Composite positive electrode material, preparation method and application thereof

Abstract

The application discloses a composite positive electrode material and a preparation method and application thereof, and belongs to the technical field of battery materials. m [B(CN)6] n , A and B are independently selected from at least one of Fe, Co, Ni, Mn, Cu and Zn, 1 The application has the advantages that compared with the prior art, a ferrocyanide complex coating layer is constructed on the surface of O2 phase lithium cobalt oxide through a low-temperature co-precipitation method, the coating layer is designed through vacancy regulation, irreversible damage of high temperature to the O2 structure can be avoided, cobalt ions dissolved out can be actively chelated and captured through cyanide functional groups, and three-dimensional ion channels of the coating layer ensure excellent rate performance of the lithium cobalt oxide material.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery technology, and particularly relates to a composite cathode material, its preparation method, and its application. Background Technology

[0002] Lithium cobalt oxide (LiCoO2) dominates the lithium-ion battery market for consumer electronics due to its high compaction density and excellent volumetric energy density. As consumers increasingly demand both longer battery life and faster charging speeds, battery cells are required to possess both high energy density and fast charging capabilities exceeding 3C. Traditional O3-phase lithium cobalt oxide faces challenges such as poor structural stability, cobalt dissolution, and oxygen evolution under 4.55V high-voltage and 3C fast charging conditions. Furthermore, the energy density gains from high doping and coating modifications of current O3-phase lithium cobalt oxide are limited, making it difficult to meet the demands.

[0003] O2-phase lithium cobalt oxide, due to its unique crystal structure, exhibits higher specific capacity, superior rate performance, and improved cycle performance resulting from a reversible structural phase transition, attracting widespread attention from academia and industry in recent years. However, the industrial application of O2-phase lithium cobalt oxide still faces the following technical challenges: High-temperature structural instability: When the temperature exceeds 250℃, the O2 phase lithium cobalt oxide undergoes an irreversible phase transition, transforming into the O3 phase structure, thus losing its intrinsic advantages.

[0004] Severe cobalt ion dissolution: During high-voltage charging and discharging, cobalt ions on the surface of O2 phase lithium cobalt oxide are easily dissolved, migrate to the negative electrode side and deposit, catalyzing the decomposition of the electrolyte, resulting in a sharp deterioration in the cell's cycle performance.

[0005] Existing modification methods are not applicable: conventional bulk doping and surface coating usually employ high-temperature solid-state methods (>500℃), which directly destroy the O2 phase structure. Some studies have attempted coating at the P2 phase sodium cobalt oxide precursor stage, but after lithium / sodium ion replacement, the coating uniformity is poor, and the binding effect on cobalt ions is limited.

[0006] Therefore, there is an urgent need to develop a modification method that can effectively suppress the dissolution of cobalt ions under high voltage without damaging its crystal structure, while maintaining the intrinsic high capacity and rate performance of O2 phase lithium cobalt oxide. Summary of the Invention

[0007] The purpose of this invention is to provide a composite cathode material, its preparation method, and its application, to solve the technical problems in the prior art where the structure of O2 phase lithium cobalt oxide is damaged due to the high-temperature coating process, and the cobalt ion dissolution is difficult to suppress under high voltage. The invention aims to achieve the technical effect of maintaining the intrinsic structure of the O2 phase and high-rate performance through heat treatment at no higher than 250°C throughout the process, and reducing the amount of cobalt dissolution by actively chelating and capturing dissolved cobalt ions using cyano vacancies.

[0008] To achieve the above objectives, the present invention provides the following technical solution: In a first aspect, the present invention provides a composite cathode material, the composite cathode material comprising: a cathode active material matrix; and a Prussian blue-like compound coating layer covering at least a portion of the surface of the cathode active material matrix; wherein the general chemical formula of the Prussian blue-like compound is A. m [B(CN)6] n A and B are each independently selected from at least one of Fe, Co, Ni, Mn, Cu, and Zn, where 1 < m ≤ 3 and 1 < n ≤ 4; and the Prussian blue compound coating layer has structural vacancies, which are cyano vacancies or unsaturated coordination sites.

[0009] Preferably, the positive electrode active material matrix is ​​lithium cobalt oxide, with the chemical formula Li. x Na y M a Co 1-a O2, wherein 0.6≤x≤0.999, 0.01≤y≤0.15, 0≤a≤0.3, and M is selected from one or more of Ni, Mn, Al, Mg, Ti, La, Y, Zr, Nb, W, Mo, Ce, and Sr, and the mass content of M is 50 to 20,000 ppm.

[0010] Preferably, the lithium cobalt oxide is O2 phase lithium cobalt oxide, and the XRD characteristic diffraction peaks of the O2 phase lithium cobalt oxide satisfy the P63 / mmc space group characteristics, and its particle size D50 is 3-25 μm.

[0011] Preferably, the preparation method of the O2 phase lithium cobalt oxide includes the following steps: Cobalt oxide, sodium carbonate, and a compound containing element M are mixed and calcined to obtain a first compound; wherein the compound containing element M is selected from one or more of oxides, hydroxides, carbonates, and acetates, the molar ratio of Na to Co is 0.6 to 1.2, and the molar ratio of M to Co is 0.0001 to 0.5. The first compound was mixed with a compound containing element M and then subjected to solid-state sintering to obtain the second compound; The second compound is mixed with a lithium salt and the mixture is slowly heated to 150–250 °C for molten salt ion exchange for 4–24 hours to obtain O2 phase lithium cobalt oxide; wherein the lithium salt is a mixture of lithium chloride and lithium nitrate, and the molar ratio of Li to Na is 2–10.

[0012] Preferably, the Prussian blue compound is a ferrocyanide complex with the general chemical formula A. m [Fe(CN)6] nA is selected from one or more of Fe, Co, Ni, Mn, Cu, and Zn, where 1 < m ≤ 3 and 1 < n ≤ 4.

[0013] Preferably, the mass of the Prussian blue compound coating layer is 0.1%-5% of the mass of the positive electrode active material matrix.

[0014] Secondly, the present invention provides a method for preparing a composite cathode material, comprising the following steps: S1. Dissolve the compound containing element A in deionized water to prepare a first solution with a concentration of 0.5-2M; S2. The positive electrode active material matrix is ​​dispersed in an aqueous solution of ferrocyanide with a concentration of 0.5-2M to obtain a second solution; S3. Place the second solution in a constant temperature water bath at 40-90℃, and add acid solution dropwise to adjust the pH to 1-3; S4. The first solution is slowly added dropwise to the second solution after pH adjustment to carry out a co-precipitation reaction. After forming a Prussian blue compound coating layer on the surface of the positive electrode active material matrix, it is washed and dried multiple times to obtain the positive electrode material precursor. S5. The cathode material precursor is subjected to low-temperature heat treatment at 100-150℃ to obtain a composite cathode material.

[0015] Preferably, in step S4, the concentration of cyano vacancies in the Prussian blue compound coating layer is controlled by at least one of controlling the molar ratio of the first solution to the second solution, introducing a complexing agent, or adjusting the coprecipitation reaction temperature. Wherein, the molar ratio of element A in the first solution to element B in the second solution is 0.25 to 3; The complexing agent is selected from at least one of sodium citrate, EDTA, and sodium dodecylbenzene sulfonate; The coprecipitation reaction temperature is 40℃~90℃.

[0016] Thirdly, the present invention provides a positive electrode sheet comprising the above-described composite positive electrode material or a composite positive electrode material obtained by the above-described preparation method.

[0017] Fourthly, the present invention provides a battery comprising the above-described positive electrode and an electrolyte containing an alkyl silicon compound, wherein the alkyl silicon compound is selected from at least one of tris(trimethylsilyl)phosphate, tris(trimethylsilyl)borate, and hexamethyldisilazane.

[0018] Compared with the prior art, the present invention has at least the following beneficial effects: This application effectively solves the technical challenge of phase transition in the O2 structure caused by surface coating of O2-phase lithium cobalt oxide. Ferrocyanide complexes themselves have excellent conductivity, which can compensate for the decrease in electronic conductivity caused by high doping of high-voltage lithium cobalt oxide. At the same time, their unique three-dimensional framework structure can build a rapid lithium-ion diffusion channel on the surface of lithium cobalt oxide and effectively suppress the volume change of the cathode during the phase transition process. This enables rapid lithium-ion insertion and extraction at high rates and maintains the stability of the surface structure, providing strong support for the fast charging design of the battery cell.

[0019] Furthermore, this application significantly improves the structural stability of the O2 phase lithium cobalt oxide cathode material by controlling the structural vacancy of ferrocyanide complexes and effectively complexing cobalt metal ions using the lone pair electrons of the cyano group. This suppresses side reactions such as the catalytic decomposition of the electrolyte by cobalt ion dissolution, thereby achieving a higher cycle life.

[0020] This application employs a low-temperature co-precipitation method to coat the surface of O2 phase lithium cobalt oxide, which can effectively avoid the irreversible loss of the O2 structure caused by conventional high-temperature solid-state methods. It also solves the technical problem that high-temperature treatment can easily lead to the transformation of the O2 phase to the O3 phase, thus fully preserving the intrinsic high specific capacity and excellent rate performance of O2 phase lithium cobalt oxide.

[0021] Furthermore, the preparation method of this application is also applicable to conventional O3 phase lithium cobalt oxide cathode materials and other battery cathode materials, such as nickel cobalt manganese cathodes, nickel cobalt aluminum cathodes, lithium-rich manganese-based cathodes, lithium manganese oxide cathodes, lithium iron phosphate cathodes, etc., and has broad application prospects. Attached Figure Description

[0022] To more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0023] Figure 1 The X-ray diffraction patterns of the cathode materials provided in Example 4 and Comparative Example 1 of this invention; Figure 2 This is a scanning electron microscope image of the cathode material provided in Comparative Example 1 of this invention; Figure 3 This is a scanning electron microscope image of the cathode material provided in Example 4 of this invention; Figure 4 This is a schematic diagram of the coating layer of the ferrocyanide-based complex with structural vacancies according to the present invention. Detailed Implementation

[0024] To make the technical problem to be solved, the technical solution, and the beneficial effects of the present invention clearer, the present invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the present invention and are not intended to limit the present invention.

[0025] In this invention, the term "and / or" describes 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, or B existing alone. A and B can be singular or plural. The character " / " generally indicates that the preceding and following related objects have an "or" relationship.

[0026] In this invention, "at least one" means one or more, and "more than one" means two or more. "At least one of the following" or similar expressions refer to any combination of these items, including any combination of single or multiple items. For example, "at least one of a, b, or c", or "at least one of a, b, and c", can both represent: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, and c can be single or multiple.

[0027] It should be understood that in various embodiments of the present invention, the order of the above-mentioned processes does not imply the order of execution. Some or all steps may be executed in parallel or sequentially. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present invention.

[0028] According to a first aspect of the present invention, the present invention provides a composite cathode material, the composite cathode material comprising: a cathode active material matrix; and a Prussian blue-like compound coating layer covering at least a portion of the surface of the cathode active material matrix; wherein the general chemical formula of the Prussian blue-like compound is A. m [B(CN)6] n A and B are independently selected from at least one of Fe, Co, Ni, Mn, Cu, and Zn, 1 < m ≤ 3, 1 < n ≤ 4; and the Prussian blue-like compound coating layer has structural vacancies, which are cyano vacancies or unsaturated coordination sites.

[0029] Among them, structural vacancies are used to chelate and capture cobalt ions dissolved from the positive electrode active material matrix.

[0030] In some embodiments of the present invention, the positive electrode active material matrix is ​​lithium cobalt oxide, with the chemical formula Li. x Na y M a Co 1-aO2, wherein 0.6≤x≤0.999, 0.01≤y≤0.15, 0≤a≤0.3, and M is selected from one or more of Ni, Mn, Al, Mg, Ti, La, Y, Zr, Nb, W, Mo, Ce, and Sr, and the mass content of element M is 50–20000 ppm. For example, the mass content of element M can be 50 ppm, 80 ppm, 100 ppm, 500 ppm, 800 ppm, 10000 ppm, 2000 ppm, 5000 ppm, 8000 ppm, 10000 ppm, 15000 ppm, 20000 ppm, etc., preferably 50–8000 ppm.

[0031] Among them, element M, as a dopant, is introduced in trace amounts (50–20,000 ppm, i.e., from tens of ppm to 2%). This content range is primarily based on its stabilizing effect on the bulk structure of the material and the balance between capacity and electrical properties: too low a content (<50 ppm) is insufficient to effectively stabilize the structure; too high a content (>20,000 ppm) leads to a decrease in specific capacity and deterioration in kinetic performance. Therefore, the range of 50–20,000 ppm is the optimal choice for achieving the best match between structural stability and electrochemical performance.

[0032] In some embodiments of the present invention, the lithium cobalt oxide is O2-phase lithium cobalt oxide, wherein the XRD characteristic diffraction peaks of the O2-phase lithium cobalt oxide satisfy the P63 / mmc space group characteristics, and its particle size D50 is 3-25 μm. For example, the particle size D50 of the O2-phase lithium cobalt oxide particles can be 3 μm, 5 μm, 10 μm, 15 μm, 20 μm, or 25 μm.

[0033] The particle size of O2 phase lithium cobalt oxide is controlled within the range of 3 to 25 μm because: particles with too small a particle size (<3 μm) have too large a specific surface area, which on the one hand requires more coating material and increases costs, and on the other hand, too many surface active sites may cause side reactions in the electrolyte, which will reduce cycle stability; particles with too large a particle size (>25 μm) will result in an excessively long solid-phase diffusion path of lithium ions inside the particles, which is not conducive to the capacity utilization at high rates. At the same time, the volume change stress of large particles during charge and discharge is greater, which can easily lead to particle cracking or even pulverization, accelerating the dissolution of cobalt ions and structural decay.

[0034] The O2 phase lithium cobalt oxide is defined by a hexagonal crystal structure with space group P63 / mmc, which clearly distinguishes it from the conventional O3 phase lithium cobalt oxide. The unique interlayer stacking mode in the hexagonal crystal structure corresponding to the P63 / mmc space group enables the O2 phase lithium cobalt oxide to exhibit smaller volume changes and higher structural reversibility during delithiation, thus demonstrating superior voltage cycle stability and rate performance compared to the O3 phase.

[0035] In some embodiments of the present invention, the method for preparing the O2 phase lithium cobalt oxide includes the following steps: Cobalt oxide, sodium carbonate, and a compound containing element M are mixed and calcined to obtain a first compound; wherein the compound containing element M is selected from one or more of oxides, hydroxides, carbonates, and acetates, the molar ratio of Na to Co is 0.6 to 1.2, and the molar ratio of M to Co is 0.0001 to 0.5. The first compound was mixed with a compound containing element M and then subjected to solid-state sintering to obtain the second compound; The second compound is mixed with a lithium salt and the mixture is slowly heated to 150–250 °C for molten salt ion exchange for 4–24 hours to obtain O2 phase lithium cobalt oxide; wherein the lithium salt is a mixture of lithium chloride and lithium nitrate, and the molar ratio of Li to Na is 2–10.

[0036] In some embodiments of the present invention, the Prussian blue compound is a ferrocyanide complex with the general chemical formula A. m [Fe(CN)6] n A is selected from one or more of Fe, Co, Ni, Mn, Cu, and Zn, where 1 < m ≤ 3 and 1 < n ≤ 4.

[0037] In some embodiments of the present invention, the mass of the Prussian blue compound coating layer is 0.1%-5% of the mass of the positive electrode active material matrix.

[0038] According to a second aspect of the present invention, the present invention provides a method for preparing a composite cathode material, comprising the following steps: S1. Dissolve the compound containing element A in deionized water to prepare a first solution with a concentration of 0.5-2M; S2. The positive electrode active material matrix is ​​dispersed in an aqueous solution of ferrocyanide with a concentration of 0.5-2M to obtain a second solution; S3. Place the second solution in a constant temperature water bath at 40-90℃, and add acid solution dropwise to adjust the pH to 1-3; S4. The first solution is slowly added dropwise to the second solution after pH adjustment to carry out a co-precipitation reaction. After forming a Prussian blue compound coating layer on the surface of the positive electrode active material matrix, it is washed and dried multiple times to obtain the positive electrode material precursor. S5. The cathode material precursor is subjected to low-temperature heat treatment at 100-150℃ to obtain a composite cathode material.

[0039] In some embodiments of the present invention, in step S4, the concentration of cyano vacancies in the Prussian blue-like compound coating layer is controlled by at least one of controlling the molar ratio of the first solution to the second solution, introducing a complexing agent, or adjusting the coprecipitation reaction temperature; wherein the molar ratio of element A in the first solution to element B in the second solution is 0.25 to 3; the complexing agent is selected from at least one of sodium citrate, EDTA, and sodium dodecylbenzenesulfonate; and the coprecipitation reaction temperature is 40°C to 90°C. For example, the molar ratio of solution 1 to solution 2 can be 0.25, 0.5, 1, 1.5, 2, 2.5, 3, etc., and the coprecipitation reaction temperature can be 40°C, 50°C, 60°C, 70°C, 80°C, etc.

[0040] In step S4, the concentration of cyano vacancies in the ferrocyanide complex is controlled by adjusting the molar ratio of the first solution to the second solution (i.e., the A / Fe molar ratio) within the range of 0.25 to 3. This range can effectively control the target vacancy concentration: if it is less than 0.25, the vacancy concentration is too low, and the effect of chelating cobalt ions is limited; if it is greater than 3, the crystallinity of the ferrocyanide complex will be significantly deteriorated or even impurity phases will be generated, which will reduce the vacancy concentration and the structural stability of the coating layer.

[0041] The temperature of the coprecipitation reaction is controlled between 40℃ and 90℃ because: First, the O2 phase lithium cobalt oxide undergoes an irreversible phase transition to the O3 phase at temperatures exceeding 250℃, thus requiring the avoidance of any high-temperature treatment. The 40℃~90℃ range is a mild low-temperature range that completely avoids damage to the O2 phase structure. Second, within this temperature range, the precipitation reaction exhibits a moderate nucleation and growth rate, ensuring that ferrocyanide complexes can uniformly and densely adhere to the surface of the O2 phase lithium cobalt oxide particles. This avoids the problems of slow reaction kinetics and incomplete coating caused by excessively low temperatures, and excessively rapid reactions and uneven coating caused by excessively high temperatures. Furthermore, this temperature range does not require special high-temperature equipment, has low energy consumption, and is easy to scale up for industrial production. It is also compatible with the subsequent drying process, which does not exceed 120℃, ensuring that the O2 phase transition temperature is not touched throughout the process.

[0042] Furthermore, the selective introduction of complexing agents during the formation of ferrocyanide complexes serves several purposes: the complexing agents can form stable complexes with transition metal ions, regulating the concentration and release rate of free metal ions in the solution, thereby slowing down the precipitation reaction rate. This allows the ferrocyanide complexes to more uniformly and densely coat the surface of O2-phase lithium cobalt oxide particles, avoiding uneven coating caused by excessively rapid local nucleation. The competitive coordination of the complexing agent also interferes with the formation of Fe(CN)6. 4- The normal coordination self-assembly process with transition metal ions further induces the formation of cyano vacancies, enabling secondary fine-tuning of vacancy concentration.

[0043] According to a third aspect of the present invention, the present invention provides a positive electrode sheet comprising the above-described composite positive electrode material or a composite positive electrode material obtained by the above-described preparation method.

[0044] According to a fourth aspect of the present invention, the present invention provides a battery comprising the above-described positive electrode and an electrolyte containing an alkyl silicon compound, wherein the alkyl silicon compound is selected from at least one of tris(trimethylsilyl)phosphate, tris(trimethylsilyl)borate, and hexamethyldisilazane.

[0045] The present invention will be further described below with reference to specific embodiments, but the scope of protection of the present invention is not limited thereto.

[0046] Example 1

[0047] Preparation of composite cathode materials Step 1: Preparation of O2 phase lithium cobalt oxide 1) Raw material mixing: Add 1 kg Co3O4, 518.9 g Na2CO3 and 1.45 g Mn2O3 to a high-speed mixer and mix thoroughly.

[0048] 2) High-temperature calcination: The mixed powder was calcined at 850 °C for 8 hours to obtain the precursor.

[0049] 3) Molten salt ion exchange: The precursor and mixed lithium salt are mixed at a Li / Na molar ratio of 3:1, and the temperature is slowly raised to 200℃ and held for 10 hours to carry out molten salt ion exchange to obtain O2 phase lithium cobalt oxide; wherein the mixed lithium salt is LiCl and LiNO3, and the molar ratio of the two is 1:1.

[0050] Step 2: Preparation of composite cathode materials S1. Solution preparation: Prepare 1 M FeCl3 aqueous solution as solution 1; prepare 1 M K4[Fe(CN)6]3 aqueous solution as solution 2.

[0051] S2. Dispersion and pH Adjustment: Take 50 g of the O2 phase lithium cobalt oxide obtained in step one, disperse it in 100 mL of solution 2, and place it in an 80 ℃ constant temperature water bath. Add an appropriate amount of sulfuric acid to the dispersion to adjust the pH to 2-3.

[0052] S3. Low-temperature coprecipitation reaction: Under stirring, 1 mL of solution 1 is slowly added dropwise to the above system to carry out a low-temperature coprecipitation reaction, generating a Fe4[Fe(CN)6]3 coating layer. The reaction temperature is 80℃.

[0053] S4. Post-processing: After the reaction is complete, the product is washed with deionized water multiple times and dried to obtain the cathode material precursor.

[0054] S5. Low-temperature heat treatment: The precursor is subjected to low-temperature heat treatment at 150 °C to finally obtain the composite cathode material.

[0055] Example 2

[0056] Unlike Example 1, the amount of solution 1 added in step S3 of step two is increased to 10 ml, while the remaining steps are the same as in Example 1.

[0057] Example 3

[0058] Unlike Example 1, the amount of solution 1 added in step S3 of step two is increased to 30 ml, while the remaining steps are the same as in Example 1.

[0059] Example 4

[0060] Unlike Example 1, the amount of solution 1 added in step S3 of step two is increased to 50 ml, while the remaining steps are the same as in Example 1.

[0061] Example 5

[0062] Unlike Example 1, in step S1 of step two, solution 1 is a 1M MnCl3 aqueous solution, and the remaining steps are the same as in Example 1.

[0063] Example 6

[0064] Unlike Example 1, in step S1 of step two, solution 1 is a 1M CoCl3 aqueous solution, and the remaining steps are the same as in Example 1.

[0065] Example 7

[0066] Unlike Example 2, in step 1), the amount of raw material mixing is increased by 1.25g Y2O3, while the remaining steps are the same as in Example 1.

[0067] Example 8

[0068] The difference from Example 2 is that in step 1), 1.45g Mn2O3 is replaced with 1.3g NiO, and the rest of the steps are the same as in Example 1.

[0069] Example 9

[0070] The difference from Example 2 is that in step 1), 1.45g Mn2O3 is replaced with 1.2g La2O3, and the rest of the steps are the same as in Example 1.

[0071] Example 10

[0072] The difference from Example 2 is that in step 1), 1.45g Mn2O3 is replaced with 2.0g Al2O3, and the rest of the steps are the same as in Example 1.

[0073] Example 11

[0074] The difference from Example 2 is that in step 1), 1.45g Mn2O3 is replaced with 1.3g WO3, and the rest of the steps are the same as in Example 1.

[0075] Example 12

[0076] The difference from Example 2 is that in step 1), 1.45g Mn2O3 is replaced with 1.3g Y2O3, and the rest of the steps are the same as in Example 1.

[0077] Comparative Example 1 Preparation of O2 phase lithium cobalt oxide 1) Raw material mixing: Add 1 kg Co3O4 and 518.9 g Na2CO3 to a high-speed mixer and mix thoroughly.

[0078] 2) High-temperature calcination: The mixed powder was calcined at 850 °C for 8 hours to obtain the precursor.

[0079] 3) Molten salt ion exchange: The precursor and mixed lithium salt are mixed at a Li / Na molar ratio of 3:1, and the temperature is slowly raised to 200℃ and held for 10 hours to carry out molten salt ion exchange to obtain O2 phase lithium cobalt oxide; wherein the mixed lithium salt is LiCl and LiNO3, and the molar ratio of the two is 1:1.

[0080] Comparative Example 2 Unlike Example 1, step two of the coating is omitted; the remaining steps are the same as in Example 1.

[0081] Comparative Example 3 Preparation of O2 phase lithium cobalt oxide Raw material mixing: Add 1 kg Co3O4, 518.9 g Na2CO3 and 1.45 g Mn2O3 to a high-speed mixer and mix thoroughly.

[0082] High-temperature calcination: The mixed powder was calcined at 850 °C for 8 hours to obtain precursor 1.

[0083] Solid-state sintering: Precursor 1 and 1g Y2O3 are added to a high-speed mixer, mixed thoroughly and uniformly, and then solid-state sintering is carried out to obtain precursor 2.

[0084] Molten salt ion exchange: Precursor 2 and mixed lithium salt are mixed at a Li / Na molar ratio of 3:1, and the temperature is slowly raised to 200℃ and held for 10 hours to carry out molten salt ion exchange to obtain O2 phase lithium cobalt oxide; wherein the mixed lithium salt is LiCl and LiNO3, and the molar ratio of the two is 1:1.

[0085] Comparative Example 4 Unlike Example 1, the temperature of low-temperature co-precipitation in step S3 of step two is 30°C, while the remaining steps are the same as in Example 1.

[0086] Comparative Example 5 Unlike Example 1, the temperature of low-temperature co-precipitation in step S3 of step two is 100°C, while the remaining steps are the same as in Example 1.

[0087] Comparative Example 6 Unlike Example 1, the temperature of the low-temperature heat treatment in step S5 of step two is 50°C, while the other steps are the same as in Example 1.

[0088] Comparative Example 7 Unlike Example 1, the temperature of the low-temperature heat treatment in step S5 of step two is 180°C, while the other steps are the same as in Example 1.

[0089] The lithium cobalt oxide cathode materials obtained in the above embodiments and comparative examples were assembled into coin cells and tested. The test results are shown in Table 1.

[0090] 1. Button cell assembly The positive electrode material, conductive agent, and binder were mixed in a mass ratio of 96:2:2, homogenized with NMP as solvent, coated onto aluminum foil, and assembled into coin cells for performance testing.

[0091] 2. Cobalt ion dissolution test The coin cell was charged at a constant current of 0.1C to 4.6V (cutoff current 0.025C) to obtain a fully charged positive electrode. The battery was disassembled, the positive electrode was placed in an aluminum-plastic casing, immersed in electrolyte, and stored in a 75℃ oven for 48 hours. After high-temperature storage of the fully charged electrode, the cobalt ion content in the electrolyte was measured.

[0092] 3. Electrical performance testing (room temperature) Routine charge-discharge test: Within the voltage range of 3.0~4.6V, charge and discharge at a constant current of 0.1C, and record the first coulombic efficiency (first efficiency) and specific capacity.

[0093] Rate performance test: Conduct 3C charging / 0.1C discharging rate tests to evaluate rate performance.

[0094] Cyclic performance testing: The above cathode materials were subjected to accelerated coin cell cycling tests at 3.0-4.6V and 1C rate to evaluate the impact of material design on battery life.

[0095] Table 1 (Continued from Table 1) As shown in Table 1: Comparing Examples 2 and 5-6, under the same coating amount of 1%, the overall performance of Fe4[Fe(CN)6]3 (956ppm, initial efficiency 97.5%, cycle retention 90%) in Example 2 is slightly better than that of Co4[Fe(CN)6]3 in Example 6, which has a Co dissolution of 1060ppm, an initial efficiency of 97.6%, and a cycle retention of 89.3%. It is also better than that of Mn4[Fe(CN)6]3 in Example 5, which has a Co dissolution of 934ppm, an initial efficiency of 97.9%, and a cycle retention of 87.5%. However, compared with Comparative Example 1, the cycling performance of the material is significantly improved after coating with Prussian blue compounds. This indicates that coating with Prussian blue compounds can effectively inhibit the dissolution of transition metals, reduce electrolyte catalysis, and thus improve electrical performance. Furthermore, the results of different transition metal ion coating layers show that the choice of transition metal ions slightly affects the structural compactness, interfacial bonding force, and ion sieving ability. Comparing Examples 2 and 7-12, under the condition of a fixed 1% Fe4[Fe(CN)6]3 coating, the suppression ability of different dopants on Co dissolution is ranked as follows: La (748 ppm) > Y (802 ppm) > Ni (853 ppm) > Al (933 ppm) > undoped (Example 2) (956 ppm) > W (987 ppm). Example 9 (La) not only exhibits the lowest Co dissolution but also maintains 97.5% first-time efficiency, 234.8 mAh / g capacity, a high rate capability (98.1%), and a high cycle retention (94.10%), demonstrating excellent stability and power performance balance. Example 12 (Y) also performs excellently (dissolution 802 ppm, rate capability 97.9%, cycle retention 92.7%). In contrast, while W boasts a rate capability of 98.5%, its specific capacity is relatively low (220.3 mAh / g), sacrificing energy density; Ni, on the other hand, exhibits a lower rate capability (96.1%).

[0096] By comparing Examples 1, 2-4 and Comparative Examples 1-3, it can be seen that without coating, the Co dissolution amount exceeds 1000 ppm regardless of whether Mn or Y is doped (Comparative Examples 1-3). When Fe4[Fe(CN)6]3 coating is introduced, the Co dissolution amount continuously decreases with the increase of coating content: it drops to 992 ppm at 0.1%, to 956 ppm at 1%, to a sharp drop to 721 ppm at 3%, and only 549 ppm at 5%. The corresponding cycle retention rate first increases and then decreases with the increase of coating content: it increases from 74.7% to 83.1% at 0.1%, to 90.1% at 1%, to 93.5% at 3%, and decreases to 91.2% at 5%. This indicates that the coating layer, as a physical barrier, can effectively isolate the interface between the electrolyte and the cathode material, reducing the dissolution and loss of Co. However, it should be noted that when the coating content reaches more than 3%, although the thermal stability and cycling performance are significantly improved, the capacity decreases (from 239.1 mAh / g to 229.5 mAh / g), indicating a trade-off between material structural stability and capacity.

[0097] Comparing Example 1 with Comparative Examples 4-5, it can be seen that at a co-precipitation reaction temperature of 30°C, the cobalt dissolution, initial efficiency, rate performance, and cycle performance of the material all decreased significantly. This is because the Prussian blue analogue reaction is incomplete at this temperature, resulting in uneven coating on the cathode material surface and damage to the matrix surface structure, thus deteriorating the overall electrochemical performance. When the co-precipitation reaction temperature is increased to 100°C, the cobalt dissolution, initial efficiency, and specific capacity of the material are comparable to those of Example 1, but the cycle retention rate is lower than that of Example 1. This indicates that the reaction kinetics of Prussian blue are affected at high temperatures. Firstly, there is a risk of increased primary particle size of Prussian blue; secondly, high temperatures lead to deviations in alkali metal content, thereby deteriorating the electrical performance of the material.

[0098] Comparing Example 1 with Comparative Examples 6-7, it can be seen that when the low-temperature treatment temperature is 50°C, the moisture in the Prussian blue analogue is difficult to remove effectively. The residual moisture will trigger side reactions, leading to a significant deterioration in the electrochemical performance of the material. After increasing the low-temperature treatment temperature to 180°C, the specific capacity and first-efficiency of the material are basically the same as in Example 1, indicating that the moisture removal is relatively sufficient. However, the amount of cobalt dissolution increases and the rate performance decreases, indicating that excessively high low-temperature treatment temperatures may damage the structural stability of the Prussian blue analogue, thereby deteriorating its ion diffusion ability.

[0099] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A composite cathode material, characterized in that, The composite cathode material includes: Positive electrode active material matrix; and A Prussian blue-like compound coating layer covering at least a portion of the surface of the positive electrode active material matrix; The general chemical formula of the Prussian blue compounds is A. m [B(CN)6] n A and B are each independently selected from at least one of Fe, Co, Ni, Mn, Cu, and Zn, where 1 < m ≤ 3 and 1 < n ≤ 4. Furthermore, the Prussian blue-like compound coating layer has structural vacancies, which are cyano vacancies or unsaturated coordination sites.

2. The composite cathode material according to claim 1, characterized in that, The positive electrode active material matrix is ​​lithium cobalt oxide, with the chemical formula Li. x Na y M a Co 1-a O2, wherein 0.6≤x≤0.999, 0.01≤y≤0.15, 0≤a≤0.3, and M is selected from one or more of Ni, Mn, Al, Mg, Ti, La, Y, Zr, Nb, W, Mo, Ce, and Sr, and the mass content of M is 50 to 20,000 ppm.

3. The composite cathode material according to claim 2, characterized in that, The lithium cobalt oxide is an O2 phase lithium cobalt oxide, and the XRD characteristic diffraction peaks of the O2 phase lithium cobalt oxide satisfy the P63 / mmc space group characteristics, and its particle size D50 is 3-25μm.

4. The composite cathode material according to claim 3, characterized in that, The preparation method of the O2 phase lithium cobalt oxide includes the following steps: Cobalt oxide, sodium carbonate, and a compound containing element M are mixed and calcined to obtain the first compound; wherein the compound containing element M is selected from one or more of oxides, hydroxides, carbonates, and acetates, the molar ratio of Na to Co is 0.6 to 1.2, and the molar ratio of M to Co is 0.0001 to 0.

5. The first compound was mixed with a compound containing element M and then subjected to solid-state sintering to obtain the second compound; The second compound is mixed with a lithium salt and the mixture is slowly heated to 150–250 °C for molten salt ion exchange for 4–24 hours to obtain O2 phase lithium cobalt oxide; wherein the lithium salt is a mixture of lithium chloride and lithium nitrate, and the molar ratio of Li to Na is 2–10.

5. The composite cathode material according to claim 1, characterized in that, The Prussian blue compounds are ferrocyanide complexes with the general chemical formula A. m [Fe(CN)6] n A is selected from one or more of Fe, Co, Ni, Mn, Cu, and Zn, where 1 < m ≤ 3 and 1 < n ≤ 4.

6. The composite cathode material according to claim 1, characterized in that, The mass of the Prussian blue compound coating layer is 0.1%-5% of the mass of the positive electrode active material matrix.

7. A method for preparing the composite cathode material according to any one of claims 1-6, characterized in that, Includes the following steps: S1. Dissolve the compound containing element A in deionized water to prepare a first solution with a concentration of 0.5-2M; S2. The positive electrode active material matrix is ​​dispersed in an aqueous solution of ferrocyanide with a concentration of 0.5-2M to obtain a second solution; S3. Place the second solution in a constant temperature water bath at 40-90℃, and add acid solution dropwise to adjust the pH to 1-3; S4. The first solution is slowly added dropwise to the second solution after pH adjustment to carry out a co-precipitation reaction. After forming a Prussian blue compound coating layer on the surface of the positive electrode active material matrix, it is washed and dried multiple times to obtain the positive electrode material precursor. S5. The cathode material precursor is subjected to low-temperature heat treatment at 100-150℃ to obtain a composite cathode material.

8. The preparation method according to claim 7, characterized in that, In step S4, the concentration of cyano vacancies in the Prussian blue compound coating layer is controlled by at least one of the following methods: controlling the molar ratio of the first solution to the second solution, introducing a complexing agent, or adjusting the coprecipitation reaction temperature. Wherein, the molar ratio of element A in the first solution to element B in the second solution is 0.25 to 3; The complexing agent is selected from at least one of sodium citrate, EDTA, and sodium dodecylbenzene sulfonate; The coprecipitation reaction temperature is 40℃~90℃.

9. A positive electrode plate, characterized in that, The composite cathode material comprising any one of claims 1-6 or the composite cathode material obtained by the preparation method of any one of claims 7-8.

10. A battery, characterized in that, The electrolyte comprises the positive electrode and electrolyte of claim 9, wherein the electrolyte contains an alkylsilane compound selected from at least one of tris(trimethylsilyl)phosphate, tris(trimethylsilyl)borate, and hexamethyldisilazane.

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