A method for preparing low-cost iron phosphate and high-density lithium iron phosphate materials

Abstract

The present invention discloses a method for preparing a low-cost iron phosphate and high-density lithium iron phosphate material. The method uses struvite as a phosphorus source and a dopant source, which not only reduces production costs but also achieves resource utilization of environmental pollutants. Acidic mine wastewater also contains trace amounts of heteroatoms such as nitrogen and sulfur, which can be in-situ doped during the preparation of lithium iron phosphate, effectively improving the electrical properties of the lithium iron phosphate. By classifying the slurry and carbon-coating it, particle growth is effectively controlled. Specifically, with a smaller amount of carbon coating, the lithium iron phosphate can grow to a greater extent. With a higher carbon content and better coating, the growth of the lithium iron phosphate is significantly inhibited, effectively preventing small particles from agglomerating into larger particles during the subsequent sintering process. This successfully achieves particle size gradation and produces a high-density lithium iron phosphate material.

Description

Technical Field

[0001] The present invention belongs to the field of lithium-ion battery positive electrode materials, and in particular relates to a method for preparing low-cost iron phosphate and high-density lithium iron phosphate materials. Background Art

[0002] Lithium iron phosphate (LiFePO4) battery materials have long been a research hotspot due to their stable structure, non-toxicity, environmental friendliness, and long cycle life. However, their low energy density has limited their development. One of the most common approaches to increasing LiFePO4's energy density is to increase its compaction density. With rising raw material prices, cost reduction has become a key issue in the lithium battery industry.

[0003] Currently, the commonly used methods to improve compaction mainly include: reducing carbon content, adjusting the sintering system and the size grading of particles. However, although reducing the carbon content or increasing the sintering temperature can increase the compaction density of the powder, it will also cause problems such as reduced capacity and poor rate performance. The size grading of particles is generally achieved by mixing slurry and sintering precursors of different particle sizes. However, it is worth noting that small particles will agglomerate and grow into large particles during the subsequent sintering process, which often fails to achieve the expected size grading effect, resulting in large performance fluctuations and inability to stabilize production, which seriously restricts the widespread application of lithium iron phosphate. Summary of the Invention

[0004] The present invention provides a method for preparing low-cost iron phosphate and high-density lithium iron phosphate materials. The method uses struvite as a phosphorus source to prepare iron phosphate, uses heteroatoms contained in acidic mine wastewater as a dopant, and achieves large and small particle grading through classified coating to prepare high-density lithium iron phosphate materials.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions:

[0006] A method for preparing low-cost ferric phosphate comprises the following steps:

[0007] 1) Add struvite to acidic mine wastewater to dissolve struvite MgNH4PO4 into H3PO4 and Mg 2+ and NH4 + , remove impurities, filter, desorb and concentrate to obtain H3PO4, Mg 2+ solution;

[0008] 2) adding an iron source to the obtained H3PO4, adding the mixed solution into a reactor, reacting in the presence of oxygen, washing, drying, and calcining to obtain iron phosphate.

[0009] In step 1) of the above method, the impurity removal method is to add one of biochar, montmorillonite, hydrotalcite, fungi or bacteria to the solution for impurity removal, and the added amount is 1-5% of the mass of the solution;

[0010] The desorption liquid is acid mine wastewater;

[0011] In one embodiment of the present invention, the impurity removal, filtration, and desorption operations are as follows: adding biochar to the solution, stirring, filtering, obtaining a filtrate containing H3PO4, adding acidic mine wastewater to the biochar filter residue, and desorbing to obtain Mg 2+ solution, wherein the amount of biochar added is 2% of the mass of the solution,

[0012] In step 2) of the above method, the iron source is at least one of ferrous chloride, ferrous sulfate, and ferrous nitrate;

[0013] The molar ratio of ferrous ions to phosphate in the iron source is 0.96-0.99:1.

[0014] The temperature of the reactor is 65-95°C, the reaction pressure is 1-5 atm, and the reaction time is 1-4 hours;

[0015] The calcination temperature is 400-700° C., and the holding time is 2-6 hours.

[0016] The present invention also provides a method for preparing a low-cost, high-density lithium iron phosphate material.

[0017] The present invention provides a method for preparing a low-cost, high-density lithium iron phosphate material, comprising the following steps:

[0018] 1) Add struvite to acidic mine wastewater to dissolve struvite MgNH4PO4 into H3PO4 and Mg 2+ and NH4 + , remove impurities, filter and desorb to obtain H3PO4, Mg 2+ solution;

[0019] 2) adding an iron source to the obtained H3PO4, adding the mixed solution to a reactor, reacting in the presence of oxygen, washing, drying, and calcining to obtain iron phosphate;

[0020] 3) Iron phosphate is mixed with lithium source, carbon source and Mg prepared in step 1) 2+ The solution is added to the acidic mine wastewater in a certain proportion, mixed to obtain a mixture, sand-milled to obtain a first slurry, and a portion of the first slurry is removed in a certain proportion for spray drying;

[0021] 4) adding a certain amount of carbon source to the remaining first slurry, mixing uniformly to obtain a second slurry, and spray drying;

[0022] 5) The spray-dried materials of the first slurry and the second slurry are uniformly mixed, sintered, and crushed to obtain a high-density lithium iron phosphate material.

[0023] In step 1) of the above method, the pH of the acid mine wastewater is 2-6, specifically 2;

[0024] The impurity removal method comprises adding one of biochar, montmorillonite, hydrotalcite, fungi or bacteria to the solution for impurity removal, with the added amount being 1-5% of the mass of the solution;

[0025] The desorption liquid is acid mine wastewater;

[0026] In one embodiment of the present invention, the impurity removal, filtration, and desorption operations are as follows: adding biochar to the solution, stirring, filtering, obtaining a filtrate containing H3PO4, adding acidic mine wastewater to the biochar filter residue, and desorbing to obtain Mg 2+ solution, wherein the amount of biochar added is 2% of the mass of the solution,

[0027] In another embodiment of the present invention, the operations of impurity removal, filtration and desorption are as follows: adding montmorillonite to the solution, stirring, filtering to obtain a filtrate containing H3PO4, adding acidic mine wastewater to the biochar filter residue, desorbing to obtain Mg 2+ solution, wherein the amount of montmorillonite added is 2% of the mass of the solution,

[0028] In another embodiment of the present invention, the operations of impurity removal, filtration and desorption are as follows: adding hydrotalcite to the solution, stirring, filtering to obtain a filtrate containing H3PO4, adding acidic mine wastewater to the hydrotalcite filter residue, desorbing to obtain Mg 2+ solution, wherein the amount of hydrotalcite added is 2% of the mass of the solution,

[0029] In step 2) of the above method, the iron source is at least one of ferrous chloride, ferrous sulfate, and ferrous nitrate;

[0030] The molar ratio of ferrous ions to phosphate in the iron source is 0.96-0.99:1.

[0031] The temperature of the reactor is 65-95°C, the reaction pressure is 1-5 atm, and the reaction time is 1-4 hours;

[0032] The calcination temperature is 400-700°C and the holding time is 2-6h;

[0033] In step 3) of the above method, the molar ratio of Li to iron phosphate in the lithium source may be 1-1.2:1;

[0034] The carbon source is one or a mixture of glucose, sucrose, starch, and polyethylene glycol;

[0035] The carbon content is 0.8-1.5% of the final lithium iron phosphate product;

[0036] The amount of acid mine drainage added is controlled to have a solid content of 45%-50%;

[0037] Mg 2+ The content is 400-2000ppm;

[0038] The particle size of the first slurry is 0.3-0.6 μm;

[0039] In step 3), the portion of the first slurry removed accounts for 1 / 10-5 / 10 of the weight of the entire first slurry, specifically 1 / 10, 2 / 10, 3 / 10, 4 / 10, or 5 / 10;

[0040] In step 4), the carbon source is a mixture of one or more of glucose, sucrose, starch, cyclodextrin, citric acid, polyethylene glycol, polyvinyl alcohol, polypropylene glycol, polyethylene oxide, polystyrene, styrene-butadiene-styrene block copolymer, and carbon nanotubes, and the carbon content is controlled to be 1.5-3.5% of the final lithium iron phosphate product;

[0041] In steps 3) and 4), the spray drying conditions are: inlet temperature is 200-240°C, outlet temperature is 80-120°C;

[0042] In step 5), the sintering temperature is 730-790° C. and the holding time is 9-15 hours;

[0043] Grind to an average particle size of 0.8-3.0 μm.

[0044] The lithium iron phosphate material prepared by the above method also falls within the protection scope of the present invention.

[0045] The powder compaction density of the lithium iron phosphate material is ≥2.45g / cm 3 .

[0046] The use of the high-density lithium iron phosphate material as a positive electrode material for lithium-ion batteries or in the preparation of positive electrode materials for lithium-ion batteries also falls within the scope of protection of the present invention.

[0047] The present invention also provides a lithium ion battery, which contains the high-density lithium iron phosphate material.

[0048] Compared with the prior art, the beneficial effects of the present invention are embodied in:

[0049] The present invention utilizes struvite as a phosphorus source and a dopant source, which not only reduces production costs but also realizes the resource utilization of environmental pollutants; acidic mine wastewater also contains trace amounts of heteroatoms such as nitrogen and sulfur, which can be in-situ doped during the preparation of lithium iron phosphate, effectively improving the electrical properties of lithium iron phosphate.

[0050] The present invention effectively controls particle growth by carbon-coating the slurry in a classified manner. Specifically, with a relatively low carbon coating, lithium iron phosphate can grow to a large extent; whereas with a higher carbon content and better coating, lithium iron phosphate growth is significantly suppressed, effectively preventing small particles from agglomerating into larger particles during the subsequent sintering process. This successfully achieves particle size gradation and produces a high-density lithium iron phosphate material.

[0051] The present invention is simple and feasible to operate, effectively improves the electrical properties and compaction properties of lithium iron phosphate, and is easy to mass produce. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] Figure 1 SEM image of lithium iron phosphate prepared in Example 4 of the present invention, 20000×;

[0053] Figure 2 SEM image of lithium iron phosphate prepared in Example 4 of the present invention, 10000×;

[0054] Figure 3 This is a SEM image of lithium iron phosphate prepared in Example 4 of the present invention, 50000×. DETAILED DESCRIPTION

[0055] The present invention will be further described in detail below in conjunction with specific embodiments. The examples provided are only for illustrating the present invention and are not intended to limit the scope of the present invention. The examples provided below can serve as a guide for further improvements by those skilled in the art and are not intended to limit the present invention in any way.

[0056] Unless otherwise specified, the experimental methods in the following examples are conventional methods and were performed according to the techniques or conditions described in the literature in the field or according to the product instructions. The materials and reagents used in the following examples, unless otherwise specified, were all commercially available.

[0057] The acid mine wastewater used in the following examples comes from a mine acid reservoir in eastern Anhui Province, contains heteroatoms nitrogen (3-10 ppm), sulfur (10-15 ppm) and metal ions Mg (1000-2000 ppm), Mn (200-300 ppm), Al (600-800 ppm), etc., and has a pH of 2.

[0058] Example 1

[0059] S1: 10 g of struvite particles with a particle size D50 of 50-70 μm were soaked in 12000 mL of acidic mine wastewater with a pH of 2, stirred for 2 h, and after the particles were completely dissolved, 2% (based on the mass of the solution) of biochar was added and continued to stir, and the filtrate containing H3PO4 was obtained by filtration. The biochar residue was desorbed with 2000 mL of acidic mine wastewater and concentrated to obtain Mg 2+ solution;

[0060] S2: Add ferrous sulfate to the H3PO4 filtrate to control the iron-phosphorus molar ratio to 0.96:1, add the mixed solution to a reactor at 65°C, introduce oxygen into the reactor, react for 3 hours, wash, dry, and calcine at 500°C for 4 hours to obtain iron phosphate;

[0061] S3: Glucose, lithium carbonate, iron phosphate prepared in S2 and Mg in S1 were added to the acid mine wastewater with a pH of 2. 2+ solution, controlling the molar ratio of Li:FePO4 to be 1:1, the carbon content (calculated based on the carbon in glucose) to be 0.8% (the carbon content is 0.8% of the final lithium iron phosphate product), Mg 2+ The content is 400ppm, and after uniform mixing, it is sand-milled to obtain a first slurry of 0.35μm, and 1 / 10 of the slurry is removed for spray drying, wherein the feed temperature is 240℃ and the outlet temperature is 80℃;

[0062] S4: adding polyethylene glycol to the remaining first slurry again, controlling the carbon content to 1.5% (based on the final lithium iron phosphate product), uniformly mixing to obtain a second slurry, and spray drying, wherein the feed temperature is 240° C. and the outlet temperature is 80° C.;

[0063] S5: The spray-dried materials in S3 and S4 are uniformly mixed, sintered at 740° C. for 12 h, and crushed to control the particle size of the finished product to 1.5-3 μm.

[0064] Example 2

[0065] S1: 10 g of struvite particles with a particle size D50 of 50-70 μm were immersed in 12000 mL of acidic mine wastewater with a pH of 2, and stirred for 2 h. After the particles were completely dissolved, 2% (based on the mass of the solution) of montmorillonite was added and continued to stir. After filtering, H3PO4 solution was obtained. The montmorillonite was desorbed by 2000 mL of acidic mine wastewater and concentrated acid to obtain Mg 2+ solution;

[0066] S2: Add ferrous nitrate to H3PO4 solution, control the iron-phosphorus molar ratio to 0.965:1, add the mixed solution to a reactor, the reactor temperature is 70°C, oxygen is introduced into the reactor, react for 4 hours, wash, dry, and calcine at 550°C for 4 hours to obtain iron phosphate;

[0067] S3: Sucrose, lithium carbonate, iron phosphate prepared in S2 and Mg in S1 were added to the acidic mine wastewater with a pH of 2. 2+ solution, controlling the molar ratio of Li:FePO4 to 1.05:1, the carbon content to 0.9% (based on the final lithium iron phosphate product), Mg 2+ The content is 800ppm, and after uniform mixing, it is sand-milled to obtain a first slurry of 0.40μm, and 1 / 5 of the slurry is removed for spray drying, wherein the feed temperature is 240℃ and the outlet temperature is 90℃;

[0068] S4: adding polyvinyl alcohol to the first slurry again, controlling the carbon content to 1.8% (based on the final lithium iron phosphate product), uniformly mixing and spray drying, wherein the feed temperature is 240° C. and the outlet temperature is 90° C.;

[0069] S5: After the spray-dried materials of the first slurry and the second slurry are uniformly mixed, they are calcined at 750° C. and kept warm for 12 h, and then pulverized to control the particle size of the finished product to be 1.5-3 μm.

[0070] Example 3

[0071] S1: 10 g of struvite particles with a particle size D50 of 50-70 μm were immersed in 12000 mL of acidic mine wastewater with a pH of 2, stirred for 2 h, and after the particles were completely dissolved, 2% (based on the mass of the solution) of biochar was added and continued to stir. After filtering, H3PO4 solution was obtained. The biochar was desorbed and concentrated through 2000 mL of acidic mine wastewater to obtain Mg 2+ solution;

[0072] S2: Add ferrous sulfate to the H3PO4 solution to control the iron-phosphorus molar ratio to 0.97:1, add the mixed solution to a reactor at 75°C, introduce oxygen into the reactor, react for 2 hours, wash, dry, and calcine at 600°C for 4 hours to obtain iron phosphate;

[0073] S3: starch, lithium carbonate, iron phosphate and Mg in S1 were added to the acidic mine wastewater with pH 2 in sequence. 2+ solution, controlling the molar ratio of Li:FePO4 to be 1.1:1, the carbon content to be 1% (based on the final lithium iron phosphate product), Mg 2+ The content is 1200ppm, and after uniform mixing, it is sand-milled to obtain a first slurry of 0.45μm, and 3 / 10 of the slurry is removed for spray drying, wherein the feed temperature is 240℃ and the outlet temperature is 100℃;

[0074] S4: adding styrene-butadiene-styrene block copolymer to the first slurry again, controlling the carbon content to 2.1% (based on the final lithium iron phosphate product), uniformly mixing and spray drying, wherein the feed temperature is 240° C. and the outlet temperature is 100° C.;

[0075] S5: After the spray-dried materials of the first slurry and the second slurry are uniformly mixed, they are calcined at 760° C. and kept warm for 12 h, and then pulverized to control the particle size of the finished product to be 1.5-3 μm.

[0076] Example 4

[0077] S1: 10 g of struvite particles with a particle size D50 of 50-70 μm were immersed in 12000 mL of acidic mine wastewater with a pH of 2, and stirred for 2 h. After the particles were completely dissolved, 2% (based on the mass of the solution) of hydrotalcite was added and continued to stir. After filtration, H3PO4 solution was obtained. The hydrotalcite was desorbed and concentrated with 2000 mL of acidic mine wastewater to obtain Mg 2+ solution;

[0078] S2: Add ferrous sulfate to the H3PO4 solution, control the iron-phosphorus molar ratio to 0.98:1, add the mixed solution to a reactor, the reactor temperature is 85°C, oxygen is introduced into the reactor, react for 2 hours, wash, dry, and calcine at 650°C for 3 hours to obtain iron phosphate;

[0079] S3: Glucose, lithium carbonate, iron phosphate and Mg in S1 were added to the acid mine wastewater with a pH of 2. 2+ solution, controlling the molar ratio of Li:FePO4 to be 1.15:1, the carbon content to be 1.1% (based on the final lithium iron phosphate product), Mg 2+ The content is 1600ppm, and after uniform mixing, it is sand-milled to obtain a first slurry of 0.5μm, and 2 / 5 of the slurry is removed for spray drying, wherein the feed temperature is 240℃ and the outlet temperature is 110℃;

[0080] S4: Add starch and polyethylene glycol to the first slurry again, control the carbon content to 2.4% (based on the final lithium iron phosphate product), mix evenly and then spray dry, wherein the feed temperature is 240°C and the outlet temperature is 110°C.

[0081] S5: After the spray-dried materials of the first slurry and the second slurry are uniformly mixed, they are calcined at 770° C. and kept warm for 12 h, and then pulverized to control the particle size of the finished product to be 1.5-3 μm.

[0082] Example 5

[0083] S1: 10 g of struvite particles with a particle size D50 of 50-70 μm were immersed in 12000 mL of acidic mine wastewater with a pH of 2, stirred for 2 h, and after all the particles were dissolved, 2% hydrotalcite (based on the mass of the solution) was added and continued to stir. After filtering, H3PO4 solution was obtained. The hydrotalcite was desorbed and concentrated in 2000 mL of acidic mine wastewater to obtain Mg 2+ solution;

[0084] S2: Add ferrous sulfate to the H3PO4 solution, control the iron-phosphorus molar ratio to 0.99:1, and keep the reactor temperature at 95°C. Introduce oxygen into the reactor, react for 2 hours, wash, dry, and calcine at 700°C for 3 hours to obtain ferric phosphate.

[0085] S3: starch, lithium carbonate, iron phosphate and Mg in S1 were added to the acidic mine wastewater with pH 2 in sequence. 2+ solution, controlling the molar ratio of Li:FePO4 to be 1.2:1, the carbon content to be 1.5% (based on the final lithium iron phosphate product), Mg 2+ The content is 2000ppm, and after uniform mixing, it is sand-milled to obtain a first slurry of 0.6μm, and 1 / 2 of the slurry is removed for spray drying, wherein the feed temperature is 240℃ and the outlet temperature is 120℃;

[0086] S4: polyethylene glycol and styrene-butadiene-styrene block copolymer are added to the first slurry again, the carbon content is controlled to be 2.8% (based on the final lithium iron phosphate product), and the mixture is evenly mixed and spray-dried, wherein the feed temperature is 240° C. and the outlet temperature is 120° C.;

[0087] S5: After the spray-dried materials of the first slurry and the second slurry are uniformly mixed, they are calcined at 780° C. and kept warm for 12 h, and then pulverized to control the particle size of the finished product to be 1.5-3 μm.

[0088] Comparative Example 1

[0089] Lithium iron phosphate (purchased from Dangsheng Technology) was used as the positive electrode material, and the lithium iron phosphate positive electrode sheet was prepared with a mass ratio of positive electrode material: conductive agent SP: polyvinylidene fluoride = 8:1:1. The battery was assembled into a CR2016 button cell and tested at 25°C with test rates of 0.2C and 1C.

[0090] Comparative Example 2

[0091] S1: Add ferrous sulfate to a solution containing H3PO4, control the iron-phosphorus molar ratio to 0.96:1, add the mixed solution to a reactor, the reactor temperature is 65°C, oxygen is introduced into the reactor, react for 3 hours, wash, dry, and calcine at 500°C for 4 hours to obtain iron phosphate;

[0092] S2: Glucose, lithium carbonate, iron phosphate prepared in S1 and MgO were added to acidic mine wastewater with a pH of 2 in sequence, and the molar ratio of Li:FePO4 was controlled to be 1:1, the carbon content (calculated as carbon in glucose) was 0.8% (calculated as the final lithium iron phosphate product), and the Mg content was 0.8%. 2+ The content is 400ppm, and after uniform mixing, it is sand-milled to obtain a first slurry of 0.35μm, and 1 / 10 of the slurry is removed for spray drying, wherein the feed temperature is 240℃ and the outlet temperature is 80℃;

[0093] S3: adding polyethylene glycol to the remaining first slurry again, controlling the carbon content to 1.5% (based on the final lithium iron phosphate product), uniformly mixing to obtain a second slurry, and spray drying, wherein the feed temperature is 240° C. and the outlet temperature is 80° C.;

[0094] S4: After the spray-dried materials in S2 and S3 are uniformly mixed, they are sintered at 740°C and kept warm for 12 hours, and crushed to control the particle size of the finished product to 1.5-3 μm.

[0095] Comparative Example 3

[0096] S1: 10 g of struvite particles with a particle size D50 of 50-70 μm were soaked in 12000 mL of acidic mine wastewater with a pH of 2, stirred for 2 h, and after the particles were completely dissolved, 2% (based on the mass of the solution) of biochar was added and continued to stir, and the filtrate containing H3PO4 was obtained by filtration. The biochar residue was desorbed with 2000 mL of acidic mine wastewater and concentrated to obtain Mg 2+ solution;

[0097] S2: Add ferrous sulfate to the H3PO4 filtrate to control the iron-phosphorus molar ratio to 0.96:1, add the mixed solution to a reactor at 65°C, introduce oxygen into the reactor, react for 3 hours, wash, dry, and calcine at 500°C for 4 hours to obtain iron phosphate;

[0098] S3: Glucose, lithium carbonate, iron phosphate prepared in S2 and Mg in S1 were added to the acidic mine wastewater with a pH of 2. 2+ solution, controlling the molar ratio of Li:FePO4 to be 1:1, the carbon content (calculated as carbon in glucose) to be 1.5% (calculated as the final lithium iron phosphate product), Mg 2+ The content is 400ppm (based on the final lithium iron phosphate product), and after uniform mixing, it is sand-milled to obtain a 0.35μm slurry, which is spray-dried, wherein the feed temperature is 240℃ and the outlet temperature is 80℃;

[0099] S4: The spray-dried material in S3 is sintered at 740° C. for 12 h, and crushed to control the particle size of the finished product to be 1.5-3 μm.

[0100] Comparative Example 4

[0101] S1: 10 g of struvite particles with a particle size D50 of 50-70 μm were immersed in 12000 mL of acidic mine wastewater with a pH of 2, and stirred for 2 h. After the particles were completely dissolved, 2% (based on the mass of the solution) of biochar was added and continued to stir. The filtrate containing H3PO4 was filtered to obtain the filtrate. The biochar residue was desorbed with 2000 mL of acidic mine wastewater and concentrated to obtain Mg 2+ solution;

[0102] S2: Add ferrous sulfate to the H3PO4 filtrate to control the iron-phosphorus molar ratio to 0.96:1, add the mixed solution to a reactor at 65°C, introduce oxygen into the reactor, react for 3 hours, wash, dry, and calcine at 500°C for 4 hours to obtain iron phosphate;

[0103] S3: Glucose, lithium carbonate, iron phosphate prepared in S2 and Mg in S1 are added to the water in sequence. 2+ solution, controlling the molar ratio of Li:FePO4 to be 1:1, the carbon content (calculated as carbon in glucose) to be 0.8% (calculated as the final lithium iron phosphate product), Mg 2 + The content is 400ppm, and after uniform mixing, it is sand-milled to obtain a first slurry of 0.35μm, and 1 / 10 of the slurry is removed for spray drying, wherein the feed temperature is 240℃ and the outlet temperature is 80℃;

[0104] S4: adding polyethylene glycol to the remaining first slurry again, controlling the carbon content to 1.5% (based on the final lithium iron phosphate product), uniformly mixing to obtain a second slurry, and spray drying, wherein the feed temperature is 240° C. and the outlet temperature is 80° C.;

[0105] S5: The spray-dried materials in S3 and S4 are uniformly mixed, sintered at 740° C. for 12 h, and crushed to control the particle size of the finished product to 1.5-3 μm.

[0106] The lithium iron phosphate materials prepared in Examples 1-5 and Comparative Examples 1-4 were used as positive electrode materials, and lithium iron phosphate positive electrode sheets were prepared with a mass ratio of positive electrode material: conductive agent SP: polyvinylidene fluoride = 8:1:1. The cells were assembled into CR2016 button batteries and tested at 25°C with test rates of 0.2C and 1C. The test results are shown in Table 1 below:

[0107] Table 1

[0108]

[0109]

[0110] From the data in Table 1, it can be seen that the lithium iron phosphate prepared by using struvite as the phosphorus source and dopant source has the same electrical performance as the existing mature products in the market, with a 1C gram capacity greater than 141mAh / g, and the powder compaction density reaches 2.60g / cm 3 It has a good application prospect.

[0111] The present invention has been described in detail above. For those skilled in the art, without departing from the purpose and scope of the present invention, and without the need to carry out unnecessary experimental conditions, the present invention can be implemented in a wide range under equivalent parameters, concentrations and conditions. Although the present invention provides specific embodiments, it should be understood that further improvements can be made to the present invention. In short, according to the principles of the present invention, this application is intended to include any changes, uses or improvements to the present invention, including changes that depart from the disclosed scope in this application and are made using conventional techniques known in the art.

Claims

1. A method for preparing a lithium iron phosphate material, comprising the following steps: 1) Add struvite to acidic mine wastewater to dissolve struvite MgNH4PO4 into H3PO4 and Mg 2+ and NH4 + , remove impurities, filter, desorb and concentrate to obtain H3PO4, Mg 2+ solution; 2) adding an iron source to the obtained H3PO4, adding the mixed solution to a reactor, reacting in the presence of oxygen, washing, drying, and calcining to obtain iron phosphate; 3) Iron phosphate is mixed with lithium source, carbon source and Mg prepared in step 1) 2+ The solution is added to the acidic mine wastewater in a certain proportion, mixed to obtain a mixture, sand-milled to obtain a first slurry, and a portion of the first slurry is removed in a certain proportion for spray drying; 4) adding a certain amount of carbon source to the remaining first slurry, mixing uniformly to obtain a second slurry, and spray drying; 5) The spray-dried materials of the first slurry and the second slurry are uniformly mixed, sintered, and crushed to obtain a high-density lithium iron phosphate material.

2. The method according to claim 1, wherein: In step 1), the pH of the acid mine wastewater is 2-6; The impurity removal method comprises adding one of biochar, montmorillonite, hydrotalcite, fungi or bacteria to the solution for impurity removal, with the added amount being 1-5% of the mass of the solution; The desorption liquid is acidic mine wastewater.

3. The method according to claim 1, wherein: In step 2), the iron source is at least one of ferrous chloride, ferrous sulfate, and ferrous nitrate; The molar ratio of ferrous ion to phosphate in the iron source is 0.96-0.99:1; The temperature of the reactor is 65-95°C, the reaction pressure is 1-5 atm, and the reaction time is 1-4 hours; The calcination temperature is 400-700° C., and the holding time is 2-6 hours.

4. The method according to claim 1, wherein: In step 3), the molar ratio of Li to iron phosphate in the lithium source is 1-1.2:1; The carbon source is one or a mixture of glucose, sucrose, starch, and polyethylene glycol; The carbon content is 0.8-1.5% of the final lithium iron phosphate product; The amount of acid mine drainage added is controlled to have a solid content of 45%-50%; Mg 2+ The content is 400-2000ppm; The particle size of the first slurry is 0.3-0.6 μm; The removed portion of the first slurry accounts for 1 / 10 to 5 / 10 of the weight of the entire first slurry.

5. The method according to claim 1, wherein: In step 4), the carbon source is a mixture of one or more of glucose, sucrose, starch, cyclodextrin, citric acid, polyethylene glycol, polyvinyl alcohol, polyglycerol, polyethylene oxide, polystyrene, styrene-butadiene-styrene block copolymer, and carbon nanotubes; The carbon content is 1.5-3.5% of the final lithium iron phosphate product.

6. The method according to claim 1, wherein: In steps 3) and 4), the spray drying conditions are: inlet temperature is 200-240°C, outlet temperature is 80-120°C; In step 5), the sintering temperature is 730-790° C. and the holding time is 9-15 hours; Grind to an average particle size of 0.8-3.0 μm.

7. The lithium iron phosphate material prepared by the method according to any one of claims 1 to 6.

8. The lithium iron phosphate material according to claim 7, characterized in that: The powder compaction density of the lithium iron phosphate material is ≥2.45g / cm 3 .

9. Use of the lithium iron phosphate material according to claim 7 or 8 as a positive electrode material for lithium ion batteries or in the preparation of a positive electrode material for lithium ion batteries.

10. A lithium ion battery comprising the lithium iron phosphate material according to claim 7 or 8.

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