Secondary battery and electronic device

Abstract

This application discloses a secondary battery comprising a positive electrode active material layer, which includes a phosphate-based positive electrode material and a lithium-rich manganese-based positive electrode material. The lithium-rich manganese-based positive electrode material includes a substrate material and a coating layer on the surface of the substrate material. The coating layer includes at least one of amorphous carbon and transition metal oxides. Based on the mass of the positive electrode active material layer, the mass content of the substrate material is 1% to 10%, and based on the mass of the substrate material, the mass content of the coating layer is 1% to 12%. This secondary battery, by introducing a lithium-rich manganese-based positive electrode material with a specific structure and content into a phosphate-based positive electrode material such as lithium iron phosphate, can effectively improve gas generation and enhance high-temperature storage performance while maintaining both rate performance and cycle performance.

Description

Technical Field

[0001] This application relates to the field of energy storage, specifically to a secondary battery and electronic device. Background Technology

[0002] With the continuous development and application of secondary battery technology, such as lithium-ion batteries, higher requirements are being placed on cathode materials, such as higher energy density, lower price, excellent cycle life, and higher rate performance. Layered lithium-rich cathode materials, with their unique high specific capacity (200mAh / g to 300mAh / g), long cycle life, and novel electrochemical charge-discharge mechanisms, have become a research hotspot for lithium-ion secondary battery cathode materials. Lithium iron phosphate modified with layered lithium-rich materials exhibits excellent cycle performance and significantly improved rate performance; however, long-term storage is accompanied by the generation of large amounts of gas, which deteriorates the secondary battery interface and greatly reduces its lifespan. Current technologies often suppress gas generation during storage of lithium-rich lithium iron phosphate hybrid materials by modifying the electrolyte with additives, but the introduction of additives makes it difficult to simultaneously achieve good rate and cycle performance of the secondary battery. Summary of the Invention

[0003] To address the problems existing in the prior art, embodiments of this application provide a secondary battery and an electronic device including the secondary battery. The secondary battery of this application, by introducing a lithium-rich manganese-based cathode material with a specific structure and content into a phosphate-based cathode material such as lithium iron phosphate, can effectively improve gas generation issues and enhance high-temperature storage performance while maintaining both rate performance and cycle performance.

[0004] In a first aspect, embodiments of this application provide a secondary battery, including a positive electrode sheet, the positive electrode sheet including a positive electrode active material layer, the positive electrode active material layer including a phosphate-based positive electrode material and a lithium-rich manganese-based positive electrode material, wherein the lithium-rich manganese-based positive electrode material includes a matrix material and a coating layer located on the surface of the matrix material, the coating layer including at least one of amorphous carbon and transition metal oxides, wherein, based on the mass of the positive electrode active material layer, the mass content of the matrix material is 1% to 10%, and based on the mass of the matrix material, the mass content of the coating layer is 1% to 12%.

[0005] In some embodiments, the matrix material comprises xLi2MnO3·(1-x)LiMO2, wherein 0.2≤x≤0.8, and M comprises at least one of Co, Ni and Mn.

[0006] Phosphate-based cathode materials, such as lithium iron phosphate (LFP), exhibit structural stability and good cycle performance, but they suffer from significant gas generation at high temperatures. While introducing larger-particle-size layered lithium-rich materials, such as HLM, can reduce side reactions with the electrolyte and improve expansion performance, the structural stability of these materials is affected by manganese leaching from their surface. Furthermore, irreversible electrochemical reactions occur at high voltages, specifically the loss of Lithium oxides within the material. + Lithium ions are extracted from the unit cell in the form of Li₂O. During discharge, these ions cannot be fully reinserted into the original unit cell, resulting in a large initial irreversible capacity, low coulombic efficiency, and poor cycle stability. This application introduces a coating layer with a specific content on the surface of a layered lithium-rich substrate material. This effectively improves the surface structure stability of the substrate material, reduces side reactions between the material surface and the electrolyte, reduces polarization and surface oxygen loss, and inhibits the continuous growth of the SEI film during cycling. This effectively improves the electrochemical performance of the material and enhances cycle performance while further reducing expansion. When the mass content of the coating layer is too low, the improvement in surface structure stability is limited, and the impact on reducing expansion and improving cycle performance is also limited. When the mass content of the coating layer is too high, the amount of active material in the same mass of substrate material is reduced, leading to a decrease in the capacity and cycle performance of the secondary battery.

[0007] In some implementations, the mass content of the coating layer is 5% to 10% based on the mass of the base material.

[0008] In some implementations, the mass content of the matrix material is 4% to 8% based on the mass of the positive electrode active material layer. If the mass content of the matrix material is too low, the improvement in the expansion performance of the secondary battery is not significant. If the mass content of the matrix material is too high, the cycle stability of the secondary battery deteriorates at high voltages.

[0009] In some embodiments, the lithium-rich manganese-based cathode material satisfies: 0 ≤ (BET2 - BET1) / BET1 ≤ 10, where BET1 = 6 / (ρ × Dv50), and ρ is the true density of the lithium-rich manganese-based cathode material in g / cm³. 3Dv50 is the particle size corresponding to a cumulative volume distribution percentage of 50% for the lithium-rich manganese-based cathode material, in μm; BET2 is the specific surface area obtained by nitrogen adsorption testing. BET1 represents the theoretical specific surface area of ​​the lithium-rich manganese-based cathode material, BET2 is the actual specific surface area, and (BET2-BET1) / BET1 represents the deviation between the theoretical and actual specific surface areas, which can measure the degree of surface unevenness. Since material uniformity is one of the factors affecting BET2, this application controls the deviation between the theoretical and actual specific surface areas within a certain range, resulting in better particle size and morphological uniformity, a smoother surface with fewer uneven structures after coating, and a smaller contact area with the electrolyte, which is beneficial for inhibiting Mn dissolution.

[0010] In some implementations, 0.1m 2 / g≤BET1≤0.5m 2 / g. In some implementations, 0.5m 2 / g≤BET2≤1.5m 2 / g.

[0011] In some embodiments, the thickness of the coating layer is from 0.3 nm to 5 nm. If the coating layer is too thick, the cycle performance of the secondary battery is poor; if the coating layer is too thin, the improvement in storage performance is not significant. In some embodiments, the thickness of the coating layer is from 0.5 nm to 4 nm.

[0012] In some embodiments, the D1v50 of the substrate material is from 3 μm to 15 μm. In some embodiments, the D2v50 of the phosphate-based cathode material is from 0.5 μm to 1.5 μm.

[0013] In some implementations, 3 ≤ D1v50 / D2v50 ≤ 15. In some implementations, 5 ≤ D1v50 / D2v50 ≤ 10.

[0014] In some embodiments, the mass content of the coating layer is 0.06% to 0.72%, based on the mass of the positive electrode active material layer.

[0015] In some embodiments, the mass content of the phosphate-based cathode material is 87% to 96% based on the mass of the cathode active material layer.

[0016] In some embodiments, the phosphate-based cathode material includes LiMn y B (1-y) At least one of PO4, wherein 0 ≤ y ≤ 1, and element B is selected from at least one of iron, cobalt, magnesium, calcium, zinc, chromium or lead.

[0017] In some embodiments, the transition metal oxide includes at least one of MgO, CuO, Al2O3, and ZrO2.

[0018] Secondly, this application also provides an electronic device that includes the aforementioned secondary battery.

[0019] The beneficial effects of the embodiments of this application are as follows:

[0020] The secondary battery of this application, by introducing a lithium-rich manganese-based cathode material with a specific structure and content into a phosphate-based cathode material such as lithium iron phosphate, can effectively improve the gas generation problem of the secondary battery and enhance its high-temperature storage performance while taking into account both rate performance and cycle performance. Detailed Implementation

[0021] The embodiments of this application will be described in detail below. These embodiments should not be construed as limiting the scope of this application.

[0022] Additionally, quantities, ratios, and other numerical values ​​are sometimes presented in range format in this application. It should be understood that such range format is for convenience and brevity, and should be interpreted flexibly to include not only numerical values ​​explicitly specified as range limits, but also all individual numerical values ​​or subranges covered within the range, as if each numerical value and subrange were explicitly specified.

[0023] In the detailed description and claims, a list of items connected by the terms "at least one of," "at least one of," "at least one of," or other similar terms may mean any combination of the listed items. For example, if items A and B are listed, then the phrase "at least one of A and B" means only A; only B; or A and B. In another example, if items A, B, and C are listed, then the phrase "at least one of A, B, and C" means only A; or only B; only C; A and B (excluding C); A and C (excluding B); B and C (excluding A); or all of A, B, and C. Item A may contain a single element or multiple elements. Item B may contain a single element or multiple elements. Item C may contain a single element or multiple elements.

[0024] Primary and secondary batteries

[0025] The secondary battery provided in this application includes a positive electrode sheet, which includes a positive electrode active material layer. The positive electrode active material layer includes a phosphate-based positive electrode material and a lithium-rich manganese-based positive electrode material. The lithium-rich manganese-based positive electrode material includes a matrix material and a coating layer located on the surface of the matrix material. The coating layer includes at least one of amorphous carbon and transition metal oxides. The matrix material has a mass content of 1% to 10% based on the mass of the positive electrode active material layer, and the coating layer has a mass content of 1% to 12% based on the mass of the matrix material.

[0026] In some embodiments, the matrix material comprises xLi₂MnO₃·(1-x)LiMO₂, wherein 0.2 ≤ x ≤ 0.8, and M comprises at least one of Co, Ni, and Mn. In some embodiments, x is a range of 0.3, 0.4, 0.5, 0.6, 0.7, or any combination of these values.

[0027] Phosphate-based cathode materials, such as lithium iron phosphate (LFP), exhibit structural stability and good cycle performance, but they suffer from significant gas generation at high temperatures. While introducing larger-particle-size layered lithium-rich materials, such as HLM, can reduce side reactions with the electrolyte and improve expansion performance, the structural stability of these materials is affected by manganese leaching from their surface. Furthermore, irreversible electrochemical reactions occur at high voltages, specifically the loss of Lithium oxides within the material. + Lithium ions are extracted from the unit cell in the form of Li₂O. During discharge, these ions cannot be fully reinserted into the original unit cell, resulting in a large initial irreversible capacity, low coulombic efficiency, and poor cycle stability. This application improves the electrochemical performance of the material by introducing a coating layer of a specific content on the surface of a layered lithium-rich substrate material. This effectively improves the surface structure stability of the substrate material, reduces side reactions between the material surface and the electrolyte, reduces polarization and surface oxygen loss, and inhibits the continuous growth of the SEI film during cycling. Furthermore, it improves cycle performance while further reducing expansion.

[0028] In some embodiments, the mass content of the coating layer is 1%, 2%, 3%, 4%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, 9.5%, 10%, 11%, or any combination of these values, based on the mass of the substrate material. When the mass content of the coating layer is too low, the improvement in surface structure stability is limited, as are the effects of reducing expansion and improving cycle performance. When the mass content of the coating layer is too high, the amount of active material in the substrate material is reduced for the same mass, resulting in a decrease in the capacity and cycle performance of the secondary battery. In some embodiments, the mass content of the coating layer is 5% to 10% based on the mass of the substrate material.

[0029] In some embodiments, the mass content of the matrix material, based on the mass of the positive electrode active material layer, is 2%, 3%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, or any combination of these values. In some embodiments, the mass content of the matrix material, based on the mass of the positive electrode active material layer, is between 4% and 8%. When the mass content of the matrix material is too low, the improvement in the expansion performance of the secondary battery is not significant. When the mass content of the matrix material is too high, the cycle stability of the secondary battery deteriorates at high voltages.

[0030] In some embodiments, the lithium-rich manganese-based cathode material satisfies: 0 ≤ (BET2 - BET1) / BET1 ≤ 10, where BET1 = 6 / (ρ × Dv50), and ρ is the true density of the lithium-rich manganese-based cathode material in g / cm³. 3 Dv50 is the particle size corresponding to a cumulative volume distribution percentage of 50% for the lithium-rich manganese-based cathode material, in μm; BET2 is the specific surface area obtained by nitrogen adsorption testing. BET1 represents the theoretical specific surface area of ​​the lithium-rich manganese-based cathode material, BET2 is the actual specific surface area, and (BET2-BET1) / BET1 represents the deviation between the theoretical and actual specific surface areas, which can measure the degree of surface unevenness. Since material uniformity is one of the factors affecting BET2, this application controls the deviation between the theoretical and actual specific surface areas within a certain range, resulting in better particle size and morphological uniformity, a smoother surface with fewer uneven structures after coating, and a smaller contact area with the electrolyte, which is beneficial for inhibiting Mn dissolution.

[0031] In some implementations, (BET2-BET1) / BET1 is a range of 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, or any combination of these values.

[0032] In some implementations, 0.1m 2 / g≤BET1≤0.5m 2 / g, for example, 0.15m 2 / g, 0.2m 2 / g, 0.25m 2 / g, 0.3m 2 / g, 0.35m 2 / g, 0.4m 2 / g or 0.45m 2 / g. In some implementations, or 0.5m 2 / g≤BET2≤1.5m 2 / g, for example, 0.6m 2 / g, 0.7m 2 / g, 0.8m 2 / g, 0.9m 2 / g, 1.0m 2 / g, 1.1m 2 / g, 1.2m 2 / g, 1.3m 2 / g or 1.4m 2 / g.

[0033] In some embodiments, the thickness of the coating layer is from 0.3 nm to 5 nm. In some embodiments, the thickness of the coating layer is a range of 0.5 nm, 0.7 nm, 1.0 nm, 1.3 nm, 1.5 nm, 1.7 nm, 2.0 nm, 2.3 nm, 2.5 nm, 2.7 nm, 3.0 nm, 3.3 nm, 3.5 nm, 3.7 nm, 4.0 nm, 4.3 nm, 4.5 nm, 4.7 nm, or any combination of these values. If the coating layer is too thick, the cycle performance of the secondary battery is poor; if the coating layer is too thin, the improvement in storage performance is not significant. In some embodiments, the thickness of the coating layer is from 0.5 nm to 4 nm.

[0034] In some embodiments, the D1v50 of the matrix material is from 3 μm to 15 μm. In some embodiments, the D1v50 of the matrix material is a range of 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, or any combination of these values. In this application, the D1v50 of the matrix material is the particle size corresponding to a cumulative volume distribution percentage of 50% for the matrix material.

[0035] In some embodiments, the D2v50 of the phosphate-based cathode material is from 0.5 μm to 1.5 μm. In some embodiments, the D2v50 of the phosphate-based cathode material is a range of 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 1.0 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.4 μm, or any combination of these values. In this application, the D2v50 of the phosphate-based cathode material is the particle size corresponding to a 50% cumulative volume distribution percentage of the matrix material.

[0036] In some embodiments, 3 ≤ D1v50 / D2v50 ≤ 15. In some embodiments, D1v50 / D2v50 is a range of 4, 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, 10, 11, 12, 13, 14, or any combination of these values. In some embodiments, 5 ≤ D1v50 / D2v50 ≤ 10.

[0037] In some embodiments, the mass content of the coating layer is from 0.06% to 0.72%, for example, 0.18%, 0.3%, 0.42%, 0.5%, 0.6%, 0.7%, or 0.71%, depending on the mass of the positive electrode active material layer.

[0038] In some embodiments, the mass content of the phosphate-based cathode material is 87% to 96% based on the mass of the cathode active material layer, for example, 88%, 89%, 90%, 91%, 92%, 93%, 94% or 95%.

[0039] In some embodiments, the phosphate-based cathode material includes LiMn y B (1-y) At least one of PO4, wherein 0 ≤ y ≤ 1, and element B is selected from at least one of iron, cobalt, magnesium, calcium, zinc, chromium, or lead. In some embodiments, y is 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, or 0.9.

[0040] In some embodiments, the amorphous carbon in the coating layer is obtained by sintering an organic carbon source. In some embodiments, the organic carbon source includes at least one selected from polydopamine, polyvinylpyrrolidone, tannic acid, citric acid, polyvinyl alcohol, polypyrrole, vitamin C, polyethylene glycol, pitch, anthracene, and aniline.

[0041] In some embodiments, the transition metal oxide includes at least one of MgO, CuO, Al2O3, and ZrO2.

[0042] In some embodiments, the preparation method of the lithium-rich manganese-based cathode material of this application includes the following steps:

[0043] S1: The matrix material and the coating layer precursor are mixed to obtain the first mixture;

[0044] S2: Sinter the first mixture.

[0045] In some embodiments, in S1, the coating precursor includes at least one of an organic carbon source and a transition metal oxide. In some embodiments, the organic carbon source includes at least one of polydopamine, polyvinylpyrrolidone, tannic acid, citric acid, polyvinyl alcohol, polypyrrole, vitamin C, polyethylene glycol, pitch, anthracene, and aniline. In some embodiments, the transition metal oxide includes at least one of MgO, CuO, Al₂O₃, and ZrO₂.

[0046] In some embodiments, the sintering temperature in S2 is from 400°C to 700°C, for example, 450°C, 500°C, 550°C, 600°C, or 650°C. In some embodiments, the sintering temperature in S2 is from 2 hours to 5 hours, for example, 3 hours or 4 hours.

[0047] In some embodiments, the positive electrode includes a current collector and a layer of positive active material disposed on the current collector.

[0048] In some embodiments, the positive electrode active material layer further includes a conductive agent. In this application, non-limiting examples of conductive agents include carbon-based materials (e.g., natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, etc.), metal-based materials (e.g., metal powder, metal fiber, etc., including, for example, copper, nickel, aluminum, silver, etc.), conductive polymers (e.g., polyphenylene derivatives), and mixtures thereof.

[0049] In some embodiments, the conductive agent is selected from conductive carbon black or carbon nanotubes.

[0050] In some embodiments, the positive electrode active material layer also includes a binder. In this application, the addition of a binder can control the adhesion between the positive electrode material and the positive electrode current collector. Too little binder may cause powder shedding from the positive electrode sheet, while too much binder may increase the processing difficulty.

[0051] In the embodiments of this application, non-limiting examples of adhesives include polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, nylon, etc.

[0052] In some embodiments of this application, the positive current collector may be aluminum (Al), but is not limited thereto.

[0053] The secondary battery provided in this application embodiment also includes a negative electrode sheet, wherein the negative electrode sheet includes a current collector and a negative electrode active material layer disposed on the current collector.

[0054] In this application embodiment, the specific type of negative electrode active material is not specifically limited and can be selected according to requirements. Specifically, the negative electrode active material is selected from natural graphite, artificial graphite, mesophase microcarbon spheres (MCMB), hard carbon, soft carbon, silicon, silicon-carbon composites, Li-Sn alloys, Li-Sn-O alloys, Sn, SnO, SnO2, and spinel-structured lithiated TiO2-Li4Ti5O. 12 One or more of Li-Al alloys. Non-limiting examples of carbon materials include crystalline carbon, amorphous carbon, and mixtures thereof. Crystalline carbon can be amorphous or flake-shaped, small flake-shaped, spherical, or fibrous natural or artificial graphite. Amorphous carbon can be soft carbon, hard carbon, mesophase pitch carbides, calcined coke, etc.

[0055] In some embodiments, the negative electrode active material layer may include a binder; the binder enhances the bonding between the negative electrode active material particles and the bonding between the negative electrode active material and the current collector.

[0056] In some embodiments, non-limiting examples of adhesives include polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymers, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, nylon, etc.

[0057] In some embodiments, the negative electrode active material layer includes a conductive material, thereby making the electrode conductive. The conductive material may include any conductive material as long as it does not cause a chemical change. Non-limiting examples of conductive materials include carbon-based materials (e.g., natural graphite, synthetic graphite, carbon black, acetylene black, Ketjen black, carbon fiber, etc.), metal-based materials (e.g., metal powders, metal fibers, such as copper, nickel, aluminum, silver, etc.), conductive polymers (e.g., polyphenylene derivatives), and mixtures thereof.

[0058] In some embodiments, the negative current collector may be selected from copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, polymer substrate coated with conductive metal, and combinations thereof.

[0059] The secondary battery provided in this application embodiment also includes an electrolyte.

[0060] In some implementations, the electrolyte includes a lithium salt and a solvent.

[0061] In some embodiments, the lithium salt includes at least one of organic or inorganic lithium salts. In some embodiments, the lithium salt includes, but is not limited to: lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium difluorophosphate (LiPO2F2), lithium bis(trifluoromethanesulfonyl)imide LiN(CF3SO2)2 (LiTFSI), lithium bis(fluorosulfonyl)imide Li(N(SO2F)2) (LiFSI), lithium bis(oxalateborate)borate LiB(C2O4)2 (LiBOB), or lithium difluorooxalateborate LiBF2(C2O4) (LiDFOB).

[0062] In some embodiments, the solvent may be selected from one or more of ethylene carbonate (EC), propylene carbonate (PC), methyl ethyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butyl carbonate (BC), fluoroethylene carbonate (FEC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), methyl ethyl sulfone (EMS), and diethyl sulfone (ESE).

[0063] The secondary battery of this application embodiment has a separator between the positive and negative electrode plates to prevent short circuits. The material and shape of the separator used in the secondary battery are not particularly limited, and can be any technology disclosed in the prior art.

[0064] In some embodiments, the diaphragm may include a substrate layer and a surface treatment layer. The substrate layer is a nonwoven fabric, membrane, or composite membrane with a porous structure, and the material of the substrate layer is selected from at least one of polyethylene, polypropylene, polyethylene terephthalate, and polyimide. Specifically, a polypropylene porous membrane, a polyethylene porous membrane, a polypropylene nonwoven fabric, a polyethylene nonwoven fabric, or a polypropylene-polyethylene-polypropylene porous composite membrane may be selected. At least one surface of the substrate layer is provided with a surface treatment layer, which may be a polymer layer or an inorganic layer, or a layer formed by mixing polymers and inorganic materials. The inorganic layer includes inorganic particles and a binder, and the inorganic particles are selected from one or a combination of several of alumina, silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, and barium sulfate. The binder is selected from one or more combinations of polyvinylidene fluoride, copolymers of polyvinylidene fluoride and hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyethylene ether, polymethyl methacrylate, polytetrafluoroethylene, and polyhexafluoropropylene. The polymer layer contains a polymer, the polymer material of which is selected from at least one of polyamide, polyacrylonitrile, acrylate polymers, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyethylene ether, polyvinylidene fluoride, and poly(vinylidene fluoride-hexafluoropropylene).

[0065] The secondary battery described in this application may include an outer packaging, which can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the secondary battery may also be a soft pack, such as a pouch-type soft pack. The material of the soft pack may be plastic, such as one or more of polypropylene (PP), polybutylene terephthalate (PBT), and polybutylene succinate (PBS).

[0066] In some embodiments, the secondary battery of this application includes, but is not limited to, a lithium-ion battery or a sodium-ion battery. In some embodiments, the secondary battery includes a lithium-ion battery.

[0067] II. Electronic Devices

[0068] The electronic device provided in the embodiments of this application includes a secondary battery of the first aspect.

[0069] The electronic devices or apparatuses described in this application are not particularly limited. In some embodiments, the electronic devices of this application include, but are not limited to, laptops, pen-based computers, mobile computers, e-book players, portable telephones, portable fax machines, portable copiers, portable printers, stereo headphones, video recorders, LCD TVs, portable cleaners, portable CD players, mini CDs, transceivers, electronic notebooks, calculators, memory cards, portable recorders, radios, backup power supplies, motors, automobiles, motorcycles, electric bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, large household batteries, and lithium-ion capacitors, etc.

[0070] Unless otherwise specified, all reagents, materials and instruments used in the following examples and comparative examples are commercially available.

[0071] Examples and Comparative Examples

[0072] Example 1-1:

[0073] Example 1-1 includes the following steps:

[0074] 1. Preparation of lithium-rich manganese-based cathode materials

[0075] Step 1: The matrix material 0.5Li2MnO3·0.5LiCoO2 with a D1v50 of 5μm is physically mixed with the modifier polyvinylpyrrolidone. The mass ratio of carbon content in polyvinylpyrrolidone to that in the matrix material is 0.3:6. During the mixing process, friction and collision occur, which allows the modifier to be uniformly dispersed and adsorbed on the surface of the matrix material to form a coating layer.

[0076] Step 2: The mixed material is then reacted and solidified by sintering to obtain the lithium-rich manganese-based cathode material, wherein the sintering temperature is 500℃ and the time is 4h.

[0077] Among them, the theoretical specific surface area (BET1) of the lithium-rich manganese-based cathode material is 0.2 g / cm³. 3 The specific surface area BET2 obtained by nitrogen adsorption test is 1 g / cm³. 3 , (BET2-BET1) / BET1=4, the thickness of the coating layer is 1nm.

[0078] 2. Preparation of secondary batteries:

[0079] (1) Preparation of the positive electrode sheet: Phosphate-based positive electrode material LiFePO4 with a Dv250 of 1 μm, the above-mentioned lithium-rich manganese-based positive electrode material, binder (PVDF), and conductive carbon black were mixed in a weight ratio of 91:6.3:1.3:1.4 to obtain a first mixture. The first mixture was added to solvent NMP and mixed uniformly under vacuum stirring to obtain a positive electrode slurry with a solid content of 75%. The positive electrode slurry was uniformly coated onto one surface of a 12 μm thick aluminum foil current collector and dried at 120°C to obtain a positive electrode sheet with a single-sided coating of positive electrode material layer with a coating thickness of 100 μm. The above steps were then repeated on the other surface of the aluminum foil to obtain a positive electrode sheet with a double-sided coating of positive electrode material layer. After drying at 120°C, the sheet was cold-pressed, then cut and welded with tabs to obtain a positive electrode sheet with dimensions of 74 mm × 867 mm for later use.

[0080] (2) Preparation of negative electrode sheet:

[0081] Artificial graphite, sodium carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), and conductive carbon black were mixed in a mass ratio of 96:1.0:1.5:1.5. Deionized water was added, and the mixture was stirred evenly under vacuum to obtain a negative electrode slurry with a solid content of 45%. The negative electrode slurry was uniformly coated onto one surface of a 10 μm thick copper foil current collector and dried at 120°C to obtain a single-sided coated negative electrode sheet with a coating thickness of 100 μm. The above steps were then repeated on the other surface of the copper foil to obtain a double-sided coated negative electrode sheet. After drying at 120°C, the sheet was cold-pressed, cut, and had tabs welded to obtain a negative electrode sheet with a size of 78 mm × 875 mm for later use.

[0082] (3) Preparation of electrolyte:

[0083] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed thoroughly at a volume ratio of 1:1:1 to obtain an organic solvent. LiPF6 was dissolved in the above organic solvent, and then vinylene carbonate was added and mixed thoroughly to obtain the electrolyte. Based on the mass of the electrolyte, the mass percentage of LiPF6 was 12.5%, and the mass percentage of vinylene carbonate was 3%.

[0084] (4) Preparation of lithium-ion batteries:

[0085] The positive electrode, separator, and negative electrode are stacked in sequence. The separator is a 14μm thick polypropylene (PP) porous membrane, which acts as a separator between the positive and negative electrodes. Then, it is wound into a bare cell, which is then placed in an aluminum-plastic film, injected with electrolyte, and sealed. After standing, formation, and shaping processes, a lithium-ion battery is obtained.

[0086] Examples 1-2 to 1-10, Examples 2-1 to 2-4, Comparative Examples 1-1 to 1-5, Comparative Examples 2-1 to 2-2

[0087] Preparation of lithium-rich manganese-based cathode materials:

[0088] The preparation process of lithium-rich manganese-based cathode materials is similar to that in Examples 1-1, except that the corresponding lithium-rich manganese-based cathode materials are prepared by adjusting the type of matrix material, the type and mass ratio of the modifying additives.

[0089] The preparation process of the positive electrode sheet is similar to that in Example 1-1, except that the corresponding positive electrode slurry is prepared by adjusting the type of phosphate-based positive electrode material and the mass ratio of phosphate-based positive electrode material, lithium-rich manganese-based positive electrode material, PVDF and conductive carbon black in the first mixture.

[0090] The specific preparation parameters are shown in Table a:

[0091] Table a

[0092]

[0093]

[0094] The preparation of the negative electrode sheet, separator, electrolyte, and lithium-ion battery is the same as in Example 1-1.

[0095] Examples 3-1 to 3-6

[0096] Preparation of lithium-rich manganese-based cathode materials

[0097] The preparation process of lithium-rich manganese-based cathode material is similar to that in Example 1-1, except that the BET1 and BET2 of the corresponding lithium-rich manganese-based cathode material are adjusted by adjusting the sintering temperature and time control in step 2.

[0098] The preparation of the positive electrode, negative electrode, separator, electrolyte, and lithium-ion battery is the same as in Example 1-1.

[0099] Examples 4-1 to 4-5

[0100] Preparation of lithium-rich manganese-based cathode materials

[0101] The preparation process of lithium-rich manganese-based cathode material is similar to that in Example 1-1, except that the thickness of the corresponding coating layer is adjusted by adjusting the sintering temperature and time in step 2.

[0102] The preparation of the positive electrode, negative electrode, separator, electrolyte, and lithium-ion battery is the same as in Example 1-1.

[0103] Examples 5-1 to 5-6

[0104] The preparation process of Examples 5-1 to 5-6 is similar to that of Example 1-1, except that different particle sizes of LiFePO4 and different particle sizes of matrix material 0.5Li2MnO3·0.5LiCoO2 are selected. For specific selection, please refer to Table 5.

[0105] Test methods

[0106] 1. Dv50 Test

[0107] Using a laser diffraction particle size distribution measuring instrument (Malvem Mastersizer 3000), and in accordance with the particle size distribution laser diffraction method GB / T19077 2016, the particle size distribution can be measured to obtain the Dv50 of the material.

[0108] 2. Cyclic performance test

[0109] Place the lithium-ion battery in a 25°C constant temperature chamber and let it stand until the lithium-ion battery reaches a constant temperature; charge it at a constant current of 0.5C to 4.55V, and charge it at a constant voltage to a current of 0.025C; discharge it at 1C to 3.0V, and take the capacity of this step as the initial capacity C0; repeat this step 400 times and record the capacity of 400 cycles as C1; calculate the capacity retention rate.

[0110] Capacity retention rate = C1 / C0 × 100%.

[0111] 3. Storage performance test

[0112] The lithium-ion battery was discharged to 3.0V at 0.5C at 25℃, then charged to 4.55V at a constant current of 0.2C, and then charged at a constant voltage of 0.025C at 4.55V. The thickness of the lithium-ion battery was measured and recorded with a micrometer and denoted as H1. The battery was then fully charged and stored at 60℃ for 60 days. After 60 days, the thickness of the lithium-ion battery was measured and recorded with a micrometer and denoted as H2.

[0113] Thickness expansion rate = (H2-H1) / H1×100%.

[0114] Test Results

[0115] Table 1

[0116]

[0117]

[0118] As shown in Table 1, in Comparative Example 1-1, when the positive electrode active material layer contains only LFP, the lithium-ion battery exhibits high cycle performance but poor expansion performance. In Comparative Example 1-2, the addition of the matrix material 0.5Li2MnO3·0.5LiCoO2 to the LFP layer resulted in a significant decrease in cycle performance compared to Comparative Example 1-1, but a substantial improvement in storage performance.

[0119] Data from Examples 1-1 to 1-6 show that by adding the lithium-rich manganese-based cathode material with the coating structure of this application to the LFP, the cycle performance of the lithium-ion battery is close to that of Comparative Example 1-1, and significantly improved compared to Comparative Example 1-2 in both cycle performance and storage performance. A comparison of Examples 1-1 to 1-6 with Comparative Examples 1-3 and 1-4 shows that both excessively low and excessively high coating content reduce the cycle performance of the secondary battery. Based on the mass of the matrix material, a coating content of 1%-12% results in good cycle performance and storage performance. Furthermore, a coating content of 5%-10% further enhances both cycle performance and storage performance. A comparison of Examples 1-7 to 1-10 with Comparative Example 1-5 shows that changing the type of lithium-rich manganese matrix material, with different types of coatings, significantly improves the cycle performance and storage performance of the lithium-ion battery.

[0120] Table 2

[0121]

[0122]

[0123] Data from Examples 2-1 to 2-4 show that adding an appropriate amount of lithium-rich manganese-based cathode material with a coating structure to the LFP base improves both the cycle performance and expansion performance of the lithium-ion battery. Furthermore, based on the quality of the cathode active material layer, a matrix material content of 4%-8% further balances the cycle performance and expansion performance of the lithium-ion battery. A comparison between Example 2-1 and Comparative Example 2-1 shows that excessive matrix material addition has limited effect on improving expansion performance, and the irreversible electrochemical reaction of the matrix material at high charging voltages leads to a significant decrease in cycle performance. A comparison between Comparative Example 2-2 and Comparative Example 1-1 shows that insufficient matrix material addition has limited effect on improving expansion performance.

[0124] Table 3

[0125]

[0126] Data from Examples 3-1 to 3-5 show that when (BET2-BET1) / BET1 is in the range of 0 to 10, the lithium-ion battery exhibits both excellent cycle performance and expansion performance, with a cycle performance exceeding 98.6%. Data from Examples 3-6 shows that when (BET2-BET1) / BET1 is too large, the surface of the lithium-rich manganese-based cathode material is not smooth enough, with a large number of uneven structures, resulting in a larger contact area with the electrolyte. This is not conducive to suppressing Mn dissolution, leading to a certain degree of decrease in cycle performance.

[0127] Table 4

[0128]

[0129] As can be seen from the data in Examples 4-1 to 4-5, lithium-ion batteries with a coating thickness of 0.3 nm to 5 nm exhibit excellent cycle performance and storage performance, especially when the coating thickness is in the range of 0.5 nm to 4 nm, both cycle performance and storage performance of lithium-ion batteries can be achieved.

[0130] Table 5

[0131]

[0132] The data in Table 5 show that when the particle size ratio of the matrix material to LFP is in the range of 3 to 15, the lithium-ion battery exhibits both excellent cycle performance and storage performance. When the particle size ratio is in the range of 5 to 10, the cycle performance and storage performance of the lithium-ion battery can be further improved. For example, in Examples 5-5, the particle size of the matrix material is too high, leading to instability under high voltage and easy dissolution of Mn, resulting in poor cycle performance. In Examples 5-6, the particle size of the matrix material is too low, resulting in a small difference in specific surface area between the matrix material and LFP, which fails to effectively reduce side reactions with the electrolyte, thus causing a decrease in cycle performance.

[0133] Although illustrative embodiments have been demonstrated and described, those skilled in the art should understand that the above embodiments should not be construed as limiting the present application, and that changes, substitutions and modifications can be made to the embodiments without departing from the spirit, principles and scope of the present application.

Claims

1. A secondary battery, comprising a positive electrode sheet, wherein the positive electrode sheet includes a positive electrode active material layer, the positive electrode active material layer comprising a phosphate-based positive electrode material and a lithium-rich manganese-based positive electrode material, wherein, The lithium-rich manganese-based cathode material includes a matrix material and a coating layer located on the surface of the matrix material. The coating layer includes at least one of amorphous carbon and transition metal oxides. Wherein, based on the mass of the positive electrode active material layer, the mass content of the matrix material is 4% to 8%, and based on the mass of the matrix material, the mass content of the coating layer is 5% to 10%. The lithium-rich manganese-based cathode material satisfies the following condition: 0 ≤ (BET2 - BET1) / BET1 ≤ 10, where BET1 = 6 / (ρ × Dv50), and ρ is the true density of the lithium-rich manganese-based cathode material in g / cm³. 3 Dv50 is the particle size corresponding to a cumulative volume distribution percentage of 50% for the lithium-rich manganese-based cathode material, in μm; BET2 is the specific surface area obtained by nitrogen adsorption testing, in m². 2 / g; Wherein, based on the mass of the positive electrode active material layer, the mass content of the phosphate-based positive electrode material is 87% to 96%.

2. The secondary battery according to claim 1, wherein, 0.1 m 2 / g≤BET1≤0.5 m 2 / g, and / or, 0.5 m 2 / g≤BET2≤1.5 m 2 / g.

3. The secondary battery according to claim 1, wherein, The secondary battery satisfies at least one of the following conditions (i) to (iii): (i) The thickness of the coating layer is from 0.3 nm to 5 nm; (ii) The D1v50 of the matrix material is 3 μm to 15 μm; (iii) The D2v50 of the phosphate-based cathode material is 0.5 μm to 1.5 μm.

4. The secondary battery according to claim 3, wherein, The thickness of the coating layer is 0.5 nm to 4 nm; and / or, 3 ≤ D1v50 / D2v50 ≤ 15.

5. The secondary battery according to claim 4, wherein, 5≤D1v50 / D2v50≤10.

6. The secondary battery according to claim 1, wherein, Based on the mass of the positive electrode active material layer, the mass content of the coating layer is from 0.06% to 0.72%.

7. The secondary battery according to claim 1, wherein, The secondary battery satisfies at least one of the following conditions (iv) to (vi): (iv) The matrix material comprises xLi2MnO3·(1-x)LiMO2, wherein 0.2≤x≤0.8, and M comprises at least one of Co, Ni and Mn; (v) The phosphate-based cathode material includes LiMn y B (1-y) At least one of PO4, wherein 0 ≤ y ≤ 1, and element B is selected from at least one of iron, cobalt, magnesium, calcium, zinc, chromium or lead; (vi) The transition metal oxide includes at least one of MgO, CuO, Al2O3 and ZrO2.

8. An electronic device comprising a secondary battery as described in any one of claims 1 to 7.

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