Positive electrode material, secondary battery and electronic apparatus

Abstract

Disclosed in the present application are a positive electrode material, a secondary battery and an electronic apparatus. The positive electrode material comprises an inner core and a carbon coating layer, the carbon coating layer coating at least part of the surface of the inner core. The inner core comprises a lithium phosphate salt, and the lithium phosphate salt comprises a first element and a second element; the first element comprises at least one of iron, manganese, cobalt or nickel; on the basis of the mass of the positive electrode material, the mass content of the first element is m%, where 32.5≤m≤37.5; the second element comprises at least one of chromium, yttrium or tungsten; on the basis of the mass of the positive electrode material, the mass content of the second element is n%, where 0.01≤n≤1. The carbon coating layer comprises carbon; on the basis of the mass of the positive electrode material, the mass content of carbon is A%, where 0.8≤A≤3. The secondary battery provided in the present application has better electrochemical performance, especially better discharge specific capacity and better low-temperature performance.

Description

Positive electrode materials, secondary batteries and electronic devices Technical Field

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

[0002] With the increasing demand for electronic products such as mobile phones, laptops, and cameras, secondary batteries are playing an increasingly important role in our daily lives as the power source for these products, thus driving the need for improved secondary battery performance. Lithium phosphate cathode materials, with their advantages of high safety, long lifespan, environmental friendliness, and low cost, have been widely used in energy storage systems, but they also suffer from relatively small discharge capacity and poor low-temperature discharge performance. Therefore, providing cathode materials with superior discharge capacity while also maintaining good low-temperature discharge performance is of paramount importance. Summary of the Invention

[0003] This application provides a positive electrode material, a secondary battery, and an electronic device. The secondary battery achieves a better discharge capacity while also improving low-temperature performance.

[0004] In a first aspect, embodiments of this application provide a cathode material, including a core and a carbon coating layer, wherein the carbon coating layer covers at least a portion of the surface of the core; the core includes a lithium phosphate salt, wherein the lithium phosphate salt includes a first element and a second element; the first element includes at least one selected from iron, manganese, cobalt, or nickel, and the mass content of the first element is m% based on the mass of the cathode material, 32.5≤m≤37.5; the second element includes at least one selected from chromium, yttrium, or tungsten, and the mass content of the second element is n% based on the mass of the cathode material, 0.01≤n≤1; the carbon coating layer includes carbon, and the mass content of the carbon element is A% based on the mass of the cathode material, 0.8≤A≤3.

[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 the discharge specific capacity and conductivity of the cathode material. Furthermore, by doping the core of the lithium phosphate cathode material containing the first element iron, manganese, cobalt, or nickel with the second element chromium, yttrium, or tungsten, the introduced second element can partially replace the sites of the first element, forming a stable solid solution structure, increasing the electronic conductivity of the material, and reducing the polarization effect at low temperatures. It can also suppress lattice contraction and phase transitions at low temperatures, reducing stress accumulation during charge and discharge, thereby improving low-temperature cycle stability. Additionally, it can introduce defects or optimize lattice parameters, reducing the activation energy of lithium-ion diffusion and improving ion transport efficiency at low temperatures. Thus, in synergy with the nitrogen-containing carbon coating layer, it can improve the specific capacity of the secondary battery while also improving low-temperature performance.

[0006] In some embodiments, the above-mentioned cathode material satisfies 0.58≤m / 57.102+n / 108.247≤0.66. Based on the above embodiments, this application can further improve the specific capacity of the secondary battery while also improving low-temperature performance by controlling the mass content relationship between the first element and the second element in the lithium phosphate cathode material within the above range.

[0007] In some embodiments, the above-mentioned cathode material satisfies at least one of the following conditions: (1) 1.2 ≤ A ≤ 2.6; (2) 34.0 ≤ m ≤ 37.0; (3) 0.1 ≤ n ≤ 0.8; (4) 0.60 ≤ m / 57.102 + n / 108.247 ≤ 0.65. Based on the above embodiments, this application adjusts the mass content of the first element and the second element or their relationship in the cathode material to meet the above range, or the mass content of nitrogen in the cathode material to meet the above range, so that the content of each element in the cathode material has a relatively balanced ratio, which can further improve the specific capacity of the secondary battery while also improving the low-temperature performance.

[0008] In some embodiments, the carbon coating layer includes nitrogen, and the mass percentage of nitrogen is B%, 0.2 ≤ B ≤ 5%, based on the mass of the cathode material. Based on the above embodiments, this application dops a specific amount of nitrogen into the carbon coating layer on the surface of the polyanionic lithium phosphate material. Nitrogen atoms have a smaller atomic radius and greater electronegativity than carbon atoms. Incorporating nitrogen into the graphite structure of carbon nanomaterials alters the atomic and electronic structure of the matrix and increases its conductivity, thereby synergistically improving the specific capacity of the secondary battery while also improving low-temperature performance.

[0009] In some embodiments, the above-mentioned cathode material satisfies 0.3≤B≤4. Based on the above embodiments, this application can further improve the specific capacity of the secondary battery while also improving low-temperature performance by further satisfying the above range in the nitrogen content of the carbon coating layer on the surface of the lithium phosphate salt material.

[0010] In some embodiments, the nitrogen element in the carbon coating layer includes pyrrole nitrogen; based on the mass of the cathode material, the mass content of the pyrrole nitrogen is C%, 0.05≤C / B≤0.2.

[0011] In some embodiments, the nitrogen element in the carbon coating layer may also be in the form of at least one of pyridine nitrogen, amino nitrogen, graphitic nitrogen, or nitrogen oxide.

[0012] Based on the above embodiments, when the nitrogen element in the carbon coating layer on the surface of the lithium phosphate material exists in the form of pyrrole nitrogen and the mass content of pyrrole nitrogen meets the above range, a delocalized conjugated system of sp2 hybrid carbon can be formed, which improves the conductivity of the carbon coating layer. At the same time, the conjugated electrons of the nitrogen-doped carbon structure coordinate with the empty orbitals of the transition metal on the surface of the core lithium phosphate material, which improves the binding force and enhances the stability of the surface-coated carbon layer. This further improves the specific capacity of the secondary battery while also improving the low-temperature performance.

[0013] In some embodiments, the D / G peak ratio of the carbon coating layer is M, where 0.90 ≤ M ≤ 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 synergistically improving the specific capacity of the secondary battery with the lithium phosphate material, while also improving low-temperature performance.

[0014] 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 relatively conducive to lithium-ion diffusion, which can further improve the specific capacity of the secondary battery while also improving low-temperature performance.

[0015] In some embodiments, the chemical formula of the above-mentioned cathode material is LiMPO4. The M element includes a first element and a second element. Based on the above embodiments, the cathode material of this application includes the above-mentioned lithium phosphate salt, which can further improve the low-temperature performance of the secondary battery.

[0016] 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.

[0017] Thirdly, embodiments of this application provide an electronic device that includes the aforementioned secondary battery. Detailed Implementation

[0018] 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.

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

[0022] The cathode material includes a core and a carbon coating layer, the carbon coating layer covering at least a portion of the surface of the core. The core includes a lithium phosphate salt, which includes a first element and a second element. 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 m%, 32.5 ≤ m ≤ 37.5%, preferably 34.0 ≤ m ≤ 37.0. For example, the mass content of the first element in the cathode material can be 32.5, 32.9, 33.8, 34.0, 34.6, 35.7, 35.9, 36.7, 37.0, 37.5, or a value within the range of any two of these values. The second element includes at least one of chromium, yttrium, or tungsten. Based on the mass of the cathode material, the mass content of the second element is n%, 0.01 ≤ n ≤ 1%, preferably 0.1 ≤ n ≤ 0.8. For example, the mass content of the second element in the cathode material can be 0.01, 0.08, 0.10, 0.18, 0.38, 0.40, 0.53, 0.66, 0.80, 0.87, 0.94, 1, or a value within any two of these ranges. The carbon coating layer includes carbon, and based on the mass of the cathode material, the mass content of the aforementioned carbon element is A%, 0.8 ≤ A ≤ 3, preferably 1.2 ≤ A ≤ 2.6. For example, the mass content of carbon in the cathode material can be 0.8, 1.0, 1.2, 1.3, 1.6, 1.9, 2.4, 2.6, 2.9, 3, or a value within any two of these ranges. By doping a second element into the core of a lithium phosphate cathode material containing the first element and coating the surface with a carbon layer, the electronic conductivity of the material can be increased, the polarization effect at low temperature can be reduced, the lattice shrinkage and phase transition at low temperature can be suppressed, and the low-temperature cycle stability can be improved. Furthermore, defects can be introduced or lattice parameters can be optimized to improve the ion transport efficiency at low temperature, thereby synergistically improving the specific capacity of the secondary battery and improving the low-temperature performance.

[0023] In some embodiments, the cathode material satisfies 0.58 ≤ m / 57.102 + n / 108.247 ≤ 0.66. In some embodiments, the cathode material satisfies 0.60 ≤ m / 57.102 + n / 108.247 ≤ 0.65. In some embodiments, the cathode material satisfies 0.58 ≤ m / 57.102 + n / 108.247 ≤ 0.64. In some embodiments, the cathode material satisfies 0.62 ≤ m / 57.102 + n / 108.247 ≤ 0.65. In some embodiments, the cathode material satisfies 0.60 ≤ m / 57.102 + n / 108.247 ≤ 0.63. By controlling the mass content relationship between the first and second elements in the lithium phosphate cathode material within the above ranges, it is possible to further improve the specific capacity of the secondary battery while also improving low-temperature performance.

[0024] In some embodiments, the carbon coating layer includes nitrogen, with the nitrogen content at a mass percentage of B%, 0.2 ≤ B ≤ 5, preferably 0.3 ≤ B ≤ 4, based on the mass of the cathode material. For example, the mass percentage of nitrogen in the cathode material can be 0.2, 0.3, 0.9, 1.4, 2.4, 2.6, 3.9, 4.0, 4.3, 5, or any combination thereof. By doping the carbon coating layer on the surface of the polyanionic lithium phosphate material with a specific amount of nitrogen, the atomic and electronic structure of the graphite matrix can be altered, increasing its conductivity, thereby synergistically improving the specific capacity of the secondary battery while also improving low-temperature performance.

[0025] In some embodiments, the nitrogen element in the carbon coating layer includes pyrrole nitrogen. Based on the mass of the cathode material, the mass content of the pyrrole nitrogen is C%, with a value of 0.05 ≤ C / B ≤ 0.2. For example, the ratio of the mass of pyrrole nitrogen to the total nitrogen mass in the cathode material can be 0.05, 0.10, 0.12, 0.13, 0.15, 0.18, 0.2, or a value within any two of these ranges. When the mass content of pyrrole nitrogen in the carbon coating layer on the surface of the lithium phosphate material meets the above range, it is possible to further improve the specific capacity of the secondary battery while also improving low-temperature performance.

[0026] In some embodiments, the D / G peak ratio of the carbon coating layer is M, where 0.90 ≤ M ≤ 1.2. For example, the D / G peak ratio of the carbon coating layer can be 0.9, 1.0, 1.1, 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 synergistically improve the specific capacity of the secondary battery while also improving low-temperature performance.

[0027] In some embodiments, the length of the positive electrode material is 1 nm, the diameter is D nm, 50 ≤ D ≤ 500, and 1 ≤ 1 / D ≤ 5. For example, the diameter of the positive electrode material can be 50, 100, 138, 216, 313, 407, 469, 500, or any value within the range of any two of these values. For example, the aspect ratio of the positive electrode 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 positive electrode material within the above ranges, the specific capacity of the secondary battery can be further improved while simultaneously improving low-temperature performance.

[0028] In some embodiments, the chemical formula of the above-mentioned cathode material is LiMPO4. The M element includes a first element and a second element. For example, the chemical formula of the cathode material could be LiFe. 0.5 Co 0.48 Cr 0.02 PO4, LiFe 0.5 Mn 0.494 W 0.006 PO4 or LiNi 0.49 Mn 0.5 Y 0.01 PO4. Based on the above embodiments, the cathode material of this application includes the above-mentioned lithium phosphate salt, which can further improve the low-temperature performance of the secondary battery.

[0029] In some embodiments, the preparation method of the above-mentioned positive electrode material includes the following steps:

[0030] (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 at a mass ratio of 5% to 8% of the total mass of the synthesized cathode material, add solvent and perform the first grinding treatment to obtain the first grinding slurry;

[0031] (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;

[0032] (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;

[0033] (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.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] negative electrode

[0041] 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.

[0042] 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.

[0043] The negative electrode material layer also includes a negative electrode binder. The negative electrode binder improves the bonding between negative electrode material particles and the bonding between the negative electrode 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 solvent used in the electrolyte or electrode manufacturing process. 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.

[0044] 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.

[0045] 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.

[0046] 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.

[0047] 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.

[0048] electrolyte

[0049] The electrolyte used in the secondary battery of this application includes a lithium salt and a non-aqueous solvent for dissolving the lithium salt.

[0050] This application does not impose any particular limitation on the type of lithium salt, as long as it achieves the purpose of this application. For example, the lithium salt may include, but is not limited to, at least one of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium difluorophosphate (LiPO2F2), lithium bis(fluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(oxalateborate)borate (LiBOB), or lithium difluorooxalateborate (LiDFOB). Based on the mass of the electrolyte, the mass percentage of the lithium salt may be from 8% to 15%, for example, the mass percentage of the lithium salt may be 8%, 9%, 10%, 11%, 12.5%, 13%, 15%, or a range of any two of these values.

[0051] 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.

[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, 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.

[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 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.

[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 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.

[0057] 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.

[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 cathode materials>

[0063] (1) Weigh out the M metal source (M metal molar ratio is Fe:Mn:Cr = 0.12:0.7697:0.0003), phosphorus source and lithium source in a molar ratio of transition M metal:phosphorus:lithium = 0.89:1:1, add polyacrylamide at a mass of 6% of the total mass of the synthesized cathode material, add deionized water and perform the first grinding treatment to obtain the first grinding slurry;

[0064] (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.

[0065] (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 the second grinding material;

[0066] (4) The second grinding material is dried, and then subjected to a second calcination treatment. It is calcined at 800℃ for 8 hours under a nitrogen atmosphere to obtain nitrogen-doped carbon-coated cathode material LiFe. 0.12 Mn 0.7697 Cr 0.0003 PO4.

[0067] <Preparation of the positive electrode>

[0068] The above steps yield the cathode material LiFe. 0.12 Mn 0.7697 Cr 0.0003PO4, conductive carbon black (a conductive agent), and polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 95:2:3. N-methylpyrrolidone (NMP) was added, and 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 used as a positive electrode current collector, and then dried to obtain a positive electrode sheet with a single-sided coating of positive electrode material. The above steps were repeated on the other surface of the aluminum foil 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 positive electrode sheets with dimensions of 74 mm × 867 mm.

[0069] 2. Preparation of electrolyte

[0070] In a dry argon atmosphere glove box, diethyl carbonate was used as the base solvent, and lithium hexafluorophosphate (LiPF6) was dissolved in the base solvent. Ethylene carbonate was added as an additive to obtain the electrolyte. Based on the total mass of the electrolyte, the mass percentage of LiPF6 was 12.5%, and the mass percentage of vinylene carbonate was 2%.

[0071] 3. Preparation of the negative electrode

[0072] Using artificial graphite as the negative electrode material, a mixture of the negative electrode 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%. This 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.

[0073] 4. Preparation of the diaphragm

[0074] A porous polyethylene film with a thickness of 15μm was used as the diaphragm.

[0075] 5. Preparation of lithium-ion batteries

[0076] 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.

[0077] 6. Testing Methods

[0078] (1) Metal element content test

[0079] 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 percentage of each metal element and nitrogen element in the solution was tested using an ICP analyzer.

[0080] 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 percentage content of each metal element and nitrogen element in the solution.

[0081] (2) Nitrogen content test

[0082] 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.

[0083] 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.

[0084] (3) Carbon content test

[0085] 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.

[0086] (4) Nitrogen element speciation test

[0087] 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);

[0088] 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).

[0089] (5) Particle morphology measurement

[0090] Remove the positive electrode from the lithium-ion battery, then cut the positive electrode using ion polishing, and observe and measure the morphology of the positive electrode material particles in the cross-section of the positive electrode using a scanning electron microscope (instrument model: ZEISS SEM, accelerating voltage: 0.1kV~30kV).

[0091] (6) Discharge capacity

[0092] Place the lithium-ion battery at 25°C and first charge it with a constant current of 0.5C until it reaches 4.35V. Then charge it with a constant voltage until the current reaches 0.05C. Finally, discharge it with a constant current of 0.2C until it reaches 2.8V.

[0093] Discharge capacity = Discharge capacity at 0.2C / Mass of cathode material.

[0094] (7) Low temperature performance test

[0095] Take a lithium-ion battery and place it in a 0℃ constant temperature test chamber for 30 minutes to allow the lithium-ion battery to reach a constant temperature. Charge it to 4.5V at a constant current of 0.5C, then charge it to 0.025C at a constant voltage of 4.5V. Let it stand for 5 minutes and record the charging capacity as C0. Then discharge it to 3.0V at a constant current of 0.5C and record the discharge capacity as C1.

[0096] Low-temperature capacity retention % = C1 / C0 × 100%.

[0097] 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 and second elements in the cathode material are shown in Table 1, and the mass content of carbon is also shown in Table 1. The performance test results of the lithium-ion batteries in each embodiment and comparative example are shown in Table 1 below.

[0098] Table 1 *In the table above, the result of the calculation of m / 57.102+n / 108.247 is rounded to two decimal places.

[0099] As shown in Table 1, the lithium-ion batteries prepared in the embodiments of this application can improve the low-temperature capacity retention and discharge capacity when the mass percentage of the first element in the cathode material satisfies 32.5 ≤ m ≤ 37.5, especially 34.0 ≤ m ≤ 37.0. Similarly, the low-temperature capacity retention and discharge capacity can be improved when the mass percentage of the second element in the cathode material satisfies 0.01 ≤ n ≤ 1, especially 0.1 ≤ n ≤ 0.8. Furthermore, the low-temperature capacity retention and discharge capacity can be improved when the carbon coating layer of the cathode material includes nitrogen and its mass content is 0.8 ≤ A ≤ 3, especially 1.2 ≤ A ≤ 2.6. When the mass content relationship between the first and second elements in the cathode material satisfies 0.58≤m / 57.102+n / 108.247≤0.66, especially 0.60≤m / 57.102+n / 108.247≤0.65, the low-temperature capacity retention rate and discharge capacity of lithium-ion batteries can be further improved.

[0100] The lithium-ion batteries in Examples 2-1 to 2-16 differ from those in Examples 1-16 only in that the type and mass content of nitrogen in the coating layer are adjusted according to Table 2, and cathode materials with different morphological parameters are selected. Specifically, the proportion of pyrrole nitrogen atoms in the carbon coating layer is controlled by adjusting the type of nitrogen-containing carbon source and the sintering process parameters.

[0101] Table 2

[0102] As shown in Table 2, the lithium-ion batteries prepared in the embodiments of this application can further improve their low-temperature capacity retention and discharge capacity when the mass percentage of nitrogen in the cathode material is controlled to meet the requirements of 0.2 ≤ B ≤ 5, especially 0.3 ≤ B ≤ 4. Similarly, controlling the mass content of pyrrole nitrogen in the carbon coating layer of the cathode material to meet the requirements of 0.05 ≤ C / B ≤ 0.2 can further improve the low-temperature capacity retention and discharge capacity. Furthermore, controlling the D / G peak ratio of the carbon coating layer of the cathode material to meet the requirements of 0.90 ≤ M ≤ 1.2 can further improve the low-temperature capacity retention and discharge capacity. Finally, controlling the diameter of the cathode material to meet the requirements of 50 ≤ D ≤ 500 and the aspect ratio to meet the requirements of 1 ≤ I / D ≤ 5 can further improve the low-temperature capacity retention and discharge capacity.

[0103] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A positive electrode material, characterized in that, It includes a core and a carbon coating layer, wherein the carbon coating layer covers at least a portion of the surface of the core; The core comprises a lithium phosphate salt, which comprises a first element and a second element; The first element includes at least one of iron, manganese, cobalt or nickel, and the mass content of the first element is m% based on the mass of the cathode material, where 32.5 ≤ m ≤ 37.5%. The second element includes at least one of chromium, yttrium, or tungsten, and the mass content of the second element is n% based on the mass of the cathode material, with 0.01 ≤ n ≤ 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 ≤ 3%.

2. The cathode material according to claim 1, characterized in that, The positive electrode material satisfies 0.58≤m / 57.102+n / 108.247≤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)1.2≤A≤2.6； (2)34.0≤m≤37.0； (3)0.1≤n≤0.8； (4)0.60≤m / 57.102+n / 108.247≤0.

65.

4. The cathode material according to any one of claims 1 to 3, characterized in that, The carbon coating layer includes nitrogen, and the mass percentage of nitrogen is B%, 0.2 ≤ B ≤ 5, based on the mass of the cathode material.

5. The cathode material according to claim 4, characterized in that, The positive electrode material satisfies 0.3≤B≤4.

6. The cathode material according to claim 5, characterized in that, The nitrogen in the carbon coating layer includes pyrrole nitrogen; Based on the mass of the cathode material, the mass content of the pyrrole nitrogen is C%, and 0.05≤C / B≤0.

2.

7. 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 is M, where 0.90 ≤ M ≤ 1.

2.

8. 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.

9. A secondary battery, characterized in that, 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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