A kind of iron phosphate material with different density in cross section and its preparation method and application in preparing lithium iron phosphate

Abstract

A ferrous phosphate material with a sparse and dense cross-section and a characteristic of large and small particle gradation is disclosed. The preparation method thereof is as follows: S1. Preparing a ferrous solution and a phosphate solution; using the ferrous solution as a base liquid, simultaneously adding a hydrogen peroxide solution and a phosphate solution to the ferrous solution to obtain a slurry A, and reacting for 40 to 70 minutes to obtain a slurry B; S2. After solid-liquid separation and washing of the slurry B, a filter cake B is obtained; the filter cake B is slurried with pure water, and a phosphoric acid solution is added, and the temperature is increased and kept warm to obtain a white slurry C; S3. After solid-liquid separation of the slurry C, a filter cake C is obtained; the filter cake C is divided into two parts, C1 and C2; the filter cake C1 is washed to a high wash water conductivity, and the filter cake C2 is washed to a low wash water conductivity, and the two parts are mixed, dried, and calcined to obtain a lithium iron phosphate material; and the lithium iron phosphate material prepared using the ferrous phosphate material with the above-mentioned large and small particle gradation has the characteristic of large and small particle gradation.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and in particular to an iron phosphate material with a sparse and dense cross-section, a preparation method thereof, and an application in preparing lithium iron phosphate. Background Art

[0002] In order to improve the compaction density of lithium iron phosphate, a commonly used technical means is to prepare lithium iron phosphate materials with a certain particle grading, and fill small-sized lithium iron phosphate particles in the gaps between large-sized lithium iron phosphate particles, thereby effectively improving the bulk density and compaction density of the material.

[0003] In order to obtain a lithium iron phosphate material with a combination of large and small particles, the prior art proposes separately preparing lithium iron phosphate with small and large particle sizes, and then mixing them in a certain proportion to obtain a lithium iron phosphate material with a large and small particle size distribution. For example, the patent entitled "A High-Density High-Performance Lithium Iron Phosphate Cathode Material and Preparation Method Thereof" with publication number CN 114497479 A, ​​on the one hand, adds a non-ionic emulsifier during the lithium iron phosphate preparation process to emulsify the slurry and increase the degree of agglomeration between the particles, and then performs a primary sintering at a higher temperature (790-840°C) to obtain lithium iron phosphate with a large particle size. On the other hand, utilizing the inhibitory effect of titanium-containing compounds on lithium iron phosphate particle growth, titanium-containing compounds are added to the slurry and a primary sintering is performed at a lower temperature (730-780°C) to prepare lithium iron phosphate with a small particle size. The two lithium iron phosphate materials of different sizes are then mixed in a certain proportion and subjected to a secondary sintering to obtain a high-density lithium iron phosphate material with a large and small particle size distribution.

[0004] However, the technical route of mixing large and small particles of lithium iron phosphate may have problems with poor batch quality stability. For example, when preparing large-particle lithium iron phosphate materials through high-temperature sintering, the high temperature produces substances such as iron phosphide, resulting in excessive magnetic foreign matter, and the high temperature causes excessive particle growth, resulting in deterioration of electrochemical performance.

[0005] Moreover, the existing technology has the problem that the particle grading characteristics of iron phosphate materials cannot be effectively inherited by lithium iron phosphate materials. The effect of preparing iron phosphate materials with particle grading on improving the compaction density of lithium iron phosphate is limited; this is because when the precursor iron phosphate is coarsely and finely ground, the secondary agglomerate particles will be broken up, and the large and small particle grading characteristics are difficult to retain. After drying and sintering, lithium iron phosphate can only partially inherit or even completely fail to inherit the large and small particle grading characteristics of the precursor. The preparation of large and small particle grading iron phosphate materials in a macroscopic sense is of limited effect on the preparation of high-compacted lithium iron phosphate materials. Summary of the Invention

[0006] In order to avoid the above technical problems, the present invention proposes an iron phosphate material with an uneven cross-section, wherein the iron phosphate material has both secondary loose particles and secondary dense particles. The secondary loose particles are composed of small particles, accounting for 70% to 90%; the secondary dense particles are composed of large particles, accounting for 10% to 30%. The secondary loose particles are particles in which primary particles are loosely agglomerated, and the secondary dense particles are particles in which primary particles are tightly agglomerated. The iron phosphate material has the characteristics of large and small particle grading.

[0007] Furthermore, a method for preparing an iron phosphate material with a dense and sparse cross-section is as follows:

[0008] S1. Prepare 0.5 mol / L to 2.0 mol / L ferrous solution and 0.5 mol / L to 2.0 mol / L phosphate solution; use the ferrous solution as the base solution, and simultaneously add the hydrogen peroxide solution and the phosphate solution to the ferrous solution to obtain slurry A. After reacting for 40 to 70 minutes, obtain yellow slurry B;

[0009] S2. After solid-liquid separation and washing, the slurry B is subjected to filter cake B. The filter cake B is slurried with pure water, and then a phosphoric acid solution is added. The mixture is heated to 80°C to 100°C and kept warm for 40 to 70 minutes to obtain a white slurry C.

[0010] S3. The slurry C is subjected to solid-liquid separation to obtain filter cake C, and the filter cake C is divided into two parts, C1 and C2. The filter cake C1 is washed until the conductivity of the washing water reaches 1000μS / cm~3000μS / cm, and the filter cake C2 is washed until the conductivity of the washing water reaches 100μS / cm~1000μS / cm. The washed filter cakes C1 and C2 are mixed, dried, and calcined to obtain an iron phosphate material with a dense and spaced cross-section; the filter cake C is an iron phosphate dihydrate filter cake; the mass ratio of C1 to the total filter cake C is 10%~30%, and the mass ratio of C2 to the total filter cake C is 70%~90%.

[0011] Furthermore, in step S1, the ferrous solution is prepared from ferrous sulfate, ferrous nitrate, ferrous chloride, iron powder, and iron sheet; the phosphate solution is prepared from ammonium monohydrogen phosphate, ammonium dihydrogen phosphate, ammonium phosphate, phosphoric acid, sodium monohydrogen phosphate, sodium dihydrogen phosphate, and sodium phosphate; the molar amount ratio of hydrogen peroxide in the hydrogen peroxide solution to the molar amount ratio of iron element in the ferrous solution is n(Fe):n(H2O2)=1:0.60~0.70; the molar amount ratio of phosphorus element in the phosphate solution to the molar amount ratio of iron element in the ferrous solution is n(Fe):n(P)=1:0.9~1:1.2; the pH of the phosphate solution is 6.5~8.5, and the pH of the ferrous solution is 2.0~4.0.

[0012] Furthermore, in step S2, the molar ratio of the phosphorus content in the phosphoric acid solution to the iron content in the filter cake B is controlled to be n(Fe):n(P)=0.20~0.60; when the filter cake B is slurried with the phosphoric acid solution and pure water in step S2, the solid content is controlled to be 8%~20%.

[0013] On the other hand, the present invention provides a lithium iron phosphate material prepared using the above-mentioned iron phosphate material with a dense and sparse cross-section. The lithium iron phosphate material has the characteristics of large and small particle grading of the iron phosphate material, including large-particle lithium iron phosphate and small-particle lithium iron phosphate. The large-particle lithium iron phosphate has a size of 500nm~1600nm, and its number accounts for 5%~25% of the entire lithium iron phosphate material; the small-particle lithium iron phosphate has a size of 50nm~400nm, and its number accounts for 95%~75% of the entire lithium iron phosphate material; wherein, the large-particle lithium iron phosphate material is sintered from dense, larger-particle iron phosphate, and the small-particle lithium iron phosphate material is sintered from loose, smaller-particle iron phosphate.

[0014] Compared with the prior art, the present invention has the following beneficial effects:

[0015] 1. The present invention regulates the density of iron phosphate by controlling the washing process of the dihydrate iron phosphate filter cake to obtain an iron phosphate material with alternating density and sparseness, ensuring that a lithium iron phosphate precursor slurry with a wide distribution and a matching of large and small particles can be produced during coarse grinding and fine grinding, and ensuring that a wide particle size distribution is produced during the sintering process of lithium iron phosphate, thereby achieving the purpose of obtaining a lithium iron phosphate material with large and small particle grading characteristics and improving the compaction density.

[0016] 2. The solution provided by the present invention can control the density of iron phosphate by controlling the washing process of the iron phosphate filter cake. The present invention divides the iron phosphate dihydrate filter cake into two parts, one part of the filter cake ensures that the conductivity at the washing end point is at a higher level, and the other part of the filter cake ensures that the conductivity at the washing end point is at a lower level; when the filter cake with higher conductivity at the washing end point is washed, a certain amount of acidic mother liquor will remain in the gaps between the primary particles of the iron phosphate dihydrate, which is conducive to promoting the formation of a dense and compact structure in the iron phosphate dehydration stage; while when the filter cake with lower conductivity at the washing end point is washed, a relatively loose structure will be formed during the dehydration process; specifically, Figure 5As shown in the figure, when the conductivity at the end point of the ferric phosphate dihydrate filter cake washing is controlled at a higher level, part of the conversion mother liquor will be adsorbed or remain between the primary particles of the ferric phosphate dihydrate, and some phosphates that have not participated in the reaction will remain in the conversion mother liquor; during the washing and drying stages, these free phosphates do not enter the crystal structure of the ferric phosphate dihydrate, but in the dehydration stage, these phosphates will re-participate in the reaction and gradually enter the crystal structure of the ferric phosphate. In this process, the fusion between the primary particles of the ferric phosphate will be promoted, thereby making the secondary particles of the ferric phosphate dense and compact.

[0017] 3. The solution provided by the present invention can improve the particle size distribution of the grinding slurry by preparing an iron phosphate material with a dense and spaced cross-section, thereby improving the particle size distribution of the lithium iron phosphate material; specifically, the iron phosphate is mixed with a lithium source and then subjected to coarse grinding and fine grinding to form a lithium iron phosphate precursor slurry with iron phosphate as the skeleton. The precursor slurry formed after grinding the loose and porous iron phosphate has a narrow particle size distribution, a small and concentrated size, while the precursor slurry formed by the iron phosphate with a dense cross-section under the same grinding conditions is larger in size. By taking advantage of the difference in grinding efficiency between loose iron phosphate and dense iron phosphate, the iron phosphate with a dense and spaced cross-section will form a precursor slurry with a wider particle size distribution and both large and small sizes after grinding. The large-scale precursor slurry produced by grinding a dense iron phosphate layer will grow into larger lithium iron phosphate particles during the sintering process; while the small-scale precursor slurry produced by grinding a loose iron phosphate layer will grow into smaller lithium iron phosphate particles during the sintering process, ultimately ensuring that the lithium iron phosphate material has a characteristic large and small particle gradation. There are many gaps between large lithium iron phosphate particles, and a certain amount of small lithium iron phosphate can effectively fill these gaps, thereby effectively improving the bulk density and compaction density of the lithium iron phosphate material. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 This is a scanning cross-sectional image of the anhydrous iron phosphate prepared in Experiment 1;

[0019] Figure 2 This is a scanning cross-sectional image of anhydrous iron phosphate prepared in Experiment 2;

[0020] Figure 3 This is a scanning cross-sectional image of the anhydrous iron phosphate prepared in Experiment 3;

[0021] Figure 4 This is a scanning cross-sectional image of the anhydrous iron phosphate prepared in Experiment 4;

[0022] Figure 5 This is a scanning cross-sectional image of the anhydrous iron phosphate prepared in Experiment 5;

[0023] Figure 6Schematic diagram showing the effect of different washing conditions on the iron-to-phosphorus ratio of iron phosphate;

[0024] Figure 7 Schematic diagram of the effect of different washing conditions on the specific surface of iron phosphate;

[0025] Figure 8 This is a scanned cross-sectional image of the lithium iron phosphate material made from the iron phosphate prepared in Experiment 3;

[0026] Figure 9 Schematic diagram of the effect of residual acidic mother liquor in the filter cake on the density of iron phosphate particles;

[0027] Figure 10 Schematic diagram of the process of making large and small particles of the iron phosphate material prepared in Example 3 of the present invention and combining it with lithium iron phosphate material. DETAILED DESCRIPTION Example 1

[0028] An iron phosphate material with a sparse and dense cross-section, and a preparation method thereof is as follows:

[0029] S1, prepare 0.5 mol / L ferrous solution and 0.5 mol / L phosphate solution; use the ferrous solution as the base solution, add the hydrogen peroxide solution and the phosphate solution to the ferrous solution at the same time to obtain slurry A, and react for 40 min to obtain yellow slurry B, which is basic ammonium ferric phosphate slurry;

[0030] S2. After solid-liquid separation and washing of the slurry B, filter cake B is obtained. Filter cake B is slurried with pure water, and then phosphoric acid solution is added. The mixture is heated to 80° C. and kept warm for 40 minutes to obtain white slurry C; the slurry C is a white ferric phosphate dihydrate slurry;

[0031] S3. The slurry C is subjected to solid-liquid separation to obtain filter cake C, which is divided into two parts, C1 and C2. The filter cake C1 is washed until the conductivity of the washing water reaches 1000 μS / cm, and the filter cake C2 is washed until the conductivity of the washing water reaches 100 μS / cm. The washed filter cakes C1 and C2 are mixed, dried, and calcined to obtain an iron phosphate material with a dense and spaced cross-section; the filter cake C is an iron phosphate dihydrate filter cake; the mass ratio of C1 to the total filter cake C is 10%, and the mass ratio of C2 to the total filter cake C is 90%.

[0032] Moreover, in step S1, the ferrous solution is prepared from ferrous sulfate; the phosphate solution is prepared from ammonium monohydrogen phosphate; the molar ratio of hydrogen peroxide in the hydrogen peroxide solution to the molar ratio of iron in the ferrous solution is n(Fe):n(H2O2)=1:0.60; the molar ratio of phosphorus in the phosphate solution to the molar ratio of iron in the ferrous solution is n(Fe):n(P)=1:0.9; the pH of the phosphate solution is 6.5, and the pH of the ferrous solution is 2.0.

[0033] Moreover, in step S2, the molar ratio of the phosphorus content in the phosphoric acid solution to the iron content in the filter cake B is controlled to be n(Fe):n(P)=0.20; when the filter cake B is slurried with the phosphoric acid solution and pure water in step S2, the solid content is controlled to be 8%. Example 2

[0034] An iron phosphate material with a sparse and dense cross-section, and a preparation method thereof is as follows:

[0035] S1. Prepare 2.0 mol / L ferrous solution and 2.0 mol / L phosphate solution; use the ferrous solution as the base solution, add the hydrogen peroxide solution and the phosphate solution to the ferrous solution simultaneously to obtain slurry A, and react for 70 min to obtain yellow slurry B; the slurry B is basic ammonium ferric phosphate slurry;

[0036] S2. After solid-liquid separation and washing of the slurry B, filter cake B is obtained. The filter cake B is slurried with pure water, and then a phosphoric acid solution is added. The mixture is heated to 100° C. and kept warm for 70 minutes to obtain a white slurry C. The slurry C is a white ferric phosphate dihydrate slurry.

[0037] S3. The slurry C is subjected to solid-liquid separation to obtain filter cake C, which is divided into two parts, C1 and C2. The filter cake C1 is washed until the conductivity of the washing water reaches 3000 μS / cm, and the filter cake C2 is washed until the conductivity of the washing water reaches 1000 μS / cm. The washed filter cakes C1 and C2 are mixed, dried, and calcined to obtain an iron phosphate material with a dense and spaced cross-section; the filter cake C is an iron phosphate dihydrate filter cake; the mass ratio of C1 to the total filter cake C is 30%, and the mass ratio of C2 to the total filter cake C is 70%.

[0038] Moreover, in step S1, the ferrous solution is prepared from ferrous nitrate; the phosphate solution is prepared from ammonium dihydrogen phosphate; the molar ratio of hydrogen peroxide in the hydrogen peroxide solution to the molar ratio of iron in the ferrous solution is n(Fe):n(H2O2)=1:0.70; the molar ratio of phosphorus in the phosphate solution to the molar ratio of iron in the ferrous solution is n(Fe):n(P)=1:1.2; the pH of the phosphate solution is 8.5, and the pH of the ferrous solution is 4.0.

[0039] Moreover, in step S2, the molar ratio of the phosphorus content in the phosphoric acid solution to the iron content in the filter cake B is controlled to be n(Fe):n(P)=0.60; when the filter cake B is slurried with the phosphoric acid solution and pure water in step S2, the solid content is controlled to be 20%. Example 3

[0040] An iron phosphate material with a dense and sparse cross-section, the preparation method is as follows:

[0041] S1. Prepare 1 mol / L ferrous solution and 1 mol / L phosphate solution; use the ferrous solution as the base solution, add the hydrogen peroxide solution and the phosphate solution to the ferrous solution simultaneously to obtain slurry A, and react for 55 minutes to obtain yellow slurry B; the slurry B is basic ammonium ferric phosphate slurry;

[0042] S2. After solid-liquid separation and washing of the slurry B, filter cake B is obtained. Filter cake B is slurried with pure water, and then phosphoric acid solution is added. The mixture is heated to 90° C. and kept warm for 40 to 70 minutes to obtain white slurry C. The slurry C is a white ferric phosphate dihydrate slurry.

[0043] S3. The slurry C is subjected to solid-liquid separation to obtain filter cake C, which is divided into two parts, C1 and C2. The filter cake C1 is washed until the conductivity of the washing water reaches 2000 μS / cm, and the filter cake C2 is washed until the conductivity of the washing water reaches 500 μS / cm. The washed filter cakes C1 and C2 are mixed, dried, and calcined to obtain an iron phosphate material with a dense and sparse cross-section; the filter cake C is an iron phosphate dihydrate filter cake; the mass ratio of C1 to the total filter cake C is 20%, and the mass ratio of C2 to the total filter cake C is 80%.

[0044] Moreover, in step S1, the ferrous solution is prepared from iron powder; the phosphate solution is prepared from ammonium phosphate; the molar ratio of hydrogen peroxide in the hydrogen peroxide solution to the molar ratio of iron in the ferrous solution is n(Fe):n(H2O2)=1:0.65; the molar ratio of phosphorus in the phosphate solution to the molar ratio of iron in the ferrous solution is n(Fe):n(P)=1:1; the pH of the phosphate solution is 7.5, and the pH of the ferrous solution is 3.0.

[0045] Moreover, in step S2, the molar ratio of the phosphorus content in the phosphoric acid solution to the iron content in the filter cake B is controlled to be n(Fe):n(P)=0.40; when the filter cake B is slurried with the phosphoric acid solution and pure water in step S2, the solid content is controlled to be 15%. Example 4

[0046] A lithium iron phosphate material is prepared using an iron phosphate material with a dense and sparse cross-section prepared in Example 3. The lithium iron phosphate material has the characteristics of a particle size gradation of the iron phosphate material, including large-particle lithium iron phosphate and small-particle lithium iron phosphate. The large-particle lithium iron phosphate has a size of 500nm~1600nm, and its number accounts for 25% of the entire lithium iron phosphate material; the small-particle lithium iron phosphate has a size of 50nm~400nm, and its number accounts for 75% of the entire lithium iron phosphate material; wherein the large-particle lithium iron phosphate material is sintered from a dense, larger-particle iron phosphate, and the small-particle lithium iron phosphate material is sintered from a loose, smaller-particle iron phosphate. Example 5

[0047] A lithium iron phosphate material is prepared using an iron phosphate material with a dense and sparse cross-section prepared in Example 3. The lithium iron phosphate material has the characteristics of a large and small particle size distribution of the iron phosphate material, including large-particle lithium iron phosphate and small-particle lithium iron phosphate. The large-particle lithium iron phosphate has a size of 500nm~1600nm, and its number accounts for 5% of the entire lithium iron phosphate material; the small-particle lithium iron phosphate has a size of 50nm~400nm, and its number accounts for 95% of the entire lithium iron phosphate material; wherein, the large-particle lithium iron phosphate material is sintered from dense, larger-particle iron phosphate, and the small-particle lithium iron phosphate material is sintered from loose, smaller-particle iron phosphate. Example 6

[0048] A lithium iron phosphate material is prepared using an iron phosphate material with a dense and sparse cross-section prepared in Example 3. The lithium iron phosphate material has the characteristics of a large and small particle size distribution of the iron phosphate material, including large-particle lithium iron phosphate and small-particle lithium iron phosphate. The large-particle lithium iron phosphate has a size of 500nm~1600nm, and its number accounts for 15% of the entire lithium iron phosphate material; the small-particle lithium iron phosphate has a size of 50nm~400nm, and its number accounts for 85% of the entire lithium iron phosphate material; wherein, the large-particle lithium iron phosphate material is sintered from dense, larger-particle iron phosphate, and the small-particle lithium iron phosphate material is sintered from loose, smaller-particle iron phosphate.

[0049] Experimental part

[0050] Experiment 1

[0051] A preparation process of an iron phosphate material having a dense and sparse cross-section includes the following steps:

[0052] (1) Prepare 200 L of ferrous sulfate solution with a pH of 3.0 and a molar concentration of 1.2 mol / L, place it in a reactor with a volume of 500 L as the bottom liquid, start feeding, and add 19.2 kg of hydrogen peroxide solution with a mass concentration of 28 wt% into the reactor within 60 min. When the hydrogen peroxide solution starts to be added, 210 L of ammonium dihydrogen phosphate solution with a molar concentration of 1.2 mol / L is added into the reactor within 60 min. After the hydrogen peroxide solution and phosphate solution are added, continue stirring and reacting for 45 min to obtain slurry B;

[0053] (2) The slurry B was filtered and washed by a filter press to obtain filter cake B, which was mixed with 300 L of pure water to prepare a slurry, and 5.5 kg of 85 wt% phosphoric acid solution was added. The mixture was then heated to 90°C and reacted at this temperature for 120 min to obtain a white slurry C.

[0054] (3) The 10% slurry C was pumped into a filter press for filter pressing, and the filter cake was washed with pure water until the conductivity of the washing water reached 2500 μS / cm to obtain filter cake C1; at the same time, the 90% slurry C was pumped into a filter press for filter pressing, and the filter cake was washed with pure water until the conductivity of the washing water reached about 600 μS / cm to obtain filter cake C2; ​​the filter cakes C1 and C2 were simultaneously dried in a flash dryer to obtain dihydrated iron phosphate, which was then calcined in a rotary kiln to obtain anhydrous iron phosphate.

[0055] Figure 1 This is a scanning electron microscope cross-sectional view of the anhydrous ferric phosphate prepared in an embodiment of the present invention. It can be observed from the figure that the anhydrous ferric phosphate prepared in Experiment 1 has a sparse and dense structure in the cross-section, and has secondary particles with relatively loose primary particle agglomeration and secondary particles with relatively dense primary particle agglomeration.

[0056] Experiment 2

[0057] A preparation process of an iron phosphate material having a dense and sparse cross-section includes the following steps:

[0058] (1) Prepare 200 L of ferrous sulfate solution with a pH of 3.0 and a molar concentration of 1.2 mol / L, place it in a reactor with a volume of 500 L as the bottom liquid, start feeding, and add 19.2 kg of hydrogen peroxide solution with a mass concentration of 28 wt% into the reactor within 60 min; when the hydrogen peroxide solution starts to be added, 210 L of ammonium dihydrogen phosphate solution with a molar concentration of 1.2 mol / L is added into the reactor within 60 min; after the hydrogen peroxide solution and phosphate solution are added, continue stirring and react for 45 min to obtain slurry B;

[0059] (2) The slurry B was filtered and washed by a filter press to obtain filter cake B, which was mixed with 300 L of pure water to prepare a slurry, and 5.5 kg of 85 wt% phosphoric acid solution was added. The mixture was then heated to 90°C and reacted at this temperature for 120 min to obtain slurry C.

[0060] (3) The above 30% slurry C is pumped into a filter press for filter pressing treatment, and the filter cake is washed with pure water until the conductivity of the washing water reaches 2500 μS / cm to obtain filter cake C1; at the same time, the above 70% slurry C is pumped into a filter press for filter pressing treatment, and the filter cake is washed with pure water until the conductivity of the washing water reaches about 600 μS / cm to obtain filter cake C2; ​​filter cakes C1 and C2 are simultaneously put into a flash dryer for drying to obtain dihydrated iron phosphate, and the dihydrated iron phosphate is calcined in a rotary kiln to obtain anhydrous iron phosphate.

[0061] Figure 2 This is a scanning electron microscope cross-sectional view of the anhydrous ferric phosphate prepared in an embodiment of the present invention. It can be observed from the figure that the anhydrous ferric phosphate prepared in Experiment 2 has a sparse and dense structure in the cross-section, and has secondary particles with relatively loose primary particle agglomeration and secondary particles with relatively dense primary particle agglomeration.

[0062] Experiment 3

[0063] A preparation process of an iron phosphate material having a dense and sparse cross-section includes the following steps:

[0064] (1) Prepare 200 L of ferrous sulfate solution with a pH of 3.0 and a molar concentration of 1.2 mol / L, place it in a reactor with a volume of 500 L as the bottom liquid, start feeding, and add 19.2 kg of hydrogen peroxide solution with a mass concentration of 28 wt% into the reactor within 60 min; when the hydrogen peroxide solution starts to be added, add 210 L of monoammonium phosphate solution with a molar concentration of 1.2 mol / L into the reactor within 60 min. After the hydrogen peroxide solution and phosphate solution are added, continue stirring and react for 45 min to obtain slurry B;

[0065] (2) After slurry B was filtered and washed by a filter press, filter cake B was obtained. Filter cake B was mixed with 300 L of pure water to prepare a slurry, and 5.5 kg of 85 wt% phosphoric acid solution was added. The mixture was then heated to 90 °C and reacted at this temperature for 120 min to obtain a white slurry C.

[0066] (3) The 20% slurry C was pumped into a filter press for filter pressing, and the filter cake was washed with pure water until the conductivity of the washing water reached 2500 μS / cm to obtain filter cake C1; at the same time, the 80% slurry C was pumped into a filter press for filter pressing, and the filter cake was washed with pure water until the conductivity of the washing water reached about 600 μS / cm to obtain filter cake C2; ​​the filter cakes C1 and C2 were simultaneously dried in a flash dryer to obtain dihydrated iron phosphate, which was then calcined in a rotary kiln to obtain anhydrous iron phosphate.

[0067] Figure 3 This is a scanning electron microscope cross-sectional view of the anhydrous ferric phosphate prepared in Experiment 3. It can be observed from the figure that the anhydrous ferric phosphate prepared in Experiment 3 has a sparse and dense structure in the cross-section, and has secondary particles with relatively loose primary particle agglomeration and secondary particles with relatively dense primary particle agglomeration.

[0068] Experiment 4

[0069] In Experiment 4, the preparation process was identical to that of Experiment 1, except that the washing process of slurry C differed from that of Experiment 2. Specifically, during the filter press washing process of slurry C in this experiment, the filter cake mass with high conductivity (2500 μS / cm) at the washing end point accounted for 5%, and the filter cake mass with low conductivity (600 μS / cm) at the washing end point accounted for 95%.

[0070] Figure 4 This is a scanning electron microscope cross-sectional view of anhydrous ferric phosphate prepared in Experiment 4 of the present invention. Figure 4 It can be observed that the anhydrous iron phosphate prepared in Experiment 4 has a loose and porous structure, and basically no secondary particles with dense cross-sections exist; compared with Experiments 1 to 3, it can be found that if too much white filter cake is washed to a lower conductivity, iron phosphate particles with dense cross-sections cannot be obtained.

[0071] Experiment 5

[0072] In Experiment 5, the preparation process was identical to that of Experiment 2, except that the washing process of slurry C differed from that of Experiment 2. Specifically, during the filter press washing process of slurry C in this experiment, the filter cake mass with high conductivity (2500 μS / cm) at the end point of washing water accounted for 35%, and the filter cake mass with low conductivity (600 μS / cm) at the end point of washing water accounted for 65%.

[0073] Figure 5 This is a scanning electron microscope cross-section of anhydrous iron phosphate prepared in Experiment 5. Figure 5 It can be observed that the anhydrous iron phosphate prepared in Experiment 5 has only a small amount of pores and has a dense and compact structure. Compared with Experiments 1 to 3, it can be found that if too much filter cake is washed to a higher conductivity, the number of iron phosphate particles with a loose cross-section will be too small.

[0074] It can be seen from Experiments 1 to 5 that Experiments 1 to 3 adopt the technical solution of the present invention, and the anhydrous ferric phosphate prepared has a structure with a sparse and dense cross-section, and has secondary particles with relatively loose primary particles and relatively dense primary particles. However, in Experiment 5, the filter cake with high conductivity at the end of washing water accounts for 35% by mass, and the filter cake with low conductivity at the end of washing water accounts for 65% by mass. The anhydrous ferric phosphate prepared has only a small amount of pores and too much dense and compact structure. In Experiment 4, the filter cake with high conductivity at the end of washing water accounts for 5% by mass, and the filter cake with low conductivity (600) at the end of washing water accounts for 10%. μS / cm) accounted for 95% by mass of the filter cake, and the prepared anhydrous ferric phosphate had a loose and porous structure, and basically no secondary particles with dense cross-sections existed; that is, starting from 10%, in the process of washing the white filter cake, washing more filter cake until the conductivity of the washing water was at a higher level could effectively increase the proportion of dense particles in the anhydrous ferric phosphate sample, but after exceeding 30%, the effect began to decline; it can be proved that the technical solution of the present invention is the best solution.

[0075] Experiment 6

[0076] The physical and chemical indexes of the iron phosphate prepared in Experiments 1 to 5 were tested, as shown in Table 1. In the washing process of the white filter cake in Experiments 1, 2, 3, 4, and 5, 10%, 30%, 20%, 5%, and 60% of the filter cake were washed to a higher conductivity (2500 μS / cm), respectively. Figure 6 As shown in Table 1, the iron-phosphorus ratios of the iron phosphates prepared in Experiment 1, Experiment 2, Experiment 3, Experiment 4, and Experiment 5 are 0.9640, 0.9613, 0.9639, 0.9680, and 0.9534, respectively. As the mass proportion of the filter cake washed to high conductivity increases, the iron-phosphorus ratio of the corresponding finished iron phosphate product shows a downward trend. This is because when the conductivity at the end of washing is at a high level, some phosphate will remain in the filter cake, which will cause the phosphorus content of the finished product to increase and the iron-phosphorus ratio to decrease. As shown in Table 1 and Figure 7 As shown in the table, the specific phosphates of the iron phosphate prepared in Experiment 1, Experiment 2, Experiment 3, Experiment 4, and Experiment 5 are 9.27, 8.35, 8.83, 8.82, and 4.21 m 2 / g. As the proportion of filter cake washed to high conductivity increases, the corresponding iron phosphate product specific gravity decreases.

[0077] Table 1. Main physical and chemical indicators of iron phosphate prepared in experiments 1 to 5

[0078]

[0079] Experiment 7

[0080] The anhydrous iron phosphate prepared in Experiments 1 to 5 was made into lithium iron phosphate material and a button cell was made. The electrochemical performance and powder compaction density were tested. The test results are shown in Table 2.

[0081] Table 2 Lithium iron phosphate performance test results

[0082]

[0083] After the iron phosphate prepared in Experiment 1, Experiment 2, Experiment 3, Experiment 4, and Experiment 5 was made into lithium iron phosphate, the compacted density of the powder was 2.547, 2.556, 2.551, 2.410, and 2.303 g / cm, respectively. 3 In experiments 1-3, 10%-30% of the white filter cake was washed to a higher endpoint conductivity, and 70%-90% of the white filter cake was washed to a lower endpoint conductivity. The prepared iron phosphate had a structure with a mix of large and small particles and a dense and sparse cross-section. Large iron phosphate particles with a dense cross-section could grow into larger lithium iron phosphate particles; small iron phosphate particles with a loose cross-section could grow into smaller lithium iron phosphate particles, thus ensuring that the lithium iron phosphate had a structure with a mix of large and small particles, thereby showing a higher compaction density (≥2.54 g / cm 3 ), 0.1C discharge capacity and 1C discharge capacity are also relatively high.

[0084] Figure 8 The scanning cross-section of the lithium iron phosphate material made from the iron phosphate prepared in Experiment 3 is shown. It can be observed that there are both large and small lithium iron phosphate particles in the figure, which further confirms that the iron phosphate prepared according to the technical solution provided by the present invention can show the structural characteristics of a combination of large and small particles after being made into lithium iron phosphate. In Experiment 4, since the iron phosphate is loose and porous inside and lacks dense and compact large iron phosphate particles, it is difficult to produce large-sized lithium iron phosphate particles when sintered into lithium iron at the same iron lithium sintering temperature as Experiments 1 to 3. Therefore, the compaction density is relatively low (2.410 g / cm 3 ). Although the sample has high 0.1C discharge capacity and 1C discharge capacity, its relatively low compaction density is not conducive to improving the energy density of the battery end. However, the compaction density of Examples 1-3 is high, and the 0.1C and 1C discharge capacities are relatively high, which is conducive to improving the energy density of lithium iron phosphate batteries. In Experiment 5, since the interior of the iron phosphate is a dense and compact structure and lacks small iron phosphate particles, when the lithium iron phosphate is sintered, small-sized particles are lacking in the lithium iron phosphate, and the gaps between large particles are not effectively filled, resulting in a low compaction density (2.303 g / cm 3 ), and because the lithium ion diffusion path of large-particle lithium iron phosphate is long, its electrochemical performance is poor.

Claims

1. An iron phosphate material with a cross-section having a density difference, characterized in that: The iron phosphate material contains both secondary loose particles and secondary dense particles. The secondary loose particles are composed of small particles, accounting for 70%-90%; the secondary dense particles are composed of large particles, accounting for 10%-30%. The secondary loose particles are particles in which the primary particles are loosely agglomerated, and the secondary dense particles are particles in which the primary particles are tightly agglomerated. The iron phosphate material has the characteristic of large and small particle gradation. The preparation method of the iron phosphate material with a dense and sparse cross-section is as follows: S1. Prepare 0.5 mol / L-2.0 mol / L ferrous solution and 0.5 mol / L-2.0 mol / L phosphate solution; Using ferrous solution as the base liquid, hydrogen peroxide solution and phosphate solution are added to the ferrous solution simultaneously to obtain slurry A. After reacting for 40-70 minutes, yellow slurry B is obtained; the slurry B is basic ammonium ferric phosphate slurry; S2. After solid-liquid separation and washing of the slurry B, filter cake B is obtained. The filter cake B is slurried with pure water, and then a phosphoric acid solution is added. The mixture is heated to 80° C.-100° C. and kept warm for 40-70 min to obtain a white slurry C. The slurry C is a white ferric phosphate dihydrate slurry. S3. The slurry C is subjected to solid-liquid separation to obtain filter cake C, which is divided into two parts, C1 and C2. The filter cake C1 is washed until the conductivity of the washing water reaches 1000μS / cm-3000μS / cm, and the filter cake C2 is washed until the conductivity of the washing water reaches 100μS / cm-1000μS / cm. The washed filter cakes C1 and C2 are mixed, dried, and calcined to obtain an iron phosphate material with a dense and spaced cross-section; the filter cake C is an iron phosphate dihydrate filter cake; the mass ratio of C1 to the total filter cake C is 10%-30%, and the mass ratio of C2 to the total filter cake C is 70%-90%.

2. The iron phosphate material with a sparse and dense cross section according to claim 1, characterized in that: In step S1, the ferrous solution is prepared from ferrous sulfate, ferrous nitrate, ferrous chloride, iron powder, and iron sheet; the phosphate solution is prepared from ammonium monohydrogen phosphate, ammonium dihydrogen phosphate, and ammonium phosphate; the molar ratio of hydrogen peroxide in the hydrogen peroxide solution to the molar ratio of iron in the ferrous solution is n(Fe):n(H2O2)=1:0.60-0.70; the molar ratio of phosphorus in the phosphate solution to the molar ratio of iron in the ferrous solution is n(Fe):n(P)=1:0.9-1:1.2; the pH of the phosphate solution is 6.5-8.5, and the pH of the ferrous solution is 2.0-4.

0.

3. The iron phosphate material with a sparse and dense cross section according to claim 1, characterized in that: In step S2, the molar ratio of the phosphorus content in the phosphoric acid solution to the iron content in the filter cake B is controlled to be n(Fe):n(P)=0.20-0.60; when the filter cake B is slurried with the phosphoric acid solution and pure water in step S2, the solid content is controlled to be 8%-20%.

4. A lithium iron phosphate material prepared using the iron phosphate material with a dense and sparse cross section according to claim 1, characterized in that: The lithium iron phosphate material has the characteristics of particle size gradation of iron phosphate material, including large-particle lithium iron phosphate and small-particle lithium iron phosphate. The size of the large-particle lithium iron phosphate is 500nm-1600nm, and the number accounts for 5%-25% of the entire lithium iron phosphate material; the size of the small-particle lithium iron phosphate is 50nm-400nm, and the number accounts for 95%-75% of the entire lithium iron phosphate material; wherein, the large-particle lithium iron phosphate material is sintered from dense, large-particle iron phosphate, and the small-particle lithium iron phosphate material is sintered from loose, small-particle iron phosphate.

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