Iron phosphate for high-density lithium iron phosphate and preparation method thereof

Abstract

The application relates to the technical field of batteries, and provides a kind of iron phosphate for high compaction lithium iron phosphate, the particle size distribution is bimodal, the tap density is high, small particles and large particles exist simultaneously, and the preparation method is as follows: S1, a phosphorus salt solution is prepared, a ferrous solution is further prepared, the phosphorus salt solution and the ferrous solution are mixed to obtain a ferrous phosphate solution; S2, the ferrous phosphate solution is heated, hydrogen peroxide is added, and slurry A composed of ferrous phosphate octahydrate and basic ammonium phosphate iron is obtained; S3, phosphoric acid solution is added to slurry A, heating and insulation are carried out, and then hydrogen peroxide is added, white ferrous phosphate dihydrate slurry B is obtained; S4, after filtration, washing and drying, ferrous phosphate dihydrate with a size particle compound structure is obtained, and after calcination, anhydrous iron phosphate with a size particle compound structure is obtained; the technical scheme provided by the application does not need to introduce a template agent and a complexing agent, and does not need to use a step-by-step method to prepare the iron phosphate material with a size particle compound structure.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of batteries, in particular to a kind of high compaction lithium iron phosphate and its preparation method. BACKGROUND

[0002] As one of the most common commercial lithium ion battery cathodes, lithium iron phosphate is increasingly concerned due to its excellent safety performance, high specific capacity and low price, and the precursor of lithium iron phosphate, iron phosphate, is also increasingly valued. In the prior art, when preparing the precursor material iron phosphate required for high compaction lithium iron phosphate, mainly including the ideas of element doping in iron phosphate material, preparing iron phosphate material with size particle complex structure, etc.

[0003] For example, the patent with publication number CN110436427 uses a two-step method to synthesize iron phosphate material with wide particle size distribution range and both flaky and granular morphology by introducing complexing agent and template agent, controlling reaction temperature and reaction pH value, etc. The flaky and granular iron phosphate forms a size particle complex structure.

[0004] The patent with publication number CN109775679 uses ferrous sulfate to prepare iron hydroxide precipitate, and then adds phosphate to convert it into smaller size iron phosphate slurry A (0.2-4 μm); at the same time, iron salt solution is used to react with calcium phosphate to generate larger size iron phosphate slurry B (3-16 μm); then slurry A and slurry B are mixed in a certain proportion, atomized and granulated, and dehydrated to form iron phosphate material with size particle complex structure, which is used for lithium iron phosphate synthesis.

[0005] The above prior art has the disadvantages of complex reaction steps, long reaction period, and the need to introduce additional template agent / complexing agent in the preparation of size particle complex structure iron phosphate. SUMMARY

[0006] To solve the above technical problems, the present application discloses a kind of high compaction lithium iron phosphate and its preparation method, provides a kind of short-range preparation method of size particle complex structure iron phosphate material, to solve the disadvantages of complex reaction steps, long reaction period, and the need to introduce additional template agent, complexing agent in the preparation of size particle complex structure iron phosphate in the prior art, specifically as follows:

[0007] A kind of high compaction lithium iron phosphate and its preparation method, the particle size distribution of the iron phosphate is bimodal, the tap density is high, there are small particles with size of 0.2-2 μm and large particles with size of 10-40 μm at the same time, and the preparation method is as follows:

[0008] S1, ammonium phosphate monohydrate and ammonium phosphate dihydride are used to prepare a phosphate salt solution, and a ferrous solution is prepared, and the phosphate salt solution and the ferrous solution are mixed to obtain a ferrous phosphate solution;

[0009] S2, the ferrous phosphate solution is warmed to 40-50 DEG C, and hydrogen peroxide solution is added, after reaction, slurry A composed of ferrous phosphate octahydrate and basic ammonium iron phosphate is obtained;

[0010] S3, phosphoric acid solution is added to slurry A, warmed to 90-100 DEG C, and kept for 30-90 min, after keeping, hydrogen peroxide is added again to ensure that there is no residual ferrous ion in the system, white ferrous phosphate dihydrate slurry B is obtained;

[0011] S4, after filtration, washing and drying of slurry B, ferrous phosphate dihydrate with size particle complex structure is obtained, after calcination of ferrous phosphate dihydrate, anhydrous ferrous phosphate with size particle complex structure is obtained.

[0012] Moreover, the phosphorus concentration in the phosphorus salt solution in step S1 is 0.8-1.4 mol / L; the iron concentration in the ferrous solution is 0.8-1.4 mol / L; the ferrous solution is prepared from ferrous sulfate, iron powder and iron scale.

[0013] Moreover, the molar ratio of iron element to phosphorus element in the ferrous solution and the phosphorus salt solution in step S1 is controlled to n(Fe):n(P)=1:1.00-1:1.05.

[0014] Moreover, the pH of the ferrous phosphate solution in step S1 is 2.0-4.0, the pH of the ferrous solution is 2.0-3.0, and the pH of the phosphorus salt solution is 3.8-4.5.

[0015] Moreover, the pH of the ferrous phosphate solution is controlled by the pH of the ferrous solution and the pH of the phosphorus salt solution.

[0016] Moreover, the molar ratio of hydrogen peroxide solution to iron element in the ferrous phosphate solution in step S2 is n(Fe):n(H2O2)=1:0.55-1:0.65.

[0017] Moreover, the hydrogen peroxide solution is added within 20-50 min, and the reaction is carried out for 20-50 min after the hydrogen peroxide is added.

[0018] Moreover, the molar ratio of phosphorus element in the added phosphoric acid solution to iron element in the slurry A solution in step S3 should be controlled to n(P):n(Fe)=0.2-0.6:1.

[0019] The present application prepares ferrous phosphate with bimodal particle size distribution and size particle complex structure by a simple method, and the corresponding lithium ferrous phosphate has high powder compaction, and the principle lies in that:

[0020] 1、The difference of reaction pH will directly affect the nucleation rate and growth rate of the crystal, affect the growth rate of each crystal face of ferrous phosphate dihydrate, and thus affect the morphology of the primary particles of ferrous phosphate dihydrate and the agglomeration degree of the secondary particles; in the present application, the ferrous solution and the phosphorus salt solution are mixed as the bottom solution, the pH is controlled in the appropriate range (pH=2.0~4.0), the supersaturation degree of the system is higher, the growth rates of each crystal face are basically the same, each crystal face grows synchronously, and a large number of small-sized round particles (as shown in Figure 9 ) are formed in the reaction process. These round particles are composed of ferrous phosphate octahydrate and ammonium ferric phosphate. After the addition of phosphoric acid and the conversion by heating, part of the ammonium ferric phosphate in these round particles will be tightly packed to form large-sized secondary large particles (as shown in Figure 10 ). Part of the ferrous phosphate octahydrate in these round particles will retain the original highly dispersed morphology to form small-sized secondary small particles. Ultimately, the ferrous phosphate material with the size particle collocation is obtained. In the case of a lower reaction pH (pH<2), the growth rate of ferrous phosphate dihydrate on some crystal faces is dominant, which will grow along the specific crystal face to form elongated or flaky primary particles. When these elongated or flaky primary particles are packed and agglomerated, they tend to be orderly arranged and interlaced at a certain angle to form a spherical particle. This kind of morphology and structure of the ferrous phosphate has a too narrow particle size distribution, there are many pores in the particle interior, and it is difficult to form tightly packed large-sized particles (as shown in Figure 5 ). In the case of a higher reaction pH (pH>4), too much ferrous phosphate octahydrate is easily formed in the system, and it is also difficult to form the structure with size particle collocation (as shown in Figure 6 ).

[0021] 2、In the present application, after the hydrogen peroxide is added, an appropriate amount of phosphoric acid solution is added to make n(P):n(Fe)=0.2:1~0.6:1, so as to promote the mutual agglomeration and tight packing of the originally dispersed particles and realize the conversion of the material crystal form from ammonium ferric phosphate to ferrous phosphate dihydrate. In this process, a large number of small particles with a size of 0.2~2 μm exist, and a large number of blocky secondary particles with a size of 10~40 μm are formed. When the amount of phosphoric acid is too small, n(P):n(Fe)<0.2:1, the small particles cannot be tightly agglomerated, and the formed large particles are less and the small particles are more, and do not have the structure characteristic of size particle collocation. When the amount of phosphoric acid is too large, n(P):n(Fe)>0.6:1, the small particles are excessively agglomerated, the formed large particles are more and the small particles are less, and do not have the structure characteristic of size particle collocation.

[0022] Compared with the prior art, the present application has the following beneficial effects:

[0023] 1. The present application provides a kind of iron phosphate with bimodal particle size distribution, high tap density and size particle complex structure, and a preparation method thereof.The technical scheme provided by the present application does not need to introduce additives such as template agent and complexing agent, which can increase the pressure of wastewater treatment, and does not need to use a step-by-step method to prepare iron phosphate material with size particle complex structure, which shortens the process flow and production cycle, and facilitates quality control.

[0024] 2. The prepared iron phosphate material has a size particle complex structure, and after being made into lithium iron phosphate material, it can ensure the size particle complex effect of lithium iron phosphate material, and the powder compaction density reaches 2.50 g / cm 3 Compared with the prior art, the compaction density is higher.

[0025] 3. Compared with the preparation technology of existing size particle complex structure iron phosphate, the present application precisely controls the size of primary iron phosphate particles and its agglomeration form by controlling the wet reaction conditions, realizes the short-range preparation of size particle complex structure iron phosphate material, significantly reduces the complexity of process flow, shortens the production cycle and reduces the production cost.

[0026] 4. In the present application, the reaction pH is controlled to be 2.0-4.0, the molar ratio of phosphorus element in phosphoric acid solution to iron element in slurry A solution should be controlled to be n(P):n(Fe)=0.2-0.6:1; the bottom liquid pH is controlled to be in the range of 2.0-4.0, to ensure that the system supersaturation is at a high level, the growth rate of each crystal face of iron phosphate dihydrate is basically the same, and the generated round primary particles are formed; on the other hand, the addition time and amount of phosphoric acid solution are controlled, under the joint action of this feeding mode and appropriate reaction conditions, the accumulation mode between primary particles is effectively controlled, to ensure that primary particles can tightly agglomerate to form blocky secondary particles. BRIEF DESCRIPTION OF DRAWINGS

[0027] Figure 1 The scanning electron microscope image of anhydrous iron phosphate prepared in Experiment 1 of the present application;

[0028] Figure 2 The particle size distribution graph of anhydrous iron phosphate prepared in Experiment 1 of the present application;

[0029] Figure 3 The scanning electron microscope image of anhydrous iron phosphate prepared in Experiment 2 of the present application;

[0030] Figure 4 The particle size distribution graph of anhydrous iron phosphate prepared in Experiment 2 of the present application;

[0031] Figure 5 The scanning electron microscope image of anhydrous iron phosphate prepared in Experiment 3 of the present application;

[0032] Figure 6 The particle size distribution diagram of the anhydrous iron phosphate prepared in the experiment 3 of the present application;

[0033] Figure 7 The scanning electron microscope diagram of the anhydrous iron phosphate prepared in the experiment 4 of the present application;

[0034] Figure 8 The particle size distribution diagram of the anhydrous iron phosphate prepared in the experiment 4 of the present application;

[0035] Figure 9 The scanning electron microscope diagram of the anhydrous iron phosphate prepared in the experiment 5 of the present application;

[0036] Figure 10 The particle size distribution diagram of the anhydrous iron phosphate prepared in the experiment 5 of the present application;

[0037] Figure 11 The scanning electron microscope diagram of the anhydrous iron phosphate prepared in the experiment 6 of the present application;

[0038] Figure 12 The scanning electron microscope diagram of the anhydrous iron phosphate prepared in the experiment 7 of the present application;

[0039] Figure 13 The scanning electron microscope diagram of the basic ammonium iron phosphate formed before adding phosphoric acid in the example 3 of the present application;

[0040] Figure 14 The scanning electron microscope diagram of the dihydrate iron phosphate formed after adding phosphoric acid in the example 3 of the present application. DETAILED DESCRIPTION Example 1

[0041] A kind of iron phosphate for high compaction lithium iron phosphate, the particle size distribution of the iron phosphate is bimodal, the tap density is high, there are small particles with size of 0.2-2 μm and large particles with size of 10-40 μm at the same time, and the preparation method is as follows:

[0042] S1, prepare a phosphorus salt solution with ammonium dihydrogen phosphate, then prepare a ferrous solution, mix the phosphorus salt solution and the ferrous solution to obtain a ferrous phosphate solution;

[0043] S2, heat the ferrous phosphate solution to 50℃, and then add a hydrogen peroxide solution, after reaction, obtain a slurry A composed of ferrous phosphate octahydrate and basic ammonium iron phosphate;

[0044] S3, add a phosphoric acid solution to the slurry A, heat to 100℃, and keep for 90 min, then add hydrogen peroxide again to ensure that there is no residual ferrous ion in the system, and obtain a white dihydrate iron phosphate slurry B;

[0045] S4, the slurry B is filtered, washed and dried to obtain iron phosphate dihydrate with a size particle compound structure, and the iron phosphate dihydrate is calcined to obtain anhydrous iron phosphate with a size particle compound structure.

[0046] Further, the concentration of phosphorus in the phosphorus salt solution is 1.4 mol / L, and the concentration of iron in the ferrous solution is 1.4 mol / L; the ferrous solution is prepared from ferrous sulfate.

[0047] Further, the molar ratio of iron and phosphorus in the ferrous solution and the phosphorus salt solution in step S1 is controlled to n(Fe):n(P)=1:1.05.

[0048] Further, the pH of the ferrous phosphate solution is 4.0, the pH of the ferrous solution is 3.0, and the pH of the phosphorus salt solution is 4.5.

[0049] Further, the pH of the ferrous phosphate solution is controlled by the pH of the ferrous solution and the phosphorus salt solution.

[0050] Further, the molar ratio of iron and hydrogen peroxide in the hydrogen peroxide solution and the ferrous phosphate solution in step S2 is n(Fe):n(H2O2)=1:0.65.

[0051] Further, the hydrogen peroxide solution is added within 20-50 min, and the reaction is carried out for 20-50 min after the completion of hydrogen peroxide feeding.

[0052] Further, the molar ratio of phosphorus in the added phosphoric acid solution and iron in the slurry A solution in step S3 should be controlled to n(P):n(Fe)=0.6:1. Example 2

[0053] A kind of high compaction iron phosphate for lithium phosphate, the particle size distribution of the iron phosphate is bimodal, the tap density is high, there are small particles with size of 0.2-2 μm and large particles with size of 10-40 μm at the same time, and the preparation method is as follows:

[0054] S1, ammonium monohydrogen phosphate is used to prepare a phosphorus salt solution, and a ferrous solution is prepared, the phosphorus salt solution and the ferrous solution are mixed to obtain a ferrous phosphate solution;

[0055] S2, the ferrous phosphate solution is heated to 40℃, and hydrogen peroxide solution is added, after reaction, slurry A composed of ferrous phosphate octahydrate and basic ammonium iron phosphate is obtained;

[0056] S3, phosphoric acid solution is added to slurry A, heated to 90℃, and kept for 30 min, after the end of the heat preservation, hydrogen peroxide is added again to ensure that there is no residual ferrous ion in the system, and white iron phosphate dihydrate slurry B is obtained;

[0057] S4, the slurry B is filtered, washed and dried to obtain iron phosphate dihydrate with a size particle compound structure, and the iron phosphate dihydrate is calcined to obtain anhydrous iron phosphate with a size particle compound structure.

[0058] Further, the concentration of phosphorus in the phosphorus salt solution is 0.8 mol / L in the step S1; the concentration of iron in the ferrous solution is 0.8 mol / L; and the ferrous solution is prepared from iron powder.

[0059] Further, the molar ratio of iron and phosphorus in the ferrous solution and the phosphorus salt solution in the step S1 is controlled to n(Fe):n(P)=1:1.

[0060] Further, the pH of the ferrous phosphate solution is 2.0, the pH of the ferrous solution is 2.0, and the pH of the phosphorus salt solution is 3.8 in the step S1.

[0061] Further, the pH of the ferrous phosphate solution is controlled by the pH of the ferrous solution and the phosphorus salt solution.

[0062] Further, the molar ratio of iron and hydrogen peroxide in the step S2 is n(Fe):n(H2O2)=1:0.55.

[0063] Further, the hydrogen peroxide solution is added within 20 min, and the reaction is carried out for 20 min after the addition of hydrogen peroxide.

[0064] Further, the molar ratio of phosphorus in the added phosphoric acid solution and iron in the slurry A solution in the step S3 should be controlled to n(P):n(Fe)=0.2:1. Example 3

[0065] A kind of high compaction iron phosphate for lithium phosphate, the particle size distribution of the iron phosphate is bimodal, the tap density is high, there are simultaneously small particles with size 0.2~2 μm and large particles with size 10~40 μm, and the preparation method is as follows:

[0066] S1, prepare a phosphorus salt solution with ammonium dihydrogen phosphate, then prepare a ferrous solution, mix the phosphorus salt solution and the ferrous solution to obtain a ferrous phosphate solution;

[0067] S2, the ferrous phosphate solution is heated to 45℃, and hydrogen peroxide solution is added, after reaction, slurry A composed of ferrous phosphate octahydrate and ammonium phosphate is obtained;

[0068] S3, add phosphoric acid solution to slurry A, heat to 95℃, and keep for 60 min, then add hydrogen peroxide to ensure that there is no residual ferrous ion in the system, and obtain white iron phosphate dihydrate slurry B;

[0069] S4, the slurry B is filtered, washed and dried to obtain iron phosphate dihydrate with a complex structure of large and small particles, and the iron phosphate dihydrate is calcined to obtain anhydrous iron phosphate with a complex structure of large and small particles.

[0070] Further, the concentration of phosphorus in the phosphorus salt solution in step S1 is 1.1 mol / L; the concentration of iron in the ferrous solution is 1.1 mol / L; and the ferrous solution is prepared from ferrous sulfate, iron powder and iron scale.

[0071] Further, the molar ratio of iron to phosphorus in the ferrous solution and the phosphorus salt solution in step S1 is controlled to n(Fe):n(P)=1:1.02.

[0072] Further, the pH of the ferrous phosphate solution in step S1 is 3.0, the pH of the ferrous solution is 2.5, and the pH of the phosphorus salt solution is 4.2.

[0073] Further, the pH of the ferrous phosphate solution is controlled by the pH of the ferrous solution and the phosphorus salt solution.

[0074] Further, the molar ratio of iron to hydrogen peroxide in the hydrogen peroxide solution and the ferrous phosphate solution in step S2 is n(Fe):n(H2O2)=1:0.60.

[0075] Further, the hydrogen peroxide solution is added within 35 min, and the reaction is carried out for 30 min after the completion of the hydrogen peroxide addition.

[0076] Further, in step S3, the molar ratio of phosphorus in the added phosphoric acid solution to iron in the slurry A solution should be controlled to n(P):n(Fe)=0.4:1.

[0077] Experimental section

[0078] Experiment 1

[0079] (1) 4L of ferrous sulfate solution with a concentration of 1 mol / L and 4L of monoammonium phosphate solution with a concentration of 1 mol / L are prepared, and mixed to obtain a ferrous phosphate solution with a pH of 2.2.

[0080] (2) The ferrous phosphate solution is heated to 50℃, and 326g of hydrogen peroxide (mass concentration of 25 wt%) is added to the ferrous phosphate solution within 30 min under stirring at a ratio of n(Fe):n(H2O2)=1:0.60, and the reaction is carried out for 30 min to obtain a slurry A composed of basic ammonium iron phosphate.

[0081] (3) Under stirring, 138 g of phosphoric acid (mass concentration of 85 wt%) was added into slurry A at a ratio of n(Fe):n(P)=1:0.3, the temperature was raised to 90°C and kept for 60 min, white slurry B composed of iron phosphate dihydrate was obtained, and hydrogen peroxide was added into slurry B before discharging slurry B to ensure that no ferrous ions exist in slurry B.

[0082] (4) After filtration, washing and drying, slurry B obtained iron phosphate dihydrate with a structure of complexed large and small particles; after calcination, iron phosphate dihydrate obtained anhydrous iron phosphate with a structure of complexed large and small particles and a bimodal particle size distribution.

[0083] Figure 1 The scanning electron microscope (SEM) image of the anhydrous iron phosphate prepared in Experiment 1 can be observed, and it can be observed that there are a large number of block-shaped large particles with a size of 10-20 μm in the anhydrous iron phosphate prepared in Experiment 1, and a large number of small particles are scattered around the large particles, which indicates that the anhydrous iron phosphate prepared in Experiment 1 has a clear structure of complexed large and small particles.

[0084] Figure 2 The particle size distribution graph of the anhydrous iron phosphate prepared in Experiment 1 can be observed, and it can be observed that the particle size distribution of the anhydrous iron phosphate prepared in Experiment 1 is bimodal, which further proves that the prepared anhydrous iron phosphate has a clear structure of complexed large and small particles.

[0085] Experiment 2

[0086] (1) A 4L ferrous sulfate solution with a ferrous ion concentration of 1.4 mol / L and a 4L ammonium phosphate solution with a phosphorus concentration of 1.4 mol / L were prepared, and mixed to obtain a ferrous phosphate solution with a pH of 2.0.

[0087] (2) The ferrous phosphate solution was heated to 40°C, and 495 g of hydrogen peroxide (mass concentration of 25 wt%) was added into the ferrous phosphate solution at a ratio of n(Fe):n(H2O2)=1:0.65 under stirring within 50 min, and reacted for 20 min to obtain slurry A composed of basic ammonium iron phosphate.

[0088] (3) Under stirring, 129 g of phosphoric acid (mass concentration of 85 wt%) was added into slurry A at a ratio of n(Fe):n(P)=1:0.2, the temperature was raised to 99°C and kept for 30 min, white slurry B composed of iron phosphate dihydrate was obtained, and hydrogen peroxide was added into slurry B before discharging slurry B to ensure that no ferrous ions exist in slurry B.

[0089] (4) After filtration, washing and drying, slurry B obtained iron phosphate dihydrate with a structure of complexed large and small particles; after calcination, iron phosphate dihydrate obtained anhydrous iron phosphate with a structure of complexed large and small particles and a bimodal particle size distribution.

[0090] Figure 3 The scanning electron microscope (SEM) image of the anhydrous ferric phosphate prepared in Experiment 2 can be observed from the SEM image that a large number of blocky large particles with a size of 10-20 μm exist in the anhydrous ferric phosphate prepared in Experiment 2, and a large number of small particles exist around the large particles, indicating that the anhydrous ferric phosphate prepared in this example has a clear structure of large and small particles.

[0091] Figure 4 The particle size distribution graph of the anhydrous ferric phosphate prepared in Experiment 2 can be observed that the particle size distribution of the anhydrous ferric phosphate prepared in Experiment 2 is bimodal, further proving that the prepared anhydrous ferric phosphate has a clear structure of large and small particles.

[0092] Experiment 3

[0093] (1) A 4L ferrous sulfate solution with a ferrous ion concentration of 0.8 mol / L and a 4L ammonium phosphate solution with a phosphorus concentration of 0.84 mol / L are prepared, and mixed to obtain a ferrous phosphate solution with a pH of 4.0.

[0094] (2) The ferrous phosphate solution is heated to 45°C, and 240 g of hydrogen peroxide (mass concentration of 25 wt%) is added to the ferrous phosphate solution in a ratio of n(Fe):n(H2O2)=1:0.55 under stirring within 20 min, and reacted for 50 min to obtain a slurry A composed of ferrous phosphate octahydrate and basic ammonium ferric phosphate.

[0095] (3) 184 g of phosphoric acid (mass concentration of 85 wt%) is added to the slurry A in a ratio of n(Fe):n(P)=1:0.5 under stirring, heated to 92°C and kept for 90 min to obtain a white slurry B composed of ferrous phosphate dihydrate, and hydrogen peroxide is added to the slurry B before the slurry B is discharged.

[0096] (4) After the slurry B is filtered, washed and dried, the ferrous phosphate dihydrate with a large and small particle complex structure is obtained; and after the ferrous phosphate dihydrate is calcined, the anhydrous ferric phosphate with a bimodal particle size distribution and a large and small particle complex structure is obtained. The anhydrous ferric phosphate prepared in Example 3 also has a large and small particle complex morphology, which will not be described here.

[0097] Figure 5 The scanning electron microscope (SEM) image of the anhydrous ferric phosphate prepared in Experiment 3 can be observed from the SEM image that a large number of blocky large particles with a size of 10-40 μm exist in the anhydrous ferric phosphate prepared in Experiment 3, and a large number of small particles exist around the large particles, indicating that the anhydrous ferric phosphate prepared in this example has a clear structure of large and small particles.

[0098] Figure 6 The particle size distribution graph of the anhydrous iron phosphate prepared in Experiment 3 can be observed to have a bimodal distribution, further proving that the anhydrous iron phosphate prepared has a clear structure of a mixture of large and small particles.

[0099] Experiment 4

[0100] In this experiment, the pH of the reaction was 1.5, and the other conditions were the same as in Experiment 1. Specifically, Step 1 of this experiment was as follows: (1) 4 L of ferrous sulfate solution with a concentration of 1 mol / L and 4 L of monoammonium phosphate solution with a concentration of 1 mol / L were prepared, and 138 g of phosphoric acid (with a mass concentration of 85 wt%) was added to the ferrous sulfate solution. After mixing the ferrous sulfate solution and the monoammonium phosphate solution, a ferrous phosphate solution with a pH of 1.5 was obtained. The other conditions were the same as in Experiment 1.

[0101] The morphology of the anhydrous iron phosphate prepared in Experiment 4 is shown in Figure 7 At a lower reaction pH, the primary particles of iron phosphate grow into a flaky shape, and the primary particles are loosely agglomerated, making it difficult to form large block-shaped particles.

[0102] Figure 8 The particle size distribution graph of the anhydrous iron phosphate prepared in Experiment 4 has a narrow particle size distribution and a unimodal distribution, further proving that the iron phosphate prepared in this comparative experiment does not have a structure of a mixture of large and small particles.

[0103] Experiment 5

[0104] In this experiment, the pH of the reaction was 4.5, and the other conditions were the same as in Experiment 1. Specifically, Step 1 of this experiment was as follows: (1) 4 L of ferrous sulfate solution with a concentration of 1 mol / L and a pH of 2.0 and 4 L of monoammonium phosphate solution with a concentration of 1 mol / L and a pH of 7.5 were prepared. After mixing the ferrous sulfate solution and the monoammonium phosphate solution, a ferrous phosphate solution with a pH of 4.5 was obtained. The other conditions were the same as in Experiment 1.

[0105] The morphology of the anhydrous iron phosphate prepared in Experiment 5 is shown in Figure 9 It can be observed that the anhydrous iron phosphate prepared in Experiment 5 has only a small amount of block-shaped particles. This is because at a higher reaction pH, excessive ferrous phosphate octahydrate is easily generated during the synthesis stage. When the ferrous phosphate octahydrate and basic ammonium iron phosphate are converted into ferrous phosphate dihydrate by adding phosphoric acid and hydrogen peroxide, it is difficult to produce more block-shaped particles.

[0106] Figure 10 The particle size distribution graph of the anhydrous iron phosphate prepared in Experiment 5 has a narrow particle size distribution and a unimodal distribution, further proving that the iron phosphate prepared in this comparative experiment does not have a structure of a mixture of large and small particles.

[0107] Experiment 6

[0108] The phosphoric acid used in this experiment is less, and in addition to the molar ratio of iron element to phosphoric acid being n(Fe):n(P)=1:0.18, the other conditions are the same as those in Experiment 2.

[0109] Figure 11 The scanning electron microscope (SEM) image of the anhydrous ferric phosphate prepared in this experiment is shown in Figure 6. As can be observed from the SEM image, the anhydrous ferric phosphate prepared in this experiment does not have a structure of mixed large and small particles.

[0110] Experiment 7

[0111] The phosphoric acid used in this experiment is more, and in addition to the molar ratio of iron element to phosphoric acid being n(Fe):n(P)=1:0.65, the other conditions are the same as those in Experiment 1.

[0112] Figure 12 The scanning electron microscope (SEM) image of the anhydrous ferric phosphate prepared in this experiment is shown in Figure 7. As can be observed from the SEM image, the anhydrous ferric phosphate prepared in this experiment has a large amount of block-shaped large particles with a size of 10-40 μm, and even some large particles with a size of more than 40 μm. Compared with Experiment 1, the ferric phosphate prepared in this experiment lacks small particles with a size of 0.2-2 μm, and does not have a structure of mixed large and small particles.

[0113] Experiment 8

[0114] The tap densities of the ferric phosphates prepared in Experiments 1-6 were tested, and the anhydrous ferric phosphates prepared in Experiments 1-6 were respectively made into lithium ferric phosphate, and the powder tap densities, 0.1C discharge capacities and 1C discharge capacities of the lithium ferric phosphate were respectively tested. The specific results are shown in Table 1.

[0115] Table 1 Performance test results of ferric phosphate and lithium ferric phosphate

[0116]

[0117] As can be seen from Table 1, the tap densities of the ferric phosphates prepared in Experiments 1-3 are respectively 1.12 g / cm 3 , 1.09 g / cm 3 and 1.11 g / cm 3 ; and the powder tap densities of the lithium ferric phosphates prepared in Experiments 1-3 are respectively 2.55 g / cm 3 , 2.52 g / cm 3 and 2.53 g / cm 3The 0.1C discharge capacities are 159.4 mAh / g, 159.7 mAh / g, and 158.5 mAh / g, respectively; the 1C discharge capacities are 146.5 mAh / g, 147.2 mAh / g, and 146.1 mAh / g, respectively.

[0118] The tap density of the prepared ferric phosphate was 0.55 g / cm³ in reactions 4 (pH=1.5) and 5 (pH=4.5), respectively. 3 0.68 g / cm 3 The compacted densities of the prepared lithium iron phosphate powder were 2.34 and 2.41 g / cm³, respectively. 3 The 0.1C discharge capacities are 159.9 mAh / g and 159.8 mAh / g, respectively; the 1C discharge capacities are 1467.1 mAh / g and 146.8 mAh / g, respectively.

[0119] In Experiment 6, the amount of phosphoric acid used was too low, and the molar ratio of iron to phosphoric acid was n(Fe):n(P) = 1:0.18. In Experiment 7, the amount of phosphoric acid used was too high, and the molar ratio of iron to phosphoric acid was n(Fe):n(P) = 1:0.65. The tap density of the prepared iron phosphate was 0.67 g / cm³. 3 1.33g / cm 3 The compacted densities of the prepared lithium iron phosphate powder were 2.41 and 2.53 g / cm³, respectively. 3 The 0.1C discharge capacities are 159.3 mAh / g and 156.2 mAh / g, respectively; the 1C discharge capacities are 146.9 mAh / g and 139.6 mAh / g, respectively. In summary, the tap density and discharge capacity of the technical solution adopted in this invention are optimal.

Claims

1. An iron phosphate for high-density lithium iron phosphate, characterized by, The iron phosphate particle size distribution is bimodal, including small particles with a size of 0.2-2 μm and large particles with a size of 10-40 μm, and the preparation method is as follows: S1, preparing a phosphorus salt solution with ammonium dihydrogen phosphate and ammonium hydrogen phosphate, and preparing a ferrous solution, mixing the phosphorus salt solution and the ferrous solution to obtain a ferrous phosphate solution; The pH of the ferrous solution is 2.0-3.0, the pH of the phosphorus salt solution is 3.8-4.5, and the pH of the ferrous phosphate solution is 2.0-4.0; S2, heating the ferrous phosphate solution to 40-50℃, and adding a hydrogen peroxide solution, after reaction, obtaining a slurry A composed of ferrous phosphate octahydrate and basic ammonium iron phosphate; The molar ratio of the hydrogen peroxide solution to the iron element in the ferrous phosphate solution is n(Fe):n(H2O2)=1:0.55-1:0.65; S3, adding a phosphoric acid solution to the slurry A, heating to 90-100℃, and keeping for 30-90 min, after keeping, adding hydrogen peroxide to ensure that there is no residual ferrous ion in the system, obtaining a white ferrous phosphate dihydrate slurry B; The molar ratio of the phosphorus element in the phosphoric acid solution to the iron element in the slurry A solution should be controlled to n(P):n(Fe)=0.2:1-0.6:1; S4, after filtering, washing and drying the slurry B, obtaining ferrous phosphate dihydrate with a complex structure of large and small particles, and after calcining the ferrous phosphate dihydrate, obtaining anhydrous ferrous phosphate with a complex structure of large and small particles.

2. The iron phosphate for high-density lithium iron phosphate of claim 1, characterized by, The concentration of phosphorus in the phosphorus salt solution in step S1 is 0.8-1.4 mol / L; the concentration of iron in the ferrous solution is 0.8-1.4 mol / L; and the ferrous solution is prepared from ferrous sulfate, iron powder and iron scale.

3. The iron phosphate for high-density lithium iron phosphate of claim 1, wherein the iron phosphate is characterized by, The molar ratio of the iron element to the phosphorus element in the ferrous solution and the phosphorus salt solution in step S1 is controlled to n(Fe):n(P)=1:1.00-1:1.

05.

4. The ferric iron phosphate for high-density lithium iron phosphate of claim 1, characterized by, The pH of the ferrous phosphate solution is controlled by the pH of the ferrous solution and the phosphorus salt solution.

5. The iron phosphate for high-density lithium iron phosphate of claim 4, wherein the iron phosphate is FePO4. The hydrogen peroxide solution is added within 20-50 min, and after the hydrogen peroxide is added, the reaction is carried out for 20-50 min.

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