Positive electrode material, electrochemical device and preparation method of positive electrode material

Abstract

The invention provides a positive electrode material, an electrochemical device and a preparation method of the positive electrode material, and belongs to the technical field of secondary batteries, and the positive electrode material comprises an inner core and a coating layer. Wherein the inner core comprises a lithium iron phosphate material, the outer side of the inner core is coated with the coating layer, the coating layer comprises a carbon material, the mass content of the carbon material in the positive electrode material is 1%-1.5%, the powder compaction density of the positive electrode material is larger than 2.6 g / cm < 3 >, and the powder resistivity of the positive electrode material is smaller than 30 omega.cm. According to the positive electrode material disclosed by the invention, the particle size of the lithium iron phosphate material is integrally reduced, the conductivity of the lithium iron phosphate material is improved, and the compaction density of the lithium iron phosphate material cannot be greatly attenuated due to the reduction of the particle size of the material by utilizing the grading of large and small particles in the lithium iron phosphate material; therefore, the positive electrode material maintains good compaction density on the premise that the overall particle size is reduced, and the energy density and the conductivity of the lithium iron phosphate positive electrode material are both improved.

Description

Technical Field

[0001] This invention relates to the field of secondary battery technology, and in particular to a cathode material, an electrochemical device, and a method for preparing the cathode material. Background Technology

[0002] In recent years, lithium-ion batteries have been widely used in 3C digital products, electric vehicles, energy storage systems, and other fields. The ever-increasing market demand has also placed higher performance requirements on them. As an important component of lithium-ion batteries, the cathode material's properties, to a certain extent, determine the overall performance of the lithium-ion battery.

[0003] Lithium iron phosphate materials occupy a large market share due to their excellent electrochemical performance and thermochemical stability. Compared with nickel-cobalt-manganese ternary layered materials and nickel-manganese spinel materials, they have better cost performance, safety and cycle stability. However, the lower energy density and conductivity of lithium iron phosphate materials also limit their further development.

[0004] Therefore, it is necessary to design a cathode material, an electrochemical device, and a method for preparing the cathode material to improve the above-mentioned problems. Summary of the Invention

[0005] This invention provides a cathode material, an electrochemical device, and a method for preparing the cathode material, which improves the technical problem of poor conductivity of lithium iron phosphate material while ensuring that the energy density of lithium iron phosphate material does not decrease due to modification.

[0006] In a first aspect, the present invention provides a cathode material comprising a core and a coating layer, wherein the carbon material comprises 1% to 1.5% by mass in the cathode material.

[0007] The core consists of lithium iron phosphate material, and a coating layer covering the outside of the core, comprising carbon material, is used. The powder compaction density of the cathode material is greater than 2.6 g / cm³. 3 The resistivity of the positive electrode material powder is less than 30 Ω·cm.

[0008] In one example of the present invention, the cathode material is doped with a metal dopant element, and the mass content of the metal dopant element in the cathode material is 500ppm to 5000ppm. The metal dopant element includes at least one of Al, Ti, Mg, Nb, Zr, Mn, Co, Zn, V, and Sn.

[0009] In a second aspect, the present invention also provides a method for preparing a cathode material, the method comprising:

[0010] The raw materials are mixed in a solvent according to a preset ratio to obtain a first slurry; the raw materials include lithium source, iron source, phosphorus source and carbon source;

[0011] The first slurry is ground to obtain a second slurry; the D50 particle size of the second slurry is 0.3 to 0.6 μm, and the D100 particle size of the second slurry is less than 2 μm;

[0012] The second slurry is dried to obtain the precursor material;

[0013] The precursor material is placed under a preset pressure environment, and the ambient temperature is raised to a first plateau temperature at a preset heating rate. The precursor material is then sintered once at the first plateau temperature. The ambient temperature is then cooled to a second plateau temperature at a preset cooling rate, and the precursor material is sintered a second time at the second plateau temperature. Finally, the precursor material is cooled to obtain the cathode material.

[0014] The preset pressure is 100-500 Pa, the temperature of the first platform is greater than or equal to 800°C, the first sintering time is 1 to 2 hours, the temperature of the second platform is 760°C to 800°C, and the second sintering time is 8 to 10 hours.

[0015] In one example of the present invention, the preset heating rate is 1.0 to 2.0 °C / min.

[0016] In one example of the present invention, the preset cooling rate is 0.4 to 0.8 °C / min.

[0017] In one example of the present invention, the temperature of the first platform is 800℃~820℃.

[0018] In one example of the present invention, the temperature of the second platform is 760°C to 780°C.

[0019] In one example of the present invention, the molar ratio of Li, Fe, and P elements in the lithium source, iron source, and phosphorus source is (1.00–1.05):(0.95–0.99):1; and the mass content of the carbon source in the raw materials is 10%–14%.

[0020] In one example of the present invention, the raw material further includes a dopant, the mass content of which is 0.05% to 0.5%; the dopant is an oxide containing a metal dopant element, the metal dopant element including at least one of Al, Ti, Mg, Nb, Zr, Mn, Co, Zn, V, and Sn.

[0021] In a third aspect, the present invention also provides an electrochemical device comprising a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the positive electrode comprises a positive current collector and a positive active material layer, and the positive active material layer comprises the positive electrode material described in any of the preceding examples, or the positive electrode material prepared by the preparation method described in any of the preceding examples.

[0022] The cathode material provided by this invention reduces the overall particle size of lithium iron phosphate material and improves its conductivity. At the same time, it utilizes the particle size distribution within the lithium iron phosphate material to ensure that the compaction density of the lithium iron phosphate material does not decrease significantly due to the reduction in particle size. This allows the cathode material to maintain good compaction density while reducing the overall particle size, thereby improving both the energy density and conductivity of the lithium iron phosphate cathode material.

[0023] This invention provides a method for preparing cathode materials. After reducing the particle size of the precursor material through grinding, the precursor material is first sintered at a relatively high temperature (greater than or equal to 800°C) for a short time under a preset pressure environment. This allows some of the precursor material to preferentially crystallize into larger lithium iron phosphate particles. Then, the temperature is lowered to a lower temperature (800°C and below) to continue sintering the precursor material for a longer time, allowing the remaining precursor material to form smaller lithium iron phosphate particles. This method produces a cathode material with a natural particle size distribution while reducing the overall particle size. In summary, this preparation method controls the size and ratio of large and small particles in the sintered lithium iron phosphate material by adjusting the furnace pressure and temperature curves during the precursor material sintering process, obtaining lithium iron phosphate material with excellent particle size distribution, thereby improving both the conductivity and energy density of the lithium iron phosphate cathode material. Attached Figure Description

[0024] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application. It is obvious that the drawings described below are merely some embodiments of this application, and those skilled in the art can obtain other embodiments based on these drawings without inventive effort.

[0025] In the attached diagram:

[0026] Figure 1 This is a scanning electron microscope image of the positive electrode material in Embodiment 1 of the present invention;

[0027] Figure 2 This is a scanning electron microscope image of the cathode material in Comparative Example 1 of the present invention;

[0028] Figure 3 This is a schematic flowchart of a method for preparing a positive electrode material in one embodiment of the present invention;

[0029] Figure 4 This is a temperature-time curve of the precursor material sintering process in Embodiment 1 of the present invention. Detailed Implementation

[0030] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that, unless otherwise specified, the following embodiments and features can be combined with each other. It should also be understood that the terminology used in the embodiments of the present invention is for describing specific implementation schemes and not for limiting the scope of protection of the present invention. Test methods in the following embodiments that do not specify specific conditions are generally performed under conventional conditions or according to the conditions recommended by the respective manufacturers.

[0031] It should be noted that the terms such as "upper", "lower", "left", "right", "middle" and "one" used in this specification are only for clarity of description and are not intended to limit the scope of the invention. Changes or adjustments to their relative relationships, without substantially altering the technical content, should also be considered as part of the scope of the invention.

[0032] In this specification, D50, D99, and D100 particle sizes are defined as the equivalent particle diameters corresponding to the cumulative proportion of particle size in the particle size distribution curve reaching 50%, 99%, and 100%, respectively. The D50 particle size can be measured, for example, by laser diffraction. The D50, D99, and D100 particle sizes can be obtained using a Malvern laser particle size analyzer (Master Size 3000). Laser diffraction can typically measure particle sizes from submicron to several millimeters, thus providing highly reproducible and high-resolution results.

[0033] To address the technical shortcomings of low lithium-ion diffusion coefficient and electronic conductivity in lithium iron phosphate (LFP) cathode materials, the industry often uses techniques such as particle nano-sizing and conductive carbon coating to modify and optimize LFP materials. However, due to the low intrinsic density of LFP materials, nano-sizing and carbon coating further reduce the compaction density of the material, negatively impacting battery energy density.

[0034] Based on this, this application provides a cathode material that, while reducing the particle size of lithium iron phosphate material, utilizes the particle size distribution within the lithium iron phosphate material to ensure that the compaction density of the lithium iron phosphate material does not decrease significantly due to the reduction in particle size, thereby maintaining a good compaction density of the cathode material and thus improving both the energy density and conductivity of the lithium iron phosphate cathode material.

[0035] like Figure 1 As shown, in one aspect, this application provides a cathode material comprising a core and a coating layer. The core comprises lithium iron phosphate material, and the coating layer, which covers the outside of the core, comprises a carbon material. The coating layer can improve the conductivity of the cathode material and reduce side reactions that occur due to direct contact between the core and the electrolyte.

[0036] The D50 particle size of the cathode material can be any value within the range of 0.7 to 1.5 μm, for example, 0.7 μm, 0.8 μm, 0.9 μm, 1 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.4 μm or 1.5 μm; the D99 particle size of the cathode material can be any value within the range of 4 to 8 μm, for example, 4 μm, 4.5 μm, 5 μm, 5.5 μm, 6 μm, 6.5 μm, 7 μm, 7.5 μm or 8 μm.

[0037] The cathode material includes particles of a first diameter and particles of a second diameter. The particle size of the first diameter particles is 1–4 μm, and the particle size of the second diameter particles is 0.2–0.6 μm. The volume ratio of the first diameter particles to the second diameter particles is 1:(1–2), for example, 1:1, 1:1.3, 1:1.5, 1:1.7, or 1:2. This cathode material utilizes the combination of the first diameter particles and the second diameter particles to increase the compaction density of the cathode material while reducing the overall particle size, thus enabling the cathode material to achieve both higher volumetric energy density and better conductivity.

[0038] The compacted density of the cathode material powder is greater than 2.6 g / cm³. 3 The compaction density of the positive electrode sheet assembled with the positive electrode material is greater than 2.7 g / cm³. 3 Meanwhile, the powder resistivity of the cathode material is less than 30 Ω·cm, and the discharge coulombic efficiency of the battery assembled with this cathode material is higher than 98% at a 0.1C rate current, and the discharge specific capacity is higher than 140 mAh / g at a 1C rate current. It is evident that this cathode material possesses both good volumetric energy density and conductivity, which can further improve the energy density and rate performance of the assembled battery.

[0039] In some embodiments, the mass content of carbon material in the coating layer in the cathode material is any value in the range of 1% to 1.5%, for example, the mass content of carbon material can be 1%, 1.1%, 1.2%, 1.3%, 1.4%, or 1.5%. Furthermore, the carbon material in the coating layer can be any one or more conventional conductive carbon materials, such as in some embodiments, where the carbon material is selected from at least one of graphene, carbon nanotubes, hard carbon, soft carbon, carbon fiber, and carbon black.

[0040] In some embodiments, the cathode material is further doped with a metal dopant element, the mass content of which is 500ppm to 5000ppm, and the metal dopant element includes at least one of Al, Ti, Mg, Nb, Zr, Mn, Co, Zn, V, and Sn.

[0041] In a second aspect, the present invention also provides a method for preparing a cathode material, such as... Figure 3 As shown, the preparation method of this cathode material includes the following steps:

[0042] S1. Mix the raw materials in a solvent according to a preset ratio to obtain the first slurry;

[0043] S2. Grind the first slurry to obtain a second slurry; the D50 particle size of the second slurry is 0.3 to 0.6 μm, and the D100 particle size of the second slurry is less than 2 μm;

[0044] S3. The second slurry is dried to obtain the precursor material;

[0045] S4. The precursor material is placed under a preset pressure environment, and the ambient temperature is raised to a first plateau temperature at a preset heating rate. The precursor material is then sintered once at the first plateau temperature. The ambient temperature is then lowered to a second plateau temperature at a preset cooling rate, and the precursor material is sintered a second time at the second plateau temperature. Finally, the precursor material is cooled to obtain the cathode material. The preset pressure is 100-500 Pa, the first plateau temperature is greater than or equal to 800°C, the first sintering time is 1-2 hours, the second plateau temperature is 760°C-800°C, and the second sintering time is 8-10 hours.

[0046] This preparation method utilizes grinding to reduce the overall particle size of the precursor material, and then sintersing the precursor material under a preset pressure environment by first heating and then gradually cooling. This allows the precursor material to first form a small amount of large-particle lithium iron phosphate material at a first plateau temperature, and then form a larger amount of small-particle lithium iron phosphate material at a second plateau temperature. Thus, by continuously sintering, a well-graded lithium iron phosphate cathode material is produced, which has both small particle size characteristics and high compaction density.

[0047] In step S1, the components of the raw materials are added to the solvent one by one from the smallest to the largest mass for mixing, and the mixture is then used to prepare a first slurry with a solid content of 30% to 40%.

[0048] In some embodiments, the molar ratio of Li, Fe, and P elements corresponding to the lithium source, iron source, and phosphorus source in the raw material is (1.00–1.05):(0.95–0.99):1. For example, the molar ratio of lithium source, iron source, and phosphorus source can be 1:0.95:1, 1:0.97:1, 1:0.99:1, 1.03:0.95:1, 1.03:0.97:1, 1.03:0.99:1, 1.05:0.95:1, 1.05:0.97:1, or 1.05:0.99:1. The mass content of carbon source in the raw material is any value within the range of 10%–14%. For example, the mass content of carbon source can be 10%, 11%, 12%, 13%, or 14%.

[0049] In step S1, the lithium source can be selected from compounds containing Li or elemental Li. Further, the lithium source is selected from one or more of lithium carbonate, lithium hydroxide, lithium oxide, and elemental lithium metal.

[0050] The iron source can be selected from compounds containing Fe, and further, the iron source can be selected from one or more of ferric phosphate, ferric oxide, ferrous oxalate, etc.

[0051] The phosphorus source can be selected from compounds containing phosphorus, and further, the phosphorus source can be selected from one or more of iron phosphate, ammonium dihydrogen phosphate, purified phosphoric acid, lithium dihydrogen phosphate, etc.

[0052] The carbon source can be an organic carbon source. For example, the carbon source can be selected from at least one of glucose, fructose, polyethylene glycol, polyvinyl alcohol, sucrose, galactose, polyvinylpyrrolidone (PVP), tannic acid, cellulose, citric acid, and ascorbic acid.

[0053] The solvent can be selected from any solvent that can dissolve the carbon source; for example, the solvent can be deionized water.

[0054] In some embodiments, when the prepared cathode material needs to be doped with a metal dopant element, the raw materials mixed in step S1 also need to contain a dopant. The mass content of the dopant in the raw materials is any value in the range of 0.05% to 0.5%, for example, it can be 0.05%, 0.1%, 0.15%, 0.2%, 0.25%, 0.3%, 0.35%, 0.4%, 0.45%, or 0.5%. The dopant is an oxide containing a metal dopant element, and the metal dopant element includes at least one of Al, Ti, Mg, Nb, Zr, Mn, Co, Zn, V, and Sn.

[0055] In some embodiments, in step S2, the first slurry is wet-milled using a nano-grinding mill to prepare a second slurry. After wet milling, the D50 particle size of the particles in the second slurry is any value in the range of 0.3 to 0.6 μm, such as 0.3 μm, 0.4 μm, 0.5 μm, or 0.6 μm; the D100 particle size of the particles in the second slurry is less than 2 μm.

[0056] In some embodiments, in step S3, the second slurry is dried and granulated using a spray dryer to obtain precursor material powder. The inlet air temperature of the spray dryer is 240–300°C, the outlet air temperature is 100–120°C, and the operating frequency of the spray dryer is 40–45 Hz.

[0057] In some embodiments, step S4 specifically includes:

[0058] S41. The precursor material is loaded into the furnace, which can be, for example, a roller kiln. High-purity nitrogen is introduced into the furnace as a protective atmosphere, and the pressure inside the furnace is controlled to be 100-500 Pa by adjusting the furnace air intake and exhaust port. The oxygen content in the entire section of the furnace is less than 10 ppm.

[0059] S42. The furnace ambient temperature is raised to the first platform temperature at a preset heating rate, and the precursor material is sintered once at the first platform temperature. The first platform temperature is greater than or equal to 800℃; the sintering time is 1 to 2 hours, such as 1 hour, 1.5 hours or 2 hours; the heating rate can be adjusted according to process requirements, for example, the heating rate can be any value in the range of 1.0 to 2.0℃ / min, such as 1℃ / min, 1.2℃ / min, 1.4℃ / min, 1.5℃ / min, 1.6℃ / min, 1.8℃ / min or 2℃ / min.

[0060] S43. After the first sintering, the furnace ambient temperature is cooled from the first platform temperature to the second platform temperature at a preset cooling rate, and the precursor material in the furnace is sintered a second time at the second platform temperature. The second platform temperature is any temperature within the range of 760℃ to 800℃, such as 760℃, 770℃, 780℃, 790℃, or 800℃; the second sintering time is 8 to 10 hours, such as 8 hours, 8.5 hours, 9 hours, 9.5 hours, or 10 hours; the cooling rate can be adjusted according to process requirements, for example, the cooling rate can be any value within the range of 0.4℃ to 0.8℃ / min, such as 0.4℃ / min, 0.5℃ / min, 0.6℃ / min, 0.7℃ / min, or 0.8℃ / min.

[0061] S44. After the secondary sintering, the furnace temperature is lowered to room temperature, and the sintered product is transferred to an air jet mill for crushing and classification, so that the D50 particle size of the sintered product is controlled to be 0.7-1.5 μm and the D99 particle size is controlled to be 4-8 μm; then, after sieving and demagnetization, the positive electrode material is obtained. In step S44, air cooling or water cooling can be used to rapidly cool the sintered product.

[0062] While high-temperature sintering can improve the crystallinity of lithium iron phosphate materials and promote particle growth, thereby increasing the compaction density of the sintered product, prolonged high-temperature sintering can easily lead to excessive particle growth, carbon layer peeling, and a decrease in compaction density. Therefore, in step S4 of this application, during the sintering process, the precursor material is first sintered briefly at a first plateau temperature greater than or equal to 800°C in step S42. During this first sintering, the carbon source in the precursor material decomposes to generate a reducing atmosphere, and other raw materials in the precursor material initially form lithium iron phosphate material under the high-temperature environment and reducing atmosphere, allowing some of the initially formed particles to further grow into larger particle sizes. Then, in step S43, the first plateau temperature is lowered to a second plateau temperature (below 800°C), and the precursor material in the furnace is subjected to a prolonged second sintering at the second plateau temperature. This allows the precursor material to complete the crystallization and particle growth of lithium iron phosphate material during the second sintering process, resulting in a cathode material with a large number of small particles in the sintered product shape. Figure 4 As shown, this application utilizes gradient sintering to obtain lithium iron phosphate cathode materials with good particle size distribution, thereby effectively improving the compaction density of the cathode material.

[0063] Furthermore, pressure sintering promotes both the crystallinity of the particles and the carbon coating effect. In this application, step S4 involves matching the temperature curve of gradient sintering and controlling the furnace pressure between 100 and 500 Pa. This effectively limits the excessive growth of cathode material particles formed during sintering, resulting in a better natural particle size distribution. If the furnace pressure is too low, the cathode material particles will experience carbon layer peeling and excessive free carbon at high temperatures, leading to a deterioration in the material's conductivity. If the furnace pressure is too high, the reducing gases produced by the decomposition of the carbon source cannot be discharged in time, and the precursor material is prone to react to generate magnetic impurities such as Fe2P, affecting the conductivity and safety performance of the cathode material.

[0064] In some embodiments, in step S4, the first platform temperature of the first sintering can be any temperature in the range of 800°C to 820°C, for example, the first platform temperature can be 800°C, 805°C, 810°C, 815°C or 820°C; the second platform temperature of the second sintering can be any temperature in the range of 760°C to 780°C, for example, the second platform temperature can be 760°C, 765°C, 770°C, 775°C or 780°C.

[0065] In a third aspect, the present invention also provides an electrochemical device, which can be a solid-state lithium-ion secondary battery or a liquid lithium-ion secondary battery. Taking a liquid lithium-ion secondary battery as an example, the electrochemical device includes a positive electrode, a negative electrode, a separator disposed between the positive and negative electrode, and an electrolyte. The positive electrode includes a positive current collector and a positive active material layer disposed on the positive current collector. The positive active material layer includes a positive conductive agent, a positive binder, and the positive electrode material described in any of the above embodiments, or the positive electrode material prepared by the preparation method described in any of the above embodiments. The negative electrode includes a negative current collector and a negative active material layer disposed on the negative current collector. The negative active material layer includes a negative electrode material, a negative conductive agent, a thickener, and a negative binder. The positive and negative electrode materials can intercalate and deintercalate lithium ions to achieve energy storage and release. The electrolyte is the carrier for lithium ion transport between the positive and negative electrodes. The separator is permeable to lithium ions but non-conductive, thereby separating the positive and negative electrodes to prevent short circuits.

[0066] It should be noted that the preparation of the positive electrode, negative electrode, separator, and electrolyte, as well as the assembly of the electrochemical device, can be carried out using conventional methods in this field. The preparation methods for the electrochemical device are described below with examples:

[0067] Positive electrode preparation: Positive electrode material, positive electrode conductive agent, and positive electrode binder are mixed in a weight ratio of (90-98):(1-4):(1-4), optionally 95:2.5:2.5. N-methylpyrrolidone (NMP) solvent is added, and the mixture is stirred under vacuum until the system is homogeneous and transparent, obtaining a positive electrode slurry. This positive electrode slurry is uniformly coated onto a positive electrode current collector aluminum foil. The aluminum foil is then air-dried at room temperature and transferred to an oven for further drying. Finally, it is cold-pressed and slit to obtain the positive electrode sheet.

[0068] The positive electrode conductive agent can be selected from at least one of conductive materials such as carbon black (Super P), acetylene black, carbon nanotubes (CNT), graphene, and carbon nanofibers (VGCF); the positive electrode binder can be selected from at least one of polyvinylidene fluoride (PVDF) or polytetrafluoroethylene (PTFE), for example, PVDF can be selected as the positive electrode binder.

[0069] Negative electrode preparation: Negative electrode material, negative electrode conductive agent, negative electrode binder, and thickener are mixed in a mass ratio of (93-98):(0.2-1.5):(1-3):(0.01-1.5). N-methylpyrrolidone or deionized water is added as a solvent, and the mixture is thoroughly stirred under vacuum to obtain a negative electrode slurry. This negative electrode slurry is uniformly coated onto a negative electrode current collector, which is then air-dried at room temperature and transferred to an oven for further drying. Finally, the negative electrode is cold-pressed and slit to obtain the negative electrode sheet.

[0070] The negative electrode active material is selected from one or more of the following: tin, artificial graphite (single-crystal graphite, polycrystalline graphite, pyrolytic graphite, graphite fiber, etc.), natural graphite (bulk graphite, flake graphite, earthy graphite, etc.), soft carbon, hard carbon, pure silicon (crystalline silicon, amorphous silicon, or organosilicon), silicon oxides, silicon carbide compounds, and nano-metal oxides (Fe2O3, CuO, SnO2, Mn3O4 nanoparticles). The negative electrode conductive agent is selected from at least one of the following conductive materials: carbon black (Super P), acetylene black, carbon nanotubes (CNT), nano-silver powder, graphene, and carbon nanofibers (VGCF). The negative electrode binder is selected from at least one of the following binder materials: polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), and styrene-butadiene rubber (SBR). The thickener is selected from carboxymethyl cellulose, which can be sodium carboxymethyl cellulose (CMC-Na) or lithium carboxymethyl cellulose (CMC-Li).

[0071] Electrolyte preparation: The electrolyte in this application can be any conventional type of electrolyte in the art. For example, a small molecule plasticizer and a lithium salt can be mixed in a mass ratio of (8-9):(1-2) to prepare the electrolyte. The small molecule plasticizer is selected from one or more combinations of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, fluorinated ethylene carbonate, dipropyl carbonate, dimethyl sulfoxide dimethoxyethane, N-methyl-2-pyrrolidone, γ-butyrolactone, and polyethylene glycol dimethyl ether; the lithium salt is selected from one or more combinations of LiBF4, LiBF6, LiAsF6, LiPF6, LiClO4, LiFSI, LiTFSI, LiB(C6H5)4, LiAlCl4, LiBr, LiCF3SO3, LiN(CF3SO2)2, and LiC(CF3SOSO2)3.

[0072] Membrane preparation: The membrane is selected from conventional porous polymer membranes in the art. For example, the membrane material can be selected from one or more combinations of polyvinylidene fluoride, polystyrene, polyarylether sulfone, polyvinyl chloride, polypropylene, polyethylene, polyamide, polyimide, polyacrylic acid, polyacetal, polycarbonate, polyester, polyetherimide, polyimide, polyketone, polyphenylene ether, polyphenylene sulfide, polymethylpentene, polysulfone nonwoven glass, glass fiber materials, ceramics, metal oxides, and composites of organic and inorganic substances. For example, in one instance, the diaphragm is a porous polyethylene (PE) or polypropylene (PP) membrane, optionally a PP / PE / PP porous membrane, with a thickness of 9 μm to 18 μm, such as 9 μm, 12 μm, 16 μm or 18 μm; an air permeability of 180 s / 100 mL to 380 s / 100 mL, such as 180 s / 100 mL, 280 s / 100 mL or 380 s / 100 mL; and a porosity of 30% to 50%, such as 30%, 40% or 50%.

[0073] Battery Assembly: Battery assembly is carried out according to conventional methods. For example, after preparation, the negative electrode, separator, and positive electrode are stacked in sequence and placed in an aluminum-plastic film to obtain a bare cell. The bare cell is then placed in a casing and thoroughly baked to ensure that its water content is below 450 ppm. The prepared electrolyte is injected into the dry cell and sealed. After formation, venting, and aging, a lithium-ion battery with the preset capacity is obtained.

[0074] The conditions for formation, venting, and aging are as follows: charging to the cutoff voltage at a rate of 0.1 to 0.33C, venting vacuum of -40 to -98 kPa, venting time of more than 10 seconds; aging temperature of 40 to 50°C, aging time of 24 to 72 hours.

[0075] The technical solution of the present invention will be described in detail below through several specific embodiments and comparative examples. Unless otherwise stated, the raw materials and reagents used in the following embodiments are commercially available products or can be prepared by conventional methods in the art.

[0076] Example 1

[0077] This embodiment provides a cathode material comprising a core and a coating layer. The core is lithium iron phosphate, and the coating layer is carbon. The carbon content in the cathode material is 1.26% by mass. The D50 particle size of this cathode material is 1.06 μm, and the D99 particle size is 5.58 μm.

[0078] The preparation process of this cathode material is as follows:

[0079] (1) Weigh 25.12 kg of battery-grade lithium carbonate (D50 = 6.22 μm) and 100 kg of nano-iron phosphate (FePO4, D50 = 7.27 μm); weigh 7 kg of anhydrous glucose and 6 kg of PEG1500 (Auk Chemical) as carbon sources; weigh 0.6 kg of nano-titanium dioxide as a dopant; add nano-titanium dioxide, anhydrous glucose, PEG1500, battery-grade lithium carbonate and nano-iron phosphate to deionized water in sequence, and stir in a mixing tank for 90 min to obtain a first slurry with a solid content of 40%.

[0080] (2) The first slurry is transferred to a pin-type nano-sand mill for grinding. The grinding endpoint D50 of the slurry is controlled to be 400nm and D100 is less than 1μm to obtain the second slurry.

[0081] (3) The second slurry is dried and granulated using a spray dryer, and the moisture content of the particles is controlled to be less than 2% to obtain the precursor material.

[0082] (4) The precursor material was loaded into the graphite sagger of the roller kiln. The protective atmosphere of the roller kiln was set to nitrogen, with air intake at the bottom and sides of the kiln. The pressure inside the kiln was controlled at 300 Pa, and the oxygen content in the entire kiln section was less than 5 ppm. The temperature inside the kiln was raised to 180℃, and then the temperature was raised to the first platform temperature of 820℃ at a heating rate of 1.5℃ / min. The material was sintered at the first platform temperature of 820℃ for 2 hours. Then the temperature was lowered to the second platform temperature of 780℃ at a cooling rate of 0.6℃ / min. The material was sintered at the second platform temperature of 780℃ for 8 hours. After the second sintering, the sintered product was rapidly cooled by water cooling. After the sintered product was cooled to room temperature, it was transferred to an air jet mill for crushing and classification to produce the cathode material. The D50 particle size of this cathode material was 1.06 μm, and the D99 particle size was 5.58 μm.

[0083] Example 2

[0084] This embodiment provides a cathode material with the same system as that in Embodiment 1. The difference between this embodiment and Embodiment 1 is that the temperature of the first platform in step (4) is 800°C.

[0085] Example 3

[0086] This embodiment provides a cathode material with the same system as in Embodiment 1. The difference between this embodiment and Embodiment 1 is that the temperature of the second platform in step (4) is 760°C.

[0087] Example 4

[0088] This embodiment provides a cathode material with the same system as in embodiment 1. The difference between this embodiment and embodiment 1 is that the furnace environment pressure is adjusted to 100 Pa in step (4).

[0089] Example 5

[0090] This embodiment provides a cathode material with the same system as that in Embodiment 1. The difference between this embodiment and Embodiment 1 is that the furnace environment pressure is adjusted to 500 Pa in step (4).

[0091] Example 6

[0092] This embodiment provides a cathode material with the same system as in Example 1. The difference between this embodiment and Example 1 is that the heating rate in step (4) is adjusted to 1℃ / min.

[0093] Example 7

[0094] This embodiment provides a cathode material with the same system as in Example 1. The difference between this embodiment and Example 1 is that the heating rate in step (4) is adjusted to 2℃ / min.

[0095] Example 8

[0096] This embodiment provides a cathode material with the same system as in Embodiment 1. The difference between this embodiment and Embodiment 1 is that the cooling rate in step (4) is adjusted to 0.4℃ / min.

[0097] Example 9

[0098] This embodiment provides a cathode material with the same system as in Embodiment 1. The difference between this embodiment and Embodiment 1 is that the cooling rate in step (4) is adjusted to 0.8℃ / min.

[0099] Comparative Example 1

[0100] This comparative example provides a cathode material with the same system as Example 1. The difference between this comparative example and Example 1 is that in step (4), a secondary sintering is not performed, but the furnace temperature is directly raised to 780°C and the precursor material is sintered at 780°C for 8 hours.

[0101] Comparative Example 2

[0102] This comparative example provides a cathode material with the same system as Example 1. The difference between this comparative example and Example 1 is that in step (4), a secondary sintering is not performed, but the furnace temperature is directly raised to 820°C and the precursor material is sintered at 820°C for 8 hours.

[0103] Comparative Example 3

[0104] This comparative example provides a cathode material with the same system as Example 1. The difference between this comparative example and Example 1 is that in step (4), no secondary sintering is performed, but the furnace temperature is directly raised to 780°C and the precursor material is sintered at 780°C for 10 hours.

[0105] Comparative Example 4

[0106] This comparative example provides a cathode material with the same system as Example 1. The difference between this comparative example and Example 1 is that the first sintering time in step (4) is 0.5 hours and the second sintering time is 9.5 hours.

[0107] Comparative Example 5

[0108] This comparative example provides a cathode material with the same system as Example 1. The difference between this comparative example and Example 1 is that the furnace environment pressure is adjusted to 50 Pa in step (4).

[0109] Comparative Example 6

[0110] This comparative example provides a cathode material with the same system as Example 1. The difference between this comparative example and Example 1 is that the furnace environment pressure is adjusted to 600 Pa in step (4).

[0111] Comparative Example 7

[0112] This comparative example provides a cathode material with the same system as Example 1. The difference between this comparative example and Example 1 is that the heating rate in step (4) is adjusted to 0.5℃ / min.

[0113] Comparative Example 8

[0114] This comparative example provides a cathode material with the same system as Example 1. The difference between this comparative example and Example 1 is that the heating rate in step (4) is adjusted to 2.5℃ / min.

[0115] Comparative Example 9

[0116] This comparative example provides a cathode material with the same system as Example 1. The difference between this comparative example and Example 1 is that the cooling rate in step (4) is adjusted to 0.2℃ / min.

[0117] Comparative Example 10

[0118] This comparative example provides a cathode material with the same system as Example 1. The difference between this comparative example and Example 1 is that the cooling rate in step (4) is adjusted to 1℃ / min.

[0119] The cathode materials prepared in Examples 1 to 9 and Comparative Examples 1 to 10 were characterized for material parameters. The cathode materials were then assembled into coin cells, and the electrochemical performance of the cathode materials was tested to verify the efficacy of the present invention. The cathode material parameter characterization results are shown in Table 1, and the electrochemical performance test results are shown in Table 2.

[0120] Particle size distribution (PSD) test of cathode materials: PSD test of cathode materials prepared in each example and comparative example was performed using a Malvern laser particle size analyzer (Master Size 3000) to obtain the D50 and D99 particle sizes of the cathode materials.

[0121] Carbon content test in cathode material: The carbon content of cathode material was measured using a high-frequency infrared carbon-sulfur analyzer (BRUKERG4ICARUS).

[0122] Powder compaction density and powder resistivity testing of cathode materials: The powder compaction density and resistivity of lithium iron phosphate materials prepared in each embodiment and comparative example were tested using a powder resistivity and compaction density tester from Yuaneng Technology. The specific testing method was as follows: a cathode material sample weighing 1±0.1g was weighed using a balance, the cathode material sample was poured into the fixture cavity, the fixture was placed in a pre-compactor to level the powder, the fixture was placed in the test stage, the 2-probe mode was selected, and the compaction density and resistivity of the sample were tested using the variable pressure test method. After the test was completed, the test pressure of 160MPa was selected to obtain the sample compaction density and powder resistivity.

[0123] Electrochemical performance testing of cathode materials: The cathode materials prepared in the examples and comparative examples were assembled into coin cells according to the national standard GB / T33822-2017. The specific assembly process of the cells is as follows:

[0124] ① The positive electrode material, PVDF binder, and acetylene black conductive agent were mixed at a mass ratio of 97:1.5:1.5, placed in NMP solvent, and stirred in a vacuum mixer until homogeneous to obtain a positive electrode slurry. The positive electrode slurry was uniformly coated onto a 10μm aluminum foil current collector, and after the current collector was dried at room temperature, it was transferred to an oven and dried at 120℃ for 10 hours. Then, the current collector was rolled and slit to obtain the positive electrode sheet; the compacted density of the positive electrode sheet was 2.0~2.4g / cm³. 3 .

[0125] ② Pure lithium metal sheets are selected as negative electrode sheets, and 12μm thick polypropylene films are selected as separators.

[0126] ③ Prepare electrolyte: Mix battery-grade ethylene carbonate, propylene carbonate, dimethyl carbonate and ethyl acetate in a mass ratio of 1:1:2:6. In an argon atmosphere glove box with a water content of less than 10 ppm, dissolve fully dried LiPF6 in the mixed organic solvent and mix evenly to obtain the electrolyte. The concentration of LiPF6 is 1 mol / L.

[0127] ④ Button cell assembly: The button cell is assembled in a glove box filled with dry argon gas. The positive electrode, separator, negative electrode and electrolyte are assembled into a button cell of model CR2016 and left to stand for 12 hours.

[0128] Scanning electron microscopy (SEM) testing of the positive electrode: Turn on the scanning electron microscope (Phenom Pure+), attach the conductive adhesive to the sample stage, spread the sample evenly on it, purge with a special gas, place the sample cup into the sample slot in the test chamber, select the test mode as "electron imaging", set an appropriate accelerating voltage of ~5kV, low beam current intensity, move the field of view to select an appropriate imaging area, and take and save the image.

[0129] Compacted density test of positive electrode sheet: The positive electrode sheet is stamped into a unit circle, and the weight of the positive electrode sheet sample is tested; the compacted density of the positive electrode sheet is obtained by subtracting the weight of the aluminum foil from the weight of the positive electrode sheet sample and dividing by the sample volume.

[0130] Battery discharge performance testing: The coin cells prepared above were subjected to electrical performance testing in a battery charge-discharge testing instrument according to the test procedure. Specifically, at room temperature of 25°C, the battery was charged at a constant current and constant voltage rate of 0.1C within the test voltage range of 2V (discharge cutoff voltage) to 3.75V (charge cutoff voltage). After charging to the cutoff current of 0.05C, the specific capacity of the battery at the 0.1C current rate was measured. Then, the battery was discharged at a constant current rate of 0.1C, and the specific capacity of the battery at the 0.1C current rate was measured. The ratio of the specific capacity of the first discharge to the specific capacity of the first charge was taken as the coulombic efficiency of the first charge-discharge cycle. Then, the battery was charged at a constant current and constant voltage rate of 0.3C until the cutoff current of 0.05C was reached. Finally, the battery was discharged at a constant current rate of 1C, and the specific capacity of the battery at the 1C current rate was measured.

[0131] Table 1: Parameters of cathode materials prepared in Examples 1 to 9 and Comparative Examples 1 to 10

[0132]

[0133]

[0134] Table 2: Performance test results of cathode material-assembled batteries in Examples 1 to 9 and Comparative Examples 1 to 10

[0135]

[0136]

[0137] Comparing the test results of Example 1 and Comparative Examples 1 to 3, it can be seen that in Comparative Examples 1 and 3, only the precursor material is sintered at a lower temperature for a longer time, making it difficult to form a naturally graded lithium iron phosphate cathode material with large and small particles after sintering. The compaction density of the resulting cathode material is significantly lower than that of the embodiments of this application, thus affecting the energy density of the battery. In Comparative Example 2, the precursor material is sintered at a higher temperature for a longer time, resulting in a significantly larger particle size of the formed lithium iron phosphate cathode material. The compaction density of the resulting cathode material is lower than that of the examples, but its conductivity is greatly reduced, thus affecting the discharge performance of the assembled battery. Compared with Comparative Examples 1 to 3, the embodiments of this application, by performing gradient sintering of the precursor material with a short time at a high temperature followed by a long time at a lower temperature, can form an excellent gradation effect of a small number of large particles and a large number of small particles in the resulting cathode material. This allows the cathode material to have both good compaction density and conductivity, thereby effectively improving the discharge performance of the assembled battery.

[0138] Furthermore, the scanning electron microscope (SEM) images of Example 1 and Comparative Example 1 also show that the cathode material prepared in this application has a large number of small particles fully filling the spaces between the large particles, resulting in excellent natural gradation of the cathode material, thus giving it both good conductivity and compaction density. In contrast, the cathode material prepared in Comparative Example 1 has fewer large particles and more small particles, leading to a lower compaction density.

[0139] Comparing the test results of Examples 1, 3 and Comparative Example 4, it can be seen that in the preparation method of Comparative Example 4, the first sintering time of the precursor material at the first plateau temperature is too short (reduced to 0.5 hours), while the second sintering time at the second plateau temperature is too long (extended to 9.5 hours). This results in too few large-diameter particles and relatively more small-diameter particles in the prepared cathode material, which leads to a decrease in the compaction density of the cathode material.

[0140] Comparing the test results of Examples 1, 4, and 5 and Comparative Examples 5 and 6, it can be seen that when the furnace pressure during the sintering process is controlled within a suitable range of 100–500 Pa, the abnormal growth of cathode material particles during sintering can be effectively limited, ensuring a good carbon coating effect on the surface of the cathode material particles. If, as shown in Comparative Example 5, the furnace pressure is too low during sintering, the large-diameter particles formed earlier will grow rapidly, leading to a decrease in the compaction density of the cathode material. Simultaneously, the precursor material will experience carbon layer peeling and excessive free carbon at high temperatures, resulting in a decrease in the carbon content of the cathode material and a decline in its conductivity. If, as shown in Comparative Example 6, the furnace pressure is too high during sintering, the reducing gases generated by the decomposition of the carbon source in the precursor material cannot be discharged in time, thus introducing magnetic impurities into the cathode material, affecting the coating effect of the carbon coating layer on the core, and leading to a deterioration in the conductivity of the cathode material.

[0141] Comparing the test results of Examples 1, 6, and 7 and Comparative Examples 7 and 8, it can be seen that, as shown in Comparative Example 7, an excessively low heating rate and excessively long heating time during the process of heating to the first plateau temperature will lead to excessive carbon source decomposition loss in the precursor material, resulting in a low carbon content in the cathode material and affecting the sphericity of the cathode material particles, thereby deteriorating the compaction density and conductivity of the cathode material. As shown in Comparative Example 8, an excessively high heating rate and excessively short heating time during the process of heating to the first plateau temperature will result in an excessively large carbon source decomposition temperature gradient in the precursor material, leading to excessive free carbon in the cathode material, poor carbon coating effect and deterioration of conductivity in the cathode material; at the same time, insufficient heating time will also lead to smaller cathode material particle size, resulting in a relatively lower compaction density of the cathode material.

[0142] Comparing the test results of Examples 1, 8, and 9 with Comparative Examples 9 and 10, it can be seen that, as shown in Comparative Example 9, if the cooling rate during the process of cooling to the second plateau temperature is too low, the high-temperature sintering time of the precursor material near the first plateau temperature will be too long, resulting in excessive decomposition of the carbon source in the precursor material, which easily forms magnetic impurities, thereby affecting the conductivity of the cathode material. As shown in Comparative Example 10, if the cooling rate during the process of cooling to the second plateau temperature is too high, the drastic cooling during the large particle growth stage at the first plateau temperature will affect the sintering neck extension, and the kinetic process of large particle formation will be "frozen," resulting in a lower large particle size and a decrease in compaction density in the cathode material.

[0143] The above embodiments are merely illustrative of the principles and effects of the present invention and are not intended to limit the invention. Any person skilled in the art can modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by those skilled in the art without departing from the spirit and technical concept disclosed in the present invention should still be covered by the claims of the present invention.

Claims

1. A positive electrode material, characterized in that, include: The core comprises lithium iron phosphate material; A coating layer, which covers the outside of the core, comprises a carbon material, wherein the carbon material has a mass content of 1% to 1.5% in the cathode material; The compacted density of the positive electrode material powder is greater than 2.6 g / cm³. 3 The resistivity of the positive electrode material powder is less than 30 Ω·cm.

2. The cathode material according to claim 1, characterized in that, The cathode material is doped with a metal dopant element, and the mass content of the metal dopant element in the cathode material is 500ppm to 5000ppm. The metal dopant element includes at least one of Al, Ti, Mg, Nb, Zr, Mn, Co, Zn, V, and Sn.

3. A method for preparing a positive electrode material, characterized in that, include: The raw materials are mixed in a solvent according to a preset ratio to obtain a first slurry; the raw materials include lithium source, iron source, phosphorus source and carbon source; The first slurry is ground to obtain a second slurry; the D50 particle size of the second slurry is 0.3 to 0.6 μm, and the D100 particle size of the second slurry is less than 2 μm; The second slurry is dried to obtain the precursor material; The precursor material is placed under a preset pressure environment, and the ambient temperature is raised to a first plateau temperature at a preset heating rate. The precursor material is then sintered at the first plateau temperature. Next, the ambient temperature is lowered to a second plateau temperature at a preset cooling rate, and the precursor material is sintered a second time at the second plateau temperature. Finally, the precursor material is cooled to obtain the cathode material. The preset pressure is 100-500 Pa, the first plateau temperature is greater than or equal to 800°C, the first sintering time is 1-2 hours, the second plateau temperature is 760°C-800°C, and the second sintering time is 8-10 hours.

4. The preparation method according to claim 3, characterized in that, The preset heating rate is 1.0 to 2.0 °C / min; and / or the preset cooling rate is 0.4 to 0.8 °C / min.

5. The preparation method according to claim 3, characterized in that, The temperature of the first platform is 800℃~820℃; and / or, the temperature of the second platform is 760℃~780℃.

6. The preparation method according to claim 3, characterized in that, The molar ratio of Li, Fe, and P elements in the lithium, iron, and phosphorus sources is (1.00–1.05):(0.95–0.99):1; the mass content of the carbon source in the raw materials is 10%–14%.

7. The preparation method according to claim 3 or 6, characterized in that, The raw material also includes a dopant, the dopant having a mass content of 0.05% to 0.5% in the raw material; the dopant is an oxide containing a metal dopant element, the metal dopant element including at least one of Al, Ti, Mg, Nb, Zr, Mn, Co, Zn, V, and Sn.

8. An electrochemical device, characterized in that, It includes a positive electrode sheet, which comprises the positive electrode material according to any one of claims 1 to 2, or the positive electrode material prepared by the preparation method according to any one of claims 3 to 7.

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