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

Abstract

The application relates to the technical field of lithium ion batteries, in particular to a phosphate-based positive electrode material, a preparation method thereof and a lithium ion battery. The preparation method comprises the following steps: dispersing a primary semi-product of the phosphate-based positive electrode material in a solvent, adding an etching agent to etch the particle surface of the primary semi-product to obtain a secondary semi-product; the crystallinity of the particle surface of the primary semi-product is lower than the crystallinity of the particle interior; mixing the secondary semi-product and additional raw materials of the phosphate-based positive electrode material, and then performing secondary sintering to obtain a finished product of the phosphate-based positive electrode material; the raw materials comprise an organic iron source; the amount of the etching agent is 1.2 wt% to 8 wt% of the primary semi-product. The phosphate-based positive electrode material prepared by the preparation method significantly improves the mass transfer problem of the lithium interface, improves the electronic conductivity, and effectively improves the cycle stability of the phosphate-based positive electrode material.

Description

Technical Field

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

[0002] Currently, phosphate-based cathode materials (such as lithium iron phosphate or lithium manganese iron phosphate) are favored by the market due to their advantages such as high safety, good cycle performance, abundant raw material sources, and low environmental pollution. However, with the development of the battery industry, the market has placed higher demands on lithium-ion battery cathode materials, and some problems have emerged in the development and application of phosphate-based cathode materials. Taking lithium manganese iron phosphate as an example, its main problems lie in its poor lithium-ion conductivity and poor electronic conductivity, which greatly limit the large-scale application of lithium manganese iron phosphate.

[0003] To address these issues, the most common optimization techniques currently employed include particle refinement, elemental doping, and carbon coating. However, these three optimization techniques often require independent, step-by-step implementation, which not only reduces production efficiency but also has limited effect on improving the electrochemical performance of phosphate-based systems.

[0004] In view of this, the present invention is hereby proposed. Summary of the Invention

[0005] The purpose of this invention is to provide a method for preparing phosphate-based cathode materials, phosphate-based cathode materials prepared by the method of this invention, and lithium-ion batteries containing phosphate-based cathode materials of this invention. The phosphate-based cathode materials prepared by the method of this invention can improve the lithium interface mass transfer problem of phosphate cathode materials and improve electronic conductivity.

[0006] To achieve the above-mentioned objectives of the present invention, one aspect of the present invention provides a method for preparing a phosphate-based cathode material, comprising the following steps:

[0007] A primary semi-finished product of phosphate-based cathode material is dispersed in a solvent, and an etchant is added to etch the surface of the particles of the primary semi-finished product to obtain a secondary semi-finished product; the crystallinity of the particle surface of the primary semi-finished product is lower than that of the particle interior.

[0008] The secondary semi-finished product is mixed with the additional raw materials of the phosphate-based cathode material and then subjected to secondary sintering to obtain the finished phosphate-based cathode material; the raw materials include an organic iron source.

[0009] The amount of the etching agent used is 1.2 wt% to 8 wt% of the primary semi-finished product.

[0010] Furthermore, the precursor of the phosphate-based cathode material is heated to 320°C to 380°C at a heating rate of 0.1°C / min to 0.5°C / min for a first sintering to obtain the first semi-finished product.

[0011] Furthermore, the etchant includes a fluorine-containing etchant.

[0012] Furthermore, the etching agent includes hydrofluoric acid.

[0013] Furthermore, the amount of hydrofluoric acid used, calculated as HF, is 3 wt% to 6 wt% of the primary semi-finished product.

[0014] Furthermore, the fluorine content on the particle surface of the finished phosphate-based cathode material is 0.5 wt% to 3 wt% relative to the mass percentage of lithium manganese iron phosphate.

[0015] Furthermore, the organic iron source comprises iron and a cyclic organic structure.

[0016] Furthermore, the organic iron source includes at least one of ferrocene, ferrocene derivatives, ferriprotoporphyrin, and ferriprotoporphyrin derivatives.

[0017] Furthermore, the temperature of the secondary sintering is 700℃~780℃, and the holding time of the secondary sintering is 4h~8h.

[0018] Furthermore, the secondary sintering includes: heating to 700℃~780℃ at a heating rate of 5℃ / min~10℃ / min, holding at that temperature for 4h~8h, and then cooling to room temperature at a rate of 1℃ / min~3℃ / min.

[0019] Furthermore, after dispersing the primary semi-finished product in a solvent, the solid content of the system is 60% to 80%.

[0020] Furthermore, the precursor of the phosphate-based cathode material also includes a doping element, which includes at least one of Ti, Mg, V, Co, In, Ni, Zr and Nb.

[0021] Furthermore, the amount of the dopant source is 0.5 wt% to 3 wt% of the mass of the finished product.

[0022] Furthermore, the raw materials for the added phosphate-based cathode material include lithium source, phosphorus source and iron source.

[0023] Furthermore, the raw materials for the added phosphate-based cathode material also include a manganese source.

[0024] In another aspect, the present invention provides a phosphate-based cathode material prepared by any of the preparation methods described above.

[0025] In another aspect, the present invention provides a lithium-ion battery comprising any of the phosphate-based cathode materials described above.

[0026] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0027] (1) The method for preparing phosphate-based cathode material of the present invention involves etching a primary semi-finished product with low surface crystallinity to dissolve the interface of the primary semi-finished product particles, while fluoride ions are adsorbed on the surface of the primary semi-finished product particles. Based on the strong electronegativity of fluorine, it can adsorb iron and manganese from organic iron sources and manganese sources in the system. During the secondary sintering process, the secondary crystallization reaction is completed. Iron and manganese from organic iron sources and manganese sources participate in the formation of phosphate substances, while the organic structure is carbonized during the secondary sintering process to form an in-situ coated highly graphitized carbon layer. Furthermore, fluoride ions are crystallized and coated into the oxygen atom sites of phosphate in the crystal lattice to achieve in-situ doping, further improving the electronic conductivity of the phosphate-based cathode material.

[0028] (2) The phosphate-based cathode material prepared by the preparation method of the present invention not only significantly improves the mass transfer problem of the lithium interface, but also improves the electronic conductivity, thereby effectively improving the cycle stability of the phosphate-based cathode material. Attached Figure Description

[0029] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0030] Figure 1 A schematic diagram of a semi-finished product obtained by hydrofluoric acid etching according to an embodiment of the present invention;

[0031] Figure 2 A schematic diagram of the finished product provided in an embodiment of the present invention;

[0032] Figure 3 The Raman spectra of the finished products obtained in Example 1 and Comparative Example 6 of this invention;

[0033] Figure 4 The images shown are SEM images of the finished products obtained in Example 1 and Comparative Example 6 of this invention. Detailed Implementation

[0034] The technical solution of the present invention will be clearly and completely described below with reference to the accompanying drawings and specific embodiments. However, those skilled in the art will understand that the embodiments described below are some embodiments of the present invention, but not all embodiments, and are only used to illustrate the present invention, and should not be regarded as limiting the scope of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall be followed. Where the manufacturers of reagents or instruments are not specified, they are all conventional products that can be purchased commercially.

[0035] A method for preparing a phosphate-based cathode material includes the following steps:

[0036] The primary semi-finished product of phosphate-based cathode material is dispersed in a solvent, and an etchant is added to etch the surface of the particles of the primary semi-finished product to obtain the secondary semi-finished product; the crystallinity of the particle surface of the primary semi-finished product is lower than that of the particle interior.

[0037] The secondary semi-finished product is mixed with the additional raw materials of the phosphate-based cathode material and then subjected to secondary sintering to obtain the finished phosphate-based cathode material; the raw materials include an organic iron source.

[0038] The amount of etchant used is 1.2wt% to 8wt% of the primary semi-finished product.

[0039] In different embodiments, the amount of etchant used can be 1.2 wt%, 2 wt%, 4 wt%, 5 wt%, 6 wt%, 8 wt%, or any combination thereof of the primary semi-finished product.

[0040] The present invention discloses a method for preparing phosphate-based cathode materials. A primary semi-finished product with low surface crystallinity is etched. Utilizing the difference in crystallinity between the surface and interior of the primary semi-finished product, the interface of the particles is etched and dissolved, while fluoride ions are adsorbed onto the surface of the particles. Based on the strong electronegativity of fluoride ions, iron and manganese from organic iron and manganese sources in the system can be adsorbed. During the secondary sintering process, a secondary crystallization reaction is completed, with iron and manganese from the organic iron and manganese sources participating in the formation of phosphate substances. The organic structure is then carbonized during the secondary sintering process to form an in-situ coated highly graphitized carbon layer. Furthermore, fluoride ions are coated into the oxygen atom sites of phosphate groups in the crystal lattice, achieving in-situ doping and further improving the electronic conductivity of the phosphate-based cathode material.

[0041] The method for preparing phosphate-based cathode materials of the present invention involves in-situ coating and doping modification simultaneously. While forming a carbon coating with high graphitization degree, non-metallic elements are also doped, which takes into account both improving lithium-ion conductivity and enhancing electronic conductivity.

[0042] refer to Figure 1 This is a schematic diagram of the hydrofluoric acid etching of a semi-finished product according to an embodiment of the present invention. The crystallinity of the surface of the semi-finished product particles is lower than that of the interior particles. Utilizing the difference in crystallinity between the surface and interior of the semi-finished product, the interface of the particles is etched and dissolved after treatment with the etchant. Simultaneously, fluoride ions are adsorbed onto the surface of the semi-finished product particles. These fluoride ions can adsorb organic iron sources in subsequent steps, etc., and undergo a secondary crystallization reaction during the secondary sintering process. Iron in the organic iron source participates in the formation of phosphate substances, and the organic structure carbonizes during the secondary sintering process to form an in-situ coated highly graphitized carbon layer; furthermore, in-situ doping of fluoride ions within the crystal lattice is achieved. (Reference) Figure 2 This is a schematic diagram of the finished phosphate-based cathode material provided in the embodiments of the present invention, wherein Ti is an optional doping element, but is not limited to it.

[0043] In a specific embodiment of the present invention, the precursor of the phosphate-based cathode material is heated to 320°C to 380°C at a heating rate of 0.1°C / min to 0.5°C / min for a single sintering to obtain a semi-finished product.

[0044] In different embodiments, during a single sintering process, the heating rate can be within the range of 0.1℃ / min, 0.2℃ / min, 0.3℃ / min, 0.4℃ / min, 0.5℃ / min, or any combination thereof, and can be raised to 320℃, 330℃, 340℃, 350℃, 360℃, 370℃, 380℃, or any combination thereof. The purpose of these single sintering conditions is to form highly crystalline LFMP or LFP within the particles. If the heating rate is too fast, the crystal structure will delaminate, with the outer layer having a higher crystallinity than the inner layer, leading to dissolution and collapse of the entire structure during subsequent etching. If the sintering temperature is too low, the corresponding crystal nuclei cannot be formed, and internal crystallization is also impossible; conversely, if the sintering temperature is too high, the external crystallinity will be higher than the internal crystallinity, similarly leading to dissolution and collapse of the structure during subsequent etching.

[0045] In a specific embodiment of the present invention, the holding time for a single sintering is 5h to 8h.

[0046] In different embodiments, the holding time for the first sintering can be 5h, 6h, 7h, 8h, or any combination thereof. This invention employs a relatively short holding time for the first sintering, resulting in a difference in crystallinity between the internal and external semi-finished products. This, combined with subsequent etching steps, keeps the internal structure stable while the surface is etched and dissolved, yielding the target semi-finished product.

[0047] In a specific embodiment of the present invention, the etchant includes a fluorinated etchant, preferably hydrofluoric acid.

[0048] In specific embodiments of the present invention, the amount of hydrofluoric acid used, calculated as HF, is 3 wt% to 6 wt% of the primary semi-finished product. In different embodiments, the amount of hydrofluoric acid used, based on the mass of HF contained therein, can be 3 wt%, 4 wt%, 5 wt%, 6 wt% of the primary semi-finished product, or any combination thereof. If the amount of hydrofluoric acid is too low, the etching effect will be insignificant, and the interface of the primary semi-finished product cannot be effectively etched and dissolved; if the amount of hydrofluoric acid is too high, it will dissolve the highly crystalline inner layer of the substrate.

[0049] The mass fraction of hydrofluoric acid can be 5% to 40%, but is not limited to this.

[0050] In a specific embodiment of the present invention, the fluorine content on the particle surface of the finished phosphate-based cathode material is 0.5 wt% to 3 wt% relative to the mass percentage of lithium manganese iron phosphate (excluding the carbon layer). The fluorine content on the particle surface of the finished product after etching can be controlled by adjusting the amount of etchant and the etching time. For example, in different embodiments, the fluorine content on the particle surface of the finished product after etching can be 0.5 wt%, 1 wt%, 1.5 wt%, 1.8 wt%, 2 wt%, 2.5 wt%, 3 wt%, or any combination thereof.

[0051] In practice, the preparation of secondary semi-finished products may include: dispersing the primary semi-finished product in a solvent, adding an etchant and mixing for 0.5 h to 1 h, and drying to obtain the secondary semi-finished product.

[0052] The mixing and stirring time can be 0.5h, 0.6h, 0.8h, 1h, or any combination thereof. Using an appropriate processing time allows for effective etching and dissolution of the interface of the semi-finished product. If the mixing and stirring time is too short, the etching effect will be insignificant, and the adsorbed fluoride ion content will be low; if the mixing and stirring time is too long, it will destroy all crystal structures.

[0053] The drying temperature in the preparation of the secondary semi-finished product can be between 70℃ and 90℃. In different embodiments, the drying temperature can be 70℃, 75℃, 80℃, 85℃, 90℃, or any combination thereof. In actual operation, the drying method can include oven drying, such as drying in an oven. The heating temperature of the oven can be adjusted according to actual needs to achieve the drying purpose.

[0054] In a specific embodiment of the present invention, the organic iron source includes iron and a cyclic organic structure. Further, the organic iron source includes at least one selected from ferrocene, ferrocene derivatives, ferrous protoporphyrin, and ferrous protoporphyrin derivatives. The cyclic organic structure may include, but is not limited to, at least one of carbocyclic organic structures and heterocyclic organic structures, and can be an alicyclic organic structure or an aromatic organic structure, preferably an aromatic organic structure.

[0055] The iron source of the present invention contains both carbon and iron, and is coated and recrystallized after being mixed with the remaining raw materials. Fluoride ions adsorbed during the etching step selectively adsorb iron and manganese ions, and secondary crystallization occurs during the secondary sintering process, resulting in the carbonization of the organic structure to form an in-situ coated highly graphitized carbon layer. The ferrocene derivatives refer to derivatives with mono- or di-substituted alkyl, ester, or acyl groups on the cyclopentadienyl group of the ferrocene, such as acetylferrocene or 1,1'-diacetylferrocene.

[0056] In a specific embodiment of the present invention, the temperature of the secondary sintering is 700℃~780℃, and the holding time of the secondary sintering is 4h~8h.

[0057] In a specific embodiment of the present invention, the secondary sintering includes: heating to 700°C to 780°C at a heating rate of 5°C / min to 10°C / min and holding at that temperature for 4 to 8 hours, followed by cooling to room temperature at a rate of 1°C / min to 3°C / min. Further, after the material has cooled to room temperature, it is pulverized to obtain the finished phosphate-based cathode material.

[0058] In different embodiments, during secondary sintering, the heating rate can be a range of 5°C / min, 6°C / min, 7°C / min, 8°C / min, 9°C / min, 10°C / min, or any combination thereof, and the temperature can be raised to 700°C, 720°C, 740°C, 750°C, 760°C, 780°C, or any combination thereof; then the temperature can be lowered to room temperature at a cooling rate of 1°C / min, 2°C / min, or 3°C / min, etc.

[0059] In a specific embodiment of the present invention, after the semi-finished product is dispersed in a solvent, the solid content of the system is 60% to 80%. The solvent used for dispersion may include water.

[0060] This invention further regulates the relationship between the amount of the primary semi-finished product and the solvent, so that the solid content in the mixed system is within the range of 60%, 65%, 70%, 75%, 80%, or any combination thereof. A suitable solid content ensures appropriate etching depth and uniformity.

[0061] In a specific embodiment of the present invention, the dopant source includes at least one of Ti, Mg, V, Co, In, Ni, Zr, and Nb. Further, the amount of the dopant source is 0.5 wt% to 3 wt% of the mass of the finished product.

[0062] In specific embodiments of the present invention, the dopant source includes at least one of the oxides and soluble salts of the dopant element. For example, when the dopant element is Ti, the dopant source includes, but is not limited to, titanium dioxide or titanium tetrachloride; when the dopant element is Mg, the dopant source includes, but is not limited to, magnesium oxide, magnesium chloride, or magnesium nitrate; when the dopant element is V, the dopant source includes, but is not limited to, vanadium dioxide; and when the dopant source is Nb, the dopant source includes, but is not limited to, niobium oxide or niobium pentoxide.

[0063] It should be noted that the finished product in this invention refers to the final product of the phosphate-based cathode material. For example, in different embodiments, the amount of dopant source can be 0.5wt%, 1wt%, 1.5wt%, 2wt%, 2.5wt%, 3wt%, or any combination thereof, of the finished product mass. Appropriate amounts of dopant elements incorporated into the crystal structure can prevent the generation of antisite defects within the crystal and increase electron conductivity.

[0064] In a specific embodiment of the present invention, the raw materials used to prepare the phosphate-based cathode material for primary semi-finished products include a lithium source, a phosphorus source, and an iron source. Further, a manganese source is also included. In the raw materials, the molar ratio of Li, Fe+Mn, and P is (0.9–1.05):1:1. That is, the phosphate-based cathode material can be lithium iron phosphate (LFP) or lithium manganese iron phosphate (LFMP).

[0065] In a specific embodiment of the present invention, the preparation of the precursor includes: mixing raw materials, carbon source, dopant element source, and solvent of a mixed phosphate-based cathode material, followed by drying and crushing to obtain a solid-phase precursor. The drying method may include oven drying, which may be accompanied by stirring, or oven drying may be used. For example, drying can be carried out by heating and stirring to remove the solvent. The heating temperature can be adjusted according to the type of solvent to achieve the effect of drying and removing the solvent.

[0066] In a specific embodiment of the present invention, the carbon source used in the preparation of the precursor includes at least one selected from PEG, glucose, sucrose, EG, and Ketjenblack. Further, the amount of carbon source used is 2 wt% to 8 wt% of the finished product mass.

[0067] In different embodiments, the amount of carbon source used can be 2 wt%, 4 wt%, 5 wt%, 6 wt%, 8 wt% of the finished product mass, or any combination thereof.

[0068] In a specific embodiment of the present invention, the solvent used in the preparation of the precursor includes at least one of water, ethanol, and acetone. The amount of solvent can be adjusted according to actual conditions, as long as it is sufficient to ensure that all materials are mixed evenly.

[0069] In a specific embodiment of the present invention, the raw materials for the added phosphate-based cathode material include a lithium source, a phosphorus source, and an iron source. Further, a manganese source is also included. The molar ratio of Li, P, and Fe+Mn in the raw materials is (0.9–1.05):1:1.

[0070] In a specific embodiment of the present invention, the molar ratio of the raw material for preparing the primary semi-finished product to the raw material for the supplemented phosphate-based cathode material is 1:(0.05~0.2), but is not limited thereto.

[0071] The raw materials used to prepare the primary semi-finished product and the supplementary raw materials are proportioned according to the elemental composition of the phosphate-based cathode material. The specific phosphorus source, iron source, lithium salt, and optional manganese source used can be the same or different, as long as the molar ratio of Li, Fe+Mn, and P in each raw material meets the requirements, and the molar ratio of the raw materials used to prepare the primary semi-finished product and the supplementary raw materials meets the aforementioned requirements. For example, in different embodiments, the molar ratio of the raw material used to prepare the primary semi-finished product to the raw material added to the phosphate-based cathode material can be 1:0.05, 1:0.1, 1:0.15, 1:0.2, or any combination thereof.

[0072] The lithium source can be at least one of Li2O, Li2CO3, LiH2PO4, LiOH·H2O, CH3COOLi, and LiNO3; the iron source used in the preparation of the primary semi-finished product of the phosphate-based cathode material can be at least one of FeCl3, Fe(NO3)3, Fe2O3, and FeSO4·7H2O; the phosphorus source can be at least one of (NH4)3PO4, LiH2PO4, and H3PO4; and the manganese source can be at least one of MnO2, Mn(NO3)2, MnSO4, and Mn3(PO4)2·3H2O.

[0073] In a specific embodiment of the present invention, the mixing method of the secondary semi-finished product and the additional phosphate-based cathode material raw materials includes ball milling. The specific conditions for ball milling are not limited, as long as the materials are mixed uniformly.

[0074] In another aspect, the present invention provides a phosphate-based cathode material prepared by any of the above-mentioned preparation methods.

[0075] In a specific embodiment of the present invention, the primary particle size of the phosphate-based cathode material is 145nm to 225nm, such as 145nm, 160nm, 175nm, 200nm, 215nm, 225nm or any combination thereof.

[0076] In a specific embodiment of the present invention, the graphitization degree I of the carbon layer of the phosphate-based cathode material is... D / I G ≤2.63, preferably ≤2.53, ≤2.25, etc., such as 2.09, 2.15, 2.23, 2.28, 2.34, 2.4 or any two of them.

[0077] In another aspect, the present invention provides a lithium-ion battery comprising any of the aforementioned phosphate-based cathode materials.

[0078] Lithium-ion batteries using the cathode material of this invention help improve battery cycle performance.

[0079] Example 1

[0080] This embodiment provides a method for preparing a phosphate-based cathode material, including the following steps:

[0081] (1) Weigh 1 mol LiNO3, 0.4 mol Fe(NO3)3, 1 mol NH4H2PO4, 0.6 mol Mn(NO3)2, dopant titanium chloride (the mass of titanium chloride is 0.5% of the mass of the final product), glucose (the mass of glucose is 5% of the mass of the final product), and 500 mL of water, mix them evenly to form a solution, then evaporate the water by heating and stirring, and then crush it to obtain a solid precursor material.

[0082] (2) The solid precursor material obtained in step (1) is placed in a sintering furnace and heated to 350°C at a heating rate of 0.3°C / min under a nitrogen atmosphere for 6 hours. After the material is cooled to room temperature, it is taken out and crushed to obtain a semi-finished product.

[0083] (3) Disperse the primary semi-finished product obtained in step (2) in water to obtain a system with a solid content of 62.5%. Then add a hydrofluoric acid aqueous solution with a mass fraction of 5wt% (the amount of hydrofluoric acid added is 5wt% of the primary semi-finished product in HF) and stir for 30 min. Then dry at 80℃ to obtain a secondary semi-finished product.

[0084] (4) Take the secondary semi-finished product from step (3) and mix it with 0.1 mol LiNO3, 0.1 mol NH4H2PO4, 0.06 mol Mn(NO3)2, 0.04 mol ferrocene and 200 mL of ultrapure water. Ball mill the mixture for 3 h, then dry it and place it in a tube furnace. Under a nitrogen atmosphere, heat the mixture to 750 °C at a heating rate of 5 °C / min and hold it at that temperature for 6 h. Then cool it to room temperature at a cooling rate of 1 °C / min and then pulverize it to obtain the finished phosphate-based cathode material LiMn. 0.6 Fe 0.39586 Ti 0.00414 PO4.

[0085] Example 2

[0086] This embodiment refers to the preparation method of Example 1, the only difference being that titanium chloride is not added in step (1), and the finished phosphate-based cathode material LiMn is obtained. 0.6 Fe 0.4 PO4.

[0087] Example 3

[0088] This embodiment refers to the preparation method of Embodiment 1, the only difference being that the sintering conditions in step (2) are different.

[0089] Step (2) of this embodiment includes: placing the solid precursor material obtained in step (1) in a sintering furnace, heating it to 320°C at a heating rate of 0.1°C / min under a nitrogen atmosphere, and holding it at that temperature for 8 hours. After the material cools to room temperature, it is taken out and crushed to obtain a semi-finished product.

[0090] Example 4

[0091] This embodiment refers to the preparation method of Embodiment 1, the only difference being that the sintering conditions in step (2) are different.

[0092] Step (2) of this embodiment includes: placing the solid precursor material obtained in step (1) in a sintering furnace, heating it to 380°C at a heating rate of 0.5°C / min under a nitrogen atmosphere, and holding it at that temperature for 5 hours. After the material cools to room temperature, it is taken out and crushed to obtain a semi-finished product.

[0093] Example 5

[0094] This embodiment refers to the preparation method of Example 1, the only difference being the amount of hydrofluoric acid used in step (3).

[0095] In step (3) of this embodiment, a hydrofluoric acid aqueous solution with a mass fraction of 5 wt% is added. The amount of hydrofluoric acid added is 3 wt% of the primary semi-finished product, calculated as HF.

[0096] Example 6

[0097] This embodiment refers to the preparation method of Example 1, the only difference being the amount of hydrofluoric acid used in step (3).

[0098] In step (3) of this embodiment, a hydrofluoric acid aqueous solution with a mass fraction of 5 wt% is added. The amount of hydrofluoric acid added is 6 wt% of the primary semi-finished product, calculated as HF.

[0099] Example 7

[0100] This embodiment refers to the preparation method of Embodiment 1, the only difference being that the iron source in step (4) is different.

[0101] The iron source used in step (4) of this embodiment is an equimolar amount of iron citrate.

[0102] Comparative Example 1

[0103] Comparative Example 1 uses the same preparation method as Example 1, except that the heating rate of the first sintering in step (2) is different.

[0104] Step (2) of Comparative Example 1 includes: placing the solid precursor material obtained in step (1) in a sintering furnace, heating it to 350°C at a heating rate of 5°C / min under a nitrogen atmosphere and holding it at that temperature for 6 hours, and then taking it out and crushing it to obtain a semi-finished product.

[0105] Comparative Example 2

[0106] Comparative Example 2 uses the same preparation method as Example 1, except that the isothermal temperature of the first sintering in step (2) is different.

[0107] Step (2) of Comparative Example 2 includes: placing the solid precursor material obtained in step (1) in a sintering furnace, heating it to 500°C at a heating rate of 0.3°C / min under a nitrogen atmosphere, and holding it at that temperature for 6 hours. After the material cools to room temperature, it is taken out and crushed to obtain a semi-finished product.

[0108] Comparative Example 3

[0109] Comparative Example 3 follows the same preparation method as Example 1, except that the amount of hydrofluoric acid used in step (3) is different.

[0110] In step (3) of Comparative Example 3, a hydrofluoric acid aqueous solution with a mass fraction of 5 wt% was added. The amount of hydrofluoric acid added, calculated as HF, was 10 wt% of the first semi-finished product.

[0111] Comparative Example 4

[0112] Comparative Example 4 uses the same preparation method as Example 1, except that the amount of hydrofluoric acid used in step (3) is different.

[0113] In step (3) of Comparative Example 4, a hydrofluoric acid aqueous solution with a mass fraction of 5 wt% was added. The amount of hydrofluoric acid added was 1 wt% of the first semi-finished product, calculated as HF.

[0114] Comparative Example 5

[0115] Comparative Example 5 uses the same preparation method as Example 1, except that the iron source in step (4) is different.

[0116] In Comparative Example 5, step (4) uses an equimolar amount of Fe(NO3)3 as the iron source. Comparative Example 6

[0117] Comparative Example 6 was prepared in the same manner as Comparative Example 5, except that glucose was added in step (4).

[0118] In step (4) of Comparative Example 6, the glucose mass is 5% of the final product mass and is mixed with the remaining materials and ball-milled.

[0119] Experimental Example 1

[0120] To compare and illustrate the performance differences of phosphate-based cathode materials prepared by different methods of the present invention, the following tests were conducted, and the test results are shown in Table 1.

[0121] Table 1. Test results for different embodiments and comparative examples.

[0122]

[0123] Note: The fluorine content on the surface of the finished particles refers to the percentage of fluorine content on the surface of the finished particles relative to the mass of lithium manganese iron phosphate (excluding the carbon layer) in the finished particles.

[0124] The degree of graphitization is determined by the peak area I of the D band measured in the Raman spectrum. D With the peak area of ​​the G band I G The ratio I D / I G I obtained D / I G The smaller the value, the higher the degree of graphitization.

[0125] like Figure 3 The image shows the Raman spectra of the finished products of Example 1 and Comparative Example 6 of this invention.

[0126] As can be seen from Examples 1 and 2, the doping element mainly affects the lithium-ion insertion / extraction rate.

[0127] Compared with Comparative Examples 1 and 2, Example 1 showed that the heating rate was too fast or the holding time was too long, resulting in high primary crystallinity and insufficient etching depth, which affected fluorine ion doping. As a result, there were not enough iron ions adsorbed in situ, the graphitization degree of the in-situ coated carbon layer was low, and the electronic conductivity was insufficient.

[0128] According to the comparison between Example 1 and Comparative Examples 3-4, too much or too little hydrofluoric acid etching will have adverse effects; when there is too much hydrofluoric acid, the etching depth is too deep, which leads to the destruction of the crystal structure; when there is too little hydrofluoric acid, the fluorine ion doping is less, which results in the graphitized carbon layer adsorbed in situ being too thin, which is not conducive to electron conduction.

[0129] Compared with Comparative Examples 5 and 6, Example 1 showed that Comparative Example 5, using only an inorganic iron source, resulted in less graphitized carbon. Comparative Example 6, using both an inorganic iron source and a non-cyclic organic carbon source, while achieving fluorine ion doping and carbon layer coating, did not achieve in-situ coating and high graphitization, leading to increased electron migration impedance and deteriorated electrical properties. Furthermore, non-in-situ coating could not effectively adsorb onto the particle surface, resulting in... Figure 4 In the figure, (a) and (b) are SEM images of the finished products obtained in Example 1 and Comparative Example 6, respectively. It can be seen from the figures that there are many free carbon particles between the finished product particles in Comparative Example 6.

[0130] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing phosphate-based cathode materials, characterized in that, Includes the following steps: A primary semi-finished product of phosphate-based cathode material is dispersed in a solvent, and an etchant is added to etch the surface of the particles of the primary semi-finished product to obtain a secondary semi-finished product; the crystallinity of the particle surface of the primary semi-finished product is lower than that of the particle interior. The secondary semi-finished product is mixed with the raw material of the added phosphate-based cathode material and then subjected to secondary sintering to obtain the finished phosphate-based cathode material. The raw material of the added phosphate-based cathode material includes a lithium source, a phosphorus source and an organic iron source. The organic iron source includes at least one of ferrocene, ferrocene derivatives, ferrous protoporphyrin and ferrous protoporphyrin derivatives. The amount of the etchant used is 1.2wt% to 8wt% of the primary semi-finished product; the etchant includes hydrofluoric acid; The precursor of the phosphate-based cathode material is heated to 320°C to 380°C at a heating rate of 0.1°C / min to 0.5°C / min for a single sintering to obtain the primary semi-finished product; the raw materials of the primary semi-finished product of the phosphate-based cathode material include lithium source, phosphorus source and iron source. The temperature for the secondary sintering is 700℃~780℃.

2. The method for preparing the phosphate-based cathode material according to claim 1, characterized in that, The amount of hydrofluoric acid used, calculated as HF, is 3 wt% to 6 wt% of the primary semi-finished product.

3. The method for preparing the phosphate-based cathode material according to claim 2, characterized in that, The fluorine content on the particle surface of the finished phosphate-based cathode material is 0.5 wt% to 3 wt% relative to the mass percentage of lithium manganese iron phosphate.

4. The method for preparing the phosphate-based cathode material according to claim 1, characterized in that, The holding time for the secondary sintering is 4h to 8h.

5. The method for preparing the phosphate-based cathode material according to claim 1, characterized in that, The secondary sintering includes: heating to 700℃~780℃ at a heating rate of 5℃ / min~10℃ / min, holding at that temperature for 4h~8h, and then cooling to room temperature at a rate of 1℃ / min~3℃ / min.

6. The method for preparing the phosphate-based cathode material according to claim 1, characterized in that, After the primary semi-finished product is dispersed in a solvent, the solid content of the system is 60% to 80%.

7. The method for preparing the phosphate-based cathode material according to claim 1, characterized in that, The precursor of the phosphate-based cathode material further includes a doping element, which includes at least one of Ti, Mg, V, Co, In, Ni, Zr and Nb.

8. The method for preparing the phosphate-based cathode material according to claim 7, characterized in that, The amount of dopant source used is 0.5wt% to 3wt% of the mass of the finished product.

9. The method for preparing the phosphate-based cathode material according to claim 1, characterized in that, The raw materials for the added phosphate-based cathode material also include a manganese source.

10. The phosphate-based cathode material prepared by the method according to any one of claims 1 to 9.

11. A lithium-ion battery, characterized in that, The phosphate-based cathode material prepared by the method of any one of claims 1 to 9 or the phosphate-based cathode material of claim 10.

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