Positive electrode material, secondary battery, and electronic device

Abstract

The present application discloses a positive electrode material, a secondary battery, and an electronic device. The positive electrode material is rod-shaped grains having an olivine structure; the average length of the positive electrode material is I nm, wherein 150≤I≤1000; the average cross-sectional diameter of the positive electrode material is D nm, wherein 50≤D≤300; the aspect ratio of each single grain of the positive electrode material is P, and based on the total number of grains of the positive electrode material, the proportion of grains having an aspect ratio of 1.5≤P≤5 is A%, wherein 50≤A≤100. The positive electrode material provided in the present application is used for secondary batteries, can increase the lithium-ion diffusion coefficient, and has excellent high-rate discharge performance and low-temperature performance.

Description

Cathode material, secondary battery and electronic device TECHNICAL FIELD

[0001] The present application relates to the technical field of energy storage, in particular to a cathode material, a secondary battery and an electronic device. BACKGROUND

[0002] The property of the cathode material is one of important factors to determine the discharge performance of the secondary battery. The olivine-type lithium phosphate salt has attracted wide attention due to its high safety, low cost, stable charge-discharge platform, stable cycle performance and environmental friendliness. However, the olivine structure material has the problems of poor electronic conductivity and slow ion diffusion rate, which seriously hinder the application of the material. Therefore, it is urgent to provide a secondary battery with good high-rate discharge performance and low-temperature discharge performance. SUMMARY

[0003] The embodiments of the present application provide a cathode material, a secondary battery and an electronic device, which achieve better high-rate discharge performance while improving low-temperature performance.

[0004] In a first aspect, the embodiments of the present application provide a cathode material, the cathode material being a rod-shaped grain with an olivine structure; the average length of the cathode material is I nm, 150≤I≤1000; and the average cross-sectional diameter of the cathode material is D nm, 50≤D≤300.

[0005] Based on the cathode material of the embodiments of the present application, the inventors find that when the cathode material is an olivine-type rod-shaped lithium phosphate salt grain and the average length and the average cross-sectional diameter thereof meet a certain range, the high-exposed (010) crystal face thereof can introduce a large number of lithium ion channels, improve the diffusion coefficient of lithium ions, and the path of the diffusion direction of lithium ions is relatively short, which shortens the diffusion path of lithium ions, can alleviate the phenomenon that the one-dimensional lithium ion diffusion path of the olivine-type lithium phosphate salt is easily blocked by lattice distortion and impurity ions, promotes the charge transfer reaction of lithium ions in the electrolyte at the cathode / electrolyte interface, and thus improves the high-rate discharge performance and low-temperature performance of the secondary battery.

[0006] In some embodiments, the aspect ratio of a single cathode material is P, and the number of particles with 1.5≤P≤5 accounts for A%, 50≤A≤100, based on the number of particles of the cathode material. Meeting the above range can improve the high-rate discharge performance and low-temperature performance of the secondary battery.

[0007] In some embodiments, the cathode material satisfies at least one of the following conditions: (1) 500≤I≤1000; (2) 50≤D≤100; (3) 70≤A≤100. Based on the above embodiments, this application can further improve the high-rate discharge performance and low-temperature performance of the secondary battery by adjusting the length, diameter and aspect ratio of the rod-shaped lithium phosphate grains to meet the above ranges.

[0008] In some embodiments, based on the number of particles in the cathode material: the proportion of particles with 2≤P≤3 is B%, 50≤B≤60; the proportion of particles with 3≤P≤5 is C%, 20≤C≤50. Based on the above embodiments, this application can improve the high-rate discharge performance and low-temperature performance of secondary batteries by controlling the distribution of lithium phosphate salt particles with a specific aspect ratio to meet the above ranges.

[0009] In some embodiments, the cathode material includes lithium phosphate, with the chemical formula LiMPO4X; wherein, element M includes one or more of iron, manganese, cobalt, or nickel, and element X includes one or more of titanium, magnesium, vanadium, chromium, copper, zinc, yttrium, zirconium, niobium, molybdenum, or tungsten. Based on the above embodiments, when the cathode material in this application uses the aforementioned lithium phosphate and contains specific metal elements, the electronic structure of the material can be altered. The incorporated metal atoms can form solid solutions with other elements, placing the impurity energy levels between the conduction band and the band gap to provide electronic conductivity. Simultaneously, doping can increase the lattice defects of the material, expand the diffusion channels of lithium ions, and reduce the resistance to lithium ion insertion / extraction, thereby improving the ionic and electronic conductivity of the material and further improving the high-rate discharge performance and low-temperature performance of the secondary battery.

[0010] In some embodiments, element X is selected from one or more of titanium, magnesium, or vanadium; based on the mass of the cathode material, the mass percentage of element X is x ppm, where 1000 ≤ x ≤ 8000. Based on the above embodiments, the cathode material in this application includes the aforementioned metal elements, and controlling their mass percentage to meet a specific range can further improve the lithium-ion diffusion coefficient, shorten the lithium-ion diffusion path, and promote the charge transfer reaction of lithium ions in the electrolyte at the cathode / electrolyte interface, thereby further improving the high-rate discharge performance and low-temperature performance of the secondary battery.

[0011] In some embodiments, the ratio of the average length I nm of the cathode material to the average cross-sectional diameter D nm of the cathode material is Q, where 7 ≤ (x / 100). ½ / 3+ 3Q≤18. Based on the above embodiments, this application controls the distribution coefficient of metal elements in the positive electrode material grains by adjusting the relationship between the mass ratio and aspect ratio of the metal elements, which can further improve the high-rate discharge performance of the secondary battery while also improving its low-temperature performance.

[0012] In some embodiments, the peak intensity ratio of peak (210) to peak (101) in the XRD pattern of the cathode material 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 high-rate discharge performance and low-temperature performance.

[0013] In some embodiments, the specific surface area of ​​the cathode material is F m 2 / g, 15≤F≤30. Based on the above embodiments, this application can further improve the high-rate discharge performance of the secondary battery while also improving its low-temperature performance by adjusting the specific surface area of ​​the cathode material to meet the above range.

[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. Attached Figure Description

[0016] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0017] Figure 1 is an XRD image of Embodiments 2-18 of this application. Embodiments of the present invention

[0018] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with embodiments and accompanying drawings. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0019] The first aspect of this application provides a secondary battery, including a positive electrode, a negative electrode, an electrolyte, and a separator.

[0020] positive electrode

[0021] <Cathode Materials>

[0022] One embodiment of this application provides a lithium phosphate-doped cathode material. The cathode material is a rod-shaped grain with an olivine structure; the average length of the cathode material is 1 nm, 150 ≤ 1 ≤ 1000. Preferably, it is 500 ≤ 1 ≤ 1000. For example, the length of the cathode material can be 150, 264, 300, 441, 500, 602, 744, 800, 976, 1000, or any value within the range of any two of these values. The average cross-sectional diameter of the cathode material is D nm, 50 ≤ D ≤ 300. Preferably, it is 50 ≤ D ≤ 100. For example, the cross-sectional diameter of the cathode material can be 50, 60, 100, 144, 197, 211, 256, 300, or any value within the range of any two of these values. The aspect ratio of a single cathode material grain is P, and based on the number of cathode material particles, the percentage of particles with a ratio of 1.5 ≤ P ≤ 5 is A%, and 50 ≤ A ≤ 100. Preferably, the ratio is 70 ≤ A ≤ 100. For example, the percentage of particles with a ratio of 1.5 ≤ P ≤ 5 in the cathode material can be 50, 56, 70, 79, 88, 90, 100, or any combination of these values. When the cathode material is an olivine-type rod-shaped lithium phosphate crystal, and its length, diameter, and aspect ratio meet specific ranges, it can promote the charge transfer reaction of lithium ions in the electrolyte at the cathode / electrolyte interface, thereby improving the high-rate discharge performance and low-temperature performance of the secondary battery.

[0023] In some embodiments, based on the particle number of the cathode material: the percentage of particles with 2 ≤ P ≤ 3 is B%, and 50 ≤ B ≤ 60. For example, the percentage of particles with 2 ≤ P ≤ 3 in the cathode material can be 50, 51, 54, 56, 57, 59, 60, or any combination of these values. The percentage of particles with 3 ≤ P ≤ 5 is C%, and 20 ≤ C ≤ 50. For example, the percentage of particles with 3 ≤ P ≤ 5 in the cathode material can be 20, 26, 30, 33, 42, 48, 50, or any combination of these values. The particle distribution of the lithium phosphate cathode material satisfying the above ranges can further improve the high-rate discharge performance and low-temperature performance of the secondary battery.

[0024] In some embodiments, the cathode material includes lithium phosphate with the chemical formula LiMPO4X; wherein element M includes one or more of iron, manganese, cobalt, or nickel, and element X includes one or more of titanium, magnesium, vanadium, chromium, copper, zinc, yttrium, zirconium, niobium, molybdenum, or tungsten. Using the above-mentioned lithium phosphate cathode material, and including specific metal elements, can improve the ionic and electronic conductivity of the material, thereby further improving the high-rate discharge performance of the secondary battery, while also improving low-temperature performance.

[0025] In some embodiments, element X is selected from one or more of titanium, magnesium, or vanadium; the mass percentage of element X is x ppm based on the mass of the cathode material, where 1000 ≤ x ≤ 8000. For example, the mass percentage of element X in the cathode material can be 1000, 2248, 2882, 3928, 6270, 7624, 8000, or any value within a range of two of these values. Lithium phosphate cathode materials comprising the aforementioned metallic elements, and controlling their mass percentage within a specific range, can further improve the high-rate discharge performance of the secondary battery while also improving low-temperature performance.

[0026] In some embodiments, the ratio of the average length I nm of the cathode material to the average cross-sectional diameter D nm of the cathode material is Q, 7 ≤ (x / 100). ½ / 3+ 3Q≤18. In some embodiments, 8≤(x / 100) ½ / 3+ 3Q≤10. In some embodiments, 17≤(x / 100) ½ / 3+ 3Q≤18. In some embodiments, 12≤(x / 100) ½ / 3+ 3Q≤15. In some embodiments, 9≤(x / 100) ½ / 3+ 3Q≤13. In some embodiments, 11≤(x / 100) ½ / 3+ 3Q≤14. The distribution coefficient of metal elements in the cathode material grains of lithium phosphate cathode materials meets the above range, which can further improve the high-rate discharge performance of secondary batteries and also improve low-temperature performance.

[0027] In some embodiments, the specific surface area of ​​the cathode material is F m 2 / g, 15≤F≤30. For example, the specific surface area of ​​the cathode material can be 15, 17, 18, 22, 23, 25, 30, or any combination of these values. When the specific surface area of ​​the cathode material meets the above range, the high-rate discharge performance of the secondary battery can be further improved, while also improving low-temperature performance.

[0028] <Preparation Methods of Cathode Materials>

[0029] (1) Weigh lithium hydroxide monohydrate, ferrous sulfate heptahydrate, and ammonium dihydrogen phosphate in the molar ratio nLi:nFe:nP, prepare an aqueous solution of a certain mass fraction, and slowly mix the three solutions while maintaining stirring conditions.

[0030] (2) A certain mass fraction of polyvinyl alcohol is added to the mixed solution as a dispersant to synthesize lithium iron phosphate, and an equal amount of glucose is added to the mixed solution as a grain growth control agent to synthesize lithium iron phosphate.

[0031] (3) Adjust the pH of the reaction slurry and keep it warm for 5h~8h to obtain lithium iron phosphate precursor A;

[0032] (4) After filtering and washing the precursor A, it is mixed evenly with the carbon source and then spray-dried to obtain lithium iron phosphate spray material B;

[0033] (5) Lithium iron phosphate spray material was calcined under a protective atmosphere to obtain lithium iron phosphate cathode material.

[0034] <Positive Electrode>

[0035] One embodiment of this application provides a 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 includes the above-described positive electrode material or a positive electrode material prepared by the above-described method.

[0036] 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, and any known conductive material can be used. Examples of positive electrode conductive materials may include, but are not limited to, acetylene black, Super-P carbon black, etc.; amorphous carbon such as needle coke; carbon nanotubes; graphene, etc. The above-mentioned positive electrode conductive materials can be used alone or in any combination.

[0037] In some embodiments, the positive electrode material layer includes a positive electrode binder; there is no particular limitation on the type of positive electrode binder, and in the case of a coating method, any material that can be dissolved or dispersed 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 elastomer-like polymers 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.

[0038] There are no restrictions on the type of solvent used to form the positive electrode slurry, as long as it is capable of dissolving or dispersing 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 and organic solvents. Examples of aqueous media can include, but are not limited to, mixtures of alcohol and water or water. Examples of organic media can 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.

[0039] 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; 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-mentioned thickeners can be used alone or in any combination.

[0040] 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 may 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.

[0041] 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.

[0042] negative electrode

[0043] 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 to prevent unintentional deposition of lithium metal on the negative electrode during charging.

[0044] 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.

[0045] The negative electrode material layer of this application also includes a negative electrode binder. The negative electrode binder can improve the bonding between the negative electrode active material particles and the bonding between the negative electrode active material and the current collector. There are no particular limitations on 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 a negative electrode slurry is prepared using an aqueous solvent, 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.

[0046] The negative electrode material layer of this application 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 can achieve 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.

[0047] 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 comprise 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 mixture 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.

[0048] 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.

[0049] 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.

[0050] electrolyte

[0051] The electrolyte used in the secondary battery of this application includes an electrolyte and a solvent for dissolving the electrolyte. The electrolyte may also include lithium salts and non-aqueous solvents. This application does not particularly limit 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(oxalatoborate)borate (LiBOB), or lithium difluorooxalatoborate (LiDFOB). Based on the mass of the electrolyte, the mass percentage of lithium salt can be from 8% to 15%, for example, the mass percentage of lithium salt can be 8%, 9%, 10%, 11%, 12.5%, 13%, 15%, or a range consisting of any two of these values. This application does not particularly limit the type of the aforementioned non-aqueous solvent, as long as it achieves the purpose of this application. For example, it may include, but is not limited to, at least one of carbonate compounds, carboxylic acid ester compounds, ether compounds, or other organic solvents. The aforementioned carbonate compounds may include, but are not limited to, at least one of chain carbonate compounds or cyclic carbonate compounds. The aforementioned chain carbonate compounds 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 aforementioned cyclic carbonate compounds may include, but are not limited to, at least one of ethylene carbonate, propylene carbonate, butylene carbonate, or ethylene ethylene carbonate. The aforementioned carboxylic acid ester compounds 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 other organic solvents mentioned above may include, but are not limited to, at least one of dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolium ketone, N-methyl-2-pyrrolidone, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, or trioctyl phosphate.

[0052] diaphragm

[0053] This application typically includes a separator between the positive and negative electrodes. The separator is used to separate the positive and negative electrode plates, prevent internal short circuits in the secondary battery, allow electrolyte ions to pass freely, and does not affect the electrochemical charging and discharging process.

[0054] 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 material of the diaphragm 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 type of diaphragm may include at least one of woven membrane, nonwoven membrane, microporous membrane, composite membrane, rolled membrane or spun membrane.

[0055] 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).

[0056] 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 thickness of the separator is greater than 1 μm, greater than 5 μm, or greater than 8 μm. In some embodiments, the thickness of the separator is less than 50 μm, less than 40 μm, or less than 30 μm. When the thickness of the separator is within the above ranges, insulation and mechanical strength can be ensured, and the rate characteristics and energy density of the secondary battery can be ensured.

[0057] This application also provides an electronic device that includes the secondary battery described in the embodiments of 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.

[0058] Example

[0059] 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.

[0060] Example 1-1

[0061] (1) Preparation of the positive electrode

[0062] <Preparation of Positive Electrode Active Materials>

[0063] 1) Weigh lithium hydroxide monohydrate, ferrous sulfate heptahydrate, and ammonium dihydrogen phosphate in a molar ratio of nLi:nFe:nP=3:1:1. Prepare aqueous solutions of 10%, 20%, and 30% by mass, respectively, and slowly mix the three solutions while maintaining stirring.

[0064] 2) Add 0.5% polyvinyl alcohol (by mass fraction of theoretically synthesized lithium iron phosphate) to the mixed solution as a dispersant, and add glucose (by molar amount of theoretically synthesized lithium iron phosphate) as a grain growth control agent.

[0065] 3) Adjust the pH of the reaction slurry to 9.0~9.5, and keep it at 150℃~200℃ for 5h~8h to obtain lithium iron phosphate precursor A;

[0066] 4) Filter and wash precursor A, mix it evenly with 10% carbon source and 1000ppm magnesium dopant, and then spray dry to obtain lithium iron phosphate spray material B;

[0067] 5) Lithium iron phosphate spray material is calcined under a protective atmosphere to obtain lithium iron phosphate cathode material.

[0068] <Preparation of the positive electrode>

[0069] The lithium iron phosphate cathode 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 cathode slurry with a solid content of 70 wt%. The cathode slurry is uniformly coated onto one surface of a 9 μm thick cathode current collector aluminum foil and dried to obtain a cathode sheet with a single-sided cathode agent layer. The above steps are repeated on the other surface of the cathode current collector aluminum foil to obtain a cathode sheet with a double-sided cathode agent layer. By adjusting the cold pressing pressure, cathode sheets with different compaction densities and surface roughness can be obtained. After cold pressing, slitting, and welding of tabs, a cathode sheet with a specification of 74 mm × 867 mm is obtained.

[0070] (2) Preparation of electrolyte

[0071] 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. The mass percentage of LiPF6 in the electrolyte was 12.5%, and the mass percentage of vinylene carbonate was 2%.

[0072] (3) Preparation of negative electrode

[0073] 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 negative electrode sheet with a single-sided coating of the negative electrode mixture layer. The above steps were repeated on the other surface of the copper foil to obtain a negative electrode sheet with a double-sided coating of the negative electrode mixture layer. 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, a negative electrode sheet with a specification of 76.6 mm × 875 mm was obtained.

[0074] (4) Preparation of the diaphragm

[0075] A porous polyethylene film with a thickness of 15μm was used as the diaphragm.

[0076] (5) Preparation of lithium-ion batteries

[0077] 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. This assembly is placed in a packaging bag, dehydrated at 80°C, injected with 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.

[0078] <Testing Methods>

[0079] (1) Structural observation methods

[0080] The lithium-ion battery prepared in the above steps was used. The positive electrode sheet was ion-polished and then cut. The crystal structure of the material layer on the surface of the electrode sheet was observed using a scanning electron microscope, and the particle length and cross-sectional diameter of the material were measured. 200 material particles were measured, and their aspect ratios were calculated, which is the P value. The proportions of particles with different aspect ratios were statistically analyzed, which are the A, B, and C values. The arithmetic mean of the lengths of the 200 material particles was calculated as the average length I nm, the arithmetic mean of the cross-sectional diameters of the 200 material particles was calculated as the average cross-sectional diameter D nm, and the arithmetic mean of the aspect ratios of the 200 material particles was calculated, which is the Q value.

[0081] Instrument model: ZEISS SEM, accelerating voltage: 0.1kV~30kV.

[0082] (2) Test methods for the content of metal elements other than lithium

[0083] Take the lithium-ion battery prepared in the above steps, dissolve the positive electrode sheet in a mixed solvent (for example, 0.4g of positive electrode sheet is dissolved in a mixed solvent of 5 ml aqua regia and 5 ml deionized water), and make up to 100mL. Then use an ICP analyzer to test the mass percentage of metal elements other than lithium in the solution.

[0084] (3) XRD image testing method (diffraction peak intensity testing method)

[0085] The lithium-ion battery prepared in the above steps was subjected to X-ray diffraction analysis to obtain XRD images of its positive electrode. Instrument model: Bruker D8 ADVANCE, target material: Cu Kα, scanning angle: 5° to 80°.

[0086] (4) 25℃ 4C rate discharge test

[0087] The lithium-ion battery prepared in the above steps was charged at 25°C with a constant current of 0.2C to 4.3V, then charged at a constant voltage until the current was less than or equal to 0.05C. After that, it was left to stand for 30 minutes, and then discharged at a constant current of 0.2C to 2.5V. The discharge capacity of the lithium-ion battery at 25°C and 0.2C was then tested. At 25°C, the lithium-ion battery was charged at a constant current of 0.2C to 4.3V, then charged at a constant voltage until the current was less than or equal to 0.05C. After that, the cell was left to stand for 60 minutes, and then discharged at a constant current of 4C to 2.5V. The discharge capacity of the lithium-ion battery at 25°C and 4C was then tested.

[0088] 25℃ 4C rate discharge capacity retention rate (%) = 25℃ 4C rate discharge capacity / 25℃ 0.2C rate discharge capacity × 100%.

[0089] (5) Low-temperature discharge performance test

[0090] Take the lithium-ion battery prepared in the above steps and place it in a high and 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 0.5C to 2.5V, then charge it at a constant current of 0.5C to 4.3V, and then charge it at a constant voltage of 4.3V until the current is 0.05C. Again, at 25°C, discharge it at a constant current of 0.5C to 2.5V, and record the discharge capacity as the initial discharge capacity. At 25°C, charge it at a constant current of 0.5C to 4.3V, and then charge it at a constant voltage of 4.3V until the current is 0.05C. Then, place the lithium-ion battery at 0°C and let it stand for 30 minutes to allow the temperature of the lithium-ion battery to match the ambient temperature. At 0°C, discharge it at a constant current of 0.5C to 2.5V, and record the discharge capacity as the low-temperature discharge capacity.

[0091] Low-temperature discharge capacity retention rate % = (low-temperature discharge capacity / initial discharge capacity) × 100%.

[0092] The lithium-ion batteries in the following examples or comparative examples differ from those in Examples 1-1 only in that materials with different average lengths, average cross-sectional diameters, and aspect ratios are screened according to Table 1 and used as the positive electrode active materials for the lithium-ion batteries. The performance test results of the lithium-ion batteries in each example and comparative example are shown in Table 1 below.

[0093] Table 1

[0094]

[0095] As shown in Table 1, the lithium-ion batteries prepared in the embodiments of this application can improve the high-rate discharge performance and low-temperature performance of the secondary battery when the average length I nm of the positive electrode material satisfies 150≤I≤1000, the average cross-sectional diameter D nm of the positive electrode material satisfies 50≤D≤300, and the percentage A% of particles with 1.5≤P≤5 satisfies 50≤A≤100. Specifically, the high-rate discharge performance and low-temperature performance of the secondary battery can be further improved when the average length of the positive electrode material further satisfies 500≤I≤1000, the average cross-sectional diameter of the positive electrode material further satisfies 50≤D≤100, or the percentage of particles with 1.5≤P≤5 further satisfies 70≤A≤100. Furthermore, the high-rate discharge performance and low-temperature performance of the secondary battery can be further improved when the percentage of particles with 2≤P≤3 in the positive electrode material satisfies 50≤B≤60 or the percentage of particles with 3≤P≤5 satisfies 20≤C≤50.

[0096] The lithium-ion batteries in Examples 2-1 to 2-25 differ from those in Examples 1-20 only in that the type of cathode material and the element content therein are adjusted according to Table 2.

[0097] Table 2

[0098]

[0099] *In the table above, in Examples 2-5 to 2-7, the two X elements were mixed in a molar ratio of 1:1.

[0100] As shown in Table 2, the lithium-ion batteries prepared in the embodiments of this application can further improve the high-rate discharge performance and low-temperature performance of the secondary battery by controlling the M element in the positive electrode material to be of a specific type. Similarly, controlling the X element in the positive electrode material to be of a specific type can further improve the high-rate discharge performance and low-temperature performance of the secondary battery. Furthermore, controlling the mass percentage of the X element in the positive electrode material to satisfy 1000 ≤ x ≤ 8000 can further improve the high-rate discharge performance and low-temperature performance of the secondary battery. Finally, a specific relationship between the mass percentage of the X element in the positive electrode material and the average length and average cross-sectional diameter of the positive electrode material satisfies 7 ≤ (x / 100). ½ When / 3+ 3Q≤18, the high-rate discharge performance and low-temperature performance of the secondary battery can be further improved. When the peak intensity ratio of the characteristic peaks in the XRD pattern of the cathode material satisfies 0.38≤E≤0.40, the high-rate discharge performance and low-temperature performance of the secondary battery can be further improved. When the specific surface area of ​​the cathode material satisfies 15≤F≤30, the high-rate discharge performance and low-temperature performance of the secondary battery can be further improved.

[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 by, The positive electrode material is a rod-shaped grain with an olivine structure; the average length of the positive electrode material is I nm, 150≤I≤1000; the average cross-sectional diameter of the positive electrode material is D nm, 50≤D≤300.

2. The positive electrode material of claim 1, wherein, The aspect ratio of a single particle of the positive electrode material is P, and the proportion of the number of particles with 1.5≤P≤5 is A%, based on the number of particles of the positive electrode material, 50≤A≤100.

3. The positive electrode material of claim 1, wherein, The positive electrode material satisfies at least one of the following conditions: （1）500≤I≤1000； （2）50≤D≤100； (3) The aspect ratio of a single particle of the positive electrode material is P, and the proportion of the number of particles with 1.5≤P≤5 is A%, based on the number of particles of the positive electrode material, 70≤A≤100.

4. The positive electrode material according to claim 3, characterized in that, Based on the number of particles of the positive electrode material: The proportion of the number of particles with 2≤P≤3 is B%, 50≤B≤60; The proportion of the number of particles with 3≤P≤5 is C%, 20≤C≤50.

5. The positive electrode material according to any one of claims 1 to 4, characterized in that, The positive electrode material comprises a lithium phosphate salt, and the chemical formula of the lithium phosphate salt is LiMPO4X; Wherein, M elements include one or more of iron elements, manganese elements, cobalt elements or nickel elements, and X elements include one or more of titanium elements, magnesium elements, vanadium elements, chromium elements, copper elements, zinc elements, yttrium elements, zirconium elements, niobium elements, molybdenum elements or tungsten elements.

6. The positive electrode material according to claim 5, characterized in that, The X element is selected from one or more of titanium elements, magnesium elements or vanadium elements; Based on the mass of the positive electrode material, the mass proportion of the X element is x ppm, and the positive electrode material satisfies at least one of the following conditions: （1）1000≤x≤8000； (2) the ratio of the average length I nm of the positive electrode material to the average cross-sectional diameter D nm of the positive electrode material is Q, 7≤(x / 100) ½ / 3+ 3Q≤18.

7. The positive electrode material according to any one of claims 1 to 4, characterized in that, In the XRD pattern of the positive electrode material, the peak intensity ratio of the (210) peak to the (101) peak is E, 0.38≤E≤0.

40.

8. The positive electrode material according to any one of claims 1 to 4, characterized in that, The specific surface area of ​​the cathode material is F m 2 / g, 15≤F≤30.

9. A secondary battery characterized by comprising: The positive electrode material layer comprises the positive electrode material according to any one of claims 1 to 8. The secondary battery according to claim 9.

10. An electronic device, comprising: ​

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