A method for preparing lithium vanadium fluorophosphate by using lithium iron phosphate waste powder

Abstract

The present application relates to the field of lithium battery material recycling, in particular to a method for preparing lithium vanadium fluorophosphate from lithium iron phosphate waste powder. The present application first removes carbon in the lithium iron phosphate waste powder by acid leaching, then adds iron powder to the leaching solution to remove copper to obtain a copper-removed solution, further adds ammonium fluoride to the copper-removed solution to precipitate impurities calcium and aluminum, after filtration and separation, adds vanadyl oxalate and ammonium sulfide to the filtrate in sequence to remove iron ions and other heavy metals, then adjusts the ratio of lithium, vanadium, phosphorus, fluorine and oxalate in the solution, and under the action of the reducing agent ammonium oxalate, a crude lithium vanadium fluorophosphate is prepared by hydrothermal synthesis method, the crude product is further slurried, washed and carbon source is added, and after high-temperature calcination, a carbon-coated lithium vanadium fluorophosphate material is obtained, which is further crushed and screened to remove iron to obtain a battery-grade lithium vanadium fluorophosphate finished product. The present application can directly prepare a battery-grade lithium vanadium fluorophosphate finished product.

Description

TECHNICAL FIELD

[0001] The application relates to the field of lithium battery material recycling, in particular to a method for preparing lithium vanadium fluorophosphate from lithium iron phosphate waste powder. BACKGROUND

[0002] In the case that lithium iron phosphate batteries are maturely used in electric vehicles, the recycling of waste batteries has become a thorny problem.

[0003] Most of the reported methods for recycling waste batteries convert iron in the batteries into iron phosphate, convert lithium in the batteries into lithium carbonate, and other elements are also recycled accordingly, but in the process of wet impurity removal, path one: the dissolved calcium and aluminum ions in the solution are removed by adjusting the pH, but in this method, due to the excessively high pH, the calcium and aluminum are precipitated, and at the same time, iron and phosphorus form Fe3(PO4)2·8H2O precipitate and are lost, path two: fluorine ions are introduced to form fluoride precipitate to remove calcium and aluminum, according to the impurity content of the battery powder, the content of fluorine ions introduced into the solution also has differences, resulting in large fluctuations in the fluorine ion content of each batch of iron phosphate and lithium carbonate products, and finally the batteries prepared from the materials have poor consistency and are prone to dangerous accidents, which is not allowed in the battery industry, in order to solve the influence of fluorine ions on the performance of the battery and reduce the loss of phosphorus and iron in the impurity removal process, based on this, it is particularly important to provide a method for preparing lithium vanadium fluorophosphate from lithium iron phosphate waste powder. SUMMARY

[0004] The application provides a method for preparing lithium vanadium fluorophosphate from lithium iron phosphate waste powder, which successfully prepares battery-grade lithium vanadium fluorophosphate and realizes the recycling of phosphorus, iron and other elements with high recovery rate.

[0005] The application achieves the above technical purpose by adopting the following technical scheme: a method for preparing lithium vanadium fluorophosphate from lithium iron phosphate waste powder, comprising the following steps:

[0006] S1, acid leaching: inorganic acid is used to leach the lithium iron phosphate waste powder, and solid-liquid separation is performed to obtain carbon residue and leaching solution, the carbon residue is washed with water and dried to obtain crude graphite powder, wherein the amount of hydrogen ions added is 2.2-3.0 times the amount of substance of lithium ions in the battery powder;

[0007] S2, copper removal: 120-150 mesh reduced iron powder is added to the leaching solution obtained in step S1, the pH of the solution is adjusted to 2.5-3.5, and solid-liquid separation is performed to obtain filter residue sponge copper and copper removal solution, and the amount of iron powder added is 0.3-1% of the mass of the leaching solution;

[0008] S3, calcium and aluminum removal: adding fluoride salt into the copper-removed solution obtained in step S2 and reacting for 1-2 hours, and then separating the solid and the liquid to obtain a filtrate 1, wherein the amount of fluoride ion added is n(F)=k*(2n(Ca)+3n(Al), k is 1.1-1.5, and n represents the amount of substance;

[0009] S4, ferrous oxalate preparation: adding VOC2O4·5H2O precipitant into the filtrate 1 obtained in step S3, and then separating the solid and the liquid to obtain a ferrous oxalate filter cake and a filtrate 2, wherein the amount of precipitant added is 1.2-2.0 times the amount of Fe ion in the solution;

[0010] The ferrous oxalate filter cake is washed, and the conductivity of the washing end point is controlled to be 200-500 μS / cm, and then the filter cake is dried to obtain battery-grade ferrous oxalate;

[0011] S5, heavy metal removal: adding ammonium sulfide solution into the filtrate 2 obtained in step S4, and then separating the solid and the liquid to obtain a filtrate 3, wherein the amount of ammonium sulfide added is 2-5% of the mass concentration of ammonium sulfate solution, and the amount of ammonium sulfide solution added is 0.5-1.5% of the mass of the filtrate 2;

[0012] S6, proportion adjustment: adding fluoride source, phosphorus source and reducing agent (NH4)2C2O4 into the filtrate 3, and then stirring uniformly to obtain a reaction solution, wherein the molar ratio of n(Li):n(V):n(P):n(F):n(C2O4 2- ) in the solution is (1.05-1.1):(0.95-1.0):(1.0-1.05):(0.9-1.0):(0.55-0.7), wherein the fluoride source is a mixture of ammonium fluoride and lithium fluoride, and the phosphorus source is any one of phosphoric acid, monobasic ammonium phosphate and dibasic ammonium phosphate;

[0013] The reaction solution is subjected to hydrothermal synthesis reaction, and then the solid and the liquid are separated, the filter cake is lithium vanadium fluorophosphate, and the filtrate is ammonium sulfate solution, and the filtrate is concentrated and crystallized to obtain ammonium sulfate crystals;

[0014] S7, washing: the lithium vanadium fluorophosphate filter cake is slurried and washed with pure water, and a carbon source is added in the last slurry washing kettle;

[0015] S8, drying and calcination: the filter cake after slurry washing is dried at 130-180 ℃ for 1.5-4 hours, and then calcined at 750-850 ℃ for 4-10 hours in a nitrogen atmosphere to obtain carbon-coated lithium vanadium fluorophosphate material;

[0016] S9, crushing: the sintered material is crushed to a particle size D50 of 0.5-2.5 μm, D90≤7 μm, and D99≤15 μm;

[0017] S10, screening and removing iron: the crushed vanadium lithium fluorophosphate material is screened and iron is removed to a content of magnetic substances <0.3 ppm, to obtain a battery-grade vanadium lithium fluorophosphate product.

[0018] The application first removes carbon in the lithium iron phosphate waste powder by acid leaching, then adds iron powder to the leaching solution to remove copper to obtain a copper-removed solution, further adds ammonium fluoride to the copper-removed solution to precipitate impurities calcium and aluminum, after filtration and separation, sequentially adds vanadyl oxalate and ammonium sulfide to the filtrate to remove iron ions and other heavy metals, then adjusts the proportions of lithium, vanadium, phosphorus, fluorine and oxalate in the solution, under the action of the reducing agent ammonium oxalate, precipitates lithium ions by hydrothermal synthesis to prepare a crude vanadium lithium fluorophosphate product, further slurries and washes the crude product and adds a carbon source, and after high-temperature calcination, obtains a carbon-coated vanadium lithium fluorophosphate material, which is further crushed and screened to remove iron to obtain a battery-grade vanadium lithium fluorophosphate product.

[0019] The application uses waste battery powder as raw material to prepare a battery-grade vanadium lithium fluorophosphate, which realizes resource recycling and reduces production cost; uses an ammonium salt system, and the ammonium sulfate mother liquor can be concentrated by MVR to obtain a high-value ammonium sulfate byproduct, without waste water generation, reducing environmental pollution; the application can fully utilize P, Fe and Li in the waste battery, and can maximize resource utilization, wherein the P recovery rate can reach 94.3% or more, the Fe recovery rate can reach 95.6% or more, and the Li leaching rate can reach 98.4% or more; the application uses vanadyl oxalate as a decontaminating agent in the iron removal step, which not only achieves the purpose of decontamination, but also introduces vanadium source, avoids subsequent addition of vanadium source, reduces process steps, and in vanadyl oxalate, vanadium is 4-valence vanadium, compared with V2O5 and NH4VO3, the required reducing agent for synthesizing vanadium lithium phosphate is less, the amount of reducing agent is reduced, the cost is saved, and the prepared vanadium lithium fluorophosphate product can directly meet the battery-grade requirements, and the further prepared lithium ion button cell has a charge / discharge specific capacity of more than 140 mAh / g at a voltage of 3.0-4.5 V and a 0.1C rate, and a first charge / discharge efficiency of more than 98%. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 The application is a process flow diagram. DETAILED DESCRIPTION

[0021] The application will be further described in detail below with reference to specific embodiments, so that those skilled in the art can more clearly understand the application. The following examples are used to illustrate the application, but are not used to limit the scope of the application. Based on the specific embodiments in the application, all other embodiments obtained by those skilled in the art without creative labor are within the protection scope of the application.

[0022] In the embodiments of the present application, all raw material components are commercially available products well known to those skilled in the art, unless otherwise specified; in the embodiments of the present application, the technical means used are conventional means well known to those skilled in the art, unless otherwise specified.

[0023] The present application provides a method for preparing lithium vanadium fluorophosphate from lithium iron phosphate waste powder, which comprises the following steps: Figure 1 , comprising the following steps:

[0024] S1, acid leaching: using inorganic acid to leach lithium iron phosphate waste powder, solid-liquid separation to obtain carbon residue and leaching solution, the carbon residue is washed and dried to obtain crude graphite powder.

[0025] In this step, the amount of hydrogen ions added is 2.2-3.0 times the amount of lithium ions in the battery powder, calculated as inorganic acid solution, and the mass ratio of inorganic acid solution to lithium iron phosphate waste powder is (3-6):1, the acid leaching conditions are 20-50℃, and the reaction time is 30-90min.

[0026] S2, copper removal: adding 120-150 mesh reduced iron powder to the leaching solution obtained in step S1, adjusting the pH of the solution to 2.5-3.5, and solid-liquid separation to obtain filter residue sponge copper and copper removal solution, the amount of iron powder added is 0.3-1% of the mass of the leaching solution.

[0027] S3, calcium and aluminum removal: adding a fluoride salt to the copper removal solution obtained in step S2 and reacting for 1-2h, and solid-liquid separation to obtain filtrate 1, wherein the amount of fluoride ions added is n(F)=k*(2n(Ca)+3n(Al), k is 1.1-1.5, and n represents the amount of substance;

[0028] In this step, the fluoride salt used can be selected from ammonium fluoride, sodium fluoride or lithium fluoride.

[0029] S4, ferrous oxalate preparation: adding VOC2O4·5H2O precipitant to the filtrate 1 obtained in step S3, solid-liquid separation to obtain ferrous oxalate filter cake and filtrate 2, wherein the amount of precipitant added is 1.2-2.0 times the amount of Fe ions in the solution;

[0030] The ferrous oxalate filter cake is washed to control the final conductivity of the washing to 200-500μS / cm, and then dried to obtain battery-grade ferrous oxalate.

[0031] S5, heavy metal removal: adding ammonium sulfide solution to the filtrate 2 obtained in step S4, solid-liquid separation after sufficient reaction to obtain filtrate 3, wherein the amount of ammonium sulfide added is 2-5% of the mass concentration of ammonium sulfate solution, and the amount of ammonium sulfide solution added is 0.5-1.5% of the mass of the filtrate 2.

[0032] S6, Adjust the ratio: Add fluorine source, phosphorus source and reducing agent (NH4)2C2O4 to filtrate 3, stir thoroughly to obtain a reaction solution, and control the ratio of n(Li):n(V):n(P):n(F):n(C2O4) in the solution. 2- )=(1.05~1.1):(0.95~1.0):(1.0~1.05):(0.9~1.0):(0.55~0.7), wherein the fluorine source is a mixture of ammonium fluoride and lithium fluoride, and the phosphorus source is any one of phosphoric acid, monoammonium phosphate and diammonium phosphate;

[0033] The above reaction solution was subjected to a hydrothermal synthesis reaction, followed by solid-liquid separation. The filter cake was lithium vanadium fluorophosphate, and the filtrate was an ammonium sulfate solution. The filtrate was concentrated and crystallized to obtain ammonium sulfate crystals.

[0034] The conditions for hydrothermal synthesis are 120–300℃ for 2–6 hours.

[0035] S7, Washing: The lithium vanadium fluorophosphate filter cake is slurried and washed with pure water, and a carbon source is added to the final slurry washing vessel;

[0036] The carbon sources are sucrose and glucose, and the carbon content of the finished product is controlled at 1.5-3%.

[0037] S8, Drying and Calcination: The filter cake after pulping and washing is dried at 130-180℃ for 1.5-4 hours, and then calcined at 750-850℃ for 4-10 hours under a nitrogen atmosphere to obtain carbon-coated lithium vanadium fluorophosphate material;

[0038] S9, Crushing: The sintered material is crushed to a particle size D50 of 0.5-2.5μm, D90≤7μm, and D99≤15μm;

[0039] S10, Iron removal by sieving: The pulverized lithium vanadium fluorophosphate material is sieved to remove iron until the content of magnetic materials is <0.3ppm, thus obtaining battery-grade lithium vanadium phosphate finished product.

[0040] Example 1

[0041] This invention provides a method for preparing lithium vanadium fluorophosphate from lithium iron phosphate waste powder, comprising the following steps:

[0042] Step 1, acid leaching: sulfuric acid is used to leach lithium iron phosphate waste powder. The acid leaching temperature is 30℃ and the reaction time is 60min. Solid-liquid separation is performed to obtain carbon slag and leachate. The carbon slag is washed and dried to obtain crude graphite powder. The mass ratio of sulfuric acid solution to lithium iron phosphate waste powder is 4:1, and the amount of hydrogen ions added is 2.5 times the amount of lithium ions in the battery powder.

[0043] Step 2, copper removal: add 120 mesh reduced iron powder to the leaching solution, adjust the pH of the solution to 2.7±0.1, and separate the solid and liquid to obtain sponge copper and copper-removed solution, the amount of iron powder added is 0.6% of the mass of the leaching solution, the unreacted iron powder is intercepted by a magnetic filter, and it is collected for reuse;

[0044] Step 3, calcium and aluminum removal: add ammonium fluoride to the copper-removed solution and react for 1 h, and then separate the solid and liquid to obtain filtrate 1, wherein the amount of fluoride added is n(F)=k*(2n(Ca)+3n(Al), k is 1.3, and n represents the amount of substance;

[0045] Step 4, ferrous oxalate preparation: add VOC2O4·5H2O precipitant to filtrate 1 and react for 60 minutes, and then separate the solid and liquid to obtain ferrous oxalate filter cake and filtrate 2, wherein the amount of precipitant added is 1.5 times the amount of Fe ions in the solution;

[0046] The ferrous oxalate filter cake is washed to control the conductivity of the washing end point to be 200-500 μS / cm, and then dried to obtain battery-grade ferrous oxalate, which is sold as a byproduct;

[0047] Step 5, heavy metal removal: add ammonium sulfide solution to filtrate 2, react for 30 min, and then separate the solid and liquid to obtain filtrate 3, wherein the concentration of the ammonium sulfide solution is 3%, and the amount of ammonium sulfide solution added is 1% of the mass of filtrate 2;

[0048] Step 6, proportion adjustment: add a fluoride source, a phosphorus source, and a reducing agent (NH4)2C2O4 to filtrate 3, and stir uniformly to obtain a reaction solution, wherein the n(Li):n(V):n(P):n(F):n(C2O4 2- ) in the solution is controlled to be 1.05:0.95:1.0:0.9:0.55, the fluoride source is a mixture of ammonium fluoride and lithium fluoride, and the phosphorus source is phosphoric acid;

[0049] The above reaction solution is stirred at 250℃±5℃ for 4 h to perform a hydrothermal synthesis reaction, the filter cake is lithium vanadium phosphate fluoride after the reaction is completed, and the filtrate is an ammonium sulfate solution, which is concentrated by MVR to obtain ammonium sulfate crystals, which can be sold as a byproduct;

[0050] Step 7, washing: the lithium vanadium phosphate fluoride filter cake is slurried and washed with pure water, and the washing is repeated three times, and the solid-liquid ratio of each washing is 2.5:1, and a carbon source sucrose is added to the last slurry washing kettle to control the carbon content in the finished product to be 2%±0.1%.

[0051] Step 8, drying and calcination: the slurry-washed filter cake is dried at 150℃ for 3 h, and then calcined at 750℃ for 5 h in a nitrogen atmosphere to obtain carbon-coated lithium vanadium phosphate fluoride material;

[0052] Step 9, crushing: the material sintered out of the furnace is crushed to a particle size D50 of 0.5-2.5 μm, D90≤7 μm, and D99≤15 μm using a jet mill;

[0053] Step 10, screening and iron removal: the crushed vanadium lithium fluorophosphate material is screened and iron is removed to a content of magnetic substances of <0.3 ppm, to obtain a battery-grade vanadium lithium fluorophosphate product.

[0054] Example 2

[0055] The embodiment of the present application provides a method for preparing vanadium lithium fluorophosphate from lithium iron phosphate waste powder, comprising the following steps:

[0056] Step 1, acid leaching: the lithium iron phosphate waste powder is leached with sulfuric acid, wherein the acid leaching temperature is 40°C, the reaction time is 60 min, and carbon residue and leaching solution are obtained by solid-liquid separation, the carbon residue is washed with water and dried to obtain crude graphite powder, wherein the mass ratio of hydrochloric acid solution to lithium iron phosphate waste powder is 5:1, and the amount of hydrogen ions added is 2.8 times the amount of substance of lithium ions in the battery powder;

[0057] Step 2, copper removal: 150 mesh reduced iron powder is added to the leaching solution, the pH of the solution is adjusted to 3.0±0.1, and solid-liquid separation is performed to obtain filter residue sponge copper and copper-removed solution, the amount of iron powder added is 1% of the mass of the leaching solution, and the unreacted iron powder is intercepted by a magnetic filter and collected for reuse;

[0058] Step 3, calcium and aluminum removal: sodium fluoride is added to the copper-removed solution and reacted for 2 h, and solid-liquid separation is performed to obtain filtrate 1, wherein the amount of fluoride ions added is n(F)=k*(2n(Ca)+3n(Al), k is 1.5, and n represents the amount of substance;

[0059] Step 4, ferrous oxalate preparation: VOC2O4·5H2O precipitant is added to the filtrate 1 and reacted for 90 min, and solid-liquid separation is performed to obtain ferrous oxalate filter cake and filtrate 2, wherein the amount of precipitant added is 2.0 times the amount of substance of Fe ions in the solution;

[0060] The ferrous oxalate filter cake is washed to control the final conductivity to 200-500 μS / cm, and then dried to obtain battery-grade ferrous oxalate, which is sold as a byproduct;

[0061] Step 5, heavy metal removal: ammonium sulfide solution is added to the filtrate 2, and solid-liquid separation is performed after reacting for 30 min to obtain filtrate 3, wherein the concentration of the ammonium sulfide solution is 5%, and the amount of ammonium sulfide solution added is 0.5% of the mass of the filtrate 2:

[0062] Step 6, adjust the proportion: add fluorine source, phosphorus source and reducing agent (NH4)2C2O4 to the filtrate 3, stir well to obtain a reaction solution, control the n(Li):n(V):n(P):n(F):n(C2O4 2- ) in the solution = 1.1:1.0:1.05:1.0:0.7, wherein the fluorine source is a mixture of ammonium fluoride and lithium fluoride, and the phosphorus source is ammonium dihydrogen phosphate;

[0063] The above reaction solution is stirred at 160°C±5°C for 5h to carry out a hydrothermal synthesis reaction. After the reaction is completed, solid-liquid separation is carried out. The filter cake is lithium vanadium fluorophosphate, and the filtrate is an ammonium sulfate solution. The filtrate is concentrated by MVR to obtain ammonium sulfate crystals, which can be sold as a byproduct;

[0064] Step 7, washing: the lithium vanadium fluorophosphate filter cake is washed with pure water by slurry washing. The washing is repeated three times. The solid-liquid ratio of each washing is 2.5:1. In the last slurry washing kettle, a carbon source, glucose, is added. The carbon content in the finished product is controlled to be 2.5%±0.1%

[0065] Step 8, drying and calcining: the filter cake after slurry washing is dried at 180°C for 1.5h, and then calcined at 800°C for 5h in a nitrogen atmosphere to obtain a carbon-coated lithium vanadium fluorophosphate material;

[0066] Step 9, crushing: the sintered material is crushed by air flow crushing equipment to a particle size D50 of 0.5-2.5μm, D90≤7μm, and D99≤15μm;

[0067] Step 10, screening and removing iron: the crushed lithium vanadium fluorophosphate material is screened and iron is removed to a magnetic substance content of <0.3ppm to obtain a battery-grade lithium vanadium phosphate product.

[0068] Comparative Example 1

[0069] This comparative example provides a method for preparing lithium vanadium fluorophosphate using lithium iron phosphate waste powder. The main difference from Example 1 is that in Step 2, the solution pH is adjusted to 2.0 with iron powder.

[0070] Comparative Example 2

[0071] This comparative example provides a method for preparing lithium vanadium fluorophosphate using lithium iron phosphate waste powder. The main difference from Example 1 is that in Step 6, the n(Li):n(V):n(P):n(F):n(C2O4 2- ) in the solution = 1.0:1.0:1.0:1.0:0.5.

[0072] Comparative Example 3

[0073] The present comparative example provides a method for preparing lithium vanadium fluorophosphate using lithium iron phosphate waste powder, which is mainly different from example 1 in that in step 8, calcination is carried out at 650°C for 5h under a nitrogen atmosphere.

[0074] Result analysis:

[0075] (1) Impurity removal analysis

[0076] In step 3 of Comparative Example 1, the effect of removing calcium and aluminum from the filtrate 1 was analyzed, and the aluminum removal rate was 47.22%, and the calcium removal rate was 63.28%. In step 3 of Example 1, the effect of removing calcium and aluminum from the filtrate 1 was analyzed, and the aluminum removal rate was 98.31%, and the calcium removal rate was 98.86%. Therefore, when the pH of the iron powder adjusting solution in step 2 is lower than the optimal range given in the present patent, the calcium removal rate in step 3 will be greatly reduced, which will ultimately result in the prepared lithium vanadium fluorophosphate product having excessive impurities and not meeting the standard of battery-grade materials.

[0077] (2) The carbon-coated lithium vanadium fluorophosphate materials prepared in Examples 1-2 and Comparative Examples 2-3 above were used as positive electrode materials to prepare lithium ion button cells, and the cells were subjected to charge-discharge tests at a voltage of 3.0-4.5V. The electrochemical performance test results are shown in Table 1.

[0078]

[0079] From the above results, it can be seen that the performance of the lithium vanadium fluorophosphate material prepared in the comparative example is much lower than that of the lithium vanadium fluorophosphate material prepared in Examples 1 and 2.

[0080] It is necessary to point out that the above examples are only limited to further illustrate and explain the technical solutions of the present application, and are not further limited to the technical solutions of the present application. The method of the present application is only a preferred embodiment, and is not used to limit the protection scope of the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A method for preparing lithium vanadium fluorophosphate using waste lithium iron phosphate powder, characterized in that, The method comprises the following steps: S1, acid leaching: using inorganic acid to leach lithium iron phosphate waste powder, solid-liquid separation to obtain carbon residue and leaching solution, the carbon residue is washed and dried to obtain crude graphite powder, wherein the amount of hydrogen ion added is 2.2-3.0 times the amount of lithium ion in the battery powder; S2, copper removal: adding 120-150 mesh reduced iron powder to the leaching solution obtained in step S1, adjusting the pH of the solution to 2.5-3.5, and solid-liquid separation to obtain filter residue sponge copper and copper-removed solution, the amount of iron powder added is 0.3-1% of the mass of the leaching solution; S3, calcium and aluminum removal: adding fluoride salt to the copper-removed solution obtained in step S2 and fully reacting for 1-2 h, and solid-liquid separation to obtain filtrate 1, wherein the amount of fluoride ion added is n(F)=k*(2n(Ca)+3n(Al), k is 1.1-1.5, and n represents the amount of substance; S4, ferrous oxalate preparation: adding VOC2O4·5H2O precipitant to the filtrate 1 obtained in step S3, and solid-liquid separation to obtain ferrous oxalate filter cake and filtrate 2, wherein the amount of precipitant added is 1.2-2.0 times the amount of Fe ion in the solution; The ferrous oxalate filter cake is washed, the terminal conductivity of the washing is controlled to be 200-500 μS / cm, and then it is dried to obtain battery-grade ferrous oxalate; S5, heavy metal removal: adding ammonium sulfide solution to the filtrate 2 obtained in step S4, fully reacting, and then solid-liquid separation to obtain filtrate 3, wherein the amount of ammonium sulfide added is 2-5% of the mass concentration of ammonium sulfate solution, and the amount of ammonium sulfide solution added is 0.5-1.5% of the mass of the filtrate 2; S6, adjusting proportion: adding fluorine source, phosphorus source and reducing agent (NH4)2C2O4 into filtrate 3, stirring uniformly, obtaining reaction solution, controlling n(Li):n(V):n(P):n(F):n(C2O4) in solution=(1.05-1.1):(0.95-1.0):(1.0-1.05):(0.9-1.0):(0.55-0.7), wherein fluorine source is mixture of ammonium fluoride and lithium fluoride, phosphorus source is any one of phosphoric acid, monobasic ammonium phosphate and dibasic ammonium phosphate; 2- S6, adjusting proportion: adding fluorine source, phosphorus source and reducing agent (NH4)2C2O4 into filtrate 3, stirring uniformly, obtaining reaction solution, controlling n(Li):n(V):n(P):n(F):n(C2O4) in solution=(1.05-1.1):(0.95-1.0):(1.0-1.05):(0.9-1.0):(0.55-0.7), wherein fluorine source is mixture of ammonium fluoride and lithium fluoride, phosphorus source is any one of phosphoric acid, monobasic ammonium phosphate and dibasic ammonium phosphate; The above reaction solution is subjected to hydrothermal synthesis reaction, and then solid-liquid separation, the filter cake is lithium vanadium fluorophosphate, and the filtrate is ammonium sulfate solution, the filtrate is concentrated and crystallized to obtain ammonium sulfate crystals; S7, washing: the lithium vanadium fluorophosphate filter cake is slurried and washed with pure water, and a carbon source is added in the last slurry washing kettle; S8, drying and calcining: the filter cake after slurry washing is dried at 130-180 ℃ for 1.5-4 h, and then calcined at 750-850 ℃ under a nitrogen atmosphere for 4-10 h to obtain carbon-coated lithium vanadium fluorophosphate material; S9, crushing: the sintered material is crushed to a particle size D50 of 0.5-2.5 μm, D90≤7 μm, and D99≤15 μm; S10, screening and iron removal: the crushed lithium vanadium fluorophosphate material is screened and iron-removed to a magnetic substance content of <0.3 ppm to obtain battery-grade lithium vanadium phosphate product.

2. The method of claim 1, wherein the lithium vanadium fluorophosphate is prepared using the lithium iron phosphate waste powder, and the lithium vanadium fluorophosphate is represented by the formula: Li3V2(PO4)2F2. The inorganic acid is any one of sulfuric acid, hydrochloric acid, and nitric acid, the acid leaching conditions are 25-50 ℃, and the reaction time is 30-90 min.

3. The method of claim 1, wherein the lithium vanadium fluorophosphate is prepared using the lithium iron phosphate waste powder, and the lithium vanadium fluorophosphate is prepared by mixing the lithium vanadium fluorophosphate waste powder with a lithium source, a vanadium source, and a fluorine source, and then performing a heat treatment. The mass ratio of the inorganic acid solution to the lithium iron phosphate waste powder is (3-6):

1.

4. The method of claim 1, wherein the lithium vanadium fluorophosphate is prepared using the lithium iron phosphate waste powder, and the lithium vanadium fluorophosphate is prepared by mixing the lithium vanadium fluorophosphate waste powder with a lithium source, a vanadium source, and a fluorine source, and then performing a heat treatment. The fluoride salt in step S3 is ammonium fluoride, sodium fluoride, or lithium fluoride.

5. The method of claim 1, wherein the lithium vanadium fluorophosphate is prepared using the lithium iron phosphate waste powder, and the lithium vanadium fluorophosphate is prepared by mixing the lithium vanadium fluorophosphate waste powder with a lithium source, a vanadium source, and a fluorine source, and then performing a heat treatment. The hydrothermal synthesis reaction in step S6 is carried out at 120-300 ℃ for 2-6 h.

6. The method of claim 1, wherein the lithium vanadium fluorophosphate is prepared using the lithium iron phosphate waste powder, and the lithium vanadium fluorophosphate is prepared by mixing the lithium iron phosphate waste powder, vanadium source, and lithium source. The carbon source in step S7 is sucrose or glucose, and the carbon content of the product is controlled to be 1.5-3%.

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