Doped iron phosphate with high iron-phosphorus ratio and specific crystal structure and preparation method thereof

Abstract

The invention provides a preparation method of doped iron phosphate with a high iron-phosphorus ratio and a specific crystal structure, and relates to the technical field of lithium ion, the preparation method comprises the following steps: S1, respectively preparing a ferrite solution, a phosphorus salt solution, a hydrogen peroxide solution and a phosphoric acid solution; s2, adding a pure water base solution into a reaction kettle, adding a phosphorus salt solution and a ferrite solution at the same time, and reacting to obtain blue-green slurry A; s3, adding a phosphoric acid solution, heating to T1, adding a hydrogen peroxide solution, and reacting to obtain white slurry B; s4, heating the white slurry B to T2, and keeping the temperature to obtain white slurry C; s5, flash evaporation calcination; on one hand, the diffusion path of lithium ions is shortened by utilizing the characteristic that the ratio of iron to phosphorus in iron phosphate is favorable for refining the particle size of lithium iron phosphate; on the other hand, a specific iron phosphate crystal structure is utilized to reduce the Li / Fe mixed arrangement phenomenon occurring in the lithium iron phosphate sintering process; meanwhile, impurity elements are doped into the iron phosphate precursor to broaden the transmission path of lithium ions and improve the electron conductivity.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium ions, and particularly to a doped iron phosphate with a high iron-to-phosphorus ratio and a specific crystal structure and a preparation method thereof. Background Art

[0002] Due to the specific olivine structure characteristics of lithium iron phosphate, its electronic conductivity and ionic conductivity are relatively low, showing poor low-temperature performance and rate performance. Currently, the most common method to improve its low-temperature performance is to increase the conductivity of lithium iron phosphate and the lithium ion diffusion rate. The main means include: (1) surface coating, (2) ion doping, and (3) particle nanosizing. Among them, coating a material with good conductivity on the surface of lithium iron phosphate can improve the conductive performance of lithium iron phosphate; ion doping in lithium iron phosphate particles can increase the one-dimensional diffusion channel of lithium ions in the crystal lattice; refining the primary particle size of lithium iron phosphate can shorten the diffusion path of lithium ions.

[0003] Existing technologies for improving the low-temperature performance of lithium iron phosphate mostly focus on improving it at the lithium iron phosphate end, and there are few technical solutions to improve it from the precursor iron phosphate end. As an important precursor for preparing lithium iron phosphate, the microscopic morphology, crystal structure, and physical and chemical indexes of iron phosphate also have a great influence on the formation and growth of lithium iron phosphate crystals during the solid-phase reaction process. For example, selecting iron phosphate with a high iron-to-phosphorus ratio and high crystallinity as the precursor is beneficial to obtaining lithium iron phosphate particles with smaller sizes, so as to achieve the purpose of refining the primary particle size of lithium iron phosphate. Summary of the Invention

[0004] Doping elements such as titanium, vanadium, manganese, and magnesium into iron phosphate is more conducive to the uniform incorporation of doping elements at the molecular level compared with doping at the lithium iron phosphate end, and can better broaden the lithium ion diffusion channel. Therefore, based on the iron phosphate end, the present invention provides a doped iron phosphate material with a high iron-to-phosphorus ratio and a specific crystal structure and a preparation method, achieving the purpose of refining the particle size of lithium iron phosphate, improving electronic conductivity and ionic conductivity, and reducing Li-Fe back-diffusion at the same time, thereby improving the low-temperature performance of lithium iron phosphate, specifically as follows: A preparation method of a doped iron phosphate with a high iron-to-phosphorus ratio and a specific crystal structure, characterized by including the following steps: S1. Raw material preparation: Prepare a ferrous salt solution with a concentration of 0.5 - 2.0 mol / L, a phosphate solution with a concentration of 0.5 - 2.0 mol / L, a hydrogen peroxide solution with a mass fraction of 10 - 30 wt%, and a phosphoric acid solution with a mass fraction of 40 - 85 wt% respectively; S2. Synthesis reaction: Add pure water as the bottom liquid into the reaction kettle, and simultaneously add the phosphate solution and the ferrous salt solution prepared in step S1 into the reaction kettle within 30 - 70 minutes. After the feeding is completed, react for 30 - 70 minutes to obtain a blue-green slurry A; S3. One-stage conversion: Add the phosphoric acid solution prepared in step S1 into the reaction kettle, then heat up to the first conversion temperature T1, add the hydrogen peroxide solution at the first conversion temperature T1, and obtain the white slurry B after reacting for 30 - 70 min; S4. Two-stage conversion: Heat up the white slurry B to the second conversion temperature T2 and keep it warm for 30 - 180 min to obtain the white slurry C; S5. Flash evaporation and calcination: Perform solid-liquid separation and washing on the white slurry C obtained in step S4 until the conductivity of the washing water ≤ 1000 μS / cm to obtain the white filter cake D; The white filter cake D is obtained through flash drying and calcination dehydration treatment to obtain the doped iron phosphate material with a high iron-phosphorus ratio and a specific crystal structure.

[0005] Further, in step S1, the ferrous salt solution is prepared from ferrous sulfate, ferrous nitrate, ferrous chloride, iron powder or iron sheet; The ferrous salt solution contains one or more corresponding metal ions of Ni, Co, Mn, Mg, V, Ti, Al, Cu, Zn, W, Cr, Zr, La, Mo, Nb, Ga, Y; The phosphate solution is prepared from ammonium monohydrogen phosphate, ammonium dihydrogen phosphate, ammonium phosphate, phosphoric acid, sodium monohydrogen phosphate, sodium dihydrogen phosphate or sodium phosphate.

[0006] Further, in step S1, the pH of the ferrous salt solution is 1.0 - 3.0, and the pH of the phosphate solution is 6.0 - 8.0.

[0007] Further, in step S2, the phosphate solution and the ferrous salt solution added into the reaction kettle satisfy n(Fe):n(P)=1:0.9 - 1:1.4.

[0008] Further, in step S3, the phosphoric acid solution and the ferrous salt solution added into the reaction kettle satisfy n(H3PO4):n(Fe)=0.35:1 - 0.10:1; The first conversion temperature T1 is 50 - 70 °C.

[0009] Further, in step S4, the hydrogen peroxide solution and the ferrous salt solution added into the reaction kettle satisfy n(Fe):n(H2O2)=1:0.6 - 1:1; Add the hydrogen peroxide solution prepared in step S1 5 min before the end of heat preservation until there is no free ferrous ion in the slurry; The second conversion temperature T2 is 85 - 100 °C.

[0010] Further, in step S4, the residual free ferrous ions are detected by potassium ferricyanide color reaction.

[0011] On the other hand, the present invention provides an iron phosphate material, and the iron-phosphorus ratio (Fe / P) of the iron phosphate material is not less than 0.970.

[0012] Furthermore, impurity elements that are beneficial to broadening the one-dimensional lithium-ion diffusion channels of lithium iron phosphate are doped into the iron phosphate material, and the impurity elements are one or more of Ni, Co, Mn, Mg, V, Ti, Al, Cu, Zn, W, Cr, Zr, La, Mo, Nb, Ga, and Y.

[0013] Furthermore, the doping amount of the impurity elements is 100 - 3000 ppm.

[0014] Furthermore, the characteristic peaks in the XRD of the iron phosphate material correspond one by one to the standard card XX, and there are no characteristic peaks of impurity phases in the XRD pattern; the relative intensity ratio I of the second-strongest characteristic peak (100) to the strongest characteristic peak (102) in the XRD pattern (100) / I (102) ≥0.240.

[0015] Furthermore, in step S5, the methods and equipment for solid-liquid separation and washing of the white slurry C are both common methods and equipment in the iron phosphate industry; the methods and equipment for flash drying and calcination dehydration of the white filter cake D are also common methods and equipment in the iron phosphate industry.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. The purpose of the present invention is to provide a high-iron-phosphorus ratio, specific crystal structure, doped iron phosphate and its preparation method; on the one hand, by utilizing the characteristics that the high-iron-phosphorus ratio of iron phosphate is beneficial to refining the particle size of lithium iron phosphate and shortening the diffusion path of lithium ions; on the other hand, by utilizing the specific crystal structure of iron phosphate to reduce the Li / Fe mixing phenomenon occurring during the solid-phase sintering reaction of lithium iron phosphate, so as to improve the discharge capacity; at the same time, by doping specific impurity elements into the iron phosphate precursor to broaden the lithium-ion transport path in lithium iron phosphate and improve the electronic conductivity of lithium iron phosphate; ultimately achieving the effect of improving the low-temperature performance and rate performance of lithium iron phosphate.

[0017] 2. By adjusting the feeding order of the hydrogen peroxide solution in the synthesis stage of iron phosphate, the present invention prepares an iron phosphate material with a high iron-to-phosphorus ratio. Specifically, the method provided by the present invention first allows the ferrous salt solution and the phosphate salt solution to react to form an iron phosphate intermediate product, and then oxidizes and converts it. The traditional iron phosphate synthesis process is to allow the ferrous solution, the phosphate salt solution, and hydrogen peroxide to come into contact and react simultaneously to form an ammonium iron(II) phosphate hydroxide intermediate product and then perform conversion. Since the theoretical iron-to-phosphorus ratio of this intermediate product, iron(II) phosphate (Fe3(PO4)2·8H2O), is 1.5:1, which is significantly higher than the theoretical iron-to-phosphorus ratio of ammonium iron(II) phosphate hydroxide (NH4Fe2(PO4)2OH·2H2O) (1:1). The conversion path with iron(II) phosphate as the intermediate product is naturally more likely to obtain an iron phosphate material with a higher iron-to-phosphorus ratio compared to the conversion path with ammonium iron(II) phosphate hydroxide as the intermediate product.

[0018] 2. The doping-type iron phosphate preparation method provided by the present invention is more conducive to the incorporation of doping elements. Specifically, the method provided by the present invention first allows the ferrous salt solution and the phosphate salt solution to react, and the pH during its synthesis process is significantly higher than that of the conventional iron phosphate process. One of the main reasons is that the conventional process forms an ammonium iron(II) phosphate hydroxide intermediate product during the synthesis process, and a large amount of NH4 + and OH - enter the precipitate, which will cause the synthesis pH to be at a relatively low level (about 1.8 - 2.2). However, the method provided by the present invention forms an iron phosphate intermediate product during the synthesis process, and NH4 + and OH - do not enter the precipitate, which ensures that the synthesis pH can be at a relatively high level (pH≥2.5). The higher the pH, the better Mn 2+ , Mg 2+ , TiO 2+ , Ni 2+ and other doped metal ions can precipitate and incorporate.

[0019] 3. The iron phosphate provided by the present invention has a better crystal structure, fewer internal crystal defects, and is more conducive to improving the rate performance and low-temperature performance of lithium iron phosphate. Specifically, the conventional iron phosphate process forms an ammonium iron(II) phosphate hydroxide intermediate product during the synthesis process and then converts it into iron(II) phosphate through a conversion reaction. During the conversion process, NH4 + and OH - are removed from the mixed phase of ammonium iron(II) phosphate hydroxide / diiron(III) bis(phosphate) dihydrate, which is more likely to cause defects inside the diiron(III) bis(phosphate) dihydrate crystal. At the same time, due to the specific crystal structure of the iron phosphate provided by the present invention, it can significantly reduce the Li / Fe mixing phenomenon generated during the sintering process of lithium iron phosphate and improve the electrochemical performance of lithium iron phosphate. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 XRD patterns of the iron phosphates prepared in each example and each comparative example; Figure 2 XRD pattern of the lithium iron phosphate prepared from the iron phosphate prepared in Example 1 and Comparative Example 3; Figure 3 Correlation between the relative intensity ratio of the second-strongest peak to the strongest peak of iron phosphate and the 1C capacity of lithium iron phosphate. Detailed implementation manners

[0021] Example 1 Example 1 of the present invention is a titanium-doped iron phosphate with a high iron-to-phosphorus ratio and a specific crystal structure and a preparation method thereof. The preparation process includes the following steps: S1. Raw material preparation: Prepare a ferrous salt solution with pH = 2.3 and a concentration of 1.4 mol / L; a phosphate salt solution with pH = 7.3 and a concentration of 1.4 mol / L; a 30 wt% hydrogen peroxide solution; and an 85 wt% phosphoric acid solution. The molar concentration of titanium element in the ferrous salt solution is 0.0662 mol / L.

[0022] S2. Synthesis reaction: Add 3 L of pure water as the bottom liquid into a 50 L reaction kettle, and simultaneously add 15 L of the ferrous salt solution and 15 L of the phosphate salt solution into the reaction kettle within 40 min. After the feeding is completed, react for 30 min to obtain a blue-green slurry A.

[0023] S3. First-stage conversion: Add 484 g of phosphoric acid solution into the reaction kettle, then heat up to the first conversion temperature of 50°C, add 1.5 L of a 30 wt% hydrogen peroxide solution, and react for 40 min to obtain a white slurry B.

[0024] S4. Second-stage conversion: Heat up the slurry to 93°C and keep it warm for 90 min to obtain a white slurry C. Add a small amount of hydrogen peroxide solution 5 min before the end of the heat preservation until no color reaction occurs after adding the potassium ferricyanide color reagent.

[0025] S5. Flash evaporation and calcination: Transfer the white slurry C obtained in the above steps into a filter press for solid-liquid separation and washing until the conductivity of the washing water ≤ 1000 μS / cm to obtain a white filter cake D. The white filter cake D is obtained as a titanium-doped iron phosphate with a high iron-to-phosphorus ratio and a specific crystal structure after flash drying and calcination dehydration treatment.

[0026] Example 2 Example 2 of the present invention is a titanium-doped iron phosphate with a high iron-to-phosphorus ratio and a specific crystal structure and a preparation method thereof. The preparation process includes the following steps: S1. Raw material preparation: Prepare a ferrous salt solution with pH = 1.9 and a concentration of 1.0 mol / L; a phosphate salt solution with pH = 7.5 and a concentration of 0.9 mol / L; a 30 wt% hydrogen peroxide solution; and an 85 wt% phosphoric acid solution. The molar concentration of titanium in the ferrous salt solution is 0.0315 mol / L.

[0027] S2. Synthesis reaction: Add 3 L of pure water as the bottom liquid into a 50 L reaction kettle, and simultaneously add 15 L of the ferrous salt solution and 16.7 L of the phosphate salt solution into the reaction kettle within 60 min. After the feeding is completed, react for 50 min to obtain a blue-green slurry A.

[0028] S3. First-stage conversion: Add 518 g of phosphoric acid solution into the reaction kettle, then heat up to the first conversion temperature of 55 °C, add 1.1 L of a 30 wt% hydrogen peroxide solution, and react for 60 min to obtain a white slurry B.

[0029] S4. Second-stage conversion: Heat up the slurry to 96 °C and keep it warm for 80 min to obtain a white slurry C. Add a small amount of hydrogen peroxide solution 5 min before the end of the insulation until no color reaction occurs after adding potassium ferricyanide color reagent.

[0030] S5. Flash evaporation and calcination: Transfer the white slurry C obtained in the above steps into a filter press for solid-liquid separation and washing until the conductivity of the washing water ≤ 1000 μS / cm to obtain a white filter cake D. The white filter cake D is obtained as titanium-doped iron phosphate with a high iron-to-phosphorus ratio and a specific crystal structure after flash drying and calcination dehydration treatment.

[0031] Example 3 Example 3 of the present invention is a titanium-doped iron phosphate with a high iron-to-phosphorus ratio and a specific crystal structure and its preparation method. The preparation process includes the following steps: S1. Raw material preparation: Prepare a ferrous salt solution with pH = 2.5 and a concentration of 0.8 mol / L; a phosphate salt solution with pH = 7.2 and a concentration of 1.0 mol / L; a 30 wt% hydrogen peroxide solution; and an 85 wt% phosphoric acid solution. The molar concentration of titanium in the ferrous salt solution is 0.0202 mol / L.

[0032] S2. Synthesis reaction: Add 3 L of pure water as the bottom liquid into a 50 L reaction kettle, and simultaneously add 15 L of the ferrous salt solution and 12 L of the phosphate salt solution into the reaction kettle within 70 min. After the feeding is completed, react for 60 min to obtain a blue-green slurry A.

[0033] S3. First-stage conversion: Add 346 g of phosphoric acid solution into the reaction kettle, then heat up to the first conversion temperature of 60 °C, add 0.9 L of a 30 wt% hydrogen peroxide solution, and react for 50 min to obtain a white slurry B.

[0034] S4, Second-stage conversion: Heat the slurry to 88 °C and hold for 140 min to obtain white slurry C. Add a small amount of hydrogen peroxide solution 5 min before the end of the holding time until no color reaction occurs after adding potassium ferricyanide color reagent.

[0035] S5, Flash evaporation and calcination: Transfer the white slurry C obtained in the above step into a filter press for solid-liquid separation and washing until the conductivity of the washing water ≤ 1000 μS / cm to obtain white filter cake D. The white filter cake D is obtained by flash drying and calcination dehydration treatment to obtain titanium-doped iron phosphate with a high iron-to-phosphorus ratio and a specific crystal structure.

[0036] Example 4 Example 4 of the present invention is a manganese-doped iron phosphate with a high iron-to-phosphorus ratio and a specific crystal structure and its preparation method. The preparation process includes the following steps: S1, Raw material preparation: Prepare a ferrous salt solution with pH = 2.8 and 1.2 mol / L respectively; a phosphate salt solution with pH = 7.0 and 1.2 mol / L respectively; a 30 wt% hydrogen peroxide solution; an 85 wt% phosphoric acid solution. The molar concentration of manganese element in the ferrous salt solution is 0.0526 mol / L.

[0037] S2, Synthesis reaction: Add 3 L of pure water as the bottom liquid into a 50 L reaction kettle, and simultaneously add 15 L of ferrous salt solution, 15 L of phosphate salt solution and 1.4 L of 30 wt% hydrogen peroxide solution into the reaction kettle within 30 min. After the feeding is completed, react for 70 min to obtain yellow slurry A.

[0038] S3, First-stage conversion: Add 374 g of phosphoric acid solution into the reaction kettle, then heat up to the first conversion temperature of 65 °C, and react for 30 min to obtain white slurry B.

[0039] S4, Second-stage conversion: Heat the slurry to 90 °C and hold for 100 min to obtain white slurry C. Add a small amount of hydrogen peroxide solution 5 min before the end of the holding time until no color reaction occurs after adding potassium ferricyanide color reagent.

[0040] S5, Flash evaporation and calcination: Transfer the white slurry C obtained in the above step into a filter press for solid-liquid separation and washing until the conductivity of the washing water ≤ 1000 μS / cm to obtain white filter cake D. The white filter cake D is obtained by flash drying and calcination dehydration treatment to obtain manganese-doped iron phosphate with a high iron-to-phosphorus ratio and a specific crystal structure.

[0041] Comparative Example 1 Comparative Example 1 of the present invention is a titanium-doped iron phosphate and its preparation method. Except that the feeding position of hydrogen peroxide is different from that in Example 1, its preparation process is completely the same as that in Example 1. The specific preparation process includes the following steps: S1. Raw material preparation: Prepare a ferrous salt solution with pH = 1.9 and 1.0 mol / L respectively; a phosphate salt solution with pH = 7.5 and 0.9 mol / L respectively; a 30 wt% hydrogen peroxide solution; an 85 wt% phosphoric acid solution. The molar concentration of titanium element in the ferrous salt solution is 0.0662 mol / L.

[0042] S2. Synthesis reaction: Add 3 L of pure water as the bottom liquid into a 50 L reaction kettle, and simultaneously add 15 L of ferrous salt solution, 16.7 L of phosphate salt solution and 1.1 L of 30 wt% hydrogen peroxide solution into the reaction kettle within 60 min. After the feeding is completed, react for 50 min to obtain yellow slurry A.

[0043] S3. First-stage conversion: Add 518 g of phosphoric acid solution into the reaction kettle, then heat up to the first conversion temperature of 55 °C, and react for 60 min to obtain white slurry B.

[0044] S4. Second-stage conversion: Heat up the slurry to 93 °C and keep it warm for 80 min to obtain white slurry C. Add a small amount of hydrogen peroxide solution 5 min before the end of the heat preservation until there is no color reaction after adding potassium ferricyanide color reagent.

[0045] S5. Flash evaporation and calcination: Transfer the white slurry C obtained in the above steps into a filter press for solid-liquid separation and washing until the conductivity of the washing water ≤ 1000 μS / cm to obtain white filter cake D. The white filter cake D is obtained by flash drying and calcination dehydration treatment to obtain titanium-doped iron phosphate.

[0046] Comparative Example 2 Comparative Example 2 of the present invention is a kind of iron phosphate and its preparation method. Except that no titanium salt is added to the ferrous salt solution, its preparation process is completely the same as that of Comparative Example 1. The specific preparation process includes the following steps: S1. Raw material preparation: Prepare a ferrous salt solution with pH = 1.9 and 1.0 mol / L respectively; a phosphate salt solution with pH = 7.5 and 0.9 mol / L respectively; a 30 wt% hydrogen peroxide solution; an 85 wt% phosphoric acid solution.

[0047] S2. Synthesis reaction: Add 3 L of pure water as the bottom liquid into a 50 L reaction kettle, and simultaneously add 15 L of ferrous salt solution, 16.7 L of phosphate salt solution and 1.1 L of 30 wt% hydrogen peroxide solution into the reaction kettle within 60 min. After the feeding is completed, react for 50 min to obtain yellow slurry A.

[0048] S3. First-stage conversion: Add 518 g of phosphoric acid solution into the reaction kettle, then heat up to the first conversion temperature of 55 °C, and react for 60 min to obtain white slurry B.

[0049] S4. Second-stage conversion: Heat the slurry to 93 °C and hold for 80 min to obtain white slurry C. Add a small amount of hydrogen peroxide solution 5 min before the end of the holding period until no color reaction occurs after adding potassium ferricyanide color reagent.

[0050] S5. Flash evaporation and calcination: Transfer the white slurry C obtained in the above step into a filter press for solid-liquid separation and washing until the conductivity of the washing water ≤ 1000 μS / cm to obtain white filter cake D. The white filter cake D is obtained as titanium-doped iron phosphate after flash drying and calcination dehydration treatment.

[0051] Comparative Example 3 Comparative Example 3 of the present invention is a doped iron phosphate with a high iron-to-phosphorus ratio and a preparation method. Except for the feeding position of hydrogen peroxide and the conversion parameters being different from those in Example 1, the preparation process is completely the same as that in Example 1. Among them, the second-stage conversion temperature is increased to 98 °C and the conversion holding time is extended to 200 min, aiming to increase the iron-to-phosphorus ratio of the prepared iron phosphate to the same level as that in Example 1. The specific preparation process includes the following steps: S1. Raw material preparation: Prepare a ferrous salt solution with pH = 1.9 and 1.0 mol / L respectively; a phosphate salt solution with pH = 7.5 and 0.9 mol / L; a 30 wt% hydrogen peroxide solution; an 85 wt% phosphoric acid solution. The molar concentration of titanium element in the ferrous salt solution is 0.0662 mol / L.

[0052] S2. Synthesis reaction: Add 3 L of pure water as the bottom liquid into a 50 L reaction kettle, and simultaneously add 15 L of ferrous salt solution, 16.7 L of phosphate salt solution, and 1.1 L of 30 wt% hydrogen peroxide solution into the reaction kettle within 60 min. After the feeding is completed, react for 50 min to obtain yellow slurry A.

[0053] S3. First-stage conversion: Add 518 g of phosphoric acid solution into the reaction kettle, then heat to the first conversion temperature of 55 °C and react for 60 min to obtain white slurry B.

[0054] S4. Second-stage conversion: Heat the slurry to 98 °C and hold for 200 min to obtain white slurry C. Add a small amount of hydrogen peroxide solution 5 min before the end of the holding period until no color reaction occurs after adding potassium ferricyanide color reagent.

[0055] S5. Flash evaporation and calcination: Transfer the white slurry C obtained in the above step into a filter press for solid-liquid separation and washing until the conductivity of the washing water ≤ 1000 μS / cm to obtain white filter cake D. The white filter cake D is obtained as titanium-doped iron phosphate with a high iron-to-phosphorus ratio after flash drying and calcination dehydration treatment.

[0056] Comparative Example 4 Comparative Example 4 of the present invention is a manganese-doped iron phosphate and its preparation method. Except that the feeding position of hydrogen peroxide is different from that in Example 1, the preparation process is exactly the same as that in Example 1. The specific preparation process includes the following steps: S1. Raw material preparation: Prepare a ferrous salt solution with pH = 2.8 and 1.2 mol / L; a phosphate solution with pH = 7.0 and 1.2 mol / L; a 30 wt% phosphoric acid solution; an 85 wt% phosphoric acid solution. The molar concentration of manganese element in the ferrous salt solution is 0.0526 mol / L.

[0057] S2. Synthesis reaction: Add 3 L of pure water as the bottom liquid into a 50 L reaction kettle. Add 15 L of the ferrous salt solution and 15 L of the phosphate solution into the reaction kettle simultaneously within 30 min. After the feeding is completed, react for 70 min to obtain a blue-green slurry A.

[0058] S3. First-stage conversion: Add 374 g of the phosphoric acid solution into the reaction kettle, then heat up to the first conversion temperature of 65 °C, add 1.4 L of a 30 wt% hydrogen peroxide solution, and react for 30 min to obtain a white slurry B.

[0059] S4. Second-stage conversion: Heat up the slurry to 90 °C and keep it warm for 100 min to obtain a white slurry C. Add a small amount of hydrogen peroxide solution 5 min before the end of the heat preservation until no color reaction occurs after adding the potassium ferricyanide color reagent.

[0060] S5. Flash evaporation and calcination: Transfer the white slurry C obtained in the above steps into a filter press for solid-liquid separation and washing until the conductivity of the washing water ≤ 1000 μS / cm to obtain a white filter cake D. The white filter cake D is obtained as manganese-doped iron phosphate after flash drying and calcination dehydration treatment.

[0061] Experimental part 1. The method provided by the present invention is beneficial to the improvement of the iron-to-phosphorus ratio: Table 1 shows the iron content, phosphorus content, and iron-to-phosphorus ratio of the iron phosphates prepared in each example and each comparative example. It can be seen from Table 1 that the iron-to-phosphorus ratios of the iron phosphates prepared in Examples 1-4 are 97.24, 97.53, 97.36, and 97.62 respectively, and their iron-to-phosphorus ratios are all at a relatively high level (Fe / P≥97.0), while the iron-to-phosphorus ratio of Comparative Example 1 is only 95.74. The iron-to-phosphorus ratio of Comparative Example 1 is significantly lower than that of Comparative Example 2 without titanium doping (Fe / P = 96.20). On the one hand, it is because the titanium element incorporated during the synthesis process of iron phosphate inhibits the growth and development of iron phosphate crystals. On the other hand, it is because the titanium element occupies the iron site, resulting in a decrease in the iron-to-phosphorus ratio. Compared with the comparative examples, the iron-to-phosphorus ratio of the examples is significantly improved. This is because the different feeding positions of hydrogen peroxide affect the chemical composition of the intermediate product, thereby affecting the iron-to-phosphorus ratio of the iron phosphate product. The method provided by the examples of the present invention is to add phosphoric acid and hydrogen peroxide to convert it into iron phosphate dihydrate (FePO4·2H2O) after the reaction of the ferrous salt solution and the phosphate solution to generate the intermediate product octahydrate ferrous phosphate (Fe3(PO4)2·8H2O). The traditional iron phosphate preparation technology represented by the comparative examples is to add hydrogen peroxide during the mixing process of the ferrous solution and the phosphate solution, and the generated intermediate product is ammonium ferric phosphate hydroxide (NH4Fe2(PO4)2OH·2H2O). The theoretical iron-to-phosphorus ratio of this intermediate product of ferrous phosphate is higher than that of ammonium ferric phosphate hydroxide. The conversion path with ferrous phosphate as the intermediate product is naturally more likely to obtain iron phosphate materials with a higher iron-to-phosphorus ratio than ammonium ferric phosphate hydroxide.

[0062] Table 1 Iron-to-phosphorus ratio data of iron phosphates prepared in each example and each comparative example

[0063] It is worth noting that Examples 4 and Comparative Example 3 used the same raw materials, and the manganese element concentration in the ferrous salt solution was 0.0526 mol / L. However, the manganese element content in the iron phosphate prepared in Example 4 (823 ppm) was significantly higher than that in Comparative Example 3 (214 ppm). This shows that the method provided by the present invention is more conducive to the precipitation and incorporation of metal ions such as 2+ Mn 2+ Mg 2+ TiO 2+ Ni

[0064] II. The method provided by the present invention can obtain iron phosphate with a specific crystal structure: Figure 1Table 2 shows the XRD patterns and related crystal data of the iron phosphates prepared in Examples 1-4 and Comparative Example 3, respectively. The iron phosphates prepared in each example and comparative example are all pure phases, and there are no characteristic peaks of other impurity phases in the XRD patterns. Among them, the full width at half maximum (FWHM) of the diffraction peaks in each example and comparative example was obtained by data fitting. The relative intensity ratios of the second-strongest peak (100) to the strongest peak (102) in Examples 1-4 and Comparative Examples 1-3 are 0.2501, 0.2425, 0.2422, 0.2546, 0.2192, 0.2145, and 0.2173, respectively. Although Comparative Example 3 and Example 1 have similar iron-to-phosphorus ratios (Fe / P = 97.24, 97.30), the relative intensity ratios of the (100) and (102) characteristic peaks between Comparative Example 3 and Example 1 differ significantly. It can be clearly observed from Figure 1 that the relative intensity of the (100) characteristic peak in the XRD pattern of Comparative Example 3 is significantly lower than that of the iron phosphates prepared in Examples 1-4. This is mainly due to the different feeding methods in the comparative example and the example, resulting in the formation of different intermediates during the synthesis of iron phosphate. Even when Comparative Example 3 and Example 1 have similar iron-to-phosphorus ratios (Fe / P ≈ 0.97), there are still significant differences in their crystal structure characteristics, which confirms the key influence of the feeding method on the formation of intermediates and thus determines the specific crystal structure characteristics of the final product.

[0065] Table 2 XRD data of the iron phosphates prepared in each example and each comparative example

[0066] III. The method provided by the present invention can obtain an iron phosphate material with smaller crystal defects and lower dislocation density: The dislocation density of the iron phosphate prepared in Example 1 is 7.35×10 12 m -2 , and the dislocation density of the iron phosphate prepared in Comparative Example 3 is 7.93×10 12 m -2 . Under the condition of the same iron-to-phosphorus ratio, the iron phosphate prepared by the method provided by the present invention has a smaller dislocation density and lower crystal defects. This is because the intermediate product generated by the traditional iron phosphate preparation technology is ammonium iron(II) phosphate hydroxide (NH4Fe2(PO4)2OH·2H2O). During the conversion of ammonium iron(II) phosphate hydroxide to iron phosphate dihydrate, NH4 + , OH - need to be discharged from the crystal interior, and the discharge process will inevitably affect the crystallinity integrity. However, the method provided by the present invention avoids the discharge of NH4 + , OH -The influence of plasma on the crystal structure. Among them, the dislocation density of the iron phosphate crystal is calculated by the following formula. First, the grain size (D) of iron phosphate is calculated by the Debye-Scherrer formula, where λ is the monochromatic Cu-Kα diffraction wavelength (1.5406 Å), β is the full width at half maximum (FWHM) of the diffraction peak, θ is the diffraction angle at the peak of the corresponding diffraction peak, and K is the Scherrer constant (0.98). After calculating the grain sizes (D1, D2,....., D i ), taking the average value can obtain the grain size of iron phosphate (D = D1 + D2 + ··· + D i ) / i. The lattice microstrain (ε) in the iron phosphate crystal can be calculated by the Stokes-Wilson equation. The dislocation density (ρ) in the iron phosphate crystal is calculated based on the grain size (D), lattice microstrain (ε), and the Burgers vector (b) of the characteristic vector determining the dislocation direction.

[0067]

[0068] IV. When using iron phosphate with a specific crystal structure as a precursor to prepare lithium iron phosphate, it can significantly reduce the Li-Fe inversion phenomenon that occurs during the sintering process of lithium iron phosphate, thereby improving the discharge capacity of lithium iron phosphate: Figure 2 The XRD patterns of lithium iron phosphate prepared from the iron phosphates of Examples 1-4, Comparative Example 1, and Comparative Example 3 are shown. In the lithium iron phosphate XRD pattern, the relative intensity ratio between the second-strongest peak (111) characteristic peak and the main-strongest peak (131) can be used to quantify the degree of Li-Fe inversion in the lithium iron phosphate material, I (111) / I (131) The larger the ratio, the lower the degree of cation mixing (Li-Fe) in the lithium iron phosphate material, and the better the electrochemical performance of the material. The I (111) / I (131) ratio of the lithium iron phosphate prepared from the iron phosphate in the examples is significantly higher than that of the comparative examples, indicating that the iron phosphate prepared in the examples can reduce the degree of cation mixing in the lithium iron phosphate material. The occurrence of this phenomenon benefits from the specific crystal structure of the iron phosphate provided by the present invention, that is, the relative intensity ratio between the second-strongest peak (100) characteristic peak and the main-strongest peak (102) of the iron phosphate is ≥ 2.40.

[0069] Figure 3 Shows the relative intensity ratio of the second-strongest peak to the main-strongest peak of iron phosphate (I (100) / I (102)The correlation with the corresponding 1C discharge capacity of lithium iron phosphate. As shown in the figure, it can be clearly observed that as the relative intensity ratio of the secondary strong peak - main strong peak of iron phosphate increases, the 1C discharge capacity of the corresponding lithium iron phosphate increases. The relative intensity ratio of the secondary strong peak - main strong peak of iron phosphate (I (100) / I (102) ) and the corresponding 1C discharge capacity of lithium iron phosphate were subjected to a correlation test. The Pearson correlation coefficient was 0.779, and Sig. (two-tailed) = 0.000 < 0.05, indicating that there is a significant positive correlation between the relative intensity ratio of the secondary strong peak - main strong peak of iron phosphate and the discharge capacity of lithium iron phosphate in a statistical sense. In summary, the method for preparing iron phosphate and the iron phosphate provided by the present invention have a unique crystal structure, and the relative intensity ratio I (100) / I (102) ≥2.40, which can significantly reduce the Li-Fe mixing phenomenon that occurs during the sintering process of lithium iron phosphate, thereby significantly improving the discharge capacity of lithium iron phosphate.

Claims

1. A preparation method of a doped iron phosphate with a high iron-to-phosphorus ratio and a specific crystal structure, characterized in that, It includes the following steps: S1. Raw material preparation: Prepare a ferrous salt solution with a concentration of 0.5 - 2.0 mol / L, a phosphate salt solution with a concentration of 0.5 - 2.0 mol / L, a hydrogen peroxide solution with a mass fraction of 10 - 30 wt%, and a phosphoric acid solution with a mass fraction of 40 - 85 wt% respectively; S2. Synthesis reaction: Add pure water into the reaction kettle as the bottom liquid, and simultaneously add the phosphate salt solution and the ferrous salt solution prepared in step S1 into the reaction kettle within 30 - 70 min. After the feeding is completed, react for 30 - 70 min to obtain a blue - green slurry A; S3. First - stage conversion: Add the phosphoric acid solution prepared in step S1 into the reaction kettle, then heat up to the first conversion temperature T1, and add the hydrogen peroxide solution at the first conversion temperature T1. After reacting for 30 - 70 min, obtain a white slurry B; S4. Second - stage conversion: Heat up the white slurry B to the second conversion temperature T2 and keep it warm for 30 - 180 min to obtain a white slurry C; S5. Flash evaporation and calcination: Perform solid - liquid separation and washing on the white slurry C obtained in step S4 until the conductivity of the washing water ≤ 1000 μS / cm to obtain a white filter cake D; The white filter cake D is obtained through flash drying and calcination dehydration treatment to obtain the doped iron phosphate material with a high iron - phosphorus ratio and a specific crystal structure.

2. The preparation method of a doped iron phosphate with a high iron-to-phosphorus ratio and a specific crystal structure according to claim 1, characterized in that, In step S1, the ferrous salt solution is prepared from ferrous sulfate, ferrous nitrate, ferrous chloride, iron powder or iron sheet; The ferrous salt solution contains one or more corresponding metal ions of Ni, Co, Mn, Mg, V, Ti, Al, Cu, Zn, W, Cr, Zr, La, Mo, Nb, Ga, Y; The phosphate salt solution is prepared from ammonium monohydrogen phosphate, ammonium dihydrogen phosphate, ammonium phosphate, phosphoric acid, sodium monohydrogen phosphate, sodium dihydrogen phosphate or sodium phosphate.

3. The preparation method of a doped iron phosphate with a high iron-to-phosphorus ratio and a specific crystal structure according to claim 1, characterized in that, In step S1, the pH of the ferrous salt solution is 1.0 - 3.0, and the pH of the phosphate salt solution is 6.0 - 8.

0.

4. The preparation method of a doped iron phosphate with a high iron-to-phosphorus ratio and a specific crystal structure according to claim 1, characterized in that, In step S2, the phosphate salt solution and the ferrous salt solution added into the reaction kettle satisfy n(Fe):n(P)=1:0.9 - 1:1.

4.

5. The preparation method of a doped iron phosphate with a high iron-to-phosphorus ratio and a specific crystal structure according to claim 1, characterized in that, In step S3, the phosphoric acid solution and the ferrous salt solution added into the reaction kettle satisfy n(H3PO4):n(Fe)=0.35:1 - 0.10:1; The first conversion temperature T1 is 50 - 70 °C.

6. The preparation method of a doped iron phosphate with a high iron-to-phosphorus ratio and a specific crystal structure according to claim 1, characterized in that, In step S4, the hydrogen peroxide solution and the ferrous salt solution added into the reaction kettle satisfy n(Fe):n(H2O2)=1:0.6 - 1:1; Add the hydrogen peroxide solution prepared in step S1 5 min before the end of heat preservation until there is no free ferrous ion in the slurry; The second conversion temperature T2 is 85 - 100 °C.

7. The preparation method of a doped iron phosphate with a high iron-to-phosphorus ratio and a specific crystal structure according to claim 6, characterized in that, In step S4, detect the residual free ferrous ions through potassium ferricyanide color reaction.

8. A ferric phosphate material prepared by the preparation method of doped ferric phosphate with a high iron-to-phosphorus ratio and a specific crystal structure according to any one of claims 1-7, characterized in that, The iron - phosphorus ratio of the iron phosphate material is not less than 0.

970.

9. The iron phosphate material according to claim 8, characterized in that, The iron phosphate material is doped with impurity elements that are beneficial to broadening the one - dimensional lithium - ion diffusion channel of lithium iron phosphate, and the impurity elements are one or more of Ni, Co, Mn, Mg, V, Ti, Al, Cu, Zn, W, Cr, Zr, La, Mo, Nb, Ga, Y.

10. The iron phosphate material according to claim 9, characterized in that, The doping amount of the impurity elements is 100 - 3000 ppm.

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