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

Abstract

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

Description

[0001] This application claims the priority of the PCT international application PCT / CN2024 / 102640, titled "Battery Cell and Its Charging Method, Battery Device and Electrical Device", filed on June 28, 2024. The entire content of this application is incorporated herein by reference. Technical Field

[0002] This application relates to a battery cell and its charging method, a battery device and an electrical device. Background Art

[0003] Battery cells have characteristics such as high capacity and long life, and are thus widely used in electronic devices, such as mobile phones, laptop computers, electric bicycles, electric vehicles, electric airplanes, electric ships, electric toy cars, electric toy ships, electric toy airplanes, and electric tools, etc. With the development of the battery field, higher requirements are put forward for the use reliability of battery cells. Summary of the Invention

[0004] This application provides a battery cell and its charging method, a battery device and an electrical device, which can reduce the risk of lithium plating during the fast charging process of the battery cell and improve the use 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 tab, a negative electrode tab, and a separator located between the positive electrode tab and the negative electrode tab. The positive electrode tab includes a positive electrode current collector and a positive electrode film layer provided on at least one side of the positive electrode current collector and containing a positive electrode active material. The positive electrode active material includes a lithium-containing phosphate with an olivine structure. The negative electrode tab includes a negative electrode current collector and a negative electrode film layer provided on at least one side of the negative electrode current collector and containing a negative electrode active material. The negative electrode film layer includes a carbon-based material. The charging process of the battery cell from 0% state of charge to 100% state of charge includes N charging steps. The difference between the maximum state of charge of any one of the N charging steps and the maximum state of charge of its adjacent charging step is less than or equal to 5% state of charge. N is a positive integer greater than or equal to 2. Among the N charging steps, the cut-off voltage of any one of the N - 1 charging steps is less than the cut-off voltage of the Nth charging step, and the cut-off voltage of the Nth charging step does not exceed 4.4V. At room temperature, the charging of the battery cell from 10% state of charge to 80% state of charge takes 5 min to 10.5 min.

[0006] Therefore, in the embodiments of the present application, during the fast charging process of the battery cell, the difference in the maximum state of charge between adjacent charging steps is less than or equal to 5% state of charge, resulting in less polarization during the charging process; before the Nth charging step, that is, the cut-off voltage of any charging step from the 1st charging step to the (N - 1)th charging step is relatively small, so that during the charging process, the voltage does not rise upwards, and basically all the lithium ions released from the positive electrode film layer can be embedded in the negative electrode film layer, and lithium deposition is not likely to occur on the surface of the negative electrode sheet, which can improve the use reliability of the battery cell.

[0007] In some embodiments, the cut-off voltage of the Nth charging step is greater than the cut-off voltage of any charging step in the (N - 1) charging steps, and the difference between the cut-off voltage of the Nth charging step and the cut-off voltage of any charging step in the (N - 1) charging steps is greater than or equal to 0.02V.

[0008] Therefore, when the battery cell in the embodiments of the present application meets the above conditions, it can further make it less likely for lithium deposition to occur on the surface of the negative electrode sheet, and can improve the use reliability of the battery cell.

[0009] In some embodiments, the cut-off voltage of the Nth charging step is greater than the cut-off voltage of any charging step in the (N - 1) charging steps, and the difference between the cut-off voltage of the Nth charging step and the cut-off voltage of any charging step in the (N - 1) charging steps is greater than or equal to 0.05V.

[0010] Therefore, when the battery cell in the embodiments of the present application meets the above conditions, it can further make it less likely for lithium deposition to occur on the surface of the negative electrode sheet, and can improve the use reliability of the battery cell.

[0011] In some embodiments, the cut-off voltage of the Nth charging step is greater than the cut-off voltage of any charging step in the (N - 1) charging steps, and the difference between the cut-off voltage of the Nth charging step and the cut-off voltage of any charging step in the (N - 1) charging steps is 0.05V to 0.2V.

[0012] Therefore, when the battery cell in the embodiments of the present application meets the above conditions, it can further make it less likely for lithium deposition to occur on the surface of the negative electrode sheet, and can improve the use reliability of the battery cell.

[0013] In some embodiments, the cut-off voltage of the Nth charging step is 3.65V to 4.4V.

[0014] Therefore, when the cut-off voltage of the Nth charging step is within the above range, the risk of lithium deposition in the battery cell is 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 can be optionally 0.10C to 0.30C.

[0016] Therefore, at the end stage of charging, the risk of lithium plating on the negative electrode plate increases. When charging at the above-mentioned rate in the Nth charging step and above, the risk of lithium plating on the negative electrode plate can be further reduced, and the reliability of the battery cell in use can be improved.

[0017] In some embodiments, the charging rate of the battery cell in the Mth charging step is 3.5C to 6C. The 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.

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

[0019] In some embodiments, the charging rate of the battery cell in the charging steps from 0% state of charge to 40% state of charge is 4C to 8C. When the battery cell is charged at the above-mentioned rate in the early stage of charging, the reliability of the battery cell in use and the fast charging performance can be improved.

[0020] In some embodiments, the constant current value of the Qth charging step in the N charging steps is less than the 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.

[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 plate is relatively small, which is beneficial to improving the reliability of the battery cell in use.

[0022] In some embodiments, the lithium-containing phosphate in the olivine structure includes phosphate particles and a coating layer. The phosphate particles include phosphate particles, and the coating layer is coated on the surface of the phosphate particles. The coating layer contains one or more elements of C, Fe, Ti, Zr, Hf, Ge, and Sn. By coating the surface of the phosphate particles with the coating layer, the conductivity of the lithium-containing phosphate in the olivine structure can be improved, the powder resistivity of the material can be reduced, and it is beneficial to the migration rate of lithium ions and the heat generation of the battery cell can be reduced.

[0023] In some embodiments, the phosphate particles include a general formula of Li x1 A y1 Me a M b P 1-c X c Y zA compound, where 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 fast ion conductor with the general formula Li 3-d Fe 2-d M2 d (PO x2 ) y2 where M2 includes one or more elements of Ti, Zr, Hf, Ge, and Sn, 0 ≤ d ≤ 1, 0 < x2 < 5, and 0 < y2 < 4.

[0025] Coating the phosphate particles with the fast ion conductor can significantly improve the transport rate of lithium ions during multiple deintercalation / insertion at the positive electrode, improve the ionic conductivity of the positive electrode active material, and thus increase the specific capacity. Further, the energy density of the corresponding battery cell is increased.

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

[0027] In some embodiments, the mass content of carbon element in the lithium-containing phosphate with olivine structure is 1% to 2%; the specific surface area of the lithium-containing phosphate with olivine structure is 5m 2 / g to 18m 2 / g; optionally 7.5m 2 / g to 14m 2 / g.

[0028] Thus, in the embodiments of the present application, the carbon element with the above mass content and the material with the above specific surface area are more conducive to the effective contact between the electrolyte and the phosphate particles of the phosphate particles, and are beneficial 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 particle size of the positive electrode active material is relatively small, the lithium ion deintercalation path in the positive electrode active material is short, and the heat generation is small; and the particle size of the positive electrode active material is not too small, and basically no agglomeration occurs during the processing and preparation process, so that the performance of the positive electrode active material is stable.

[0030] In some embodiments, the lithium-containing phosphate with an olivine structure is in a granular form, and the lithium-containing phosphate with an olivine structure includes secondary particles formed by agglomeration of primary particles, and the average particle size of the primary particles is 200 nm to 500 nm. The average particle size of the primary particles is relatively small, the lithium ion deintercalation path in the positive electrode active material is short, and the heat generation is less.

[0031] In some embodiments, the positive electrode film layer further includes one or more of ternary materials, lithium phosphate, lithium hydrogen phosphate, lithium sulfate, lithium sulfite, lithium molybdate, lithium oxalate, lithium titanate, lithium tetraborate, lithium metasilicate, lithium metamanganate, lithium tartrate, lithium citrate, lithium nickelate and lithium ferrite. The above materials can supplement lithium ions for the positive electrode film layer, make up for the irreversible lithium ion loss in the system, increase the capacity, and thus increase the energy density of the battery cell.

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

[0033] In some embodiments, the compaction density of the positive electrode film layer of the battery cell at 100% charge state is 2.50 g / cm 3 Up to 2.80g / cm 3 , optional 2.55g / cm 3 Up to 2.70g / cm 3 When the compaction density of the positive electrode film layer 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 layer is densely packed, the contact resistance between particles is small, which can further reduce the resistance of the pole piece, thereby reducing heat generation.

[0034] In some embodiments, the single-sided coating weight of the negative electrode film layer is 90 mg / 1540.25 mm 2 Up to 170mg / 1540.25mm2 ; Optional: 110mg / 1540.25mm 2 Up to 150mg / 1540.25mm 2 When the single-sided coating weight of the negative electrode film layer is within the above range, the heat generated per unit area of the negative electrode sheet will not be too large, and the energy density of the battery cell can be improved.

[0035] In some embodiments, the compaction density of the negative electrode film layer of the battery cell at 100% charge state is 1.15 g / cm 3 Up to 1.36g / cm 3 ; Optional: 1.25g / cm 3 Up to 1.36g / cm 3 When the compaction density of the negative electrode film layer is within the above range, it is beneficial to improve the energy density of the battery cell; and because the negative electrode active materials in the negative electrode film layer are densely packed, the contact resistance between particles is small, which can further reduce the resistance of the pole piece, thereby reducing heat generation.

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

[0037] In some embodiments, the graphite particles include artificial graphite and a carbon coating layer, the artificial graphite includes secondary particles formed by the aggregation of multiple primary particles; the carbon coating layer is coated on the surface of the artificial graphite. The carbon coating layer has more end faces and defects, so that the number of sites that can be used to insert and remove lithium ions is greater, so that the conductivity of the carbon coating layer is relatively excellent, which can reduce the internal resistance of the negative electrode plate and reduce the heat generation of the battery cell.

[0038] In some embodiments, the mass content of the carbon coating layer is 2% to 5% based on the mass of the graphite particles. When the mass content of the carbon coating layer is within the above range, the internal resistance of the negative electrode plate 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, 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, the carbon-based material in the second negative electrode film layer includes graphite particles, and 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, in the embodiments of the present application, there are differences in the particle sizes of the first negative electrode film layer and the second negative electrode film layer, which 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 relatively high, and the bottleneck of fast charging mainly lies in the second negative electrode film layer. In the embodiments of the present application, the particle size of the second negative electrode film layer is relatively small, which can shorten the solid-phase transmission path of lithium ions, improve the fast charging performance, and can also improve the problem of lithium deposition on the surface layer 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 9.5 μm to 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, the 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 layer is 7.8 μm to 14.3 μm. When the volume-average particle size Dv50 of the negative electrode active material in the second negative electrode film layer is within the above range, the tortuosity of lithium ion transmission can be reduced, and the fast charging performance of the battery cell can be improved.

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

[0044] In some embodiments, the tapped 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 . When the tapped density of the carbon-based material in the first negative electrode film layer is within a suitable range, the fast charging performance of the battery cell can be improved.

[0045] In some embodiments, the tapped 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 . When the tapped density of the carbon-based material in the second negative electrode film layer is within a suitable range, the energy density of the battery cell can be improved.

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

[0047] In some embodiments, after the battery cell undergoes 10 full charge test cycles at the beginning of life (BOL), the thickness of the first negative electrode film layer is 15 μm to 65 μm. When the thickness of the first negative electrode film layer is within the above range, the first negative electrode film layer and the second negative electrode film layer can regulate and increase the gradient pore difference between the upper and lower layers, reduce the tortuosity of lithium ion transport, and improve the fast charging ability of the battery cell.

[0048] In some embodiments, after the battery cell undergoes 10 full charge test cycles at the beginning of life (BOL), the thickness of the second negative electrode film layer is 15 μm to 65 μm. When the thickness of the second negative electrode film layer is within the above range, the first negative electrode film layer and the second negative electrode film layer can regulate and increase the gradient pore difference between the upper and lower layers, reduce the tortuosity of lithium ion transport, and improve the fast charging ability of the battery cell.

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

[0050] In some embodiments, after the battery cell undergoes a full charge test at the end of life (EOL), the thickness of the second negative electrode film layer is 15 μm to 70 μm. When the thickness of the second negative electrode film layer is within the above range, the first negative electrode film layer and the second negative electrode film layer can regulate and increase the gradient pore difference between the upper and lower layers, reduce the tortuosity of lithium ion transport, and improve the fast charging ability 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. 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 the present application, the mass content of the second lithium-containing binder in the second negative electrode film layer is relatively high, and the number of free-moving lithium ions provided by the second lithium-containing binder for the second negative electrode film layer is relatively more, 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 mass of 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, the rate of lithium ion deintercalation can be improved, and the fast charging performance of the battery cell can be improved.

[0054] In some embodiments, the mass content of lithium element in the first lithium-containing binder is 3% to 10%, optionally 3% to 8%. When the mass content of lithium element is within the above range, the number of free-moving lithium ions in the negative electrode film layer can be relatively large, which can further shorten the distance for lithium ions to diffuse to the surface of the negative electrode film layer, improve the intercalation and deintercalation rate of lithium ions, 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 mass of the second negative electrode film layer is 0.1% to 1%. When the mass content of lithium element in the second lithium-containing binder is within the above range, the intercalation and deintercalation rate of lithium ions is improved, and the fast charging performance of the battery cell is improved.

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

[0057] In some embodiments, the first lithium-containing binder includes a lithium acrylate-acrylonitrile-acrylamide-2-hydroxyethyl acrylate copolymer, and the lithium acrylate-acrylonitrile-acrylamide-2-hydroxyethyl acrylate copolymer is derived from lithium acrylate monomer, acrylonitrile monomer, acrylamide monomer and 2-hydroxyethyl acrylate monomer, and the molar ratio of lithium acrylate monomer, acrylonitrile monomer, acrylamide monomer and 2-hydroxyethyl acrylate monomer is 30% to 50%: 15% to 45%: 5% to 20%: 20% to 35%.

[0058] Thus, the lithium-containing binder of the above material can provide a certain number of lithium ions for the negative electrode film layer, improve the fast charging performance of the battery cell; and it is not easy to swell during charge and discharge, with a stable structure, so that the cycle performance of the negative electrode film layer is improved during fast charge and discharge.

[0059] In some embodiments, the second lithium-containing binder includes a lithium acrylate-acrylonitrile-acrylamide-2-hydroxyethyl acrylate copolymer, and the lithium acrylate-acrylonitrile-acrylamide-2-hydroxyethyl acrylate copolymer is derived from lithium acrylate monomer, acrylonitrile monomer, acrylamide monomer and 2-hydroxyethyl acrylate monomer, and the molar ratio of lithium acrylate monomer, acrylonitrile monomer, acrylamide monomer and 2-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 amount of lithium ions for the negative electrode film layer, improving the fast charging performance of the battery cell; moreover, it is not prone to swelling during charge and discharge, with a stable structure, enabling the cycle performance of the negative electrode film layer to be improved during fast charge and discharge.

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

[0062] In some embodiments, the separator includes a base film with a porous structure, and the porosity of the base film is 20% to 70%. When the porosity of the separator in the embodiments of the present 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 cell, and thus reduce heat generation.

[0063] In some embodiments, the separator includes a base film with a porous structure, and the porosity of the base film is 35% to 60%. When the porosity of the separator in the embodiments of the present 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 cell, 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 and thus reduce 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 and thus reduce heat generation.

[0066] In some embodiments, the separator includes a base film and a functional layer provided 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, and the second inorganic particles are attached to the surface of the non-fluoropolymer particles and / or dispersed inside the non-fluoropolymer particles. The first functional layer and the second functional layer 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 with the base film.

[0068] In some embodiments, the first inorganic particles include one or more of silica, alumina, boehmite, barium sulfate, calcium oxide, titanium oxide, zinc oxide, magnesium oxide, zirconium oxide, and tin oxide. The above-mentioned 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 silica, alumina, boehmite, barium sulfate, calcium oxide, titanium oxide, zinc oxide, magnesium oxide, zirconium oxide, and tin oxide. The above-mentioned 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 5 nm to 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 compression modulus of the composite particles.

[0071] In some embodiments, the conductivity of the electrolyte is 13 mS / cm to 20 mS / cm. When the conductivity of the electrolyte 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.

[0072] In some embodiments, the viscosity of the electrolyte at room temperature is 2.3 mPa·s to 3.5 mPa·s. When the viscosity of the electrolyte 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.

[0073] In some embodiments, the density of the electrolyte at room temperature is 1.05 g / mL to 1.35 g / mL. When the density of the electrolyte 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.

[0074] In some embodiments, the carboxylic acid ester solvent includes a chain carboxylic acid ester solvent, and the mass content of the chain carboxylic acid ester solvent in the organic solvent is greater than or equal to 5% and less than 75%, optionally greater than or equal to 10%, optionally 30% to 70%, 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 small, which is beneficial to the migration of lithium ions.

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

[0076] Formula I,

[0077] In Formula I,

[0078] R1 includes a hydrogen atom, a halogen atom, a C1-C5 alkyl group, or a C1-C5 haloalkyl group.

[0079] R2 includes a C1-C5 alkyl group or a C1-C5 haloalkyl group.

[0080] Thus, in the embodiments of the present application, the chain carboxylic ester solvent has a relatively high conductivity, which is beneficial to improving the fast charging ability of the battery cell.

[0081] In some embodiments, R1 includes a hydrogen atom, a halogen atom, a C1-C3 alkyl group, or a C1-C3 haloalkyl group.

[0082] In some embodiments, R2 includes a C1-C3 alkyl group or a C1-C3 haloalkyl group.

[0083] In some embodiments, the chain carboxylic ester solvent includes one or more of the compounds represented by Formula I-1 to Formula I-8.

[0084]

[0085] In some embodiments, the organic solvent further includes a carbonate solvent, and the carbonate solvent includes one or more of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate. The above carbonate solvent and the chain carboxylic ester solvent are used in combination, which improves the conductivity of the electrolyte and is beneficial to the migration of lithium ions.

[0086] In some embodiments, the carbonate solvent includes one or more of ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate.

[0087] In some embodiments, the mass content of the carbonate solvent in the organic solvent is 30% to 70%, and may be 30% to 50%. The carbonate solvent with the above mass content can further improve the conductivity of the electrolyte and is beneficial to the migration of lithium ions.

[0088] In some embodiments, the electrolyte further includes an additive, and the additive includes one or more of a carbonate additive, a sulfur-containing additive, and a lithium salt additive. The above additive can improve the interfacial film performance on the positive electrode side and / or the negative electrode side, which is beneficial to improving the fast charging performance of the battery cell and improving the cycle performance.

[0089] In some embodiments, the carbonate additive includes one or more of vinylene carbonate VC and fluoroethylene carbonate FEC.

[0090] In some embodiments, the sulfur-containing additive includes one or more of vinylene sulfate DTD, bis(vinylsulfonyl)ethylene 2-DTD, butylene sulfite BS, 1,3-propane sultone PS, ethylene sulfite ES, and methylene methylene disulfonate MMDS.

[0091] In some embodiments, the lithium salt additive includes one or more of lithium difluorophosphate LiPO2F2, lithium difluorooxalate borate LiDFOB, lithium tetrafluoroborate LiBF4, and lithium bis(oxalato)borate LiBOB.

[0092] In some embodiments, the mass content of the additive in the electrolyte is 1% to 10%, and optionally 2% to 8%. The additive with the above mass content can effectively improve the interfacial film performance on the positive electrode side and / or the negative electrode side, is beneficial to improving the fast charging performance of the battery cell, and improves the cycle performance.

[0093] In some embodiments, the electrolyte further includes a lithium salt, and the lithium salt includes one or more of fluorosulfonylimide salts and lithium hexafluorophosphate LiPF6. The above lithium salts are easy to dissociate, which is beneficial to the rapid migration of lithium ions; and the electrolyte system is relatively stable and not easy to decompose, which can improve the cycle performance of the battery cell.

[0094] In some embodiments, the fluorosulfonylimide salt includes one or more of lithium bis(fluorosulfonyl)imide LiFSI and lithium bis(trifluoromethylsulfonyl)imide LiTFSI.

[0095] In some embodiments, the lithium salt includes lithium bis(fluorosulfonyl)imide LiFSI and lithium hexafluorophosphate LiPF6. The molar concentration of lithium bis(fluorosulfonyl)imide LiFSI is 0.2 mol / L to 0.5 mol / L, and the molar concentration of lithium hexafluorophosphate LiPF6 is 0.5 mol / L to 1.0 mol / L.

[0096] In some embodiments, the ratio of the molar concentration of lithium bis(fluorosulfonyl)imide to the molar concentration of lithium hexafluorophosphate LiPF6 is 0.2 to 1.0.

[0097] In some embodiments, the material of the housing includes steel, and the thickness of the housing is 0.1 mm to 0.5 mm, and optionally 0.2 mm to 0.35 mm. When the thickness of the housing is within the above range, the mechanical strength of the housing is relatively high, which can improve the usage reliability and cycle performance of the battery cell; and the housing occupies less space and there is more internal space in the housing, which is beneficial to improving the energy density of the battery cell.

[0098] In some embodiments, the overcurrent area of a single said first electrode terminal is 25 mm 2 to 315 mm 2; The overcurrent area of the first electrode terminal is relatively large, resulting in a small resistance, which is beneficial to reducing the overall internal resistance of the battery cell.

[0099] In some embodiments, the overcurrent area of a single said second electrode terminal is 25 mm 2 to 315 mm 2 . The overcurrent area of the second electrode terminal is relatively large, resulting in a small resistance, which is beneficial to reducing the overall internal resistance of the battery cell.

[0100] In some embodiments, the battery cell includes a first electrode terminal, the positive electrode tab includes a positive electrode ear, and the first electrode terminal and the positive electrode ear are directly welded. Direct welding can reduce the resistance at the connection, which is beneficial to reducing the overall internal resistance of the battery cell.

[0101] In some embodiments, the battery cell includes a second electrode terminal, the negative electrode tab includes a negative electrode ear, and the second electrode terminal and the negative electrode ear are directly welded. Direct welding can reduce the resistance at the connection, which is beneficial to reducing the overall internal resistance of the battery cell.

[0102] Second, the present application proposes a battery device, which includes the battery cell of any one of the first aspects of the present application.

[0103] In some embodiments, at room temperature, the charging time of the battery device from 10% state of charge to 80% state of charge is 5 min to 10.5 min. The charging speed of the battery device is relatively fast, which is more beneficial to improving the fast charging ability.

[0104] Third, the present application proposes an electrical device, which includes the battery device of any one of the second aspects of the present application.

[0105] Fourth, the present application proposes a charging method for a battery cell. The charging method includes charging the battery cell in a first state of charge so that the increase in the state of charge of the battery cell is 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, where 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, where the cut-off voltage of any step before charging to the second state of charge is less than the cut-off voltage of the step of charging to 100% state of charge, and the cut-off voltage of the step of charging to 100% state of charge does not exceed the theoretical voltage upper limit of the lithium-containing phosphate with olivine structure. Description of the Drawings

[0106] To more clearly illustrate the technical solutions of the embodiments of the present application, the following will briefly introduce the accompanying drawings required for use in the embodiments of the present application. Obviously, the accompanying drawings described below are only some embodiments of the present application. For those of ordinary skill in the art, without creative efforts, other accompanying drawings can be obtained based on the accompanying drawings.

[0107] Figure 1 Schematic structural diagram of a battery cell provided in some embodiments of the present application;

[0108] Figure 2 Explosion schematic diagram of a battery cell provided in some embodiments of the present application;

[0109] Figure 3 Schematic structural diagram of a battery module provided in some embodiments of the present application;

[0110] Figure 4 Schematic structural diagram of a battery pack provided in some embodiments of the present application;

[0111] Figure 5 Schematic structural diagram of an electrical device provided in some embodiments of the present application.

[0112] The accompanying drawings are not necessarily drawn to actual scale.

[0113] The description of the reference numerals is as follows:

[0114] 1. Electrical device; 2. Battery pack; 3. Controller; 4. Motor; 5. Box body; 5a. First box body part; 5b. Second box body part; 5c. Accommodating space; 6. Battery module;

[0115] 7. Battery cell;

[0116] 10. Electrode assembly; 111. First tab; 112. Second tab; 12. Main body part;

[0117] 20. Outer shell; 21. Shell; 22. End cover;

[0118] 31. First electrode terminal; 32. Second electrode terminal. Detailed implementation manners

[0119] Hereinafter, embodiments of the battery cell, its charging method, battery device, and electrical device of the present application will be specifically disclosed in detail with appropriate reference to the accompanying drawings. However, there may be cases where unnecessary details are omitted. For example, there may be cases where details of well-known matters are omitted and repeated descriptions of actually identical structures are omitted. This is to prevent the following description from becoming unnecessarily lengthy and to facilitate the understanding of those skilled in the art. In addition, the accompanying drawings and the following description are provided to enable those skilled in the art to fully understand the present application and are not intended to limit the subject matter recited in the claims.

[0120] The "range" disclosed in the present application is defined in the form of a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundary of a particular range. The ranges defined in this way can include or exclude the end values and can be combined arbitrarily, 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, ranges of 60 to 110 and 80 to 120 are also contemplated. In addition, if the minimum range values 1 and 2 are listed, and if the maximum range values 3, 4, and 5 are listed, then the following ranges are all contemplated: 1 to 3, 1 to 4, 1 to 5, 2 to 3, 2 to 4, and 2 to 5. In the present application, unless otherwise specified, the numerical range "a to b" represents an abbreviated representation of any real number combination between a and b, where a and b are both real numbers. For example, the numerical range "0 to 5" means that all real numbers between "0 to 5" have been fully listed herein, and "0 to 5" is only an abbreviated representation of these numerical combinations. In addition, when a certain parameter is expressed as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, the integers 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0121] If there is no special instruction, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0122] If there is no special instruction, all technical features and optional technical features of the present application can be combined with each other to form a new technical solution.

[0123] If there is no special instruction, all steps of the present application can be carried out in sequence or randomly, preferably in sequence. For example, if a method includes steps (a) and (b), it means that the method can include steps (a) and (b) carried out in sequence, or can also include steps (b) and (a) carried out in sequence. For example, if it is mentioned that the method may further include step (c), it means that step (c) can be added to the method in any order. For example, the method can include steps (a), (b), and (c), or can also include steps (a), (c), and (b), or can also include steps (c), (a), and (b), etc.

[0124] The battery cell includes a positive electrode plate and a negative electrode plate. During the charging process, lithium ions in the positive electrode film layer of the positive electrode plate are removed and migrate to the negative electrode plate, and are embedded in the negative electrode film layer of the negative electrode plate after obtaining electrons. The degree of lithium intercalation in each region of the negative electrode film layer of the negative electrode plate may vary, especially during rapid charging, and the difference in the degree of lithium intercalation is greater, resulting in differences in the state of charge (SOC) corresponding to different regions of the negative electrode plate; correspondingly, a part of the region of the positive electrode film layer is in a lithium-rich state, and another part of the region is in a lithium-poor state.

[0125] The positive electrode film layer includes lithium-containing phosphate with an olivine structure. This type of material has an obvious voltage plateau during the charging process. The positive electrode voltage remains unchanged in the early stage of charging, and the positive electrode voltage upturns at the end of charging, causing the lithium ions in the lithium-rich state to rapidly escape, increasing the local lithium ion activity, and the lithium deposition rate being greater than the lithium intercalation rate, resulting in lithium deposition on the surface of the negative electrode plate and causing problems with the reliability of the battery cell during use.

[0126] In view of the above problems, the embodiments of the present application design the system of the battery cell so that lithium deposition basically does not occur during the charging process of the battery cell, which can improve the reliability of the battery cell during use.

[0127] Battery cell

[0128] In a first aspect, an embodiment of the present application provides a battery cell.

[0129] The battery cell includes an electrolyte and an electrode assembly. The electrode assembly includes a positive electrode plate, a negative electrode plate, and a separator located between the positive electrode plate and the negative electrode plate. The positive electrode plate includes a positive electrode current collector and a positive electrode film layer provided on at least one side of the positive electrode current collector and containing a positive electrode active material. The positive electrode active material includes lithium-containing phosphate with an olivine structure. The negative electrode plate includes a negative electrode current collector and a negative electrode film layer provided on at least one side of the negative electrode current collector. The negative electrode film layer includes a carbon-based material.

[0130] The charging process of the battery cell from 0% state of charge to 100% state of charge includes N charging steps. The difference between the maximum state of charge of any charging step in the N charging steps and the maximum state of charge of its 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;

[0131] In the N charging steps, the cut-off voltage of any charging step in N - 1 charging steps is less than the cut-off voltage of the Nth charging step, and the cut-off voltage of the Nth charging step does not exceed 4.4V.

[0132] In other words, the charging method of the battery cell includes:

[0133] Charge the battery cell in the first state of charge so that the increase in the state of charge of the battery cell is less than or equal to 5% state of charge; that is, in each charging step of the battery cell, the difference between the maximum state of charge and the minimum state of charge of the battery cell is less than or equal to 5% state of charge; optionally, the first state of charge is less than or equal to 5% state of charge.

[0134] Repeat the above steps at least once until the second state of charge is reached, where the second state of charge is greater than or equal to 95% state of charge and less than 100% state of charge.

[0135] Charge the battery cell in the second state of charge to 100% state of charge, where the cut-off voltage for the step of charging to 100% state of charge does not exceed the theoretical voltage upper limit of the lithium-containing phosphate with an olivine structure, and the cut-off voltage for any step before reaching the third state of charge is less than the cut-off voltage for the step of charging to 100% state of charge. Correspondingly, the step of charging to 100% state of charge corresponds to the Nth charging step, that is, the last charging step; any step before reaching the second state of charge refers to the N - 1 charging steps.

[0136] In the embodiments of the present application, N is a positive integer greater than or equal to 2, optionally 20 to 30, such as 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30 or a range composed of any two of the above values.

[0137] The N charging steps refer to the 1st charging step to the Nth charging step; the N - 1 charging steps refer to the 1st charging step to the N - 1th charging step, and any charging step in the N - 1 charging steps refers to any charging step from the 1st charging step to the N - 1th charging step. The Nth charging step is the last charging step of the N charging steps.

[0138] During the charging process of the battery cell, the difference in the maximum state of charge between adjacent charging steps is less than or equal to 5% state of charge, resulting in less polarization during the charging process; before the Nth charging step, that is, the cut-off voltage of any charging step from the 1st charging step to the (N - 1)th charging step is relatively small, so that during the charging process, the voltage does not rise sharply, and the lithium ions released from the positive electrode film layer can basically be embedded in the negative electrode film layer, and lithium deposition is not likely to occur on the surface of the negative electrode plate, which can improve the reliability of the battery cell during use. During the charging process of the battery cell, the state of charge of the battery cell can be used as the basis for jumping to the next charging step. The increase in the state of charge in each step does not exceed 5% state of charge. When the increase in the state of charge reaches 5% state of charge, it will jump to the next charging step; of course, when the increase in the state of charge does not reach 5% state of charge, it can also jump to the next charging step, which can be specifically set according to the actual charging strategy. For example, when the increase in the state of charge is set to 4% state of charge, it will jump to the next charging step when the state of charge increases by 4% state of charge in each charging step. However, it should be noted that the cut-off voltage of any charging step in the (N - 1)th charging step is less than the cut-off voltage of the Nth charging step.

[0139] In the embodiment of the present application, the cut-off voltage of any charging step in the (N - 1)th charging step does not exceed the cut-off voltage of the Nth charging step. In other words, the cut-off voltage of the Nth charging step is greater than or equal to the cut-off voltage of any charging step in the (N - 1)th charging step. Optionally, the cut-off voltage of the Nth charging step is greater than the cut-off voltage of any charging step in the (N - 1)th charging step. Further optionally, the difference between the cut-off voltage of the Nth charging step and the cut-off voltage of any charging step in the (N - 1)th charging step is greater than or equal to 0.02V; even further optionally, the difference between the cut-off voltage of the Nth charging step and the cut-off voltage of any charging step in the (N - 1)th charging step is greater than or equal to 0.05V. In some embodiments, the cut-off voltage of the Nth charging step is greater than the cut-off voltage of any charging step in the (N - 1)th charging step, and the difference between the cut-off voltage of the Nth charging step and the cut-off voltage of any charging step in the (N - 1)th charging step is greater than or equal to 0.05V and less than or equal to 0.2V.

[0140] The embodiment of the present application uses a lithium-containing phosphate in olivine structure, whose charging cut-off voltage is less than or equal to 4.4V. In the last step of charging the battery device, the cut-off voltage is less than or equal to 4.4V, and in the remaining steps, it is less than 4.4V. During the charging process of a battery cell, the cut-off voltage of each charging step can be used as the basis for jumping to the next charging step. For example, it is set that the cut-off voltage of each of the N - 1 charging steps is less than the cut-off voltage of the Nth charging step. Specifically, the cut-off voltage of the Nth charging step is 3.65V, and the cut-off voltage of each of the N - 1 charging steps is 3.6V. When charging to 3.6V during any charging process in the N - 1 charging steps, it jumps to the next charging step; it should be noted that the increase in the state of charge in each charging step is less than or equal to 5% state of charge.

[0141] In the embodiment of the present application, the difference between the maximum state of charge of any charging step among the N charging steps and the maximum state of charge of 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 the range composed of any two of the above values.

[0142] In the embodiment of the present application, the lithium-containing phosphate in olivine structure can include a lithium iron phosphate system or a lithium manganese iron phosphate system.

[0143] Exemplarily, the difference between the cut-off voltage of the Nth charging step and the cut-off voltage of any charging step among 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 the range composed of any two of the above values.

[0144] When the difference between the cut-off voltage of the Nth charging step and the cut-off voltage of any charging step among the N - 1 charging steps is within the above range, the risk of lithium plating on the negative electrode can be further reduced, and the reliability of use of the battery cell can be improved.

[0145] In the embodiments of the present application, the cut-off voltage of the Nth charging step does not exceed the upper limit of the theoretical voltage of the lithium-containing phosphate with an olivine structure. In other words, the cut-off voltage of the Nth charging step is less than or equal to the upper limit of the theoretical voltage of the lithium-containing phosphate with an olivine structure. In some embodiments, the cut-off voltage of the Nth charging step is from 3.65 V to 4.4 V. Exemplarily, the cut-off voltage of the Nth charging step is 3.65 V, 3.7 V, 3.75 V, 3.8 V, 3.85 V, 3.9 V, 3.95 V, 4 V, 4.05 V, 4.1 V, 4.15 V, 4.2 V, 4.25 V, 4.3 V, 4.35 V, 4.4 V or a range composed of any two of the above values.

[0146] When the cut-off 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.

[0147] In some embodiments, the charging rate in the Nth charging step is a charging rate between 0.05 C and 0.30 C; optionally, the charging rate in the Nth charging step is a charging rate between 0.10 C and 0.30 C. Exemplarily, the charging rate in the Nth charging step is 0.05 C, 0.10 C, 0.15 C, 0.20 C, 0.25 C, 0.30 C or a range composed of any two of the above values.

[0148] At the end of charging, the risk of lithium plating on the negative electrode plate increases. When the Nth charging step is charged at the above rate, the risk of lithium plating on the negative electrode plate can be further reduced, and the service reliability of the battery cell can be improved.

[0149] In some embodiments, the battery cell is charged at a rate between 3.5 C and 6 C in the Mth charging step. In the Mth charging step, the state of charge of the battery cell includes a 50% state of charge. M is less than N, and M is a positive integer greater than or equal to 1. Exemplarily, the charging rate of the battery cell in the Mth charging step is 3.5 C, 4 C, 4.5 C, 5 C, 5.5 C, 6 C or a range composed of any two of the above values. For example, M is 8, 9, 10, 11, 12, 13, 14 or 15, etc. When M is 10, it is charged at any charging rate between 3.5 C and 6 C in the 10th charging step.

[0150] In the middle stage of charging, the risk of lithium plating on the negative electrode plate is relatively small, and the charging rate can be increased by high-rate charging; for example, when the battery cell is charged at the above rate in the Mth charging step, the service reliability and fast charging performance of the battery cell can be improved.

[0151] In some embodiments, the battery cell is charged at a rate of 4C to 8C in any charging step from 0% state of charge (SOC) to 40% SOC. Exemplarily, if the adjacent charging steps are spaced 5% SOC apart, from 0% SOC to 5% SOC, from 5% SOC to 10% SOC, from 10% SOC to 15% SOC... in any charging step from 35% SOC to 40% SOC, the charging rate is 4C, 4.5C, 5C, 5.5C, 6C, 6.5C, 7C, 7.5C, 8C or a range composed of any two of the above values. The SOC of the battery cell in the P-th charging step includes 40% SOC, the charging rate from the 1st charging step to the P-th charging step is 4C to 8C, P is less than M, and P is a positive integer greater than or equal to 1. Exemplarily, P is 6, 7, 8 or 9, etc. When P is 8, in the 9 charging steps from the 1st charging step to the 9th charging step, any charging step is charged at a rate of 4C to 8C.

[0152] In the early stage of charging, the risk of lithium plating on the negative electrode plate is relatively small, and the charging rate can be increased by high-rate charging; for example, when the battery cell is charged at the above rate in the early stage of charging, the reliability and fast charging performance of the battery cell can be improved.

[0153] Optionally, the battery cell is charged at a rate of 4C to 8C in the charging steps from 10% SOC to 40% SOC. Exemplarily, the charging rate in the charging steps from 10% SOC to 40% SOC of the battery cell is 4C, 4.5C, 5C, 5.5C, 6C, 6.5C, 7C, 7.5C, 8C or a range composed of any two of the above values. The SOC of the battery cell in the P2-th charging step includes 40% SOC, the SOC of the battery cell in the P1-th charging step includes 10% SOC, the charging rate from the P1-th charging step to the P2-th 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.

[0154] In some embodiments, the constant current value of the Q-th charging step in 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, N. In any charging step among the N charging steps, the constant current value is less than the constant current value of the previous charging step. As the SOC increases, the risk of lithium plating on the negative electrode plate is relatively small, which is beneficial to improving the reliability of the battery cell.

[0155] With the development of the battery cell field, the requirements for the fast charging performance of battery cells are gradually increasing. The embodiments of the present application are particularly applicable to battery cells for fast charging. For example, in an environment of 30°C, the charging time of the battery cell from 10% state of charge to 80% state of charge is less than or equal to 10.5 min, and can be optionally 5 min to 10.5 min. Exemplarily, the charging time of the battery cell from 10% state of charge to 80% state of charge is 10.5 min, 10 min, 9.5 min, 9 min, 8.5 min, 8 min, 7.5 min, 7 min, 6.5 min, 6 min, 5.5 min, 5 min or the range composed of any two of the above values.

[0156] In some embodiments, during the charging process of the battery cell from 10% state of charge to 80% state of charge, the difference between the maximum state of charge of any charging step 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 the range composed of any two of the above values. This charging process can be carried out at room temperature, such as 30°C.

[0157] In some embodiments, the volumetric energy density of the battery cell is 390 Wh / L to 500 Wh / L, and can be optionally 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 the range composed of any two of the above values. The volumetric energy density of the battery cell is relatively high.

[0158] The charging upper limit voltage and the discharge cut-off voltage of the battery cell are different according to different cathode active materials. For example, when the phosphate material includes lithium iron phosphate, the charging upper limit voltage (i.e., the theoretical voltage upper limit) is 3.65 V, the discharge cut-off voltage is 2.0 V, or the charging upper limit voltage is 3.7 V, or the charging upper limit voltage is 3.8 V; and for another example, when the phosphate material includes lithium manganese iron phosphate, the charging upper limit voltage is 4.2 V, the discharge cut-off voltage is 2.5 V, or the charging upper limit voltage is 4.25 V, or the charging upper limit voltage is 4.35 V, or the charging upper limit voltage is 4.4 V.

[0159] In the embodiments of the present application, the volumetric energy density of the battery cell has the meaning well-known in the art and can be detected by using the equipment and methods well-known in the art. Taking the upper charge limit voltage of the battery cell as 3.65V and the discharge cut-off voltage of the battery cell as 2.0V as an example, the battery cell is placed at 25°C and charged at a constant current of 0.33C to 3.65V, and then charged at a constant voltage to 0.05C; discharged at a constant current of 0.33C to 2.0V, and record the discharge capacity A0 at this time, unit: Ah; use a caliper to measure the length, width and height of the battery cell (generally calculated based on the outer shell size of the battery device, excluding the height of the electrode terminal and the insulating film outside the shell), and calculate the volume V0 of the single battery device, unit: L; the volumetric energy density VED of the battery cell = (A0 × discharge platform voltage) / V0, unit: Wh / L.

[0160] [Positive electrode plate]

[0161] The positive electrode plate includes a positive current collector and a positive electrode film layer provided on at least one surface of the positive current collector and including a positive electrode active material. For example, the positive current collector has two surfaces opposite to each other in its own thickness direction, and the positive electrode film layer is provided on any one or both of the two opposite surfaces of the positive current collector.

[0162] Next, taking the upper charge limit voltage as 3.65V and the discharge cut-off voltage as 2.0V as an example, the state of the battery cell will be described. In the embodiments of the present application, the 0% state of charge SOC and the 100% state of charge SOC are defined as follows:

[0163] The battery cell is charged at a constant current rate of 0.33C to the upper limit voltage of the battery cell, and then charged at a constant voltage to 0.05C, corresponding to the 100% SOC state of the battery cell; the battery cell is discharged at a constant current rate of 0.33C to the cut-off voltage, corresponding to the 0% SOC state of the battery cell.

[0164] In some embodiments, when the battery cell is in the 100% state of charge SOC, the tap density of the positive electrode film layer is 2.50 g / cm 3 to 2.80 g / cm 3 ; it can be optionally 2.55 g / cm 3 to 2.70 g / cm 3 . Exemplarily, when the battery cell is in the 100% state of charge SOC, the tap density of the positive electrode film layer is 2.50 g / cm 3 , 2.52 g / cm 3 , 2.55 g / cm 3 , 2.56 g / cm 3 , 2.57 g / cm 3 , 2.58 g / cm 3 , 2.60 g / cm3 , 2.62 g / cm 3 , 2.65 g / cm 3 , 2.68 g / cm 3 , 2.70 g / cm 3 , 2.72 g / cm 3 , 2.75 g / cm 3 , 2.78 g / cm 3 , 2.80 g / cm 3 or a range composed of any two of the above values.

[0165] When the compaction density of the positive electrode film layer is within the above range, it is beneficial to improve the energy density of the single battery cell; and since the positive electrode active materials in the positive electrode film layer are stacked relatively closely, the contact resistance between particles is small, which can further reduce the resistance of the electrode sheet, thereby reducing heat generation.

[0166] In some embodiments, the single-sided coating weight of the positive electrode film layer is 200 mg / 1540.25 mm 2 to 370 mg / 1540.25 mm 2 ; optionally 240 mg / 1540.25 mm 2 to 330 mg / 1540.25 mm 2 . Exemplarily, the single-sided coating weight of the positive electrode film layer is 200 mg / 1540.25 mm 2 , 210 mg / 1540.25 mm 2 , 220 mg / 1540.25 mm 2 , 230 mg / 1540.25 mm 2 , 240 mg / 1540.25 mm 2 , 250 mg / 1540.25 mm 2 , 260 mg / 1540.25 mm 2 , 270 mg / 1540.25 mm 2 , 280 mg / 1540.25 mm 2 , 290 mg / 1540.25 mm 2 , 300 mg / 1540.25 mm 2 , 310 mg / 1540.25 mm 2 , 320 mg / 1540.25 mm 2 , 330 mg / 1540.25 mm 2 , 340 mg / 1540.25 mm 2 , 350 mg / 1540.25 mm 2 , 360 mg / 1540.25 mm 2, 370 mg / 1540.25 mm 2 Or a range composed of any two of the above values.

[0167] When the single-sided coating weight of the positive electrode film layer is within the above range, the heat generation per unit area of the positive electrode plate will not be too large, and it can take into account the improvement of the energy density of the battery cell.

[0168] In the embodiments of the present application, the compaction density of the positive electrode film layer of the battery cell in the 100% state of charge (SOC) can be detected by the following method. Disassemble the positive electrode plate from the battery cell in the 100% state of charge (SOC), and measure the compaction density of the positive electrode film layer. For example, take a single-sided coated positive electrode plate (if it is a double-sided coated plate, the positive electrode film layer on one side can be wiped off first), punch it into small round pieces with an area of S1, weigh it, record it as M1, and measure its thickness H1. Then wipe off the positive electrode film layer of the above-mentioned weighed positive electrode plate, weigh the weight of the positive electrode current collector, record it as M0, and measure its thickness H0. The single-sided coating weight of the positive electrode film layer = (the weight M1 of the positive electrode plate - the weight M0 of the positive electrode current collector) / S1, the thickness of the positive electrode film layer = the thickness H1 of the positive electrode plate - the thickness H0 of the positive electrode current collector, and the compaction density of the positive electrode film layer = the single-sided coating weight of the positive electrode film layer / the thickness of the positive electrode film layer.

[0169] In some embodiments, the powder resistivity of the positive electrode active material is from 1 Ω·cm to 27.5 Ω·cm. Optionally, it is less than or equal to 20 Ω·cm. Optionally, it is less than or equal to 11 Ω·cm. Exemplarily, the powder resistivity of the positive electrode active material 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 composed of any two of the above values.

[0170] The relatively low powder resistivity of the positive electrode active material makes the resistance of the positive electrode plate relatively low and the heat generation of the battery cell less.

[0171] In the embodiments of the present application, the powder resistivity of the material has the meaning well-known in the art, and can be detected by the methods and equipment well-known in the art. For example, according to the test standard GB / T30835-2014, use a PRCD1100 powder resistivity meter for testing.

[0172] In some embodiments, the powder compaction density of the positive electrode active material under 30000 N is 2.46 g / cm 3 to 2.8 g / cm 3Exemplarily, the powder tap density of the positive electrode active material under 30000 N is 2.46 g / cm 3 、2.47 g / cm 3 、2.48 g / cm 3 、2.49 g / cm 3 、2.5 g / cm 3 、2.51 g / cm 3 、2.55 g / cm 3 、2.58 g / cm 3 、2.60 g / cm 3 、2.65 g / cm 3 、2.68 g / cm 3 、2.70 g / cm 3 、2.72 g / cm 3 、2.75 g / cm 3 、2.78 g / cm 3 、2.80 g / cm 3 or a range composed of any two of the above values.

[0173] When the powder tap density of the positive electrode active material under 30000 N is within the above range, the energy density of the battery cell can be improved, and since the positive electrode active material in the positive electrode film layer can be stacked more closely, the contact resistance between particles is small, the resistance of the electrode sheet can be further reduced, thereby reducing heat generation.

[0174] In the embodiments of the present application, the powder tap density of the material has the meaning well-known in the art, and can be detected by methods and equipment well-known in the art. Detection is carried out according to the test standard GB / T24533-2009. For example, a certain amount of positive electrode active material is taken as a sample and added to a mold with a bottom area of 1.327 cm 2 in the UTM7305 type electronic pressure testing machine, pressurized to 3000 kg (equivalent to 30000 N), kept under pressure for 30 s, then depressurized, kept for 10 s, and then the powder tap density of the positive electrode active material under the action of 30000 N is recorded and calculated.

[0175] In some embodiments, the charging specific capacity of the positive electrode active material at a rate of 0.1C is from 150 mAh / g to 170 mAh / g, optionally from 157 mAh / g to 170 mAh / g. Exemplarily, the charging specific capacity of the positive electrode active material at a rate of 0.1C 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 composed of any two of the above values.

[0176] When the charging specific capacity of the positive electrode active material at a rate of 0.1C is within the above range, the energy density of the battery cell is relatively high.

[0177] In the embodiments of the present application, the specific capacity of the active material has the meaning well-known in the art and can be tested by the equipment and methods well-known in the art. The test method for the initial Coulomb efficiency and the initial discharge specific capacity in Appendix G of the national standard GB / T 24533-2019 can be adopted. Using metallic lithium as the negative electrode and the sample electrode sheet containing the above materials as the positive electrode, a half-button cell is assembled. Under the condition of 23°C ± 2°C, the half-button cell is placed on a battery tester or other test equipment with equivalent performance, and the discharging capacity is obtained through charge and discharge at a rate of 0.1C. Then, the capacity is divided by the mass of the active material of the electrode sheet to obtain the charging specific capacity parameter.

[0178] In some embodiments, the mass fraction of the lithium-containing phosphate in the olivine structure in the positive electrode active material can be greater than or equal to 80% and less than or equal to 100%. It can be considered that the positive electrode active material of the present application is a lithium-containing phosphate system with an olivine structure. When the mass fraction of the lithium-containing phosphate in the olivine structure is less than 100%, the positive electrode active material can further include common positive electrode active materials, for example, it can include but is not limited to at least one of lithium-containing transition metal oxides. Examples of the lithium-containing transition metal oxides can 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.

[0179] Optionally, the mass fraction of the lithium-containing phosphate in the olivine structure in the positive electrode active material is 100%.

[0180] In the embodiments of the present application, the lithium-containing phosphate in the olivine structure may be phosphate particles or a material obtained by coating and modifying them. For example, the lithium-containing phosphate in the olivine structure includes phosphate particles and a coating layer, the coating layer covers the surface of the phosphate particles, and the coating layer contains one or more elements among C, Fe, Ti, Zr, Hf, Ge, and Sn.

[0181] By coating the surface of the phosphate particles with a coating layer, the conductivity of the lithium-containing phosphate in the olivine structure can be improved, the powder resistivity of the material can be reduced, and it is beneficial to the migration rate of lithium ions, improving the fast charging ability of the battery and reducing the heat generation of the battery cell.

[0182] In some embodiments, the phosphate particles include a compound with the general formula Li x1 A y1 Me a M b P 1-c X c Y z where 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 phosphate particles have relatively excellent cycle stability, which is beneficial to improving the cycle performance of the battery cell.

[0183] Exemplarily, the phosphate particles include one or more of LiFePO4, LiMnPO4, LiNiPO4, and LiCoPO4. The battery cell will be accompanied by the deintercalation and consumption of active ions such as Li during the charging and discharging process, and the molar content of Li in the battery cell is different when it is discharged to different states. In the enumeration of positive electrode active materials LiFePO4, LiMnPO4, LiNiPO4, LiCoPO4, etc., the molar content of Li is the initial state of the material, that is, the state before feeding. The positive electrode active material is used in the battery system, and the molar content of Li may change after the charge and discharge cycle. In the enumeration of positive electrode active materials LiFePO4, LiMnPO4, LiNiPO4, LiCoPO4, etc. in the embodiment of the present application, the molar content of oxygen O is only a theoretical state value. The release of lattice oxygen will cause the molar content of oxygen O to change. In fact, the molar content of oxygen O will fluctuate. The above situations are all within the scope of protection of the present application.

[0184] In some embodiments, the coating layer includes a general formula of Li 3-d Fe 2-d M2 d (PO x2 ) y2 The fast ion conductor M2 includes one or more elements of Ti, Zr, Hf, Ge and Sn, 0≤d≤1, 0<x2<5, 0<y2<4.

[0185] Exemplarily, the fast ion conductor is a material having a NASICON structure, for example, including one or more of lithium iron titanium phosphate Li2FeTi(PO4)3, lithium iron zirconium phosphate Li2FeZr(PO4)3, and lithium iron tin phosphate Li2FeSn(PO4)3.

[0186] Fast ion conductors with NASICON structures are materials with ultrafast ion conduction capabilities, rich three-dimensional lithium ion diffusion and transmission channels, and have the advantages of high ion conduction efficiency and strong structural stability during multiple lithium removal and insertion processes. Coating fast ion conductors with NASICON structures on the surface of phosphate particles can significantly increase the transmission rate of lithium ions during multiple lithium removal / insertion at the positive terminal, improve the ionic conductivity of the positive electrode active material, and improve the fast charging capability of the battery cell. In addition, it can also increase the gram capacity and the energy density of the corresponding battery cell.

[0187] In some embodiments, the coating layer further includes carbon.

[0188] The carbon element and the fast ion conductor can be arranged in layers. For example, the carbon element serves as an independent carbon coating layer, and the fast ion conductor serves as an independent fast ion conductor layer. The carbon coating layer can coat the surface of phosphate particles, and the fast ion conductor layer is located on the surface of the carbon coating layer, that is, the fast ion conductor layer is located on the side of the carbon coating layer away from the phosphate particles. Or, the fast ion conductor layer can coat the surface of phosphate particles, and the carbon coating layer is located on the surface of the fast ion conductor layer, that is, 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 arranged in the same layer.

[0189] Optionally, the carbon coating layer can be formed by carbonizing an organic carbon source (such as glucose, polyethylene glycol, etc.) on the surface of the fast ion conductor layer. The carbon coating layer can partially coat the fast ion conductor layer or completely coat the fast ion conductor layer. The setting of the carbon coating layer can significantly improve the electronic conductivity of phosphate particles, make up for the defect of poor electronic conduction performance of phosphate particles, and improve the energy density of a single battery.

[0190] Specifically, the setting of the carbon coating layer endows the positive electrode active material of the present application with the following advantages:

[0191] The carbon coating layer in the positive electrode active material of the present application provides a suitable channel for the transmission of electrons, can significantly improve the conduction rate of electrons during multiple de-lithiation and lithiation processes, improve the electronic conductivity of lithium-containing phosphates, improve the charging ability of the corresponding single battery, and can also improve the energy density.

[0192] The carbon coating layer of the positive electrode active material of the present application is loose and porous, which enables the electrolyte to make full and effective contact with the lithium-containing phosphates, thereby improving the transmission rate of lithium ions at the phase interface and improving the charging ability of a single battery.

[0193] Coating a layer of carbon coating layer on the surface of lithium-containing phosphates can not only improve the conductivity of lithium-containing phosphates, but also improve the structural stability of the positive electrode active material, effectively alleviate the iron dissolution phenomenon of the positive electrode active material during the long-term storage and cyclic use of a single battery, and thus improve the cycle life of a single battery.

[0194] The positive electrode active material of the present application uses lithium-containing phosphates as the base material, giving full play to the advantages of low cost, high use reliability, and good cycle stability of lithium-containing phosphates. At the same time, the coating layers (fast ion conductor layer and carbon coating layer) are used to solve the disadvantages of poor electronic conductivity and ion conductivity. The single battery prepared from the positive electrode active material of the present application can improve the energy density of the single battery on the premise of excellent cycle performance.

[0195] In the embodiments of the present application, the content of elements in the positive electrode active material has the meaning well-known in the art, and can be detected by equipment and methods well-known in the art. For example, referring to EPA 6010D-2014, it is tested by inductively coupled plasma atomic emission spectrometry, and determined by inductively coupled plasma optical emission spectrometry (ICP-OES, instrument model: Thermo ICAP7400). After discharging the battery cell to 0% state of charge (SOC), the positive electrode plate is disassembled, washed with DMC and dried, and then after removing impurities by high-temperature calcination, 0.4 g of the positive electrode active material is weighed, and 10 ml (50% concentration) of aqua regia is added thereto. Then it is placed on a flat plate at 180 °C for 30 min. After digestion on the flat plate, it is fixed to a volume of 100 mL, and quantitative testing is carried out by the standard curve method.

[0196] In some embodiments, the graphitization degree of the positive electrode active material is 0.15 to 0.32, and can be optionally 0.19 to 0.26. Exemplarily, the graphitization degree 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 the range composed of any two of the above values.

[0197] When the graphitization degree 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 plate, and thus reduce the heat generation of the battery cell.

[0198] In the embodiments of the present application, the higher the graphitization degree of the material, the lower the degree of disorder, and it can be tested according to the general rules of X-ray diffraction analysis method of JIS / K 0131-1996.

[0199] In some embodiments, the mass content of carbon element in the lithium-containing phosphate with olivine structure is 1% to 2%, and the specific surface area of the lithium-containing phosphate with olivine structure is 5 m 2 / g to 18 m 2 / g.

[0200] Optionally, the mass content of carbon element in the lithium-containing phosphate with olivine structure is 1% to 2%, and the specific surface area of the lithium-containing phosphate with olivine structure is 7.5 m 2 / g to 14 m 2 / g.

[0201] Exemplarily, the mass content of carbon element in the lithium-containing 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 the range composed of any two of the above values.

[0202] Exemplarily, the specific surface area of the lithium-containing phosphate with an olivine structure is 5 m 2 / g, 6 m 2 / g, 7 m 2 / g, 8 m 2 / g, 9 m 2 / g, 10 m 2 / g, 11 m 2 / g, 12 m 2 / g, 13 m 2 / g, 14 m 2 / g, 15 m 2 / g, 16 m 2 / g, 17 m 2 / g, 18 m 2 / g or a range composed of any two of the above values.

[0203] Carbon mainly exists in the form of a carbon coating layer in the coating layer. The carbon coating layer is loose and porous, which is beneficial to increasing the specific surface area of the material, more beneficial to the effective contact between the electrolyte and the phosphate particles, and beneficial to the transmission 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 the lithium-containing phosphate with an olivine structure, which is beneficial to enhancing the ionic conductivity and electronic conductivity of the lithium-containing phosphate with an olivine structure, and can improve the rapid charging ability and energy density of the battery cell.

[0204] In the embodiments of the present application, the specific surface area of the material has the meaning well-known in the art, and can be detected by using the equipment and methods well-known in the art. For example, it can be detected according to the test standard GB / T 19587-2017. Taking the positive electrode active material as a sample, the specific surface area is tested by a Tri-Star 3020 specific surface area and pore size analyzer of Micromeritics Company, USA.

[0205] In some embodiments, the volume distribution particle size of the positive electrode active material satisfies: 1 µm ≤ Dv50 ≤ 2 µm, 0.4 µm ≤ Dv10 ≤ 0.7 µm.

[0206] Exemplarily, 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 a range composed of any two of the above values.

[0207] Exemplarily, 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 a range composed of any two of the above values.

[0208] The particle size of the positive electrode active material is relatively small, the lithium deintercalation / insertion path of lithium ions in the positive electrode active material is short, and the heat generation is less. Moreover, the particle size of the above positive electrode active material is not too small, and basically no agglomeration occurs during the processing and preparation process, making the performance of the positive electrode active material stable.

[0209] In the embodiments of the present application, the volume average particle size Dv50 of the material refers to the particle size corresponding to 50% in the volume distribution, and the volume average particle size Dv10 of the material refers to the particle size corresponding to 10% in the volume distribution. It can be detected by using equipment and methods well-known in the art. For example, taking the positive electrode active material as a sample, according to the test standard GB / T 19077-2016, the Dv50 and Dv10 of the particles are tested by a Mastersizer 2000E laser particle size analyzer, etc.

[0210] When the positive electrode active material includes other materials in addition to the lithium-containing phosphate with an 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.

[0211] In some embodiments, the lithium-containing phosphate with an olivine structure is in a granular shape. The lithium-containing phosphate with an olivine structure includes secondary particles, and the secondary particles include a plurality of primary particles. The average particle size of the primary particles is from 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 composed of any two of the above values.

[0212] The average particle size of the primary particles is relatively small, the lithium deintercalation / insertion path of lithium ions in the positive electrode active material is short, and the heat generation is less.

[0213] In the embodiments of the present application, the secondary particles refer to agglomerated particles formed by the aggregation of two or more primary particles. The primary particles and the secondary particles can be easily distinguished by experimental means (such as taking SEM images using a scanning electron microscope), and the average particle size of the primary particles can be obtained by testing in the SEM image of the scanning electron microscope. The SEM test parameters can be set as: the working voltage (EHT) is 10.00 kV, using an InLens detector, the working distance is 4.6 mm, and the magnification is 1000X.

[0214] In some embodiments, the positive electrode film layer further includes one or more of ternary materials, lithium phosphate, lithium hydrogen phosphate, lithium sulfate, lithium sulfite, lithium molybdate, lithium oxalate, lithium titanate, lithium tetraborate, lithium metasilicate, lithium manganate, lithium tartrate, trilithium citrate, lithium nickelate, and lithium ferrate. The above materials can be used as lithium supplements. The lithium supplement can supplement lithium ions to the positive electrode film layer, make up for the irreversible loss of lithium ions in the system, and improve the capacity, thereby improving the energy density of the battery cell.

[0215] Optionally, the ternary material includes Li x3 A y3 Ni a3 Co b3 Mn c M3 (1-a3-b3-c3) Y3 z3 , where 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 several of Na, K, and Mg, M3 includes one or several 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, Ce, and Y3 includes one or several of O and F.

[0216] Exemplarily, the ternary materials include at least one of 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 O2.

[0217] In some embodiments, the mass content of the lithium supplement in the positive electrode film layer is 0.5% to 5%, such as 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, or a range composed of any two of the above values. When the mass content of the lithium supplement is within the above range, it can supplement lithium ions to the positive electrode film layer, make up for the irreversible lithium ion loss in the system, and improve the capacity, thereby improving the energy density of the battery cell.

[0218] The lithium supplement can be in the same layer as the positive electrode active material or in different layers. When the lithium supplement and the positive electrode active material are in different layers, the lithium supplement can be in the lithium supplement layer, and the positive electrode active material can be in the positive electrode active material layer. In other words, the positive electrode film layer includes a lithium supplement 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 supplement layer can be located between the positive electrode active material layer and the positive electrode current collector. Or, the lithium supplement 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 supplement layer and the positive electrode current collector. Optionally, the lithium supplement layer can be located between the positive electrode active material layer and the positive electrode current collector. During the charge and discharge cycle of the battery cell, the lithium supplement in the lithium supplement layer can be gradually released into the system to make up for the lithium loss in the battery system.

[0219] In some embodiments, the positive electrode film layer may further optionally include a positive electrode conductive agent. There is no particular limitation on the type of the positive electrode conductive agent in the embodiments of the present application. As an example, the positive electrode conductive agent includes at least one of superconducting carbon, conductive graphite, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. In some embodiments, based on the mass of the positive electrode film layer, the mass content of the positive electrode conductive agent is ≤5%.

[0220] In some embodiments, the positive electrode film layer may further optionally include a positive electrode binder. There is no particular limitation on the type of the positive electrode binder in the embodiments of the present application. As an example, the positive electrode binder may include at least one of polyvinylidene fluoride, polytetrafluoroethylene, vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, polyacrylic acid, and fluorinated acrylate resins. In some embodiments, based on the mass of the positive electrode film layer, the mass content of the positive electrode binder is ≤5%.

[0221] In some embodiments, the positive current collector can be a metal foil or a composite current collector. As an example of the metal foil, at least one of aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy foils can be used. The composite current collector can include a polymer material base layer and a metal material layer formed on at least one surface of the polymer material base layer. As an example, the metal material of the metal material layer can include at least one of aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy. As an example, the polymer material base layer can include at least one of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE).

[0222] In some embodiments, the ratio of the thickness of the positive current collector to the thickness of the single-sided positive electrode film layer is from 0.05 to 0.3. Exemplarily, the ratio of the thickness of the positive current collector to the thickness of the single-sided positive electrode film layer 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 composed of any two of the above values.

[0223] When the ratio of the thickness of the positive current collector to the thickness of the single-sided positive electrode film layer is within the above range, the fast charging ability and energy density of the battery cell can be improved.

[0224] In some embodiments, the thickness of the positive current collector is from 10 μm to 15 μm, and can be optionally from 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 composed of any two of the above values.

[0225] When the thickness of the positive current collector is within the above range, the current-carrying capacity of the positive current collector is relatively excellent, and the battery cell can have a high energy density.

[0226] In the embodiments of the present application, the thicknesses of the positive electrode film layer and the positive current collector have meanings well-known in the art, and can be detected by devices and methods well-known in the art. For example, the thickness of the positive electrode plate is measured with a micrometer, the film layer on the surface of the positive current collector is removed, and the thickness of the positive current collector is measured with a micrometer. When the positive electrode film layer is coated on one side, the thickness of the positive electrode film layer is the thickness of the positive electrode plate minus the thickness of the positive current collector. When the positive electrode film layer is coated on both sides, the thickness of the positive electrode film layer is (the thickness of the positive electrode plate minus the thickness of the positive current collector) / 2.

[0227] The positive electrode film layer is usually formed by coating a positive electrode slurry on a positive electrode current collector and then drying and cold pressing. The positive electrode slurry is usually formed by dispersing a positive electrode active material, an optional conductive agent, an optional binder, and any other components in a solvent and stirring evenly. The solvent can be N-methylpyrrolidone (NMP), but is not limited thereto.

[0228] The positive electrode tab does not exclude other additional functional layers besides the positive electrode film layer. For example, in some embodiments, the positive electrode tab of the embodiment of the present application further includes a positive electrode conductive layer disposed on the surface of the positive electrode current collector and sandwiched between the positive electrode current collector and the positive electrode film layer. In some other embodiments, the positive electrode tab of the embodiment of the present application further includes a protective layer covering the surface of the positive electrode film layer.

[0229] In some embodiments, the positive electrode tab further includes a positive electrode conductive layer located between the positive electrode film layer and the positive electrode current collector. The positive electrode conductive layer can further improve the conductivity of the positive electrode tab and reduce the heat generation of the positive electrode tab, thereby reducing the heat generation of the battery cell.

[0230] In some embodiments, the thickness of the positive electrode conductive layer is from 0.5 μm to 2 μm. Exemplarily, 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 a range composed of any two of the above values.

[0231] When the thickness of the positive electrode conductive layer is within the above range, it can further improve the conductivity of the positive electrode tab, reduce the heat generation of the positive electrode tab, thereby reducing the heat generation of the battery cell, and can also take into account the improvement of the energy density of the battery cell.

[0232] In the embodiment of the present application, the thickness of the positive electrode conductive layer has the meaning well known in the art and can be detected by using equipment and methods well known in the art. For example, tomography is performed on the positive electrode tab to directly measure the thickness of the positive electrode conductive layer.

[0233] In some embodiments, the positive electrode conductive layer includes one or more of a positive electrode conductive agent and a positive electrode binder.

[0234] Optionally, the mass content of the positive electrode conductive agent in the positive electrode conductive layer is from 30% to 50%. Exemplarily, the mass content of the positive electrode conductive agent is 30%, 35%, 40%, 45%, 50% or a range composed of any two of the above values.

[0235] Exemplarily, 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 tab and reducing the heat generation of the battery cell.

[0236] Optionally, the mass content of the positive electrode binder in the positive electrode conductive layer is 50% to 70%. Exemplarily, 50%, 60%, 65%, 70% or a range composed of any two of the above values.

[0237] Exemplarily, the positive electrode binder includes one or more of polyvinylidene fluoride, polytetrafluoroethylene, vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, polyacrylic acid, and fluorinated acrylate resins. The positive electrode binder in the positive electrode conductive layer can improve the adhesion performance between the positive electrode current collector and the positive electrode film layer, and improve the structural stability of the positive electrode plate.

[0238] [Negative electrode plate]

[0239] The negative electrode plate includes a negative electrode current collector and a negative electrode film layer provided on at least one surface of the negative electrode current collector and including a negative electrode active material. For example, the negative electrode current collector has two surfaces opposite to each other in its own thickness direction, and the negative electrode film layer is provided on any one or both of the two opposite surfaces of the negative electrode current collector.

[0240] In some embodiments, when the battery cell is in a 100% charged state, the tap density of the negative electrode film layer is 1.15 g / cm 3 to 1.36 g / cm 3 , optionally 1.25 g / cm 3 to 1.36 g / cm 3 . Exemplarily, when the battery cell is in a 100% charged state, the tap density of the negative electrode film layer is 1.15 g / cm 3 , 1.18 g / cm 3 , 1.20 g / cm 3 , 1.22 g / cm 3 , 1.25 g / cm 3 , 1.28 g / cm 3 , 1.3 g / cm 3 , 1.32 g / cm 3 , 1.35 g / cm 3 , 1.36 g / cm 3 or a range composed of any two of the above values.

[0241] When the tap density of the negative electrode film layer is within the above range, it is beneficial to improve the energy density of the battery cell, and since the negative electrode active materials in the negative electrode film layer are stacked relatively tightly, the contact resistance between particles is small, which can further reduce the resistance of the electrode plate, thereby reducing heat generation.

[0242] In the embodiments of the present application, the compaction density of the negative electrode film layer in the 100% charged state is a well-known meaning in the art, and can be detected by using well-known equipment and methods in the art. The detection method is the same as the compaction density test method of the positive electrode film layer described above.

[0243] In some embodiments, the single-sided coating weight of the negative electrode film layer is 90 mg / 1540.25 mm 2 to 170 mg / 1540.25 mm 2 , and can be optionally 110 mg / 1540.25 mm 2 to 150 mg / 1540.25 mm 2 . Exemplarily, the single-sided coating weight of the negative electrode film layer is 90 mg / 1540.25 mm 2 , 92 mg / 1540.25 mm 2 , 95 mg / 1540.25 mm 2 , 96 mg / 1540.25 mm 2 , 100 mg / 1540.25 mm 2 , 102 mg / 1540.25 mm 2 , 104 mg / 1540.25 mm 2 , 105 mg / 1540.25 mm 2 , 108 mg / 1540.25 mm 2 , 110 mg / 1540.25 mm 2 , 112 mg / 1540.25 mm 2 , 114 mg / 1540.25 mm 2 , 115 mg / 1540.25 mm 2 , 116 mg / 1540.25 mm 2 , 118 mg / 1540.25 mm 2 , 120 mg / 1540.25 mm 2 , 122 mg / 1540.25 mm 2 , 125 mg / 1540.25 mm 2 , 128 mg / 1540.25 mm 2 , 130 mg / 1540.25 mm 2 , 132 mg / 1540.25 mm 2 , 135 mg / 1540.25 mm 2 , 137 mg / 1540.25 mm 2 , 140 mg / 1540.25 mm 2 , 142 mg / 1540.25 mm 2, 145 mg / 1540.25 mm 2 , 148 mg / 1540.25 mm 2 , 150 mg / 1540.25 mm 2 , 152 mg / 1540.25 mm 2 , 155 mg / 1540.25 mm 2 , 160 mg / 1540.25 mm 2 , 165 mg / 1540.25 mm 2 , 170 mg / 1540.25 mm 2 Or a range composed of any two of the above values.

[0244] When the single-sided coating weight of the negative electrode film layer is within the above range, the heat generation per unit area of the negative electrode sheet will not be too large, and it can also take into account the improvement of the energy density of the battery cell.

[0245] In the embodiments of the present application, the single-sided coating weight of the negative electrode film layer has the meaning well-known in the art, and can be detected by the equipment and methods well-known in the art. The detection method is as described in the single-sided coating weight test method of the film layer above.

[0246] In some embodiments, the powder resistivity of the negative electrode active material is from 0.005 Ω·cm to 0.043 Ω·cm, and can be optionally 0.04 Ω·cm. Exemplarily, the powder resistivity of the negative electrode active material 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 composed of any two of the above values.

[0247] The relatively low powder resistivity of the negative electrode active material results in a relatively low resistance of the negative electrode sheet and less heat generation of the battery cell.

[0248] In the embodiments of the present application, the powder resistivity of the negative electrode active material has the meaning well-known in the art, and can be detected by the equipment and methods well-known in the art. The detection method is as described in the powder resistivity test method of the positive electrode active material above.

[0249] In some embodiments, the powder compaction density of the negative electrode active material under a pressure of 20000 N is 1.5 g / cm 3 to 1.85 g / cm 3 , and can be optionally 1.55 g / cm 3 to 1.65 g / cm 3 . Exemplarily, the powder compaction density of the negative electrode active material under a pressure of 20000 N is 1.5 g / cm 3, 1.55 g / cm 3 , 1.6 g / cm 3 , 1.65 g / cm 3 , 1.7 g / cm 3 , 1.75 g / cm 3 , 1.8 g / cm 3 , 1.85 g / cm 3 or a range composed of any two of the above values.

[0250] When the powder compaction density of the negative electrode active material is within the above range under 20,000 N, the energy density of the battery cell can be improved. Moreover, since the negative electrode active material in the negative electrode film layer can be stacked more closely and the contact resistance between particles is small, the resistance of the electrode sheet can be further reduced, thereby reducing heat generation.

[0251] In the embodiments of the present application, the powder compaction density of the material has the meaning well known in the art and can be detected by methods and equipment well known in the art, and is detected according to the test standard GB / T24533 - 2009. As an example, a certain amount of negative electrode active material is taken as a sample and added to a mold with a bottom area of 1.327 cm 2 in the UTM7305 type electronic pressure testing machine, pressurized to 2000 kg (equivalent to 20,000 N), held for 30 s, then depressurized, held for 10 s, and then the powder compaction density of the negative electrode active material under the action of 20,000 N is recorded and calculated.

[0252] In some embodiments, the charging specific 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 charging specific 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 composed of any two of the above values.

[0253] When the charging specific 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.

[0254] In the embodiments of the present application, the charging specific capacity of the negative electrode active material at a rate of 0.1C has the meaning well-known in the art and can be detected by the equipment and methods well-known in the art. The detection method is the same as the charging specific capacity test method of the positive electrode active material at a rate of 0.1C described above.

[0255] In some embodiments, the negative electrode active material includes a carbon-based material. The carbon-based material has high cycle stability and can improve the cycle performance of the battery cell. Optionally, the mass ratio 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%.

[0256] The positive electrode active material of the present application is mainly a lithium-containing phosphate system with an olivine structure, and the negative electrode active material is mainly a carbon-based material system. When used in combination, the battery cell has excellent cycle performance.

[0257] Optionally, the carbon-based material includes graphite particles, and the graphitization degree of the graphite particles is 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 the range composed of any two of the above values.

[0258] When the graphitization degree of the graphite particles is within the above range, the graphite particles have excellent electrical conductivity, can reduce the heat generation of the negative electrode sheet and the battery cell, and can improve the fast charging performance of the battery cell.

[0259] In some embodiments, the graphite particles include artificial graphite and a carbon coating layer. The artificial graphite includes secondary particles, and the secondary particles include a plurality of primary particles. The plurality of primary particles aggregate to form secondary particles, and the carbon coating layer coats the surface of the artificial graphite. Amorphous carbon refers to a transitional carbon material with a very low degree of graphitization crystallization, approximately in an amorphous form (or without a fixed shape and periodic structural rules). In the present application, amorphous carbon refers to the product after carbonization treatment of an organic carbon source.

[0260] The artificial graphite includes secondary particles. In the artificial graphite, there are more migration paths for lithium ions, and the migration path in the primary particles is shorter, which can improve the migration rate of lithium ions. The carbon coating layer has more end faces and defects, resulting in a larger number of sites where lithium ions can be intercalated and deintercalated, making the electrical conductivity of the carbon coating layer excellent, and can reduce the internal resistance of the negative electrode sheet and the heat generation of the battery cell.

[0261] Optionally, based on the mass of the graphite particles, the mass content of the carbon coating layer is 2% to 5%. Exemplarily, the mass content of the carbon coating layer is 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5% or the range composed of any two of the above values.

[0262] When the mass content of the carbon coating layer 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.

[0263] In the embodiments of the present application, the graphite particles can be prepared by methods well-known in the art. For example, the preparation method includes: providing artificial graphite and an organic carbon source, mixing the two, and after carbonization treatment, a carbon coating layer is formed on at least a part of the surface of the artificial graphite particles.

[0264] Optionally, the organic carbon source includes one or more of coal tar pitch, petroleum pitch, phenolic resin, and coconut shell. Further optionally, the organic carbon source includes petroleum pitch. Optionally, the softening point of the coal tar pitch and petroleum pitch is below 250 °C.

[0265] Optionally, the carbonization treatment temperature is 700 °C to 1800 °C. Optionally, the carbonization treatment temperature is 1000 °C to 1300 °C. When the carbonization treatment temperature is within a suitable range, the organic carbon source can be carbonized, and a coating layer containing amorphous carbon is formed on at least a part of the surface of the artificial graphite.

[0266] Optionally, the carbonization treatment time is 1 h to 6 h.

[0267] In the embodiments of the present application, the morphology and mass content of amorphous carbon have meanings well-known in the art, and can be detected by equipment and methods well-known in the art. For example, the confirmation of the amorphous carbon layer can be obtained by using conventional methods for testing the crystal form of carbon substances in the art. For example, Raman spectroscopy can be used for testing and analysis. The formation of the amorphous carbon layer can be analyzed according to the characteristic peak information of the carbon component in the spectrum (such as the intensity ratio of the D peak to the G peak, I D / G ). Among them, both the D peak and the G peak are Raman characteristic peaks of the carbon atom crystal. The D peak represents the defects of the carbon atom crystal. The more defects, the greater the intensity of the D peak. The intensity of the D peak can reflect the content of the amorphous (disordered stacking) region. The G peak represents the in-plane stretching vibration of the sp2 hybridization of carbon atoms. The intensity of the G peak can reflect the content of the graphitized (layered structure) region; as the degree of disorder of carbon atoms increases, the intensity ratio of the D peak to the G peak also increases. It can be determined whether an amorphous carbon layer is formed according to whether the I D / G (intensity ratio of the D peak to the G peak) of the surface composition of the particles before and after coating constitutes a statistically significant change (such as p < 0.05, p < 0.01, etc.), that is, according to the deviation degree of the layer structure introduced after coating from the I D / G before coating. This is easy for those skilled in the art to judge. It is also possible to compare the Raman spectrum I D / G of a specific structural layer of the particle to be tested with the standard Raman spectrum I D / G of graphite to determine whether it is an amorphous carbon layer.

[0268] The graphitization degree of the graphite particles can be detected by the detection method of the graphitization degree of the cathode active material.

[0269] In the embodiments of the present application, regarding the mass ratio of the "amorphous carbon layer" in the graphite particles, it can be obtained by combining TEM (transmission electron microscopy) morphology observation and data analysis. Cut the negative electrode sheet, scrape the powder sample from the cross-section, conduct TEM tests, and compare the graphite particles with and without the amorphous carbon layer coated. On the surface of the graphite particles coated with the amorphous carbon layer, an obvious interface can be observed. Thus, the thickness of the amorphous carbon layer at this position can be estimated from the TEM photos. Analyses can be carried out from multiple different cross-section positions, and the average value is taken as the thickness of the amorphous carbon layer. Assuming that the graphite particles are spherical, according to the thickness of the amorphous carbon layer and the particle size statistical data of the graphite particles, the volume ratio 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; further, according to the average mass of a single graphite particle, the volume of artificial graphite (which can be deduced or can also be statistically obtained from the TEM test photos), and the density of artificial graphite, the mass content of the amorphous carbon layer in the graphite particles can be estimated.

[0270] In some embodiments, the carbon-based material may further include natural graphite. Specifically, the carbon-based material may include graphite particles, or the carbon-based material may include graphite particles and natural graphite. Optionally, the carbon-based material is graphite particles.

[0271] In some embodiments, the negative electrode active material may further include a silicon-based material. The introduction of the silicon-based material can improve the capacity of the negative electrode active material and increase the energy density of the battery cell.

[0272] 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%, and may be 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 the range composed of any two of the above values.

[0273] When the mass content of silicon element in the silicon-based material is within the above range, it can improve the capacity of the negative electrode active material, and can make the negative electrode active material have better cycle stability, thereby improving the energy density and cycle performance of the battery cell.

[0274] Optionally, the silicon-based material may include at least one of elemental silicon, silicon oxide, silicon-carbon composite, silicon-nitrogen composite, and silicon alloy material.

[0275] In some embodiments, in addition to the above-mentioned carbon-based material and optional silicon-based material, the negative electrode active material may further include at least one of a tin-based material and lithium titanate. The tin-based material may include at least one of elemental tin, tin oxide, and tin alloy material.

[0276] In this application, the qualitative and quantitative determination of each substance or element can be detected by suitable equipment and methods known to those skilled in the art. Relevant detection methods can refer to domestic and foreign detection standards, domestic and foreign enterprise standards, etc. And those skilled in the art can also adaptively change certain detection steps / instrument parameters, etc. from the perspective of detection accuracy to obtain more accurate detection results. One detection method can be used for qualitative or quantitative determination, or several detection methods can be used in combination for qualitative or quantitative determination.

[0277] For example, the carbon-based material in this application can be combined with the general rules of JIS / K0131-1996 X-ray diffraction analysis method to perform X-ray powder diffraction testing and qualitative analysis on the negative electrode sheet or the negative electrode active material.

[0278] Artificial graphite and natural graphite can be distinguished by the SEM cross-sectional view taken by scanning electron microscope (SEM). There are voids between the flake structures in the SEM cross-sectional view of natural graphite, while the SEM cross-sectional view of artificial graphite is dense and seamless; or they can be distinguished by the XRD spectrum obtained by X-ray diffraction method (XRD). There are obvious 2H phase and 3R phase in the XRD spectrum of natural graphite, while only 2H phase exists in the XRD spectrum of artificial graphite.

[0279] In the embodiments of this application, the negative electrode film layer includes at least one layer of film layer, and a single-layer film layer can be used, or at least two layers of film layers can be used. Optionally, the negative electrode film layer includes at least two layers of film layers.

[0280] When the negative electrode film layer adopts a single-layer film layer, the negative electrode active material in the negative electrode film layer includes a carbon-based material, and optionally also includes a silicon-based material. When a single-layer film layer is adopted, the volume average particle size Dv50 of the negative electrode active material is 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 the range composed of any two of the above values.

[0281] When the negative electrode film layer adopts at least two layers of 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 can be located in one of the at least two layers of film layers, or can be located in at least two of the at least two layers of film layers. The negative electrode film layer can include two layers of film layers, three layers of film layers, four layers of film layers, or even more layers of film layers.

[0282] 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. 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 facing away from the negative electrode current collector. The carbon-based material in the second negative electrode film layer includes graphite particles.

[0283] The interface between the first negative electrode film layer and the second negative electrode film layer can be regular or irregular; optionally it is irregular.

[0284] Optionally, the carbon-based material in the first negative electrode film layer further includes natural graphite.

[0285] The negative electrode film layer includes at least two layers of film layers. Layered coating is beneficial to improving the fast charging performance of the battery cell. Especially when there are differences between the first negative electrode film layer and the second negative electrode film layer, it is possible to construct pore differences in the negative electrode film layer, reduce the tortuosity of lithium ion transport, and improve the fast charging performance of the battery cell.

[0286] 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. Further optionally, 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 to improving 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.

[0287] There are differences in the particle sizes of the particles in the first negative electrode film layer and the second negative electrode film layer, which 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 high, and the bottleneck of fast charging mainly lies in the second negative electrode film layer. In the embodiments of the present application, the particle size of the particles in the second negative electrode film layer is relatively small, which can shorten the solid-phase transmission path of lithium ions, improve the fast charging performance, and can improve the problem of lithium deposition on the surface layer of the negative electrode sheet.

[0288] Optionally, the negative electrode active material in the first negative electrode film layer is in the form of particles, and its volume average particle size Dv50 is from 9.5 μm to 18.5 μm, and can be optionally from 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 the range composed 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 from 9.5 μm to 18.5 μm, and can be optionally from 9.5 μm to 14.6 μm.

[0289] 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, on the one hand, it can shorten the solid-phase transmission path of lithium ions and improve the fast charging performance; on the other hand, the material is not prone to agglomeration during the preparation process, which can improve the stability of the material.

[0290] Optionally, the negative electrode active material in the second negative electrode film layer is in the form of particles, and its volume average particle size Dv50 is from 7.8 μm to 14.3 μm, and can be optionally from 7.8 μm to 11.3 μm. Exemplarily, 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 the range composed 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 from 7.8 μm to 14.3 μm, and can be optionally from 7.8 μm to 11.3 μm.

[0291] When the volume average particle size Dv50 of the negative electrode active material in the second negative electrode film layer is within the above range, on the one hand, it can shorten the solid-phase transmission path of lithium ions and improve the fast charging performance; on the other hand, the material is not prone to agglomeration during the preparation process, which can improve the stability of the material; on the other hand, the cooperation between the negative electrode active material in the second negative electrode film layer and the negative electrode active material in the first negative electrode film layer within the above volume average particle size range is beneficial to constructing the gradient pore difference between the second negative electrode film layer and the first negative electrode film layer, reducing the tortuosity of lithium ion transmission, and improving the fast charging performance of the battery cell.

[0292] In the embodiments of the present application, the volume average particle size Dv50 of the negative electrode active material has the meaning well-known in the art and can be detected by using the equipment and methods well-known in the art. Its detection method is the same as the test method for the volume average particle size Dv50 of the positive electrode active material described above.

[0293] Optionally, the tapped density of the carbon-based material in the first negative electrode film layer is less than or equal to the tapped density of the carbon-based material in the second negative electrode film layer. The tapped density can reflect the packing density of the active material in the film layer. When the tapped density of the carbon-based material in the second negative electrode film layer is greater than the tapped density of the carbon-based material in the first negative electrode film layer, the second negative electrode film layer is more densely packed, resulting in an increase in the energy density of the battery cell; the packing of the first negative electrode film layer is relatively sparse and the pores are more abundant, which can improve the fast charging performance of the battery cell. When the negative electrode active material includes graphite particles, the tapped density of the graphite particles in the first negative electrode film layer is less than or equal to the tapped density of the graphite particles in the second negative electrode film layer.

[0294] Optionally, the tapped 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 , for example 0.82 g / cm 3 , 0.85 g / cm 3 , 0.88 g / cm 3 , 0.90 g / cm 3 , 0.92 g / cm 3 , 0.95 g / cm 3 , 0.98 g / cm 3 , 1.00 g / cm 3 , 1.05 g / cm 3 , 1.08 g / cm 3 , 1.10 g / cm 3 , 1.12 g / cm 3 , 1.15 g / cm 3 , 1.18 g / cm 3 , 1.20 g / cm 3 , 1.21 g / cm 3Or a range composed of any two of the above values. When the tapped density of the carbon-based material in the first negative electrode film layer is within a suitable range, the fast charging performance of the battery cell can be improved.

[0295] Optionally, the tapped 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 , such as 0.90 g / cm 3 , 0.92 g / cm 3 , 0.95 g / cm 3 , 0.98 g / cm 3 , 1.00 g / cm 3 , 1.05 g / cm 3 , 1.08 g / cm 3 , 1.10 g / cm 3 , 1.12 g / cm 3 , 1.15 g / cm 3 , 1.18 g / cm 3 , 1.20 g / cm 3 , 1.21 g / cm 3 , 1.22 g / cm 3 , 1.23 g / cm 3 , 1.24 g / cm 3 , 1.25 g / cm 3 Or a range composed of any two of the above values. When the tapped density of the carbon-based material in the second negative electrode film layer is within a suitable range, the energy density of the battery cell can be improved.

[0296] In the embodiments of the present application, the tapped density of the material has the meaning well known in the art and can be measured by the instruments and methods known in the art. For example, reference can be made to GB / T5162-2006 and a powder tapped density tester can be used for measurement. The test instrument can be BT-301 produced by Dandong BETOP.

[0297] Optionally, the ratio of the thickness of the second negative electrode film layer to the thickness of the first negative electrode film layer is from 3:7 to 7:3, and can be optionally from 4:6 to 5:5. Exemplarily, the ratio of the thickness of the second negative electrode film layer to the thickness of the first negative electrode film layer is 3:7, 4:6, 5:5, 6:4, 7:3 or a range composed of any two of the above values. By adjusting the thickness ratio of the first negative electrode film layer and the second negative electrode film layer, the gradient pore difference between the upper and lower layers can be further increased, the tortuosity of lithium ion transport can be reduced, and the fast charging ability of the battery cell can be improved.

[0298] In some embodiments, after the battery cell undergoes 10 full charge test cycles at the beginning of life (BOL), the thickness of the first negative electrode film layer is 15 μm to 65 μm, such as 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 the range composed of any two of the above values. When the thickness of the first negative electrode film layer is within the above range, the first negative electrode film layer and the second negative electrode film layer can regulate and increase the gradient pore difference between the upper and lower layers, reduce the tortuosity of lithium ion transport, and improve the fast charging ability of the battery cell.

[0299] In some embodiments, after the battery cell undergoes 10 full charge test cycles at the beginning of life (BOL), the thickness of the second negative electrode film layer is 15 μm to 65 μm, such as 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 the range composed of any two of the above values. When the thickness of the second negative electrode film layer is within the above range, the first negative electrode film layer and the second negative electrode film layer can regulate and increase the gradient pore difference between the upper and lower layers, reduce the tortuosity of lithium ion transport, and improve the fast charging ability of the battery cell.

[0300] In the embodiments of the present application, for example, taking the upper charge limit voltage of the battery cell as 3.65 V and the discharge cut-off voltage of the battery cell as 2.0 V as an example for illustration,

[0301] The specific steps of the BOL full charge test are as follows: At 25 °C, charge at a charging rate of 0.33 C of the battery nominal capacity to 3.65 V, then charge at a constant voltage of 3.65 V to 0.05 C, stand for 10 min, then discharge at a discharge rate of 0.33 C to 2.0 V, stand for 10 min. The above one charge and discharge is one cycle, and cycle 10 times. Then charge at a charging rate of 0.33 C of the nominal capacity to 3.65 V, and then charge at a constant voltage of 3.65 V to 0.05 C to obtain the BOL full charge state. In the BOL full charge state, disassemble the negative electrode plate, use a tomography electron microscope to observe the cross-section in the thickness direction of the middle area of the negative electrode plate, distinguish the regions of the first negative electrode film layer and the second negative electrode film layer according to the interface between them, and measure their thicknesses respectively. For example, measure the thicknesses of 10 positions of the first negative electrode film layer, and calculate their average value as the average value of the first negative electrode film layer; measure the thicknesses of 10 positions of the second negative electrode film layer, and calculate their average value as the average value of the second negative electrode film layer.

[0302] In some embodiments, after the end-of-life (EOL) full charge test of the battery cell, the thickness of the first negative electrode film layer is 15 μm to 70 μm, such as 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 a range composed of any two of the above values. When the thickness of the first negative electrode film layer is within the above range, the first negative electrode film layer and the second negative electrode film layer can regulate and increase the gradient pore difference between the upper and lower layers, reduce the tortuosity of lithium ion transport, and improve the fast charging ability of the battery cell.

[0303] In some embodiments, after the end-of-life (EOL) full charge test of the battery cell, the thickness of the second negative electrode film layer is 15 μm to 70 μm, such as 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 a range composed of any two of the above values. When the thickness of the second negative electrode film layer is within the above range, the first negative electrode film layer and the second negative electrode film layer can regulate and increase the gradient pore difference between the upper and lower layers, reduce the tortuosity of lithium ion transport, and improve the fast charging ability of the battery cell.

[0304] In the embodiments of the present application, for example, taking the upper charging limit voltage of the battery cell as 3.65 V and the discharge cut-off voltage of the battery cell as 2.0 V as an example for illustration,

[0305] The specific steps of the EOL full charge test are as follows:

[0306] At 60 °C, charge at a charging rate of 0.33C of the battery's nominal capacity until 3.65V, then charge at a constant voltage of 3.65V until 0.05C, let it stand for 10 min, then discharge at a discharge rate of 0.33C until 2.0V, let it stand for 10 min. One charge and discharge cycle as described above is considered one cycle, and the test is stopped until the battery capacity decays to 80% of the nominal capacity. Then, at 25 °C, charge at a constant current of 0.33C until 3.65V, and charge at a constant voltage of 0.05C until 3.65V, which is the fully charged state at EOL. In the fully charged state at EOL, disassemble the negative electrode sheet, use a tomography electron microscope to observe the cross-section in the thickness direction of the middle area of the negative electrode sheet, distinguish the regions of the first negative electrode film layer and the second negative electrode film layer according to their interfaces, and measure their thicknesses respectively. For example, measure the thicknesses at 10 positions of the first negative electrode film layer, and calculate their average value as the average value of the first negative electrode film layer; measure the thicknesses at 10 positions of the second negative electrode film layer, and calculate their average value as the average value of the second negative electrode film layer.

[0307] In some embodiments, when the negative electrode film layer adopts a single-layer film layer, the negative electrode film layer further includes a lithium-containing binder. Optionally, the mass content of the lithium-containing binder relative to the mass of the negative electrode film layer is 0.1% to 1%. Exemplarily, the mass content of the lithium-containing binder relative to the mass of the 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 the range composed of any two of the above values. The lithium element in the lithium-containing binder can exist in the form of ions, 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, increase the insertion and extraction rate of lithium ions, and improve the fast charging performance of the battery cell. Optionally, the negative electrode film layer can further include a negative electrode binder. For example, the negative electrode binder includes at least one of styrene-butadiene rubber (SBR), water-soluble unsaturated resin SR-1B, water-based acrylic resin (e.g., polyacrylic acid PAA, polymethacrylic acid PMAA, sodium polyacrylate PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), and carboxymethyl chitosan (CMCS).

[0308] Optionally, the mass content of lithium element in the lithium-containing binder is 3% to 10%. Exemplarily, the mass content of lithium element in the lithium-containing binder is 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10% or the range composed of any two of the above values. The mass content of lithium element is calculated based on the mass of the lithium-containing binder. When the mass content of lithium element is within the above range, the number of freely moving lithium ions in the negative electrode film layer can be relatively large, which can further shorten the distance for lithium ions to diffuse to the surface of the negative electrode film layer, increase the insertion and extraction rate of lithium ions, and improve the fast charging performance of the battery cell.

[0309] Exemplarily, the lithium-containing binder includes a lithium acrylate-acrylonitrile-acrylamide-2-hydroxyethyl acrylate copolymer, which is derived from lithium acrylate monomer, acrylonitrile monomer, acrylamide monomer and 2-hydroxyethyl acrylate monomer, and the molar ratio of lithium acrylate monomer, acrylonitrile monomer, acrylamide monomer and 2-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 2-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 materials can provide a certain number of lithium ions for the negative electrode film layer, improving the fast charging performance of the battery cell; moreover, it is not prone to swelling during the charge and discharge process, with a stable structure, so that the cycle performance of the negative electrode film layer is improved during the fast charge and discharge process.

[0311] In some other embodiments, when the negative electrode film layer adopts at least two layers of film layers, the negative electrode film layer further includes a lithium-containing binder.

[0312] Optionally, the first negative electrode film layer further includes a first lithium-containing binder, the second negative electrode film layer further includes a second lithium-containing binder, and 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. Further 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.

[0313] The mass content of the second lithium-containing binder in the second negative electrode film layer is relatively high, and the number of freely movable lithium ions provided by the second lithium-containing binder for the second negative electrode film layer is relatively more, which can further improve the fast charging performance of the battery cell.

[0314] Optionally, the mass content of the first lithium-containing binder relative to the mass of the first negative electrode film layer is 0.1% to 1%. Exemplarily, the mass content of the first lithium-containing binder relative to the mass of 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 a range composed of any two of the above values. The lithium element in the first lithium-containing binder can exist in the form of ions, which can increase the number of freely movable 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, increase the deintercalation rate of lithium ions, and improve the fast charging performance of the battery cell.

[0315] Optionally, the mass content of lithium element in the first lithium-containing binder is 3% to 10%, and can be optionally 3% to 8%. Exemplarily, the mass content of lithium element in the first lithium-containing binder is 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10% or a range composed of any two of the above values. When the mass content of lithium element is within the above range, the number of free-moving lithium ions in the negative electrode film layer can be relatively large, which can further shorten the distance for lithium ions to diffuse to the surface of the negative electrode film layer, improve the intercalation and deintercalation rate of lithium ions, and improve the fast charging performance of the battery cell.

[0316] Exemplarily, the first lithium-containing binder includes a lithium acrylate-acrylonitrile-acrylamide-2-hydroxyethyl acrylate copolymer, and the lithium acrylate-acrylonitrile-acrylamide-2-hydroxyethyl acrylate copolymer is derived from a lithium acrylate monomer, an acrylonitrile monomer, an acrylamide monomer, and a 2-hydroxyethyl acrylate monomer. The molar ratio of the lithium acrylate monomer, the acrylonitrile monomer, the acrylamide monomer, and the 2-hydroxyethyl acrylate monomer is 30% to 50%: 15% to 45%: 5% to 20%: 20% to 35%. For example, the molar ratio of the lithium acrylate monomer, the acrylonitrile monomer, the acrylamide monomer, and the 2-hydroxyethyl acrylate monomer is 35%: 30%: 15%: 20%, or 40%, 20%, 10%, 30%, or 45%, 15%, 20%, 20%, etc.

[0317] The lithium-containing binder of the above material can provide a certain number of lithium ions for the negative electrode film layer, improving the fast charging performance of the battery cell; and it is not prone to swelling during charge and discharge, with a stable structure, so that the cycle performance of the negative electrode film layer is improved during fast charge and discharge.

[0318] Optionally, the mass content of the second lithium-containing binder relative to the mass of the second negative electrode film layer is 0.1% to 1%. Exemplarily, the mass content of the second lithium-containing binder relative to the mass of 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 a range composed of any two of the above values. The lithium element in the second lithium-containing binder can exist in the form of ions, which can increase the number of free-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 intercalation and deintercalation rate of lithium ions, and improve the fast charging performance of the battery cell.

[0319] The first lithium-containing binder and the second lithium-containing binder can be made of the same material or different materials.

[0320] Optionally, the mass content of lithium element in the second lithium-containing binder is 3% to 10%, and can be optionally 3% to 8%. Exemplarily, the mass content of lithium element in the second lithium-containing binder is 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10% or the range composed of any two of the above values. When the mass content of lithium element is within the above range, the number of free-moving lithium ions in the negative electrode film layer can be relatively large, which can further shorten the distance for lithium ions to diffuse to the surface of the negative electrode film layer, improve the intercalation and deintercalation rate of lithium ions, and improve the fast charging performance of the battery cell.

[0321] Exemplarily, the second lithium-containing binder includes a lithium acrylate-acrylonitrile-acrylamide-2-hydroxyethyl acrylate copolymer, and the lithium acrylate-acrylonitrile-acrylamide-2-hydroxyethyl acrylate copolymer is derived from lithium acrylate monomer, acrylonitrile monomer, acrylamide monomer and 2-hydroxyethyl acrylate monomer, and the molar ratio of lithium acrylate monomer, acrylonitrile monomer, acrylamide monomer and 2-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 2-hydroxyethyl acrylate monomer is 35%: 30%: 15%: 20%, or 40%, 20%, 10%, 30%, or 45%, 15%, 20%, 20%, etc.

[0322] The lithium-containing binder of the above material can provide a certain number of lithium ions for the negative electrode film layer, improving the fast charging performance of the battery cell; and it is not easy to swell during the charge and discharge process, with a stable structure, so that the cycle performance of the negative electrode film layer is improved during the fast charge and discharge process.

[0323] 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, water-based acrylic resin (for example, polyacrylic acid PAA, polymethacrylic acid PMAA, sodium polyacrylate PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA) and carboxymethyl chitosan (CMCS).

[0324] 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.

[0325] In some embodiments, the negative electrode film layer may further optionally include a negative electrode conductive agent. There is no particular limitation on the type of the negative electrode conductive agent in the embodiments of the present application. As an example, the negative electrode conductive agent may include at least one of superconducting carbon, conductive graphite, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. In some embodiments, based on the total weight of the negative electrode film layer, the mass content of the negative electrode conductive agent is ≤5%.

[0326] In some embodiments, the negative electrode film layer may further optionally include a negative electrode binder. There is no particular limitation on the type of the negative electrode binder in the embodiments of the present application. As an example, the negative electrode binder may include at least one of styrene-butadiene rubber (SBR), water-soluble unsaturated resin SR-1B, water-based acrylic resin (for example, polyacrylic acid PAA, polymethacrylic acid PMAA, sodium polyacrylate PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), and carboxymethyl chitosan (CMCS). In some embodiments, based on the total weight of the negative electrode film layer, the mass content of the negative electrode binder is ≤5%.

[0327] In some embodiments, the negative electrode film layer may further optionally include other additives. As an example, the other additives may include thickeners, dispersants, etc., such as sodium carboxymethyl cellulose (CMC-Na), PTC thermistor materials, etc. In some embodiments, based on the total weight of the negative electrode film layer, the mass content of the other additives is ≤2%.

[0328] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. As an example of the metal foil, at least one foil of copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy may be used. The composite current collector may include a polymer material base layer and a metal material layer formed on at least one surface of the polymer material base layer. As an example, the metal material in the metal material layer may include at least one of copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy. As an example, the polymer material base layer may include at least one of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE).

[0329] In some embodiments, the thickness of the negative electrode current collector is 4 μm to 6 μm. Exemplarily, the thickness of the negative electrode current collector is 4 μm, 4.5 μm, 5 μm, 5.5 μm, 6 μm or a range composed of any two of the above values.

[0330] 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 relatively excellent, and the battery cell can have a relatively high energy density.

[0331] In the embodiments of the present application, the thickness of the negative electrode current collector has the meaning well-known in the art and can be detected by the equipment and methods well-known in the art. For example, the film layer on the surface of the negative electrode current collector is washed off with an organic solvent such as water, and the thickness of the positive electrode current collector is measured with a micrometer.

[0332] The negative electrode film layer is usually formed by coating a negative electrode slurry on the negative electrode current collector and then drying and cold pressing. The negative electrode slurry is usually formed by dispersing a negative electrode active material, an optional conductive agent, an optional binder, and other optional additives in a solvent and stirring evenly. The solvent can be N-methylpyrrolidone (NMP) or deionized water, but is not limited thereto.

[0333] 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 the embodiments of the present application further includes a negative electrode conductive layer disposed on the surface of the negative electrode current collector and sandwiched between the negative electrode current collector and the negative electrode film layer. In some other embodiments, the negative electrode sheet of the embodiments of the present application further includes a protective layer covering the surface of the negative electrode film layer.

[0334] 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, reduce the heat generation of the negative electrode sheet, and thus reduce the heat generation of the battery cell.

[0335] In some embodiments, the thickness of the negative electrode conductive layer is 0.5 μm to 2 μm. Exemplarily, 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 a range composed of any two of the above values.

[0336] When the thickness of the negative electrode conductive layer is within the above range, it can further improve the conductivity of the negative electrode sheet, reduce the heat generation of the negative electrode sheet, and thus reduce the heat generation of the battery cell; and it can also take into account the improvement of the energy density of the battery cell.

[0337] In the embodiments of the present application, the thickness of the negative electrode conductive layer has the meaning well-known in the art and can be detected by the equipment and methods well-known in the art. The testing method of the negative electrode conductive layer described above can be adopted.

[0338] 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 bonding performance between the negative electrode current collector and the negative electrode film layer and improve the structural stability of the negative electrode sheet.

[0339] In some embodiments, the negative electrode conductive layer may further optionally include other additives. By way of example, the other additives may include thickeners such as sodium carboxymethyl cellulose (CMC), PTC thermistor materials, and the like.

[0340] Optionally, the mass content of the negative electrode conductive agent in the negative electrode conductive layer is 20% to 40%. Exemplarily, the mass content of the negative electrode conductive agent is 20%, 25%, 30%, 35%, 40% or a range composed of any two of the above values.

[0341] Exemplarily, 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.

[0342] Optionally, the mass content of the negative electrode binder in the negative electrode conductive layer is 60% to 80%. Exemplarily, 60%, 65%, 70%, 75%, 80% or a range composed of any two of the above values.

[0343] Exemplarily, 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.

[0344] In some embodiments, the ratio CB of the capacity of the negative electrode film layer per unit area to the capacity of the positive electrode film layer per unit area in the battery cell is 1.05 to 1.30, and may be optionally 1.07 to 1.15. Exemplarily, the ratio CB of the capacity of the negative electrode film layer per unit area to the capacity of the positive electrode film layer per unit area in the 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 composed of any two of the above values.

[0345] When the ratio CB of the capacity of the negative electrode film layer per unit area to the capacity of the positive electrode film layer per unit area in the battery cell is within the above range, there are sufficient sites in the negative electrode film layer for lithium insertion, which can reduce the risk of lithium plating; and is beneficial for fast charging.

[0346] In the embodiments of the present application, the meaning of the CB value is well-known in the art, and it can be detected by using well-known equipment and methods in the art. For example, the capacity of the negative electrode film layer per unit area and the capacity of the positive electrode film layer per unit area are calculated respectively, and then the ratio of the two is calculated to obtain the CB value.

[0347] Specifically, taking the battery charging upper limit voltage of 3.65V and the battery discharge cut-off voltage of 2.0V as an example for illustration,

[0348] The capacity of the positive electrode film layer per unit area refers to the actual de-lithiation capacity of the positive electrode active material. The test method is as follows: Disassemble the battery in a PRS340 / 11-119-11 Braun glove box, take the positive electrode plate, and assemble it into a CR2430 type half-button battery of positive electrode-lithium sheet. The area of the positive electrode plate used is amm 2 , where the electrolyte is a solution of 1M LiPF6 in EC / EMC / DEC = 3 / 5 / 2 (mass ratio); then let the assembled half-button battery stand for 3 h, the test is carried out at 25 °C, first charge (Charge) and de-lithiate at a voltage range of 2.0 V to 3.65 V with 0.1C, and then discharge (Discharge) and intercalate lithium to 2.0 V with 0.05C, cycle 2 times, and record the discharge and charge capacity of the second cycle as Y mAh. The actual length of the positive electrode plate designed for the battery is b mm, the width is c mm, and the number of sides of the positive electrode active material coated on the positive electrode current collector is d. Then the capacity of the positive electrode film layer per unit area = Y / a * b * c * d.

[0349] Specifically, the capacity of the negative electrode film layer per unit area refers to the actual intercalation capacity of the negative electrode active material. The test method is: Disassemble the battery in a PRS340 / 11-119-11 Braun glove box, take the negative electrode plate, and assemble it into a CR2430 type half-button battery of negative electrode-lithium sheet. The area of the negative electrode plate used is fmm 2 , where the electrolyte is a solution of 1M LiPF6 in EC / EMC / DEC = 3 / 5 / 2 (mass ratio); then let the assembled half-button battery stand for 3 h, the test is carried out at 25 °C, first discharge (Discharge) and intercalate lithium at a voltage range of 2 V - 0 V with 0.1C, and then charge (Discharge) and de-lithiate to 2 V with 0.05C, cycle 2 times, and record the discharge and charge capacity of the second cycle as Z mAh. The actual length of the negative electrode plate designed for the battery is h mm, the width is i mm, and the number of sides of the negative electrode active material coated on the negative electrode current collector is d. Then the intercalation capacity of the negative electrode = Z / f * h * i * d.

[0350] [Separator membrane]

[0351] In the embodiments of the present application, the separator membrane includes a base film with a porous structure.

[0352] In some embodiments, the base film includes at least one of glass fiber, non-woven 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.

[0353] Optionally, the polyolefin includes at least one of polyethylene, polypropylene, and polyvinylidene fluoride.

[0354] In some embodiments, the porosity of the base film is 20% to 70%, optionally 35% to 60%. Exemplarily, the porosity of the base film is 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70% or a range consisting of any two of the above values.

[0355] When the porosity of the base film in the embodiment of the present application is within the above range, the migration ability of lithium ions in the isolation film can be improved, and the internal resistance of the battery cell can be further reduced, thereby reducing heat generation.

[0356] In the embodiments of the present application, porosity refers to the percentage of the pore volume in the separator to the total volume of the separator. The porosity can be tested in accordance with the standard GB / T36363-2018 "Polyolefin separator for battery monomers". It should be noted that the actual test process can be slightly different from the standard test process according to the difference in test instruments, test errors, and in order to eliminate the test influence on porosity as much as possible, so as to obtain a more accurate test value.

[0357] In some embodiments, the thickness of the base film is 5 μm to 12 μm, and may be no more than 9 μm, and may be 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.

[0358] 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.

[0359] In the embodiment of the present application, the isolation film may be a base film; optionally, the isolation film further comprises a functional layer disposed on at least one side of the base film, and the functional layer may comprise inorganic particles to improve the heat resistance of the isolation film. Optionally, the functional layer is disposed on both sides of the base film.

[0360] 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, the first functional layer includes first inorganic particles, the second functional layer is located on the other side of the base film, the second functional layer includes composite particles, the composite particles include second inorganic particles and multiple non-fluoropolymer particles, the second inorganic particles are attached to the surface of the non-fluoropolymer particles and / or dispersed in the interior of the non-fluoropolymer particles.

[0361] The first functional layer and the second functional layer have good heat resistance and can improve the heat resistance of the isolation film.

[0362] Optionally, the first functional layer may include a binder, optionally including at least one of a fluorine-containing binder or a polyacrylic acid binder, such as polyvinylidene fluoride.

[0363] Optionally, the first inorganic particles include one or more of silica, alumina, boehmite, barium sulfate, calcium oxide, titanium oxide, zinc oxide, magnesium oxide, zirconium oxide, and tin oxide. The above first inorganic particles can improve the heat resistance of the first functional layer.

[0364] In the embodiments of the present application, the meaning of the thickness of the base film is the meaning well known in the art, and it can be detected by using the meaning and equipment well known in the art. For example, a newly prepared separator can be taken as a sample, or a battery cell that has been fully discharged (discharged to the lower cut-off voltage so that the charged state of the battery is 0% SOC) can be disassembled in reverse, the separator is obtained from the battery cell, and the separator is dried and used as a sample. The separator is cut off with an ion beam cutter to form a cross-section; subsequently, a scanning electron microscope is used to measure the thickness of the cross-section of the separator and its respective layers.

[0365] The non-fluoropolymer particles in the second functional layer refer to polymers that are non-fluorinated polymers. For example, the non-fluoropolymer particles include acrylate copolymers. Optionally, the acrylate copolymers include acrylate-acrylonitrile-acrylamide-acrylonitrile copolymers. The acrylate copolymers have excellent adhesion properties and relatively high adhesion stability with the base film. The molar ratio of each monomer in the copolymer can be any ratio, such as a molar ratio of 35%:30%:15%:20%, or 40%, 20%, 10%, 30%, or 45%, 15%, 20%, 20%, etc.

[0366] The second inorganic particles in the composite particles make it difficult for the non-fluoropolymer particles to adhere to each other due to the high-temperature treatment during the granulation process, resulting in pores in the composite particles, which is beneficial to the transport of lithium ions and improves the ion-conducting ability of the separator; and the second inorganic particles can also increase the compression modulus of the composite particles. During the charge and discharge process, the composite particles are not easily deformed, making the structure of the separator more stable, which can improve the kinetic performance of the battery cell and the fast charging performance. Optionally, compared with the first functional layer, the second functional layer is disposed closer to the negative electrode tab. Since the composite particles are not easily deformed, the separator basically does not cause side effects such as extrusion to the negative electrode tab, making the kinetic performance of the negative electrode tab stable. Correspondingly, the first functional layer is disposed closer to the positive electrode tab.

[0367] Optionally, the second inorganic particles include one or more of silica, alumina, boehmite, barium sulfate, calcium oxide, titanium oxide, zinc oxide, magnesium oxide, zirconium oxide, and tin oxide; optionally, the second inorganic particles include silica. The above-mentioned second inorganic particles can improve the heat resistance of the second functional layer and can form composite particles in cooperation with the non-fluoropolymer, further improving the cycle stability and kinetic performance of the separator membrane and improving the cycle performance and fast charging performance of the battery cell.

[0368] The average particle size of the second inorganic particles is 5 nm to 100 nm, optionally 10 nm to 100 nm, and optionally 5 nm to 20 nm. Exemplarily, 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 a range composed of any 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 compression modulus of the composite particles.

[0369] In the embodiments of the present application, the average particle size of the second inorganic particles has the meaning well-known in the art and can be detected by the equipment and methods well-known in the art. For example, after obtaining the separator membrane and drying the separator membrane as a sample, the separator membrane is cut off with an ion beam cutter to form a cross-section; subsequently, a scanning electron microscope is used to measure the particle size of the second inorganic particles in the separator membrane, and the particle sizes of multiple, such as 50, second inorganic particles are measured, and the average value is calculated as the average particle size of the second inorganic particles.

[0370] In some embodiments, the ionic conductivity of the separator membrane is 0.3 mS / cm to 0.6 mS / cm. Exemplarily, the ionic conductivity of the separator membrane 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 composed of any two of the above values.

[0371] When the ionic conductivity of the separator membrane is within the above range, it can further improve the migration ability of lithium ions in the separator membrane and improve the fast charging performance of the battery cell.

[0372] In the embodiments of the present application, the ionic conductivity of the separator membrane has the meaning well-known in the art and can be detected by the equipment and methods well-known in the art. For example,

[0373] Preparation of 2025-type button battery for testing: In a vacuum glove box, place a lithium sheet into the negative electrode case of the battery, add 150 μL of electrolyte. The electrolyte is a solution of 1 M LiPF6 in EC / EMC / DEC = 3 / 5 / 2 (mass ratio), then place the separator (with an area of 3.14 cm 2 , with a thickness of 12 μm) to make it close to the lithium sheet, add another 25 μL of electrolyte, and finally place the positive electrode sheet (the positive electrode sheet can be the positive electrode sheet in Example 1) on it and seal. Take out the assembled button battery from the vacuum glove box and place it for 24 h for the next test.

[0374] Testing: On an electrochemical workstation, conduct tests in the frequency range of 10 -1 ~10 6 Hz to obtain the separator resistance R b , and calculate the ionic conductivity σ (unit: mS / cm) through the following formula,

[0375] σ = L / (R b ×S)

[0376] where: R b is the separator resistance, and L and S are the thickness and area of the separator to be measured respectively.

[0377] [Electrolyte]

[0378] In some embodiments, the battery cell further includes an electrolyte.

[0379] During the charge and discharge process of the battery cell, active ions such as lithium ions are embedded and extracted back and forth between the positive electrode sheet and the negative electrode sheet, and the electrolyte plays a role in conducting active ions between the positive electrode sheet and the negative electrode sheet.

[0380] In the embodiments of the present application, the conductivity of the electrolyte at room temperature is 13 mS / cm to 20 mS / cm, and can be 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 the range composed of any two of the above values.

[0381] 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 can improve the fast charging performance of the battery cell.

[0382] In the embodiments of the present application, the conductivity of the electrolyte at room temperature, such as 25 °C, is the ionic conductivity, and it can be detected by using the equipment and methods well-known in the art. For example, it can be tested with reference to the industry standard HG-T 4067-2015.

[0383] 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 the range composed of any two of the above values.

[0384] 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.

[0385] In the embodiments of the present application, the viscosity of the electrolyte has the meaning well-known in the art and can be detected by using the equipment and methods well-known in the art. For example, it can be detected in accordance with GB / T10247-2008.

[0386] In some embodiments, the density of the electrolyte at room temperature, such as 25 °C, is from 1.05 g / mL to 1.35 g / mL. Exemplarily, the density of the electrolyte 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 the range composed of any two of the above values.

[0387] When the density of the electrolyte 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.

[0388] In the embodiments of the present application, the density of the electrolyte has the meaning well-known in the art and can be detected by using the equipment and methods well-known in the art. For example, it can be tested with reference to GB / T 2013-2010.

[0389] The electrolyte includes an organic solvent and an electrolyte salt. The types of the organic solvent and the electrolyte salt are not specifically limited and can be selected according to actual needs.

[0390] In some embodiments, the organic solvent includes a chain carboxylic acid ester solvent, and the mass content of the chain carboxylic acid ester solvent relative to the mass of the organic solvent is greater than or equal to 5% and less than or equal to 75%, optionally greater than or equal to 10% and less than or equal to 75%, optionally 30% to 70%, and 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 composed of any two of the above values.

[0391] When the mass content of the chain carboxylic acid ester solvent is within the above range, the viscosity of the electrolyte system is relatively small, which is beneficial to the migration of lithium ions.

[0392] In some embodiments, the chain carboxylic acid ester solvent includes a compound represented by Formula I,

[0393] Formula I,

[0394] In Formula I,

[0395] R1 includes a hydrogen atom, a halogen atom, a C1-C5 alkyl group or a C1-C5 haloalkyl group,

[0396] R2 includes a C1-C5 alkyl group or a C1-C5 haloalkyl group.

[0397] The above chain carboxylic acid ester solvent has a high conductivity, which is beneficial to improving the fast charging ability of the battery monomer.

[0398] Optionally, R1 includes a hydrogen atom, a halogen atom, a C1-C3 alkyl group or a C1-C3 haloalkyl group. Further optionally, R1 includes a hydrogen atom, a halogen atom, a C1-C2 alkyl group or a C1-C2 haloalkyl group.

[0399] Optionally, R2 includes a C1-C3 alkyl group or a C1-C3 haloalkyl group. Further optionally, R2 includes a C1-C2 alkyl group or a C1-C2 haloalkyl group.

[0400] In the above embodiments, the halogen atom includes one or more of a fluorine atom, a chlorine atom, a bromine atom and an iodine atom. Optionally, the halogen atom includes a fluorine atom.

[0401] In the above embodiments, the haloalkyl group includes one or more of a fluoroalkyl group, a chloroalkyl group, a bromoalkyl group and an iodoalkyl group. Optionally, the haloalkyl group includes a fluoroalkyl group.

[0402] Exemplarily, the chain carboxylic acid ester solvent includes one or more of the compounds represented by Formula I-1 to Formula I-8,

[0403]

[0404] In some embodiments, the organic solvent further includes carbonate solvents.

[0405] Optionally, the carbonate solvents include one or more of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate. Further optionally, the carbonate solvents include one or more of ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate. The above carbonate solvents and chain carboxylic ester solvents are used in combination, so that the conductivity of the electrolyte at room temperature is improved, which is beneficial to the migration of lithium ions.

[0406] Further optionally, the mass content of the carbonate solvents in the organic solvent is 30% to 70%, and can be 30% to 50%. Exemplarily, the mass content of the carbonate solvents in the organic solvent is 30%, 31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%, 40%, 45%, 48%, 50%, 55%, 60%, 65%, 70% or a range composed of any two of the above values. The carbonate solvents with the above mass content can further improve the conductivity of the electrolyte at room temperature, which is beneficial to the migration of lithium ions.

[0407] Exemplarily, the carbonate solvents include one or more of ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate, and the mass content of the carbonate solvents is 30% to 50%.

[0408] In some embodiments, the electrolyte further contains additives. The additives can include negative electrode film-forming additives, can also include positive electrode film-forming additives, and can also include additives that can improve certain performances of the battery, such as additives for improving the overcharge performance of the battery, additives for improving the high-temperature performance of the battery, additives for improving the low-temperature power performance of the battery, etc.

[0409] In some embodiments, the additives include one or more of carbonate additives, sulfur-containing additives, and lithium salt additives, and can be at least two. The above additives can improve the interfacial film performance on the positive electrode side and / or the negative electrode side, which is beneficial to improving the fast charging performance of the battery monomer and improving the cycling performance.

[0410] In some embodiments, the mass content of the additives in the electrolyte is 1% to 10%, optionally 2% to 8%, and further optionally 3.5% to 8%. Exemplarily, the mass content of the additives in the electrolyte is 1%, 2%, 3%, 3.5%, 4%, 5%, 6%, 7%, 8%, 9%, 10% or a range composed of any two of the above values.

[0411] The additives with the above mass contents can effectively improve the interfacial film properties on the positive electrode side and / or the negative electrode side, which is beneficial to enhancing the fast charging performance of the battery cell and improving the cycling performance.

[0412] Exemplarily, the carbonate additives include one or more of vinylene carbonate (VC) and fluoroethylene carbonate (FEC).

[0413] Exemplarily, the sulfur-containing additives include one or more of ethylene sulfate (DTD), bis(ethylene sulfate) (2-DTD), butene sulfite (BS), 1,3-propane sultone (PS), ethylene sulfite (ES), and methylene methyl disulfonate (MMDS).

[0414] Optionally, the lithium salt additives include one or more of lithium difluorophosphate (LiPO2F2), lithium difluoro(oxalato)borate (LiDFOB), lithium tetrafluoroborate (LiBF4), and lithium bis(oxalato)borate (LiBOB).

[0415] Optionally, the mass content of vinylene carbonate (VC) in the electrolyte is 0.5% to 9%, and can be optionally 2% to 6%.

[0416] Optionally, the mass content of fluoroethylene carbonate (FEC) in the electrolyte is 0.1% to 4%, and can be optionally 0.5% to 3%.

[0417] 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%.

[0418] 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%.

[0419] In some embodiments, the electrolyte salt includes a lithium salt, and the lithium salt includes one or more of fluorosulfonimide salts and lithium hexafluorophosphate (LiPF6). The above lithium salts are easy to dissociate, which is beneficial to the rapid migration of lithium ions, and the electrolyte system is relatively stable and not easy to decompose, which can improve the cycling performance of the battery cell.

[0420] Optionally, the fluorosulfonimide salts include one or more of lithium bis(fluorosulfonyl)imide (LiFSI) and lithium bis(trifluoromethylsulfonyl)imide (LiTFSI).

[0421] Optionally, the lithium salt includes lithium bis(fluorosulfonyl)imide (LiFSI) and lithium hexafluorophosphate (LiPF6). The molar concentration of lithium bis(fluorosulfonyl)imide (LiFSI) is 0.2 mol / L to 0.5 mol / L, and the molar concentration of lithium hexafluorophosphate (LiPF6) is 0.5 mol / L to 1.0 mol / L.

[0422] Exemplarily, 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.

[0423] Exemplarily, 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.

[0424] Exemplarily, 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.

[0425] Optionally, the ratio of the molar concentration of lithium bis(fluorosulfonyl)imide to the molar concentration of lithium hexafluorophosphate LiPF6 is 0.2 to 1.0, and can be optionally 0.2 to 0.5. Exemplarily, the ratio of the molar concentration of lithium bis(fluorosulfonyl)imide to the molar concentration of lithium hexafluorophosphate LiPF6 is 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1 or a range composed of any two of the above values.

[0426] In the embodiments of the present application, the types and contents of inorganic components / lithium salt concentrations in the electrolyte have meanings well-known in the art, and can be detected by devices and methods well-known in the art. For example, reference can be made to the standard JY / T 020-1996 "General Rules for Ion Chromatographic Analysis Methods" to qualitatively or quantitatively analyze the inorganic components / lithium salt concentrations in the electrolyte by ion chromatography. In the embodiments of the present application, freshly prepared electrolyte can be taken as a sample, free electrolyte of a fresh battery can be taken as a sample, or a discharged battery (discharged to the lower cut-off voltage such that the charged state of the battery is approximately 0% SOC) can be disassembled in reverse, and the free electrolyte obtained from the battery can be taken as a sample for detection by ion chromatography.

[0427] In the embodiments of the present application, the types and contents of organic components in the electrolyte have meanings well-known in the art, and can be detected by devices and methods well-known in the art. For example, reference can be made to GB / T 9722-2006 "General Rules for Gas Chromatography of Chemical Reagents" to qualitatively and quantitatively analyze the organic components in the electrolyte by gas chromatography. In the embodiments of the present application, freshly prepared electrolyte can be taken as a sample, free electrolyte of a fresh battery can be taken as a sample, or a discharged battery (discharged to the lower cut-off voltage such that the charged state of the battery is approximately 0% SOC) can be disassembled in reverse, and the free electrolyte obtained from the battery can be taken as a sample for detection by ion chromatography.

[0428] In the embodiments of the present application, after quantitatively and qualitatively detecting each component in the electrolyte, each component is classified. Chain carboxylic ester solvents and carbonate solvents (such as ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate) are used as constituent components of the organic solvent. Based on the mass of the organic solvent being 100%, the mass content of each component is calculated.

[0429] Vinylene carbonate additives (such as vinylene carbonate and fluoroethylene carbonate), sulfur-containing additives, and lithium salt additives are used as additives for the electrolyte. Based on the mass of the electrolyte being 100%, the mass content of each component is calculated.

[0430] In some embodiments, the battery cell satisfies: 2.45 g / Ah ≤ d / A ≤ 3.5 g / Ah, and optionally 2.45 g / Ah ≤ d / A ≤ 3.3 g / Ah, where d represents the mass of the electrolyte in the battery cell, in 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 composed of any two of the above values.

[0431] d / A can reflect the liquid retention ability of the electrolyte. When d / A is within the above range, the electrolyte can play a good wetting role on the positive electrode plate and the negative electrode plate, and can also improve the migration rate of lithium ions in the liquid phase, which is beneficial to improving the fast charging ability of the battery cell.

[0432] In the embodiments of the present application, d / A of the battery cell can be understood as the liquid retention coefficient, and can be detected by using equipment and methods well-known in the art. For example, taking the upper limit voltage of battery charging as 3.65 V and the cut-off voltage of battery discharging as 2.0 V in accordance with GB / T 31486-2015 "Power Battery Electrical Performance Requirements and Test Methods for Electric Vehicles" for illustration.

[0433] At 25°C, the battery cell is charged to 3.65 V at 0.33 C, then charged at a constant voltage until 0.05 C, and then discharged at a constant current of 0.33 C to 2.0 V. The discharged capacity A is used as the denominator. The battery cell is weighed as M0, and then the positive electrode plate, negative electrode plate, separator, and electrolyte are disassembled. The free electrolyte is in a bag. All the above solid components are placed in an oven at 60°C and baked for more than 4 hours (including but not limited to the positive electrode plate, negative electrode plate, and separator, but also other mechanical parts contributing to M0 in the disassembled battery cell). Then, all the components of the battery cell are weighed as M1. The weight difference between M0 and M1 is used as the numerator. The liquid retention coefficient is equal to the value obtained by dividing the weight difference d between M0 and M1 by the capacity A.

[0434] In some embodiments, the positive electrode sheet, the separator, and the negative electrode sheet can be made into an electrode assembly through a winding process and / or a stacking process.

[0435] Figure 1 and Figure 2 shows a schematic structural diagram of a battery cell.

[0436] In some embodiments, the battery cell 7 may include a housing 20.

[0437] In some embodiments, the housing 20 of the battery cell 7 may be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The housing 20 of the battery cell 7 may also be a soft package, such as a pouch soft package. The material of the soft package may be plastic, such as at least one of polypropylene (PP), polybutylene terephthalate (PBT), and polybutylene succinate (PBS).

[0438] The housing 20 is a hollow structure, and the housing 20 can be used to encapsulate the above-mentioned electrode assembly 10 and the electrolyte.

[0439] The preparation method of the battery cell 7 according to the embodiments of the present application is well-known. In some embodiments, the positive electrode sheet, the separator, the negative electrode sheet, and the electrolyte can be assembled to form the battery cell 7. As an example, the positive electrode sheet, the separator, and the negative electrode sheet can be formed into an electrode assembly 10 through a winding process and / or a stacking process, the electrode assembly 10 is placed in the housing 20, the electrolyte is injected after drying, and after processes such as vacuum packaging, standing, formation, and shaping, the battery cell 7 is obtained.

[0440] In some embodiments, the housing 20 includes a housing body 21 and an end cover 22, the housing body 21 has an opening, and the end cover 22 covers the opening.

[0441] The shape of the housing body 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 body 21 are cuboid structures.

[0442] In some embodiments, the material of the housing body 21 includes steel, and the mechanical strength of the steel is relatively high and it is not easy to deform, which can improve the use reliability of the battery cell 7. In the embodiments of the present application, steel refers to the material with the highest proportion in the housing body 21.

[0443] Optionally, the thickness of the housing 21 is from 0.1 mm to 0.5 mm, and optionally from 0.2 mm to 0.35 mm. Exemplarily, the thickness of the housing 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 a range composed of any two of the above values. When the thickness of the housing 21 is within the above range, the mechanical strength of the housing 21 is relatively high, which can improve the reliability of use of the battery cell 7; and the housing 21 occupies less space and has more internal space, which is beneficial to improving the energy density of the battery cell 7.

[0444] Viewed from the outer shape of the electrode assembly 10, the electrode assembly 10 includes a main body portion 12, a first tab 111 and a second tab 112, and the first tab 111 and the second tab 112 protrude from the main body portion 12. The first tab 111 is the portion of the first electrode sheet where the active material layer is not coated, and the second tab 112 is the portion of the second electrode sheet where the active material layer is not coated. The first tab 111 and the second tab 112 are used to lead out the current in the main body portion 12. The polarities of the first electrode sheet and the second electrode sheet are opposite. In other words, one of the first electrode sheet and the second electrode sheet is the positive electrode sheet, and the other of the first electrode sheet and the second electrode sheet is the negative electrode sheet.

[0445] Taking the first tab 111 as the negative tab and the second tab 112 as the positive tab as an example for illustration; the negative current collector portion of the negative electrode sheet where the active material layer is not coated is the negative tab, and the active material coated on the negative current collector of the negative electrode sheet constitutes the negative electrode film layer. The negative electrode film layer and the negative current collector coated with the active material are part of the main body portion 12. The positive current collector portion of the positive electrode sheet where the active material layer is not coated is the positive tab, and the active material coated on the positive current collector of the positive electrode sheet constitutes the positive electrode film layer. The positive electrode film layer and the positive current collector coated with the active material are part of the main body portion 12. Of course, the first tab 111 can be the positive electrode sheet, and the second tab 112 can be the negative electrode sheet.

[0446] The first tab 111 and the second tab 112 can extend from the same side of the main body portion 12, or can extend from opposite sides respectively.

[0447] Optionally, the number of the first tabs 111 located on the same side of the main body portion 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 tab 111.

[0448] Optionally, the number of the second tabs 112 located on the same side of the main body portion 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 tab 112.

[0449] In some embodiments, the battery cell 7 further includes a first electrode terminal 31, and the first electrode terminal 31 is electrically connected to the first tab 111. Optionally, the first electrode terminal 31 and the first tab 111 are welded. The first electrode terminal 31 and the first tab 111 can be connected through an adapter, or can be connected without using an adapter. Optionally, the first electrode terminal 31 and the first tab 111 are not connected through an adapter, that is, the first electrode terminal 31 and the first tab 111 are directly welded, which can reduce the resistance at the connection, and is beneficial to reducing the overall internal resistance of the battery cell 7. When the first tab 111 is a negative tab, the first electrode terminal 31 is a negative terminal. When the first tab 111 is a positive tab, the first electrode terminal 31 is a positive terminal.

[0450] In some embodiments, the battery cell 7 further includes a second electrode terminal 32, and the second electrode terminal 32 is electrically connected to the second tab 112. Optionally, the second electrode terminal 32 and the second tab 112 are welded. The second electrode terminal 32 and the second tab 112 can be connected through an adapter, or can be connected without using an adapter. Optionally, the second electrode terminal 32 and the second tab 112 are not connected through an adapter, that is, the second electrode terminal 32 and the second tab 112 are directly welded, which can reduce the resistance at the connection, and is beneficial to reducing the overall internal resistance of the battery cell 7. When the second tab 112 is a negative tab, the second electrode terminal 32 is a negative terminal. When the second tab 112 is a positive tab, the second electrode terminal 32 is a positive terminal.

[0451] Optionally, the number of the first electrode terminals 31 on the same side of the main body portion 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.

[0452] Further optionally, the current-carrying area of a single first electrode terminal 31 is greater than or equal to 25 mm 2 and can be 25 mm 2 to 315 mm 2 . The current-carrying area of the first electrode terminal 31 can be understood as the cross-sectional area of the first electrode terminal 31, and this cross-section is perpendicular to the thickness direction of the end cover 22.

[0453] Exemplarily, the current-carrying area of a single first electrode terminal 31 can be 25 mm 2 , 30 mm 2 , 35 mm 2 , 40 mm 2 , 45 mm 2 , 50 mm 2 , 55 mm 2 , 60 mm 2 , 62 mm 2 , 64 mm 2, 65 mm 2 , 70 mm 2 , 75 mm 2 , 80 mm 2 , 85 mm 2 , 90 mm 2 , 95 mm 2 , 100 mm 2 , 105 mm 2 , 110 mm 2 , 115 mm 2 , 120 mm 2 , 125 mm 2 , 130 mm 2 , 135 mm 2 , 140 mm 2 , 145 mm 2 , 150 mm 2 , 155 mm 2 , 160 mm 2 , 165 mm 2 , 170 mm 2 , 175 mm 2 , 180 mm 2 , 185 mm 2 , 190 mm 2 , 195 mm 2 , 200 mm 2 , 205 mm 2 , 210 mm 2 , 215 mm 2 , 220 mm 2 , 225 mm 2 , 230 mm 2 , 235 mm 2 , 240 mm 2 , 245 mm 2 , 250 mm 2 , 255 mm 2 , 260 mm 2 , 265 mm 2 , 270 mm 2 , 275 mm 2 , 280 mm 2 , 285 mm 2 , 290 mm 2 , 295 mm 2 , 300 mm 2 , 305 mm 2 , 310 mm 2 , 315 mm 2 or a range formed by any two of the above values.

[0454] Optionally, the number of the second electrode terminals 32 on the same side of the main body 12 is at least one, optionally at least two. The at least two second electrode terminals 32 can increase the over-current capacity of the second electrode terminals 32.

[0455] Further optionally, the over-current area of a single second electrode terminal 32 is greater than or equal to 25 mm 2 , optionally 25 mm 2 to 315 mm 2 . The over-current area of the second electrode terminal 32 can be understood as the cross-sectional area of the second electrode terminal 32, and this cross-section is perpendicular to the thickness direction of the end cover 22.

[0456] Exemplarily, the over-current area of a single second electrode terminal 32 can be 25 mm 2 , 30 mm 2 , 35 mm 2 , 40 mm 2 , 45 mm 2 , 50 mm 2 , 55 mm 2 , 60 mm 2 , 62 mm 2 , 64 mm 2 , 65 mm 2 , 70 mm 2 , 75 mm 2 , 80 mm 2 , 85 mm 2 , 90 mm 2 , 95 mm 2 , 100 mm 2 , 105 mm 2 , 110 mm 2 , 115 mm 2 , 120 mm 2 , 125 mm 2 , 130 mm 2 , 135 mm 2 , 140 mm 2 , 145 mm 2 , 150 mm 2 , 155 mm 2 , 160 mm 2 , 165 mm 2 , 170 mm 2 , 175 mm 2 , 180 mm 2 , 185 mm 2 , 190 mm 2 , 195 mm 2 , 200 mm 2 , 205 mm2 , 210 mm 2 , 215 mm 2 , 220 mm 2 , 225 mm 2 , 230 mm 2 , 235 mm 2 , 240 mm 2 , 245 mm 2 , 250 mm 2 , 255 mm 2 , 260 mm 2 , 265 mm 2 , 270 mm 2 , 275 mm 2 , 280 mm 2 , 285 mm 2 , 290 mm 2 , 295 mm 2 , 300 mm 2 , 305 mm 2 , 310 mm 2 , 315 mm 2 or a range composed of any two of the above values.

[0457] As Figure 3 shown, in some embodiments of the present application, the battery cells 7 according to the implementation manner of the present application can be assembled into a battery module 6. The number of battery cells 7 included 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.

[0458] If there are multiple battery cells 7, the multiple battery cells 7 can be connected in series, in parallel, or in a series-parallel combination. A series-parallel combination means that there are both series and parallel connections among the multiple battery cells 7. The multiple battery cells 7 can be directly connected in series, in parallel, or in a series-parallel combination together, and then the whole formed by the multiple battery cells 7 is accommodated in the accommodation part of the battery module 6; of course, it is also possible that the multiple battery cells 7 are first connected in series, in parallel, or in a series-parallel combination to form a battery module 6, and then the multiple battery modules 6 are connected in series, in parallel, or in a series-parallel combination to form a whole and are accommodated in the accommodation part. Optionally, the battery module 6 can also include an accommodation part having an accommodation space, and the multiple battery cells 7 are accommodated in this accommodation space.

[0459] As Figure 4 shown, in some implementation manners, the above-mentioned battery module 6 can also be assembled into a battery pack 2. The number of battery modules 6 included in the battery pack 2 can be adjusted according to the application and capacity of the battery pack. The multiple battery modules 6 can be arranged in the box body 5 in any manner. The battery device can be in the form of a battery pack 2 or a battery module 6, etc.

[0460] The battery pack 2 may include a housing 5 and a plurality of battery modules 6 disposed in the housing 5. The housing 5 includes a first housing portion 5a and a second housing portion 5b. 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 receiving the battery modules 6. The plurality of battery modules 6 may be arranged in the housing 5 in any manner.

[0461] The first housing portion 5a and the second housing portion 5b cover each other, and the first housing portion 5a and the second housing portion 5b jointly define a receiving space 5c for receiving battery cells. The second housing portion 5b may be a hollow structure with one end open, and the first housing portion 5a is a plate-like structure. The first housing portion 5a covers the open side of the second housing portion 5b to form the housing 5 with the receiving space 5c; both the first housing portion 5a and the second housing portion 5b may also be hollow structures with one side open, and the open side of the first housing portion 5a covers the open side of the second housing portion 5b to form the housing 5 with the receiving space 5c. Of course, the first housing portion 5a and the second housing portion 5b may be of various shapes, such as a cylinder, a cuboid, etc.

[0462] To improve the sealing performance after the connection between the first housing portion 5a and the second housing portion 5b, a sealing member, such as sealant, sealing ring, etc., may also be provided between the first housing portion 5a and the second housing portion 5b.

[0463] Assuming that the first housing portion 5a covers the top of the second housing portion 5b, the first housing portion 5a may also be referred to as an upper cover, and the second housing portion 5b may also be referred to as a lower housing.

[0464] The battery pack 2 or any battery cell constituting the battery pack 2 also includes a plurality of charging steps from 40% state of charge to 80% state of charge. 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 of charging to 80% state of charge is any value from 2.5C to 5C. For example, it may be 2.7C.

[0465] In some embodiments, the charging time of the battery pack 2 or any battery cell constituting the battery pack 2 from 10% state of charge to 80% state of charge is less than or equal to 10.5 min, and may be selected from 5 min to 10.5 min. The temperature of the external environment where the battery pack 2 is located is room temperature, such as 30°C. Exemplarily, the charging time of the battery pack 2 from 10% state of charge to 80% state of charge is 10.5 min, 10 min, 9.5 min, 9 min, 8.5 min, 8 min, 7.5 min, 7 min, 6.5 min, 6 min, 5.5 min, 5 min or any range composed of any two of the above values.

[0466] Power consumption device

[0467] In a second aspect of the embodiments of the present application, an electrical device is provided. The electrical device includes at least one of a battery cell, a battery module, or a battery pack according to the embodiments of the present application. The battery cell, the battery module, or the battery pack can be used as the power source of the electrical device or as the energy storage unit of the electrical device. The electrical device can be a vehicle, a mobile phone, a portable device, a laptop computer, a ship, a spacecraft, an electric toy, an electric tool, and the like. The vehicle can be a fuel vehicle, a gas vehicle, or a new energy vehicle. The new energy vehicle can be a pure electric vehicle, a hybrid electric vehicle, or an extended-range electric vehicle, etc.; the spacecraft includes an airplane, a rocket, a space shuttle, a spaceship, and the like; the electric toy includes a stationary or mobile electric toy, for example, a game console, an electric vehicle toy, an electric ship toy, an electric airplane toy, and the like; the electric tool includes a metal cutting electric tool, a grinding electric tool, an assembly electric tool, and a railway electric tool, for example, an electric drill, an electric grinder, an electric wrench, an electric screwdriver, a hammer drill, an impact drill, a concrete vibrator, and a planer, and the like. The embodiments of the present application do not impose special restrictions on the above-mentioned electrical devices.

[0468] The electrical device can select a battery cell, a battery module, or a battery pack according to its usage requirements.

[0469] Figure 5 It is a schematic diagram of an electrical device 1 as an example. The electrical device 1 is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. In order to meet the high-power and high-energy density requirements of the electrical device 1, a battery pack or a battery module can be used.

[0470] A battery pack 2 is provided inside the electrical device 1. The battery pack 2 can be provided at the bottom, the head, or the tail of the electrical device 1. The battery pack 2 can be used for power supply of the electrical device 1. For example, the battery pack 2 can be used as the operating power source of the electrical device 1 and can also be used as the driving power source of the electrical device 1, replacing or partially replacing fuel or natural gas to provide driving power for the electrical device 1.

[0471] The electrical device 1 may further include a controller 3 and a motor 4. The controller 3 is used to control the battery pack 2 to supply power to the motor 4. For example, it is used for the working power requirements during the start, navigation, and driving of the electrical device 1.

[0472] Another example of an electrical device can be a mobile phone, a tablet computer, a laptop computer, etc. This electrical device usually requires thin and light design, and a battery cell can be used as the power source.

[0473] In some embodiments, the charging time of the electrical device from 10% state of charge to 80% state of charge is less than or equal to 10.5 min, optionally 5 min to 10.5 min, and the temperature of the external environment where the electrical device is located is room temperature, such as 30 °C. Exemplarily, the charging time of the battery pack 2 from 10% state of charge to 80% state of charge is 10.5 min, 10 min, 9.5 min, 9 min, 8.5 min, 8 min, 7.5 min, 7 min, 6.5 min, 6 min, 5.5 min, 5 min or the range composed of any two of the above values. The electrical device can adopt the same charging strategy as the battery pack 2 or the battery cell.

[0474] Embodiment

[0475] The following examples more specifically describe the content disclosed in the embodiments of the present application. These examples are only for illustrative purposes, because various modifications and changes within the scope of the content disclosed in the embodiments of the present application are obvious to those skilled in the art. Unless otherwise stated, all parts, percentages, and ratios reported in the following examples are based on mass, and all reagents used in the examples are commercially available or synthesized according to conventional methods and can be used directly without further treatment, and the instruments used in the examples are all commercially available.

[0476] Example 1 Preparation of the battery pack

[0477] 1. Preparation of the positive electrode plate

[0478] The positive electrode plate includes a positive current collector, a positive conductive layer on the positive current collector, and a positive electrode 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 formed by uniformly coating a mixture of a positive conductive agent, superconducting carbon, a positive binder, polyvinylidene fluoride (PVDF), and a solvent, N-methylpyrrolidone (NMP), on the surface of the current collector, with a thickness of 1 μm. The mass content of the positive conductive agent in the positive conductive layer is 40%, and the mass content of the positive binder is 60%.

[0479] The positive electrode film layer includes a film layer formed by uniformly coating a positive electrode slurry (with a solvent of N-methylpyrrolidone NMP) on the surface of the positive conductive layer and then drying and cold pressing. The positive electrode film layer includes a positive electrode active material, a binder, polyvinylidene fluoride (PVDF), and a conductive agent, acetylene black, with a weight ratio of 97:2:1.

[0480] 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. The coating layer 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.

[0481] The single-sided coating weight of the positive electrode film layer is 300 mg / 1540.25 mm 2 。

[0482] 2. Preparation of the negative electrode sheet

[0483] The negative electrode sheet includes a negative electrode current collector, a negative electrode conductive layer on the negative electrode current collector, and a negative electrode film layer. The negative electrode current collector is a copper foil with a thickness of 5 μm. The negative electrode conductive layer on the negative electrode current collector is a film layer formed by uniformly coating a mixture of a negative electrode conductive agent, superconducting carbon, a negative electrode binder, styrene-butadiene rubber (SBR), a thickener, sodium carboxymethyl cellulose (CMC-Na), and a solvent, water, on the surface of the negative electrode current collector, with a thickness of 1 μm. The mass content of the negative electrode conductive agent in the negative electrode conductive layer is 35%, the mass content of the negative electrode binder in the negative electrode conductive layer is 60%, and the mass content of the thickener in the negative electrode conductive layer is 5%.

[0484] The negative electrode film layer includes a film layer formed by uniformly coating a negative electrode slurry (with deionized water as the solvent) on the surface of the negative electrode conductive layer, followed by drying and cold pressing.

[0485] The single-sided coating weight of the negative electrode film layer is 138 mg / 1540.25 mm 2 。

[0486] 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.

[0487] The first negative electrode film layer includes graphite particles, conductive agent acetylene black, a first lithium-containing binder (copolymer of lithium acrylate - acrylonitrile - acrylamide - 2-hydroxyethyl acrylate), negative electrode binder styrene-butadiene rubber, and thickener sodium carboxymethyl cellulose with a mass ratio of 96.5:0.5:0.5:1.5:1. The mass content of lithium element in the first lithium-containing binder is 4.8%. The Dv50 of the graphite particles is 11.3 μm. The graphite particles include artificial graphite and a carbon coating layer, and the carbon coating layer covers the surface of the artificial graphite, with the mass content of amorphous carbon being 3.5%.

[0488] The second negative electrode film layer includes graphite particles, conductive agent acetylene black, a second lithium-containing binder (copolymer of lithium acrylate - acrylonitrile - acrylamide - 2-hydroxyethyl acrylate), negative electrode binder styrene-butadiene rubber, and thickener sodium carboxymethyl cellulose with a mass ratio of 97.5:0.5:0.5:0.5:1. The mass content of lithium element in the second lithium-containing binder is 4.8%. The Dv50 of the graphite particles is 11.3 μm. The graphite particles include artificial graphite and a carbon coating layer, and the carbon coating layer covers the surface of the artificial graphite, with the mass content of amorphous carbon being 3.5%.

[0489] 3. Separator

[0490] The separator membrane includes a base film, which is a 7-μm polyethylene film layer with a porosity of 42%.

[0491] 4. Preparation of the electrolyte

[0492] The electrolyte includes an organic solvent, a lithium salt, and an additive.

[0493] The organic solvent includes 60% chain carboxylic ester solvent (ethyl acetate) and 40% carbonate solvent (30% ethylene carbonate EC, 10% dimethyl carbonate). The mass content of each component in the organic solvent is calculated based on the mass of the organic solvent.

[0494] Based on the mass of the electrolyte, the mass content of the additive is 6.5%, which includes vinylene carbonate VC, fluoroethylene carbonate FEC, ethylene sulfite ES, and lithium difluorooxalate borate LiDFOB with a mass ratio of 5:0.5:0.5:0.5.

[0495] The lithium salt includes 1 mol / L lithium hexafluorophosphate LiPF6.

[0496] The conductivity of the electrolyte at room temperature is 16.4 mS / cm.

[0497] 5. Preparation of the battery cell

[0498] Stack the above positive electrode sheet, separator membrane, and negative electrode sheet in sequence, with the separator membrane between the positive electrode sheet and the negative electrode sheet to play an isolation role, obtaining an electrode assembly; place the electrode assembly in an outer packaging shell, inject the electrolyte after drying, and through processes such as vacuum packaging, standing, forming, and shaping, obtain the battery cell. The compaction density of the positive electrode film layer at 100% SOC is 2.72 g / cm 3 and the compaction density of the negative electrode film layer at 100% SOC is 1.26 g / cm 3 .

[0499] 6. Preparation of the battery pack

[0500] Divide multiple battery cells into two groups, connect them in series within each group, and then connect the two groups in parallel to assemble into a battery module. Then connect the battery module and the battery management system BMS, and assemble them into a box to form a battery pack. The battery management system BMS is configured to monitor the charging state and / or discharging state of multiple battery cells and can adjust the charging current and voltage as needed.

[0501] It should be noted that the battery pack can be charged independently, and each independent battery cell in the battery pack can also be charged independently. Both the battery pack and the battery cell can adopt the following charging strategy for charging. Next, the charging strategy will be described taking the battery pack as an example.

[0502] Charging Strategy 1

[0503] After assembling the battery cell prepared in Example 1 with the battery management system BMS into a battery pack, charging is carried out at an external temperature of 30°C. The charging steps include the following steps:

[0504] Constant current charge from 0% SOC to 5% SOC at 5.0C;

[0505] Constant current charge from 5% SOC to 10% SOC at 5.0C;

[0506] Constant current charge from 10% SOC to 15% SOC at 5.0C;

[0507] Constant current charge from 15% SOC to 20% SOC at 5.0C;

[0508] Constant current charge from 20% SOC to 25% SOC at 5.0C;

[0509] Constant current charge from 25% SOC to 30% SOC at 5.0C;

[0510] Constant current charge from 30% SOC to 35% SOC at 5.0C;

[0511] Constant current charge from 35% SOC to 40% SOC at 5.0C;

[0512] Constant current charge from 40% SOC to 45% SOC at 4.6C;

[0513] Constant current charge from 45% SOC to 50% SOC at 4.3C;

[0514] Constant current charge from 50% SOC to 55% SOC at 4.0C;

[0515] Constant current charge from 55% SOC to 60% SOC at 3.7C;

[0516] Constant current charge from 60% SOC to 65% SOC at 3.4C;

[0517] Constant current charge from 65% SOC to 70% SOC at 3.1C;

[0518] Constant current charge from 70% SOC to 75% SOC at 2.9C;

[0519] Constant current charge from 75% SOC to 80% SOC at 2.7C;

[0520] Constant current charge from 80% SOC to 85% SOC at 1.8C;

[0521] Constant current charge from 85% SOC to 90% SOC at 1.3C;

[0522] Charge from 90% SOC to 95% SOC at a constant current of 0.7C;

[0523] Charge from 95% SOC to 98% SOC at a constant current of 0.33C;

[0524] Charge from 98% SOC to 100% SOC at a constant current of 0.1C.

[0525] The cut-off voltage of the last charging step in the above charging steps is 3.65V. The difference between the cut-off voltage of any one of the N - 1 charging steps and the cut-off voltage of the last charging step is less than or equal to 0.05V. The cut-off voltage of each of the N - 1 charging steps is 3.6V, and the difference in the maximum state of charge between two adjacent charging steps is less than or equal to 5% SOC.

[0526] Charging strategy 2

[0527] After assembling the battery cells prepared in Example 1 with the battery management system BMS into a battery pack, charge at an external temperature of 30°C. The charging steps include the following steps:

[0528] Charge from 0% SOC to 5% SOC at a constant current of 5.0C;

[0529] Charge from 5% SOC to 10% SOC at a constant current of 5.0C;

[0530] Charge from 10% SOC to 15% SOC at a constant current of 5.0C;

[0531] Charge from 15% SOC to 20% SOC at a constant current of 5.0C;

[0532] Charge from 20% SOC to 25% SOC at a constant current of 5.0C;

[0533] Charge from 25% SOC to 30% SOC at a constant current of 5.0C;

[0534] Charge from 30% SOC to 35% SOC at a constant current of 5.0C;

[0535] Charge from 35% SOC to 40% SOC at a constant current of 5.0C;

[0536] Charge from 40% SOC to 45% SOC at a constant current of 4.6C;

[0537] Charge from 45% SOC to 50% SOC at a constant current of 4.3C;

[0538] Charge from 50% SOC to 55% SOC at a constant current of 4.0C;

[0539] Charge from 55% SOC to 60% SOC at a constant current of 3.7C;

[0540] Charge from 60% SOC to 65% SOC at a constant current of 3.4C;

[0541] Charge from 65% SOC to 70% SOC at a constant current of 3.1C;

[0542] Charge from 70% SOC to 75% SOC at a constant current of 2.9C;

[0543] Charge from 75% SOC to 80% SOC at a constant current of 2.7C;

[0544] Charge from 80% SOC to 85% SOC at a constant current of 1.8C;

[0545] Charge from 85% SOC to 90% SOC at a constant current of 1.3C;

[0546] Charge from 90% SOC to 95% SOC at a constant current of 0.7C;

[0547] Charge from 95% SOC to 98% SOC at a constant current of 0.33C;

[0548] Charge from 98% SOC to 100% SOC at a constant current of 0.01C.

[0549] The cut-off voltage of the last charging step in the above charging steps is 3.65V. The difference between the cut-off voltage of any charging step in the N-1 charging steps and the cut-off voltage of the last charging step is less than or equal to 0.05V. The cut-off voltage of each charging step in the N-1 steps is 3.6V, and the difference in the maximum state of charge between two adjacent charging steps is less than or equal to 5% SOC.

[0550] Charging strategy 3

[0551] After assembling the battery cells prepared in Example 1 with the battery management system BMS into a battery pack, charge at an external temperature of 30°C. The charging steps include the following steps:

[0552] Charge from 0% SOC to 5% SOC at a constant current of 5.0C;

[0553] Charge from 5% SOC to 10% SOC at a constant current of 5.0C;

[0554] Charge from 10% SOC to 15% SOC at a constant current of 5.0C;

[0555] Charge from 15% SOC to 20% SOC at a constant current of 5.0C;

[0556] Charge from 20% SOC to 25% SOC at a constant current of 5.0C;

[0557] Charge from 25% SOC to 30% SOC at a constant current of 5.0C;

[0558] Charge from 30% SOC to 35% SOC at a constant current of 5.0C;

[0559] Charge from 35% SOC to 40% SOC at a constant current of 5.0C;

[0560] Charge from 40% SOC to 45% SOC at a constant current of 4.6C;

[0561] Charge from 45% SOC to 50% SOC at a constant current of 4.3C;

[0562] Charge from 50% SOC to 55% SOC at a constant current of 4.0C;

[0563] Charge from 55% SOC to 60% SOC at a constant current of 3.7C;

[0564] Charge from 60% SOC to 65% SOC at a constant current of 3.4C;

[0565] Charge from 65% SOC to 70% SOC at a constant current of 3.1C;

[0566] Charge from 70% SOC to 75% SOC at a constant current of 2.9C;

[0567] Charge from 75% SOC to 80% SOC at a constant current of 2.7C;

[0568] Charge from 80% SOC to 85% SOC at a constant current of 1.8C;

[0569] Charge from 85% SOC to 90% SOC at a constant current of 1.3C;

[0570] Charge from 90% SOC to 95% SOC at a constant current of 0.7C;

[0571] Charge from 95% SOC to 98% SOC at a constant current of 0.33C;

[0572] Charge from 98% SOC to 100% SOC at a constant current of 0.05C.

[0573] The cut-off voltage of the last charging step in the above charging steps is 3.65V. The difference between the cut-off voltage of any one of the N - 1 charging steps and the cut-off voltage of the last charging step is less than or equal to 0.05V. The cut-off voltage of each of the N - 1 steps is 3.6V, and the difference in the maximum state of charge between two adjacent charging steps is less than or equal to 5% SOC.

[0574] Charging strategy 4

[0575] After assembling the battery cell prepared in Example 1 with the battery management system BMS into a battery pack, charging was carried out at an external temperature of 30 °C. The charging steps included the following steps:

[0576] Constant current charge at 5.0C from 0% SOC to 5% SOC;

[0577] Constant current charge at 5.0C from 5% SOC to 10% SOC;

[0578] Constant current charge at 5.0C from 10% SOC to 15% SOC;

[0579] Constant current charge at 5.0C from 15% SOC to 20% SOC;

[0580] Constant current charge at 5.0C from 20% SOC to 25% SOC;

[0581] Constant current charge at 5.0C from 25% SOC to 30% SOC;

[0582] Constant current charge at 5.0C from 30% SOC to 35% SOC;

[0583] Constant current charge at 5.0C from 35% SOC to 40% SOC;

[0584] Constant current charge at 4.6C from 40% SOC to 45% SOC;

[0585] Constant current charge at 4.3C from 45% SOC to 50% SOC;

[0586] Constant current charge at 4.0C from 50% SOC to 55% SOC;

[0587] Constant current charge at 3.7C from 55% SOC to 60% SOC;

[0588] Constant current charge at 3.4C from 60% SOC to 65% SOC;

[0589] Constant current charge at 3.1C from 65% SOC to 70% SOC;

[0590] Constant current charge at 2.9C from 70% SOC to 75% SOC;

[0591] Constant current charge at 2.7C from 75% SOC to 80% SOC;

[0592] Constant current charge at 1.8C from 80% SOC to 85% SOC;

[0593] Constant current charge at 1.3C from 85% SOC to 90% SOC;

[0594] Charge from 90% SOC to 95% SOC at a constant current of 0.7C;

[0595] Charge from 95% SOC to 98% SOC at a constant current of 0.33C;

[0596] Charge from 98% SOC to 100% SOC at a constant current of 0.3C.

[0597] The cut-off voltage of the last charging step in the above charging steps is 3.65V. The difference between the cut-off voltage of any charging step in the N - 1 charging steps and the cut-off voltage of the last charging step is less than or equal to 0.05V. The cut-off voltage of each charging step in the N - 1 steps is 3.6V, and the difference in the maximum state of charge between two adjacent charging steps is less than or equal to 5% SOC.

[0598] Charging strategy 5

[0599] After assembling the battery cells prepared in Example 1 with the battery management system BMS into a battery pack, charge at an external temperature of 30°C. The charging steps include the following:

[0600] Charge from 0% SOC to 5% SOC at a constant current of 5.0C;

[0601] Charge from 5% SOC to 10% SOC at a constant current of 5.0C;

[0602] Charge from 10% SOC to 15% SOC at a constant current of 5.0C;

[0603] Charge from 15% SOC to 20% SOC at a constant current of 5.0C;

[0604] Charge from 20% SOC to 25% SOC at a constant current of 5.0C;

[0605] Charge from 25% SOC to 30% SOC at a constant current of 5.0C;

[0606] Charge from 30% SOC to 35% SOC at a constant current of 5.0C;

[0607] Charge from 35% SOC to 40% SOC at a constant current of 5.0C;

[0608] Charge from 40% SOC to 45% SOC at a constant current of 4.6C;

[0609] Charge from 45% SOC to 50% SOC at a constant current of 4.3C;

[0610] Charge from 50% SOC to 55% SOC at a constant current of 4.0C;

[0611] Charge from 55% SOC to 60% SOC at a constant current of 3.7C;

[0612] Charge from 60% SOC to 65% SOC at a constant current of 3.4C;

[0613] Charge from 65% SOC to 70% SOC at a constant current of 3.1C;

[0614] Charge from 70% SOC to 75% SOC at a constant current of 2.9C;

[0615] Charge from 75% SOC to 80% SOC at a constant current of 2.7C;

[0616] Charge from 80% SOC to 85% SOC at a constant current of 1.8C;

[0617] Charge from 85% SOC to 90% SOC at a constant current of 1.3C;

[0618] Charge from 90% SOC to 95% SOC at a constant current of 0.7C;

[0619] Charge from 95% SOC to 98% SOC at a constant current of 0.33C;

[0620] Charge from 98% SOC to 100% SOC at a constant current of 0.1C.

[0621] The cut-off voltage of the last charging step in the above charging steps is 3.65V. The difference between the cut-off voltage of any charging step in the N-1 charging steps and the cut-off voltage of the last charging step is less than or equal to 0.02V. The cut-off 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.

[0622] Charging strategy 6

[0623] After assembling the battery cells prepared in Example 1 with the battery management system BMS into a battery pack, charge at an external temperature of 30°C. The charging steps include the following steps:

[0624] Charge from 0% SOC to 5% SOC at a constant current of 5.0C;

[0625] Charge from 5% SOC to 10% SOC at a constant current of 5.0C;

[0626] Charge from 10% SOC to 15% SOC at a constant current of 5.0C;

[0627] Charge from 15% SOC to 20% SOC at a constant current of 5.0C;

[0628] Charge from 20% SOC to 25% SOC at a constant current of 5.0C;

[0629] Charge from 25% SOC to 30% SOC at a constant current of 5.0C;

[0630] Charge from 30% SOC to 35% SOC at a constant current of 5.0C;

[0631] Charge from 35% SOC to 40% SOC at a constant current of 5.0C;

[0632] Charge from 40% SOC to 45% SOC at a constant current of 4.6C;

[0633] Charge from 45% SOC to 50% SOC at a constant current of 4.3C;

[0634] Charge from 50% SOC to 55% SOC at a constant current of 4.0C;

[0635] Charge from 55% SOC to 60% SOC at a constant current of 3.7C;

[0636] Charge from 60% SOC to 65% SOC at a constant current of 3.4C;

[0637] Charge from 65% SOC to 70% SOC at a constant current of 3.1C;

[0638] Charge from 70% SOC to 75% SOC at a constant current of 2.9C;

[0639] Charge from 75% SOC to 80% SOC at a constant current of 2.7C;

[0640] Charge from 80% SOC to 85% SOC at a constant current of 1.8C;

[0641] Charge from 85% SOC to 90% SOC at a constant current of 1.3C;

[0642] Charge from 90% SOC to 95% SOC at a constant current of 0.7C;

[0643] Charge from 95% SOC to 98% SOC at a constant current of 0.33C;

[0644] Charge from 98% SOC to 100% SOC at a constant current of 0.1C.

[0645] The cut-off voltage of the last charging step in the above charging steps is 3.65V. The difference between the cut-off voltage of any one of the N - 1 charging steps and the cut-off voltage of the last charging step is less than or equal to 0.08V. The cut-off voltage of each of 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.

[0646] Charging Strategy 7 (Slow Charging Comparison)

[0647] After assembling the battery monomer prepared in Example 1 with the battery management system BMS into a battery pack, charging is carried out at an external temperature of 30°C. The charging steps include the following steps:

[0648] Constant current charge from 0% SOC to 30% SOC at 3.0C;

[0649] Constant current charge from 30% SOC to 35% SOC at 2.8C;

[0650] Constant current charge from 35% SOC to 40% SOC at 2.6C;

[0651] Constant current charge from 40% SOC to 45% SOC at 2.4C;

[0652] Constant current charge from 45% SOC to 50% SOC at 2.2C;

[0653] Constant current charge from 50% SOC to 55% SOC at 2.0C;

[0654] Constant current charge from 55% SOC to 60% SOC at 1.8C;

[0655] Constant current charge from 60% SOC to 65% SOC at 1.6C;

[0656] Constant current charge from 65% SOC to 70% SOC at 1.4C;

[0657] Constant current charge from 70% SOC to 75% SOC at 1.3C;

[0658] Constant current charge from 75% SOC to 80% SOC at 1.2C;

[0659] Constant current charge from 80% SOC to 85% SOC at 0.8C;

[0660] Constant current charge from 85% SOC to 90% SOC at 0.6C;

[0661] Constant current charge from 90% SOC to 95% SOC at 0.4C;

[0662] Constant current charge from 95% SOC to 98% SOC at 0.33C;

[0663] Constant current charge from 98% SOC to 100% SOC at 0.3C.

[0664] The cut-off voltage of the last charging step in the above charging steps is 3.65V. The difference between the cut-off voltage of any one of the N-1 charging steps and the cut-off voltage of the last charging step is equal to 0V, that is, the cut-off voltage of each charging step is 3.65V, and the difference in the maximum state of charge between two adjacent charging steps is less than or equal to 5% SOC.

[0665] Charging strategy 8

[0666] After assembling the battery cells prepared in Example 1 with the battery management system (BMS) into a battery pack, charging is carried out at an external temperature of 30°C. The charging steps include the following:

[0667] Constant current charging at 5.0C from 0% SOC to 5% SOC;

[0668] Constant current charging at 5.0C from 5% SOC to 10% SOC;

[0669] Constant current charging at 5.0C from 10% SOC to 15% SOC;

[0670] Constant current charging at 5.0C from 15% SOC to 20% SOC;

[0671] Constant current charging at 5.0C from 20% SOC to 25% SOC;

[0672] Constant current charging at 5.0C from 25% SOC to 30% SOC;

[0673] Constant current charging at 5.0C from 30% SOC to 35% SOC;

[0674] Constant current charging at 5.0C from 35% SOC to 40% SOC;

[0675] Constant current charging at 5C from 40% SOC to 45% SOC;

[0676] Constant current charging at 4.3C from 45% SOC to 50% SOC;

[0677] Constant current charging at 4.0C from 50% SOC to 55% SOC;

[0678] Constant current charging at 3.7C from 55% SOC to 60% SOC;

[0679] Constant current charging at 3.4C from 60% SOC to 65% SOC;

[0680] Constant current charging at 3.1C from 65% SOC to 70% SOC;

[0681] Constant current charging at 2.9C from 70% SOC to 75% SOC;

[0682] Constant current charging at 2.7C from 75% SOC to 80% SOC;

[0683] Constant current charging at 1.8C from 80% SOC to 85% SOC;

[0684] Charge from 85% SOC to 90% SOC at a constant current of 1.3C;

[0685] Charge from 90% SOC to 95% SOC at a constant current of 0.7C;

[0686] Charge from 95% SOC to 98% SOC at a constant current of 0.33C;

[0687] Charge from 98% SOC to 100% SOC at a constant current of 0.33C.

[0688] The cut-off voltage of the last charging step in the above charging steps is 3.65V. The difference between the cut-off voltage of any one of the N-1 charging steps and the cut-off voltage of the last charging step is equal to 0V, that is, the cut-off voltage of each charging step is 3.65V, and the difference in the maximum state of charge between two adjacent charging steps is less than or equal to 5% SOC.

[0689] Preparation of the battery pack in Example 2

[0690] The battery cells and the battery pack are prepared by a method similar to that in Example 1. Different from Example 1, the positive active material includes lithium iron manganese phosphate and a coating layer. The coating layer is coated on the surface of lithium iron manganese phosphate, and the coating layer includes lithium iron titanium phosphate Li2FeTi(PO4)3 and amorphous carbon.

[0691] Charging strategy 9

[0692] After assembling the battery cells prepared in Example 2 with the battery management system BMS into a battery pack, charge at an external temperature of 30°C. The charging steps include the following steps:

[0693] Charge from 0% SOC to 5% SOC at a constant current of 5.0C;

[0694] Charge from 5% SOC to 10% SOC at a constant current of 5.0C;

[0695] Charge from 10% SOC to 15% SOC at a constant current of 5.0C;

[0696] Charge from 15% SOC to 20% SOC at a constant current of 5.0C;

[0697] Charge from 20% SOC to 25% SOC at a constant current of 5.0C;

[0698] Charge from 25% SOC to 30% SOC at a constant current of 5.0C;

[0699] Charge from 30% SOC to 35% SOC at a constant current of 5.0C;

[0700] Charge from 35% SOC to 40% SOC at a constant current of 5.0C;

[0701] Charge from 40% SOC to 45% SOC at a constant current of 5.0 C;

[0702] Charge from 45% SOC to 50% SOC at a constant current of 4.3 C;

[0703] Charge from 50% SOC to 55% SOC at a constant current of 4.0 C;

[0704] Charge from 55% SOC to 60% SOC at a constant current of 3.7 C;

[0705] Charge from 60% SOC to 65% SOC at a constant current of 3.4 C;

[0706] Charge from 65% SOC to 70% SOC at a constant current of 3.1 C;

[0707] Charge from 70% SOC to 75% SOC at a constant current of 2.9 C;

[0708] Charge from 75% SOC to 80% SOC at a constant current of 2.7 C;

[0709] Charge from 80% SOC to 85% SOC at a constant current of 1.8 C;

[0710] Charge from 85% SOC to 90% SOC at a constant current of 1.3 C;

[0711] Charge from 90% SOC to 95% SOC at a constant current of 0.7 C;

[0712] Charge from 95% SOC to 98% SOC at a constant current of 0.33 C;

[0713] Charge from 98% SOC to 100% SOC at a constant current of 0.33 C.

[0714] In the above charging steps, the cut-off voltage of the last charging step is 4.25 V. The difference between the cut-off voltage of any one of the N - 1 charging steps and the cut-off voltage of the last charging step is less than or equal to 0.05 V. The cut-off voltage of each of the N - 1 charging steps is 4.2 V. The difference in the maximum state of charge between two adjacent charging steps is less than or equal to 5% SOC.

[0715] Performance Test

[0716] 1. Lithium deposition area test of battery cells

[0717] After cycling each battery pack of each embodiment 20 times according to its respective charge and discharge strategy, the discharge is carried out using the following strategy: discharge at a constant current of 0.33 C to 2.0 V, then fully charge to 100% SOC according to the corresponding charging strategy, disassemble the negative electrode plate in the battery pack, unfold the negative electrode plate, observe the lithium deposition area (grayish-white area), and measure the lithium deposition area.

[0718] No lithium plating: The lithium plating area < 0.05%.

[0719] Slight lithium plating: The lithium plating area < 2%.

[0720] Severe lithium plating: The lithium plating area ≥ 2%.

[0721] Test results

[0722] The test results are shown in Table 1.

[0723] Table 1

[0724]

[0725] As can be seen from Table 1, when using a slower charging strategy (charging strategy 7), although there is basically no lithium plating on the negative electrode sheet, its charging time is relatively long and it cannot meet the requirements of fast charging.

[0726] When using a faster charging strategy (charging strategy 8), when charging to the cut-off voltage in each step, the risk of lithium plating in each step is relatively large, resulting in severe lithium plating on the negative electrode sheet during the entire charging process, which is likely to shorten the service life of the battery pack, etc.

[0727] During the charging process of the battery pack according to the embodiment of the present application, the difference in the maximum state of charge between adjacent charging steps is less than or equal to 5% state of charge, making the polarization during the charging process relatively small; before the Nth charging step, that is, in any charging step from the 1st charging step to the N - 1th charging step, the cut-off voltage is relatively small, so that during the charging process, the voltage will not rise sharply, and the lithium ions released from the positive electrode film layer can basically be embedded in the negative electrode film layer, and it is not easy to occur lithium plating on the surface of the negative electrode sheet, which can improve the use reliability of the battery cell.

[0728] Moreover, the embodiment of the present application is applicable to different phosphate systems, such as lithium iron phosphate, lithium manganese iron phosphate, lithium manganese phosphate, etc. Their corresponding battery cut-off voltages are different, which means that the embodiment of the present application is applicable to batteries with different cut-off voltages, such as batteries less than or equal to 4.4V, and the battery system can be selected from 3.65V to 4.4V. When the above battery systems adopt the charging strategy of the present application, they can all achieve fast charging and reduce the risk of lithium plating.

[0729] Although the illustrative embodiments have been demonstrated and described, those skilled in the art should understand that the above embodiments should not be construed as limitations of the present application, and changes, substitutions, and modifications can be made to the embodiments without departing from the spirit, principle, and scope of the present application.

Claims

1. A battery cell, characterized in that, It includes an electrolyte and an electrode assembly. The electrode assembly includes a positive electrode sheet, a negative electrode sheet, and a separator membrane located between the positive electrode sheet and the negative electrode sheet. The positive electrode sheet includes a positive current collector and a positive electrode film layer provided on at least one side of the positive current collector and containing a positive electrode active material. The positive electrode active material includes a lithium-containing phosphate with an olivine structure. The negative electrode sheet includes a negative current collector and a negative electrode film layer provided on at least one side of the negative current collector and containing a negative electrode active material. The negative electrode film layer includes a carbon-based material. The charging process of the battery cell from 0% state of charge to 100% state of charge includes N charging steps. The difference between the maximum state of charge of any one of the N charging steps and the maximum state of charge of its adjacent charging step is less than or equal to 5% state of charge. N is a positive integer greater than or equal to 2. Among the N charging steps, the cut-off voltage of any one of the N - 1 charging steps is less than the cut-off voltage of the Nth charging step, and the cut-off voltage of the Nth charging step does not exceed 4.4V. At room temperature, the charging time of the battery cell from 10% state of charge to 80% state of charge is 5 min to 10.5 min.

2. The battery cell according to claim 1, wherein, The cut-off voltage of the Nth charging step is greater than the cut-off voltage of any one of the N - 1 charging steps, and the difference between the cut-off voltage of the Nth charging step and the cut-off voltage of any one of the N - 1 charging steps is greater than or equal to 0.02V.

3. The battery cell according to claim 1 or 2, characterized in that, The cut-off voltage of the Nth charging step is greater than the cut-off voltage of any one of the N - 1 charging steps, and the difference between the cut-off voltage of the Nth charging step and the cut-off voltage of any one of the N - 1 charging steps is greater than or equal to 0.05V.

4. The battery cell according to claim 3, characterized in that, The cut-off voltage of the Nth charging step is greater than the cut-off voltage of any one of the N - 1 charging steps, and the difference between the cut-off voltage of the Nth charging step and the cut-off voltage of any one of the N - 1 charging steps is 0.05V to 0.2V.

5. The battery cell according to claim 1, characterized in that, The cut-off voltage of the Nth charging step is 3.65V to 4.4V.

6. The battery cell according to claim 1, characterized in that The charging rate of the Nth charging step is 0.05C to 0.30C.

7. The battery cell according to claim 6, wherein The charging rate of the Nth charging step is 0.1C to 0.30C.

8. The battery cell according to claim 1, characterized in that, The charging rate of the battery cell in the Mth charging step is 3.5C to 6C. In the Mth charging step, the state of charge of the battery cell includes 50% state of charge. M is less than N and M is a positive integer greater than or equal to 1.

9. The battery cell according to claim 1, characterized in that, The charging rate of any one of the charging steps of the battery cell from 0% state of charge to 40% state of charge is 4C to 8C.

10. The battery cell according to claim 1, characterized in that, The constant current value of the Qth charging step among the N charging steps is less than the 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.

11. The battery cell according to claim 1, characterized in that, The lithium-containing phosphate with an olivine structure includes: phosphate particles, and a coating layer coated on the surface of the phosphate particles. The coating layer contains one or more elements of C, Fe, Ti, Zr, Hf, Ge, and Sn.

12. The battery cell according to claim 11, characterized in that, The phosphate particles comprise a compound of the general formula Li x1 A y1 Me a M b P 1-c X c Y z where 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 comprises one or more of Na, K, Mg, Me comprises one or more of Mn, Fe, Co, Ni, M comprises 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 comprises one or more of S, Si, Cl, B, C, N, and Y comprises one or more of O, F.

13. The battery cell according to claim 11 or 12, characterized in that, The coating layer includes a fast ion conductor with the general formula Li 3- d Fe 2-d M2 d (PO x2 ) y2 , where M2 includes one or more elements of Ti, Zr, Hf, Ge, and Sn, 0 ≤ d ≤ 1, 0 < x2 < 5, and 0 < y2 < 4.

14. The battery cell according to claim 11, characterized in that, The mass content of carbon element in the lithium-containing phosphate of olivine structure is 1% to 2%. The specific surface area of the lithium-containing phosphate with olivine structure is 5 m 2 / g to 18 m 2 / g.

15. The battery cell according to claim 1, characterized in that, The single-sided coating weight of the positive electrode film layer is 200 mg / 1540 mm 2 to 370 mg / 1540 / mm 2 ; and / or When the battery cell is in a 100% state of charge, the tap density of the positive electrode film layer is 2.50 g / cm 3 to 2.80 g / cm 3 .

16. The battery cell according to claim 1, wherein The single-sided coating weight of the negative electrode film layer is 90 mg / 1540 mm 2 to 170 mg / 1540 mm 2 ; and / or When the battery cell is in a 100% state of charge, the tap density of the negative electrode film layer is 1.15 g / cm 3 to 1.36 g / cm 3 .

17. The battery cell according to claim 16, wherein When the battery cell is in a 100% state of charge, the tap density of the negative electrode film layer is 1.25 g / cm 3 to 1.36 g / cm 3 .

18. The battery cell according to claim 1, wherein the carbon-based material includes graphite particles, and the graphitization degree of the graphite particles is 92.0% to 94.5%.

19. The battery cell according to claim 18, characterized in that, The graphite particles include: artificial graphite, including secondary particles; and a carbon coating layer coated on the surface of the artificial graphite.

20. The battery cell according to claim 18 or 19, characterized in that, The negative electrode film layer includes: a first negative electrode film layer disposed on the surface of the negative electrode current collector, the first negative electrode film layer includes 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 includes 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 the volume average particle diameter Dv50 of the graphite particles in the first negative electrode film layer is greater than or equal to the volume average particle diameter Dv50 of the graphite particles in the second negative electrode film layer.

21. 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.

22. The battery cell according to claim 20, wherein, 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.

23. The battery cell according to claim 22, characterized in that, The tapped 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 tapped 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 .

24. The battery cell according to claim 20, characterized in that, The ratio of the thickness of the second negative electrode film layer to the thickness of the first negative electrode film layer is 3:7 to 7:

3.

25. The battery cell according to claim 24, characterized in that, The ratio of the thickness of the second negative electrode film layer to the thickness of the first negative electrode film layer is 4:6 to 6:

4.

26. The battery cell according to claim 20, wherein, After the battery cell undergoes 10 full charge test cycles at the beginning of life (BOL), the thickness of the first negative electrode film layer is 15 μm to 65 μm, and / or the thickness of the second negative electrode film layer is 15 μm to 65 μm.

27. The battery cell according to claim 20, characterized in that, After the battery cell undergoes a full charge test at the end of life (EOL), the thickness of the first negative electrode film layer is 15 μm to 70 μm, and / or the thickness of the second negative electrode film layer is 15 μm to 70 μm.

28. The battery cell according to claim 20, wherein The first negative electrode film layer further includes a first lithium-containing binder, the second negative electrode film layer further includes a second lithium-containing binder, and 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.

29. The battery cell according to claim 28, wherein the mass content of the first lithium-containing binder relative to the mass of the first negative electrode film layer is 0.1% to 1%, and / or the mass content of the second lithium-containing binder relative to the mass of the second negative electrode film layer is 0.1% to 1%.

30. The battery cell according to claim 28 or 29, wherein the mass content of lithium element in the first lithium-containing binder is 3% to 10%, and / or the mass content of lithium element in the second lithium-containing binder is 3% to 10%.

31. The battery cell according to claim 28, wherein, The first lithium-containing binder includes a lithium acrylate-acrylonitrile-acrylamide-2-hydroxyethyl acrylate copolymer, and the lithium acrylate-acrylonitrile-acrylamide-2-hydroxyethyl acrylate copolymer is derived from a lithium acrylate monomer, an acrylonitrile monomer, an acrylamide monomer, and a 2-hydroxyethyl acrylate monomer. The molar ratio of the lithium acrylate monomer, the acrylonitrile monomer, the acrylamide monomer, and the 2-hydroxyethyl acrylate monomer is 30% to 50%: 15% to 45%: 5% to 20%: 20% to 35%; and / or The second lithium-containing binder includes a lithium acrylate-acrylonitrile-acrylamide-2-hydroxyethyl acrylate copolymer, and the lithium acrylate-acrylonitrile-acrylamide-2-hydroxyethyl acrylate copolymer is derived from a lithium acrylate monomer, an acrylonitrile monomer, an acrylamide monomer, and a 2-hydroxyethyl acrylate monomer. The molar ratio of the lithium acrylate monomer, the acrylonitrile monomer, the acrylamide monomer, and the 2-hydroxyethyl acrylate monomer is 30% to 50%: 15% to 45%: 5% to 20%: 20% to 35%.

32. The battery cell according to claim 1, wherein, The negative electrode active material further includes a silicon-based material, and the content of silicon element in the silicon-based material is 0.3% to 10.0% based on the mass of the negative electrode active material.

33. The battery cell according to claim 1, characterized in that, The separator includes a base film with a porous structure, and the porosity of the base film is 20% to 70%; and / or the thickness of the base film is 6 μm to 12 μm.

34. The battery cell according to claim 33, wherein The separator further includes a functional layer disposed on at least one side of the base film, and the functional layer includes: A first functional layer located on one side of the base film, and the first functional layer includes first inorganic particles, A second functional layer located on the other side of the base film, and the second functional layer includes composite particles. The composite particles include second inorganic particles and a plurality of non-fluoropolymer particles, and the second inorganic particles are attached to the surface of the non-fluoropolymer particles and / or dispersed inside the non-fluoropolymer particles.

35. The battery cell according to claim 34, wherein The non-fluoropolymer particles include an acrylate copolymer.

36. The battery cell according to claim 34 or 35, characterized in that, 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, and / or 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.

37. The battery cell according to claim 34, characterized in that, The average particle size of the second inorganic particles is 5 nm to 100 nm.

38. The battery cell according to claim 1, wherein, The battery cell further includes an electrolyte, The viscosity of the electrolyte at room temperature is 2.3 mPa·s to 3.5 mPa·s; and / or The conductivity of the electrolyte at room temperature is 13 mS / cm to 20 mS / cm; and / or The density of the electrolyte at room temperature is 1.05 g / mL to 1.35 g / mL.

39. The battery cell according to claim 1, wherein, The electrolyte includes an organic solvent, and the organic solvent includes one or more of carbonate solvents and carboxylate solvents.

40. The battery cell according to claim 39, wherein The carboxylate solvent includes a chain carboxylate solvent, and the mass content of the chain carboxylate solvent relative to the mass of the organic solvent is greater than or equal to 5% and less than 75%.

41. The battery cell according to claim 1, characterized in that, 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.

42. The battery cell according to claim 1, characterized in that, The battery cell includes a positive terminal, the positive electrode plate includes a positive electrode tab, and the positive terminal and the positive electrode tab are directly welded; and / or The battery cell includes a negative terminal, the negative electrode plate includes a negative electrode tab, and the negative terminal and the negative electrode tab are directly welded.

43. The battery cell according to claim 42, wherein The overcurrent area of a single positive terminal is 25 mm 2 to 315 mm 2 ; and / or The overcurrent area of a single said negative terminal is 25 mm 2 to 315 mm 2 .

44. The battery cell according to claim 1, wherein, It further includes a housing, the electrode assembly is accommodated in the housing, the material of the housing includes steel, and the thickness of the housing is 0.1 mm to 0.5 mm.

45. A battery device, characterized in that, It includes the battery cell according to any one of claims 1 to 44.

46. The battery device according to claim 45, wherein, At room temperature, the charging time of the battery device from 10% state of charge to 80% state of charge is 5 min to 10.5 min.

47. An electrical device, characterized in that, It includes the battery device according to claim 45 or 46.

48. A charging method for a battery cell, characterized in that, Applicable to the battery cell according to any one of claims 1-44, including: Charging the battery cell in the first state of charge so that the increase in the state of charge of the battery cell is less than or equal to 5% state of charge; Repeating the above steps at least once until the battery cell is charged to the 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 cut-off voltage of any step before charging to the second state of charge is less than the cut-off voltage of the step of charging to 100% state of charge, and the cut-off voltage of the step of charging to 100% state of charge does not exceed the theoretical voltage upper limit of the lithium-containing phosphate in the olivine structure.

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