A modification method for a high-capacity lithium iron vanadium phosphate cathode material doped with zinc and magnesium ions

Abstract

A modification method for a high-capacity lithium iron vanadium phosphate cathode material doped with zinc and magnesium ions, which belongs to the technical field of cathode materials for lithium-ion batteries. Method: First, weigh raw materials according to the molar ratio of lithium, vanadium, iron, phosphorus, zinc, magnesium and carbon; Second, prepare a carbon-coated zinc and magnesium ion-doped lithium iron vanadium phosphate precursor material; Third, prepare a zinc and magnesium ion-doped lithium iron vanadium phosphate cathode material with the chemical formula Li<subgt;x+3< / subgt;Fe<subgt;x‑< / subgt;<subgt;y< / subgt;Mg<subgt;y< / subgt>V<subgt;2‑z< / subgt>Zn<subgt;z< / subgt;(PO<subgt;4< / subgt;)<subgt;3< / subgt; / C, where x = 1 to 5, y = 0.01 to 0.2, and z = 0.01 to 0.05. The present invention improves the electrochemical performance of the material, has a high capacity retention rate, and good cycle stability. The preparation process is simple. While ensuring that the cathode material has good specific capacity and capacity retention rate, it shortens the production cycle, reduces costs, reduces potential safety hazards during production, the equipment is inexpensive, there is no release of toxic and harmful substances, and it is suitable for industrial production.

Description

Technical Field

[0001] The present invention belongs to the technical field of cathode materials for lithium-ion batteries, and particularly relates to a modification method for a high-capacity zinc-magnesium ion-doped lithium iron vanadium phosphate cathode material. Background Art

[0002] Polyanion-type compounds are a general term for a series of compounds containing tetrahedral or octahedral anion structural units (XOm)n- (X = P, S, As, Mo, and W). Due to their good lithium intercalation performance and excellent safety, these compounds are most promising to become the cathode materials for the new generation of lithium-ion batteries. Among them, LiFePO4 has become a research hotspot because of its rich raw material sources, stable discharge voltage, good thermal stability, and excellent cycling characteristics. However, LiFePO4 not only has low electronic conductivity and tap density, but also has a relatively low discharge voltage, which limits its further development and application. The Li3V2(PO4)3 compound with a monoclinic NASICON (Sodium Super Ion Conductor) structure has a higher diffusion coefficient than LiFePO4 because there is enough space to conduct alkali metal ions such as Na and Li. Moreover, Li3V2(PO4)3 has a much more stable structure than transition metal oxides. Even when the molar ratio of Li + to transition metal is greater than 1, it still has extraordinary stability. Thus, Li3V2(PO4)3 has become a new research hotspot and is regarded as the most promising cathode material for electric vehicle lithium-ion batteries because of its rich raw material sources, low cost, good high-rate discharge performance, high safe charging cut-off voltage, good safety, high discharge voltage, and high specific energy.

[0003] For lithium iron vanadium phosphate materials, due to the inherent problem of low electronic conductivity of polyanion-type phosphates, doping metal ions helps to improve the preferential conduction path of electrons, thereby improving the rate capability and cycling performance of the electrode materials. At present, the doping of metal ions in lithium iron vanadium phosphate materials mainly uses solid-phase ball milling method and sol-gel method. The preparation of the existing publicly reported lithium iron vanadium phosphate materials doped with metal ions usually has a high ball milling speed, high energy consumption, and also has a high temperature and long time during calcination, resulting in high energy consumption and increased synthesis cost. Lithium-ion batteries face cost problems during commercialization. There are many mature lithium iron vanadium phosphate cathode materials on the market, and the market competition is fierce. Those with outstanding cost performance have an advantage. Therefore, the modified synthesis of lithium iron vanadium phosphate materials doped with metal ions needs to have obvious advantages in both performance and cost to compete in the market. Summary of the Invention

[0004] The purpose of the present invention is to solve the above-mentioned existing technical problems, and provide a modification method for a high-capacity zinc-magnesium ion-doped lithium iron vanadium phosphate cathode material.

[0005] A modification method for a high-capacity zinc- and magnesium-ion-doped lithium iron vanadium phosphate cathode material, which is realized according to the following steps:

[0006] I. Weighing materials:

[0007] According to the molar ratio of lithium, vanadium, iron, phosphorus, zinc, magnesium to carbon being (3.675 - 6.825):(0.95 - 1):(1.8 - 5):5.5:(0.01 - 0.05):(0.01 - 0.2):(4 - 8), weigh lithium source, vanadium source, iron source, phosphorus source, zinc source, magnesium source and carbon source respectively;

[0008] II. Preparing a carbon-coated zinc- and magnesium-ion-doped lithium iron vanadium phosphate precursor material:

[0009] Add the weighed lithium source, vanadium source, iron source, phosphorus source, zinc source, magnesium source and carbon source into deionized water and stir evenly to obtain a mixture. Then disperse the mixture into a dispersion medium, and then place it in a ball mill for ball milling for 7 - 10 h to obtain a slurry. After drying and grinding, a carbon-coated zinc- and magnesium-ion-doped lithium iron vanadium phosphate precursor material is obtained;

[0010] III. Preparing a zinc- and magnesium-ion-doped lithium iron vanadium phosphate cathode material:

[0011] Transfer the above-mentioned carbon-coated zinc- and magnesium-ion-doped lithium iron vanadium phosphate precursor material to a tubular furnace, and carry out gradient high-temperature calcination under an inert gas atmosphere. After the calcination is completed, naturally cool it to room temperature. After grinding and sieving, a high-capacity zinc- and magnesium-ion-doped lithium iron vanadium phosphate cathode material with the chemical formula Li x+3 Fe x-y Mg y V 2-z Zn z (PO4)3 / CC is obtained, where x = 1 - 5, y = 0.01 - 0.2, z = 0.01 - 0.05, and thus the modification method is completed.

[0012] Furthermore, in step I, the lithium source is lithium carbonate; the vanadium source is vanadium pentoxide; the iron source is ferrous oxalate dihydrate; the phosphorus source is ammonium dihydrogen phosphate; the zinc source is zinc oxide; the magnesium source is magnesium nitrate, magnesium acetate, magnesium sulfate or magnesium oxide; the carbon source is sucrose, glucose or citric acid monohydrate.

[0013] Furthermore, in step II, the dispersion medium is an aqueous hydrogen peroxide solution with a volume fraction of 1% - 5%; the dispersion method is electromagnetic stirring.

[0014] Furthermore, in step II, the ratio of the total mass of the lithium source, vanadium source, iron source, phosphorus source, magnesium source, zinc source and carbon source to the volume of the dispersion medium is 1 g:(5 mL - 15 mL).

[0015] Further, the temperature of the ball milling in Step 2 is 20 - 25°C, and the rotation speed is 400 - 600 r / min.

[0016] Further, the ball milling in Step 2 uses steel balls, agate balls or zirconia balls, and the mass ratio of balls to materials is 20:1.

[0017] Further, the drying in Step 2: drying in a forced air drying oven at 80 - 120°C for 12 - 15 h; the grinding: grinding in an agate mortar for 20 - 30 min.

[0018] Further, the gradient high-temperature calcination in Step 3: heating up to 300 - 400°C at a rate of 4 - 8°C / min, and holding for 6 - 10 h; after the holding ends, heating up to 750 - 850°C at a rate of 4 - 8°C / min, and continuing to hold for 6 - 10 h.

[0019] Further, the inert gas in Step 3 is argon; the sieving uses a 400-mesh sieve.

[0020] The beneficial technical effects of the present invention are as follows:

[0021] (1) In the present invention, an aqueous solution of hydrogen peroxide is used as a ball milling dispersant, which completely dissolves the raw materials and enables them to be fully mixed during the ball milling process, obtaining an LFMVZP / C sample with small and relatively uniform particle sizes. Due to the addition of hydrogen peroxide, vanadium pentoxide forms a wet gel, improving the uniformity of the raw materials at the molecular level, thus making the raw materials mix more uniformly during the ball milling process. In addition, since hydrogen peroxide can accelerate the formation of ammonium vanadate, this can improve the reaction activity of the precursor and shorten the sintering time.

[0022] (2) Magnesium ion doping

[0023] The particles of the sample prepared by magnesium ion doping are concentratedly distributed at about 1 μm. After doping, the particle diameter of the material becomes smaller, which is beneficial to the penetration of the electrolyte and plays a positive role in the electrochemical performance of the material. It shows that the doping of Mg 2+ can refine the particle size of the LFMVZP / C material, so the diffusion distance of lithium ions in the LFMVZP / C lattice structure is shortened, and thus the electrochemical performance of the material can be improved.

[0024] (3) Zinc ion doping

[0025] The charge-discharge test of the lithium iron vanadium phosphate cathode material doped with zinc ions shows that the prepared LFMVZP / C has a relatively high initial discharge specific capacity and capacity retention rate. The initial discharge specific capacity at a current density of 2C is 118.35 mAh g -1The reversible capacity. After 50 cycles, the LFMVZP / C material can release 117.43 mAh g -1 The reversible capacity. The Coulomb efficiency during the cycling process is always close to 100%, with a high capacity retention rate of 99.22%, and good cycling stability.

[0026] (4) The preparation process of the present invention is simple. While ensuring that the cathode material has good specific capacity and capacity retention rate, it shortens the production cycle, reduces costs, reduces potential safety hazards during the production process, has low equipment prices, and does not release toxic and harmful substances, showing obvious advantages in both performance and cost, and is suitable for industrial production.

[0027] The present invention is applicable to the modified preparation of high-capacity zinc and magnesium ion-doped lithium iron vanadium phosphate cathode materials. Description of the Drawings

[0028] Figure 1 It is the XRD pattern of the zinc and magnesium ion-doped lithium iron vanadium phosphate cathode material in Example 1;

[0029] Figure 2 It is the scanning electron microscope image of the zinc and magnesium ion-doped lithium iron vanadium phosphate cathode material in Example 1;

[0030] Figure 3 The first figure in it is the CV curve of the LFVP / C material in Comparative Example 1, and the second figure is the CV curve of the zinc and magnesium ion-doped lithium iron vanadium phosphate cathode material in Example 1;

[0031] Figure 4 It is the 2C constant current cycling performance diagram of the zinc and magnesium ion-doped lithium iron vanadium phosphate cathode material in Example 1;

[0032] Figure 5 It is the AC impedance spectrum diagram of the zinc and magnesium ion-doped lithium iron vanadium phosphate cathode material in Example 1. Detailed Embodiments

[0033] The technical solution of the present invention is not limited to the following specific embodiments listed, and also includes any combination between the specific embodiments.

[0034] Specific Embodiment 1: A method for modifying a high-capacity zinc and magnesium ion-doped lithium iron vanadium phosphate cathode material, which is realized according to the following steps:

[0035] I. Weighing materials:

[0036] Weigh the lithium source, vanadium source, iron source, phosphorus source, zinc source, magnesium source and carbon source respectively according to the molar ratio of lithium, vanadium, iron, phosphorus, zinc, magnesium to carbon being (3.675 - 6.825):(0.95 - 1):(1.8 - 5):5.5:(0.01 - 0.05):(0.01 - 0.2):(4 - 8).

[0037] II. Prepare a carbon-coated zinc and magnesium ion-doped lithium iron vanadium phosphate precursor material:

[0038] Add the weighed lithium source, vanadium source, iron source, phosphorus source, zinc source, magnesium source and carbon source to deionized water and stir evenly to obtain a mixture. Then disperse the mixture in a dispersion medium, and then place it in a ball mill for ball milling for 7 - 10 h to obtain a slurry. After drying and grinding, a carbon-coated zinc and magnesium ion-doped lithium iron vanadium phosphate precursor material is obtained;

[0039] III. Prepare a zinc and magnesium ion-doped lithium iron vanadium phosphate cathode material:

[0040] Transfer the above carbon-coated zinc and magnesium ion-doped lithium iron vanadium phosphate precursor material to a tubular furnace, and perform gradient high-temperature calcination under an inert gas atmosphere. After the calcination is completed, naturally cool it to room temperature, and after grinding and sieving, a high-capacity zinc and magnesium ion-doped lithium iron vanadium phosphate cathode material with the chemical formula Li x+3 Fe x-y Mg y V 2-z Zn z (PO4)3 / C is obtained, where x = 1 - 5, y = 0.01 - 0.2, z = 0.01 - 0.05, and thus the modification method is completed.

[0041] Specific Embodiment 2: The difference between this embodiment and Specific Embodiment 1 is that in Step I, the lithium source is lithium carbonate; the vanadium source is vanadium pentoxide; the iron source is ferrous oxalate dihydrate; the phosphorus source is ammonium dihydrogen phosphate; the zinc source is zinc oxide; the magnesium source is magnesium nitrate, magnesium acetate, magnesium sulfate or magnesium oxide; the carbon source is sucrose, glucose or citric acid monohydrate. Other steps and parameters are the same as those in Specific Embodiment 1.

[0042] Specific Embodiment 3: The difference between this embodiment and Specific Embodiment 1 is that in Step II, the dispersion medium is an aqueous hydrogen peroxide solution with a volume fraction of 1% - 5%; the dispersion method is electromagnetic stirring. Other steps and parameters are the same as those in Specific Embodiment 1.

[0043] Specific Embodiment 4: The difference between this embodiment and Specific Embodiment 1 is that in Step II, the ratio of the total mass of the lithium source, vanadium source, iron source, phosphorus source, magnesium source, zinc source and carbon source to the volume of the dispersion medium is 1 g:(5 mL - 15 mL). Other steps and parameters are the same as those in Specific Embodiment 1.

[0044] Embodiment 5: The difference between this embodiment and Embodiment 1 is that in Step 2, the temperature of the ball milling is 20 - 25°C, and the rotation speed is 400 - 600 r / min. Other steps and parameters are the same as those in Embodiment 1.

[0045] Embodiment 6: The difference between this embodiment and Embodiment 1 is that in Step 2, the ball milling uses steel balls, agate balls or zirconia balls, and the mass ratio of balls to materials is 20:1. Other steps and parameters are the same as those in Embodiment 1.

[0046] Embodiment 7: The difference between this embodiment and Embodiment 1 is that in Step 2, the drying is carried out in a forced-air drying oven at 80 - 120°C for 12 - 15 h; the grinding is carried out in an agate mortar for 20 - 30 min. Other steps and parameters are the same as those in Embodiment 1.

[0047] Embodiment 8: The difference between this embodiment and Embodiment 1 is that in Step 3, the gradient high-temperature calcination is carried out by heating to 300 - 400°C at a rate of 4 - 8°C / min and holding for 6 - 10 h; after the holding is completed, it is heated to 750 - 850°C at a rate of 4 - 8°C / min and held for another 6 - 10 h. Other steps and parameters are the same as those in Embodiment 1.

[0048] Embodiment 9: The difference between this embodiment and Embodiment 1 is that in Step 3, the inert gas is argon; the sieving uses a 400-mesh sieve. Other steps and parameters are the same as those in Embodiment 9.

[0049] Embodiment 10: The difference between this embodiment and Embodiment 1 is that in Step 3, the high-capacity zinc and magnesium ion-doped lithium iron vanadium phosphate cathode material has the chemical formula Li x+3 Fe x-y Mg y V 2-z Zn z (PO4)3 / C, and x = 4, y = 0.08, z = 0.05. Other steps and parameters are the same as those in Embodiment 1.

[0050] The beneficial effects of the present invention are verified through the following examples:

[0051] Example 1:

[0052] A modification method of a high-capacity zinc and magnesium ion-doped lithium iron vanadium phosphate cathode material is realized according to the following steps:

[0053] 1. Weighing materials:

[0054] Weigh the lithium source, vanadium source, iron source, phosphorus source, zinc source, magnesium source and carbon source respectively according to the molar ratio of lithium, vanadium, iron, phosphorus, zinc, magnesium to carbon being 5.775:0.975:3.92:5.5:0.025:0.08:6.325;

[0055] II. Prepare a carbon-coated zinc and magnesium ion-doped lithium iron vanadium phosphate precursor material:

[0056] Add the weighed lithium source, vanadium source, iron source, phosphorus source, zinc source, magnesium source and carbon source to deionized water and stir evenly to obtain a mixture. Then disperse the mixture in a dispersion medium and place it in a ball mill for ball milling for 7 h to obtain a slurry. After drying and grinding, a carbon-coated zinc and magnesium ion-doped lithium iron vanadium phosphate precursor material is obtained;

[0057] III. Prepare a zinc and magnesium ion-doped lithium iron vanadium phosphate cathode material:

[0058] Transfer the above carbon-coated zinc and magnesium ion-doped lithium iron vanadium phosphate precursor material to a tube furnace and carry out gradient high-temperature calcination under an inert gas atmosphere. After the calcination is completed, naturally cool it to room temperature. After grinding and sieving, a high-capacity zinc and magnesium ion-doped lithium iron vanadium phosphate cathode material with the chemical formula Li x+3 Fe x-y Mg y V 2-z Zn z (PO4)3 / C, where x = 1 - 5, y = 0.01 - 0.2, z = 0.01 - 0.05, thus completing the modification method.

[0059] In step I of this embodiment, the lithium source is lithium carbonate; the vanadium source is vanadium pentoxide; the iron source is ferrous oxalate dihydrate; the phosphorus source is ammonium dihydrogen phosphate; the zinc source is zinc oxide; the magnesium source is magnesium oxide; the carbon source is glucose.

[0060] In step II of this embodiment, the dispersion medium is an aqueous hydrogen peroxide solution with a volume fraction of 3%; the dispersion method is electromagnetic stirring, and the stirring time is 0.5 h.

[0061] In step II of this embodiment, the ratio of the total mass of the lithium source, vanadium source, iron source, phosphorus source, magnesium source, zinc source and carbon source to the volume of the dispersion medium is 1 g:(5 mL - 15 mL).

[0062] In step II of this embodiment, the temperature of the ball milling is 25 °C and the rotation speed is 4000 r / min.

[0063] In step II of this embodiment, the ball milling uses steel balls, agate balls or zirconia balls, and the ball-to-material mass ratio is 20:1.

[0064] The drying in Step 2 of this example: drying at 80 °C for 12 h in a forced-air drying oven; the grinding: grinding in an agate mortar for 20 min.

[0065] The gradient high-temperature calcination in Step 3 of this example: heating to 350 °C at a rate of 5 °C / min and holding for 6 h; after the holding ends, heating to 800 °C at a rate of 5 °C / min and continuing to hold for 8 h.

[0066] The inert gas in Step 3 of this example is argon; the sieving uses a 400-mesh sieve.

[0067] The carbon-coated zinc and magnesium ion-doped lithium iron vanadium phosphate (LFMVZP / C) precursor material obtained in Step 2 of this example is in the form of green powder.

[0068] The high-capacity zinc and magnesium ion-doped lithium iron vanadium phosphate cathode material in Step 3 of this example has the chemical formula Li x+3 Fe x- y Mg y V 2-z Zn z (PO4)3 / C, where x = 4, y = 0.08, and z = 0.05.

[0069] Comparative Example 1:

[0070] The method provided in Example 1 is used, with the only difference being that Mg and Zn metal compounds are not added to the raw materials, and finally a lithium-ion battery cathode material with the general formula Li x+3 Fe x-y Mg y V 2-z Zn z (PO4)3 / C (x = 4, y = 0, z = 0) is obtained.

[0071] The material obtained in this example is LFVP / C material.

[0072] Comparative Example 2:

[0073] The method provided in Example 1 is used, with the only difference being that the molar ratio of Fe:V:Zn:Mg in the raw materials is 4:1:0.075:0.08, and finally a lithium-ion battery cathode material with the general formula Li x+3 Fe x-y Mg y V 2-z Zn z (PO4)3 / C (x = 4, y = 0.08, z = 0.075) is obtained.

[0074] Comparative Example 3:

[0075] The method provided in Example 1 is adopted, with the only difference being that the molar ratio of Fe:V:Zn:Mg in the raw materials is 3:1:0.05:0.06, and finally a cathode material for lithium-ion batteries with the general formula Li x+3 Fe x-y Mg y V 2-z Zn z (PO4)3 / C(x = 3, y = 0.06, z = 0.05) is obtained.

[0076] Comparative Example 4:

[0077] The method provided in Example 1 is adopted, with the only difference being that the molar ratio of Fe:V:Zn:Mg in the raw materials is 2:1:0.05:0.04, and finally a cathode material for lithium-ion batteries with the general formula Li x+3 Fe x-y Mg y V 2-z Zn z (PO4)3 / C(x = 2, y = 0.04, z = 0.05) is obtained.

[0078] The LFMVZP / C materials (i.e., high-capacity zinc and magnesium ion-doped lithium iron vanadium phosphate cathode materials) obtained in Example 1 and Comparative Examples 2 to 4 are used as cathode materials for lithium-ion batteries. They are mixed with a conductive agent (acetylene black) and a binder (PVDF) in a volume ratio of 8:1:1 to obtain a slurry, which is then coated on a metal aluminum current collector to obtain a cathode. A polyolefin is used as the separator, and metallic lithium is used as the counter electrode. The electrolyte uses 1 mol / L LiPF6 as the solute, a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) with a volume ratio of 1:1 as the solvent, and 5% fluoroethylene carbonate (FEC) as an additive. The assembly of the lithium-ion battery is completed in a glove box filled with argon.

[0079] After the above-assembled lithium-ion battery is left standing for 12 h, a constant-current charge-discharge test is carried out. The charge-discharge voltage window is 2.5 - 4.3 V, and the specific capacity, rate performance, and long-cycle performance of the cathode of the lithium-ion battery are measured at room temperature.

[0080] The LFMVZP / C material obtained in Example 1 is subjected to structural characterization, and the results are as Figures 1 to 2 shown:

[0081] Figure 1 is the XRD pattern of the zinc and magnesium ion-doped lithium iron vanadium phosphate cathode material in Example 1; as Figure 1As shown, that is, all the diffraction peak positions of the LFMVZP / C material are consistent with those of the standard cards LVP JCPDS#:80-1515 and LFP JCPDS#:81-1173. It belongs to the monoclinic phase structure of the pnma space group and no impurity peaks are detected, indicating that the introduction of zinc and magnesium ions does not affect the crystal structure of the material.

[0082] Figure 2 Figure 4 is the scanning electron microscope image of the lithium iron vanadium phosphate cathode material doped with zinc and magnesium ions in Example 1; as Figure 2 shown, the sample particles prepared by doping Zn 2+ and Mg 2+ are concentratedly distributed at about 2 μm. The sample particle size is small, which is beneficial to the penetration of the electrolyte and plays a positive role in the electrochemical performance of the material. It shows that the doping of Zn 2+ and Mg 2+ can refine the particle size of the LFVP / C material. Therefore, the diffusion distance of lithium ions in the LFVP / C lattice structure is shortened, and thus the electrochemical performance of the material can be improved.

[0083] Figure 3 Figure 5 is the CV curve of the lithium iron vanadium phosphate cathode material doped with zinc and magnesium ions in Example 1; as Figure 3 shown, the CV curve has 4 pairs of obvious redox peaks. The redox peaks of the LFMVZP / C material are sharp and symmetric, the oxidation peak intensity is higher, and the reduction peak area is larger, which can provide more reversible capacity during the cycling process.

[0084] Figure 4 Figure 6 is the 2C constant current cycling performance of the lithium iron vanadium phosphate cathode material doped with zinc and magnesium ions in Example 1; as Figure 4 shown, it can be seen that for the lithium ion battery prepared with the LFMVZP / C material, the initial discharge specific capacity at a 2C current density is 118.35 mAh g -1 of reversible capacity. After 50 cycles, the LFMVZP / C material can discharge 117.43 mAh g -1 of reversible capacity. The Coulomb efficiency during the cycling process is close to 100% all the time, with a high capacity retention rate of 99.22% and good cycling stability. The effect comparison between Example 1 and Comparative Example 1 is shown in Table 1.

[0085] Table 1: Data table of the effects of Application Example 1 and Comparative Example 1

[0086]

[0087] Figure 5 Figure 7 is the AC impedance spectrum of the lithium iron vanadium phosphate cathode material doped with zinc and magnesium ions in Example 1; as Figure 5As shown, it can be seen that the modified lithium iron vanadium phosphate material has a smaller charge transfer resistance (Rct), faster lithium ion insertion / extraction kinetics, and easier electrode reactions.

[0088] The embodiments described above are only descriptions of the preferred embodiments of the present invention and do not limit the scope of the present invention. Without departing from the design spirit of the present invention, various deformations and improvements made by those of ordinary skill in the art to the technical solutions of the present invention shall fall within the protection scope determined by the claims of the present invention.

Claims

1. A modification method for a high-capacity lithium iron vanadium phosphate cathode material doped with zinc and magnesium ions, characterized in that It is implemented according to the following steps: I. Weighing materials: Weigh lithium source, vanadium source, iron source, phosphorus source, zinc source, magnesium source and carbon source respectively according to the molar ratio of lithium, vanadium, iron, phosphorus, zinc, magnesium to carbon being (3.675 - 6.825):(0.95 - 1):(1.8 - 5):5.5:(0.01 - 0.05):(0.01 - 0.2):(4 - 8); II. Preparing carbon-coated zinc and magnesium ion-doped lithium iron vanadium phosphate precursor material: Add the weighed lithium source, vanadium source, iron source, phosphorus source, zinc source, magnesium source and carbon source into deionized water and stir evenly to obtain a mixture. Then disperse the mixture into a dispersion medium, and then place it in a ball mill for ball milling for 7 - 10 h to obtain a slurry. After drying and grinding, obtain the carbon-coated zinc and magnesium ion-doped lithium iron vanadium phosphate precursor material; III. Preparing zinc and magnesium ion-doped lithium iron vanadium phosphate cathode material: Transfer the above carbon-coated zinc and magnesium ion-doped lithium iron vanadium phosphate precursor material into a tube furnace, and carry out gradient high-temperature calcination under an inert gas atmosphere. After the calcination is completed, it is naturally cooled to room temperature, and then ground and sieved to obtain a high-capacity zinc and magnesium ion-doped lithium iron vanadium phosphate cathode material with the chemical formula Li x+3 Fe x-y Mg y V 2-z Zn z (PO4)3 / C, where x = 1 - 5, y = 0.01 - 0.2, z = 0.01 - 0.05, thus completing the said modification method; Among them, the lithium source described in step I is lithium carbonate; the vanadium source is vanadium pentoxide; the iron source is ferrous oxalate dihydrate; the phosphorus source is ammonium dihydrogen phosphate; the zinc source is zinc oxide; the magnesium source is magnesium nitrate, magnesium acetate, magnesium sulfate or magnesium oxide; the carbon source is sucrose, glucose or citric acid monohydrate; The dispersion medium described in step II is an aqueous hydrogen peroxide solution with a volume fraction of 1% - 5%; the dispersion method is electromagnetic stirring; The ratio of the total mass of the lithium source, vanadium source, iron source, phosphorus source, magnesium source, zinc source and carbon source described in step II to the volume of the dispersion medium is 1 g:(5 mL - 15 mL); The temperature of the ball milling described in step II is 20 - 25 °C, and the rotation speed is 400 - 600 r / min; The ball milling described in step II uses steel balls, agate balls or zirconia balls, and the mass ratio of balls to materials is 20:1; The drying described in step II: dry in a blast drying oven at 80 - 120 °C for 12 - 15 h; the grinding: grind in an agate mortar for 20 - 30 min; The gradient high-temperature calcination described in step III: heat up to 300 - 400 °C at a rate of 4 - 8 °C / min and keep warm for 6 - 10 h; after the heat preservation ends, heat up to 750 - 850 °C at a rate of 4 - 8 °C / min and continue to keep warm for 6 - 10 h; The inert gas described in step III is argon; the sieving uses a 400-mesh sieve; In step three, the high-capacity zinc- and magnesium-doped lithium iron vanadium phosphate cathode material has the chemical formula Li x+3 Fe x-y Mg y V 2-z Zn z (PO4)3 / C, where x = 4, y = 0.08, and z = 0.05.

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