Preparation method of spherical high-compaction lithium iron phosphate

Abstract

The invention discloses a preparation method of a spherical high-compaction lithium iron phosphate material. The preparation method comprises the following steps: 1) mixing iron powder with phosphoric acid to obtain a ferrous dihydrogen phosphate solution; 2) adding an oxidizing agent containing air, filtering and washing to obtain iron phosphate dihydrate slurry, and further drying to obtain an iron phosphate dihydrate solid; 3) adding a lithium source, a carbon source and iron phosphate dihydrate raw materials, mixing and sintering; and (3) mixing the sintered powder and a carbon source, and performing secondary sintering. Iron phosphate dehydrate is adopted as a raw material, and the spherical lithium iron phosphate material is prepared from the cluster-shaped iron phosphate precursor, so that the cost of the high-compaction lithium iron phosphate material is reduced.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of lithium ion battery cathode material lithium iron phosphate, and particularly relates to a preparation method of spherical lithium iron phosphate material. BACKGROUND

[0002] Lithium iron phosphate (LiFePO4, LFP) is one of the lithium ion battery cathode materials, and is considered as an ideal cathode material applicable to electric vehicles due to its stable olivine structure, good reliability of lithium extraction and good thermal stability. However, the low compaction density of lithium iron phosphate affects the overall energy density of the battery. With the adjustment of the national new energy vehicle subsidy policy, the battery manufacturers have higher and higher requirements for the cost control and lithium iron phosphate material, especially the compaction density of lithium iron phosphate. In order to pursue higher energy density, the material manufacturers must further improve the compaction density of the material to meet the frequent use of electric vehicles. There are also certain requirements for the discharge capacity of lithium iron phosphate material. Therefore, high-capacity and high-compaction lithium iron phosphate material will be the development trend in the future.

[0003] There are mainly three kinds of production processes for industrialized lithium iron phosphate in China, including high-temperature solid-phase method, carbon thermal reduction method and sol-gel method. The high-temperature solid-phase method mainly produces lithium iron phosphate by using ferrous oxalate as raw material; the carbon thermal reduction method mainly produces lithium iron phosphate by using iron oxide and iron phosphate as raw material; and the sol-gel method mainly produces lithium iron phosphate by using iron nitrate as raw material. The lithium iron phosphate process has the characteristics of simple process flow, high product compaction density and high specific capacity, and is the mainstream process route of lithium iron phosphate.

[0004] In the prior art, patent CN201810611876.8 discloses a preparation method of high-compaction lithium iron phosphate. The sintering temperature of lithium iron phosphate in the preparation process is 770-780℃, and it needs to be sintered twice. High-temperature sintering is easy to cause the magnetic substance to exceed the standard, and the 0.1C discharge capacity is only 156mAh / g, which is low. The preparation method has the disadvantages of complex preparation steps, high energy consumption and low capacity.

[0005] In the prior art, the high-temperature sintering is generally used to make the primary particles grow to achieve the purpose of high compaction. However, when the sintering temperature exceeds 765℃, the risk of exceeding the standard of the magnetic substance is caused, resulting in unqualified products. The high sintering temperature increases the energy consumption of industrialization, and also makes the primary particles excessively large, resulting in low discharge capacity of the material and failing to achieve the purpose of high compaction and high capacity. When the sintering temperature is too low, the fusion and growth of the primary particles cannot be promoted, and the effect of high compaction cannot be achieved. Therefore, a simple high-compaction lithium iron phosphate preparation method is needed to solve the problems of complex preparation process and poor performance in the prior art. SUMMARY

[0006] The main objective of this invention is to provide a method for preparing spherical high-compact lithium iron phosphate, which has high compaction density, high discharge capacity, simple process, and is easy to industrialize, thereby solving the problems of low compaction, complex process, and high cost in the existing technology.

[0007] To achieve the above-mentioned objectives, the technical solution of the present invention is as follows:

[0008] A method for preparing spherical high-pressure lithium iron phosphate includes the following steps:

[0009] (1) Add iron powder to dilute phosphoric acid and dissolve to obtain ferrous dihydrogen phosphate solution;

[0010] (2) Add the above-mentioned ferrous dihydrogen phosphate solution and air oxidant to a stirred reactor. After the reaction is completed, a slurry is obtained. After filtration, washing and drying, ferric phosphate dihydrate raw material is obtained.

[0011] (3) After mixing lithium salt, first carbon source and iron phosphate dihydrate raw material, the first sintering process is carried out to obtain the first spherical lithium iron phosphate precursor;

[0012] (4) After mixing lithium salt, second carbon source and iron phosphate dihydrate raw material, the second spherical lithium iron phosphate precursor is obtained by sand milling and spray drying.

[0013] (5) After mixing the first spherical lithium iron phosphate precursor, the second spherical lithium iron phosphate precursor and the third carbon source, the spherical lithium iron phosphate finished product is obtained by the second sintering process.

[0014] In the present invention, in step (1), the mass concentration of the dilute phosphoric acid is 20% to 30 wt%, and the molar ratio of the phosphoric acid to the iron powder in step (1) is 2 to 3:1, based on pure phosphoric acid.

[0015] In step (1), preferably, the temperature of the dissolution process is 60-70℃, and preferably, the dissolved ferrous dihydrogen phosphate solution is subjected to solid-liquid separation by a filter press.

[0016] In step (2), the flow rate of the air oxidant is 150-350 mL / min / (1g iron powder), for example 180 mL / min / g, 200 mL / min / g, 300 mL / min / g, 330 mL / min / g, the stirring speed in the reactor is 100-700 rpm, the reaction temperature is 50-90℃, the reaction time is 2-10 h, and the air oxidant is added by means of induction.

[0017] In step (2), the slurry is filtered using a filter press, the filter cake is washed with deionized water until the conductivity of the wash water is ≤200μS / cm, and the filter cake is dried using a rotary flash desiccant with a moisture content of ≤0.5%.

[0018] In step (3), the temperature in the first sintering conditions is 600-800℃, the atmosphere is nitrogen, the sintering time is 1-10h, and the heating rate is 1-10℃ / min. Preferably, the temperature in the first sintering conditions is 700-800℃, the sintering time is 1-4h, and the heating rate is 2-5℃ / min.

[0019] In steps (3) to (5), the first / second / third carbon source is one or more of glucose, PEG and citric acid, and the lithium salt is one of lithium hydroxide and lithium carbonate. Preferably, the lithium salt is lithium carbonate.

[0020] In step (3), the preferred mass of the first carbon source is 3% to 5% of the sum of the masses of lithium salt and ferric phosphate dihydrate, and the mass ratio of lithium salt to ferric phosphate dihydrate is 1:3 to 5.

[0021] In step (4), the sand milling step is carried out by using a horizontal sand mill and a vertical sand mill to control the final sand mill slurry D50 to be 0.3μm~0.5μm;

[0022] In step (4), the feed temperature of the spray is 150–220°C, and the discharge temperature of the spray is 90–110°C. Preferably, the mass of the second carbon source is 4%–10% of the sum of the masses of lithium salt and ferric phosphate dihydrate. The mass ratio of lithium salt to ferric phosphate dihydrate is 1:3–5.

[0023] In step (5), the mass ratio of the first spherical lithium iron phosphate precursor to the second lithium iron phosphate precursor is 1:0.7 to 1.5. Preferably, the mass ratio of the third carbon source to the sum of the masses of the first and second spherical lithium iron phosphate precursors is 0.03 to 0.06:1.

[0024] In step (5), the second sintering temperature is 600-800℃, the atmosphere is nitrogen, the sintering time is 1-10h, and the heating rate is 1-10℃ / min. Preferably, the second sintering conditions are a temperature of 720-800℃, a sintering time of 1-4h, and a heating rate of 2-5℃ / min.

[0025] The beneficial effects of this invention are as follows:

[0026] (1) Using iron-phosphate clusters of dihydrate iron phosphate as raw material, spherical lithium iron phosphate is produced in a one-step process. This eliminates the sintering process of dihydrate iron phosphate in the iron phosphate lithium iron phosphate production process, reducing the cost by approximately RMB 800 per ton of lithium iron phosphate compared to the traditional iron phosphate production line. This achieves low energy consumption, low cost, and clean production of lithium iron phosphate, and has good prospects for promotion. In the first sintering process, dihydrate iron phosphate dehydrates to form clusters of anhydrous iron phosphate. Simultaneously, the lithium source decomposes and is uniformly adsorbed onto the surface of the anhydrous iron phosphate, forming a spherical lithium iron phosphate precursor with high phase purity, which is more suitable as a raw material for high-density lithium iron phosphate.

[0027] (2) Using iron-based clustered dihydrate iron phosphate as raw material, spherical lithium iron phosphate can be produced in one step, which improves the sphericity of the particles and increases the compaction density of lithium iron phosphate products. Attached Figure Description

[0028] Figure 1 This is a SEM image of the clustered ferric phosphate dihydrate synthesized in this invention;

[0029] Figure 2 SEM image of the synthesized iron phosphate dihydrate for Comparative Example 1. Detailed Implementation

[0030] To facilitate understanding of the present invention, the following description, in conjunction with embodiments, will further illustrate the invention. It should be understood that the following embodiments are merely for a better understanding of the invention and do not imply that the invention is limited to these embodiments.

[0031] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The term "and / or" may be used herein to include any and all combinations of one or more of the associated listed items.

[0032] Main raw material sources

[0033] Lithium carbonate, battery grade, GEM Co., Ltd.

[0034] Iron powder, Hangzhou Yitong New Materials Co., Ltd.;

[0035] Glucose, Luzhou Biotechnology Co., Ltd.;

[0036] Polyethylene glycol 4000, Liaoning Aoke Chemical Co., Ltd.;

[0037] Main testing methods

[0038] SEM (Semiconductor Electron Microscopy): In this invention, the Sirion 200 (20kV) and INSPECT F50 from FEI Corporation of the United States are used to observe the microstructure of the sample through high-energy electron beam imaging technology.

[0039] The fabrication of lithium-ion batteries:

[0040] In this embodiment, lithium iron phosphate, PVDF, and a conductive agent were mixed uniformly at a mass ratio of 96:2:2 to prepare a positive electrode slurry. Using aluminum foil as a current collector, the slurry was coated onto the aluminum foil to a thickness of 20 μm, and then vacuum dried at 120°C for 12 hours. After thorough drying, the electrode sheet was cut into small pieces with a diameter of 16 mm, thus completing the positive electrode sheet fabrication.

[0041] The battery was assembled in a glove box filled with high-purity argon gas (water and oxygen <0.1ppm). 1M LiPF6 and a 1 / 1 / 1 volume ratio of ethyl carbonate (EC) / dimethyl carbonate (DMC) / diethyl carbonate (DEC) were used as the electrolyte. The battery casing was CR2025, with lithium metal sheets and Celgard 2400 microporous membranes serving as the anode and separator, respectively. After drying at 105℃, the positive electrode was obtained by roll pressing. The prepared positive electrode was then assembled with lithium sheets, the separator, and the electrolyte to form a CR2025 coin cell.

[0042] Example 1

[0043] (1) 85wt% industrial phosphoric acid was diluted to 20wt% with deionized water. 5500g of 20wt% dilute phosphoric acid was added to a 20L jacketed reactor and the temperature of the oil bath in the jacket of the 20L reactor was controlled at 70℃. 300g of iron powder was slowly added to the dilute phosphoric acid and dissolved under stirring to generate ferrous dihydrogen phosphate solution. The hydrogen gas generated during the dissolution process was extracted by a blower and discharged at high altitude. The ferrous dihydrogen phosphate solution was separated into solid and liquid by a filter press to obtain a pure ferrous dihydrogen phosphate solution.

[0044] (2) 1130g of purified ferrous solution was further transferred into a 20L jacketed reactor by a pump. The temperature of the oil bath in the jacket of the 20L reactor was controlled at 60℃. Compressed air (flow rate 333mL / min / g) was continuously introduced for oxidation. The stirring speed was 100r / min. After holding the temperature for 2h, ferric phosphate dihydrate crystal slurry was obtained. The ferric phosphate dihydrate slurry was filtered through a plate and frame filter press and the filter cake was dried by a rotary flash dryer. After flash drying, ferric phosphate dihydrate solid was obtained.

[0045] (3) Take 412.87g of solid iron phosphate dihydrate, add 84g of lithium carbonate, and then add 15g of glucose and polyethylene glycol 4000. The mass ratio of glucose to polyethylene glycol 4000 is 1:2 (the total carbon content accounts for 3.02% of the mass of iron phosphate and lithium carbonate). After mixing evenly, sinter at 790℃ for 10h in a box furnace under a nitrogen atmosphere at a heating rate of 2℃ / min to obtain the first spherical lithium iron phosphate precursor.

[0046] (4) Take 897g of ferric phosphate dihydrate slurry (solid content 30%), add 129.5g of lithium carbonate, and then add 47g of glucose and polyethylene glycol 4000. The mass ratio of glucose to polyethylene glycol 4000 is 1:3 (the total carbon content accounts for 8.04% of the mass of ferric phosphate and lithium carbonate). After mixing evenly, grind the slurry to D50 = 0.3μm in a horizontal sand mill. Spray dry the sand-milled slurry. The feed temperature of the spray is 180℃ and the discharge temperature of the spray is 95℃.

[0047] (5) 600g of the first spherical lithium iron phosphate precursor, 400g of the second spherical lithium iron phosphate precursor, and 60g of the third carbon source glucose were mixed and sintered in a box furnace to obtain spherical lithium iron phosphate finished product. The sintering temperature was 750℃, the sintering time was 4h, and the heating rate was 2℃ / min. The sintered product was crushed by an airflow pulverizer. The crushed product was then subjected to subsequent screening, demagnetization, and packaging steps to obtain the final product lithium iron phosphate. The product quality is shown in Table 1.

[0048] Example 2

[0049] (1) 85wt% industrial phosphoric acid was diluted to 30wt% with deionized water. 8000g of 30wt% dilute phosphoric acid was added to a 20L jacketed reactor and the oil bath temperature of the 20L reactor jacket was controlled at 60℃. 600g of iron powder was slowly added to the dilute phosphoric acid and dissolved under stirring to generate ferrous dihydrogen phosphate solution. The hydrogen gas generated during the dissolution process was extracted by a blower and discharged at high altitude. The ferrous dihydrogen phosphate solution was separated into solid and liquid by a filter press to obtain a pure ferrous dihydrogen phosphate solution.

[0050] (2) 1360g of purified ferrous solution was further transferred into a 20L jacketed reactor by a pump. The temperature of the oil bath in the jacket of the 20L reactor was controlled at 70℃. Compressed air (flow rate 300mL / min / g) was continuously introduced for oxidation. The stirring speed was 450r / min. After holding the temperature for 5h, ferric phosphate dihydrate crystal slurry with specific surface area and particle size meeting the requirements was obtained. The ferric phosphate dihydrate slurry was filtered by a plate and frame filter press and flash dried to obtain ferric phosphate dihydrate solid.

[0051] (3) Take 322.9g of solid iron phosphate dihydrate according to the product formula requirements, add 101.07g of lithium carbonate, and then add 15.6g of glucose. The mass ratio of glucose to polyethylene glycol 4000 is 1:1 (the total carbon content accounts for 3.68% of the mass of iron phosphate and lithium carbonate). After mixing evenly, sinter at 750℃ for 4h in a box furnace under nitrogen atmosphere at a heating rate of 3℃ / min to obtain the first spherical lithium iron phosphate precursor.

[0052] (4) Take 897g of ferric phosphate dihydrate slurry (solid content 30%) according to the product formula requirements, add 84g of lithium carbonate, and then add 17.06g of polyethylene glycol 4000 (the total carbon content accounts for 4.83% of the mass of ferric phosphate and lithium carbonate). After mixing evenly, grind the slurry to D50 = 0.4μm in a sand mill. Spray dry the sand-milled slurry. The feed temperature of the spray is 150℃ and the discharge temperature of the spray is 100℃.

[0053] (5) 290g of the first spherical lithium iron phosphate precursor, 225g of the second spherical lithium iron phosphate precursor, and 19g of the third carbon source glucose were mixed and sintered in a box furnace to obtain spherical lithium iron phosphate finished product. The sintering temperature was 800℃, the sintering time was 2h, and the heating rate was 3℃ / min. The sintered product was crushed by a crushing device, and the crushed product was further screened, demagnetized, and packaged to obtain the final product lithium iron phosphate. The product quality is shown in Table 1.

[0054] Example 3

[0055] (1) 85wt% industrial phosphoric acid was diluted to 20wt% with deionized water. 15000g of 20wt% dilute phosphoric acid was added to a 20L jacketed reactor and the oil bath temperature of the 20L reactor jacket was controlled at 65℃. 600g of iron powder was slowly added to the dilute phosphoric acid and dissolved under stirring to generate ferrous dihydrogen phosphate solution. The hydrogen gas generated during the dissolution process was extracted by a blower and discharged at high altitude. The ferrous dihydrogen phosphate solution was separated into solid and liquid by a filter press to obtain a pure ferrous dihydrogen phosphate solution.

[0056] (2) 906g of purified ferrous solution was further transferred into a 20L jacketed reactor by a pump. The temperature of the oil bath in the jacket of the 20L reactor was controlled at 90℃. Compressed air (flow rate 200mL / min / g) was continuously introduced for oxidation. The stirring speed was 320r / min. After holding the temperature for 10h, ferric phosphate dihydrate crystal slurry with specific surface area and particle size meeting the requirements was obtained. The ferric phosphate dihydrate slurry was filtered by a plate and frame filter press and flash dried to obtain ferric phosphate dihydrate solid.

[0057] (3) Take 503.3g of solid iron phosphate dihydrate according to the product formula requirements, add 129.5g of lithium carbonate, and then add 29g of glucose and polyethylene glycol 4000. The mass ratio of glucose to polyethylene glycol 4000 is 1:2 (the total carbon content accounts for 4.58% of the mass of iron phosphate and lithium carbonate). After mixing evenly, sinter at 780℃ for 6h in a box furnace under nitrogen atmosphere at a heating rate of 3℃ / min to obtain the first spherical lithium iron phosphate precursor.

[0058] (4) Take 1400g of ferric phosphate dihydrate slurry (solid content 30%) according to the product formula requirements, add 101.67g of lithium carbonate, and then add 40g of glucose and polyethylene glycol 4000. The mass ratio of glucose to polyethylene glycol 4000 is 1:3 (the total carbon content accounts for 7.67% of the mass of ferric phosphate and lithium carbonate). After mixing evenly, grind the slurry to D50 = 0.38μm in a sand mill. Spray dry the sand-milled slurry. The feed temperature of the spray is 200℃ and the discharge temperature of the spray is 105℃.

[0059] (5) 200g of the first spherical lithium iron phosphate precursor, 300g of the second spherical lithium iron phosphate precursor, and 22.5g of the third carbon source glucose were mixed and sintered in a box furnace to obtain spherical lithium iron phosphate finished product. The sintering temperature was 740℃, the sintering time was 4h, and the heating rate was 2℃ / min. The sintered product was crushed by a crushing device, and the crushed product was further screened, demagnetized, and packaged to obtain the final product lithium iron phosphate. The product quality is shown in Table 1.

[0060] Example 4

[0061] (1) 85wt% industrial phosphoric acid was diluted to 25wt% with deionized water. 13334.4g of 25wt% dilute phosphoric acid was added to a 20L jacketed reactor and the oil bath temperature of the 20L reactor jacket was controlled at 65℃. 800g of iron powder was slowly added to the dilute phosphoric acid and dissolved under stirring to generate ferrous dihydrogen phosphate solution. The hydrogen gas generated during the dissolution process was extracted by a blower and discharged at high altitude. The ferrous dihydrogen phosphate solution was separated into solid and liquid by a filter press to obtain a pure ferrous dihydrogen phosphate solution.

[0062] (2) 1208g of purified ferrous solution was further transferred into a 20L jacketed reactor by a pump. The temperature of the oil bath in the jacket of the 20L reactor was controlled at 80℃. Compressed air (flow rate 212.5mL / min / g) was continuously introduced for oxidation. The stirring speed was 700r / min. After holding at the temperature for 4h, ferric phosphate dihydrate crystal slurry was obtained. The ferric phosphate dihydrate slurry was filtered through a plate and frame filter press and the filter cake was dried by a rotary flash dryer. After flash drying, ferric phosphate dihydrate solid was obtained.

[0063] (3) Take 366.67g of solid iron phosphate dihydrate according to the product formula requirements, add 82.77g of lithium carbonate, and then add 18.67g of glucose (the total carbon content accounts for 4.15% of the mass of iron phosphate and lithium carbonate). After mixing evenly, sinter at 700℃ for 10h in a box furnace under nitrogen atmosphere at a heating rate of 5℃ / min to obtain the first spherical lithium iron phosphate precursor.

[0064] (4) Take 1033.3g of ferric phosphate dihydrate slurry (solid content 30%) according to the product formula requirements, add 67.33g of lithium carbonate, and then add 37.33g of glucose and polyethylene glycol 4000. The mass ratio of glucose to polyethylene glycol 4000 is 1:2 (the total carbon content accounts for 9.89% of the mass of ferric phosphate and lithium carbonate). After mixing evenly, grind the slurry to D50 = 0.42μm in a sand mill. Spray dry the sand-milled slurry. The feed temperature of the spray is 210℃ and the discharge temperature of the spray is 98℃.

[0065] (5) Mix 300g of the first spherical lithium iron phosphate precursor, 400g of the second spherical lithium iron phosphate precursor, and 25g of the third carbon source glucose, and sinter them in a box furnace to obtain spherical lithium iron phosphate finished product. The sintering temperature is 720℃, the sintering time is 4h, and the heating rate is 5℃ / min. The sintered product is then crushed by a crushing device. The crushed product is then subjected to subsequent screening, demagnetization, and packaging steps to obtain the final product lithium iron phosphate. The product quality is shown in Table 1.

[0066] Example 5

[0067] (1) 85wt% industrial phosphoric acid was diluted to 20wt% with deionized water. 12000g of 20wt% dilute phosphoric acid was added to a 20L jacketed reactor and the oil bath temperature of the 20L reactor jacket was controlled at 65℃. 600g of iron powder was slowly added to the dilute phosphoric acid and dissolved under stirring to generate ferrous dihydrogen phosphate solution. The hydrogen gas generated during the dissolution process was extracted by a blower and discharged at high altitude. The ferrous dihydrogen phosphate solution was separated into solid and liquid by a filter press to obtain a pure ferrous dihydrogen phosphate solution.

[0068] (2) 906.4g of purified ferrous solution was further transferred into a 20L jacketed reactor by pump. The oil bath temperature of the 20L reactor jacket was controlled at 90℃. Compressed air (flow rate 183.3mL / min / g) was continuously introduced for oxidation. The stirring speed was 200r / min. After holding at the temperature for 2h, ferric phosphate dihydrate crystal slurry was obtained. The ferric phosphate dihydrate slurry was filtered through a plate and frame filter press and the filter cake was dried by a rotary flash dryer. After flash drying, ferric phosphate dihydrate solid was obtained.

[0069] (3) Take 329.57g of solid iron phosphate dihydrate according to the product formula requirements, add 83.87g of lithium carbonate, and then add 14.67g of glucose (containing a total carbon content of 3.55% of the mass of iron phosphate and lithium carbonate). After mixing evenly, sinter at 750℃ for 4h in a box furnace under a nitrogen atmosphere at a heating rate of 3℃ / min to obtain the first spherical lithium iron phosphate precursor.

[0070] (4) Take 873.3g of ferric phosphate dihydrate slurry (solid content 30%) according to the product formula requirements, add 54.3g of lithium carbonate, and then add 21.3g of glucose and polyethylene glycol 4000. The mass ratio of glucose to polyethylene glycol 4000 is 1:1 (the total carbon content accounts for 6.74% of the mass of ferric phosphate and lithium carbonate). After mixing evenly, grind the slurry to D50 = 0.45μm in a sand mill. Spray dry the sand-milled slurry. The feed temperature of the spray is 190℃ and the discharge temperature of the spray is 100℃.

[0071] (5) 400g of the first spherical lithium iron phosphate precursor, 600g of the second spherical lithium iron phosphate precursor, and 45g of the third carbon source glucose were mixed and sintered in a box furnace to obtain spherical lithium iron phosphate finished product. The sintering temperature was 780℃, the sintering time was 3h, and the heating rate was 3℃ / min. The sintered product was then crushed by a crushing device. The crushed product was then subjected to subsequent screening, demagnetization, and packaging steps to obtain the final product lithium iron phosphate. The product quality is shown in Table 1.

[0072] Comparative Example 1

[0073] The preparation process is the same as in Example 1, except that in step (2), the compressed air flow rate is 100 mL / min / g for oxidation, while the other preparation parameters are the same. The quality of the lithium iron phosphate product obtained is shown in Table 1.

[0074] Comparative Example 2

[0075] The preparation process is the same as in Example 1, except that in step (2), the compressed air flow rate is 1333.3 mL / min / g for oxidation, while the other preparation parameters are the same. The quality of the lithium iron phosphate product obtained is shown in Table 1.

[0076] Comparative Example 3

[0077] The preparation process is the same as in Example 1, except that in step (2), solid iron phosphate dihydrate is obtained and then dehydrated by sintering at 600°C for 2 hours in a rotary kiln to obtain solid iron phosphate anhydrous. In steps (3) and (4), solid iron phosphate anhydrous is used to replace solid iron phosphate dihydrate and slurry iron phosphate dihydrate, respectively. The other preparation parameters are the same. The quality of the lithium iron phosphate product obtained is shown in Table 1.

[0078] The fabrication of lithium-ion batteries:

[0079] The lithium iron phosphate prepared in the above comparative examples were assembled into 2025 coin cells according to the coin cell preparation steps described above. The batteries prepared in Examples 1-4 and Comparative Examples 1-2 were subjected to performance tests, and their discharge capacities at 0.1C and 1C were tested. The test data are detailed in Table 1.

[0080] Table 1. Performance test data of the batteries prepared in Examples 1-4 and Comparative Examples 1-2.

[0081]

[0082] It is readily understood that the above embodiments are merely illustrative examples for clear explanation and do not imply that the invention is limited thereto. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A method for preparing spherical high-pressure lithium iron phosphate, comprising the following steps: (1) Add iron powder to dilute phosphoric acid and dissolve to obtain ferrous dihydrogen phosphate solution; (2) Add the above-mentioned ferrous dihydrogen phosphate solution and air oxidant to a stirred reactor. After the reaction is completed, a slurry is obtained. After filtration, washing and drying, ferric phosphate dihydrate raw material is obtained. (3) After mixing lithium salt, first carbon source and iron phosphate dihydrate raw material, the first sintering process is carried out to obtain the first spherical lithium iron phosphate precursor; (4) After mixing lithium salt, second carbon source and iron phosphate dihydrate raw material, the second spherical lithium iron phosphate precursor is obtained by sand milling and spray drying. (5) After mixing the first spherical lithium iron phosphate precursor, the second spherical lithium iron phosphate precursor and the third carbon source, the spherical lithium iron phosphate finished product is obtained by the second sintering process.

2. The preparation method according to claim 1, characterized in that, In step (1), the mass concentration of the dilute phosphoric acid is 20% to 30 wt%, and the molar ratio of the phosphoric acid to the iron powder in step (1) is 2 to 3:1, based on pure phosphoric acid.

3. The preparation method according to claim 1 or 2, characterized in that, In step (2), the flow rate of the air oxidant is 150-350 mL / min / (1g iron powder), the stirring speed in the reactor is 100-700 rpm, the reaction temperature is 50-90℃, and the reaction time is 2-10h.

4. The preparation method according to any one of claims 1-3, characterized in that, In step (2), the slurry is filtered using a filter press, the filter cake is washed with deionized water until the conductivity of the wash water is ≤200μS / cm, and the filter cake is dried using a rotary flash desiccant with a moisture content of ≤0.5%.

5. The preparation method according to any one of claims 1-4, characterized in that, In step (3), the temperature in the first sintering conditions is 600-800℃, the atmosphere is nitrogen, the sintering time is 1-10h, and the heating rate is 1-10℃ / min. Preferably, the temperature in the first sintering conditions is 700-800℃, the sintering time is 1-4h, and the heating rate is 2-5℃ / min.

6. The preparation method according to any one of claims 1-5, characterized in that, In steps (3) to (5), the first / second / third carbon source is one or more of glucose, PEG and citric acid, and the lithium salt is one of lithium hydroxide and lithium carbonate. Preferably, the lithium salt is lithium carbonate.

7. The preparation method according to any one of claims 1-6, characterized in that, In step (3), the preferred mass of the first carbon source is 3% to 5% of the sum of the masses of lithium salt and ferric phosphate dihydrate, and the mass ratio of lithium salt to ferric phosphate dihydrate is 1:3 to 5.

8. The preparation method according to any one of claims 1-7, characterized in that, In step (4), the grinding step uses a horizontal sand mill or a vertical sand mill to grind the material, controlling the final grinding slurry D50 to be 0.3μm to 0.5μm; and / or, the feed temperature of the spray is 150 to 220℃, and the discharge temperature of the spray is 90 to 110℃. Preferably, the mass of the second carbon source is 4% to 10% of the sum of the masses of lithium salt and ferric phosphate dihydrate. The mass ratio of lithium salt to ferric phosphate dihydrate is 1:3 to 5.

9. The preparation method according to any one of claims 1-8, characterized in that, In step (5), the mass ratio of the first spherical lithium iron phosphate precursor to the second lithium iron phosphate precursor is 1:0.7 to 1.5; preferably, the mass of the third carbon source is the sum of the masses of the first and second spherical lithium iron phosphate precursors = 0.03 to 0.06:

1.

10. The preparation method according to any one of claims 1-9, characterized in that, In step (5), the second sintering temperature is 600-800℃, the atmosphere is nitrogen, the sintering time is 1-10h, and the heating rate is 1-10℃ / min. Preferably, the second sintering conditions are a temperature of 720-800℃, a sintering time of 1-4h, and a heating rate of 2-5℃ / min.

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