodiments, the negative electrode active material may include, in addition to the carbon-based materials and optionally silicon-based materials described above, at least one of tin-based materials and lithium titanate. Tin-based materials may include at least one of elemental tin, tin oxides, and tin alloys.

[0269] The qualitative and quantitative analysis of each substance or element in this application can be performed using suitable equipment and methods known to those skilled in the art. Relevant testing methods can be referenced from domestic and international testing standards and enterprise standards. Furthermore, those skilled in the art can adaptively modify certain testing steps / instrument parameters from the perspective of testing accuracy to obtain more accurate results. One testing method can be used for qualitative or quantitative analysis, or several testing methods can be used in combination for qualitative or quantitative determination.

[0270] For example, the carbon-based materials in this application can be subjected to X-ray powder diffraction tests and qualitative analysis on the negative electrode sheet or negative electrode active material in accordance with the general rules of X-ray diffraction analysis in JIS / K0131-1996.

[0271] Artificial graphite and natural graphite can be distinguished by SEM cross-sectional images taken by scanning electron microscope (SEM). Natural graphite has gaps between the sheet-like structures in its SEM cross-section, while artificial graphite has a dense and seamless SEM cross-section. Alternatively, they can be distinguished by XRD patterns obtained by X-ray diffraction (XRD). Natural graphite has obvious 2H and 3R phases in its XRD pattern, while artificial graphite only has the 2H phase in its XRD pattern.

[0272] In the embodiments of this application, the negative electrode film layer includes at least one film layer, which can be a single film layer or at least two film layers. Optionally, the negative electrode film layer includes at least two film layers.

[0273] When a single-layer negative electrode film is used, the negative electrode active material in the negative electrode film includes a carbon-based material, and optionally also includes a silicon-based material. When a single-layer film is used, the volume average particle size Dv50 of the negative electrode active material is from 8.2 μm to 13.5 μm. Exemplarily, the volume average particle size Dv50 of the negative electrode active material is 8.2 μm, 8.5 μm, 8.8 μm, 9 μm, 9.2 μm, 9.5 μm, 9.8 μm, 10 μm, 10.2 μm, 10.5 μm, 10.8 μm, 11 μm, 11.2 μm, 11.5 μm, 11.8 μm, 12 μm, 12.2 μm, 12.5 μm, 12.8 μm, 13 μm, 13.2 μm, 13.5 μm, or a range consisting of any two of the above values.

[0274] When the negative electrode film layer comprises at least two film layers, the negative electrode active material in the negative electrode film layer includes a carbon-based material, and optionally also includes a silicon-based material. The silicon-based material may be located in one of the at least two film layers, or in at least two of the at least two film layers. The negative electrode film layer may include two film layers, three film layers, four film layers, or even more film layers.

[0275] In some embodiments, the negative electrode film layer includes a first negative electrode film layer and a second negative electrode film layer. The first negative electrode film layer is disposed on the surface of the negative electrode current collector, and the carbon-based material in the first negative electrode film layer includes graphite particles. The second negative electrode film layer is connected to the side of the first negative electrode film layer away from the negative electrode current collector, and the carbon-based material in the second negative electrode film layer includes graphite particles.

[0276] The interface between the first negative electrode film and the second negative electrode film can be regular or irregular; it may optionally be irregular.

[0277] Optionally, the carbon-based material in the first negative electrode film layer may also include natural graphite.

[0278] The negative electrode film consists of at least two layers, and layered coating is beneficial for improving the fast charging performance of the battery cell. In particular, when there are differences between the first and second negative electrode films, it can create porosity differences in the negative electrode films, reduce the tortuosity of lithium-ion transport, and improve the fast charging performance of the battery cell.

[0279] Optionally, the volume average particle size Dv50 of the negative electrode active material in the first negative electrode film layer is greater than or equal to the volume average particle size Dv50 of the negative electrode active material in the second negative electrode film layer. More preferably, the volume average particle size Dv50 of the negative electrode active material in the first negative electrode film layer is greater than the volume average particle size Dv50 of the negative electrode active material in the second negative electrode film layer, which is beneficial for increasing the compaction density of the negative electrode film layer. When the negative electrode active material includes graphite particles, the volume average particle size Dv50 of the graphite particles in the first negative electrode film layer is greater than or equal to the volume average particle size Dv50 of the graphite particles in the second negative electrode film layer.

[0280] The difference in particle size between the first and second negative electrode layers can improve the fast charging performance of the battery cell. Specifically, during fast charging, the overpotential of the second negative electrode layer is usually higher, and the bottleneck of fast charging is mainly in the second negative electrode layer. However, in the embodiments of this application, the particle size of the second negative electrode layer is relatively small, which can shorten the solid-phase transport path of lithium ions, improve fast charging performance, and improve the problem of lithium deposition on the surface of the negative electrode sheet.

[0281] Optionally, the negative electrode active material in the first negative electrode film layer is particulate, and its volume average particle size Dv50 is 9.5 μm to 18.5 μm, optionally 9.5 μm to 14.6 μm. Exemplarily, the volume average particle size of the negative electrode active material in the first negative electrode film layer is 9.5 μm, 10 μm, 10.5 μm, 11 μm, 11.5 μm, 12 μm, 12.5 μm, 13 μm, 13.5 μm, 14 μm, 14.5 μm, 14.6 μm, 15 μm, 15.5 μm, 16 μm, 16.5 μm, 17 μm, 17.5 μm, 18 μm, 18.5 μm, or a range consisting of any two of the above values. When the first negative electrode film layer includes graphite particles, the volume average particle size Dv50 of the graphite particles in the first negative electrode film layer is 9.5 μm to 18.5 μm, and can be selected as 9.5 μm to 14.6 μm.

[0282] When the volume average particle size Dv50 of the negative electrode active material in the first negative electrode film layer is within the above range, it can shorten the solid-phase transport path of lithium ions and improve fast charging performance. On the other hand, the material is less prone to agglomeration during the preparation process, which can improve the stability of the material.

[0283] Optionally, the negative electrode active material in the second negative electrode film layer is particulate, and its volume average particle size Dv50 is 7.8 μm to 14.3 μm, optionally 7.8 μm to 11.3 μm. For example, the volume average particle size Dv50 of the negative electrode active material is 7.8 μm, 8.0 μm, 8.2 μm, 8.5 μm, 8.8 μm, 9 μm, 9.2 μm, 9.5 μm, 9.8 μm, 10 μm, 10.2 μm, 10.5 μm, 10.8 μm, 11 μm, 11.3 μm, 11.2 μm, 11.5 μm, 11.8 μm, 12 μm, 12.2 μm, 12.5 μm, 12.8 μm, 13 μm, 13.2 μm, 13.5 μm, 13.8 μm, 14 μm, 14.1 μm, 14.3 μm, or a range of any two of the above values. When the second negative electrode film layer includes graphite particles, the volume average particle size Dv50 of the graphite particles in the second negative electrode film layer is 7.8 μm to 14.3 μm, and can be selected as 7.8 μm to 11.3 μm.

[0284] When the volume average particle size Dv50 of the negative electrode active material in the second negative electrode film is within the above-mentioned range, it can shorten the solid-phase transport path of lithium ions and improve fast charging performance. On the other hand, the material is less prone to agglomeration during the preparation process, which can improve the stability of the material. Furthermore, the combination of the negative electrode active material in the second negative electrode film within the above-mentioned volume average particle size range and the negative electrode active material in the first negative electrode film is conducive to building a gradient porosity difference between the second negative electrode film and the first negative electrode film, reducing the tortuosity of lithium ion transport, and improving the fast charging performance of the battery cell.

[0285] In the embodiments of this application, the volume average particle size Dv50 of the negative electrode active material has a meaning known in the art and can be detected using equipment and methods known in the art, such as the volume average particle size Dv50 test method for positive electrode active materials described above.

[0286] Optionally, the tap density of the carbon-based material in the first negative electrode film layer is less than or equal to the tap density of the carbon-based material in the second negative electrode film layer. Tap density reflects the compactness of the active material within the film layer. When the tap density of the carbon-based material in the second negative electrode film layer is greater than that in the first negative electrode film layer, the second negative electrode film layer is more densely packed, thus improving the energy density of the battery cell. Conversely, the first negative electrode film layer is relatively sparsely packed with more pores, which enhances the fast-charging performance of the battery cell. When the negative electrode active material includes graphite particles, the tap density of the graphite particles in the first negative electrode film layer is less than or equal to that in the second negative electrode film layer.

[0287] Optionally, the tap density of the carbon-based material in the first negative electrode film layer is 0.82 g / cm³. 3 Up to 1.21 g / cm 3 For example, 0.82 g / cm³ 3 0.85g / cm 3 0.88g / cm 3 0.90g / cm 3 0.92g / cm 3 0.95g / cm 3 0.98g / cm 3 1.00g / cm 3 1.05g / cm 3 1.08g / cm 3 1.10 g / cm 3 1.12 g / cm 3 1.15g / cm 3 1.18 g / cm 3 1.20g / cm 3 1.21 g / cm 3Or it can be a range consisting of any two of the above values. When the tap density of the carbon-based material in the first negative electrode film is within a suitable range, it can improve the fast charging performance of the battery cell.

[0288] Optionally, the tap density of the carbon-based material in the second negative electrode film is 0.90 g / cm³. 3 Up to 1.25 g / cm 3 For example, 0.90 g / cm³ 3 0.92g / cm 3 0.95g / cm 3 0.98g / cm 3 1.00g / cm 3 1.05g / cm 3 1.08g / cm 3 1.10 g / cm 3 1.12 g / cm 3 1.15g / cm 3 1.18 g / cm 3 1.20g / cm 3 1.21 g / cm 3 1.22g / cm 3 1.23g / cm 3 1.24 g / cm 3 1.25g / cm 3 Or it can be a range consisting of any two of the above values. When the tap density of the carbon-based material in the second negative electrode film is within a suitable range, it can improve the energy density of the battery cell.

[0289] In the embodiments of this application, the tap density of the material has a meaning known in the art and can be determined using instruments and methods known in the art. For example, it can be determined using a powder tap density tester according to GB / T5162-2006. The testing instrument can be Dandong Baite BT-301.

[0290] Optionally, the thickness ratio of the second negative electrode film to the first negative electrode film is 3:7 to 7:3, and optionally 4:6 to 5:5. For example, the thickness ratio of the second negative electrode film to the first negative electrode film is 3:7, 4:6, 5:5, 6:4, 7:3, or any range of two of the above values. By adjusting the thickness ratio of the first and second negative electrode films, the gradient porosity difference between the upper and lower layers can be further increased, reducing the tortuosity of lithium-ion transport and improving the fast charging capability of the battery cell.

[0291] In some embodiments, after 10 full-charge cycles during the Beginning of Life (BOL) test, the thickness of the first negative electrode film layer is between 15 μm and 65 μm, for example, 15 μm, 17 μm, 18 μm, 20 μm, 25 μm, 30 μm, 35 μm, 40 μm, 45 μm, 50 μm, 55 μm, 57 μm, 58 μm, 60 μm, 61 μm, 62 μm, 63 μm, 64 μm, 65 μm, or any combination of two of the above values. When the thickness of the first negative electrode film layer is within the above range, the first and second negative electrode film layers can be modulated to increase the gradient porosity difference between the upper and lower layers, reduce the tortuosity of lithium-ion transport, and improve the fast-charging capability of the battery cell.

[0292] In some implementations, after 10 full-charge cycles during the Beginning of Life (BOL) test, the thickness of the second negative electrode film is between 15 μm and 65 μm, for example, 15 μm, 17 μm, 18 μm, 20 μm, 25 μm, 30 μm, 35 μm, 40 μm, 45 μm, 50 μm, 55 μm, 57 μm, 58 μm, 60 μm, 61 μm, 62 μm, 63 μm, 64 μm, 65 μm, or any combination of two of the above values. When the thickness of the second negative electrode film is within the above range, the first and second negative electrode films can be modulated to increase the gradient porosity difference between the upper and lower layers, reduce the tortuosity of lithium-ion transport, and improve the fast-charging capability of the battery cell.

[0293] In the embodiments of this application, for example, the upper limit charging voltage of a single battery cell is 3.65V and the lower limit discharging voltage of a single battery cell is 2.0V, as described below.

[0294] The BOL full charge test procedure is as follows: At 25℃, charge the battery to 3.65V at a charging rate of 0.33C (the nominal capacity), then charge it to 0.05C at a constant voltage of 3.65V, let it rest for 10 minutes, then discharge it to 2.0V at a discharging rate of 0.33C, let it rest for 10 minutes. One charge-discharge cycle constitutes one cycle; complete 10 cycles. Then charge the battery to 3.65V again at a charging rate of 0.33C (the nominal capacity), and then charge it to 0.05C at a constant voltage of 3.65V. With the BOL fully charged, the negative electrode sheet is disassembled. A tomographic scanning electron microscope is used to observe the cross-section of the thickness direction of the middle region of the negative electrode sheet. The two regions are distinguished according to the interface between the first negative electrode film layer and the second negative electrode film layer. The thickness of each is measured separately. For example, the thickness of the first negative electrode film layer is measured at 10 locations, and the average value is calculated as the average value of the first negative electrode film layer. The thickness of the second negative electrode film layer is measured at 10 locations, and the average value is calculated as the average value of the second negative electrode film layer.

[0295] In some embodiments, after a full-charge test at the end of life (EOL) of the battery cell, the thickness of the first negative electrode film is between 15 μm and 70 μm, for example, 15 μm, 16 μm, 17 μm, 18 μm, 19 μm, 20 μm, 21 μm, 22 μm, 23 μm, 24 μm, 25 μm, 26 μm, 27 μm, 28 μm, 29 μm, 30 μm, 35 μm, 40 μm, 43 μm, 45 μm, 50 μm, 55 μm, 60 μm, 65 μm, 66 μm, 67 μm, 68 μm, 69 μm, 70 μm, or any combination of two of the above values. When the thickness of the first negative electrode film is within the above range, the first and second negative electrode films can be modulated to increase the gradient porosity difference between the upper and lower layers, reduce the tortuosity of lithium-ion transport, and improve the fast charging capability of the battery cell.

[0296] In some embodiments, after the battery cell undergoes an end-of-life (EOL) full-charge test, the thickness of the second negative electrode film is between 15 μm and 70 μm, for example, 15 μm, 16 μm, 17 μm, 18 μm, 19 μm, 20 μm, 21 μm, 22 μm, 23 μm, 24 μm, 25 μm, 26 μm, 27 μm, 28 μm, 29 μm, 30 μm, 35 μm, 40 μm, 43 μm, 45 μm, 50 μm, 55 μm, 60 μm, 65 μm, 66 μm, 67 μm, 68 μm, 69 μm, 70 μm, or any combination of two of the above values. When the thickness of the second negative electrode film is within the above range, the first and second negative electrode films can be modulated to increase the gradient porosity difference between the upper and lower layers, reduce the tortuosity of lithium-ion transport, and improve the fast charging capability of the battery cell.

[0297] In the embodiments of this application, for example, the upper limit charging voltage of a single battery cell is 3.65V and the lower limit discharging voltage of a single battery cell is 2.0V, as described below.

[0298] The specific steps for the EOL full charge test are as follows:

[0299] At 60℃, charge the battery to 3.65V at a charging rate of 0.33C (the nominal capacity), then charge it to 0.05C at a constant voltage of 3.65V, let it stand for 10 minutes, then discharge it to 2.0V at a discharging rate of 0.33C, and let it stand for 10 minutes. One charge-discharge cycle is one cycle. Continue the test until the battery capacity decreases to 80% of the nominal capacity. Then, at 25°C, it is charged to 3.65V with a constant current of 0.33C and then charged to 3.65V with a constant voltage of 0.05C, which is the EOL full charge state. In the EOL full charge state, the negative electrode is disassembled, and the cross-section of the thickness direction of the middle region of the negative electrode is observed using a tomographic scanning electron microscope. The two regions are distinguished according to the interface between the first negative electrode film layer and the second negative electrode film layer. The thickness of each is measured. For example, the thickness of the first negative electrode film layer is measured at 10 locations, and the average value is calculated as the average value of the first negative electrode film layer; the thickness of the second negative electrode film layer is measured at 10 locations, and the average value is calculated as the average value of the second negative electrode film layer.

[0300] In some embodiments, when the negative electrode film is a single layer, the negative electrode film further includes a lithium-containing binder. Optionally, the mass content of the lithium-containing binder relative to the negative electrode film is 0.1% to 1%. Exemplarily, the mass content of the lithium-containing binder relative to the negative electrode film is 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, or any combination of two of the above values. The lithium element in the lithium-containing binder can exist in ionic form, which can increase the number of freely moving lithium ions in the negative electrode film, shorten the distance that lithium ions diffuse to the surface of the negative electrode film, improve the lithium ion insertion / extraction rate, and improve the fast charging performance of the battery cell. Optionally, the negative electrode film layer may further include a negative electrode binder, such as at least one of styrene-butadiene rubber (SBR), water-soluble unsaturated resin SR-1B, waterborne acrylic resins (e.g., polyacrylic acid PAA, polymethacrylic acid PMAA, sodium polyacrylate PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), and carboxymethyl chitosan (CMCS).

[0301] Optionally, the lithium content in the lithium-containing binder is 3% to 10% by mass. For example, the lithium content in the lithium-containing binder is 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or any combination of two of the above values. The lithium content is calculated based on the mass of the lithium-containing binder. When the lithium content is within the above range, a relatively large number of freely moving lithium ions can be achieved in the negative electrode film, further shortening the distance that lithium ions diffuse to the surface of the negative electrode film, increasing the lithium ion insertion / extraction rate, and improving the fast-charging performance of the battery cell.

[0302] For example, lithium-containing binders include lithium acrylate-acrylonitrile-acrylamide-hydroxyethyl acrylate copolymers, which are derived from lithium acrylate monomers, acrylonitrile monomers, acrylamide monomers, and hydroxyethyl acrylate monomers, wherein the molar ratio of lithium acrylate monomers, acrylonitrile monomers, acrylamide monomers, and hydroxyethyl acrylate monomers is 30% to 50%: 15% to 45%: 5% to 20%: 20% to 35%. For example, the molar ratio of lithium acrylate monomers, acrylonitrile monomers, acrylamide monomers, and hydroxyethyl acrylate monomers is 35%: 30%: 15%: 20%, or 40%, 20%, 10%, 30%, or 45%, 15%, 20%, 20%, etc.

[0303] The lithium-containing binder of the above-mentioned material can provide a certain number of lithium ions to the negative electrode film layer, thereby improving the fast charging performance of the battery cell; moreover, it is not prone to swelling during charging and discharging, and its structure is stable, which improves the cycle performance of the negative electrode film layer during fast charging and discharging.

[0304] In other embodiments, where the negative electrode film layer comprises at least two layers, the negative electrode film layer further includes a lithium-containing binder.

[0305] Optionally, the first negative electrode film layer further includes a first lithium-containing binder, and the second negative electrode film layer further includes a second lithium-containing binder, wherein the mass content of the first lithium-containing binder relative to the mass of the first negative electrode film layer is less than or equal to the mass content of the second lithium-containing binder relative to the mass of the second negative electrode film layer. More optionally, the mass content of the first lithium-containing binder relative to the mass of the first negative electrode film layer is less than the mass content of the second lithium-containing binder relative to the mass of the second negative electrode film layer.

[0306] The second lithium-containing binder has a relatively high mass content in the second negative electrode film layer. The second lithium-containing binder provides a relatively larger number of freely movable lithium ions to the second negative electrode film layer, which can further improve the fast charging performance of the battery cell.

[0307] Optionally, the mass content of the first lithium-containing binder relative to the first negative electrode film layer is 0.1% to 1%. For example, the mass content of the first lithium-containing binder relative to the first negative electrode film layer is 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, or any combination of two of the above values. The lithium element in the first lithium-containing binder can exist in ionic form, which can increase the number of freely moving lithium ions in the negative electrode film layer, shorten the distance for lithium ions to diffuse to the surface of the negative electrode film layer, improve the lithium ion insertion / extraction rate, and improve the fast charging performance of the battery cell.

[0308] Optionally, the lithium content in the first lithium-containing binder is 3% to 10% by mass, and optionally 3% to 8%. For example, the lithium content in the first lithium-containing binder is 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or any combination of two of the above values. When the lithium content is within the above range, a relatively large number of freely moving lithium ions can be achieved in the negative electrode film layer, further shortening the distance that lithium ions diffuse to the surface of the negative electrode film layer, increasing the lithium ion insertion / extraction rate, and improving the fast charging performance of the battery cell.

[0309] For example, the first lithium-containing binder comprises a lithium acrylate-acrylonitrile-acrylamide-hydroxyethyl acrylate copolymer, which is derived from lithium acrylate monomer, acrylonitrile monomer, acrylamide monomer, and hydroxyethyl acrylate monomer, wherein the molar ratio of lithium acrylate monomer, acrylonitrile monomer, acrylamide monomer, and hydroxyethyl acrylate monomer is 30% to 50%: 15% to 45%: 5% to 20%: 20% to 35%. For example, the molar ratio of lithium acrylate monomer, acrylonitrile monomer, acrylamide monomer, and hydroxyethyl acrylate monomer is 35%: 30%: 15%: 20%, or 40%, 20%, 10%, 30%, or 45%, 15%, 20%, 20%, etc.

[0310] The lithium-containing binder of the above-mentioned material can provide a certain number of lithium ions to the negative electrode film layer, thereby improving the fast charging performance of the battery cell; moreover, it is not prone to swelling during charging and discharging, and its structure is stable, which improves the cycle performance of the negative electrode film layer during fast charging and discharging.

[0311] Optionally, the mass content of the second lithium-containing binder relative to the second negative electrode film layer is 0.1% to 1%. For example, the mass content of the second lithium-containing binder relative to the second negative electrode film layer is 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, or any combination of two of the above values. The lithium element in the second lithium-containing binder can exist in ionic form, which can increase the number of freely moving lithium ions in the negative electrode film layer, shorten the distance for lithium ions to diffuse to the surface of the negative electrode film layer, improve the lithium ion insertion / extraction rate, and improve the fast charging performance of the battery cell.

[0312] The first lithium-containing binder and the second lithium-containing binder can be made of the same material or different materials.

[0313] Optionally, the lithium content in the second lithium-containing binder is 3% to 10% by mass, and optionally 3% to 8%. For example, the lithium content in the second lithium-containing binder is 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or any combination of two of the above values. When the lithium content is within the above range, a relatively large number of freely moving lithium ions can be achieved in the negative electrode film layer, further shortening the distance that lithium ions diffuse to the surface of the negative electrode film layer, increasing the lithium ion insertion / extraction rate, and improving the fast charging performance of the battery cell.

[0314] For example, the second lithium-containing binder includes a lithium acrylate-acrylonitrile-acrylamide-hydroxyethyl acrylate copolymer, which is derived from lithium acrylate monomer, acrylonitrile monomer, acrylamide monomer, and hydroxyethyl acrylate monomer, wherein the molar ratio of lithium acrylate monomer, acrylonitrile monomer, acrylamide monomer, and hydroxyethyl acrylate monomer is 30% to 50%: 15% to 45%: 5% to 20%: 20% to 35%. For example, the molar ratio of lithium acrylate monomer, acrylonitrile monomer, acrylamide monomer, and hydroxyethyl acrylate monomer is 35%: 30%: 15%: 20%, or 40%, 20%, 10%, 30%, or 45%, 15%, 20%, 20%, etc.

[0315] The lithium-containing binder of the above-mentioned material can provide a certain number of lithium ions to the negative electrode film layer, thereby improving the fast charging performance of the battery cell; moreover, it is not prone to swelling during charging and discharging, and its structure is stable, which improves the cycle performance of the negative electrode film layer during fast charging and discharging.

[0316] In some embodiments, the first negative electrode film layer further includes a negative electrode binder, and the second negative electrode film layer further includes a negative electrode binder. The negative electrode binder in the first negative electrode film layer and the negative electrode binder in the second negative electrode film layer each independently include at least one of styrene-butadiene rubber (SBR), water-soluble unsaturated resin SR-1B, waterborne acrylic resins (e.g., polyacrylic acid PAA, polymethacrylic acid PMAA, sodium polyacrylate PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), and carboxymethyl chitosan (CMCS).

[0317] In some embodiments, the total content of the first lithium-containing binder and the negative electrode binder in the first negative electrode film layer is greater than the total content of the second lithium-containing binder and the negative electrode binder in the second negative electrode film layer, and the mass content of the first lithium-containing binder relative to the mass of the first negative electrode film layer is less than the mass content of the second lithium-containing binder relative to the mass of the second negative electrode film layer.

[0318] In some embodiments, the negative electrode film layer may optionally include a negative electrode conductive agent. This application does not impose particular limitations on the type of negative electrode conductive agent. As an example, the negative electrode conductive agent may include at least one selected from superconducting carbon, conductive graphite, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. In some embodiments, the mass content of the negative electrode conductive agent is ≤5% based on the total weight of the negative electrode film layer.

[0319] In some embodiments, the negative electrode film layer may optionally include a negative electrode binder. This application does not impose particular limitations on the type of negative electrode binder. As an example, the negative electrode binder may include at least one of styrene-butadiene rubber (SBR), water-soluble unsaturated resin SR-1B, waterborne acrylic resins (e.g., polyacrylic acid PAA, polymethacrylic acid PMAA, sodium polyacrylate PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), and carboxymethyl chitosan (CMCS). In some embodiments, the mass content of the negative electrode binder is ≤5% based on the total weight of the negative electrode film layer.

[0320] In some embodiments, the negative electrode film layer may optionally include other additives. As examples, other additives may include thickeners, dispersants, etc., such as sodium carboxymethyl cellulose (CMC-Na), PTC thermistor materials, etc. In some embodiments, the mass content of other additives is ≤2% based on the total weight of the negative electrode film layer.

[0321] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. Examples of metal foils include at least one foil selected from copper, copper alloys, nickel, nickel alloys, titanium, titanium alloys, silver, and silver alloys. The composite current collector may include a polymeric material substrate and a metal material layer formed on at least one surface of the polymeric material substrate. As an example, the metal material layer may include at least one selected from copper, copper alloys, nickel, nickel alloys, titanium, titanium alloys, silver, and silver alloys. As an example, the polymeric material substrate may include at least one selected from polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE).

[0322] In some embodiments, the thickness of the negative electrode current collector is 4 μm to 6 μm. For example, the thickness of the negative electrode current collector is 4 μm, 4.5 μm, 5 μm, 5.5 μm, 6 μm, or any range of two of the above values.

[0323] When the thickness of the negative electrode current collector is within the above range, the current-carrying capacity of the negative electrode current collector is excellent, and it can enable the battery cell to have a high energy density.

[0324] In the embodiments of this application, the thickness of the negative current collector has a meaning known in the art and can be detected using equipment and methods known in the art. For example, the film layer on the surface of the negative current collector can be washed away with organic solvents such as water, and the thickness of the positive current collector can be measured with a micrometer.

[0325] The negative electrode film is typically formed by coating a negative electrode slurry onto a negative electrode current collector, followed by drying and cold pressing. The negative electrode slurry is usually formed by dispersing the negative electrode active material, optional conductive agent, optional binder, and other optional additives in a solvent and stirring until homogeneous. The solvent can be N-methylpyrrolidone (NMP) or deionized water, but is not limited to these.

[0326] The negative electrode sheet does not exclude other additional functional layers besides the negative electrode film layer. For example, in some embodiments, the negative electrode sheet of this application further includes a negative electrode conductive layer sandwiched between the negative electrode current collector and the negative electrode film layer and disposed on the surface of the negative electrode current collector. In other embodiments, the negative electrode sheet of this application further includes a protective layer covering the surface of the negative electrode film layer.

[0327] In some embodiments, the negative electrode sheet further includes a negative electrode conductive layer, which is located between the negative electrode film layer and the negative electrode current collector. The negative electrode conductive layer can further improve the conductivity of the negative electrode sheet and reduce the heat generation of the negative electrode sheet, thereby reducing the heat generation of the battery cell.

[0328] In some embodiments, the thickness of the negative electrode conductive layer is from 0.5 μm to 2 μm. For example, the thickness of the negative electrode conductive layer can be 0.5 μm, 0.8 μm, 1 μm, 1.2 μm, 1.5 μm, 1.6 μm, 1.8 μm, 2 μm, or any combination of two of the above values.

[0329] When the thickness of the negative electrode conductive layer is within the above range, the conductivity of the negative electrode sheet can be further improved, the heat generation of the negative electrode sheet can be reduced, thereby reducing the heat generation of the battery cell; and the energy density of the battery cell can also be improved.

[0330] In the embodiments of this application, the thickness of the negative electrode conductive layer has a meaning known in the art and can be detected using equipment and methods known in the art, such as the test method for the negative electrode conductive layer described above.

[0331] In some embodiments, the negative electrode conductive layer includes one or more of a negative electrode conductive agent and a negative electrode binder. The negative electrode conductive agent in the negative electrode conductive layer can improve the conductivity of the negative electrode conductive layer, thereby improving the conductivity of the negative electrode sheet and reducing the heat generation of the battery cell; the negative electrode binder in the negative electrode conductive layer can improve the adhesion between the negative electrode current collector and the negative electrode film layer, thereby improving the structural stability of the negative electrode sheet.

[0332] In some embodiments, the negative electrode conductive layer may optionally include other additives. As an example, other additives may include thickeners, such as sodium carboxymethyl cellulose (CMC), PTC thermistor materials, etc.

[0333] Optionally, the mass content of the negative electrode conductive agent in the negative electrode conductive layer is 20% to 40%. For example, the mass content of the negative electrode conductive agent is 20%, 25%, 30%, 35%, 40%, or any combination of two of the above values.

[0334] For example, the negative electrode conductive agent includes one or more of superconducting carbon, conductive graphite, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0335] Optionally, the negative electrode binder has a mass content of 60% to 80% in the negative electrode conductive layer. For example, 60%, 65%, 70%, 75%, 80%, or any range of two of the above values.

[0336] For example, the negative electrode binder includes one or more of styrene-butadiene rubber (SBR), water-soluble unsaturated resin SR-1B, water-based acrylic resin, polyvinyl alcohol, sodium alginate, and carboxymethyl chitosan.

[0337] In some embodiments, the ratio CB of the capacity of the negative electrode film per unit area to the capacity of the positive electrode film per unit area in a single battery cell is from 1.05 to 1.30, and optionally from 1.07 to 1.15. Exemplarily, the ratio CB of the capacity of the negative electrode film per unit area to the capacity of the positive electrode film per unit area in a single battery cell is 1.05, 1.07, 1.1, 1.12, 1.15, 1.18, 1.2, 1.22, 1.25, 1.28, 1.3, or a range of any two of the above values.

[0338] When the ratio CB of the capacity of the negative electrode film per unit area to the capacity of the positive electrode film per unit area in a single battery cell is within the above range, there are sufficient sites in the negative electrode film for lithium insertion, which can reduce the risk of lithium plating; and it is also conducive to fast charging.

[0339] In the embodiments of this application, the CB value has a well-known meaning in the art and can be detected using well-known equipment and methods in the art. For example, the capacity of the negative electrode film per unit area and the capacity of the positive electrode film per unit area can be calculated respectively, and the ratio between the two can be calculated to obtain the CB value.

[0340] Specifically, this explanation will be based on an example where the upper limit of battery charging voltage is 3.65V and the battery discharge cutoff voltage is 2.0V.

[0341] The capacity per unit area of ​​the positive electrode film refers to the actual lithium-depleting capacity of the positive electrode active material. The testing method is as follows: The battery is disassembled in a Braun glove box (PRS340 / 11-119-11), the positive electrode sheet is removed, and assembled into a CR2430 model semi-button battery with a positive electrode and lithium sheet. The positive electrode sheet area used is a mm². 2 The electrolyte is a 1M LiPF6 solution in EC / EMC / DEC = 3 / 5 / 2 (mass ratio). The assembled semi-coin cells are then left to stand for 3 hours. The test is conducted at 25°C. The cells are first charged at 0.1C in the voltage range of 2.0V to 3.65V to remove lithium, and then discharged at 0.05C to insert lithium to 2.0V. This cycle is repeated twice. The discharge capacity of the second cycle is recorded as YmAh. The actual battery design has a positive electrode length of bmm and a width of cmm. The number of surfaces of the positive electrode active material coated on the positive electrode current collector is d. Therefore, the capacity of the positive electrode film per unit area is Y / a*b*c*d.

[0342] Specifically, the capacity per unit area of ​​the negative electrode film refers to the actual lithium intercalation capacity of the negative electrode active material. The testing method is as follows: The battery is disassembled in a PRS340 / 11-119-11 Braun glove box, the negative electrode sheet is removed, and it is assembled into a CR2430 model semi-button battery with a negative electrode and lithium sheet. The area of ​​the negative electrode sheet used is fmm². 2 The electrolyte is a solution of 1M LiPF6 in EC / EMC / DEC = 3 / 5 / 2 (mass ratio). The assembled semi-coin cells are then left to stand for 3 hours. The test is carried out at 25°C. Lithium insertion is performed by discharging at 0.1C in the voltage range of 2V-0V, followed by lithium extraction by charging at 0.05C to 2V. This cycle is repeated twice. The discharge capacity of the second cycle is recorded as ZmAh. The actual battery design has a negative electrode length of h mm and a width of i mm. The number of surfaces of the negative electrode active material coated on the negative electrode current collector is d. Therefore, the lithium insertion capacity of the negative electrode is Z / f*h*i*d.

[0343] [Isolation membrane]

[0344] In this embodiment, the separator includes a porous base membrane.

[0345] In some embodiments, the base film includes at least one of glass fiber, nonwoven fabric, and polyolefin. The base film can be a single-layer film or a multi-layer composite film, without particular limitation. When the base film is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0346] Optionally, the polyolefin includes at least one of polyethylene, polypropylene, and polyvinylidene fluoride.

[0347] In some embodiments, the porosity of the base membrane is 20% to 70%, optionally 35% to 60%. Exemplarily, the porosity of the base membrane is 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, or any range of two of the above values.

[0348] When the porosity of the base film in the embodiments of this application is within the above-mentioned range, it can enhance the migration ability of lithium ions in the separator, further reduce the internal resistance of the battery cell, and thus reduce heat generation.

[0349] In this embodiment, porosity refers to the percentage of the volume of the pores in the separator to the total volume of the separator. Porosity can be tested according to the standard GB / T36363-2018 "Polyolefin Separators for Battery Cells". It should be noted that the actual testing process may differ slightly from the standard due to differences in testing instruments, testing errors, and to minimize the impact on porosity testing, in order to obtain more accurate test values.

[0350] In some embodiments, the thickness of the base film is from 5 μm to 12 μm, optionally not exceeding 9 μm, optionally from 6 μm to 9 μm. Exemplarily, the thickness of the base film is 6 μm, 6.5 μm, 7 μm, 7.5 μm, 8 μm, 8.5 μm, 9 μm, 9.5 μm, 10 μm, 10.5 μm, 11 μm, 11.5 μm, 12 μm, or a range consisting of any two of the above values.

[0351] When the thickness of the base film is within the above range, the migration path of lithium ions in the base film is shorter, which can further reduce the internal resistance of the battery cell and thus reduce heat generation.

[0352] In this embodiment, the separator can be a base film; optionally, the separator further includes a functional layer disposed on at least one side of the base film, the functional layer including inorganic particles to improve the heat resistance of the separator. Optionally, the functional layer is disposed on both sides of the base film.

[0353] In some embodiments, the functional layer includes a first functional layer and a second functional layer. The first functional layer is located on one side of the base film and includes first inorganic particles. The second functional layer is located on the other side of the base film and includes composite particles. The composite particles include second inorganic particles and a plurality of non-fluoropolymer particles. The second inorganic particles are attached to the surface of the non-fluoropolymer particles and / or dispersed inside the non-fluoropolymer particles.

[0354] The first and second functional layers have good heat resistance, which can improve the heat resistance of the separator.

[0355] Optionally, the first functional layer may include an adhesive, optionally including at least one of a fluorinated adhesive or a polyacrylic adhesive, such as polyvinylidene fluoride.

[0356] Optionally, the first inorganic particles include one or more of silicon oxide, aluminum oxide, boehmite, barium sulfate, calcium oxide, titanium oxide, zinc oxide, magnesium oxide, zirconium oxide, and tin oxide. These first inorganic particles can improve the heat resistance of the first functional layer.

[0357] In the embodiments of this application, the thickness of the base film has a meaning known in the art and can be detected using known meanings and equipment. For example, a newly prepared separator can be taken as a sample, or a battery cell that has been completely discharged (discharged to the lower limit cutoff voltage so that the battery's state of charge is 0% SOC) can be disassembled in reverse, and the separator can be obtained from the battery cell. After drying the separator, it can be used as a sample. The separator can be cut with an ion beam cutter to form a cross section. Subsequently, the thickness of the separator and its various layers can be measured using a scanning electron microscope.

[0358] In the second functional layer, the non-fluorinated polymer particles refer to polymers that are non-fluorinated polymers. For example, non-fluorinated polymer particles include acrylate copolymers. Optionally, acrylate copolymers include acrylate-acrylonitrile-acrylamide-propylene copolymers. Acrylate copolymers have excellent adhesion properties and high adhesion stability to the base film. The molar ratio of each monomer in the copolymer can be arbitrary, for example, a molar ratio of 35%:30%:15%:20%, or 40%, 20%, 10%, 30%, or 45%, 15%, 20%, 20%, etc.

[0359] The second inorganic particle in the composite particles prevents the non-fluoropolymer particles from sticking together due to the high-temperature treatment during granulation, creating porosity within the composite particles. This facilitates lithium-ion transport and enhances the ion-conductivity of the separator. Furthermore, the second inorganic particle increases the compressive modulus of the composite particles, reducing their deformation during charging and discharging. This results in a more stable separator structure, improving the kinetic performance of the battery cells and enhancing fast-charging performance. Optionally, compared to the first functional layer, the second functional layer is positioned closer to the negative electrode. Because the composite particles are less prone to deformation, the separator exerts minimal pressure or other side effects on the negative electrode, ensuring stable kinetic performance. Conversely, the first functional layer is positioned closer to the positive electrode.

[0360] Optionally, the second inorganic particles include one or more of silicon oxide, aluminum oxide, boehmite, barium sulfate, calcium oxide, titanium oxide, zinc oxide, magnesium oxide, zirconium oxide, and tin oxide; optionally, the second inorganic particles include silicon oxide. The aforementioned second inorganic particles can improve the heat resistance of the second functional layer and can combine with non-fluoropolymers to form composite particles, further improving the cycle stability and kinetic performance of the separator, and improving the cycle performance and fast-charging performance of the battery cells.

[0361] The average particle size of the second inorganic particles is 5 nm to 100 nm, optionally 10 nm to 100 nm, or optionally 5 nm to 20 nm. For example, the average particle size of the second inorganic particles is 5 nm, 10 nm, 15 nm, 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm, 50 nm, 55 nm, 60 nm, 65 nm, 70 nm, 75 nm, 80 nm, 85 nm, 90 nm, 95 nm, 100 nm, or any combination of two of the above values. When the average particle size of the second inorganic particles is within the above range, it is beneficial to improve the heat resistance and compressive modulus of the composite particles.

[0362] In the embodiments of this application, the average particle size of the second inorganic particles has a meaning known in the art and can be detected using equipment and methods known in the art. For example, after obtaining the separator membrane, the separator membrane is dried and used as a sample. The separator membrane is cut with an ion beam cutter to form a cross-section. Subsequently, the particle size of the second inorganic particles in the separator membrane is measured using a scanning electron microscope. The particle size of multiple, for example, 50, second inorganic particles is measured, and their average value is calculated as the average particle size of the second inorganic particles.

[0363] In some embodiments, the ionic conductivity of the separator is from 0.3 mS / cm to 0.6 mS / cm. Exemplarily, the ionic conductivity of the separator is 0.3 mS / cm, 0.35 mS / cm, 0.4 mS / cm, 0.45 mS / cm, 0.5 mS / cm, 0.55 mS / cm, 0.6 mS / cm, or a range consisting of any two of the above values.

[0364] When the ionic conductivity of the separator is within the above range, the lithium-ion migration ability of the separator can be further improved, thereby enhancing the fast charging performance of the battery cell.

[0365] In this application embodiment, the ionic conductivity of the separator has a meaning known in the art and can be detected using equipment and methods known in the art, for example,

[0366] Preparation of the 2025-type button cell for testing: In a vacuum glove box, a lithium sheet was placed in the negative electrode shell of the battery, and 150 μL of electrolyte was added. The electrolyte was a solution of 1 M LiPF6 in EC / EMC / DEC = 3 / 5 / 2 (mass ratio). Then, a separator (with an area of ​​3.14 cm²) was placed inside. 2 The electrode (12 μm thick) is tightly attached to the lithium sheet, then 25 μL of electrolyte is added, and finally the positive electrode (which can be the positive electrode from Example 1) is placed on top and sealed. The assembled button battery is removed from the vacuum glove box and left for 24 hours for further testing.

[0367] Test: At an electrochemical workstation, at 10 -1 ~10 6 The isolation film resistance R was obtained by testing within a frequency range of Hz. b The ionic conductivity σ (unit: mS / cm) is calculated using the following formula.

[0368] σ=L / (R b ×S)

[0369] Where: R b Let L be the resistance of the isolation membrane, and S be the thickness and area of ​​the isolation membrane under test, respectively.

[0370] Electrolyte

[0371] In some implementations, the battery cell also includes an electrolyte.

[0372] During the charging and discharging process of a single battery cell, active ions such as lithium ions are inserted and extracted back and forth between the positive and negative electrode plates, and the electrolyte plays the role of conducting active ions between the positive and negative electrode plates.

[0373] In this embodiment of the application, the conductivity of the electrolyte at room temperature is 13 mS / cm to 20 mS / cm, optionally 15 mS / cm to 20 mS / cm. Exemplarily, the conductivity of the electrolyte at room temperature is 13 mS / cm, 13.5 mS / cm, 14 mS / cm, 14.5 mS / cm, 15 mS / cm, 15.5 mS / cm, 16 mS / cm, 16.5 mS / cm, 17 mS / cm, 17.5 mS / cm, 18 mS / cm, 18.5 mS / cm, 19 mS / cm, 19.5 mS / cm, 20 mS / cm, or any range of two of the above values.

[0374] When the conductivity of the electrolyte at room temperature, such as 25°C, is within the above range, the migration rate of lithium ions in the electrolyte is relatively high, which can further reduce the internal resistance of the battery cell, thereby reducing heat generation and improving the fast charging performance of the battery cell.

[0375] In the embodiments of this application, the conductivity of the electrolyte at room temperature, such as 25°C, is the ionic conductivity, which can be detected using equipment and methods known in the art, such as by referring to industry standard HG-T 4067-2015.

[0376] In some embodiments, the viscosity of the electrolyte at room temperature is from 2.3 mPa·s to 3.5 mPa·s. Exemplarily, the viscosity of the electrolyte is 2.3 mPa·s, 2.4 mPa·s, 2.5 mPa·s, 2.6 mPa·s, 2.7 mPa·s, 2.8 mPa·s, 2.9 mPa·s, 3.0 mPa·s, 3.1 mPa·s, 3.2 mPa·s, 3.3 mPa·s, 3.4 mPa·s, 3.5 mPa·s, or a range consisting of any two of the above values.

[0377] When the viscosity of the electrolyte at room temperature, such as 25°C, is within the above range, the migration rate of lithium ions in the electrolyte is relatively high, which can further reduce the internal resistance of the battery cell, thereby reducing heat generation and improving the fast charging performance of the battery cell.

[0378] In the embodiments of this application, the viscosity of the electrolyte has a meaning known in the art and can be detected using equipment and methods known in the art, such as in accordance with GB / T10247-2008.

[0379] In some embodiments, the electrolyte has a density of 1.05 g / mL to 1.35 g / mL at room temperature, such as 25°C. Exemplarily, the electrolyte density is 1.05 g / mL, 1.10 g / mL, 1.15 g / mL, 1.2 g / mL, 1.25 g / mL, 1.3 g / mL, 1.35 g / mL, or a range of any two of the above values.

[0380] When the electrolyte density is within the above range, the migration rate of lithium ions in the electrolyte is relatively high, which can further reduce the internal resistance of the battery cell, thereby reducing heat generation and improving the fast charging performance of the battery cell.

[0381] In the embodiments of this application, the density of the electrolyte has a meaning known in the art and can be detected using equipment and methods known in the art, such as referring to GB / T 2013-2010 for testing.

[0382] Electrolytes consist of organic solvents and electrolyte salts. The types of organic solvents and electrolyte salts are not specifically limited and can be selected according to actual needs.

[0383] In some embodiments, the organic solvent includes a chain carboxylic acid ester solvent, wherein the chain carboxylic acid ester solvent comprises, by mass, 5% and 75% of the organic solvent, optionally 10% and 75%, optionally 30% to 70%, or optionally 50% to 70%. Exemplarily, the mass content of the chain carboxylic acid ester solvent is 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, or a range of any two of the above values.

[0384] When the mass content of chain carboxylic acid ester solvents is within the above range, the viscosity of the electrolyte system is relatively low, which is conducive to the migration of lithium ions.

[0385] In some embodiments, the chain carboxylic acid ester solvent includes compounds represented by Formula I.

[0386] In formula I,

[0387] R1 includes a hydrogen atom, a halogen atom, a C1 to C5 alkyl group, or a C1 to C5 haloalkyl group.

[0388] R2 includes C1 to C5 alkyl or C1 to C5 haloalkyl.

[0389] The aforementioned chain-like carboxylic acid ester solvents have high conductivity, which is beneficial for improving the fast charging capability of battery cells.

[0390] Optionally, R1 includes a hydrogen atom, a halogen atom, a C1 to C3 alkyl group, or a C1 to C3 haloalkyl group. Further optionally, R1 includes a hydrogen atom, a halogen atom, a C1 to C2 alkyl group, or a C1 to C2 haloalkyl group.

[0391] Optionally, R2 comprises a C1 to C3 alkyl group or a C1 to C3 haloalkyl group. More optionally, R2 comprises a C1 to C2 alkyl group or a C1 to C2 haloalkyl group.

[0392] In the above embodiments, the halogen atom includes one or more of fluorine, chlorine, bromine and iodine atoms, and optionally, the halogen atom includes fluorine atom.

[0393] In the above embodiments, the halogenated alkyl group includes one or more of fluoroalkyl, chloroalkyl, bromoalkyl and iodoalkyl groups, and optionally, the halogenated alkyl group includes fluoroalkyl.

[0394] For example, the chain carboxylic acid ester solvent includes one or more compounds of formula I-1 to formula I-8.

[0395] In some embodiments, the organic solvent also includes carbonate solvents.

[0396] Optionally, the carbonate solvent includes one or more of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate. More preferably, the carbonate solvent includes one or more of ethylene carbonate, dimethyl carbonate, and methyl ethyl carbonate. The combined use of the above-mentioned carbonate solvents and chain carboxylic acid ester solvents improves the conductivity of the electrolyte at room temperature, which is beneficial for lithium ion migration.

[0397] Further optionally, the carbonate solvent in the organic solvent has a mass content of 30% to 70%, optionally 30% to 50%. Exemplarily, the mass content of the carbonate solvent in the organic solvent is 30%, 31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%, 40%, 45%, 48%, 50%, 55%, 60%, 65%, 70%, or any combination of two of the above values. The carbonate solvent at the above mass content can further improve the conductivity of the electrolyte at room temperature, which is beneficial for lithium ion migration.

[0398] For example, the carbonate solvent includes one or more of ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate, and the carbonate solvent content is 30% to 50% by mass.

[0399] In some embodiments, the electrolyte also contains additives, which may include negative electrode film-forming additives, positive electrode film-forming additives, and additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature performance, and additives that improve battery low-temperature power performance.

[0400] In some embodiments, the additive comprises one or more of carbonate additives, sulfur-containing additives, and lithium salt additives, optionally at least two. These additives can improve the interfacial film performance on the positive and / or negative electrode sides, which is beneficial for improving the fast-charging performance of individual battery cells and enhancing cycle performance.

[0401] In some embodiments, the additive has a mass content of 1% to 10% in the electrolyte, optionally 2% to 8%, and more preferably 3.5% to 8%. Exemplarily, the additive has a mass content of 1%, 2%, 3%, 3.5%, 4%, 5%, 6%, 7%, 8%, 9%, 10% in the electrolyte, or a range of any two of the above values.

[0402] The additives mentioned above can effectively improve the interfacial film performance on the positive and / or negative electrode sides, which is beneficial to improving the fast charging performance of battery cells and improving cycle performance.

[0403] For example, carbonate additives include one or more of vinylene carbonate (VC) and fluoroethylene carbonate (FEC).

[0404] For example, sulfur-containing additives include one or more of vinyl sulfate DTD, vinyl disulfate 2-DTD, butenyl sulfite BS, 1,3-propanesulfonate lactone PS, vinyl sulfite ES, and methylene disulfonate MMDS.

[0405] Optionally, the lithium salt additive includes one or more of lithium difluorophosphate (LiPO2F2), lithium difluorooxalate borate (LiDFOB), lithium tetrafluoroborate (LiBF4), and lithium dioxalate borate (LiBOB).

[0406] Optionally, the mass content of vinylene carbonate (VC) in the electrolyte is 0.5% to 9%, and optionally 2% to 6%.

[0407] Optionally, the mass content of fluoroethylene carbonate (FEC) in the electrolyte is 0.1% to 4%, and optionally 0.5% to 3%.

[0408] Optionally, the mass content of vinylene carbonate (VC) in the electrolyte is 0.5% to 9%, and the mass content of fluoroethylene carbonate (FEC) in the electrolyte is 0.1% to 4%.

[0409] Further optionally, the mass content of vinylene carbonate (VC) in the electrolyte is 2% to 6%, and the mass content of fluoroethylene carbonate (FEC) in the electrolyte is 0.5% to 3%.

[0410] In some embodiments, the electrolyte salt includes a lithium salt, which includes one or more of fluorosulfonylimide salts and lithium hexafluorophosphate (LiPF6). These lithium salts are readily dissociated, facilitating rapid lithium-ion migration, and the electrolyte system is relatively stable and not easily decomposed, thus improving the cycle performance of the battery cells.

[0411] Optionally, the fluorosulfonyl imide salt includes one or more of lithium bisfluorosulfonyl imide (LiFSI) and lithium bistrifluoromethanesulfonate (LiTFSI).

[0412] Optionally, the lithium salt includes lithium bis(fluorosulfonyl)imide (LiFSI) and lithium hexafluorophosphate (LiPF6), wherein the molar concentration of lithium bis(fluorosulfonyl)imide (LiFSI) is from 0.2 mol / L to 0.5 mol / L, and the molar concentration of lithium hexafluorophosphate (LiPF6) is from 0.5 mol / L to 1.0 mol / L.

[0413] For example, the molar concentration of lithium bis(fluorosulfonyl)imide (LiFSI) is 0.4 mol / L to 0.5 mol / L, and the molar concentration of lithium hexafluorophosphate (LiPF6) is 0.7 mol / L.

[0414] For example, the molar concentration of lithium bis(fluorosulfonyl)imide (LiFSI) is 0.5 mol / L, and the molar concentration of lithium hexafluorophosphate (LiPF6) is 0.5 mol / L.

[0415] For example, the molar concentration of lithium bis(fluorosulfonyl)imide (LiFSI) is 0.2 mol / L, and the molar concentration of lithium hexafluorophosphate (LiPF6) is 0.8 mol / L.

[0416] Optionally, the molar ratio of lithium bis(fluorosulfonyl)imide to lithium hexafluorophosphate (LiPF6) is from 0.2 to 1.0, and optionally from 0.2 to 0.5. Exemplarily, the molar ratio of lithium bis(fluorosulfonyl)imide to lithium hexafluorophosphate (LiPF6) is 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, or a range of any two of the above values.

[0417] In the embodiments of this application, the types and contents of inorganic components / lithium salt concentrations in the electrolyte are well-known in the art and can be detected using well-known equipment and methods. For example, the concentrations of inorganic components / lithium salts in the electrolyte can be qualitatively or quantitatively analyzed by ion chromatography using the standard JY / T020-1996 "General Rules for Ion Chromatography Analysis". In the embodiments of this application, freshly prepared electrolyte can be used as a sample, the free electrolyte of a fresh battery can be used as a sample, or a battery that has been completely discharged (discharged to the lower limit cutoff voltage so that the battery's state of charge is approximately 0% SOC) can be disassembled in reverse, and the free electrolyte obtained from the battery can be used as a sample for detection by ion chromatography analysis.

[0418] In the embodiments of this application, the types and contents of organic components in the electrolyte are defined in the art and can be detected using equipment and methods known in the art. For example, the organic components in the electrolyte can be qualitatively and quantitatively analyzed by gas chromatography using GB / T9722-2006 "General Rules for Gas Chromatography of Chemical Reagents". In the embodiments of this application, freshly prepared electrolyte can be used as a sample, the free electrolyte of a fresh battery can be used as a sample, or a battery that has been completely discharged (discharged to the lower limit cutoff voltage so that the battery's state of charge is approximately 0% SOC) can be disassembled in reverse, and the free electrolyte obtained from the battery can be used as a sample for detection using ion chromatography.

[0419] In this embodiment, after quantitative and qualitative detection of each component in the electrolyte, the components are classified. Chain-like carboxylic acid ester solvents and carbonate solvents (e.g., ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate) are considered as components of the organic solvent. The mass content of each component is calculated with the mass of the organic solvent representing 100%.

[0420] Carbonate additives (such as vinylene carbonate and fluoroethylene carbonate), sulfur-containing additives, and lithium salt additives are used as additives in the electrolyte. The mass content of each component is calculated based on the mass of the electrolyte as 100%.

[0421] In some embodiments, the battery cell satisfies: 2.45 g / Ah ≤ d / A ≤ 3.5 g / Ah, optionally 2.45 g / Ah ≤ d / A ≤ 3.3 g / Ah, where d represents the mass of electrolyte in the battery cell in grams (g), and A represents the rated capacity of the battery cell in Ah. Exemplarily, d / A can be 3.5 g / Ah, 3.3 g / Ah, 3.2 g / Ah, 3.0 g / Ah, 2.8 g / Ah, 2.5 g / Ah, 2.45 g / Ah, or a range of any two of the above values.

[0422] d / A reflects the electrolyte's liquid retention capacity. When d / A is within the above range, the electrolyte can effectively wet the positive and negative electrode plates and improve the migration rate of lithium ions in the liquid phase, which is beneficial to improving the fast charging capability of the battery cell.

[0423] In the embodiments of this application, the d / A ratio of a single battery cell can be understood as the liquid retention coefficient, which can be tested using equipment and methods known in the art. For example, it can be described using GB / T31486-2015 "Electrical Performance Requirements and Test Methods for Power Batteries for Electric Vehicles", taking a battery charging upper limit voltage of 3.65V and a battery discharge cut-off voltage of 2.0V as an example.

[0424] At 25°C, the battery cell is charged to 3.65V at 0.33C, then charged at a constant voltage to 0.05C, and then discharged at a constant current of 0.33C to 2.0V. The discharged capacity A is used as the denominator. The battery cell is weighed as M0. Then, the positive electrode, negative electrode, separator, and electrolyte are disassembled, with the free electrolyte remaining in a bag. All the solid components are placed in a 60°C oven and baked for at least 4 hours (including but not limited to the positive electrode, negative electrode, separator, and other mechanical parts of the disassembled battery cell that contribute to M0). Then, all components of the battery cell are weighed again as M1, with the weight difference between M0 and M1 as the numerator. The liquid retention coefficient is equal to the weight difference d between M0 and M1 divided by the capacity A.

[0425] In some implementations, the positive electrode, separator, and negative electrode can be fabricated into an electrode assembly using a winding process and / or a stacking process.

[0426] Figures 1 and 2 show schematic diagrams of the structure of a single battery cell.

[0427] In some embodiments, the battery cell 7 may include a housing 20.

[0428] In some embodiments, the casing 20 of the battery cell 7 can be a rigid casing, such as a hard plastic casing, an aluminum casing, a steel casing, etc. The casing 20 of the battery cell 7 can also be a pouch, such as a pouch-type pouch. The material of the pouch can be plastic, such as at least one of polypropylene (PP), polybutylene terephthalate (PBT), and polybutylene succinate (PBS).

[0429] The outer shell 20 is a hollow structure, and the outer shell 20 can be used to encapsulate the electrode assembly 10 and the electrolyte.

[0430] The method for preparing the battery cell 7 according to the embodiments of this application is well known. In some embodiments, a positive electrode, a separator, a negative electrode, and an electrolyte can be assembled to form a battery cell 7. As an example, the positive electrode, the separator, and the negative electrode can be formed into an electrode assembly 10 by a winding process and / or a stacking process. The electrode assembly 10 is placed in a housing 20, dried, and then injected with electrolyte. After vacuum sealing, settling, formation, shaping, and other processes, the battery cell 7 is obtained.

[0431] In some embodiments, the housing 20 includes a housing 21 and an end cap 22, the housing 21 having an opening and the end cap 22 closing the opening.

[0432] The shape of the housing 21 can be determined according to the specific shape of the electrode assembly 10. For example, if the electrode assembly 10 is a cylindrical structure, a cylindrical housing can be selected; if the electrode assembly 10 is a cuboid structure, a cuboid housing can be selected. Optionally, both the electrode assembly 10 and the housing 21 are cuboid structures.

[0433] In some embodiments, the casing 21 is made of steel, which has high mechanical strength, is not easily deformed, and can improve the reliability of the battery cell 7. In the embodiments of this application, steel refers to the material with the highest proportion in the casing 21.

[0434] Optionally, the thickness of the casing 21 is 0.1 mm to 0.5 mm, and optionally 0.2 mm to 0.35 mm. For example, the thickness of the casing 21 is 0.1 mm, 0.15 mm, 0.2 mm, 0.25 mm, 0.3 mm, 0.35 mm, 0.4 mm, 0.45 mm, 0.5 mm, or any combination of two of the above values. When the thickness of the casing 21 is within the above range, the mechanical strength of the casing 21 is high, which improves the reliability of the battery cell 7; and the casing 21 occupies less space, while having more internal space, which is beneficial for increasing the energy density of the battery cell 7.

[0435] From the external shape of the electrode assembly 10, the electrode assembly 10 includes a main body 12, a first electrode tab 111, and a second electrode tab 112, which protrude from the main body 12. The first electrode tab 111 is the portion of the first electrode sheet that is not coated with an active material layer, and the second electrode tab 112 is the portion of the second electrode sheet that is not coated with an active material layer. The first electrode tab 111 and the second electrode tab 112 are used to draw current from the main body 12. The first electrode sheet and the second electrode sheet have opposite polarities; in other words, one of the first electrode sheet and the second electrode sheet is a positive electrode sheet, and the other of the first electrode sheet and the second electrode sheet is a negative electrode sheet.

[0436] Taking the first tab 111 as the negative electrode tab and the second tab 112 as the positive electrode tab as an example, the portion of the negative electrode current collector not coated with an active material layer in the negative electrode sheet is the negative electrode tab. The active material coated on the negative electrode current collector in the negative electrode sheet constitutes the negative electrode film layer. The negative electrode film layer and the negative electrode current collector coated with the active material are part of the main body 12. Similarly, the portion of the positive electrode current collector not coated with an active material layer in the positive electrode sheet is the positive electrode tab. The active material coated on the positive electrode current collector in the positive electrode sheet constitutes the positive electrode film layer. The positive electrode film layer and the positive electrode current collector coated with the active material are part of the main body 12. Of course, the first tab 111 can be a positive electrode sheet, and the second tab 112 can be a negative electrode tab.

[0437] The first tab 111 and the second tab 112 can extend from the same side of the main body 12, or they can extend from opposite sides respectively.

[0438] Optionally, the number of first tabs 111 located on the same side of the main body 12 is at least one, and optionally at least two. At least two first tabs 111 can increase the current carrying capacity of the first tabs 111.

[0439] Optionally, the number of second tabs 112 located on the same side of the main body 12 is at least one, and optionally at least two. At least two second tabs 112 can increase the current carrying capacity of the second tabs 112.

[0440] In some embodiments, the battery cell 7 further includes a first electrode terminal 31, which is electrically connected to the first tab 111. Optionally, the first electrode terminal 31 and the first tab 111 are welded together. The first electrode terminal 31 and the first tab 111 can be connected via an adapter, or they can be connected without an adapter. Optionally, the first electrode terminal 31 and the first tab 111 are directly welded together without an adapter, which can reduce the resistance at the connection point and help reduce the overall internal resistance of the battery cell 7. When the first tab 111 is the negative electrode, the first electrode terminal 31 is the negative terminal. When the first tab 111 is the positive electrode, the first electrode terminal 31 is the positive terminal.

[0441] In some embodiments, the battery cell 7 further includes a second electrode terminal 32, which is electrically connected to the second tab 112. Optionally, the second electrode terminal 32 and the second tab 112 are welded together. The second electrode terminal 32 and the second tab 112 can be connected via an adapter, or they can be connected without an adapter. Optionally, the second electrode terminal 32 and the second tab 112 are directly welded together without an adapter, which can reduce the resistance at the connection point and help reduce the overall internal resistance of the battery cell 7. When the second tab 112 is the negative electrode, the second electrode terminal 32 is the negative terminal. When the second tab 112 is the positive electrode, the second electrode terminal 32 is the positive terminal.

[0442] Optionally, the number of first electrode terminals 31 located on the same side of the main body 12 is at least one, and can be at least two. At least two first electrode terminals 31 can increase the current carrying capacity of the first electrode terminals 31.

[0443] Further optionally, the current-passing area of ​​a single first electrode terminal 31 is greater than or equal to 25 mm². 2 25mm is optional 2 Up to 315mm 2 The current-passing area of ​​the first electrode terminal 31 can be understood as the cross-sectional area of ​​the first electrode terminal 31, which is perpendicular to the thickness direction of the end cap 22.

[0444] For example, the current-carrying area of ​​a single first electrode terminal 31 can be 25 mm². 2 30mm 2 35mm 2 40mm 2 45mm 2 50mm 2 55mm 2 60mm 2 62mm 2 64mm 2 65mm 2 70mm 2 75mm 2 80mm 2 85mm 2 90mm 2 95mm 2 100mm 2 105mm 2 110mm 2 115mm 2 120mm 2 125mm 2 130mm 2135mm 2 140mm 2 145mm 2 150mm 2 155mm 2 160mm 2 165mm 2 170mm 2 175mm 2 180mm 2 185mm 2 190mm 2 195mm 2 200mm 2 205mm 2 210mm 2 215mm 2 220mm 2 225mm 2 230mm 2 235mm 2 240mm 2 245mm 2 250mm 2 255mm 2 260mm 2 265mm 2 270mm 2 275mm 2 280mm 2 285mm 2 290mm 2 295mm 2 300mm 2 305mm 2 310mm 2 315mm 2 Or a range consisting of any two of the above values.

[0445] Optionally, the number of second electrode terminals 32 located on the same side of the main body 12 is at least one, and optionally at least two. At least two second electrode terminals 32 can increase the current carrying capacity of the second electrode terminals 32.

[0446] Further optionally, the current-passing area of ​​a single second electrode terminal 32 is greater than or equal to 25 mm². 2 25mm is optional 2 Up to 315mm 2 The current-passing area of ​​the second electrode terminal 32 can be understood as the cross-sectional area of ​​the second electrode terminal 32, which is perpendicular to the thickness direction of the end cap 22.

[0447] For example, the current-carrying area of ​​a single second electrode terminal 32 can be 25 mm². 2 30mm 2 35mm 2 40mm 2 45mm 2 50mm 2 55mm 2 60mm 2 62mm 2 64mm 2 65mm 2 70mm 2 75mm 2 80mm 2 85mm 2 90mm 2 95mm 2 100mm 2 105mm 2 110mm 2 115mm 2 120mm 2 125mm 2 130mm 2 135mm 2 140mm 2 145mm 2 150mm 2 155mm 2 160mm 2 165mm 2 170mm 2 175mm 2 180mm 2 185mm 2 190mm 2 195mm 2 200mm 2 205mm 2 210mm 2 215mm 2 220mm 2 225mm 2 230mm 2 235mm 2 240mm 2 245mm 2 250mm 2 255mm 2 260mm 2 265mm 2 270mm 2 275mm2 280mm 2 285mm 2 290mm 2 295mm 2 300mm 2 305mm 2 310mm 2 315mm 2 Or a range consisting of any two of the above values.

[0448] As shown in Figure 3, in some embodiments of this application, the battery cell 7 according to the implementation of this application can be assembled into a battery module 6. The number of battery cells 7 contained in the battery module 6 can be one or more, and the specific number can be adjusted according to the application and capacity of the battery module 6.

[0449] If there are multiple battery cells 7, they can be connected in series, parallel, or in a mixed configuration. A mixed configuration means that some battery cells 7 are connected in series while others are connected in parallel. Multiple battery cells 7 can be directly connected in series, parallel, or in a mixed configuration, and then the entire assembly of the multiple battery cells 7 is housed within the housing of the battery module 6. Alternatively, multiple battery cells 7 can first be connected in series, parallel, or in a mixed configuration to form the battery module 6, and then the multiple battery modules 6 can be connected in series, parallel, or in a mixed configuration to form a whole, which is then housed within the housing. Optionally, the battery module 6 may also include a housing with a accommodating space, within which multiple battery cells 7 are housed.

[0450] As shown in Figure 4, in some embodiments, the battery modules 6 can also be assembled into a battery pack 2. The number of battery modules 6 contained in the battery pack 2 can be adjusted according to the application and capacity of the battery pack. Multiple battery modules 6 can be arranged in any way in the housing 5. The battery device can be in the form of a battery pack 2 or battery modules 6, etc.

[0451] The battery pack 2 may include a housing 5 and a plurality of battery modules 6 disposed within the housing 5. The housing 5 includes a first housing portion 5a and a second housing portion 5b, and the housing 5 has a receiving space 5c. The first housing portion 5a is used to cover the second housing portion 5b and form a closed space for accommodating the battery modules 6. The plurality of battery modules 6 can be arranged in the housing 5 in any manner.

[0452] The first housing portion 5a and the second housing portion 5b overlap each other, and together they define a receiving space 5c for accommodating a single battery cell. The second housing portion 5b can be a hollow structure with one open end, and the first housing portion 5a can be a plate-like structure. The first housing portion 5a covers the open side of the second housing portion 5b to form a housing 5 with the receiving space 5c. Alternatively, both the first housing portion 5a and the second housing portion 5b can be hollow structures with one open side, with the open side of the first housing portion 5a covering the open side of the second housing portion 5b to form a housing 5 with the receiving space 5c. Of course, the first housing portion 5a and the second housing portion 5b can be of various shapes, such as cylinders, cuboids, etc.

[0453] To improve the sealing performance after the first housing part 5a and the second housing part 5b are connected, a sealing element, such as sealant or sealing ring, can also be provided between the first housing part 5a and the second housing part 5b.

[0454] Assuming that the first box section 5a covers the top of the second box section 5b, the first box section 5a can also be called the upper box cover, and the second box section 5b can also be called the lower box.

[0455] The charging process from 40% state of charge to 80% state of charge in the battery pack 2 or any individual battery cell comprising the battery pack 2 also includes multiple charging steps. The charging rate of any charging step is less than the charging rate of any charging step from 10% state of charge to 40% state of charge, and the charging rate of the step to 80% state of charge is any value from 2.5C to 5C, for example, it can be 2.7C.

[0456] In some embodiments, the charging time for the battery pack 2 or any individual battery cell comprising the battery pack 2 from 10% state of charge to 80% state of charge is less than or equal to 10.5 minutes, optionally ranging from 5 minutes to 10.5 minutes, and the ambient temperature of the external environment in which the battery pack 2 is located is room temperature, for example, 30°C. Exemplarily, the charging time for the battery pack 2 from 10% state of charge to 80% state of charge is 10.5 minutes, 10 minutes, 9.5 minutes, 9 minutes, 8.5 minutes, 8 minutes, 7.5 minutes, 7 minutes, 6.5 minutes, 6 minutes, 5.5 minutes, 5 minutes, or a range of any two of the above values.

[0457] Electrical appliances

[0458] A second aspect of this application provides an electrical device, which includes at least one of the battery cell, battery module, or battery pack described in this application. The battery cell, battery module, or battery pack can be the power source of the electrical device or the energy storage unit of the electrical device. The electrical device can be a vehicle, mobile phone, portable device, laptop computer, ship, spacecraft, electric toy, or power tool, etc. Vehicles can be gasoline-powered vehicles, natural gas-powered vehicles, or new energy vehicles; new energy vehicles can be pure electric vehicles, hybrid electric vehicles, or range-extended electric vehicles, etc.; spacecraft include airplanes, rockets, space shuttles, and spacecraft, etc.; electric toys include stationary or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric airplane toys, etc.; power tools include metal cutting power tools, grinding power tools, assembly power tools, and railway power tools, such as electric drills, electric grinders, electric wrenches, electric screwdrivers, electric hammers, impact drills, concrete vibrators, and electric planers, etc. This application does not impose any special limitations on the above-described electrical device.

[0459] Electrical devices can be equipped with individual battery cells, battery modules, or battery packs depending on their usage requirements.

[0460] Figure 5 is a schematic diagram of an example electrical device 1. This electrical device 1 is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. To meet the high power and high energy density requirements of this electrical device 1, a battery pack or battery module can be used.

[0461] The electrical device 1 is equipped with a battery pack 2, which can be located at the bottom, head, or tail of the electrical device 1. The battery pack 2 can be used to supply power to the electrical device 1. For example, the battery pack 2 can serve as the operating power source for the electrical device 1, and can also serve as the driving power source for the electrical device 1, replacing or partially replacing fuel oil or natural gas to provide driving power for the electrical device 1.

[0462] Electrical device 1 may also include controller 3 and motor 4. Controller 3 is used to control battery pack 2 to supply power to motor 4, for example, to meet the power needs of electrical device 1 during startup, navigation and driving.

[0463] Another example of an electrical device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use a single battery cell as their power source.

[0464] In some embodiments, the charging time for the electrical device from 10% state of charge to 80% state of charge is less than or equal to 10.5 minutes, optionally ranging from 5 minutes to 10.5 minutes, and the ambient temperature of the electrical device is room temperature, for example, 30°C. Exemplarily, the charging time for battery pack 2 from 10% state of charge to 80% state of charge is 10.5 minutes, 10 minutes, 9.5 minutes, 9 minutes, 8.5 minutes, 8 minutes, 7.5 minutes, 7 minutes, 6.5 minutes, 6 minutes, 5.5 minutes, 5 minutes, or a range of any two of the above values. The electrical device can employ the same charging strategy as battery pack 2 or individual battery cells.

[0465] Example

[0466] The following embodiments describe the contents disclosed in this application in more detail. These embodiments are merely illustrative, as various modifications and variations will be apparent to those skilled in the art within the scope of the disclosure of the embodiments of this application. Unless otherwise stated, all parts, percentages, and ratios reported in the following embodiments are based on mass, and all reagents used in the embodiments are commercially available or synthesized by conventional methods and can be used directly without further processing, and the instruments used in the embodiments are commercially available.

[0467] Example 1: Preparation of the battery pack

[0468] 1. Preparation of positive electrode sheet

[0469] The positive electrode sheet includes a positive current collector, a positive conductive layer on the positive current collector, and a positive film layer. The positive current collector is an aluminum foil with a thickness of 10 μm. The positive conductive layer on the positive current collector is a film layer with a thickness of 1 μm formed by uniformly mixing the positive conductive agent superconducting carbon, the positive binder polyvinylidene fluoride (PVDF), and the solvent N-methylpyrrolidone (NMP) and then coating it on the surface of the current collector. The positive conductive layer contains 40% positive conductive agent by mass and 60% positive binder by mass.

[0470] The positive electrode film layer comprises a film layer formed by uniformly coating a positive electrode slurry (solvent being N-methylpyrrolidone, NMP) onto the surface of a positive electrode conductive layer, followed by drying and cold pressing. The positive electrode film layer comprises positive electrode active material, binder polyvinylidene fluoride (PVDF), and conductive agent acetylene black in a weight ratio of 97:2:1.

[0471] The positive electrode active material includes lithium iron phosphate and a coating layer. The coating layer is coated on the surface of the lithium iron phosphate and includes lithium titanium iron phosphate (Li2FeTi(PO4)3) and amorphous carbon. The Dv50 of the positive electrode active material is 1.6 μm, and the Dv10 is 0.64 μm.

[0472] The single-sided coating weight of the positive electrode film is 300 mg / 1540.25 mm. 2 .

[0473] 2. Preparation of negative electrode sheet

[0474] The negative electrode sheet includes a negative current collector, a negative conductive layer on the negative current collector, and a negative film layer. The negative current collector is a copper foil with a thickness of 5 μm. The negative conductive layer on the negative current collector is a film layer with a thickness of 1 μm formed by coating a mixture of superconducting carbon as a negative conductive agent, styrene-butadiene rubber (SBR) as a negative binder, sodium carboxymethyl cellulose (CMC-Na) as a thickener, and water as a solvent onto the surface of the negative current collector. The negative conductive agent has a mass content of 35% in the negative conductive layer, the negative binder has a mass content of 60% in the negative conductive layer, and the thickener has a mass content of 5% in the negative conductive layer.

[0475] The negative electrode film layer comprises a film layer formed by uniformly coating a negative electrode slurry (with deionized water as the solvent) onto the surface of a negative electrode conductive layer, followed by drying and cold pressing.

[0476] The single-sided coating weight of the negative electrode film is 138 mg / 1540.25 mm. 2 .

[0477] The negative electrode film layer includes a first negative electrode film layer and a second negative electrode film layer. The first negative electrode film layer is located on the surface of the negative electrode conductive layer, and the second negative electrode film layer is located on the surface of the first negative electrode film layer.

[0478] The first negative electrode film layer comprises graphite particles in a mass ratio of 96.5:0.5:0.5:1.5:1, conductive agent acetylene black, a first lithium-containing binder (lithium acrylate-acrylonitrile-acrylamide-hydroxyethyl acrylate copolymer), negative electrode binder styrene-butadiene rubber, and thickener sodium carboxymethyl cellulose; the lithium content in the first lithium-containing binder is 4.8% by mass; the Dv50 of the graphite particles is 11.3 μm, and the graphite particles include artificial graphite and a carbon coating layer, with the carbon coating layer coating the surface of the artificial graphite, and the mass content of amorphous carbon is 3.5%.

[0479] The second negative electrode film layer comprises graphite particles in a mass ratio of 97.5:0.5:0.5:0.5:1, conductive agent acetylene black, a second lithium-containing binder (lithium acrylate-acrylonitrile-acrylamide-hydroxyethyl acrylate copolymer), negative electrode binder styrene-butadiene rubber, and thickener sodium carboxymethyl cellulose; the lithium content in the second lithium-containing binder is 4.8% by mass; the Dv50 of the graphite particles is 11.3 μm, and the graphite particles include artificial graphite and a carbon coating layer, with the carbon coating layer coating the surface of the artificial graphite, and the mass content of amorphous carbon is 3.5%.

[0480] 3. Separating membrane

[0481] The separator includes a base membrane, which is a 7μm polyethylene film layer with a porosity of 42%.

[0482] 4. Preparation of electrolyte

[0483] The electrolyte consists of organic solvents, lithium salts, and additives.

[0484] The organic solvents include 60% chain carboxylic acid ester solvents (ethyl acetate) and 40% carbonate solvents (30% ethylene carbonate EC, 10% dimethyl carbonate). The mass content of each component in the organic solvents is calculated based on the mass of the organic solvents.

[0485] Based on the mass of the electrolyte, the additive content is 6.5%, which includes vinylene carbonate (VC), fluoroethylene carbonate (FEC), vinyl sulfite (ES), and lithium difluorooxalate borate (LiDFOB) in a mass ratio of 5:0.5:0.5:0.5.

[0486] The lithium salts include 1 mol / L lithium hexafluorophosphate (LiPF6).

[0487] The electrolyte has a conductivity of 16.4 mS / cm at room temperature.

[0488] 5. Preparation of battery cells

[0489] The positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes to provide isolation, thus obtaining the electrode assembly. The electrode assembly is then placed in an outer packaging shell, dried, and injected with electrolyte. After vacuum sealing, settling, formation, and shaping, a single battery cell is obtained. The compaction density of the positive electrode film at 100% SOC is 2.72 g / cm³. 3 The compaction density of the negative electrode film at 100% SOC is 1.26 g / cm³. 3 .

[0490] 6. Battery Pack Preparation

[0491] Multiple battery cells are divided into two groups, each group is connected in series, and the two groups are connected in parallel to assemble into a battery module. The battery module is then connected to the battery management system (BMS) and assembled into a housing to form a battery pack. The BMS is configured to monitor the charging and / or discharging states of multiple battery cells and can adjust the charging current and voltage as needed.

[0492] It should be noted that the battery pack can be charged independently, and each individual battery cell within the battery pack can also be charged independently. Both the battery pack and individual battery cells can be charged using the following charging strategies. The charging strategies will be explained below using the battery pack as an example.

[0493] Charging Strategy 1

[0494] After assembling the battery cells prepared in Example 1 with the battery management system (BMS) into a battery pack, the battery was charged at an ambient temperature of 30°C. The charging process included the following steps:

[0495] Charge from 0% SOC to 5% SOC at a constant current of 5.0C;

[0496] Charge from 5% SOC to 10% SOC at a constant current of 5.0C;

[0497] Charge from 10% SOC to 15% SOC at a constant current of 5.0C;

[0498] Charge from 15% SOC to 20% SOC at a constant current of 5.0C;

[0499] Charge from 20% SOC to 25% SOC at a constant current of 5.0C;

[0500] Charge from 25% SOC to 30% SOC at a constant current of 5.0C;

[0501] Charge from 30% SOC to 35% SOC at a constant current of 5.0C;

[0502] Charge from 35% SOC to 40% SOC at a constant current of 5.0C;

[0503] Charge from 40% SOC to 45% SOC at a constant current of 4.6C;

[0504] Charge from 45% SOC to 50% SOC at a constant current of 4.3C;

[0505] Charge from 50% SOC to 55% SOC at a constant current of 4.0C;

[0506] Charge from 55% SOC to 60% SOC at a constant current of 3.7C;

[0507] Charge from 60% SOC to 65% SOC at a constant current of 3.4C;

[0508] Charge from 65% SOC to 70% SOC at a constant current of 3.1C;

[0509] Charge from 70% SOC to 75% SOC at a constant current of 2.9C;

[0510] Charge from 75% SOC to 80% SOC at a constant current of 2.7C;

[0511] Charge from 80% SOC to 85% SOC at a constant current of 1.8C;

[0512] Charge from 85% SOC to 90% SOC at a constant current of 1.3C;

[0513] Charge from 90% SOC to 95% SOC at a constant current of 0.7C;

[0514] Charge from 95% SOC to 98% SOC at a constant current of 0.33C;

[0515] Charge from 98% SOC to 100% SOC at a constant current of 0.1C.

[0516] The cutoff voltage of the last charging step in the above charging steps is 3.65V. The difference between the cutoff voltage of any charging step in the N-1 charging steps and the cutoff voltage of the last charging step is less than or equal to 0.05V. The cutoff voltage of each charging step in the N-1 steps is 3.6V. The difference between the maximum state of charge of two adjacent charging steps is less than or equal to 5%SOC.

[0517] Charging Strategy 2

[0518] After assembling the battery cells prepared in Example 1 with the battery management system (BMS) into a battery pack, the battery was charged at an ambient temperature of 30°C. The charging process included the following steps:

[0519] Charge from 0% SOC to 5% SOC at a constant current of 5.0C;

[0520] Charge from 5% SOC to 10% SOC at a constant current of 5.0C;

[0521] Charge from 10% SOC to 15% SOC at a constant current of 5.0C;

[0522] Charge from 15% SOC to 20% SOC at a constant current of 5.0C;

[0523] Charge from 20% SOC to 25% SOC at a constant current of 5.0C;

[0524] Charge from 25% SOC to 30% SOC at a constant current of 5.0C;

[0525] Charge from 30% SOC to 35% SOC at a constant current of 5.0C;

[0526] Charge from 35% SOC to 40% SOC at a constant current of 5.0C;

[0527] Charge from 40% SOC to 45% SOC at a constant current of 4.6C;

[0528] Charge from 45% SOC to 50% SOC at a constant current of 4.3C;

[0529] Charge from 50% SOC to 55% SOC at a constant current of 4.0C;

[0530] Charge from 55% SOC to 60% SOC at a constant current of 3.7C;

[0531] Charge from 60% SOC to 65% SOC at a constant current of 3.4C;

[0532] Charge from 65% SOC to 70% SOC at a constant current of 3.1C;

[0533] Charge from 70% SOC to 75% SOC at a constant current of 2.9C;

[0534] Charge from 75% SOC to 80% SOC at a constant current of 2.7C;

[0535] Charge from 80% SOC to 85% SOC at a constant current of 1.8C;

[0536] Charge from 85% SOC to 90% SOC at a constant current of 1.3C;

[0537] Charge from 90% SOC to 95% SOC at a constant current of 0.7C;

[0538] Charge from 95% SOC to 98% SOC at a constant current of 0.33C;

[0539] Charge from 98% SOC to 100% SOC at a constant current of 0.01C.

[0540] The cutoff voltage of the last charging step in the above charging steps is 3.65V. The difference between the cutoff voltage of any charging step in the N-1 charging steps and the cutoff voltage of the last charging step is less than or equal to 0.05V. The cutoff voltage of each charging step in the N-1 steps is 3.6V. The difference between the maximum state of charge of two adjacent charging steps is less than or equal to 5%SOC.

[0541] Charging Strategy 3

[0542] After assembling the battery cells prepared in Example 1 with the battery management system (BMS) into a battery pack, the battery was charged at an ambient temperature of 30°C. The charging process included the following steps:

[0543] Charge from 0% SOC to 5% SOC at a constant current of 5.0C;

[0544] Charge from 5% SOC to 10% SOC at a constant current of 5.0C;

[0545] Charge from 10% SOC to 15% SOC at a constant current of 5.0C;

[0546] Charge from 15% SOC to 20% SOC at a constant current of 5.0C;

[0547] Charge from 20% SOC to 25% SOC at a constant current of 5.0C;

[0548] Charge from 25% SOC to 30% SOC at a constant current of 5.0C;

[0549] Charge from 30% SOC to 35% SOC at a constant current of 5.0C;

[0550] Charge from 35% SOC to 40% SOC at a constant current of 5.0C;

[0551] Charge from 40% SOC to 45% SOC at a constant current of 4.6C;

[0552] Charge from 45% SOC to 50% SOC at a constant current of 4.3C;

[0553] Charge from 50% SOC to 55% SOC at a constant current of 4.0C;

[0554] Charge from 55% SOC to 60% SOC at a constant current of 3.7C;

[0555] Charge from 60% SOC to 65% SOC at a constant current of 3.4C;

[0556] Charge from 65% SOC to 70% SOC at a constant current of 3.1C;

[0557] Charge from 70% SOC to 75% SOC at a constant current of 2.9C;

[0558] Charge from 75% SOC to 80% SOC at a constant current of 2.7C;

[0559] Charge from 80% SOC to 85% SOC at a constant current of 1.8C;

[0560] Charge from 85% SOC to 90% SOC at a constant current of 1.3C;

[0561] Charge from 90% SOC to 95% SOC at a constant current of 0.7C;

[0562] Charge from 95% SOC to 98% SOC at a constant current of 0.33C;

[0563] Charge from 98% SOC to 100% SOC at a constant current of 0.05C.

[0564] The cutoff voltage of the last charging step in the above charging steps is 3.65V. The difference between the cutoff voltage of any charging step in the N-1 charging steps and the cutoff voltage of the last charging step is less than or equal to 0.05V. The cutoff voltage of each charging step in the N-1 steps is 3.6V. The difference between the maximum state of charge of two adjacent charging steps is less than or equal to 5%SOC.

[0565] Charging Strategy 4

[0566] After assembling the battery cells prepared in Example 1 with the battery management system (BMS) into a battery pack, the battery was charged at an ambient temperature of 30°C. The charging process included the following steps:

[0567] Charge from 0% SOC to 5% SOC at a constant current of 5.0C;

[0568] Charge from 5% SOC to 10% SOC at a constant current of 5.0C;

[0569] Charge from 10% SOC to 15% SOC at a constant current of 5.0C;

[0570] Charge from 15% SOC to 20% SOC at a constant current of 5.0C;

[0571] Charge from 20% SOC to 25% SOC at a constant current of 5.0C;

[0572] Charge from 25% SOC to 30% SOC at a constant current of 5.0C;

[0573] Charge from 30% SOC to 35% SOC at a constant current of 5.0C;

[0574] Charge from 35% SOC to 40% SOC at a constant current of 5.0C;

[0575] Charge from 40% SOC to 45% SOC at a constant current of 4.6C;

[0576] Charge from 45% SOC to 50% SOC at a constant current of 4.3C;

[0577] Charge from 50% SOC to 55% SOC at a constant current of 4.0C;

[0578] Charge from 55% SOC to 60% SOC at a constant current of 3.7C;

[0579] Charge from 60% SOC to 65% SOC at a constant current of 3.4C;

[0580] Charge from 65% SOC to 70% SOC at a constant current of 3.1C;

[0581] Charge from 70% SOC to 75% SOC at a constant current of 2.9C;

[0582] Charge from 75% SOC to 80% SOC at a constant current of 2.7C;

[0583] Charge from 80% SOC to 85% SOC at a constant current of 1.8C;

[0584] Charge from 85% SOC to 90% SOC at a constant current of 1.3C;

[0585] Charge from 90% SOC to 95% SOC at a constant current of 0.7C;

[0586] Charge from 95% SOC to 98% SOC at a constant current of 0.33C;

[0587] Charge from 98% SOC to 100% SOC at a constant current of 0.3C.

[0588] The cutoff voltage of the last charging step in the above charging steps is 3.65V. The difference between the cutoff voltage of any charging step in the N-1 charging steps and the cutoff voltage of the last charging step is less than or equal to 0.05V. The cutoff voltage of each charging step in the N-1 steps is 3.6V. The difference between the maximum state of charge of two adjacent charging steps is less than or equal to 5%SOC.

[0589] Charging Strategy 5

[0590] After assembling the battery cells prepared in Example 1 with the battery management system (BMS) into a battery pack, the battery was charged at an ambient temperature of 30°C. The charging process included the following steps:

[0591] Charge from 0% SOC to 5% SOC at a constant current of 5.0C;

[0592] Charge from 5% SOC to 10% SOC at a constant current of 5.0C;

[0593] Charge from 10% SOC to 15% SOC at a constant current of 5.0C;

[0594] Charge from 15% SOC to 20% SOC at a constant current of 5.0C;

[0595] Charge from 20% SOC to 25% SOC at a constant current of 5.0C;

[0596] Charge from 25% SOC to 30% SOC at a constant current of 5.0C;

[0597] Charge from 30% SOC to 35% SOC at a constant current of 5.0C;

[0598] Charge from 35% SOC to 40% SOC at a constant current of 5.0C;

[0599] Charge from 40% SOC to 45% SOC at a constant current of 4.6C;

[0600] Charge from 45% SOC to 50% SOC at a constant current of 4.3C;

[0601] Charge from 50% SOC to 55% SOC at a constant current of 4.0C;

[0602] Charge from 55% SOC to 60% SOC at a constant current of 3.7C;

[0603] Charge from 60% SOC to 65% SOC at a constant current of 3.4C;

[0604] Charge from 65% SOC to 70% SOC at a constant current of 3.1C;

[0605] Charge from 70% SOC to 75% SOC at a constant current of 2.9C;

[0606] Charge from 75% SOC to 80% SOC at a constant current of 2.7C;

[0607] Charge from 80% SOC to 85% SOC at a constant current of 1.8C;

[0608] Charge from 85% SOC to 90% SOC at a constant current of 1.3C;

[0609] Charge from 90% SOC to 95% SOC at a constant current of 0.7C;

[0610] Charge from 95% SOC to 98% SOC at a constant current of 0.33C;

[0611] Charge from 98% SOC to 100% SOC at a constant current of 0.1C.

[0612] The cutoff voltage of the final charging step in the above charging process is 3.65V. The difference between the cutoff voltage of any charging step in the N-1 charging steps and the cutoff voltage of the final charging step is less than or equal to 0.02V. The cutoff voltage of each charging step in the N-1 steps is 3.63V. The difference in the maximum state of charge between two adjacent charging steps is less than or equal to 5% SOC.

[0613] Charging Strategy 6

[0614] After assembling the battery cells prepared in Example 1 with the battery management system (BMS) into a battery pack, the battery was charged at an ambient temperature of 30°C. The charging process included the following steps:

[0615] Charge from 0% SOC to 5% SOC at a constant current of 5.0C;

[0616] Charge from 5% SOC to 10% SOC at a constant current of 5.0C;

[0617] Charge from 10% SOC to 15% SOC at a constant current of 5.0C;

[0618] Charge from 15% SOC to 20% SOC at a constant current of 5.0C;

[0619] Charge from 20% SOC to 25% SOC at a constant current of 5.0C;

[0620] Charge from 25% SOC to 30% SOC at a constant current of 5.0C;

[0621] Charge from 30% SOC to 35% SOC at a constant current of 5.0C;

[0622] Charge from 35% SOC to 40% SOC at a constant current of 5.0C;

[0623] Charge from 40% SOC to 45% SOC at a constant current of 4.6C;

[0624] Charge from 45% SOC to 50% SOC at a constant current of 4.3C;

[0625] Charge from 50% SOC to 55% SOC at a constant current of 4.0C;

[0626] Charge from 55% SOC to 60% SOC at a constant current of 3.7C;

[0627] Charge from 60% SOC to 65% SOC at a constant current of 3.4C;

[0628] Charge from 65% SOC to 70% SOC at a constant current of 3.1C;

[0629] Charge from 70% SOC to 75% SOC at a constant current of 2.9C;

[0630] Charge from 75% SOC to 80% SOC at a constant current of 2.7C;

[0631] Charge from 80% SOC to 85% SOC at a constant current of 1.8C;

[0632] Charge from 85% SOC to 90% SOC at a constant current of 1.3C;

[0633] Charge from 90% SOC to 95% SOC at a constant current of 0.7C;

[0634] Charge from 95% SOC to 98% SOC at a constant current of 0.33C;

[0635] Charge from 98% SOC to 100% SOC at a constant current of 0.1C.

[0636] The cutoff voltage of the final charging step in the above charging process is 3.65V. The difference between the cutoff voltage of any charging step in the N-1 charging steps and the cutoff voltage of the final charging step is less than or equal to 0.08V. The cutoff voltage of each charging step in the N-1 steps is 3.57V. The difference in the maximum state of charge between two adjacent charging steps is less than or equal to 5% SOC.

[0637] Charging Strategy 7 (Slow Charging Comparison)

[0638] After assembling the battery cells prepared in Example 1 with the battery management system (BMS) into a battery pack, the battery was charged at an ambient temperature of 30°C. The charging process included the following steps:

[0639] Charge from 0% SOC to 30% SOC at a constant current of 3.0C;

[0640] Charge from 30% SOC to 35% SOC at a constant current of 2.8C;

[0641] Charge from 35% SOC to 40% SOC at a constant current of 2.6C;

[0642] Charge from 40% SOC to 45% SOC at a constant current of 2.4C;

[0643] Charge from 45% SOC to 50% SOC at a constant current of 2.2C;

[0644] Charge from 50% SOC to 55% SOC at a constant current of 2.0C;

[0645] Charge from 55% SOC to 60% SOC at a constant current of 1.8C;

[0646] Charge from 60% SOC to 65% SOC at a constant current of 1.6C;

[0647] Charge from 65% SOC to 70% SOC at a constant current of 1.4C;

[0648] Charge from 70% SOC to 75% SOC at a constant current of 1.3C;

[0649] Charge from 75% SOC to 80% SOC at a constant current of 1.2C;

[0650] Charge from 80% SOC to 85% SOC at a constant current of 0.8C;

[0651] Charge from 85% SOC to 90% SOC at a constant current of 0.6C;

[0652] Charge from 90% SOC to 95% SOC at a constant current of 0.4C;

[0653] Charge from 95% SOC to 98% SOC at a constant current of 0.33C;

[0654] Charge from 98% SOC to 100% SOC at a constant current of 0.3C.

[0655] The cutoff voltage of the last charging step in the above charging steps is 3.65V. The difference between the cutoff voltage of any charging step in the N-1 charging steps and the cutoff voltage of the last charging step is equal to 0V. That is, the cutoff voltage of each charging step is 3.65V. The difference between the maximum state of charge of two adjacent charging steps is less than or equal to 5% SOC.

[0656] Charging Strategy 8

[0657] After assembling the battery cells prepared in Example 1 with the battery management system (BMS) into a battery pack, the battery was charged at an ambient temperature of 30°C. The charging process included the following steps:

[0658] Charge from 0% SOC to 5% SOC at a constant current of 5.0C;

[0659] Charge from 5% SOC to 10% SOC at a constant current of 5.0C;

[0660] Charge from 10% SOC to 15% SOC at a constant current of 5.0C;

[0661] Charge from 15% SOC to 20% SOC at a constant current of 5.0C;

[0662] Charge from 20% SOC to 25% SOC at a constant current of 5.0C;

[0663] Charge from 25% SOC to 30% SOC at a constant current of 5.0C;

[0664] Charge from 30% SOC to 35% SOC at a constant current of 5.0C;

[0665] Charge from 35% SOC to 40% SOC at a constant current of 5.0C;

[0666] Charge from 40% SOC to 45% SOC at a constant current of 5C.

[0667] Charge from 45% SOC to 50% SOC at a constant current of 4.3C;

[0668] Charge from 50% SOC to 55% SOC at a constant current of 4.0C;

[0669] Charge from 55% SOC to 60% SOC at a constant current of 3.7C;

[0670] Charge from 60% SOC to 65% SOC at a constant current of 3.4C;

[0671] Charge from 65% SOC to 70% SOC at a constant current of 3.1C;

[0672] Charge from 70% SOC to 75% SOC at a constant current of 2.9C;

[0673] Charge from 75% SOC to 80% SOC at a constant current of 2.7C;

[0674] Charge from 80% SOC to 85% SOC at a constant current of 1.8C;

[0675] Charge from 85% SOC to 90% SOC at a constant current of 1.3C;

[0676] Charge from 90% SOC to 95% SOC at a constant current of 0.7C;

[0677] Charge from 95% SOC to 98% SOC at a constant current of 0.33C;

[0678] Charge from 98% SOC to 100% SOC at a constant current of 0.33C.

[0679] The cutoff voltage of the last charging step in the above charging steps is 3.65V. The difference between the cutoff voltage of any charging step in the N-1 charging steps and the cutoff voltage of the last charging step is equal to 0V. That is, the cutoff voltage of each charging step is 3.65V. The difference between the maximum state of charge of two adjacent charging steps is less than or equal to 5% SOC.

[0680] Example 2: Preparation of the battery pack

[0681] The battery cells and battery packs were prepared using a similar method to Example 1. The difference from Example 1 is that the positive electrode active material includes lithium manganese iron phosphate and a coating layer. The coating layer is coated on the surface of lithium manganese iron phosphate and includes lithium titanium iron phosphate Li2FeTi(PO4)3 and amorphous carbon.

[0682] Charging Strategy 9

[0683] After assembling the battery cells prepared in Example 2 with the battery management system (BMS) into a battery pack, the battery was charged at an ambient temperature of 30°C. The charging process included the following steps:

[0684] Charge from 0% SOC to 5% SOC at a constant current of 5.0C;

[0685] Charge from 5% SOC to 10% SOC at a constant current of 5.0C;

[0686] Charge from 10% SOC to 15% SOC at a constant current of 5.0C;

[0687] Charge from 15% SOC to 20% SOC at a constant current of 5.0C;

[0688] Charge from 20% SOC to 25% SOC at a constant current of 5.0C;

[0689] Charge from 25% SOC to 30% SOC at a constant current of 5.0C;

[0690] Charge from 30% SOC to 35% SOC at a constant current of 5.0C;

[0691] Charge from 35% SOC to 40% SOC at a constant current of 5.0C;

[0692] Charge from 40% SOC to 45% SOC at a constant current of 5.0C;

[0693] Charge from 45% SOC to 50% SOC at a constant current of 4.3C;

[0694] Charge from 50% SOC to 55% SOC at a constant current of 4.0C;

[0695] Charge from 55% SOC to 60% SOC at a constant current of 3.7C;

[0696] Charge from 60% SOC to 65% SOC at a constant current of 3.4C;

[0697] Charge from 65% SOC to 70% SOC at a constant current of 3.1C;

[0698] Charge from 70% SOC to 75% SOC at a constant current of 2.9C;

[0699] Charge from 75% SOC to 80% SOC at a constant current of 2.7C;

[0700] Charge from 80% SOC to 85% SOC at a constant current of 1.8C;

[0701] Charge from 85% SOC to 90% SOC at a constant current of 1.3C;

[0702] Charge from 90% SOC to 95% SOC at a constant current of 0.7C;

[0703] Charge from 95% SOC to 98% SOC at a constant current of 0.33C;

[0704] Charge from 98% SOC to 100% SOC at a constant current of 0.33C.

[0705] The cutoff voltage of the last charging step in the above charging steps is 4.25V. The difference between the cutoff voltage of any charging step in the N-1 charging steps and the cutoff voltage of the last charging step is less than or equal to 0.05V. The cutoff voltage of each charging step in the N-1 steps is 4.2V. The difference between the maximum state of charge of two adjacent charging steps is less than or equal to 5%SOC.

[0706] Performance testing

[0707] 1. Lithium plating area test of individual battery cells

[0708] After each embodiment's battery pack was cycled 20 times according to its respective charge-discharge strategy, the discharge strategy was as follows: constant current discharge at 0.33C to 2.0V, followed by full charge to 100% SOC according to the corresponding charging strategy. The negative electrode plate in the battery pack was then disassembled, unfolded, and the cleavage region (grayish-white area) was observed and its area measured.

[0709] No lithium plating: lithium plating area < 0.05%.

[0710] Slight lithium plating: lithium plating area <2%.

[0711] Severe lithium plating: lithium plating area ≥2%.

[0712] Test Results

[0713] The test results are shown in Table 1.

[0714] Table 1

[0715] As can be seen from Table 1,

[0716] A slower charging strategy (charging strategy 7) is adopted. Although the negative electrode plate does not have much lithium deposition, the charging time is long and cannot meet the requirements of fast charging.

[0717] Using a faster charging strategy (charging strategy 8), charging to the cutoff voltage at each step increases the risk of lithium plating at each step, resulting in severe lithium plating on the negative electrode during the entire charging process, which can easily shorten the battery pack's lifespan.

[0718] In the battery pack of this application embodiment, during the charging process, the difference in the maximum state of charge between adjacent charging steps is less than or equal to 5%, resulting in less polarization during the charging process. Before the Nth charging step, that is, the cutoff voltage of any charging step from the 1st to the (N-1)th charging step is relatively small, so that the voltage will not rise during the charging process. This allows the lithium ions extracted from the positive electrode film to be basically embedded in the negative electrode film, making it less likely for lithium to be deposited on the surface of the negative electrode sheet, thereby improving the reliability of the battery cell.

[0719] Furthermore, the embodiments of this application are applicable to different phosphate systems, such as lithium iron phosphate, lithium manganese iron phosphate, and lithium manganese phosphate, which have different corresponding battery cutoff voltages. This means that the embodiments of this application are applicable to batteries with different cutoff voltages, such as less than or equal to 4.4V, and can be selected as battery systems from 3.65V to 4.4V. The above battery systems can all achieve fast charging and reduce the risk of lithium plating when using the charging strategy of this application.

[0720] Although illustrative embodiments have been demonstrated and described, those skilled in the art should understand that the above embodiments should not be construed as limiting the present application, and that changes, substitutions and modifications can be made to the embodiments without departing from the spirit, principles and scope of the present application.

Claims

A battery cell includes an electrolyte and an electrode assembly, the electrode assembly including a positive electrode tab, a negative electrode tab, and a separator film between the positive electrode tab and the negative electrode tab, the positive electrode tab including a positive electrode current collector and a positive electrode film layer disposed on at least one side of the positive electrode current collector and containing a positive electrode active material, the positive electrode active material including a lithium-containing phosphate of olivine structure, the negative electrode tab including a negative electrode current collector and a negative electrode film layer disposed on at least one side of the negative electrode current collector and containing a negative electrode active material, the negative electrode film layer including a carbon-based material, a charging process of the battery cell from 0% state of charge to 100% state of charge includes N charging steps, a difference between a maximum state of charge of any one of the N charging steps and a maximum state of charge of an adjacent charging step of the N charging steps is less than or equal to 5% state of charge, N is a positive integer greater than or equal to 2; a cutoff voltage of any one of N-1 charging steps of the N charging steps is less than a cutoff voltage of the Nth charging step, the cutoff voltage of the Nth charging step is no more than 4.4 V, a charging time of the battery cell from 10% state of charge to 80% state of charge at room temperature is 5 min to 10.5 min. The battery cell of claim 1, wherein, the cutoff voltage of the Nth charging step is greater than the cutoff voltage of any one of the N-1 charging steps, and a difference between the cutoff voltage of the Nth charging step and the cutoff voltage of any one of the N-1 charging steps is greater than or equal to 0.02 V. The battery cell according to claim 1 or 2, wherein, the cutoff voltage of the Nth charging step is greater than the cutoff voltage of any one of the N-1 charging steps, and a difference between the cutoff voltage of the Nth charging step and the cutoff voltage of any one of the N-1 charging steps is greater than or equal to 0.05 V. The battery cell of claim 3, wherein, the cutoff voltage of the Nth charging step is greater than the cutoff voltage of any one of the N-1 charging steps, and a difference between the cutoff voltage of the Nth charging step and the cutoff voltage of any one of the N-1 charging steps is 0.05 V to 0.2 V. The battery cell of any one of claims 1 to 4, wherein, the cutoff voltage of the Nth charging step is 3.65 V to 4.4 V. The battery cell of any one of claims 1 to 5, wherein, a charging rate of the Nth charging step is 0.05 C to 0.30 C. The battery cell of claim 6, wherein, a charging rate of the Nth charging step is 0.1 C to 0.30 C. The battery cell of any one of claims 1 to 7, wherein, a charging rate of the battery cell in the Mth charging step is 3.5 C to 6 C, a state of charge of the battery cell in the Mth charging step includes 50% state of charge, M is less than N, and M is a positive integer greater than or equal to 1. The battery cell of any one of claims 1 to 8, wherein, a charging rate in any one of the charging steps of the battery cell from 0% state of charge to 40% state of charge is 4 C to 8 C. The battery cell of any one of claims 1 to 9, wherein, a constant current value of the Qth charging step of the N charging steps is less than a constant current value of the Q-1th charging step, Q is less than or equal to N, and Q is a positive integer greater than or equal to 2. The battery cell of any one of claims 1 to 10, wherein, the lithium-containing phosphate of olivine structure includes: a phosphate particle, and a coating layer coated on a surface of the phosphate particle, the coating layer containing one or more elements of C, Fe, Ti, Zr, Hf, Ge, and Sn. The battery cell of claim 11, wherein, The phosphate particles include a compound of a general formula of Li x1 A y1 Me a M b P 1-c X c Y z , wherein 0.5≤x1≤1.3, 0≤y1≤1.3, and 0.9≤x1+y1≤1.3, 0.9≤a≤1.5, 0≤b≤0.5, and 0.9≤a+b≤1.5, 0≤c≤0.5, 3≤z≤5, A includes one or more of Na, K, Mg, Me includes one or more of Mn, Fe, Co, Ni, M includes one or more of B, Mg, Al, Si, P, S, Ca, Sc, Ti, V, Cr, Cu, Zn, Sr, Y, Zr, Nb, Mo, Cd, Sn, Sb, Te, Ba, Ta, W, Yb, La, Ce, X includes one or more of S, Si, Cl, B, C, N, and Y includes one or more of O, F. The battery cell according to claim 11 or 12, wherein The coating layer includes a general formula of Li 3-d Fe 2- d M2 d (PO x2 ) y2 a fast ion conductor of a general formula of Li M2Ti2-x2Snx2P2-y2O12, M2 includes one or more elements of Ti, Zr, Hf, Ge, and Sn, 0≤d≤1, 0 The battery cell according to any one of claims 11 to 13, wherein The meso-carbon element content of the lithium-containing phosphate of olivine structure is 1% to 2% by mass; The lithium-containing phosphate of olivine structure has a specific surface area of 5 m 2 / g to 18 m 2 / g. The battery cell according to any one of claims 1 to 14, wherein The single-side coating weight of the positive electrode film layer is 200 mg / 1540 mm 2 to 370 mg / 1540 / mm 2 ; and / or The compacted density of the positive electrode film layer is 2.50 g / cm 3 to 2.80 g / cm 3 . The battery cell according to any one of claims 1 to 15, wherein The single-side coating weight of the negative electrode film layer is 90 mg / 1540 mm 2 to 170 mg / 1540 mm 2 ; and / or The compacted density of the negative electrode film layer is 1.15 g / cm 3 to 1.36 g / cm 3 . The battery cell of claim 16, wherein, The compacted density of the negative electrode film layer is 1.25 g / cm 3 to 1.36 g / cm 3 . The battery cell of any one of claims 1 to 17, wherein, The carbon-based material includes graphite particles, and a degree of graphitization of the graphite particles is 92.0% to 94.5%. The battery cell of claim 18, wherein, The graphite particles include: Artificial graphite including secondary particles; and A carbon coating layer coated on a surface of the artificial graphite. The battery cell according to claim 18 or 19, wherein The negative electrode film layer includes: A first negative electrode film layer disposed on a surface of the negative electrode current collector, the first negative electrode film layer including a carbon-based material, and A second negative electrode film layer connected to a side of the first negative electrode film layer facing away from the negative electrode current collector, the second negative electrode film layer including a carbon-based material, The carbon-based material in the first negative electrode film layer and the carbon-based material in the second negative electrode film layer each independently include graphite particles, and a volume average particle diameter Dv50 of the graphite particles in the first negative electrode film layer is greater than or equal to a volume average particle diameter Dv50 of the graphite particles in the second negative electrode film layer. The battery cell according to claim 20, wherein The volume average particle diameter Dv50 of the graphite particles in the first negative electrode film layer is 9.5 μm to 18.5 μm, and / or The volume average particle diameter Dv50 of the graphite particles in the second negative electrode film layer is 7.8 μm to 14.3 μm. The battery cell according to claim 20 or 21, wherein A tap density of the carbon-based material in the first negative electrode film layer is less than or equal to a tap density of the carbon-based material in the second negative electrode film layer. The battery cell of claim 22, wherein, The tap density of the carbon-based material in the first negative electrode film layer is 0.82 g / cm 3 to 1.21 g / cm 3 , and / or The tap density of the carbon-based material in the second negative electrode film layer is 0.90 g / cm 3 to 1.25 g / cm 3 . The battery cell of any one of claims 20 to 23, wherein, A ratio of a thickness of the second negative electrode film layer to a thickness of the first negative electrode film layer is 3:7 to 7:

3. The battery cell of claim 24, wherein, A ratio of a thickness of the second negative electrode film layer to a thickness of the first negative electrode film layer is 4:6 to 6:

4. The battery cell of any one of claims 20-25, wherein, After the battery cell is subjected to a full charge test for 10 cycles at a beginning of life (BOL), a thickness of the first negative electrode film layer is 15 μm to 65 μm, and / or a thickness of the second negative electrode film layer is 15 μm to 65 μm. The battery cell of any one of claims 20-26, wherein, After the battery cell is subjected to a full charge test at an end of life (EOL), a thickness of the first negative electrode film layer is 15 μm to 70 μm, and / or a thickness of the second negative electrode film layer is 15 μm to 70 μm. The battery cell of any one of claims 20-27, wherein, The first negative electrode film layer further includes a first lithium-containing binder, and the second negative electrode film layer further includes a second lithium-containing binder, and a mass content of the first lithium-containing binder with respect to a mass of the first negative electrode film layer is less than or equal to a mass content of the second lithium-containing binder with respect to a mass of the second negative electrode film layer. The battery cell according to claim 28, wherein The mass content of the first lithium-containing binder with respect to a mass of the first negative electrode film layer is 0.1% to 1%, and / or The mass content of the second lithium-containing binder with respect to a mass of the second negative electrode film layer is 0.1% to 1%. The battery cell according to claim 28 or 29, wherein A mass content of lithium element in the first lithium-containing binder is 3% to 10%, and / or A mass content of lithium element in the second lithium-containing binder is 3% to 10%. The battery cell according to any one of claims 28 to 30, wherein the first lithium-containing binder comprises a lithium acrylate-acrylonitrile-acrylamide-hydroxyethyl acrylate copolymer derived from lithium acrylate monomers, acrylonitrile monomers, acrylamide monomers, and hydroxyethyl acrylate monomers in a molar ratio of 30% to 50%: 15% to 45%: 5% to 20%: 20% to 35%; and / or the second lithium-containing binder comprises a lithium acrylate-acrylonitrile-acrylamide-hydroxyethyl acrylate copolymer derived from lithium acrylate monomers, acrylonitrile monomers, acrylamide monomers, and hydroxyethyl acrylate monomers in a molar ratio of 30% to 50%: 15% to 45%: 5% to 20%: 20% to 35%. The battery cell of any one of claims 1 to 31, wherein, The negative electrode active material further comprises a silicon-based material, a content of silicon element in the silicon-based material being 0.3% to 10.0% based on a mass of the negative electrode active material. The battery cell of any one of claims 1 to 32, wherein, The separator film comprises a base film of a porous structure, a porosity of the base film being 20% to 70%; and / or a thickness of the base film being 6 µm to 12 µm. The battery cell of claim 33, wherein, The separator film further comprises a functional layer provided on at least one side of the base film, the functional layer comprising: a first functional layer on one side of the base film, the first functional layer comprising first inorganic particles, a second functional layer on the other side of the base film, the second functional layer comprising composite particles, the composite particles comprising second inorganic particles and a plurality of non-fluoropolymer particles, the second inorganic particles being attached to surfaces of the non-fluoropolymer particles and / or dispersed inside the non-fluoropolymer particles. The battery cell of claim 34, wherein, The non-fluoropolymer particles comprise an acrylate-based copolymer. The battery cell of claim 34 or 35, wherein, The first inorganic particles comprise one or more of silicon oxide, aluminum oxide, boehmite, barium sulfate, calcium oxide, titanium oxide, zinc oxide, magnesium oxide, zirconium oxide, and tin oxide, and / or The second inorganic particles comprise one or more of silicon oxide, aluminum oxide, boehmite, barium sulfate, calcium oxide, titanium oxide, zinc oxide, magnesium oxide, zirconium oxide, and tin oxide. The battery cell of any one of claims 34-36, wherein, An average particle diameter of the second inorganic particles is 5 nm to 100 nm. The battery cell according to any one of claims 1 to 37, the battery cell further comprising an electrolyte, a viscosity of the electrolyte at room temperature is 2.3 mPa·s to 3.5 mPa·s; and / or an electrical conductivity of the electrolyte at room temperature is 13 mS / cm to 20 mS / cm; and / or a density of the electrolyte at room temperature is 1.05 g / mL to 1.35 g / mL. The battery cell according to any one of claims 1 to 38, the electrolyte comprising an organic solvent, the organic solvent comprising one or more of a carbonate-based solvent and a carboxylate-based solvent. The battery cell of claim 39, wherein, The carboxylic acid ester-based solvent includes a chain carboxylic acid ester-based solvent, and the mass content of the chain carboxylic acid ester-based solvent relative to the mass of the organic solvent is greater than or equal to 5% and less than 75%. The battery cell of any one of claims 1-40, wherein, The ratio of the negative electrode capacity per unit area to the positive electrode capacity per unit area in the battery cell is 1.05 to 1.

20. The battery cell of any one of claims 1-41, wherein, The battery cell includes a positive electrode terminal, the positive electrode tab includes a positive electrode tab, and the positive electrode terminal and the positive electrode tab are directly welded; and / or The battery cell includes a negative electrode terminal, the negative electrode tab includes a negative electrode tab, and the negative electrode terminal and the negative electrode tab are directly welded. The battery cell of claim 42, wherein, The overcurrent area of the single positive terminal is 25mm 2 to 315mm 2 ; and / or The overcurrent area of the single negative terminal is 25mm 2 to 315mm 2 . The battery cell according to any one of claims 1 to 43, further comprising a shell, the shell containing the electrode assembly, the material of the shell comprising steel, and the thickness of the shell being 0.1 mm to 0.5 mm. A battery device comprising the battery cell according to any one of claims 1 to 44. The battery device of claim 45, wherein, The charging time of the battery device from 10% state of charge to 80% state of charge at room temperature is 5 min to 10.5 min. An electric device comprising the battery device according to claim 45 or 46. A charging method of a battery cell, comprising: charging a battery cell in a first state of charge, so that the state of charge of the battery cell increases by less than or equal to 5% state of charge; repeating the above steps at least once until the battery cell is charged to a second state of charge, wherein the second state of charge is greater than the first state of charge, and the second state of charge is greater than or equal to 95% state of charge and less than 100% state of charge; charging the battery cell in the second state of charge to 100% state of charge, wherein the cutoff voltage of any step before charging to the second state of charge is less than the cutoff voltage of the step of charging to 100% state of charge, and the cutoff voltage of the step of charging to 100% state of charge is not more than the theoretical voltage upper limit of the olivine structure lithium-containing phosphate.

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