Carbon-coated ferrous phosphate and its preparation method, cathode materials and their preparation methods, cathode plates and secondary batteries

Abstract

This application provides a carbon-coated ferrous phosphate and its preparation method, a cathode material and its preparation method, a cathode electrode sheet, and a secondary battery, belonging to the field of lithium-ion battery technology. The preparation method of carbon-coated ferrous phosphate includes: mixing an organic iron salt, a phosphorus source, a complexing agent, and water to obtain a mixed solution; spray-drying the mixed solution to obtain a dry powder; and sintering the dry powder to obtain carbon-coated ferrous phosphate. This application aims to solve the technical problem of excessively large primary particle size of lithium iron phosphate in existing lithium iron phosphate preparation processes.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, specifically to a carbon-coated ferrous phosphate and its preparation method, a cathode material and its preparation method, a cathode electrode sheet, and a secondary battery. Background Technology

[0002] The main cathode materials for lithium-ion batteries include lithium cobalt oxide, lithium manganese oxide, nickel-manganese-cobalt ternary materials, and lithium iron phosphate. Among them, lithium iron phosphate (LiFePO4) has become a research hotspot due to its advantages such as high safety, stable cycle performance, low price, stable discharge platform, and environmental friendliness.

[0003] Currently, the mainstream lithium iron phosphate (LFP) preparation process uses iron phosphate and lithium carbonate as precursors, which are mixed with a carbon source, ground, and then calcined. However, this process has the following problems: when grinding the precursor materials (iron phosphate, lithium carbonate, etc.), the small particle size causes agglomeration of materials such as iron phosphate. As a result, during high-temperature calcination, the agglomerated iron phosphate is prone to fuse and grow into large single crystal particles, leading to a larger primary particle size of lithium iron phosphate and affecting the material's capacity. Summary of the Invention

[0004] In view of the technical problems existing in the background art, this application provides a carbon-coated ferrous phosphate and its preparation method, a cathode material and its preparation method, a cathode electrode sheet and a secondary battery, aiming to solve the technical problem of the large particle size of primary lithium iron phosphate particles in the existing lithium iron phosphate preparation process.

[0005] In a first aspect, embodiments of this application provide a method for preparing carbon-coated ferrous phosphate, comprising the following steps: Organic iron salt, phosphorus source, complexing agent, and water are mixed to obtain a mixed solution; The mixed solution was spray-dried to obtain a dry powder; The dry powder was sintered to obtain carbon-coated ferrous phosphate.

[0006] In the technical solution of this application embodiment, a mixed solution formed by organic iron salt, phosphorus source, complexing agent and water is spray-dried, and then the resulting dry powder is sintered to obtain anhydrous ferrous phosphate with a carbon layer on the surface. This process is short, produces no wastewater, and the carbon-coated ferrous phosphate has a small particle size, high purity and uniform carbon layer coating, resulting in better activity. When using this carbon-coated ferrous phosphate as a precursor to synthesize lithium iron phosphate cathode materials, not only is it not necessary to add an additional carbon source, but also, due to the presence of the carbon layer, the mutual fusion and agglomeration of adjacent precursor particles is suppressed during grinding and mixing, thereby avoiding the growth of cathode material particles. This helps to improve the capacity, rate performance and electrochemical performance at low temperatures of the cathode material. Moreover, since the size of the cathode material can be controlled by controlling the size of the carbon-coated ferrous phosphate, it is also beneficial to better control quality and improve the quality stability of the cathode material preparation process.

[0007] In some embodiments, the molar ratio of iron in the organic iron salt to phosphorus in the phosphorus source is 3:(2.02~2.08).

[0008] In this embodiment, controlling the Fe / P molar ratio within the above range helps to regulate the elemental ratio in the product and obtain ferrous phosphate with a suitable elemental ratio.

[0009] In some embodiments, the density of the mixed solution is 1.15~1.30 g / mL.

[0010] In this embodiment, controlling the density of the mixed solution within the above-mentioned range can regulate the content of organic iron salt and phosphorus source in the mixed solution, keeping them within a suitable range. This avoids oversaturation due to excessively high content of the two, which would lead to crystal precipitation, or excessive water content due to excessively low content, which would increase the load on spray drying. This helps to improve production efficiency and reduce costs.

[0011] In some embodiments, the concentration of the complexing agent in the mixed solution is 0.005~0.1mol / L.

[0012] In this embodiment, a complexing agent is added to suppress precipitation and ensure that the mixed solution exists in solution form before spray drying, thereby ensuring the size consistency and small particle size distribution of the dry powder. To ensure that the complexing agent plays its full role and ensures the stability of the mixed solution, the amount of complexing agent added can be adjusted to control the concentration of the complexing agent in the mixed solution within the above-mentioned range.

[0013] In some embodiments, the pH of the mixed solution is less than or equal to 2.5.

[0014] In this embodiment, controlling the pH of the mixed solution within the aforementioned range provides a suitable environment for the complexing agent to function. It is understood that if the pH of the mixed solution obtained after mixing the organic iron salt, phosphorus source, and complexing agent is not within the aforementioned range, phosphoric acid can be used for pH adjustment.

[0015] In some embodiments, the organic iron salt includes at least one of ferrous gluconate, ferrous citrate, ferrous citrate, ferrous ascorbate, and ferrous succinate; the phosphorus source includes at least one of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, ammonium phosphate, phytic acid, triethyl phosphate, and vinylphosphonic acid; and the complexing agent includes at least one of ethylenediaminetetraacetic acid, diisobutyltriaminepentaacetic acid, and aminotriacetic acid.

[0016] In this embodiment, the above-mentioned organic iron salt and phosphorus source compound are widely available and easy to obtain, and are water-soluble, so they can be dissolved well in water; the above-mentioned complexing agent compound is widely available and easy to obtain, and has a good complexing effect.

[0017] In some embodiments, the water content in the dry powder is less than or equal to 1% by mass; the D50 particle size of the dry powder is 3~30μm.

[0018] In this embodiment, controlling the water content of the dry powder obtained by spray drying within the above-mentioned range helps to regulate the water vapor generated during the sintering stage, thereby improving the phase purity of carbon-coated ferrous phosphate; by controlling the particle size of the dry powder within the above-mentioned range, it helps to regulate the size of carbon-coated ferrous phosphate to obtain a smaller size product.

[0019] In some embodiments, during spray drying, the inlet air temperature is 200~350℃, the drying temperature is 110~130℃, and the outlet temperature is 60~90℃; during spray drying, the particle size of the spray droplets is controlled to be 5~20μm.

[0020] In this embodiment, by adjusting the various temperature parameters of spray drying, the moisture content of the product can be controlled, thereby controlling the product morphology and dispersibility; by adjusting the droplet size, the dispersibility and size of the dry powder can be controlled.

[0021] In some embodiments, the sintering process includes: heating to 400-600°C at a heating rate of 30-80°C / h, holding at that temperature for 5-10 hours, and then cooling to a temperature less than or equal to 80°C.

[0022] In this embodiment, controlling the various process parameters of the sintering process within the above-mentioned range can promote the mutual reaction and fusion of raw materials and crystallization to form crystals.

[0023] Secondly, embodiments of this application provide a carbon-coated ferrous phosphate, prepared by the method described above.

[0024] In the technical solution of this application embodiment, carbon-coated ferrous phosphate uses anhydrous ferrous phosphate as the core and a carbon layer as the coating layer on the surface of the anhydrous ferrous phosphate. The carbon-coated ferrous phosphate has a small particle size, high purity, and uniform carbon layer coating, resulting in better activity. When using the carbon-coated ferrous phosphate as a precursor to synthesize lithium iron phosphate cathode materials, not only is it not necessary to add an additional carbon source, but also, due to the presence of the carbon layer, the mutual fusion and agglomeration of adjacent precursor particles is suppressed during grinding and mixing, thereby avoiding the growth of cathode material particles. This helps to improve the capacity, rate performance, and electrochemical performance at low temperatures of the cathode material. Furthermore, since the size of the cathode material can be controlled by controlling the size of the carbon-coated ferrous phosphate, it is also beneficial to better control quality and improve the quality stability of the cathode material preparation process.

[0025] In some embodiments, the D50 particle size of the carbon-coated ferrous phosphate is 10~30 μm.

[0026] In this embodiment, the carbon-coated ferrous phosphate has a small size.

[0027] In some embodiments, the carbon content in the carbon-coated ferrous phosphate is 3.25-6.25% by mass.

[0028] In this embodiment, controlling the carbon content within the above range can ensure that the core is fully coated to fully improve the conductivity and electrochemical stability of the material, while avoiding damage to the carbon layer due to grinding during the cathode material synthesis process.

[0029] Thirdly, embodiments of this application provide a positive electrode material prepared from carbon-coated ferrous phosphate as described above.

[0030] In the technical solution of this application embodiment, the positive electrode material obtained by using carbon-coated ferrous phosphate as a precursor has the advantages of small primary particle size, high specific capacity, high rate performance, and good electrochemical performance at low temperature, due to the protection of the carbon layer avoiding the problem of particle agglomeration during the grinding stage.

[0031] In some embodiments, the D50 particle size of the cathode material is 0.8~2μm; and the mass percentage of carbon in the cathode material is 1.3~2%.

[0032] In this embodiment, the cathode material has a small particle size and advantages such as high specific capacity, high rate performance, and good electrochemical performance at low temperature; at the same time, the carbon content in the cathode material is controlled within the above range to ensure that its lithium iron phosphate core is fully coated.

[0033] Fourthly, embodiments of this application provide a method for preparing the cathode material as described above, comprising the following steps: A slurry is prepared by mixing carbon-coated ferrous phosphate, a metal source, and water. The slurry is calcined to obtain the cathode material; The metal source includes a lithium source or a mixture of a lithium source and a doped metal source.

[0034] In the technical solution of this application embodiment, using carbon-coated ferrous phosphate as a precursor to synthesize lithium iron phosphate cathode materials not only eliminates the need for additional carbon sources, but also, due to the presence of the carbon layer, suppresses the mutual fusion and agglomeration of adjacent precursor particles during grinding and mixing. This prevents cathode material particle growth and helps improve the capacity, rate performance, and electrochemical performance at low temperatures of the cathode material. Simultaneously, since the size of the cathode material can be controlled by controlling the size of the carbon-coated ferrous phosphate, it also facilitates better quality control and improves the quality stability of the cathode material preparation process. It is understood that the metal source can be a lithium source, thus producing a carbon-coated, undoped lithium iron phosphate material. Alternatively, the metal source can be a mixture of a lithium source and a doped metal source, where the doped metal source refers to a compound used to provide the doped metal element, thus producing a carbon-coated, doped lithium iron phosphate material.

[0035] In some embodiments, the lithium source includes at least one of lithium carbonate, lithium oxalate, lithium acetate, lithium dihydrogen phosphate, and lithium phosphate; the doped metal source includes at least one of titanium source and vanadium source, wherein the titanium source includes at least one of titanium dioxide and tetrabutyl titanate, and the vanadium source includes at least one of vanadium pentoxide and ammonium metavanadate.

[0036] In this embodiment, the lithium source is common and readily available, and can be well integrated and dispersed with carbon-coated ferrous phosphate; the various titanium and vanadium compounds can provide titanium and vanadium elements for doping, which helps to broaden ion migration channels and improve kinetic performance.

[0037] In some embodiments, the molar ratio of iron in the carbon-coated ferrous phosphate to lithium in the lithium source is 1:(1.01~1.05); the molar ratio of the doped metal element in the lithium source to iron in the carbon-coated ferrous phosphate is greater than 0 and less than or equal to 0.1.

[0038] In this embodiment, controlling the molar ratio of carbon-coated ferrous phosphate and lithium within the above-mentioned range helps to construct lithium iron phosphate materials with suitable element ratios, which helps to improve the electrochemical performance of the materials; controlling the amount of dopant added within the above-mentioned range helps to increase the doping amount in the product.

[0039] In some embodiments, the calcination temperature is 750~850℃, and the calcination time is 6~12h.

[0040] In this embodiment, controlling the calcination temperature and time within the above range helps to promote the formation and reconstruction of the crystal structure, resulting in a more ordered and complete crystal structure, thereby improving the electrochemical performance of the cathode material.

[0041] Fifthly, embodiments of this application provide a positive electrode sheet, comprising the positive electrode material described above, or the positive electrode material prepared by the preparation method described above.

[0042] In this embodiment, the positive electrode sheet contains the aforementioned positive electrode material, thus possessing the advantages of high specific capacity, high rate performance, and good electrochemical performance at low temperatures.

[0043] Sixthly, embodiments of this application provide a secondary battery, including the positive electrode sheet described above.

[0044] In this embodiment, the secondary battery includes the aforementioned positive electrode, thus possessing advantages such as high specific capacity, high rate performance, and good electrochemical performance at low temperatures.

[0045] Seventhly, embodiments of this application provide an electrical device including the secondary battery described above.

[0046] In this embodiment, the electrical device includes the aforementioned secondary battery, thus possessing advantages such as high specific capacity, high rate performance, and good electrochemical performance at low temperatures.

[0047] The above description is only an overview of the technical solution of this application. In order to better understand the technical means of this application and to implement it in accordance with the contents of the specification, and to make the above and other objects, features and advantages of this application more obvious and understandable, the following are specific embodiments of this application. Attached Figure Description

[0048] To more clearly illustrate the technical solutions of this application, the accompanying drawings used in this application will be briefly described below. Obviously, the drawings described below are merely some embodiments of this application. For those skilled in the art, other drawings can be obtained from these drawings without any creative effort.

[0049] Figure 1 This is a schematic flowchart of a method for preparing carbon-coated ferrous phosphate according to an embodiment of this application; Figure 2 This is a SEM image of carbon-coated ferrous phosphate prepared in Example 1 at the first magnification. Figure 3This is a SEM image of carbon-coated ferrous phosphate prepared in Example 1 at the second magnification. Figure 4 This is a TEM image of the carbon-coated ferrous phosphate obtained in Example 1; Figure 5 The XRD pattern of carbon-coated ferrous phosphate prepared in Example 1; Figure 6 This is a schematic flowchart of a method for preparing a positive electrode material according to an embodiment of this application. Detailed Implementation

[0050] The embodiments of the technical solution of this application will now be described in detail with reference to the accompanying drawings. These embodiments are only used to more clearly illustrate the technical solution of this application and are therefore merely examples, and should not be used to limit the scope of protection of this application.

[0051] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the application; the terms “comprising” and “having”, and any variations thereof, in the specification and claims of this application are intended to cover non-exclusive inclusion.

[0052] In the description of the embodiments of this application, technical terms such as "first" and "second" are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly specifying the number, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, "multiple" means two or more, unless otherwise explicitly defined.

[0053] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0054] In the description of the embodiments in this application, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, and B existing alone. Additionally, the character " / " in this document generally indicates that the preceding and following related objects have an "or" relationship.

[0055] In the description of the embodiments of this application, the term "multiple" refers to two or more (including two), similarly, "multiple sets" refers to two or more (including two sets), and "multiple pieces" refers to two or more (including two pieces).

[0056] In the description of the embodiments of this application, the technical terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," "counterclockwise," "axial," "radial," and "circumferential" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing the embodiments of this application and simplifying the description, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the embodiments of this application.

[0057] In the description of the embodiments of this application, unless otherwise expressly specified and limited, technical terms such as "installation," "connection," "joining," and "fixing" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. For those skilled in the art, the specific meaning of the above terms in the embodiments of this application can be understood according to the specific circumstances.

[0058] Currently, the mainstream lithium iron phosphate (LFP) preparation process uses iron phosphate and lithium carbonate as precursors, which are mixed with a carbon source, ground, and then calcined. However, this process has the following problems: when grinding the precursor materials (iron phosphate, lithium carbonate, etc.), the small particle size causes agglomeration of materials such as iron phosphate. As a result, during high-temperature calcination, the agglomerated iron phosphate is prone to fuse and grow into large single crystal particles, leading to a larger primary particle size of lithium iron phosphate and affecting the material's capacity.

[0059] In view of this, this application provides a carbon-coated ferrous phosphate and its preparation method, a cathode material and its preparation method, a cathode electrode sheet, and a secondary battery. It proposes using carbon-coated ferrous phosphate as a precursor, thereby controlling the size of the cathode material by controlling the size of the carbon-coated ferrous phosphate. This effectively solves the technical problem of the large particle size of primary lithium iron phosphate particles in the existing lithium iron phosphate preparation process, which helps to improve the capacity, rate performance, and electrochemical performance at low temperatures of the cathode material, and also helps to improve the capacity, rate performance, and electrochemical performance at low temperatures of the cathode electrode sheet, secondary battery, and power-consuming device.

[0060] Firstly, embodiments of this application provide a method for preparing carbon-coated ferrous phosphate; please refer to [link to relevant documentation]. Figure 1The preparation method includes the following steps: Step S10: Mix the organic iron salt, phosphorus source, complexing agent, and water to obtain a mixed solution.

[0061] Step S20: Spray dry the mixed solution to obtain dry powder.

[0062] Step S30: The dry powder is sintered to obtain carbon-coated ferrous phosphate.

[0063] In the technical solution of this application embodiment, a mixed solution formed by organic iron salt, phosphorus source, complexing agent and water is spray-dried, and then the resulting dry powder is sintered to obtain anhydrous ferrous phosphate with a carbon layer on the surface. This process is short, produces no wastewater, and the carbon-coated ferrous phosphate has a small particle size, high purity and uniform carbon layer coating, resulting in better activity. When using this carbon-coated ferrous phosphate as a precursor to synthesize lithium iron phosphate cathode materials, not only is it not necessary to add an additional carbon source, but also, due to the presence of the carbon layer, the mutual fusion and agglomeration of adjacent precursor particles is suppressed during grinding and mixing, thereby avoiding the growth of cathode material particles. This helps to improve the capacity, rate performance and electrochemical performance at low temperatures of the cathode material. Moreover, since the size of the cathode material can be controlled by controlling the size of the carbon-coated ferrous phosphate, it is also beneficial to better control quality and improve the quality stability of the cathode material preparation process.

[0064] Specifically, in this preparation method, water-soluble organic iron salts and phosphorus sources are used as raw materials, and a complexing agent is added simultaneously to inhibit precipitation formation and maintain solution stability. This allows the components to mix in ionic form, resulting in more uniform material mixing and smaller powder size obtained from spray drying. Consequently, smaller-sized products can be formed. Furthermore, due to the more uniform distribution of organic ions and the better coating effect of the organic carbon source itself, the carbon layer formed after sintering is not only more uniform but also has a better coating effect. Compared with the synthesis method using inorganic iron salts, phosphorus sources, and carbon sources as raw materials, the carbon-coated ferrous phosphate primary particles prepared by this method are smaller in size and have higher activity.

[0065] Furthermore, in some embodiments, the molar ratio of iron in the organic iron salt to phosphorus in the phosphorus source is 3:(2.02~2.08); for example, it can be 3:2.02, 3:2.03, 3:2.04, 3:2.05, 3:2.06, 3:2.07, 3:2.08, or any value between any two of the above.

[0066] In this embodiment, controlling the Fe / P molar ratio within the above range helps to regulate the elemental ratio in the product and obtain ferrous phosphate with a suitable elemental ratio.

[0067] Furthermore, in some embodiments, the density of the mixed solution is 1.15~1.30 g / mL; for example, it can be 1.15 g / mL, 1.17 g / mL, 1.20 g / mL, 1.22 g / mL, 1.25 g / mL, 1.28 g / mL, 1.30 g / mL, or any value between any two of the above.

[0068] In this embodiment, controlling the density of the mixed solution within the above-mentioned range can regulate the content of organic iron salt and phosphorus source in the mixed solution, keeping them within a suitable range. This avoids oversaturation due to excessively high content of the two, which would lead to crystal precipitation, or excessive water content due to excessively low content, which would increase the load on spray drying. This helps to improve production efficiency and reduce costs.

[0069] Furthermore, in some embodiments, the concentration of the complexing agent in the mixed solution is 0.005~0.1 mol / L; for example, it can be 0.005 mol / L, 0.008 mol / L, 0.01 mol / L, 0.03 mol / L, 0.05 mol / L, 0.08 mol / L, 0.1 mol / L, or any value between any two of the above.

[0070] When the system contains only phosphorus source and iron salt, and the pH of the system fluctuates, the two are prone to react to form ferrous phosphate and precipitate directly. In this embodiment, a complexing agent is added to inhibit precipitation and ensure that the mixed solution exists in solution form before spray drying, thereby ensuring the size consistency and small particle size distribution of the dry powder. To ensure that the complexing agent plays a full role and ensures the stability of the mixed solution, the amount of complexing agent added can be adjusted to control the concentration of the complexing agent in the mixed solution within the above range.

[0071] Furthermore, in some embodiments, the pH of the mixed solution is less than or equal to 2.5.

[0072] In this embodiment, controlling the pH of the mixed solution within the aforementioned range provides a suitable environment for the complexing agent to function. It is understood that if the pH of the mixed solution obtained after mixing the organic iron salt, phosphorus source, and complexing agent is not within the aforementioned range, phosphoric acid can be used for pH adjustment.

[0073] Furthermore, in some embodiments, the organic iron salt may include, but is not limited to, at least one of ferrous gluconate, ferric citrate, ferrous citrate, ferrous ascorbate, and ferrous succinate; the phosphorus source may include, but is not limited to, at least one of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, ammonium phosphate, phytic acid, triethyl phosphate, and vinylphosphonic acid; and the complexing agent may include, but is not limited to, at least one of ethylenediaminetetraacetic acid, diisobutyltriaminepentaacetic acid, and aminotriacetic acid.

[0074] In this embodiment, the aforementioned organic iron salts and phosphorus source compounds are widely available and readily obtained, and are water-soluble, dissolving well in water. Furthermore, since the organic ions of the organic iron salts act as a carbon source during the reaction, the method of this application is not stringent in its requirement that the iron source be a ferrous salt, thus helping to expand the range of selectable organic iron salts. The aforementioned complexing agent compounds are widely available and readily obtained, and exhibit good complexing effects.

[0075] Furthermore, in some embodiments, the water content in the dry powder is less than or equal to 1% by mass; the D50 particle size of the dry powder is 3~30μm.

[0076] In this embodiment, controlling the water content of the dry powder obtained by spray drying within the above-mentioned range helps to regulate the water vapor generated during the sintering stage, thereby improving the phase purity of carbon-coated ferrous phosphate; by controlling the particle size of the dry powder within the above-mentioned range, it helps to regulate the size of carbon-coated ferrous phosphate to obtain a smaller size product.

[0077] Furthermore, in some embodiments, during the spray drying process, the inlet air temperature is 200~350℃, for example, it can be 200℃, 220℃, 250℃, 280℃, 300℃, 330℃, 350℃, or any value between any two of the above; the drying temperature is 110~130℃, for example, it can be 110℃, 115℃, 120℃, 125℃, 130℃, or any value between any two of the above; the outlet temperature is 60~90℃, for example, it can be 60℃, 70℃, 80℃, 90℃, or any value between any two of the above; during the spray drying process, the particle size of the spray droplets is controlled to be 5~20μm, for example, it can be 5μm, 8μm, 10μm, 13μm, 15μm, 17μm, 20μm, or any value between any two of the above.

[0078] In this embodiment, by controlling the various temperature parameters of spray drying, the moisture content of the product can be controlled, thereby controlling the morphology and dispersibility of the product; by controlling the droplet size, the dispersibility and size of the dry powder can be controlled, avoiding the decrease in dry powder dispersibility or the size not meeting the requirements due to the droplet size being too small, and avoiding the formation of large particles due to the droplets sticking together, thereby affecting the size of the dry powder.

[0079] Further, in some embodiments, the sintering process includes: heating to 400-600°C at a heating rate of 30-80°C / h, holding at that temperature for 5-10 hours, and then cooling to a temperature less than or equal to 80°C. The heating rate can be 30°C / h, 40°C / h, 50°C / h, 60°C / h, 70°C / h, 80°C / h, or any value between any two of these values; the sintering temperature can be 400°C, 450°C, 500°C, 550°C, 600°C, or any value between any two of these values; and the sintering time can be 5 hours, 6 hours, 7 hours, 8 hours, 9 hours, 10 hours, or any value between any two of these values. It is understood that the sintering process can be carried out in a protective atmosphere, which includes, but is not limited to, nitrogen, argon, or a nitrogen / argon mixture. During the sintering process, the furnace pressure is maintained at 10~200Pa, and the humidity in the heat preservation section is ≤0.5%. After the material is discharged from the furnace, it is vacuum-packed after being screened. The screen used can be an 80~200 mesh screen.

[0080] In this embodiment, controlling the various process parameters of the sintering process within the above-mentioned range can promote the mutual reaction and fusion of raw materials and crystallization to form crystals.

[0081] Secondly, embodiments of this application provide a carbon-coated ferrous phosphate, prepared by the method described above.

[0082] In the technical solution of this application embodiment, carbon-coated ferrous phosphate uses anhydrous ferrous phosphate as the core and a carbon layer as the coating layer on the surface of the anhydrous ferrous phosphate. The carbon-coated ferrous phosphate has a small particle size, high purity, and uniform carbon layer coating, resulting in better activity. When using the carbon-coated ferrous phosphate as a precursor to synthesize lithium iron phosphate cathode materials, not only is it not necessary to add an additional carbon source, but also, due to the presence of the carbon layer, the mutual fusion and agglomeration of adjacent precursor particles is suppressed during grinding and mixing, thereby avoiding the growth of cathode material particles. This helps to improve the capacity, rate performance, and electrochemical performance at low temperatures of the cathode material. Furthermore, since the size of the cathode material can be controlled by controlling the size of the carbon-coated ferrous phosphate, it is also beneficial to better control quality and improve the quality stability of the cathode material preparation process.

[0083] Furthermore, in some embodiments, the D50 particle size of the carbon-coated ferrous phosphate is 10~30μm; for example, it can be 10μm, 15μm, 20μm, 25μm, 30μm, or any value between the two above.

[0084] In this embodiment, the carbon-coated ferrous phosphate has a small size.

[0085] Furthermore, in some embodiments, the carbon mass percentage in the carbon-coated ferrous phosphate is 3.25% to 6.25%; for example, it can be 3.25%, 3.5%, 4%, 4.5%, 4.75%, 5%, 5.5%, 6%, 6.25%, or any value between any two of the above.

[0086] In this embodiment, controlling the carbon content within the above range can ensure that the core is fully coated to fully improve the conductivity and electrochemical stability of the material, while avoiding damage to the carbon layer due to grinding during the cathode material synthesis process.

[0087] Thirdly, embodiments of this application provide a positive electrode material prepared from carbon-coated ferrous phosphate as described above.

[0088] In the technical solution of this application embodiment, the cathode material obtained using carbon-coated ferrous phosphate as a precursor avoids the size increase problem caused by particle agglomeration during the grinding stage due to the protection of the carbon layer, resulting in a smaller primary particle size and advantages such as high specific capacity, high rate performance, and good electrochemical performance at low temperatures. It is understood that this cathode material can be a carbon-coated, undoped lithium iron phosphate material, or a carbon-coated, doped lithium iron phosphate material; this application does not limit this.

[0089] Furthermore, in some embodiments, the D50 particle size of the cathode material is 0.8~2μm; for example, it can be 0.8μm, 1μm, 1.2μm, 1.5μm, 1.7μm, 2μm, or any value between any two of the above. The carbon mass percentage in the cathode material is 1.3~2%; for example, it can be 1.3%, 1.5%, 1.8%, 2%, or any value between any two of the above.

[0090] In this embodiment, the cathode material has a small particle size and advantages such as high specific capacity, high rate performance, and good electrochemical performance at low temperature; at the same time, the carbon content in the cathode material is controlled within the above range to ensure that its lithium iron phosphate core is fully coated.

[0091] Fourthly, this application provides a method for preparing the cathode material as described above. Please refer to [link to relevant documentation]. Figure 6 The preparation method includes the following steps: Step S1: Mix carbon-coated ferrous phosphate, a metal source, and water to obtain a slurry; Step S2: Calcine the slurry to obtain the positive electrode material; The metal source includes a lithium source or a mixture of a lithium source and a doped metal source.

[0092] In the technical solution of this application embodiment, using carbon-coated ferrous phosphate as a precursor to synthesize lithium iron phosphate cathode materials not only eliminates the need for additional carbon sources, but also, due to the presence of the carbon layer, suppresses the mutual fusion and agglomeration of adjacent precursor particles during grinding and mixing. This prevents cathode material particle growth and helps improve the capacity, rate performance, and electrochemical performance at low temperatures of the cathode material. Simultaneously, since the size of the cathode material can be controlled by controlling the size of the carbon-coated ferrous phosphate, it also facilitates better quality control and improves the quality stability of the cathode material preparation process. It is understood that the metal source can be a lithium source, thus producing a carbon-coated, undoped lithium iron phosphate material. Alternatively, the metal source can be a mixture of a lithium source and a doped metal source, where the doped metal source refers to a compound used to provide the doped metal element, thus producing a carbon-coated, doped lithium iron phosphate material.

[0093] Furthermore, in some embodiments, the lithium source may include, but is not limited to, at least one of lithium carbonate, lithium oxalate, lithium acetate, lithium dihydrogen phosphate, and lithium phosphate; the doped metal source may include, but is not limited to, at least one of titanium source and vanadium source, the titanium source may include, but is not limited to, at least one of titanium dioxide and tetrabutyl titanate, and the vanadium source may include, but is not limited to, at least one of vanadium pentoxide and ammonium metavanadate.

[0094] In this embodiment, the lithium source is common and readily available, and can be well integrated and dispersed with carbon-coated ferrous phosphate; the various titanium and vanadium compounds can provide titanium and vanadium elements for doping, which helps to broaden ion migration channels and improve kinetic performance.

[0095] Furthermore, the metal source contains phosphorus; for example, the lithium source contains phosphorus, or at least one of the lithium source and the doped metal source contains phosphorus. This serves to supplement phosphorus, thereby better optimizing the elemental ratio in the cathode material. In some embodiments, the amount of carbon-coated ferrous phosphate and the amount of metal source added can also satisfy the requirement that the molar ratio of iron to phosphorus is close to or equal to 1:1.

[0096] Furthermore, in some embodiments, the molar ratio of iron in the carbon-coated ferrous phosphate to lithium in the lithium source is 1:(1.01~1.05), for example, it can be 1:1.01, 1:1.02, 1:1.03, 1:1.04, 1:1.05, or any value between the above two; the molar ratio of the doped metal element in the lithium source to the iron in the carbon-coated ferrous phosphate is greater than 0 and less than or equal to 0.1.

[0097] In this embodiment, controlling the molar ratio of carbon-coated ferrous phosphate and lithium within the above-mentioned range helps to construct lithium iron phosphate materials with suitable element ratios, which helps to improve the electrochemical performance of the materials; controlling the amount of dopant added within the above-mentioned range helps to increase the doping amount in the product.

[0098] Further, in some embodiments, the calcination temperature is 750~850℃, for example, it can be 750℃, 760℃, 780℃, 800℃, 820℃, 840℃, 850℃, or any value between two of the above; the calcination time is 6~12h; for example, it can be 6h, 7h, 8h, 10h, 12h, or any value between two of the above. It is understood that the calcination process is carried out in a protective atmosphere, which includes, but is not limited to, nitrogen, argon, or a nitrogen / argon mixture. During the calcination process, the furnace pressure is maintained at 10~200Pa. After calcination, the product can be pulverized until its D50 particle size is 0.8~2μm, thus obtaining the cathode material.

[0099] In this embodiment, controlling the calcination temperature and time within the aforementioned range helps to promote the formation and reconstruction of the crystal structure, resulting in a more ordered and complete crystal structure, thereby improving the electrochemical performance of the cathode material. Fifthly, embodiments of this application provide a cathode sheet, comprising the cathode material described above, or the cathode material prepared by the preparation method described above.

[0100] In this embodiment, the positive electrode sheet contains the aforementioned positive electrode material, thus possessing the advantages of high specific capacity, high rate performance, and good electrochemical performance at low temperatures.

[0101] Sixthly, embodiments of this application provide a secondary battery, including the positive electrode sheet described above.

[0102] In this embodiment, the secondary battery includes the aforementioned positive electrode, thus possessing advantages such as high specific capacity, high rate performance, and good electrochemical performance at low temperatures.

[0103] Seventhly, embodiments of this application provide an electrical device including the secondary battery described above.

[0104] The electrical devices provided in this application embodiment can be, but are not limited to, mobile phones, tablets, laptops, electric toys, power tools, electric vehicles, electric cars, ships, spacecraft, etc. Among them, electric toys can include stationary or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric airplane toys, etc., and spacecraft can include airplanes, rockets, space shuttles, and spacecraft, etc.

[0105] The following are some specific embodiments. It should be noted that the embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they shall be performed in accordance with the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be obtained commercially.

[0106] I. Preparation Method Example 1 (1) Preparation of carbon-coated ferrous phosphate: In step S10, according to the molar ratio of iron in the organic iron salt to phosphorus in the phosphorus source being 3:2.05, the concentration of the complexing agent in the mixed solution being 0.008 mol / L, and the density of the mixed solution being 1.23 g / mL, ferrous gluconate, phosphoric acid, and EDTA are added to pure water, dissolved, and stirred to obtain a clear mixed solution with a pH of 1.32.

[0107] In step S20, the mixed solution is spray-dried to obtain a dry powder with a D50 particle size of 15 μm and a moisture content of ≤1.0 wt%. During the spray drying process, the following process parameters are controlled: inlet air temperature is 280℃, drying temperature is 120℃, spray droplet particle size is 15.5 μm, and outlet temperature is 75℃.

[0108] Step S30: Under nitrogen protection, the dry powder is sintered. During the sintering process, the furnace pressure is maintained at 100 Pa and the humidity in the heat preservation section is ≤0.5%. The sintering process is as follows: the temperature is raised to 500℃ at a heating rate of 60℃ / h, and then held for 8 hours. After cooling to a material temperature ≤80℃, the material is discharged and screened through an 80-mesh ultrasonic vibrating screen to obtain carbon-coated ferrous phosphate.

[0109] (2) Preparation of lithium iron phosphate materials: In step S1, the carbon-coated ferrous phosphate, lithium phosphate and nano titanium dioxide prepared in step (1) are mixed evenly according to the molar ratio of iron in carbon-coated ferrous phosphate, lithium in lithium phosphate and titanium in nano titanium dioxide as 3:3.06:0.05, and then pure water is added for gelatinization to obtain a slurry.

[0110] Step S2: The slurry is placed in a roller furnace for calcination at a temperature of 790℃ for 9 hours and a furnace pressure of 50 Pa to obtain calcined material. The calcined material is then pulverized to a D50 particle size of 1.4 micrometers to obtain the cathode material.

[0111] Example 2 (1) Preparation of carbon-coated ferrous phosphate: In step S10, according to the molar ratio of iron in the organic iron salt to phosphorus in the phosphorus source being 3:2.08, the concentration of the complexing agent in the mixed solution being 0.008 mol / L, and the density of the mixed solution being 1.15 g / mL, ferric citrate, ammonium dihydrogen phosphate, and nitric acid were added to pure water, dissolved and stirred, and the pH was adjusted to obtain a clear mixed solution with a pH of 2.0.

[0112] In step S20, the mixed solution is spray-dried to obtain a dry powder with a D50 particle size of 11.7 μm and a moisture content of ≤1.0 wt%. During the spray drying process, the following process parameters are controlled: inlet air temperature of 350℃, drying temperature of 130℃, spray droplet size of 20 μm, and outlet temperature of 90℃.

[0113] Step S30: Under nitrogen protection, the dry powder is sintered. During the sintering process, the furnace pressure is maintained at 100 Pa and the humidity in the heat preservation section is ≤0.5%. The sintering process is as follows: the temperature is raised to 400℃ at a heating rate of 30℃ / h, and then held for 10h. After cooling to the material temperature ≤80℃, the material is discharged and screened through a 100-mesh ultrasonic vibrating screen to obtain carbon-coated ferrous phosphate.

[0114] (2) Preparation of lithium iron phosphate materials: In step S1, the carbon-coated ferrous phosphate, lithium dihydrogen phosphate and nano titanium dioxide obtained in step (1) are mixed evenly according to the molar ratio of iron in carbon-coated ferrous phosphate, lithium dihydrogen phosphate and nano titanium dioxide of 3:3.03:0.05, and then pure water is added to gelatinize and obtain slurry.

[0115] Step S2: The slurry is placed in a roller furnace for calcination at a temperature of 750℃ for 12 hours and a furnace pressure of 50 Pa to obtain calcined material. The calcined material is then pulverized to a D50 particle size of 0.8 micrometers to obtain the cathode material.

[0116] Example 3 (1) Preparation of carbon-coated ferrous phosphate: In step S10, according to the molar ratio of iron in the organic iron salt to phosphorus in the phosphorus source being 3:2.02, the concentration of the complexing agent in the mixed solution being 0.008 mol / L, and the density of the mixed solution being 1.30 g / mL, ferrous succinate, phosphoric acid, and diisobutyltriaminepentaacetic acid are added to pure water, dissolved and stirred to obtain a clear mixed solution with a pH of 1.4.

[0117] In step S20, the mixed solution is spray-dried to obtain a dry powder with a D50 particle size of 3.4 μm and a moisture content of ≤1.0 wt%. During the spray drying process, the following process parameters are controlled: inlet air temperature is 200℃, drying temperature is 110℃, spray droplet particle size is 5 μm, and discharge temperature is 60℃.

[0118] Step S30: Under nitrogen protection, the dry powder is sintered. During the sintering process, the furnace pressure is maintained at 100 Pa and the humidity in the heat preservation section is ≤0.5%. The sintering process is as follows: the temperature is raised to 600℃ at a heating rate of 80℃ / h, held for 5 hours, and then cooled to ≤80℃ before being discharged. The material is then screened through a 200-mesh ultrasonic vibrating screen to obtain carbon-coated ferrous phosphate.

[0119] (2) Preparation of lithium iron phosphate materials: In step S1, the carbon-coated ferrous phosphate, lithium phosphate and titanate obtained in step (1) are mixed evenly according to the molar ratio of iron in carbon-coated ferrous phosphate, lithium phosphate and titanate n-butyl ester as 3:3.15:0.05, and then pure water is added for gelatinization to obtain a slurry.

[0120] Step S2: The slurry is placed in a roller furnace for calcination at a temperature of 850℃ for 6 hours and a furnace pressure of 50Pa to obtain calcined material. The calcined material is then pulverized to a D50 particle size of 2 micrometers to obtain the cathode material.

[0121] Example 4 This embodiment is basically the same as Embodiment 1, except that in step S10, the concentration of the complexing agent in the mixed solution is changed to 0.004 mol / L. All other steps and conditions remain unchanged.

[0122] Example 5 This embodiment is basically the same as Embodiment 1, except that in step S10, the concentration of the complexing agent in the mixed solution is changed to 0.005 mol / L. All other steps and conditions remain unchanged.

[0123] Example 6 This embodiment is basically the same as Embodiment 1, except that in step S10, the concentration of the complexing agent in the mixed solution is changed to 0.1 mol / L. All other steps and conditions remain unchanged.

[0124] Example 7 This embodiment is basically the same as Embodiment 1, except that in step S10, the concentration of the complexing agent in the mixed solution is changed to 0.11 mol / L. All other steps and conditions remain unchanged.

[0125] Example 8 This embodiment is basically the same as Embodiment 1, except that in step S10, the phosphorus source is changed to ammonium phosphate, and the pH of the mixed solution is adjusted to 2.8. All other steps and conditions remain unchanged.

[0126] Example 9 This embodiment is basically the same as Embodiment 1, except that in step S10, the organic iron salt is replaced with ferrous citrate. All other steps and conditions remain unchanged.

[0127] Example 10 This embodiment is basically the same as Embodiment 1, except that nano-titanium dioxide is not added in step S1. All other steps and conditions remain unchanged.

[0128] Comparative Example 1 This comparative example is basically the same as Example 1, except that step (1) is omitted in this comparative example, and step S1 in step (2) is changed to: Based on the molar ratio of Li:Fe:P:Ti of 3.06:3:3.09:0.05 and the mass ratio of ferric phosphate to glucose of 1:2.6, ferric phosphate (with an iron-to-phosphorus ratio of 0.972 and a BET of 8.5m) was added. 2 The mixture (g) of titanium dioxide, lithium carbonate, glucose, and nano-titanium dioxide is mixed together, then water is added to form a slurry with a solid content of 35%. The slurry is then nano-milled until the particle size is 320 nm, and then spray-dried to obtain a dried material. This dried material is used for calcination in step S2.

[0129] Apart from that, all other steps and conditions remain unchanged.

[0130] Comparative Example 2 This comparative example is basically the same as Example 1, except that inorganic iron salt, organic carbon source, phosphorus source and water are used as raw materials in this comparative example. Accordingly, step S10 is changed to: the iron source is ferrous oxalate, and glucose is introduced as an organic carbon source. The number of moles of iron and glucose added is the same as the number of moles of ferrous gluconate added in Example 1.

[0131] Apart from that, all other steps and conditions remain unchanged.

[0132] The final ferrous phosphate contained a large number of impurities, mainly ferrous oxide, ferric oxide and pyrophosphate, and the purity of ferrous phosphate was only 72%.

[0133] Comparative Example 3 This comparative example is basically the same as Example 1, except that the complexing agent EDTA is not added in step S10. All other steps and conditions remain unchanged.

[0134] Comparative Example 4 This comparative example is basically the same as Example 10, except that step (1) is omitted in this comparative example, and step S1 in step (2) is changed to: Based on the Fe:P:Li molar ratio of 3:3.05:3.06 and the ferric phosphate to glucose mass ratio of 1:2.6, ferric phosphate (with an iron-to-phosphorus ratio of 0.972 and a BET of 8.5m) was prepared. 2 The mixture (g) of lithium carbonate and glucose is mixed together, then water is added to form a slurry with a solid content of 35%. The slurry is then nano-milled to a particle size of 320 nm, and finally spray-dried to obtain a dried material. This dried material is used for calcination in step S2.

[0135] Apart from that, all other steps and conditions remain unchanged.

[0136] II. Testing Methods (1) Performance testing of carbon-coated ferrous phosphate and cathode material 1. Element content: Inductively coupled plasma optical emission spectrometer (ICP-OES) was used to test the carbon-coated ferrous phosphate and cathode material to detect the content of each element in the material, and an infrared carbon-sulfur analyzer was used to test the carbon content. The test results are shown in Table 1 and Table 2.

[0137] 2. Microstructure characterization of carbon-coated ferrous phosphate: Field emission scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to characterize the carbon-coated ferrous phosphate. The test results are as follows: Figures 2 to 4 As shown.

[0138] 3. Phase purity: The purity was determined using a Rigaku X-ray diffractometer (XRD) from Japan. The test results are shown below. Figure 5 .

[0139] 4. Particle size of primary particles: obtained by SEM testing and analysis using Nano Measurer 1.2 software. The test results are shown in Table 3.

[0140] 5. D10, D50 and D90 particle size detection: Malvern laser particle size analyzer was used for testing, and the test results are shown in Table 3.

[0141] 6. Specific surface area BET test: The specific surface area was determined by the gas adsorption BET method. The test results are shown in Table 4.

[0142] 7. Loose packing density test: The test was conducted using the funnel method in accordance with GB / T 31057.1-2014. The test results are shown in Table 4.

[0143] 8. Detection of ferric ion content: The redox titration method was used for detection, and the test results are shown in Table 4.

[0144] 9. Moisture content detection: The moisture content was tested using the loss on drying method at a temperature of 150℃. The test results are shown in Table 4.

[0145] 10. Powder resistivity test: The four-probe method was used for testing at a pressure of 8 MPa. The test results are shown in Table 4.

[0146] 11. Compacted density (PD) test: The compacted density was tested using a UTM7305 battery powder compaction density tester provided by Shenzhen Sansi Zongheng Technology Co., Ltd. The test pressure was 3T and the pressing time was 30s. The test results are shown in Table 4.

[0147] 12. pH test: Refer to GB / T 9724-2007 General Rules for pH determination of chemical reagents. The test results are shown in Table 2.

[0148] (2) Performance testing of secondary batteries The positive electrode materials prepared in each embodiment and comparative example were mixed with conductive carbon black and binder (PVDF5130) at a mass ratio of 92:4:4, respectively. PVDF was dissolved in N-methylpyrrolidone (NMP) to form a slurry. The slurry was coated onto a 12μm thick aluminum foil to obtain an electrode sheet. The electrode sheet was dried in a 100℃ vacuum oven. Subsequently, based on the electrode compaction density (2.30g / mL), the electrode sheet was rolled to a certain thickness, and then punched to a diameter of 15mm. After obtaining the dried electrode sheet, CR2032 button half-cells were assembled using an LG2400 / 1000TS glove box manufactured by Wiegand Gas Purification Technology (Suzhou) Co., Ltd.

[0149] The CR2032 button half-cell was tested using a battery performance testing system (model: CT2001A) manufactured by Wuhan Landian Electronics Technology Co., Ltd. The test temperature was 25±1℃ and the voltage range was 2.00-3.75V. The test results are shown in Table 5.

[0150] (3) Exhaust gas content test During the calcination and heat preservation process in step S2 of Example 1 and Comparative Example 1, samples were taken from the roller furnace to detect the component content of the gas in the heat preservation section. The results are shown in Table 5. The method for detecting the gas component content is GB / T 8979-2008 "Pure Nitrogen, High-Purity Nitrogen and Ultra-Purity Nitrogen".

[0151] III. Analysis of Test Results for Each Embodiment and Comparative Example Table 1. Elemental content of carbon-coated ferrous phosphate

[0152] Table 2 Elemental content and pH of cathode materials

[0153] Table 3

[0154] Table 4

[0155] Table 5

[0156] Table 6

[0157] Results analysis: See Figures 2 to 4 As can be seen, the carbon-coated ferrous phosphate obtained in Example 1 consists of irregular, dense particles with a primary particle size of approximately 200 nm, and its surface is coated with a carbon layer approximately 30 nm thick. This indicates that the preparation method of this application can produce ferrous phosphate with a small size and a carbon layer on its surface. Further, see [reference needed]. Figure 5 As can be seen, the material has a high degree of crystallinity and a relatively pure phase. Combining the impurity content data in Table 1 and the ferric ion content data in Table 4, it can be concluded that the method of this application can produce carbon-coated ferrous phosphate with high purity.

[0158] Furthermore, comparing Example 1 and Comparative Example 1, and Example 10 and Comparative Example 4, it can be seen that Example 1 has a higher compaction density, 0.1C initial discharge capacity and initial efficiency than Comparative Example 1, a smaller primary particle size and a narrower secondary particle size distribution than Comparative Example 1. Example 10 has a higher compaction density, 0.1C initial discharge capacity and initial efficiency than Comparative Example 4, and a smaller primary particle size than Comparative Example 4. This indicates that the carbon-coated ferrous phosphate prepared by the method of this application helps to prevent particle agglomeration, regulate the size of the cathode material, and thus improve the electrochemical performance of the cathode material.

[0159] Furthermore, comparing Example 1 and Comparative Example 2, the primary particles of carbon-coated ferrous phosphate prepared in Example 1 have smaller particle size and better battery performance, indicating that compared with the synthesis method using inorganic iron salts, phosphorus sources, and carbon sources as raw materials, the primary particles of carbon-coated ferrous phosphate prepared by this method are smaller in size and have higher activity.

[0160] Furthermore, comparing Examples 1, 4 to 7, and Comparative Example 3, it can be seen that Examples 1, 5, and 6 exhibit better performance in terms of particle size distribution and electrochemical performance. This indicates that adding a complexing agent and controlling its concentration within the range of 0.005~0.1 mol / L helps maintain the stability of the mixed solution, ensuring that the mixed solution exists in solution form before spray drying, thereby ensuring the size consistency and small particle size distribution of the dry powder and improving the electrical performance of the cathode material. When the amount of complexing agent is too small, the complexation is incomplete, resulting in a low concentration of complexed metal ions in the mixed solution, causing the precipitation rate of metal ions to be too fast, thus leading to insufficient stability. When the amount of complexing agent is too large, the concentration of complexed metal ions will be too high, resulting in incomplete precipitation of metal ions, thus causing an imbalance in the ratio.

[0161] Furthermore, Example 1 has a narrower particle size distribution, smaller primary particle size, and better electrochemical performance than Example 8, indicating that controlling the pH of the mixed solution below 2.5 is more conducive to maintaining the stability of the mixed solution, thereby ensuring the size consistency and small particle size distribution of the dry powder.

[0162] Furthermore, the data comparison in Table 6 shows that the content of gas components in the calcination furnace corresponding to Example 1 is lower than that in Comparative Example 1, indicating that the method of this application generates less waste gas and is more environmentally friendly. In addition, the waste gas in Example 1 contains significantly lower levels of CO, CO2, and H2. This may be because, in comparison, the raw materials in Comparative Example 1 tend to generate a large amount of water vapor and carbon dioxide during calcination. These components can easily enter the pores formed on the surface of the product due to carbon reduction, and then be reduced to form CO and H2, which is not conducive to improving the purity of the phase.

[0163] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A method for preparing carbon-coated ferrous phosphate, characterized in that, Includes the following steps: Organic iron salt, phosphorus source, complexing agent, and water are mixed to obtain a mixed solution; The mixed solution was spray-dried to obtain a dry powder; The dry powder was sintered to obtain carbon-coated ferrous phosphate.

2. The method for preparing carbon-coated ferrous phosphate according to claim 1, characterized in that, The molar ratio of iron in the organic iron salt to phosphorus in the phosphorus source is 3:(2.02~2.08); and / or, The density of the mixed solution is 1.15~1.30 g / mL; and / or, In the mixed solution, the concentration of the complexing agent is 0.005~0.1 mol / L; and / or, The pH of the mixed solution is less than or equal to 2.5; and / or, The organic ferric salt includes at least one of ferrous gluconate, ferrous citrate, ferrous citrate, ferrous ascorbate, and ferrous succinate; and / or, The phosphorus source includes at least one selected from phosphoric acid, diammonium dihydrogen phosphate, diammonium hydrogen phosphate, ammonium phosphate, phytic acid, triethyl phosphate, and vinylphosphonic acid; and / or, The complexing agent includes at least one of ethylenediaminetetraacetic acid, diisobutyltriaminepentaacetic acid, and aminotriacetic acid; and / or The dry powder contains less than or equal to 1% water by mass; and / or, The D50 particle size of the dry powder is 3~30μm; And / or, The sintering process includes: heating to 400-600°C at a heating rate of 30-80°C / h, holding at that temperature for 5-10 hours, and then cooling to a temperature less than or equal to 80°C.

3. The method for preparing carbon-coated ferrous phosphate according to claim 2, characterized in that, During the spray drying process, the inlet air temperature is 200~350℃, the drying temperature is 110~130℃, and the outlet temperature is 60~90℃; and / or, During the spray drying process, the droplet size is controlled to be 5~20μm.

4. A carbon-coated ferrous phosphate, characterized in that, It is prepared by the preparation method according to any one of claims 1 to 3.

5. The carbon-coated ferrous phosphate according to claim 4, characterized in that, The carbon-coated ferrous phosphate has a D50 particle size of 10~30 μm; and / or, The carbon-coated ferrous phosphate contains 3.25% to 6.25% carbon by mass.

6. A positive electrode material, characterized in that, It is prepared by carbon-coated ferrous phosphate as described in claim 4 or 5; wherein the D50 particle size of the cathode material is 0.8~2μm, and the mass percentage of carbon in the cathode material is 1.3~2%.

7. A method for preparing the cathode material according to claim 6, characterized in that, Includes the following steps: A slurry is prepared by mixing carbon-coated ferrous phosphate, a metal source, and water. The slurry is calcined to obtain the cathode material; The metal source includes a lithium source or a mixture of a lithium source and a doped metal source.

8. The method for preparing the cathode material according to claim 7, characterized in that, The lithium source includes at least one of lithium carbonate, lithium oxalate, lithium acetate, lithium dihydrogen phosphate, and lithium phosphate; and / or, The molar ratio of iron in the carbon-coated ferrous phosphate to lithium in the lithium source is 1:(1.01~1.05); and / or, The molar ratio of the doped metal element in the lithium source to the iron element in the carbon-coated ferrous phosphate is greater than 0 and less than or equal to 0.1; and / or, The doped metal source includes at least one of a titanium source and a vanadium source; the titanium source includes at least one of titanium dioxide and tetrabutyl titanate; the vanadium source includes at least one of vanadium pentoxide and ammonium metavanadate; and / or, The calcination temperature is 750~850℃, and the calcination time is 6~12h.

9. A positive electrode sheet, characterized in that, Includes the cathode material as described in claim 6.

10. A secondary battery, characterized in that, Includes the positive electrode sheet as described in claim 9.

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