Positive electrode material, secondary battery and electronic apparatus

Abstract

Disclosed in the present application are a positive electrode material, a secondary battery and an electronic apparatus. The positive electrode material comprises a lithium phosphate salt, the lithium phosphate salt comprising a first element and a second element; the first element comprises cobalt and nickel; on the basis of the total mass of the positive electrode material, the mass percentage of the first element is a%, and 35.5≤a≤37.5; the second element comprises tungsten, vanadium and chromium; and on the basis of the total mass of the positive electrode material, the mass percentage of the second element is b%, and 0.1≤b≤1.5. The positive electrode material is rod-shaped grains having an olivine structure, wherein the length-to-diameter ratio of a single grain of the positive electrode material is P, and, on the basis of the number of grains of the positive electrode material, the proportion of the number of grains whose P≥1.5 is A%, and 50≤A≤100; the average length of the positive electrode material is I nm, and 150≤I≤1000. The positive electrode material provided in the present application can improve the energy density of secondary batteries, and improve both low-temperature discharge performance and high-rate discharge performance.

Description

Positive electrode materials, secondary batteries and electronic devices Technical Field

[0001] This application relates to the field of energy storage technology, and in particular to a cathode material, a secondary battery, and an electronic device. Background Technology

[0002] As the core of modern energy storage technology, the performance improvement of lithium-ion batteries has always been a research hotspot. Lithium phosphate, as a cathode material for lithium-ion batteries, possesses advantages such as ultra-long lifespan, safety, no memory effect, and environmental friendliness. However, it suffers from problems such as slow lithium-ion diffusion at low temperatures and severe accumulation of solid-phase diffusion under long-pulse conditions. Therefore, proposing a low-temperature discharge performance optimization scheme for lithium phosphate cathode materials has become an urgent task to improve this situation. Summary of the Invention

[0003] This application provides a cathode material, a secondary battery, and an electronic device that achieves superior energy density while also improving high-rate discharge performance and low-temperature performance.

[0004] In a first aspect, embodiments of this application provide a cathode material, including lithium phosphate, which includes a first element and a second element; the first element includes cobalt and nickel, and based on the total mass of the cathode material, the mass percentage of the first element is a%, 35.5≤a≤37.5%; the second element includes tungsten, vanadium, and chromium, and based on the total mass of the cathode material, the mass percentage of the second element is b%, 0.1≤b≤1.5%; the cathode material is rod-shaped grains with an olivine structure, the aspect ratio of a single cathode material is P, and based on the number of cathode material particles, the percentage of particles with P≥1.5 is A%, 50≤A≤100%.

[0005] The lithium phosphate cathode material based on the embodiments of this application further includes the aforementioned first and second elements, which can modify the lithium phosphate material, change the electronic structure of the material, and give it a high redox potential, i.e., a high energy density. At the same time, it can form a (010) crystal plane with a high exposure structure, thereby expanding the one-dimensional lithium-ion diffusion channel of the lithium phosphate material into a multi-dimensional short-path diffusion channel, thereby increasing the number of lithium-ion diffusion channels and accelerating the diffusion rate. In addition, when the cathode material has a specific aspect ratio and the number of particles within the above range, it has a morphology that is more conducive to lithium-ion transport, thereby comprehensively improving the rate of lithium-ion transport and deintercalation in the cathode, and improving the low-temperature discharge performance and high-rate discharge performance of the secondary battery.

[0006] In some embodiments, the above-mentioned cathode material satisfies at least one of the following conditions: (1) 36.2 ≤ a ≤ 36.8; (2) 0.3 ≤ b ≤ 1; (3) 0.615 ≤ a / 58.9 + b / 183.8 ≤ 0.628; (4) the average length of the rod-shaped grains is 1 nm, and 150 ≤ 1 ≤ 1000. Based on the above embodiments, the secondary battery can be further made to have a higher energy density, while improving both low-temperature discharge performance and high-rate discharge performance.

[0007] In some embodiments, in the above-mentioned cathode material, the mass percentage ratio of cobalt to nickel is x, and the ratio of the sum of the mass percentages of vanadium and chromium to the mass percentage of tungsten is y, where 0.22≤x≤6.73, 0.4≤y≤9.0, and 0.15≤x / (0.9959y)≤8.87. Based on the above embodiments, when the mass percentage relationship of the above-mentioned metal elements in the cathode material is controlled within the above range, the proportion distribution of metal elements in the cathode material can be more conducive to lithium-ion transport, further enabling the secondary battery to have a higher energy density, while also improving low-temperature discharge performance and high-rate discharge performance.

[0008] In some embodiments, the average cross-sectional diameter of the cathode material is D nm, where 50 ≤ D ≤ 300. Based on the above embodiments, when the average cross-sectional diameter of the cathode material is within the above range, it has a morphology that is more conducive to lithium-ion transport, which can further improve the low-temperature discharge performance and high-rate discharge performance of the secondary battery.

[0009] In some embodiments, based on the particle number of the above-mentioned cathode material: the proportion of particles with 2≤P≤5 is B%, 50≤B≤80; the proportion of particles with 4≤P≤5 is C%, 20≤C≤50. Based on the above embodiments, this application further enables the secondary battery to have a higher energy density by controlling the distribution of the number of particles with a specific aspect ratio in the cathode material to meet the above range, while also improving low-temperature discharge performance and high-rate discharge performance.

[0010] In some embodiments, the slope of the conductivity graph of the above-mentioned positive electrode material during the voltage boosting process from 200 MPa to 350 MPa is K×10. -5 S / (cm·MPa), 4≤K≤20. Based on the above embodiments, the slope of the conductivity image of the cathode material of this application is within the above range, indicating that the cathode material of this application has good slip and contact properties, which is beneficial to electron transport between material particles, thereby further improving the low-temperature discharge performance and high-rate discharge performance of the secondary battery.

[0011] In some embodiments, in the XRD pattern of the above-mentioned cathode material, the peak intensity ratio of peak (210) to peak (101) is E, where 0.38 ≤ E ≤ 0.40. Based on the above embodiments, when the peak intensity ratio of the two peaks in the XRD pattern of the cathode material of this application meets the above range, the secondary battery has better low-temperature discharge performance and high-rate discharge performance.

[0012] In some embodiments, the specific surface area of ​​the above-mentioned positive electrode material is F m 2 / g, 15≤F≤30. Based on the above embodiments, when the specific surface area of ​​the cathode material of this application is within the above range, the low-temperature discharge performance and high-rate discharge performance of the secondary battery can be further improved.

[0013] In some embodiments, the cathode material comprises LiMPO4, wherein the M element includes cobalt, nickel, tungsten, vanadium, and chromium. Based on the above embodiments, when the lithium phosphate salt includes the aforementioned metal elements, it has a morphology that is beneficial to lithium-ion transport, which can improve the low-temperature discharge performance and high-rate discharge performance of the secondary battery.

[0014] Secondly, embodiments of this application provide a secondary battery, including a positive electrode sheet, the positive electrode sheet including a positive current collector and a positive electrode material layer located on at least one surface of the positive current collector, the positive electrode material layer including the aforementioned positive electrode material.

[0015] Thirdly, embodiments of this application provide an electronic device that includes the aforementioned secondary battery. Detailed Implementation

[0016] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0017] The first aspect of this application provides a secondary battery, including a positive electrode, a negative electrode, an electrolyte, and a separator.

[0018] positive electrode

[0019] The positive electrode includes a positive current collector and a positive electrode material layer disposed on at least one surface of the positive current collector, the positive electrode material layer including a positive electrode material.

[0020] The cathode material includes lithium phosphate, which comprises a first element and a second element. The first element includes cobalt and nickel, and the mass percentage of the first element is a% based on the total mass of the cathode material, 35.5 ≤ a ≤ 37.5, preferably 36.2 ≤ a ≤ 36.8. For example, the mass percentage of the first element in the cathode material can be 35.5, 35.9, 36.2, 36.5, 36.8, 37.0, 37.5, or any value within the range of any two of these values. The second element includes tungsten, vanadium, and chromium, and the mass percentage of the second element is b% based on the total mass of the cathode material, 0.1 ≤ b ≤ 1.5, preferably 0.3 ≤ b ≤ 1. For example, the mass percentage of the second element in the cathode material can be 0.1, 0.2, 0.3, 0.5, 0.8, 0.9, 1.0, 1.5, or any value within the range of any two of these values. In some embodiments, 0.615 ≤ a / 58.9 + b / 183.8 ≤ 0.628. In some embodiments, 0.621 ≤ a / 58.9 + b / 183.8 ≤ 0.623. In some embodiments, 0.622 ≤ a / 58.9 + b / 183.8 ≤ 0.626. In some embodiments, 0.625 ≤ a / 58.9 + b / 183.8 ≤ 0.627. In some embodiments, 0.624 ≤ a / 58.9 + b / 183.8 ≤ 0.628. The cathode material is rod-shaped grains with an olivine structure. The aspect ratio of a single cathode material grain is P. Based on the number of cathode material particles, the percentage of particles with P ≥ 1.5 is A%, and 50 ≤ A ≤ 100. Based on the number of particles in the cathode material, the percentage of particles with P ≥ 1.5 can be 50, 59, 71, 78, 84, 95, 100, or any combination of these values. The average length of the rod-shaped grains in the cathode material is 1 nm, 150 ≤ 1 ≤ 1000. For example, the average length of the rod-shaped grains in the cathode material can be 150, 181, 300, 488, 668, 800, 940, 1000, or any combination of these values. The lithium phosphate cathode material further includes the aforementioned first and second elements, which can modify the lithium phosphate material, change its electronic structure, and give it a higher energy density and a morphology more conducive to lithium-ion transport, thereby comprehensively improving the rate of lithium-ion transport and deintercalation in the cathode, and improving the low-temperature discharge performance and high-rate discharge performance of the secondary battery.

[0021] In some embodiments, the mass percentage ratio of cobalt to nickel in the cathode material is x, and the ratio of the sum of the mass percentages of vanadium and chromium to the mass percentage of tungsten is y, where 0.22 ≤ x ≤ 6.73, 0.4 ≤ y ≤ 9.0, and 0.15 ≤ x / (0.9959y) ≤ 8.87. For example, the mass percentage ratio of cobalt to nickel in the cathode material can be 0.22, 0.79, 1.86, 2.70, 3.01, 4.54, 5.10, 6.04, 6.73, or any value within the range of any two of these values. For example, the mass percentage ratio of the sum of the mass percentages of vanadium and chromium to the mass percentage of tungsten can be 0.4, 0.8, 1.9, 3.4, 5.2, 6.0, 7.4, 8.5, 9.0, or any value within the range of any two of these values. In some embodiments, 0.15 ≤ x / (0.9959y) ≤ 2.25. In some embodiments, 3.35 ≤ x / (0.9959y) ≤ 8.87. In some embodiments, 5.13 ≤ x / (0.9959y) ≤ 6.61. In some embodiments, 2.24 ≤ x / (0.9959y) ≤ 4.80. In some embodiments, 3.25 ≤ x / (0.9959y) ≤ 5.17. By controlling the mass percentage of the aforementioned metal elements in the cathode material within the above ranges, the secondary battery can further achieve a higher energy density, while simultaneously improving both low-temperature discharge performance and high-rate discharge performance.

[0022] In some embodiments, the average cross-sectional diameter of the cathode material is D nm, where 50 ≤ D ≤ 300. For example, the average cross-sectional diameter of the cathode material can be 50, 84, 110, 125, 163, 199, 258, 295, 300, or any combination of these values. When the average cross-sectional diameter of the cathode material is within the above range, it has a morphology more conducive to lithium-ion transport, which can further improve the low-temperature discharge performance and high-rate discharge performance of the secondary battery.

[0023] In some embodiments, based on the number of particles in the cathode material: the percentage of particles with a ratio of 2 ≤ P ≤ 5 is B%, and 50 ≤ B ≤ 80. For example, the percentage of particles with a ratio of 2 ≤ P ≤ 5 can be 50, 51, 57, 61, 65, 69, 72, 77, 80, or any combination of these values. The percentage of particles with a ratio of 4 ≤ P ≤ 5 is C%, and 20 ≤ C ≤ 50. For example, the percentage of particles with a ratio of 4 ≤ P ≤ 5 can be 20, 21, 27, 31, 35, 39, 42, 50, or any combination of these values. By controlling the distribution of particles with a specific aspect ratio in the cathode material to meet the above ranges, the secondary battery can achieve a higher energy density while improving both low-temperature discharge performance and high-rate discharge performance.

[0024] In some embodiments, the slope of the conductivity graph of the cathode material during the boost process from 200 MPa to 350 MPa is K×10. -5 S / (cm·MPa), 4≤K≤20. For example, the slope of the conductivity graph of the cathode material under the above conditions can be 4, 5, 8, 10, 15, 17, 20, or any combination of these values. A slope in the conductivity graph of the cathode material within the above range indicates that the cathode material of this application has good slip and contact properties, which is beneficial for electron transport between material particles, thereby further improving the low-temperature discharge performance and high-rate discharge performance of the secondary battery.

[0025] In some embodiments, the peak intensity ratio of the (210) peak to the (101) peak in the XRD pattern of the cathode material is E, where 0.38 ≤ E ≤ 0.40. For example, the peak intensity ratio of the (210) peak to the (101) peak in the XRD pattern of the cathode material can be 0.38, 0.39, 0.40, or any value within the range of any two of these values. When the peak intensity ratio of the two peaks in the XRD pattern of the cathode material meets the above range, the secondary battery exhibits better low-temperature discharge performance and high-rate discharge performance.

[0026] In some embodiments, the specific surface area of ​​the cathode material is Fm. 2 / g, 15≤F≤30. For example, the specific surface area of ​​the cathode material can be 15, 17, 19, 20, 22, 25, 28, 30, or any combination of these values. When the specific surface area of ​​the cathode material is within the above range, the low-temperature discharge performance and high-rate discharge performance of the secondary battery can be further improved.

[0027] In some embodiments, the cathode material comprises LiMPO4, wherein the M element includes cobalt, nickel, tungsten, vanadium, and chromium. For example, the cathode material could be LiCo. 0.77 Ni 0.2 V 0.01 Cr 0.015 W 0.005 PO4, LiCo 0.48 Ni 0.49 V 0.01 Cr 0.013 W 0.007 PO4, LiCo 0.28 Ni 0.70 V 0.002 Cr 0.01 W 0.008 PO4, etc. When lithium phosphate salts include the aforementioned metal elements, they exhibit morphologies that are favorable for lithium-ion transport, which can improve the low-temperature discharge performance and high-rate discharge performance of secondary batteries.

[0028] In some embodiments, the positive electrode material layer includes a positive electrode conductive material. There is no limitation on the type of positive electrode conductive material; any known conductive material can be used. Examples of positive electrode conductive materials include, but are not limited to, acetylene black, Super-P carbon black, amorphous carbon such as needle coke, carbon nanotubes, and graphene. The above-mentioned positive electrode conductive materials can be used alone or in any combination.

[0029] In some embodiments, the positive electrode material layer includes a positive electrode binder. There are no particular limitations on the type of positive electrode binder; in the case of a coating method, any material that is soluble or dispersible in the liquid medium used during electrode manufacturing is acceptable. Examples of positive electrode adhesives may include, but are not limited to, one or more of the following: resin-based polymers such as polyethylene, polypropylene, polyethylene terephthalate, polymethyl methacrylate, polyimide, aromatic polyamide, cellulose, and nitrocellulose; rubber-like polymers such as styrene-butadiene rubber, nitrile rubber, fluororubber, isoprene rubber, polybutadiene rubber, and ethylene-propylene rubber; thermoplastic elastomers such as styrene-butadiene-styrene block copolymers or their hydrides, ethylene-propylene-diene terpolymers, styrene-ethylene-butadiene-ethylene copolymers, and styrene-isoprene-styrene block copolymers or their hydrides; soft resin-like polymers such as syndiotactic-1,2-polybutadiene, polyvinyl acetate, ethylene-vinyl acetate copolymers, and propylene-α-olefin copolymers; fluorinated polymers such as polyvinylidene fluoride, polytetrafluoroethylene, fluorinated polyvinylidene fluoride, and polytetrafluoroethylene-ethylene copolymers; and polymer compositions with alkali metal ion conductivity. The above-mentioned positive electrode adhesives may be used alone or in any combination.

[0030] There are no restrictions on the type of solvent used to form the positive electrode slurry, as long as it can dissolve or disperse the positive electrode active material, conductive material, positive electrode binder, and thickener used as needed. Examples of solvents used to form the positive electrode slurry can include any of aqueous solvents and organic solvents. Examples of aqueous media include, but are not limited to, mixtures of alcohol and water or water. Examples of organic media include, but are not limited to, aliphatic hydrocarbons such as hexane; aromatic hydrocarbons such as benzene, toluene, xylene, and methylnaphthalene; heterocyclic compounds such as quinoline and pyridine; ketones such as acetone, methyl ethyl ketone, and cyclohexanone; esters such as methyl acetate and methyl acrylate; amines such as diethylenetriamine and N,N-dimethylaminopropylamine; ethers such as diethyl ether, propylene oxide, and tetrahydrofuran; amides such as N-methylpyrrolidone, dimethylformamide, and dimethylacetamide; and aprotic polar solvents such as hexamethylphosphoramide and dimethyl sulfoxide.

[0031] Thickeners are typically used to adjust the viscosity of slurries. In the case of aqueous media, thickeners and styrene-butadiene rubber latex can be used for slurry preparation. There are no particular limitations on the types of thickeners, but examples include, but are not limited to, carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, ethyl cellulose, polyvinyl alcohol, oxidized starch, phosphorylated starch, casein, and their salts. The above thickeners can be used alone or in any combination.

[0032] There are no particular limitations on the type of positive electrode current collector; it can be any known material suitable for use as a positive electrode current collector. Examples of positive electrode current collectors include, but are not limited to, metallic materials such as aluminum, stainless steel, nickel plating, titanium, and tantalum; and materials such as carbon cloth and carbon paper. In some embodiments, the positive electrode current collector is a metallic material. In some embodiments, the positive electrode current collector is aluminum.

[0033] To reduce the electronic contact resistance between the positive current collector and the positive electrode material layer, the surface of the positive current collector may include a conductive additive or a conductive coating. Examples of conductive additives include, but are not limited to, carbon and precious metals such as gold, platinum, and silver. Examples of conductive coatings may include a mixture layer containing inorganic oxides, conductive agents, and binders.

[0034] negative electrode

[0035] The negative electrode includes a negative electrode current collector and a negative electrode material layer disposed on at least one surface of the negative electrode current collector, the negative electrode material layer containing a negative electrode active material. In some embodiments, the rechargeable capacity of the negative electrode active material is greater than the discharge capacity of the positive electrode active material, in order to reduce the problem of lithium metal deposition at the negative electrode during charging.

[0036] Negative electrode active materials may include natural graphite, artificial graphite, mesophase microcarbon spheres (MCMB), silicon, silicon-carbon composites, and SiO2. w(0.5 < w < 1.6), Li-Sn alloy, Li-Sn-O alloy, Sn, SnO, SnO2, spinel-structured lithium titanate Li4Ti5O 12 At least one of Li-Al alloys or metallic lithium. Optionally, the negative electrode active material may further include amorphous carbon materials, which may be soft carbon (low-temperature calcined carbon), hard carbon, mesophase pitch carbide, or calcined coke, etc.

[0037] The negative electrode material layer also includes a negative electrode binder. The negative electrode binder improves the bonding between the negative electrode active material particles and the bonding between the negative electrode active material and the current collector. This application does not particularly limit the type of negative electrode binder, as long as it is a material stable to the electrolyte or the solvent used in electrode manufacturing. In some embodiments, the negative electrode binder includes a resin binder. Examples of resin binders include, but are not limited to, fluoropolymers, polyacrylonitrile (PAN), polyimide resins, acrylic resins, polyolefin resins, etc. When using an aqueous solvent to prepare the negative electrode slurry, the negative electrode binder includes, but is not limited to, carboxymethyl cellulose (CMC) or its salts, styrene-butadiene rubber (SBR), polyacrylic acid (PAA) or its salts, polyvinyl alcohol, etc.

[0038] The negative electrode material layer also includes a conductive agent. This application does not impose any particular limitation on the type of negative electrode conductive agent, as long as it achieves the purpose of this application. For example, the negative electrode conductive agent can be at least one of acetylene black, Ketjen black, carbon nanotubes, carbon fibers, carbon dots, or graphene, etc., and the aforementioned carbon nanotubes can include, but are not limited to, at least one of single-walled carbon nanotubes or multi-walled carbon nanotubes.

[0039] This application does not impose any particular limitation on the negative electrode current collector, as long as it achieves the purpose of this application. For example, the negative electrode current collector may include copper foil, aluminum foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, or a polymer substrate coated with a conductive metal. The conductive metal includes, but is not limited to, copper, nickel, or titanium, and the polymer substrate material includes, but is not limited to, at least one of polyethylene, polypropylene, ethylene-propylene copolymer, polyethylene terephthalate, polyethylene terephthalate, or poly(p-phenylene terephthalate). In this application, there are no particular limitations on the thickness of the negative electrode current collector and the negative electrode material layer, as long as it achieves the purpose of this application. For example, the thickness of the negative electrode current collector is 4 μm to 12 μm, and the thickness of the single-sided negative electrode material layer is 30 μm to 160 μm. In this application, the negative electrode material layer may be disposed on one surface or on two surfaces in the thickness direction of the negative electrode current collector. It should be noted that the "surface" here can be the entire area of ​​the negative electrode current collector or only a part of it. This application has no particular restrictions, as long as the purpose of this application can be achieved.

[0040] This application does not impose any particular limitation on the compaction density of the negative electrode sheet, as long as it achieves the purpose of this application. For example, the compaction density of the negative electrode sheet can be 1.0 g / cm³. 3 Up to 1.85 g / cm 3 This application does not impose any particular limitation on the cold pressing pressure of the negative electrode sheet, as long as the purpose of this application can be achieved. For example, the cold pressing pressure of the negative electrode sheet can be from 3 tons to 30 tons.

[0041] Optionally, the negative electrode sheet may further include a conductive layer, which is located between the negative electrode current collector and the negative electrode material layer. This application does not impose any particular limitation on the composition of the conductive layer, and it can be a conductive layer commonly used in the art. The conductive layer includes a conductive agent and a binder. This application does not impose any particular limitation on the conductive agent and binder in the conductive layer, and it can be at least one of the aforementioned conductive agents and binders. This application does not impose any particular limitation on the mass ratio of the conductive agent to the binder in the conductive layer; those skilled in the art can choose according to actual needs, as long as the purpose of this application is achieved. This application does not impose any particular limitation on the thickness of the conductive layer, as long as the purpose of this application is achieved; for example, the thickness of the conductive layer is 1 μm to 10 μm.

[0042] electrolyte

[0043] The electrolyte used in the secondary battery of this application includes a lithium salt and a non-aqueous solvent for dissolving the lithium salt.

[0044] This application does not impose any particular limitation on the type of lithium salt, as long as it achieves the purpose of this application. For example, the lithium salt may include, but is not limited to, at least one of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium difluorophosphate (LiPO2F2), lithium bis(fluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(oxalateborate)borate (LiBOB), or lithium difluorooxalateborate (LiDFOB). Based on the mass of the electrolyte, the mass percentage of the lithium salt may be from 8% to 15%, for example, the mass percentage of the lithium salt may be 8%, 9%, 10%, 11%, 12.5%, 13%, 15%, or a range of any two of these values.

[0045] This application does not impose any particular limitation on the types of non-aqueous solvents mentioned above, as long as they can achieve the purpose of this application. For example, they may include, but are not limited to, at least one of carbonate compounds, carboxylic acid ester compounds, ether compounds, or other organic solvents. The carbonate compounds mentioned above may include, but are not limited to, at least one of chain carbonate compounds or cyclic carbonate compounds. The chain carbonate compounds mentioned above may include, but are not limited to, at least one of dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, or methyl ethyl carbonate. The cyclic carbonate compounds mentioned above may include, but are not limited to, at least one of ethylene carbonate, propylene carbonate, butyl carbonate, or ethylene ethylene carbonate. The carboxylic acid ester compounds mentioned above may include, but are not limited to, at least one of methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanolactone, valproic acid lactone, or caprolactone. The aforementioned ether compounds may include, but are not limited to, at least one of ethylene glycol dimethyl ether, dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1-ethoxy-1-methoxyethane, 2-methyltetrahydrofuran, or tetrahydrofuran. The aforementioned other organic solvents may include, but are not limited to, at least one of dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolium ketone, N-methyl-2-pyrrolidone, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, or trioctyl phosphate.

[0046] diaphragm

[0047] This application typically includes a separator between the positive and negative electrodes. The separator is used to separate the positive and negative electrode plates, reducing the problem of internal short circuits in the secondary battery, allowing electrolyte ions to pass freely, and not affecting the electrochemical charging and discharging process.

[0048] This application does not impose any particular limitation on the diaphragm, as long as it can achieve the purpose of this application. For example, the diaphragm material may include, but is not limited to, at least one of polyethylene (PE), polyolefin (PO) based on polypropylene (PP), polyester (e.g., polyethylene terephthalate (PET) film), cellulose, polyimide (PI), polyamide (PA), spandex, or aramid; the diaphragm type may include at least one of woven membrane, nonwoven membrane, microporous membrane, composite membrane, rolled membrane, or spun membrane.

[0049] In this application, the diaphragm may include a substrate and a surface treatment layer. The substrate may be a nonwoven fabric or composite membrane with a porous structure, and the material of the substrate may include at least one of polyethylene, polypropylene, polyethylene terephthalate, or polyimide. Optionally, 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 used. Optionally, a surface treatment layer is provided on at least one surface of the substrate. The surface treatment layer may be a polymer layer or an inorganic layer, or a layer formed by mixing polymers and inorganic materials. For example, the inorganic layer includes inorganic particles and a binder. This application does not have any particular limitation on the aforementioned inorganic particles, and may include at least one 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, or barium sulfate. This application does not have any particular limitation on the aforementioned binders, and may include at least one of the aforementioned binders. The polymer layer contains a polymer, the polymer material of which includes at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyvinylpyrrolidone, polyvinyl ether, polyvinylidene fluoride, or poly(vinylidene fluoride-hexafluoropropylene).

[0050] In this application, the pore size of the separator is from 0.01 μm to 1 μm, and the thickness is from 5 μm to 50 μm. In some embodiments, the separator thickness is greater than 1 μm, greater than 5 μm, or greater than 8 μm. In some embodiments, the separator thickness is less than 50 μm, less than 40 μm, or less than 30 μm. When the separator thickness is within the above ranges, insulation and mechanical strength can be ensured, and the rate performance and energy density of the secondary battery can be guaranteed.

[0051] This application also provides an electronic device, which includes the secondary battery described in this application. Electronic devices 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.

[0052] Example

[0053] The following examples, using lithium-ion batteries as an example, provide more specific illustrations of the implementation methods of the secondary battery of this application. Those skilled in the art will understand that the preparation methods described in this application are merely examples, and any other suitable preparation methods are within the scope of this application. Various tests and evaluations were conducted according to the methods described below. Furthermore, unless otherwise specified, "parts" and "%" refer to mass measurements.

[0054] Example 1-1

[0055] 1. Preparation of the positive electrode

[0056] <Preparation of Positive Electrode Active Materials>

[0057] (1) Weigh lithium hydroxide monohydrate, cobalt sulfate heptahydrate, nickel sulfate hexahydrate and ammonium dihydrogen phosphate in a molar ratio of nLi:n(Co+Ni):nP = 3:1:1. Prepare a 10% aqueous solution of lithium hydroxide monohydrate, prepare a 20% aqueous solution of cobalt sulfate heptahydrate and nickel sulfate hexahydrate by mixing them in a ratio of nCo:nNi = 10.9:51.1, and prepare a 30% aqueous solution of ammonium dihydrogen phosphate. Keep stirring and slowly mix the three solutions.

[0058] (2) Add 0.5% polyvinyl alcohol (by mass fraction of theoretically synthesized cathode material) to the mixed solution as a dispersant, and add glucose (by molar amount of theoretically synthesized cathode material) as a grain growth control agent.

[0059] (3) Adjust the pH of the reaction slurry to 9.0-9.5 and keep it at 150℃-200℃ for 5-8 hours to obtain cathode material precursor A;

[0060] (4) Filter and wash the precursor A, and weigh out the dopant with 10% carbon source, 4300ppm tungsten, 850ppm vanadium and 850ppm chromium according to the mass fraction of the synthesized cathode material. Mix them evenly and then spray dry to obtain cathode material spray material B.

[0061] (5) The cathode material B was calcined under a protective atmosphere to obtain the cathode material LiCo. 0.173 Ni 0.817 W 0.004 V 0.003 Cr 0.003 PO4.

[0062] <Preparation of the positive electrode>

[0063] The positive electrode active material, conductive carbon black, and polyvinylidene fluoride (PVDF) prepared in the above steps are mixed at a mass ratio of 95:2:3. N-methylpyrrolidone (NMP) is added, and the mixture is stirred evenly under vacuum to obtain a positive electrode slurry with a solid content of 70 wt%. The positive electrode slurry is uniformly coated onto one surface of a 9 μm thick aluminum foil for the positive electrode current collector, and then dried to obtain a positive electrode sheet with a single-sided coating of positive electrode material. The above steps are repeated on the other surface of the aluminum foil for the positive electrode current collector to obtain a positive electrode sheet with a double-sided coating of positive electrode material. By adjusting the cold pressing pressure, positive electrode sheets with different compaction densities and surface roughness can be obtained. After cold pressing, slitting, and welding of electrode tabs, the sheets are dried to obtain a positive electrode sheet with a size of 74 mm × 867 mm.

[0064] 2. Preparation of electrolyte

[0065] In a dry argon atmosphere glove box, diethyl carbonate was used as the base solvent, and lithium hexafluorophosphate (LiPF6) was dissolved in the base solvent. Ethylene carbonate was added as an additive to obtain the electrolyte. Based on the total mass of the electrolyte, the mass percentage of LiPF6 was 12.5%, and the mass percentage of vinylene carbonate was 2%.

[0066] 3. Preparation of the negative electrode

[0067] Using artificial graphite as the negative electrode active material, a mixture of the negative electrode active material, styrene-butadiene rubber (SBR), polyacrylic acid (PAA), carbon nanotubes (CNTs), and carboxymethyl cellulose (CMC) was prepared in a mass ratio of 95.8:2.4:0.5:0.5:0.8. Deionized water was then added as a solvent and the mixture was stirred until homogeneous, resulting in a negative electrode slurry with a solid content of 45 wt%. The negative electrode slurry was uniformly coated onto one surface of a 6 μm thick copper foil current collector, and then dried to obtain a single-sided coated negative electrode sheet. The above steps were repeated on the other surface of the copper foil to obtain a double-sided coated negative electrode sheet. By adjusting the cold pressing pressure, negative electrode sheets with different compaction densities and surface roughness could be obtained. After cold pressing, slitting, and welding of tabs, the sheets were dried to obtain negative electrode sheets with dimensions of 76.6 mm × 875 mm.

[0068] 4. Preparation of the diaphragm

[0069] A porous polyethylene film with a thickness of 15μm was used as the diaphragm.

[0070] 5. Preparation of lithium-ion batteries

[0071] The positive electrode, negative electrode, and separator are stacked sequentially, with the separator positioned between the positive and negative electrodes for isolation. The electrode assembly is then wound to form an electrode assembly. The electrode assembly is placed in a packaging bag, dehydrated at 80°C, injected with the aforementioned electrolyte, and sealed. After processing including formation hot pressing (at 80°C / 1–2.5 MPa for 0.5–2 hours), degassing, edge trimming, and capacity testing, a lithium-ion battery is obtained.

[0072] 6. Testing Methods

[0073] (1) Structural observation methods

[0074] Take a lithium-ion battery, remove the positive electrode, polish it with ion polishing, cut it open, and observe the crystal structure of the material layer on the electrode surface using a scanning electron microscope. Measure the particle length and cross-sectional diameter of the material. Measure 200 material particles and calculate their aspect ratios, i.e., the P value. Statistically determine the percentage of particles with different aspect ratios, i.e., the A, B, and C values. Calculate the arithmetic mean of the lengths of the 200 material particles as the average length I nm, and calculate the arithmetic mean of the cross-sectional diameters of the 200 material particles as the average cross-sectional diameter D nm.

[0075] Instrument model: ZEISS SEM, accelerating voltage: 0.1kV~30kV.

[0076] (2) Element content testing methods

[0077] Take the lithium-ion battery, remove the positive electrode, dissolve it in a mixed solvent (for example, use a mixed solvent of 5 ml aqua regia and 5 ml deionized water for 0.4 g of positive electrode), bring the volume to 100 mL, and then use an ICP analyzer to test the mass percentage of each element in the solution.

[0078] (3) Conductivity graph

[0079] A lithium-ion battery was taken, and the positive electrode sheet was removed, dissolved in NMP solvent, filtered, and dried to obtain positive electrode powder. The positive electrode powder was ultrasonically dispersed in alcohol for 10 minutes, allowed to stand for 2 hours, the upper suspension was discarded, and the bottom precipitate was obtained and dried to obtain the positive electrode material. A conductivity meter was used, with a pressure range of 0 MPa to 350 MPa, and a conductivity value was measured for every 10 MPa increase in pressure. A conductivity graph was obtained by plotting pressure as the x-axis and conductivity as the y-axis, and the slope from 200 MPa to 350 MPa was calculated.

[0080] (4) Diffraction peak intensity test

[0081] A lithium-ion battery was used, and its positive electrode was subjected to X-ray diffraction analysis to obtain XRD images. Instrument model: Bruker D8 ADVANCE, target material: Cu Kα, scanning angle: 5° to 80°.

[0082] (5) Low-temperature discharge performance test

[0083] Take a lithium-ion battery and place it in a high-low temperature chamber. Adjust the temperature to 25°C and let it stand for 30 minutes to allow the lithium-ion battery to reach a constant temperature. Discharge the lithium-ion battery at a constant current of 0.3C to 2.5V, then charge it at a constant current of 0.3C to 5.2V, and then charge it at the same voltage until the current reaches 0.05C. Again, at 25°C, discharge it at a constant current of 0.3C to 2.5V; record this discharge capacity as the initial discharge capacity. At 25°C, charge it at a constant current of 0.3C to 5.2V, and then charge it at the same voltage until the current reaches 0.05C. Then, place the lithium-ion battery at 0°C and let it stand for 30 minutes to allow the battery temperature to match the ambient temperature. At 0°C, discharge it at a constant current of 0.3C to 2.5V; record this discharge capacity as the low-temperature discharge capacity.

[0084] Low-temperature discharge capacity retention rate % = (low-temperature discharge capacity / initial discharge capacity) × 100%.

[0085] (6) 25℃ 4C rate discharge test

[0086] A lithium-ion battery was charged at 25°C with a constant current rate of 0.2C to 5.2V, then charged at a constant voltage rate until the current was less than or equal to 0.05C. After that, the battery was left to stand for 30 minutes, and then discharged at a constant current rate of 0.2C to 2.5V. The discharge capacity of the lithium-ion battery at 25°C and a 0.2C rate was measured. At 25°C, the lithium-ion battery was charged at a constant current rate of 0.2C to 5.2V, then charged at a constant voltage rate until the current was less than or equal to 0.05C. After that, the battery cell was left to stand for 60 minutes, and then discharged at a constant current rate of 4C to 2.5V. The discharge capacity of the lithium-ion battery at 25°C and a 4C rate was measured.

[0087] 25℃ 4C rate discharge capacity retention % = 25℃ 4C rate discharge capacity / 25℃ 0.2C rate discharge capacity × 100%.

[0088] (7) Energy density

[0089] Take a lithium-ion battery, charge it at 25℃ with a constant current of 0.2C to 5.2V, then charge it with a constant voltage until the current is less than or equal to 0.05C, and then discharge it at a constant current of 0.2C to 2.8V. Test the discharge energy E of the lithium-ion battery at 25℃ and 0.2C. Let it stand for 30 minutes, then charge it at a constant current of 0.2C for 2.5 hours. Measure the cell thickness T and the cell cross-sectional area S.

[0090] Energy density Wh / L = E / (T×S).

[0091] The lithium-ion batteries in the following embodiments or comparative examples differ from those in Examples 1-1 only in that the mass percentages of the first and second elements in the cathode material, the percentage of particles with P ≥ 1.5 in the cathode material particles, and the average length of the cathode material are adjusted according to Table 1. The performance test results of the lithium-ion batteries in each embodiment and comparative example are shown in Table 1 below.

[0092] Table 1

[0093] As shown in Table 1, the lithium-ion batteries prepared in the embodiments of this application can improve the low-temperature discharge capacity retention rate and high-rate discharge capacity retention rate, and increase the energy density of the lithium-ion battery, when the mass percentage content of the first element in the positive electrode material satisfies 35.5≤a≤37.5, especially 36.2≤a≤36.8. Similarly, when the mass percentage content of the second element in the positive electrode material satisfies 0.1≤b≤1.5, especially 0.3≤b≤1, the low-temperature discharge capacity retention rate and high-rate discharge capacity retention rate, and increase the energy density of the lithium-ion battery. Furthermore, when the proportion of particles with P≥1.5 in the positive electrode material satisfies 50≤A≤100, the low-temperature discharge capacity retention rate and high-rate discharge capacity retention rate, and increase the energy density of the lithium-ion battery, and when the average length of the positive electrode material satisfies 150≤I≤1000, the low-temperature discharge capacity retention rate and high-rate discharge capacity retention rate, and increase the energy density of the lithium-ion battery, can all be improved. When the mass relationship between the first and second elements in the cathode material satisfies 0.615≤a / 58.9+b / 183.8≤0.628, the low-temperature discharge capacity retention rate and rate discharge capacity retention rate of lithium-ion batteries can be improved, thereby increasing the energy density of lithium-ion batteries.

[0094] The lithium-ion batteries in Examples 2-1 to 2-11 differ from those in Examples 1-15 only in that the mass ratio of cobalt and nickel, and the ratio of the sum of the mass percentages of vanadium and chromium to the mass percentage of tungsten are adjusted according to Table 2.

[0095] Table 2

[0096] *In the table above, Co% + Ni% = 36.4%, W% + V% + Cr% = 0.6%. x = Co% / Ni%, y = (V% + Cr%) / W%. The results for x and y are rounded to two decimal places.

[0097] As shown in Table 2, the lithium-ion batteries prepared in this application can improve the low-temperature discharge capacity retention rate and high-rate discharge capacity retention rate, and increase the energy density of the lithium-ion battery, when the mass percentage ratio of cobalt to nickel is adjusted to 0.22 ≤ x ≤ 6.73. Similarly, when the ratio of the sum of the mass percentages of vanadium and chromium to the mass percentage of tungsten is adjusted to 0.4 ≤ y ≤ 9.0, the low-temperature discharge capacity retention rate and high-rate discharge capacity retention rate, and the energy density of the lithium-ion battery, can be improved. Furthermore, when the relationship between x and y is adjusted to 0.15 ≤ x / (0.9959y) ≤ 8.87, the low-temperature discharge capacity retention rate and high-rate discharge capacity retention rate, and the energy density of the lithium-ion battery, can be improved.

[0098] The lithium-ion batteries in Examples 3-1 to 3-17 differ from those in Examples 2-8 only in that the number distribution of particles with different aspect ratios in the cathode material particles is adjusted according to Table 3.

[0099] Table 3

[0100] As shown in Table 3, the lithium-ion batteries prepared in the embodiments of this application can improve the low-temperature discharge capacity retention rate and high-rate discharge capacity retention rate, and increase the energy density of the lithium-ion battery, when the proportion of particles with 2≤P≤5 in the cathode material is controlled to meet the condition of 50≤B≤80. Similarly, when the proportion of particles with 4≤P≤5 in the cathode material is controlled to meet the condition of 20≤C≤50, the low-temperature discharge capacity retention rate and high-rate discharge capacity retention rate, and the energy density of the lithium-ion battery, can be improved. Furthermore, when the average cross-sectional diameter of the cathode material is controlled to meet the condition of 50≤D≤300, the low-temperature discharge capacity retention rate and high-rate discharge capacity retention rate, and the energy density of the lithium-ion battery, can be improved. Finally, when the slope coefficient of the conductivity graph of the cathode material meets the condition of 4≤K≤20, the lithium-ion battery exhibits excellent performance in terms of low-temperature discharge capacity retention rate, high-rate discharge capacity retention rate, and energy density. When the peak intensity ratio of (210) peak to (101) peak in the XRD pattern of the cathode material satisfies 0.38≤E≤0.40, the lithium-ion battery exhibits excellent low-temperature discharge capacity retention, high-rate discharge capacity retention, and energy density. When the specific surface area of ​​the cathode material is adjusted to satisfy 15≤F≤30, the lithium-ion battery exhibits excellent low-temperature discharge capacity retention, high-rate discharge capacity retention, and energy density.

[0101] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A positive electrode material, characterized in that, Including lithium phosphate salts, which include a first element and a second element; The first element includes cobalt and nickel, and the mass percentage of the first element is a% based on the total mass of the cathode material, with a content of 35.5 ≤ a ≤ 37.5%; the second element includes tungsten, vanadium and chromium, and the mass percentage of the second element is b% based on the total mass of the cathode material, with a content of 0.1 ≤ b ≤ 1.5%. The cathode material is a rod-shaped grain with an olivine structure. The aspect ratio of a single cathode material is P. Based on the number of cathode material particles, the percentage of particles with P ≥ 1.5 is A%, and 50 ≤ A ≤ 100.

2. The cathode material according to claim 1, characterized in that, The cathode material satisfies at least one of the following conditions: (1)36.2≤a≤36.8； (2)0.3≤b≤1； (3)0.615≤a / 58.9+b / 183.8≤0.628; (4) The average length of the rod-shaped grains is 1 nm, 150≤1≤1000.

3. The cathode material according to claim 1 or 2, characterized in that, In the cathode material, the mass percentage ratio of cobalt to nickel is x, and the mass percentage ratio of the sum of vanadium and chromium to tungsten is y, where 0.22≤x≤6.73, 0.4≤y≤9.0, and 0.15≤x / (0.9959y)≤8.

87.

4. The cathode material according to claim 1 or 2, characterized in that, Based on the number of particles in the cathode material: The percentage of particles with 2≤P≤5 is B%, and 50≤B≤80. The percentage of particles with 4≤P≤5 is C%, and the percentage of particles with 20≤C≤50 is 20%.

5. The cathode material according to claim 1 or 2, characterized in that, The average cross-sectional diameter of the cathode material is D nm, where 50 ≤ D ≤ 300.

6. The cathode material according to claim 1 or 2, characterized in that, The slope of the conductivity graph of the positive electrode material during the voltage ramp-up process from 200 MPa to 350 MPa is K×10. -5 S / (cm·MPa), 4≤K≤20.

7. The cathode material according to claim 1 or 2, characterized in that, In the XRD pattern of the cathode material, the peak intensity ratio of (210) peak to (101) peak is E, 0.38≤E≤0.

40.

8. The positive electrode material according to claim 1, characterized in that, The specific surface area of ​​the cathode material is Fm. 2 / g, 15≤F≤30.

9. A secondary battery, characterized in that, It includes a positive electrode sheet, the positive electrode sheet comprising a positive current collector and a positive electrode material layer located on at least one surface of the positive current collector; The cathode material layer comprises the cathode material according to any one of claims 1 to 8.

10. An electronic device, characterized in that, Includes the secondary battery as described in claim 9.

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