Carbon-coated lithium iron phosphate positive electrode material with double-doped core-shell structure and preparation method of carbon-coated lithium iron phosphate positive electrode material

Abstract

The invention relates to the technical field of lithium ion batteries, and discloses a double-doped core-shell structure carbon-coated lithium iron phosphate positive electrode material and a preparation method thereof.The method comprises the steps of ion complexing and time-controlled aging, metal ion precipitation kinetics regulation and control through a polyhydroxy alcohol complexing agent, and time-controlled aging, the atomic-scale uniform distribution of doping elements in the lithium iron phosphate precursor is realized; chemical anchor points are pre-constructed on the surface of the precursor, and an activation layer rich in P-O-C groups is formed on the surface of the precursor through phosphorus-containing organic gas treatment; pulse type multistage gas supply chemical vapor deposition is carried out, and a functional gradient coating layer with the inner layer rich in boron and the outer layer rich in graphitized carbon is constructed on the surface of the nuclear material. Through the synergistic effect of uniform bulk phase doping and interface chemical bonding, the charge transfer resistance and ion diffusion impedance of the material are remarkably reduced, and the obtained positive electrode material has high first discharge specific capacity, excellent high-rate charge-discharge performance, long cycle stability and low-temperature performance.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, specifically to a dual-doped core-shell structure carbon-coated lithium iron phosphate cathode material and its preparation method. Background Technology

[0002] Lithium-ion batteries have become a major energy solution for portable electronic devices, electric vehicles, and energy storage systems due to their high energy density, long cycle life, and environmental friendliness. Cathode materials are the core component determining the electrochemical performance of lithium-ion batteries. Among them, olivine-type lithium iron phosphate is considered one of the most promising cathode materials due to its high theoretical specific capacity, stable structure, wide availability of raw materials, low cost, and good safety.

[0003] However, the commercial application of lithium iron phosphate (LFP) materials still faces challenges due to their inherent limitations. Their low intrinsic electronic conductivity and slow one-dimensional lithium-ion diffusion coefficient jointly restrict the electrochemical reaction kinetics of the material. These inherent kinetic hysteresis problems directly lead to poor rate performance of LFP batteries in practical applications, making it difficult to meet the demands of high-power charging and discharging. Simultaneously, at low temperatures, charge transport and ion migration capabilities further deteriorate, causing significant capacity decay and limiting their application in wide temperature ranges.

[0004] To improve the kinetic performance of lithium iron phosphate (LFP), existing technologies typically employ modification strategies such as ion doping and surface carbon coating. Regarding ion doping, although introducing high-valence metal cations can improve the intrinsic electronic conductivity of the material to some extent, conventional solid-state methods or simple liquid-phase co-precipitation methods struggle to achieve atomically uniform distribution of dopant elements within the LFP crystal lattice. Local aggregation or segregation of dopant ions not only fails to effectively improve the overall conductivity network but may also form impurity phases, becoming obstacles to lithium-ion diffusion, thus preventing the modification effect from meeting expectations.

[0005] Regarding surface carbon coating, while constructing a conductive carbon layer on the particle surface is an effective way to improve the overall conductivity of the electrode, its effectiveness is highly dependent on the interface quality between the carbon layer and the lithium iron phosphate core material. In existing technologies, the coating layer formed by the physical mixing and sintering of the carbon source and the core material is mostly physically attached to the core material through van der Waals forces. This interfacial bonding force is relatively weak. During repeated charge and discharge of the battery, as the lithium iron phosphate particles expand and contract, the carbon coating layer is prone to cracking or even peeling, leading to the failure of the conductive network between particles, a sharp increase in interfacial impedance, and ultimately, rapid capacity decay. In addition, a single-structure carbon layer cannot simultaneously achieve close contact with the core material, efficient electron transport, and structural stability of the outer layer, which also limits further improvement in the overall electrochemical performance of the material. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides a dual-doped core-shell carbon-coated lithium iron phosphate cathode material and its preparation method. This solves the problem that existing lithium iron phosphate materials suffer from uneven bulk doping and weak interfacial bonding between the conductive coating layer and the core material, resulting in sluggish electrochemical kinetics and consequently, inadequate rate performance, low-temperature performance, and cycle stability for demanding applications.

[0007] To achieve the above objectives, the present invention provides the following technical solution: a dual-doped core-shell structure carbon-coated lithium iron phosphate cathode material, wherein the chemical formula of the lithium iron phosphate cathode material is... ; The total mole fraction of titanium and boron The value ranges from 0.01 to 0.04.

[0008] Preferably, the mole fraction of titanium mole fraction of boron molar ratio : The ratio is 3:1 to 5:1.

[0009] Preferably, the steps include: S1. Dissolve the iron, lithium, phosphorus, titanium and boron sources in a mixed solvent, add a polyhydroxy alcohol complexing agent, adjust the pH to 7.5-8.5, and perform constant temperature aging treatment at 60-70℃ for 2-4 hours. S2. The solution that has been aged at a constant temperature is subjected to a solvothermal reaction at 180-220℃ for 10-12 hours to obtain a titanium and boron doped lithium iron phosphate precursor. S3. The lithium iron phosphate precursor is reacted with a phosphorus-containing organic gas at 250-350°C for 30-60 minutes. S4. The precursor that has been treated by contact reaction with phosphorus-containing organic gas is first pre-sintered at 350-450℃ in a nitrogen atmosphere, and then the main sintering is carried out at 700-800℃ in a mixed atmosphere of hydrogen and argon to obtain titanium and boron doped lithium iron phosphate core material. S5. At 650-750℃, the titanium and boron doped lithium iron phosphate core material is subjected to chemical vapor deposition using a pulsed multi-stage gas supply mode. The pulsed multi-stage gas supply mode includes an inner layer deposition stage and an outer layer deposition stage. In the inner layer deposition stage, the time proportion of borane introduced is higher than that of acetylene, and in the outer layer deposition stage, the time proportion of acetylene introduced is higher than that of borane. S6. After chemical vapor deposition is completed, anneal at 500-600℃ for 1-2 hours under argon protection.

[0010] Preferably, in step S1, the polyhydroxy alcohol complexing agent is mannitol.

[0011] Preferably, in step S1, the mixed solvent is a mixture of ethylene glycol and deionized water, wherein the volume ratio of ethylene glycol to deionized water is 1:1 to 1:3.

[0012] Preferably, in step S1, the lithium source is lithium carbonate, and the lithium carbonate is added in excess by 3% to 5% relative to the molar amount required for theoretical stoichiometry. Preferably, in step S3, the phosphorus-containing organic gas is triethyl phosphite vapor.

[0013] Preferably, in step S5, the specific method of the pulsed multi-stage gas supply mode is as follows: During the inner layer deposition stage, a first gas pulse and a second gas pulse are alternately introduced at a preset cycle. The first gas pulse is a mixture of borane and argon, and its duration accounts for 65-75% of the cycle. The second gas pulse is a mixture of acetylene and argon, and its duration accounts for 25-35% of the cycle. During the outer layer deposition stage, the first gas pulse and the second gas pulse are alternately introduced through the preset cycle, wherein the duration of the first gas pulse accounts for 25-35% of the cycle and the duration of the second gas pulse accounts for 65-75% of the cycle.

[0014] Preferably, the preset period is 100-150 seconds.

[0015] Preferably, in step S4, during the main sintering stage, the volume percentage of hydrogen in the hydrogen-argon mixed atmosphere is 3-7%.

[0016] This invention provides a dual-doped core-shell structured carbon-coated lithium iron phosphate cathode material and its preparation method. It has the following beneficial effects: 1. This invention utilizes the coordination effect of polyhydroxy alcohol complexing agents and metal ions through ion complexation and time-controlled aging steps. Combined with the control of pH value and aging temperature, it can effectively regulate the hydrolysis and precipitation kinetics of each metal ion, achieving synchronous and uniform co-precipitation of each component at the molecular level. This ensures that titanium and boron dopants can achieve atomic-level uniform distribution within the lithium iron phosphate precursor during subsequent solvothermal reactions, laying the material foundation for the subsequent formation of nuclear materials with small lattice distortion and unobstructed ion diffusion channels.

[0017] 2. This invention utilizes a precursor surface chemical anchoring pre-construction step to pre-build an activation layer rich in POC groups on the precursor surface before the formation of the lithium iron phosphate crystalline phase. These groups provide highly active reaction sites for the formation of Li-OC chemical bonds between the carbon coating layer and the lithium iron phosphate core material during subsequent high-temperature processing, thereby significantly enhancing the bonding strength of the core-shell interface and reducing the interfacial charge transport impedance.

[0018] 3. In this invention, the boron-rich atmosphere in the inner layer deposition stage is conducive to the formation of inner carbon with close contact with the core material, high boron doping concentration, and high electronic conductivity; while the acetylene-rich atmosphere in the outer layer deposition stage is conducive to the formation of outer carbon with a high degree of graphitization and stable structure. By utilizing this functionally graded carbon layer structure, both interfacial conductivity and outer layer structural integrity are taken into account. Combined with the subsequent interfacial enhancement annealing treatment, a core-shell structure with stable interface and efficient charge transport is finally obtained. Detailed Implementation

[0019] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0020] The main raw materials and reagents used in the following examples and comparative examples have the following sources and specifications. Reagents not specifically mentioned are all commercially available analytical grade or higher grade products.

[0021] Ferric phosphate, CAS: 13463-10-0; Lithium carbonate, CAS: 554-13-2; Phosphoric acid, CAS: 7664-38-2; Tetrabutyl titanate, CAS: 5593-70-4; Boric acid, CAS: 10043-35-3; Ethylene glycol, CAS: 107-21-1; Mannitol, CAS: 69-65-8; Ammonia solution, CAS: 1336-21-6; Triethyl phosphite, CAS: 122-52-1; Conductive carbon black, CAS: 1333-86-4; Polyvinylidene fluoride, CAS: 24937-79-9; N-Methylpyrrolidone, CAS: 872-50-4; Aluminum foil, CAS: 7429-90-5; Lithium metal sheet, CAS: 7439-93-2; Polypropylene diaphragm, CAS: 9003-07-0; The electrolyte is composed of lithium hexafluorophosphate (CAS:21324-40-3) at a concentration of 1 mol / L, and a solvent consisting of a mixture of ethylene carbonate (CAS:96-49-1), dimethyl carbonate (CAS:616-38-6), and ethyl methyl carbonate (CAS:623-53-0) in a volume ratio of 1:1:1.

[0022] Examples 1-4: Example 1:

[0023] This embodiment provides a target chemical formula of The specific steps for preparing the positive electrode material are as follows: (1) Weigh 0.97 mol of ferric phosphate, 0.025 mol of tetrabutyl titanate, 0.005 mol of boric acid, and 0.995 mol of phosphoric acid according to the stoichiometric ratio, and add them to 1000 mL of a 1:2 mixture of ethylene glycol and deionized water. Then add 0.52 mol of lithium carbonate (4% excess relative to the theoretical stoichiometric molar amount) and mannitol (0.5 times the total molar amount of metal ions). Adjust the pH of the solution to 8.0 with ammonia water under magnetic stirring. Then transfer the mixed solution to a water bath and age it at 65°C for 3 hours.

[0024] (2) The aged solution obtained in step (1) was transferred to a high-pressure reactor and subjected to a solvothermal reaction at 200°C for 11 hours. After the reaction was completed, it was naturally cooled to room temperature. The product was centrifuged and washed three times with deionized water and anhydrous ethanol, and then dried in a vacuum oven at 80°C for 12 hours to obtain the titanium / boron dual-doped lithium iron phosphate precursor.

[0025] (3) Place the above precursor in a tube furnace and heat it to 300°C under an argon atmosphere. Use a bubbling method to introduce triethyl phosphite vapor into the tube furnace and react it with the precursor for 45 minutes.

[0026] (4) The precursor treated in step (3) was first pre-sintered at 400°C under a nitrogen atmosphere at a heating rate of 5°C / min and held for 2 hours. Then, the atmosphere was switched to a hydrogen / argon mixed atmosphere (hydrogen fraction of 5%) and the temperature was increased to 750°C at a heating rate of 5°C / min and held for 6 hours for main sintering. After sintering, the furnace was cooled to room temperature to obtain titanium / boron dual-doped lithium iron phosphate core material.

[0027] (5) Place the above-mentioned nuclear material in a fluidized bed reactor and heat it to 700℃ under argon protection. Chemical vapor deposition is performed using a pulsed multi-stage gas supply mode with a preset cycle of 120 seconds. Inner layer deposition stage (lasting 30 minutes): In each cycle, a borane / argon mixture is first introduced for 84 seconds (70% of the cycle), followed by an acetylene / argon mixture for 36 seconds (30% of the cycle). Outer layer deposition stage (lasting 30 minutes): In each cycle, a borane / argon mixture is first introduced for 36 seconds (30% of the cycle), followed by an acetylene / argon mixture for 84 seconds (70% of the cycle).

[0028] (6) After completing step (5), under argon protection, adjust the temperature to 550℃ and hold for 1.5 hours for annealing. After the treatment, cool the furnace to room temperature to obtain the final product, denoted as S1.

[0029] Example 2:

[0030] This embodiment provides a target chemical formula of Methods for preparing cathode materials.

[0031] The preparation method is basically the same as in Example 1, except that the amount of raw materials in step (1) is adjusted to 0.99 mol of ferric phosphate, 0.008 mol of tetrabutyl titanate, 0.002 mol of boric acid and 0.998 mol of phosphoric acid, so as to achieve a total molar fraction of titanium and boron of x+y of 0.01. All other steps and process parameters are exactly the same as in Example 1. The final product obtained is denoted as S2.

[0032] Example 3:

[0033] This embodiment provides a target chemical formula of Methods for preparing cathode materials.

[0034] The preparation method is basically the same as in Example 1, except that the amount of raw materials in step (1) is adjusted to 0.96 mol of ferric phosphate, 0.03 mol of tetrabutyl titanate, 0.01 mol of boric acid and 0.99 mol of phosphoric acid, so as to achieve a total molar fraction of titanium and boron of x+y of 0.04. All other steps and process parameters are exactly the same as in Example 1. The final product obtained is denoted as S3.

[0035] Example 4:

[0036] This embodiment provides a method for preparing a cathode material with the same target chemical formula as in Example 1. The raw material ratios and main preparation process are exactly the same as in Example 1, with the only difference being adjustments to some process parameters, as detailed below: The constant temperature aging conditions in step (1) are: temperature 70℃, time 4 hours; The main sintering conditions in step (4) are: temperature 800℃; The chemical vapor deposition temperature in step (5) is 650°C; All other steps and process parameters were exactly the same as in Example 1. The final product obtained was denoted as S4.

[0037] Comparative Examples 1-6: Comparative Example 1: Compared with Example 1, the difference is that steps (1), (3), and (5) are not performed; after obtaining the nuclear material in step (4), it is physically mixed with glucose at a mass ratio of 9:1, and then carbon-coated by sintering at 700°C for 4 hours under an argon atmosphere. The rest are the same. The final product obtained is denoted as CS1.

[0038] Comparative Example 2: Compared with Example 1, the difference is that the ion complexation and time-controlled aging in step (1) are not performed. The raw materials are directly mixed and then the solvothermal reaction in step (2) is carried out. All other steps are the same. The final product is denoted as CS2.

[0039] Comparative Example 3: Compared with Example 1, the difference is that the precursor surface chemical anchoring preconstruction in step (3) is not performed, but all other steps are the same. The final product obtained is denoted as CS3.

[0040] Comparative Example 4: Compared to Example 1, the difference lies in that the pulsed multi-stage gas supply in step (5) is replaced with conventional continuous gas supply. Specifically, at 700°C, a borane / argon mixture and an acetylene / argon mixture are mixed at a constant volumetric flow rate ratio (the same as the total flow rate ratio of the two gases in Example 1), and then continuously introduced into the reactor for 60 minutes, with all other steps remaining the same. The final product obtained is denoted as CS4.

[0041] Comparative Example 5: Compared with Example 1, the difference is that the amount of raw materials fed in step (1) is adjusted so that the total molar fraction of titanium and boron x+y is 0.005, while the rest are the same. The final product obtained is denoted as CS5.

[0042] Comparative Example 6: Compared with Example 1, the difference is that the amount of raw materials fed in step (1) is adjusted so that the total molar fraction of titanium and boron x+y is 0.05, while the rest are the same. The final product obtained is denoted as CS6.

[0043] Test Example 1-2: Test Example 1: Assembly of button cells: The cathode materials (S1-S4, CS1-CS6) prepared in Examples 1-4 and Comparative Examples 1-6 were used as active materials to assemble CR2032 coin cells.

[0044] First, the electrode slurry is prepared: the active material, conductive carbon black SuperP, and polyvinylidene fluoride (PVDF) binder are mixed in N-methylpyrrolidone (NMP) solvent at a mass ratio of 80:10:10. The mixture is stirred for 8 hours using a planetary ball mill to form a uniform electrode slurry with a solid content of 40%.

[0045] Next, the electrode sheet was prepared: the above slurry was uniformly coated onto a 15 μm thick aluminum foil current collector using a doctor blade coater, controlling the wet film thickness of the coating to be 120 μm. The coated aluminum foil was dried in an 80°C forced-air oven for 2 hours, and then transferred to a 120°C vacuum oven for further drying for 12 hours to completely remove the solvent. After drying, the electrode sheet was compacted using a roller press, controlling the compaction density of the active material to be 2.2 g / cm³. Finally, the compacted electrode sheet was cut into circular positive electrode sheets with a diameter of 14 mm using a punching machine.

[0046] Finally, battery assembly was carried out in a glove box filled with argon gas and where the water and oxygen content were both below 0.1 ppm. Using the prepared positive electrode as the working electrode, a lithium metal sheet as the counter electrode, and a polypropylene membrane as the separator, electrolyte was injected to assemble a CR2032 coin cell, which was then left to stand for 12 hours.

[0047] Electrochemical performance testing: The electrochemical performance of the assembled coin cells was tested using the LANDCT2001A battery testing system at 25°C. The specific test items are as follows: The first constant current charge-discharge test was conducted at a current density of 0.1 C within a voltage range of 2.5–4.2 V, and the first discharge specific capacity and first coulombic efficiency were recorded.

[0048] After charging the battery to 4.2 V at 0.1 C, it was discharged at constant current to 2.5 V at current densities of 0.2 C, 0.5 C, 1 C, 2 C, 5 C and 10 C, respectively, and the discharge specific capacity at each rate was recorded.

[0049] Cycling performance: Constant current charge-discharge cycles were performed at a current density of 1 C within a voltage range of 2.5–4.2 V, and the capacity retention rate was recorded after 200 cycles.

[0050] Low temperature performance: The fully charged battery was placed in a constant temperature chamber at -20℃ for 4 hours, and then discharged to 2.5 V at a current density of 0.5 C. Its discharge specific capacity was recorded, and the ratio of its discharge specific capacity to that at 0.5 C in a 25℃ environment was calculated.

[0051] Test results: The performance data obtained from the above tests are summarized in Table 1.

[0052] Table 1. Comparison of electrochemical performance data between each example and the comparative example

[0053] As can be seen from the test data in Table 1, the samples of Examples S1-S4 showed significant differences compared with the samples of Comparative Examples CS1-CS6 in terms of initial discharge specific capacity, rate performance, cycle stability, and low-temperature performance.

[0054] A comparison of data from Example S1 and Comparative Example CS2 shows that by performing the ion complexation and time-controlled aging steps, the initial discharge specific capacity, capacity utilization at a high rate of 10C, and capacity retention after cycling of the final material are all improved. This is because this step regulates the precipitation kinetics of metal ions through the coordination effect of the complexing agent, promoting the uniform atomic-level distribution of titanium and boron dopants within the precursor during subsequent solvothermal reactions, thus providing a foundation for forming a nuclear material structure with low defect concentration and complete ion diffusion channels.

[0055] A comparison of data from Example S1 and Comparative Example CS3 shows that the rate performance and cycling stability of the material are further improved by performing a precursor surface chemical anchoring pre-construction step. This step pre-constructs an activation layer rich in POC groups on the precursor surface. These groups provide reaction sites for the subsequent interfacial chemical bonding between the carbon coating layer and the lithium iron phosphate core material. This chemical bonding reduces the charge transport impedance at the core-shell interface and enhances the structural stability of the interface, thereby slowing down the capacity decay of the material at high current densities and maintaining structural integrity during long-term cycling.

[0056] The data comparison between Example S1 and Comparative Examples CS4, CS5, and CS6 jointly confirms the effectiveness of the specific process in this technical solution. Compared with CS4, which uses continuous gas supply, S1, which uses pulsed multi-stage gas supply, has a higher high-rate capacity ratio and cycle capacity retention rate. This is attributed to the functionally graded coating structure formed by the pulsed process, which has an inner boron-rich inner layer and an outer graphitized carbon-rich outer layer. Meanwhile, compared with CS5, which has a total doping amount below 0.01, and CS6, which has a total doping amount above 0.04, Examples S1, S2, and S3, with a total doping amount in the range of 0.01 to 0.04, all achieved higher initial discharge capacity and overall electrochemical performance, indicating that this doping range is a suitable range for obtaining the expected technical effects.

[0057] Test Example 2: Test method: Electrochemical kinetics characteristics of the coin cell assembled in Test Example 1 and subjected to three activation cycles were tested using an electrochemical workstation.

[0058] The battery was charged to 50% state of charge (SOC) and left to rest for 1 hour before testing. The test frequency range was 100 kHz to 0.01 Hz, and the applied AC disturbance signal amplitude was 5 mV. The Nyquist plot obtained from the test was fitted using ZView software. Equivalent circuit model, extracting the charge transfer resistance ( )value.

[0059] Cyclic voltammetry was performed at a scan rate of 0.1 mV / s within a voltage range of 2.5 V to 4.2 V. The potential difference between the oxidation and reduction peaks in the third scan curve was recorded. .

[0060] Test results: The dynamic parameters obtained from the above tests are summarized in Table 2.

[0061] Table 2. Comparison of electrochemical kinetic parameters between each example and the comparative example

[0062] As can be seen from the test data in Table 2, the charge transfer resistance of samples S1-S4 in Examples ( ) and peak potential difference ( All were lower than those of the comparative samples CS1-CS6.

[0063] A comparison of data from Example S1 and Comparative Example CS2 shows that performing the ion complexation and time-controlled aging steps improves the material's... and The numerical values ​​are reduced. This step utilizes the coordination effect of the complexing agent on the metal ions to achieve atomically uniform distribution of titanium and boron dopants in the precursor in subsequent reactions. This distribution reduces lattice defects and impurity phase formation in the final core material, thereby lowering the energy barrier for lithium ion migration in the solid matrix, manifested as a change in electrochemical reaction kinetic parameters.

[0064] A comparison of data from Example S1 with comparative examples CS3 and CS4 shows that the interface treatment process has a direct impact on charge transfer resistance. Compared to CS3, which did not undergo surface prefabrication, S1's... The value is significantly reduced because the precursor surface chemical anchoring pre-construction step forms an activation layer rich in POC groups on the core material surface. These groups form interfacial chemical bonds with the carbon coating layer during subsequent processing, providing a low-impedance path for electron transport at the core-shell interface. Compared to CS4, which uses continuous gas supply, S1, which uses pulsed multi-stage gas supply, also exhibits lower [resistance / impedance]. The value is attributed to the gradient structure formed by the process, which has an inner boron-rich layer and an outer graphitized carbon-rich layer, further optimizing the charge transport efficiency at the interface.

[0065] The comparison of data from Examples S1, S2, and S3 with Comparative Examples CS5 and CS6 demonstrates the effectiveness of the composition range. When the total molar fraction of titanium and boron is below 0.01 (CS5) or above 0.04 (CS6), the material... and The values ​​are all higher than those of the example samples within this range. Too low a total doping concentration will not sufficiently alter the intrinsic conductivity of the material; too high a total doping concentration may introduce excessive lattice distortion or impurity phases, hindering charge and ion transport. Therefore, a doping range of 0.01 to 0.04 is a necessary condition for obtaining the kinetic properties shown in Table 2.

Claims

1. A dual-doped core-shell structured carbon-coated lithium iron phosphate cathode material, characterized in that, The chemical formula of the lithium iron phosphate cathode material is: ; The total mole fraction of titanium and boron The value ranges from 0.01 to 0.

04.

2. The lithium iron phosphate cathode material with a dual-doped core-shell structure and carbon coating according to claim 1, characterized in that, The mole fraction of titanium mole fraction of boron molar ratio : The ratio is 3:1 to 5:

1.

3. A method for preparing a carbon-coated lithium iron phosphate cathode material with a dual-doped core-shell structure as described in claim 1 or 2, characterized in that, Includes the following steps: S1. Dissolve the iron, lithium, phosphorus, titanium and boron sources in a mixed solvent, add a polyhydroxy alcohol complexing agent, adjust the pH to 7.5-8.5, and perform constant temperature aging treatment at 60-70℃ for 2-4 hours. S2. The solution that has been aged at a constant temperature is subjected to a solvothermal reaction at 180-220℃ for 10-12 hours to obtain a titanium and boron doped lithium iron phosphate precursor. S3. The lithium iron phosphate precursor is reacted with a phosphorus-containing organic gas at 250-350°C for 30-60 minutes. S4. The precursor that has been treated by contact reaction with phosphorus-containing organic gas is first pre-sintered at 350-450℃ in a nitrogen atmosphere, and then the main sintering is carried out at 700-800℃ in a mixed atmosphere of hydrogen and argon to obtain titanium and boron doped lithium iron phosphate core material. S5. At 650-750℃, the titanium and boron doped lithium iron phosphate core material is subjected to chemical vapor deposition using a pulsed multi-stage gas supply mode. The pulsed multi-stage gas supply mode includes an inner layer deposition stage and an outer layer deposition stage. In the inner layer deposition stage, the time proportion of borane introduced is higher than that of acetylene, and in the outer layer deposition stage, the time proportion of acetylene introduced is higher than that of borane. S6. After chemical vapor deposition is completed, anneal at 500-600℃ for 1-2 hours under argon protection.

4. The method according to claim 3, characterized in that, In step S1, the polyhydroxy alcohol complexing agent is mannitol.

5. The method according to claim 3, characterized in that, In step S1, the mixed solvent is a mixture of ethylene glycol and deionized water, wherein the volume ratio of ethylene glycol to deionized water is 1:1 to 1:

3.

6. The method according to claim 3, characterized in that, In step S1, the lithium source is lithium carbonate, and the lithium carbonate is added in excess by 3% to 5% relative to the molar amount required by theoretical stoichiometry.

7. The method according to claim 3, characterized in that, In step S3, the phosphorus-containing organic gas is triethyl phosphite vapor.

8. The method according to claim 3, characterized in that, In step S5, the specific method of the pulsed multi-stage gas supply mode is as follows: During the inner layer deposition stage, a first gas pulse and a second gas pulse are alternately introduced at a preset cycle. The first gas pulse is a mixture of borane and argon, and its duration accounts for 65-75% of the cycle. The second gas pulse is a mixture of acetylene and argon, and its duration accounts for 25-35% of the cycle. During the outer layer deposition stage, the first gas pulse and the second gas pulse are alternately introduced through the preset cycle, wherein the duration of the first gas pulse accounts for 25-35% of the cycle and the duration of the second gas pulse accounts for 65-75% of the cycle.

9. The method according to claim 8, characterized in that, The preset period is 100-150 seconds.

10. The method according to claim 3, characterized in that, In step S4, during the main sintering stage, the volume percentage of hydrogen in the hydrogen-argon mixed atmosphere is 3-7%.

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