Phosphate-based positive electrode material, preparation method thereof and lithium ion battery

Abstract

The application provides a phosphate-based positive electrode material, a preparation method thereof and a lithium ion battery, and the preparation method comprises the following steps: (1) mixing raw materials of the phosphate-based positive electrode material, a first carbon source and a solvent, and performing drying, crushing and first calcination to obtain one-step calcined particles; (2) mixing a second carbon source and the one-step calcined particles in step (1), and performing heat treatment, wherein the second carbon source is sublimated and adhered to the surface of the one-step calcined particles in the process of the heat treatment, and then performing second calcination to obtain the phosphate-based positive electrode material. The phosphate-based positive electrode material provided by the application is coated with a dense carbon film on the outer surface, and can better isolate the electrolyte and the phosphate-based positive electrode material after forming a lithium ion battery, thereby improving the high-temperature performance of the lithium ion battery.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery materials technology, and in particular to a phosphate-based cathode material, its preparation method, and a lithium-ion battery. Background Technology

[0002] Lithium-ion batteries, with their advantages of high energy density, environmental friendliness, and long lifespan, have been widely used in portable energy sources, electric bicycles, energy storage power supplies, and electric vehicles. Currently, the main cathode materials for lithium-ion batteries include lithium cobalt oxide (LiCoO2), lithium manganese oxide (LiMn2O4), lithium metal phosphate series, and ternary lithium batteries.

[0003] However, in lithium-ion batteries using phosphate-based cathode materials, the cathode material is in a delithiation state when fully charged, resulting in increased valence of transition metal ions and enhanced oxidizability, making it prone to oxidizing the electrolyte. The graphite anode material is in a lithium-intercalated state, exhibiting poor stability and readily reacting with the electrolyte. Therefore, the battery's performance deteriorates when stored at full charge, with particularly significant degradation under high-temperature conditions.

[0004] Therefore, it is necessary to develop new phosphate-based cathode materials and their preparation methods to improve their high-temperature performance. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a phosphate-based cathode material, its preparation method, and a lithium-ion battery. Specifically, it provides a method to improve the high-temperature storage performance of phosphate-based lithium-ion batteries. By adding a readily sublimable second carbon source, a dense carbon film is formed on the surface of the cathode material particles through in-situ chemical vapor deposition, thereby achieving isolation between the phosphate-based cathode material and the electrolyte and improving high-temperature performance.

[0006] To achieve this objective, the present invention adopts the following technical solution:

[0007] In a first aspect, the present invention provides a method for preparing a phosphate-based cathode material, the method comprising:

[0008] (1) Mix the raw materials, first carbon source and solvent of phosphate-based cathode material, and then dry, crush and calcinate to obtain calcined particles;

[0009] (2) Mix the second carbon source and the calcined particles from step (1), and heat-treat them. During the heat treatment, the second carbon source is sublimated and attached to the surface of the calcined particles. Then, the mixture is calcined again to obtain the phosphate-based cathode material.

[0010] The method for preparing phosphate-based cathode materials provided by this invention adds a first carbon source in step (1) of mixing raw materials and forming a precursor, which has a carbothermic reduction effect, thereby reducing the Fe content in the raw materials. 3+ and or Mn3+ Reduced to Fe 2+ and or Mn 2+ Simultaneously, the first carbon source is uniformly distributed in the precursor, and after carbonization, it can isolate the cathode material particles in situ, preventing the agglomeration of phosphate-based cathode material particles and inhibiting the growth of crystal particles. Then, the second carbon source and the first carbon particles are mixed in a solid-state manner. Through heat treatment, the second carbon source is sublimated in situ and escapes into the gas phase to the surface of the cathode material particles, where it adheres. Then, a second calcination is performed to carbonize the second carbon source adhering to the surface, forming a dense coating film, thereby improving the high-temperature performance of the lithium-ion battery.

[0011] Preferably, the first calcination in step (1) is carried out under a nitrogen atmosphere.

[0012] Preferably, the first calcination includes a first heating stage and a first isothermal stage.

[0013] Preferably, the heating rate of the first heating stage is 3 to 9 °C / min, for example, it can be 3 °C / min, 3.7 °C / min, 4.4 °C / min, 5 °C / min, 5.7 °C / min, 6.4 °C / min, 7 °C / min, 7.7 °C / min, 8.4 °C / min or 9 °C / min, but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0014] Preferably, the final temperature of the first heating stage is 400 to 700°C, for example, it can be 400°C, 434°C, 467°C, 500°C, 534°C, 567°C, 600°C, 634°C, 667°C or 700°C, but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0015] Preferably, the duration of the first constant temperature stage is 2 to 12 hours, for example, it can be 2 hours, 4 hours, 5 hours, 6 hours, 7 hours, 8 hours, 9 hours, 10 hours, 11 hours or 12 hours, but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0016] Preferably, in step (2), the second carbon source accounts for 3 to 6% of the mass of the calcined particles, for example, it can be 3%, 3.4%, 3.7%, 4%, 4.4%, 4.7%, 5%, 5.4%, 5.7% or 6%, etc., but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0017] The present invention preferably controls the quality of the second carbon source within the aforementioned range, which can avoid the degradation of the electrical performance of the cathode material caused by excessive second carbon source content, and can also ensure the high-temperature performance of the cathode material. When the content of the second carbon source is too low, the uniformity of the coated carbon film will be poor, with areas not coated with the second carbon source and the carbon film thickness not within the ideal range, resulting in a decrease in the high-temperature performance of the prepared lithium-ion battery; when the content of the second carbon source is too high, the dense second carbon film will be too thick, hindering lithium-ion diffusion and causing a decrease in the electrical performance of the cathode material.

[0018] Preferably, the sublimation point of the second carbon source is 100 to 300°C, for example, it can be 100°C, 120°C, 140°C, 160°C, 180°C, 210°C, 230°C, 250°C, 270°C or 300°C, but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0019] Preferably, the second carbon source comprises any one or a combination of at least two of naphthalene, anthracene, 1,4-naphthoquinone, benzoic acid, salicylic acid, phthalic anhydride, 2,4,6-triisopropyl-1,3,5-trioxane, camphor, or caffeine. Typical but non-limiting combinations include naphthalene and anthracene, 1,4-naphthoquinone and anthracene, naphthalene and 1,4-naphthoquinone, benzoic acid and anthracene, naphthalene and benzoic acid, phthalic anhydride and salicylic acid, 2,4,6-triisopropyl-1,3,5-trioxane and caffeine, and phthalic anhydride and camphor. More preferably, it comprises any one or a combination of at least two of naphthalene, anthracene, or benzoic acid.

[0020] The present invention preferably uses the above-mentioned substances as the second carbon source. These substances have a sublimation point below the carbonization temperature, and the sublimation temperature does not affect the performance of the phosphate-based cathode material itself.

[0021] Preferably, the second carbon source comprises free anthraquinone compounds.

[0022] Preferably, the heat treatment in step (2) includes a second heating stage and a second isothermal stage.

[0023] Preferably, the heating rate of the second heating stage is 1 to 3 °C / min, for example, it can be 1 °C / min, 1.3 °C / min, 1.5 °C / min, 1.7 °C / min, 1.9 °C / min, 2.2 °C / min, 2.4 °C / min, 2.6 °C / min, 2.8 °C / min or 3 °C / min, but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0024] Preferably, the final temperature of the second heating stage is 100-300℃, for example, it can be 100℃, 120℃, 145℃, 160℃, 180℃, 210℃, 230℃, 250℃, 270℃ or 300℃, etc., but is not limited to the listed values, and other unlisted values ​​in this range are also applicable.

[0025] The challenge of the technical solution of this invention lies in enabling the second carbon source to be carbonized as soon as possible after sublimation and deposition on the surface of the phosphate-based cathode material. It is necessary not only to avoid the direct volatilization of the second carbon source, but also to avoid the situation where the second carbon source volatilizes again after sublimation and deposition on the particle surface. Therefore, this invention preferably controls the heating rate of the second heating stage within the above-mentioned range, in conjunction with the heating rate of the second calcination. The heat treatment can ensure that the second carbon source is uniformly deposited on the surface of the phosphate-based cathode material, and the second calcination can effectively ensure that it is carbonized in situ to form a dense coating film.

[0026] Preferably, the isothermal time of the second isothermal stage is 1 to 2 hours, for example, it can be 1 hour, 1.2 hours, 1.3 hours, 1.4 hours, 1.5 hours, 1.6 hours, 1.7 hours, 1.8 hours, 1.9 hours or 2 hours, but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0027] Preferably, the second calcination in step (3) includes a third heating stage and a third isothermal stage.

[0028] Preferably, the heating rate of the third heating stage is higher than the heating rate of the second heating stage.

[0029] Preferably, the heating rate of the third heating stage is 10-15℃ / min, for example, it can be 10℃ / min, 10.6℃ / min, 11.2℃ / min, 11.7℃ / min, 12.3℃ / min, 12.8℃ / min, 13.4℃ / min, 13.9℃ / min, 14.5℃ / min or 15℃ / min, etc., but is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0030] Preferably, the final temperature of the third heating stage is 400-750℃, for example, it can be 400℃, 435℃, 470℃, 515℃, 550℃, 590℃, 630℃, 670℃, 710℃ or 750℃, but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0031] Preferably, the isothermal time of the third isothermal stage is 4 to 8 hours, for example, it can be 4 hours, 4.5 hours, 4.9 hours, 5.4 hours, 5.8 hours, 6.3 hours, 6.7 hours, 7.2 hours, 7.6 hours or 8 hours, but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0032] Preferably, the raw materials of the phosphate-based cathode material in step (1) include a lithium source, a phosphorus source and an iron source, and the raw materials of the phosphate-based cathode material may optionally also include a manganese source.

[0033] The raw materials for the phosphate-based cathode material described in this invention can also be selected according to the specific composition of the phosphate-based cathode material. For example, when titanium is present, the raw materials can also contain a titanium source, such as titanium dioxide.

[0034] Preferably, the lithium source includes any one or a combination of at least two of Li2CO3, LiH2PO4, LiOH·H2O, CH3COOLi, or LiNO3.

[0035] Preferably, the iron source includes any one or a combination of at least two of Fe(NO3)3, FeCl3, Fe2O3, or FeSO4·7H2O.

[0036] Preferably, the phosphorus source includes any one or a combination of at least two of (NH4)3PO4, LiH2PO4, or H3PO4.

[0037] Preferably, the manganese source is any one or a combination of at least two of MnO2, Mn(NO3)2, MnSO4 or Mn3(PO4)2·3H2O.

[0038] Preferably, the solvent in step (1) includes water.

[0039] Preferably, the first carbon source in step (1) includes any one or a combination of at least two of glucose, sucrose, PVDF, carbon black, PEG, paraffin, graphite or graphene, wherein typical but non-limiting combinations are combinations of glucose and sucrose, combinations of PVDF and sucrose, combinations of glucose and PVDF, combinations of glucose and carbon black, and combinations of PEG and carbon black.

[0040] In this invention, PVDF refers to polyvinylidene fluoride, and PEG refers to polyethylene glycol.

[0041] Preferably, the first carbon source accounts for 5 to 15% of the mass of the raw material of the phosphate-based cathode material. It can be 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, or 15%, etc., but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0042] The present invention does not impose any special restrictions on the mixing in the above process. Any device and method known to those skilled in the art for mixing can be used. Adjustments can also be made according to the actual process. For example, it can be stirring and mixing, or a combination of different methods.

[0043] The present invention does not impose any special restrictions on the drying process described above. Any device and method known to those skilled in the art for drying can be used. Adjustments can also be made according to the actual process. For example, it can be air drying, vacuum drying, oven drying, or freeze drying, or a combination of different methods.

[0044] The present invention does not impose any special restrictions on the crushing process described above. Any crushing device and method known to those skilled in the art can be used, and adjustments can be made according to the actual process. For example, it can be grinding, extrusion crushing, splitting crushing, or impact crushing, or a combination of different methods.

[0045] In a second aspect, the present invention provides a phosphate-based cathode material, wherein the phosphate-based cathode material is prepared by the preparation method of the phosphate-based cathode material described in the first aspect.

[0046] The phosphate-based cathode material provided by the second aspect of the present invention has a dense carbon coating film and exhibits better high-temperature performance.

[0047] Preferably, the particle size of the phosphate-based cathode material is 80-200 nm, for example, it can be 80 nm, 90 nm, 100 nm, 110 nm, 120 nm, 130 nm, 140 nm, 150 nm, 180 nm or 200 nm, but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0048] Preferably, the phosphate-based cathode material includes phosphate-based cathode material particles and a carbon film coating the phosphate-based cathode material particles.

[0049] Preferably, the mass fraction of carbon in the phosphate-based cathode material is 0.8% to 3.0%, for example, it can be 0.8%, 0.9%, 1.0%, 1.2%, 1.3%, 1.5%, 1.8%, 2.0%, 2.2%, 2.5%, or 3.0%, but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0050] Preferably, the carbon film includes a first carbon film and a second carbon film.

[0051] Preferably, the thickness of the carbon film is 2 to 3 nm, for example, it can be 2 nm, 2.2 nm, 2.3 nm, 2.4 nm, 2.5 nm, 2.6 nm, 2.7 nm, 2.8 nm, 2.9 nm or 3 nm, but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0052] Preferably, the thickness of the first carbon film is 1 to 2.5 nm, for example, it can be 1 nm, 1.1 nm, 1.2 nm, 1.3 nm, 1.5 nm, 1.8 nm, 2.0 nm, 2.2 nm, 2.3 nm or 2.5 nm, but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0053] Preferably, the thickness of the second carbon film is 0.5 to 1 nm, for example, it can be 0.5 nm, 0.6 nm, 0.65 nm, 0.7 nm, 0.75 nm, 0.8 nm, 0.85 nm, 0.9 nm, 0.95 nm or 1.0 nm, but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0054] Preferably, the surface roughness Ra of the carbon film is ≤1.0 nm, for example, it can be 1.0 nm, 0.9 nm, 0.8 nm, 0.7 nm, 0.6 nm or 0.5 nm, etc., but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0055] Thirdly, the present invention provides a lithium-ion battery, wherein the lithium-ion battery comprises the phosphate-based cathode material described in the second aspect.

[0056] The lithium-ion battery provided by the third aspect of the present invention has excellent high-temperature performance and is suitable for temperature environments of 30 to 60°C. In particular, its performance remains relatively good in high-temperature environments of 40 to 60°C.

[0057] Preferably, the lithium-ion battery is a cylindrical lithium-ion battery.

[0058] Compared with the prior art, the present invention has at least the following beneficial effects:

[0059] (1) The method for preparing phosphate-based cathode materials provided by the present invention uses an easily sublimable second carbon source to form a dense carbon film on the surface of cathode material particles by chemical vapor deposition, which isolates the positive electrode active components from direct contact with the electrolyte, thereby significantly improving the high-temperature storage performance of the battery.

[0060] (2) The phosphate-based cathode material particles provided by the present invention are coated with a dense carbon film, which improves the high-temperature storage performance. The capacity retention rate of the lithium-ion battery at 100% SOC state after 7 days of storage at 60°C is preferably above 98.8%, the capacity recovery rate is preferably above 99.6%, and the first charge capacity of the lithium-ion battery is above 148.3mAh and the first discharge capacity is above 139.8mAh.

[0061] (3) The lithium-ion battery provided by the present invention can be used well in high-temperature environments, thus broadening the application scenarios. Detailed Implementation

[0062] To facilitate understanding of the present invention, the following embodiments are provided. Those skilled in the art should understand that these embodiments are merely illustrative and should not be construed as limiting the scope of the invention.

[0063] It should be understood that in the description of this invention, the terms "first," "second," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Therefore, a feature defined with "first," "second," etc., may explicitly or implicitly include one or more of that feature. In the description of this invention, unless otherwise stated, "a plurality of" means two or more.

[0064] Example 1

[0065] This embodiment provides a method for preparing a phosphate-based cathode material, the method comprising the following steps:

[0066] (1) The raw materials of mixed phosphate-based cathode material (1888.14g LiNO3, 1892.32g Fe(NO3)3, 3000g NH4H2PO4 and 3266.93g Mn(NO3)2), the first carbon source (482.27g sucrose) and 15071.09g water were used to form a slurry. The dried sample was crushed in a soymilk maker and then calcined in a nitrogen atmosphere at a temperature of 5℃ / min from room temperature to 400℃ for 12h to obtain calcined particles.

[0067] (2) Mix the second carbon source (benzoic acid) and the calcined particles from step (1), wherein the second carbon source accounts for 3% of the mass of the calcined particles. The mixed sample is then pulverized and heat-treated in a nitrogen atmosphere by raising the temperature from room temperature to 200°C at a rate of 1°C / min for 1 hour. During the heat treatment, the second carbon source sublimates and adheres to the surface of the calcined particles. The sample is then calcined again by raising the temperature to 650°C at a rate of 10°C / min for 6 hours. After pulverization, LiMn is obtained. 0.7 Fe 0.3 PO4 active component.

[0068] Example 2

[0069] This embodiment provides a method for preparing a phosphate-based cathode material. The difference between this method and that of Embodiment 1 is that step (2) includes: mixing a second carbon source (benzoic acid) and the calcined particles from step (1), wherein the second carbon source accounts for 3% of the mass of the calcined particles. The mixed sample is then pulverized and heat-treated in a nitrogen atmosphere by heating from room temperature to 200°C at 3°C / min for 2 hours. During the heat treatment, the second carbon source sublimates and adheres to the surface of the calcined particles. The sample is then calcined again by heating to 650°C at 15°C / min for 8 hours, and finally pulverized to obtain LiMn. 0.7 Fe 0.3 PO4 active component.

[0070] Example 3

[0071] This embodiment provides a method for preparing a phosphate-based cathode material, the method comprising the following steps:

[0072] (1) A slurry was formed by mixing raw materials of phosphate-based cathode material (1094.32g LiOH·H2O, 624.71g Fe2O3, 1800g NH4H2PO4, 2132.12g Mn3(PO4)2·3H2O and 208.38g TiO2), a first carbon source (482.27g glucose) and 15071.09g water. The slurry was dried, and the dried sample was crushed by a crusher. Then, it was calcined in a nitrogen atmosphere from room temperature to 700℃ at a rate of 3℃ / min for 2 hours to obtain calcined particles.

[0073] (2) The second carbon source (naphthalene) and the calcined particles from step (1) are mixed, wherein the second carbon source accounts for 3.5% of the mass of the calcined particles. The mixed sample is pulverized and then heat-treated in a nitrogen atmosphere by raising the temperature from room temperature to 300°C at 2°C / min and holding it at that temperature for 1 hour. During the heat treatment, the second carbon source sublimates and adheres to the surface of the calcined particles. Then, the temperature is raised to 750°C at 15°C / min and held at that temperature for 4 hours for a second calcination. The sample is then pulverized to obtain LiMn. 0.6 Fe 0.3 Ti 0.1 PO4 active component.

[0074] Example 4

[0075] This embodiment provides a method for preparing a phosphate-based cathode material, the method comprising the following steps:

[0076] (1) A slurry was formed by mixing raw materials of phosphate-based cathode material (1404.63g FeCl3, 3000g LiH2PO4 and 3051.11g MnSO4), first carbon source (192.91g carbon black) and 15071.09g water. The slurry was dried, and the dried sample was crushed by a crusher. Then, it was calcined in a nitrogen atmosphere from room temperature to 700℃ at 3℃ / min for 2h to obtain calcined particles.

[0077] (2) The second carbon source (anthracene) and the calcined particles from step (1) are mixed, wherein the second carbon source accounts for 6% of the mass of the calcined particles. The mixed sample is then pulverized and heat-treated in a nitrogen atmosphere by raising the temperature from room temperature to 100°C at 3°C / min for 2 hours. During the heat treatment, the second carbon source sublimates and adheres to the surface of the calcined particles. The sample is then calcined again by raising the temperature to 450°C at 10°C / min for 7.5 hours. After pulverization, LiMn is obtained. 0.7 Fe 0.3 PO4 active component.

[0078] Example 5

[0079] This embodiment provides a method for preparing a phosphate-based cathode material. Except for step (2), in which the second carbon source (benzoic acid) accounts for 1% of the mass of the calcined particles, the preparation method is the same as in Example 1.

[0080] Example 6

[0081] This embodiment provides a method for preparing a phosphate-based cathode material. Except for step (2), in which the second carbon source (benzoic acid) accounts for 8% of the mass of the calcined particles, the preparation method is the same as in Example 1.

[0082] Example 7

[0083] This embodiment provides a method for preparing a phosphate-based cathode material. The preparation method is the same as in Example 1, except that the second carbon source in step (2) is replaced with camphor.

[0084] Example 8

[0085] This embodiment provides a method for preparing a phosphate-based cathode material. Except for the heating rate of the second calcination in step (2) being only 5°C / min, the preparation method is the same as in Example 1.

[0086] Example 9

[0087] This embodiment provides a method for preparing a phosphate-based cathode material. Except for the heating rate of 5°C / min in step (2), the preparation method is the same as that in Example 1.

[0088] Example 10

[0089] This embodiment provides a method for preparing a phosphate-based cathode material. Except for the heating rate of 0.3℃ / min in step (2), the preparation method is the same as that in Example 1.

[0090] Comparative Example 1

[0091] This comparative example provides a method for preparing a phosphate-based cathode material. The preparation method is the same as in Example 1, except that the second carbon source in step (2) is replaced with sucrose.

[0092] Comparative Example 2

[0093] This comparative example provides a method for preparing a phosphate-based cathode material. Except for step (2), in which heat treatment is not performed and a second calcination is performed directly, the preparation method is the same as in Example 1.

[0094] Comparative Example 3

[0095] This comparative example provides a method for preparing a phosphate-based cathode material. The preparation method is the same as that in Example 1, except that the second calcination is not performed in step (2).

[0096] Comparative Example 4

[0097] This comparative example provides a method for preparing a phosphate-based cathode material. The preparation method is the same as in Example 1 except that a first carbon source (sucrose) is not added in step (1).

[0098] Comparative Example 5

[0099] This comparative example provides a method for preparing a phosphate-based cathode material. The preparation method is essentially the same as in Example 1, except that both the first carbon source and the second carbon source are added to step (1). The specific steps are as follows:

[0100] (1) The raw materials of the mixed phosphate-based cathode material (1888.14g LiNO3, 1892.32g Fe(NO3)3, 3000g NH4H2PO4 and 3266.93g Mn(NO3)2), the first carbon source (482.27g sucrose), the second carbon source (benzoic acid) in the same amount as in Example 1, and 15071.09g water were dried. The dried sample was crushed in a soymilk maker and then calcined in a nitrogen atmosphere at a temperature of 5℃ / min from room temperature to 400℃ for 12h to obtain calcined particles.

[0101] (2) The calcined particles were heat-treated in a nitrogen atmosphere by heating from room temperature to 200°C at a rate of 1°C / min and holding at that temperature for 1 hour. During the heat treatment, the second carbon source was sublimated and attached to the surface of the calcined particles. Then, the temperature was increased to 650°C at a rate of 10°C / min and held at that temperature for 6 hours for a second calcination. After pulverization, LiMn was obtained. 0.7 Fe 0.3 PO4 active component.

[0102] Surface roughness testing: The surface of the active component of the cathode material is observed using an atomic force microscope to determine its surface roughness.

[0103] Film thickness testing: Film thickness was tested using a transmission electron microscope.

[0104] Preparation of cylindrical lithium-ion batteries: 2400g of the active components of the electrode material prepared in the examples and comparative examples, 300g of the conductive agent acetylene black, and 300g of the binder polyvinylidene fluoride (PVDF) were added to 2400g of N-methylpyrrolidone solution (NMP solution) and stirred in a vacuum mixer for 4 hours to obtain a positive electrode slurry. The positive electrode slurry was uniformly coated on aluminum foil, dried, and rolled to form a positive electrode sheet. Graphite was coated on copper foil, dried, and rolled to form a negative electrode sheet, and then assembled into a cylindrical lithium-ion battery.

[0105] Charge and discharge test: The prepared cylindrical lithium-ion batteries from the examples and comparative examples were charged and discharged once at 1C standard using a lithium-ion battery charge and discharge test system at 25±0.5℃. The batteries were stored at 60℃ for 7 days at 100% SOC, discharged at 1C at 25±0.5℃, and then charged and discharged again at 1C standard.

[0106] The results of the above tests are shown in Table 1.

[0107] Table 1

[0108]

[0109] In Table 1, "-" indicates that there is no relevant data.

[0110] The following points can be observed from Table 1:

[0111] (1) As can be seen from the comprehensive examples 1 to 4, the method for preparing phosphate-based cathode materials provided by the present invention can produce cathode materials with excellent high-temperature performance. The lithium-ion batteries prepared by it can be stored at 60°C for 7 days at 100% SOC, with a capacity retention rate of over 98.8% and a capacity recovery rate of over 99.6%. Moreover, the first charge capacity is over 148.3mAh and the first discharge capacity is over 139.8mAh.

[0112] (2) It can be seen from the combined results of Example 1 and Comparative Example 1 that benzoic acid was selected as the second carbon source in Example 1. Compared with sucrose selected as the second carbon source in Comparative Example 1, the surface roughness in Example 1 was only 0.7 nm, while the surface roughness in Comparative Example 1 was as high as 1.8 nm. Moreover, the capacity retention rate of the lithium-ion battery stored at 60°C for 7 days at 100% SOC was only 80.2%, and the capacity recovery rate was only 91.7%. This shows that by selecting organic materials with sublimation points within a specific temperature range for the second coating, the present invention can produce phosphate-based cathode materials with better high-temperature performance.

[0113] (3) As can be seen from the combined examples 1, 2-3 and 5, the process of first mixing the first carbon source to prepare calcined particles, then mixing the second carbon source and sequentially heat-treating and calcining the carbon film with low surface roughness (meaning fewer surface pores and a denser coating) in the present invention is as follows: In Example 1, the capacity retention rate of the lithium-ion battery stored at 60°C for 7 days at 100% SOC was as high as 98.7% and the capacity recovery rate was as high as 99.6%, while in Comparative Examples 2-3 and 5, the capacity retention rates were only 96.4%, 50.8% and 73.3%, respectively, and the capacity recovery rates were only 97.3%, 64.1% and 76.4%, respectively. This shows that the present invention significantly improves the high-temperature performance of phosphate-based cathode materials by designing a specific process flow and using a second carbon source with a sublimation point at a specific temperature.

[0114] (4) It can be seen from the combined examples of Example 1 and Comparative Example 4 that the role of the first carbon source is very important. In Comparative Example 4, sucrose is not added as the first carbon source. In the first calcination stage, the trivalent iron in the raw material cannot be reduced to divalent iron. Moreover, manganese and / or iron are easily oxidized during the roasting process, and lithium manganese iron phosphate crystals cannot be synthesized. Instead, other impurities are generated, making it difficult to obtain phosphate-based cathode materials.

[0115] (5) As can be seen from the combined examples 1 and 5-6, the benzoic acid content in Example 1 is 3%, which is higher than the benzoic acid content in Examples 5-6, which is 1% and 8% respectively. The first charge-discharge capacity in Example 1 is high, and the high-temperature capacity retention rate and recovery rate are also high. In contrast, the capacity retention rate in Example 5 is only 76.3%, and the capacity recovery rate is only 82.9%. The first charge-discharge capacity in Example 6 is significantly lower than that in the examples. This shows that by limiting the content of the second carbon source to a specific range, the present invention is more conducive to balancing high-temperature performance and first charge-discharge capacity, and obtains a phosphate-based cathode material with better performance.

[0116] (6) As can be seen from Examples 1 and 8-10, the heating rate of the heat treatment and calcination process has a significant impact on the thickness and density of the second carbon film. In Examples 8-10, due to the heating rate being too fast or too slow, the capacity retention rates were only 70.0%, 78.5% and 60.8%, respectively. This shows that by controlling the heating rate of the heat treatment and the second calcination within a specific range, the present invention can obtain a dense second carbon film, which significantly improves the high-temperature performance of phosphate-based cathode materials.

[0117] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A method for preparing a phosphate-based cathode material, characterized in that, The preparation method includes: (1) Mix the raw materials, first carbon source and solvent of phosphate-based cathode material, and then dry, crush and calcinate to obtain calcined particles; (2) The second carbon source and the calcined particles described in step (1) are mixed in a solid phase manner and subjected to heat treatment. During the heat treatment, the second carbon source is sublimated and attached to the surface of the calcined particles by in-situ chemical vapor deposition. The mixture is then subjected to a second calcination to obtain the phosphate-based cathode material. The sublimation point of the second carbon source is 100~300℃. The second carbon source includes any one or a combination of at least two of the following: naphthalene, anthracene, 1,4-naphthoquinone, benzoic acid, salicylic acid, phthalic anhydride, 2,4,6-triisopropyl-1,3,5-trioxane, camphor, or caffeine. The phosphate-based cathode material includes phosphate-based cathode material particles and a carbon film coating the phosphate-based cathode material particles. The carbon film includes a first carbon film and a second carbon film, and the surface roughness Ra of the carbon film is ≤1.0 nm.

2. The preparation method according to claim 1, characterized in that, In step (1), the first calcination is carried out under a nitrogen atmosphere.

3. The preparation method according to claim 1, characterized in that, The first calcination includes a first heating stage and a first isothermal stage.

4. The preparation method according to claim 3, characterized in that, The heating rate in the first heating stage is 3~9℃ / min.

5. The preparation method according to claim 3, characterized in that, The final temperature of the first heating stage is 400~700℃.

6. The preparation method according to claim 3, characterized in that, The duration of the first isothermal stage is 2 to 12 hours.

7. The preparation method according to claim 1, characterized in that, In step (2), the second carbon source accounts for 3 to 6% of the mass of the calcined particles.

8. The preparation method according to claim 1, characterized in that, The second carbon source includes any one or a combination of at least two of naphthalene, anthracene, or benzoic acid.

9. The preparation method according to claim 1, characterized in that, The heat treatment in step (2) includes a second heating stage and a second isothermal stage.

10. The preparation method according to claim 9, characterized in that, The heating rate in the second heating stage is 1~3℃ / min.

11. The preparation method according to claim 9, characterized in that, The final temperature of the second heating stage is 100~300℃.

12. The preparation method according to claim 9, characterized in that, The temperature control period for the second constant temperature stage is 1 to 2 hours.

13. The preparation method according to claim 9, characterized in that, The second calcination includes a third heating stage and a third isothermal stage.

14. The preparation method according to claim 13, characterized in that, The heating rate in the third heating stage is higher than that in the second heating stage.

15. The preparation method according to claim 13, characterized in that, The heating rate in the third heating stage is 10~15 ℃ / min.

16. The preparation method according to claim 13, characterized in that, The final temperature of the third heating stage is 400~750 ℃.

17. The preparation method according to claim 13, characterized in that, The isothermal period of the third isothermal stage is 4 to 8 hours.

18. The preparation method according to claim 1, characterized in that, The raw materials for the phosphate-based cathode material mentioned in step (1) include lithium source, phosphorus source and iron source.

19. The preparation method according to claim 18, characterized in that, The raw materials for the phosphate-based cathode material also include a manganese source.

20. The preparation method according to claim 1, characterized in that, In step (1), the first carbon source includes any one or a combination of at least two of glucose, sucrose, PVDF, carbon black, PEG, paraffin, graphite or graphene.

21. The preparation method according to claim 1, characterized in that, The first carbon source accounts for 5-15% of the mass of the raw materials of the phosphate-based cathode material.

22. A phosphate-based cathode material, characterized in that, The phosphate-based cathode material is prepared by the method for preparing the phosphate-based cathode material according to any one of claims 1 to 21; The phosphate-based cathode material includes phosphate-based cathode material particles and a carbon film coating the phosphate-based cathode material particles; the surface roughness Ra of the carbon film is ≤1.0 nm.

23. The phosphate-based cathode material according to claim 22, characterized in that, The particle size of the phosphate-based cathode material is 80~200 nm.

24. The phosphate-based cathode material according to claim 22, characterized in that, The mass fraction of carbon in the phosphate-based cathode material is 0.8-3.0%.

25. The phosphate-based cathode material according to claim 22, characterized in that, The thickness of the carbon film is 2~3 nm.

26. The phosphate-based cathode material according to claim 22, characterized in that, The thickness of the first carbon film is 1~2.5 nm.

27. The phosphate-based cathode material according to claim 22, characterized in that, The thickness of the second carbon film is 0.5~1 nm.

28. A lithium-ion battery, characterized in that, The lithium-ion battery includes the phosphate-based cathode material as described in any one of claims 22 to 27.

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