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 comprises a core and a carbon coating layer, wherein the core comprises a lithium phosphate salt, and the lithium phosphate salt comprises a first element, a second element, and a third element; the first element comprises at least one of an iron element, a manganese element, a cobalt element, or a nickel element, the second element comprises at least one of a molybdenum element, a zinc element, or a calcium element, and the third element comprises at least one of a boron element or a chlorine element. On the basis of the mass of the positive electrode material, the mass content of the first element is M1％, wherein 32.5≤M1≤37.5; the mass content of the second element is M2％, wherein 0.01≤M2≤1; and the mass content of the third element is M3％, wherein 0.005≤M3≤0.1. The carbon coating layer comprises a carbon element, and on the basis of the mass of the positive electrode material, the mass content of the carbon element is A%, wherein 0.8≤A≤8. The secondary battery provided in the present application has better discharge performance, and particularly has better rate discharge performance and longer calendar life.

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] Lithium phosphate cathode materials, with their high safety, environmental friendliness, and economic advantages, have become the mainstream technology in the energy storage field. However, their insufficient high-rate charge-discharge efficiency in fast-charging scenarios and the accelerated capacity decay caused by structural degradation during long-term cycling severely restrict their application expansion in scenarios such as new energy vehicles and high-power energy storage. Therefore, developing novel lithium phosphate cathode systems that combine high-rate performance with long-cycle stability has become a core research direction for the current iteration of rechargeable battery technology. Summary of the Invention

[0003] In view of this, embodiments of this application provide a cathode material, a secondary battery, and an electronic device that achieves superior rate discharge performance while also improving calendar life.

[0004] In a first aspect, embodiments of this application provide a cathode material, including a core and a carbon coating layer. The core includes a lithium phosphate salt, which comprises a first element, a second element, and a third element. The carbon coating layer covers at least a portion of the surface of the core. The first element includes at least one of iron, manganese, cobalt, or nickel, and its mass content is M1% based on the mass of the cathode material, with a content of 32.5 ≤ M1 ≤ 37.5%. The second element includes at least one of molybdenum, zinc, or calcium, and its mass content is M2% based on the mass of the cathode material, with a content of 0.01 ≤ M2 ≤ 1. The third element includes at least one of boron or chlorine, and its mass content is M3% based on the mass of the cathode material, with a content of 0.005 ≤ M3 ≤ 0.1. The carbon coating layer includes carbon, and its mass content is A% based on the mass of the cathode material, with a content of 0.8 ≤ A ≤ 8.

[0005] Based on the cathode material of this application embodiment, coating the surface of the polyanionic lithium phosphate material with a carbon layer can increase its conductivity and improve its rate discharge performance. Furthermore, the specific amounts of molybdenum, zinc, or calcium doped into the lithium phosphate material can partially replace some sites of iron, manganese, cobalt, or nickel, forming high-energy transition metal-oxygen bonds. This alters the crystal electronic structure of the lithium phosphate material, providing a smoother lithium-ion diffusion path and further improving conductivity, thereby improving the rate discharge performance of the secondary battery and reducing self-discharge during long-term storage. In addition, the introduced boron or chlorine can suppress lattice relaxation and microcrack formation during long-term storage, synergistically reducing self-discharge caused by increased polarization during long-term storage and extending calendar life.

[0006] In some embodiments, the above-mentioned cathode material satisfies 0.57≤M1 / 57.102+M2 / 67.003+M3 / 23.132≤0.66. Based on the above embodiments, this application, by controlling the mass content relationship of the first element, the second element, and the third element in the lithium phosphate cathode material within the above range, can further improve the ion transport efficiency of the material and improve the rate discharge stability; at the same time, the first element, the second element, and the third element synergistically suppress self-discharge caused by increased polarization during long-term storage, thereby extending the calendar lifetime.

[0007] In some embodiments, the above-mentioned cathode material satisfies at least one of the following conditions: (1) 34≤M1≤37; (2) 0.1≤M2≤0.8; (3) 0.60≤M1 / 57.102+M2 / 67.003+M3 / 23.132≤0.65; (4) 1.2≤A≤6. Based on the above embodiments, this application further satisfies the above ranges by adjusting the mass content of the first element and the second element in the cathode material and the content relationship of the first element, the second element and the third element, or by further satisfying the above ranges for the mass content of carbon element in the cathode material, thereby balancing the proportion of each element content in the cathode material, improving the rate discharge performance of the cathode material while extending the calendar life.

[0008] In some embodiments, the carbon coating layer includes nitrogen. Based on the mass of the cathode material, the mass content of nitrogen is B%, 0.03 ≤ B ≤ 5%. Based on the above embodiments, the carbon coating layer on the surface of the lithium phosphate cathode material of this application further contains nitrogen, which can form a delocalized conjugated system of sp2 hybrid carbon. The conjugated electrons in the conjugated system can coordinate with the empty transition metal orbitals on the surface of the core lithium phosphate material, improving the stability of the surface coating layer, thereby maintaining efficient ion conduction in the material, extending calendar lifetime, and improving rate discharge performance.

[0009] In some embodiments, the above-mentioned cathode material satisfies 0.6 ≤ B ≤ 3.4. Based on the above embodiments, this application further satisfies the above range by adjusting the mass content of nitrogen in the carbon coating layer of the cathode material, which can further improve the rate discharge performance while extending the calendar life.

[0010] In some embodiments, the nitrogen element in the carbon coating layer comprises at least one of pyridine nitrogen, amino nitrogen, pyrrole nitrogen, graphitic nitrogen, or nitrogen oxide. In some embodiments, the nitrogen element in the carbon coating layer comprises graphitic nitrogen; based on the mass of the cathode material, the mass content of graphitic nitrogen is C%, 0.2≤C / B≤0.6. Based on the above embodiments, when the nitrogen element in the carbon coating layer on the surface of the lithium phosphate material comprises the above-mentioned forms, especially including a specific amount of amino nitrogen, a better delocalized conjugated system of sp2 hybrid carbon can be formed, further improving rate discharge performance while extending calendar lifetime.

[0011] In some embodiments, the length of the cathode material is 1 nm, the diameter is D nm, 50≤D≤500, and 1≤1 / D≤5. Based on the above embodiments, this application, by controlling the morphology parameters of the cathode material within the above range, enables the cathode material to have a morphology size that is relatively conducive to lithium-ion diffusion, thereby further improving rate discharge performance while extending calendar lifetime.

[0012] In some embodiments, the D / G peak ratio of the carbon coating layer is N, where 0.9 ≤ N ≤ 1.2. Based on the above embodiments, the D / G peak ratio of the carbon coating layer on the surface of the lithium phosphate material of this application meets the above range, which can balance the conductivity and structural stability of the carbon coating layer, thereby improving both rate discharge performance and low-temperature performance.

[0013] In some embodiments, the chemical formula of the above-mentioned cathode material is LiMPO4. The M element includes a first element, a second element, and a third element. Based on the above embodiments, the cathode material of this application includes the above-mentioned lithium phosphate salt, which can further improve the rate discharge performance of the cathode material while extending its calendar life.

[0014] Secondly, embodiments of this application provide a secondary battery, including a positive electrode, the positive electrode including a positive electrode current collector and a positive electrode material layer located on at least one surface of the positive electrode current collector; the positive electrode material layer includes 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 a core and a carbon coating layer. The core includes a lithium phosphate salt, which includes a first element, a second element, and a third element. The carbon coating layer covers at least a portion of the surface of the core. The first element includes at least one of iron, manganese, cobalt, or nickel. Based on the mass of the cathode material, the mass content of the first element is M1%, 32.5 ≤ M1 ≤ 37.5, preferably 34 ≤ M1 ≤ 37. For example, the mass content of the first element in the cathode material can be 32.5, 32.8, 33.0, 33.6, 33.9, 34.0, 35.0, 35.2, 35.9, 36.1, 36.7, 37, 37.1, 37.5, or a value within any two of these ranges. The second element includes at least one of molybdenum, zinc, or calcium. Based on the mass of the cathode material, the mass content of the second element is M2%, 0.01 ≤ M2 ≤ 1, preferably 0.1 ≤ M2 ≤ 0.8. For example, the mass content of the second element in the cathode material can be 0.01, 0.03, 0.1, 0.21, 0.33, 0.49, 0.53, 0.66, 0.74, 0.80, 0.98, 1, or a value within any two of these ranges. The third element includes at least one of boron or chlorine, and the mass content of the third element is M3% based on the mass of the cathode material, where 0.005 ≤ M3 ≤ 0.1. For example, the mass content of the third element in the cathode material can be 0.005, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, or a value within any two of these ranges. The carbon coating layer includes carbon, and the mass content of the carbon element is A% based on the mass of the cathode material, where 0.8 ≤ A ≤ 8, preferably 1.2 ≤ A ≤ 6. For example, the mass content of carbon in the cathode material can be 0.8, 1.0, 1.2, 1.3, 1.6, 1.8, 1.9, 2.2, 2.6, 2.8, 2.9, 3.1, 4.6, 6, 8, or any combination of these values. Coating the surface of polyanionic lithium phosphate materials with a carbon layer can increase their conductivity and improve rate discharge performance. Furthermore, doping the lithium phosphate material with specific amounts of molybdenum, zinc, or calcium can further improve conductivity, thereby improving the rate discharge performance of the secondary battery and reducing self-discharge during long-term storage. In addition, the introduction of boron or chlorine can synergistically reduce self-discharge caused by increased polarization during long-term storage, extending calendar life.

[0021] In some embodiments, the positive electrode material satisfies 0.57 ≤ M1 / 57.102 + M2 / 67.003 + M3 / 23.132 ≤ 0.66. Preferably, it satisfies 0.60 ≤ M1 / 57.102 + M2 / 67.003 + M3 / 23.132 ≤ 0.65. In some embodiments, the positive electrode material satisfies 0.59 ≤ M1 / 57.102 + M2 / 67.003 + M3 / 23.132 ≤ 0.65. In some embodiments, the positive electrode material satisfies 0.61 ≤ M1 / 57.102 + M2 / 67.003 + M3 / 23.132 ≤ 0.63. In some embodiments, the positive electrode material satisfies 0.64 ≤ M1 / 57.102 + M2 / 67.003 + M3 / 23.132 ≤ 0.65. By controlling the mass content relationship of the first, second, and third elements in the lithium phosphate cathode material within the above range, the ion transport efficiency of the material can be further improved, and the rate discharge stability can be enhanced. At the same time, the first, second, and third elements synergistically suppress self-discharge caused by increased polarization during long-term storage, thus extending the calendar lifetime.

[0022] In some embodiments, the carbon coating layer includes nitrogen. Based on the mass of the cathode material, the nitrogen content is B%, 0.03 ≤ B ≤ 5%, preferably 0.6 ≤ B ≤ 3.4%. For example, the mass content of nitrogen in the cathode material can be 0.03, 0.3, 0.6, 0.8, 1.6, 2.1, 2.3, 2.9, 3.4, 4.0, 4.2, 4.6, 5, or a range of any two of these values. The carbon coating layer on the surface of the lithium phosphate cathode material further contains nitrogen, which can improve the stability of the surface coating layer, thereby maintaining efficient ion conduction in the material, extending calendar life, and improving rate discharge performance.

[0023] In some embodiments, the nitrogen element in the carbon coating layer comprises at least one of pyridine nitrogen, amino nitrogen, pyrrole nitrogen, graphitic nitrogen, or nitrogen oxide. In some embodiments, the nitrogen element in the carbon coating layer comprises graphitic nitrogen; the mass content of graphitic nitrogen is C%, 0.2 ≤ C / B ≤ 0.6, based on the mass of the cathode material. For example, the mass ratio of graphitic nitrogen to total nitrogen in the cathode material layer is 0.20, 0.25, 0.30, 0.36, 0.36, 0.40, 0.46, 0.50, 0.54, 0.57, 0.6, or a value within any two of these ranges. When the nitrogen element in the carbon coating layer on the surface of the lithium phosphate material comprises the above-mentioned forms, especially when it comprises a specific amount of amino nitrogen, it is possible to further improve rate discharge performance while extending calendar life.

[0024] In some embodiments, the length of the cathode material is 1 nm, the diameter is D nm, 50 ≤ D ≤ 500, and 1 ≤ 1 / D ≤ 5. For example, the length of the cathode material can be 50, 76, 99, 152, 192, 256, 276, 349, 385, 440, 464, 500, or any value within the range of any two of these values. For example, the aspect ratio of the cathode material can be 1, 2, 3, 4, 5, or any value within the range of any two of these values. By controlling the morphology parameters of the cathode material within the above ranges, it is possible to further improve rate discharge performance while extending calendar life.

[0025] In some embodiments, the D / G peak ratio of the carbon coating layer is N, where 0.9 ≤ N ≤ 1.2. For example, the D / G peak ratio of the carbon coating layer can be 0.9, 0.93, 0.95, 0.97, 1.00, 1.04, 1.08, 1.10, 1.12, 1.15, 1.19, 1.2, or a value within any two of these ranges. When the D / G peak ratio of the carbon coating layer on the surface of the lithium phosphate material meets the above range, it can balance the conductivity and structural stability of the carbon coating layer, thereby improving both rate discharge performance and low-temperature performance.

[0026] In some embodiments, the chemical formula of the above-mentioned cathode material is LiMPO4. Here, element M includes a first element, a second element, and a third element. For example, the chemical formula of the cathode material can be:

[0027] LiFe 0.415 Mn 0.482 Zn 0.0002 PO4B 0.0007 LiCo 0.225 Mn 0.662 Ca 0.0002 PO4Cl 0.0002 or LiCo 0.52 Ni 0.323 Mo 0.0002 PO4Cl 0.0002 Based on the above embodiments, the cathode material of this application includes the aforementioned lithium phosphate salt, which can improve rate discharge performance while extending calendar life.

[0028] In some embodiments, the preparation method of the above-mentioned positive electrode material includes the following steps:

[0029] (1) Weigh out the M metal source (phosphate, oxide, carbonate, oxalate), phosphorus source and lithium source in the ratio of M metal: phosphorus: lithium = (0.89-1.115):1:(1~1.03), add nitrogen-containing carbon source with a mass ratio of 5% to 8% of the total mass of the synthesized cathode material, add non-metallic element dopant, add solvent and perform the first grinding treatment to obtain the first grinding slurry;

[0030] (2) The first grinding slurry is dried and then subjected to the first calcination treatment. It is calcined at 500℃~600℃ for 4h~8h under nitrogen atmosphere to obtain pre-calcined material;

[0031] (3) Add 5% to 15% of the total mass of the synthesized cathode material to the pre-burned material for a second grinding and mixing process to obtain the second grinding material;

[0032] (4) The second grinding material is dried and then subjected to a second calcination treatment. The material is calcined at 700℃~800℃ for 5h~10h under a nitrogen atmosphere to obtain the positive electrode material.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] 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.

[0039] negative electrode

[0040] 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.

[0041] Negative electrode active materials may include natural graphite, artificial graphite, mesophase microcarbon spheres (MCMB), silicon, silicon-carbon composites, and SiO2. x(0.5 < x < 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.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] 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.

[0047] electrolyte

[0048] The electrolyte used in the secondary battery of this application includes a lithium salt and a non-aqueous solvent for dissolving the lithium salt.

[0049] 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 content of the lithium salt may be from 8% to 15%, for example, the mass content of the lithium salt may be 8%, 9%, 10%, 11%, 12.5%, 13%, 15%, or a range of any two of these values.

[0050] 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.

[0051] diaphragm

[0052] 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.

[0053] 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.

[0054] 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).

[0055] 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.

[0056] 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.

[0057] Example

[0058] 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.

[0059] Example 1-1

[0060] 1. Preparation of the positive electrode

[0061] <Preparation of cathode materials>

[0062] (1) Weigh out the M metal source (M metal molar ratio is Fe:Mo = 0.89:0.0002), phosphorus source and lithium source in a molar ratio of transition M metal:phosphorus:lithium = 0.89:1:1, add polyacrylamide (6% of the total mass of the synthesized cathode material), add lithium chloride (molar ratio is Fe:Mo:Cl = 0.89:0.0002:0.0002), add deionized water and perform the first grinding treatment to obtain the first grinding slurry;

[0063] (2) The first grinding slurry is dried and then subjected to the first calcination treatment. It is calcined at 600°C for 8 hours under a nitrogen atmosphere to obtain the pre-calcined material.

[0064] (3) Add a nitrogen-containing carbon source accounting for 12% of the total mass of the synthesized cathode material to the pre-burned material for a second grinding and mixing treatment to obtain a second grinding material;

[0065] (4) After the second grinding material is dried, it undergoes a second calcination treatment, calcining at 800℃ for 8 hours under a nitrogen atmosphere to obtain nitrogen-doped carbon-coated cathode material LiFe. 0.89 Mo 0.0002 PO4Cl 0.0002 .

[0066] <Preparation of the positive electrode>

[0067] The above steps yield the cathode material LiFe. 0.89 Mo 0.0002 PO4Cl 0.0002Conductive carbon black and polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 95:2:3, and N-methylpyrrolidone (NMP) was added. The mixture was stirred evenly under vacuum to obtain a positive electrode slurry with a solid content of 70 wt%. The positive electrode slurry was uniformly coated onto one surface of a 9 μm thick aluminum foil current collector and dried to obtain a positive electrode sheet with a single-sided coating of positive electrode material. The above steps were repeated on the other surface of the aluminum foil 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 could be obtained. After cold pressing, slitting, and welding of tabs, the sheets were dried to obtain a positive electrode sheet with a size of 74 mm × 867 mm.

[0068] 2. Preparation of electrolyte

[0069] 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 electrolyte contained 12.5% ​​LiPF6, 2% vinylene carbonate, and the remainder was diethyl carbonate, based on the total mass of the electrolyte.

[0070] 3. Preparation of the negative electrode

[0071] 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.

[0072] 4. Preparation of the diaphragm

[0073] A porous polyethylene film with a thickness of 15μm was used as the diaphragm.

[0074] 5. Preparation of lithium-ion batteries

[0075] 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.

[0076] 6. Testing Methods

[0077] (1) Metal element content test

[0078] For the initial cathode material, the cathode material was dissolved separately using a mixed solvent (for example, 0.4g of material was dissolved in a mixed solvent of 10ml aqua regia (nitric acid and hydrochloric acid mixed in a 1:1 ratio) and 2ml HF), and the volume was adjusted to 100mL. Then, the mass content of each metal element and nitrogen element in the solution was tested using an ICP analyzer.

[0079] For the positive electrode, take a lithium-ion battery, scrape off the positive electrode material from 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 material), bring the volume to 100 mL, and then use an ICP analyzer to test the mass content of each metal element and nitrogen element in the solution.

[0080] (2) Nitrogen content test

[0081] For the initial cathode material, a certain amount of cathode material is weighed, the sample is wrapped in a container, and then the mass content of nitrogen in the sample is tested using an elemental analyzer.

[0082] For the initial positive electrode, take a lithium-ion battery, scrape off the positive electrode material from the positive electrode, weigh a certain amount of positive electrode material, wrap the sample in a container, and then use an elemental analyzer to test the mass content of nitrogen in the sample.

[0083] (3) Carbon content test

[0084] A lithium-ion battery is used. The positive electrode material is scraped off from the positive electrode sheet and placed in the crucible of the analyzer. It is then heated in a high-frequency induction furnace to promote the reaction of carbon with oxygen to produce carbon dioxide. The generated carbon dioxide gas is passed through an infrared detection cell. The concentration of carbon dioxide gas is determined using the principle of infrared absorption, and the carbon content concentration in the positive electrode material layer is calculated. Testing instrument: High-frequency infrared carbon-sulfur analyzer. Model: Shanghai Dekai DK-666.

[0085] (4) Nitrogen element speciation test

[0086] For the initial cathode material, the photoelectron spectroscopy of the material sample was measured using an XPS instrument in a vacuum environment to obtain the photoelectron peak spectrum of nitrogen (N1s);

[0087] For the positive electrode, a lithium-ion battery was taken, the positive electrode was removed and cut along its thickness. The material sample in the positive electrode was then subjected to photoelectron spectroscopy (XPS) in a vacuum environment to obtain the photoelectron peak spectrum of nitrogen (N1s). Nitrogen elements were distinguished by differences in XPS binding energy: pyridine nitrogen (Npyri, binding energy ≥ 398 eV, < 399 eV), amino nitrogen (Namine, binding energy ≥ 399 eV, < 400 eV), pyrrole nitrogen (Npyrro, binding energy ≥ 400 eV, < 401 eV), graphitic nitrogen (Ngrap, binding energy ≥ 401 eV, < 402 eV), and nitrogen oxide (Noxide, binding energy ≥ 402 eV, < 405 eV).

[0088] (5) Particle morphology measurement

[0089] For the positive electrode material, the morphology of the positive electrode material particles in the cross section of the positive electrode sheet was observed and measured using a scanning electron microscope (instrument model: ZEISS SEM, accelerating voltage: 0.1kV~30kV). The length of the positive electrode material is the diameter of the smallest circumscribed circle of the positive electrode material particles, and the diameter of the positive electrode material is the diameter of the largest inscribed circle of the positive electrode material particles.

[0090] For the positive electrode, take the lithium-ion battery, remove the positive electrode, and then cut the positive electrode using ion polishing. Use a scanning electron microscope (instrument model: ZEISS SEM, accelerating voltage: 0.1kV~30kV) to observe and measure the morphology of the positive electrode material particles in the cross-section of the positive electrode. The length of the positive electrode material is the diameter of the smallest circumscribed circle of the positive electrode material particles, and the diameter of the positive electrode material is the diameter of the largest inscribed circle of the positive electrode material particles.

[0091] (6) Calendar lifespan

[0092] Take a lithium-ion battery, place it in a constant temperature environment of 25℃ and let it stand for 30 minutes. Charge it to 5V with a constant current of 0.5C, then charge it to 0.025C with a constant voltage of 5V. Then discharge it to 2.5V with a constant current of 0.5C. Record the discharge capacity at this time as the initial discharge capacity.

[0093] Take a lithium-ion battery, place it in a constant temperature environment of 25℃ for 30 minutes, charge it with a constant current of 0.5C to 5V, then charge it with a constant voltage of 5V to a current of 0.025C, and then store it in a constant temperature chamber of 25℃ for 300 days. After that, take it out and discharge it with a constant current of 0.5C to 2.5V. Record the discharge capacity at this time as the storage discharge capacity. Calculate the storage capacity retention rate of the lithium-ion battery and use it as an indicator to evaluate the calendar life of the lithium-ion battery.

[0094] Storage capacity retention rate % = Storage discharge capacity / Initial discharge capacity × 100%.

[0095] (7) Rate Discharge Test

[0096] Take a lithium-ion battery, place it at 25℃, charge it to 5V at a constant current rate of 0.2C, then charge it at a constant voltage rate until the current is less than or equal to 0.05C, then let it stand for 30 minutes, and then discharge it to 2.5V at a constant current rate of 0.2C. The discharge capacity of the lithium-ion battery at 25℃ and 0.2C rate is then measured.

[0097] Take a lithium-ion battery, place it at 25℃, charge it at a constant current rate of 0.2C to 5V, then charge it at a constant voltage rate until the current is less than or equal to 0.05C, and then discharge it at a constant current rate of 2C to 2.5V. The discharge capacity of the lithium-ion battery at the 2C rate is then measured.

[0098] 2C rate discharge capacity retention % = 2C rate discharge capacity / 0.2C rate discharge capacity × 100%.

[0099] The lithium-ion batteries in the following embodiments or comparative examples differ from those in Examples 1-1 only in that the types and mass contents of the first, second, and third elements in the cathode material, and the mass content of carbon in the cathode material, are adjusted according to Table 1. The types and contents of each element are controlled by adjusting the preparation parameters during the cathode material preparation process. The performance test results of the lithium-ion batteries in each embodiment and comparative example are shown in Table 1 below.

[0100] Table 1

[0101] *In the table above, the calculation result of M1 / 57.102+M2 / 67.003+M3 / 23.132 is rounded to two decimal places; the proportions shown in Examples 1-2, 1-3 and 1-4 are all element molar ratios.

[0102] As shown in Table 1, the lithium-ion batteries prepared in the embodiments of this application can improve the rate discharge capacity retention and calendar life when the mass content of the first element in the cathode material satisfies 32.5≤M1≤37.5, especially 34≤M1≤37. Similarly, when the mass content of the second element in the cathode material satisfies 0.01≤M2≤1, especially 0.1≤M2≤0.8, the rate discharge capacity retention and calendar life can be improved. Furthermore, when the mass content of the third element in the cathode material satisfies 0.005≤M3≤0.1, the rate discharge capacity retention and calendar life can be improved. Finally, when the mass content of carbon in the cathode material satisfies 0.8≤A≤8, especially 1.2≤A≤6, the rate discharge capacity retention and calendar life can be improved.

[0103] In particular, the lithium-ion battery prepared in the embodiments of this application can further improve the rate discharge capacity retention rate and calendar life of the lithium-ion battery when the relationship between the mass content of the first element, the second element and the third element in the cathode material satisfies 0.57≤M1 / 57.102+M2 / 67.003+M3 / 23.132≤0.66, especially 0.60≤M1 / 57.102+M2 / 67.003+M3 / 23.132≤0.65.

[0104] The lithium-ion batteries in Examples 2-1 to 2-13 differ from those in Examples 1-17 only in that cathode materials with different morphological parameters are selected according to Table 2, and the mass content of nitrogen and the proportion of graphite nitrogen in the cathode materials are adjusted. The proportion of graphite nitrogen in the carbon coating layer is controlled by adjusting the type of nitrogen-containing carbon source and the sintering process parameters.

[0105] Table 2

[0106] As shown in Table 2, the lithium-ion batteries prepared in the embodiments of this application can further improve their rate discharge capacity retention and calendar life when the mass content of nitrogen in the cathode material is adjusted to meet the requirements of 0.03≤B≤5, especially 0.6≤B≤3.4. Similarly, adjusting the proportion of graphite nitrogen in the cathode material to the total nitrogen to meet the requirements of 0.2≤C / B≤0.6 can further improve the rate discharge capacity retention and calendar life. Furthermore, when the diameter of the cathode material meets the requirements of 50≤D≤500 and the aspect ratio meets the requirements of 1≤I / D≤5, the rate discharge capacity retention and calendar life can be further improved.

[0107] 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 principles of this application should be included within the protection scope of this application.

Claims

1. A positive electrode material, characterized in that, It includes a core and a carbon coating layer, the core comprising a lithium phosphate salt comprising a first element, a second element and a third element, and the carbon coating layer covering at least a portion of the surface of the core; The first element includes at least one of iron, manganese, cobalt or nickel, and the mass content of the first element is M1% based on the mass of the cathode material, where 32.5 ≤ M1 ≤ 37.5%. The second element includes at least one of molybdenum, zinc, or calcium, and the mass content of the second element is M2% based on the mass of the cathode material, where 0.01 ≤ M2 ≤ 1. The third element includes at least one of boron or chlorine, and the mass content of the third element is M3% based on the mass of the cathode material, where 0.005 ≤ M3 ≤ 0.1%. The carbon coating layer includes carbon elements, and the mass content of the carbon elements is A% based on the mass of the cathode material, with a mass content of 0.8 ≤ A ≤ 8%.

2. The cathode material according to claim 1, characterized in that, The positive electrode material satisfies 0.57≤M1 / 57.102+M2 / 67.003+M3 / 23.132≤0.

66.

3. The cathode material according to claim 2, characterized in that, The cathode material satisfies at least one of the following conditions: (1)34≤M1≤37； (2)0.1≤M2≤0.8； (3)0.60≤M1 / 57.102+M2 / 67.003+M3 / 23.132≤0.65; (4)1.2≤A≤6。 4. The cathode material according to any one of claims 1 to 3, characterized in that, The carbon coating layer includes nitrogen. Based on the mass of the cathode material, the mass content of nitrogen is B%, 0.03≤B≤5.

5. The positive electrode material according to claim 4, characterized in that, The positive electrode material satisfies 0.6≤B≤3.

4.

6. The cathode material according to claim 4, characterized in that, The nitrogen element in the carbon coating layer includes graphitic nitrogen; Based on the mass of the cathode material, the mass content of graphite nitrogen is C%, and 0.2≤C / B≤0.

6.

7. The cathode material according to any one of claims 1 to 3, characterized in that, The length of the positive electrode material is 1 nm, the diameter is D nm, 50≤D≤500, and 1≤I / D≤5.

8. The cathode material according to any one of claims 1 to 3, characterized in that, The ratio of the D / G peaks of the carbon coating layer is N, where 0.9 ≤ N ≤ 1.

2.

9. A secondary battery, characterized in that, It includes a positive electrode, the positive electrode comprising a positive electrode current collector and a positive electrode material layer located on at least one surface of the positive electrode 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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