Lithium manganese iron phosphate positive electrode material with low specific surface area, high compaction and high capacity

Abstract

The invention discloses a lithium manganese iron phosphate positive electrode material with low specific surface area, high compaction and high capacity, which is prepared by the following steps: placing a manganese source, an iron source, a phosphorus source and a lithium source in deionized water, stirring, grinding and drying to obtain lithium manganese iron phosphate precursor powder; calcining the lithium iron manganese phosphate precursor powder in an inert atmosphere at a low temperature of 450-550 DEG C to prepare a small-particle-size lithium iron manganese phosphate crystal nucleus; calcining the lithium iron manganese phosphate precursor powder in an inert atmosphere at a high temperature of 650-750 DEG C to prepare a large-particle-size lithium iron manganese phosphate crystal nucleus; the temperature difference between the high-temperature condition and the low-temperature condition is not less than 200 DEG C; mixing the large-particle-size lithium manganese iron phosphate crystal nucleus with the small-particle-size lithium manganese iron phosphate crystal nucleus according to a specific mass ratio to prepare a lithium manganese iron phosphate pre-sintered material; calcining the lithium manganese iron phosphate pre-sintered material and a carbon source in an inert atmosphere at 750-800 DEG C to prepare a lithium manganese iron phosphate positive electrode material; according to the lithium manganese iron phosphate positive electrode material, high compactness and low specific surface area can be achieved while the electrochemical performance is ensured, so that the overall structural stability of the battery is improved.

Description

Technical Field

[0001] The present invention belongs to the field of cathode materials for batteries, and particularly relates to a lithium iron manganese phosphate cathode material with low specific surface area, high tap density and high capacity. Background Art

[0002] LiMn x Fe 1-x PO 4 (0 < x < 1) (LMFP) has a high-voltage discharge plateau of 4.1 V, which is much higher than the 3.4 V discharge plateau of LiFePO 4 (LFP). The high voltage further brings a high energy density. The energy density of LMFP is increased by 15% - 20% compared with that of LFP. The energy density of LMFP can reach the level of ternary 523 or even ternary 622, showing significant advantages over LFP. However, the electronic conductivity of LMFP differs from that of LFP by 10 -4 S / cm, resulting in its inferior electrochemical performance compared with LFP.

[0003] To solve the problem of poor electrochemical performance, currently, mainly three improvement measures are adopted: grain nanocrystallization, carbon coating and ion doping. The essence of grain nanocrystallization of LMFP is to shorten the Li + ion diffusion path by reducing the grain size, so as to improve the migration of Li + ions for efficient deintercalation and intercalation, and improve the rate performance of the battery. At the same time, grain nanocrystallization also increases the specific surface area of LMFP, increases the reaction area with the electrolyte, and improves the electrochemical performance. The principle of carbon coating is to construct a conductive network by using conductive carbon to construct a fast conductive network with lithium iron manganese phosphate, enabling electrons to migrate rapidly between active substances during charge and discharge, reducing the internal resistance and charge-discharge polarization of the battery, and also increasing the specific surface area of LMFP. Ion doping is to introduce defects, increase active sites, and improve the conductivity and ion diffusion performance. Mg 2+ 、V 5+ 、Ti 4+ are the three most widely used elements, and the introduction of V 5 + 、Ti 4+ will cause the nanocrystallization of LMFP grains and increase the specific surface area of LMFP.

[0004] Although grain nanocrystallization, carbon coating and ion doping improve the electrochemical performance, the specific surface area of the prepared LMFP is 20 - 30 m 2 / g, much higher than the specific surface area of LFP which is 9 - 13 m 2 / g. The nanocrystallization of grains results in a tap density of its powder of 1.8 - 2.0 g / cm 3 , much lower than the tap density of LFP powder which is 2.5 - 2.6 g / cm 3, while the cathode material with a large BET is prone to particle agglomeration and uneven dispersion during the slurry mixing process, and the poor coating uniformity brings more serious problems such as poor slurry mixing and coating processability at the front end of cell preparation and low volumetric energy density, seriously affecting its application.

[0005] Based on this, there is an urgent need to study a lithium iron manganese phosphate cathode material, which can not only effectively solve the problem of poor electrochemical performance of lithium iron manganese phosphate, but also avoid the increase in specific surface area and the decrease in compaction, and solve the problems of poor slurry mixing and coating processability at the front end of cell preparation and low volumetric energy density. Summary of the Invention

[0006] Object of the Invention: The technical problem to be solved by the present invention is to provide a lithium iron manganese phosphate cathode material, which has a low specific surface area, high compaction and high capacity, and can solve the problems of poor slurry mixing and coating processability at the front end of cell preparation and low volumetric energy density.

[0007] Technical Solution: A lithium iron manganese phosphate cathode material with a low specific surface area, high compaction and high capacity of the present invention is characterized by being prepared by the following steps:

[0008] (1) Placing a manganese source, an iron source, a phosphorus source and a lithium source in deionized water, and obtaining a lithium iron manganese phosphate precursor powder through stirring, grinding and drying;

[0009] (2) Calcining the lithium iron manganese phosphate precursor powder under an inert atmosphere at a low temperature of 450 - 550 °C for 4 - 15 h to obtain small particle size lithium iron manganese phosphate crystal nuclei;

[0010] (3) Calcining the lithium iron manganese phosphate precursor powder under an inert atmosphere at a high temperature of 650 - 750 °C for 4 - 15 h to obtain large particle size lithium iron manganese phosphate crystal nuclei;

[0011] (4) Mixing the large particle size lithium iron manganese phosphate crystal nuclei prepared in step (3) with the small particle size lithium iron manganese phosphate crystal nuclei prepared in step (2) according to a mass ratio of x:(1 - x), where 0.05 < x < 0.45, to obtain a lithium iron manganese phosphate pre-calcined material;

[0012] (5) Placing the lithium iron manganese phosphate pre-calcined material and a carbon source in deionized water, after stirring, grinding and drying, and calcining at 750 - 800 °C under an inert atmosphere for 10 - 25 h to obtain the lithium iron manganese phosphate cathode material;

[0013] Wherein, in step (2) and step (3), the temperature difference between the high temperature condition and the low temperature condition is not less than 200 °C.

[0014] The present invention adopts two - stage calcination to prepare a lithium iron manganese phosphate cathode material with low specific surface area, high tap density and high capacity. In the first stage, high - temperature and low - temperature conditions (with a temperature difference of at least 200°C between them) are respectively adopted to obtain large - particle lithium iron manganese phosphate crystal nuclei with uniform and smooth - round particles and small lithium iron manganese phosphate crystal nuclei with non - uniform particles and rough surfaces; and the two are mixed and blended in a specific proportion to form crystal nuclei with size gradation. Then, during the second high - temperature calcination, the two kinds of lithium iron manganese phosphate crystal nuclei grow further simultaneously. The large - particle lithium iron manganese phosphate crystal nuclei are formed into large - particle lithium iron manganese phosphate after one - stage sintering and then further formed after two - stage sintering. The small - particle lithium iron manganese phosphate crystal nuclei are also formed into small - particle lithium iron manganese phosphate after one - stage sintering and then further formed after two - stage sintering. Finally, a lithium iron manganese phosphate cathode material with size - particle gradation can be formed, improving the tap density and electrical properties of the prepared cathode material, reducing the specific surface area, solving the poor slurry - making and coating processability at the front end of cell preparation, and increasing the volume energy density. Among them, when the temperature difference between the two is 200°C or more, the particle size of the small - particle lithium iron manganese phosphate crystal nuclei is significantly different from that of the large - particle lithium iron manganese phosphate crystal nuclei. Then, after the mixture of the large - and small - particle lithium iron manganese phosphate crystal nuclei is sintered at high temperature for the second time, the crystal nuclei grow simultaneously to form the closest packing.

[0015] And at the same time, during the second high - temperature calcination, the secondary calcination temperature is close to the temperature for forming large - particle lithium iron manganese phosphate crystal nuclei. The volume growth rate of the large - particle lithium iron manganese phosphate crystal nuclei is small, while the crystal growth rate of the small - particle lithium iron manganese phosphate crystal nuclei is large, enabling the small - particle lithium iron manganese phosphate crystal nuclei to continuously fill the gaps between the large - particle lithium iron manganese phosphate crystal nuclei; based on this, a carbon source is introduced as a reducing agent. In the whole system, the addition of the carbon source can reduce Fe 3+ (ion radius is about 0.064nm) ions in the lithium iron manganese phosphate crystal nuclei to Fe 2+ (ion radius is about 0.078nm), further promoting the total unit cell volume of the small - particle lithium iron manganese phosphate crystal nuclei, which is much more numerous than that of the large - particle lithium iron manganese phosphate, to be greater than the total unit cell volume of the large - particle ones. On the basis of their simultaneous growth, the small - particle lithium iron manganese phosphate crystal nuclei with a large total crystal volume can further fill the gaps between the large and small particles, thereby obtaining a lithium iron manganese phosphate cathode material with low specific surface area and high tap density.

[0016] Furthermore, the molar ratio of the manganese source, iron source, phosphorus source, and lithium source used in the lithium iron manganese phosphate cathode material of the present invention can be a:(1 - a):b:c, where 0.05 ≤ a ≤ 0.95; 1 ≤ b ≤ 1.05; 1 ≤ c ≤ 1.08.

[0017] Furthermore, the manganese source used in the lithium iron manganese phosphate cathode material of the present invention includes at least one of manganese carbonate, manganese tetroxide, manganese salts, manganese iron oxides or manganese iron precursors. The iron source includes at least one of iron phosphate, iron tetroxide, iron salts, manganese iron oxides or manganese iron precursors. The phosphorus source includes at least one of phosphoric acid, ammonium dihydrogen phosphate, lithium phosphate or ammonium monophosphate. The lithium source includes at least one of lithium carbonate, lithium hydroxide, lithium sulfate, lithium nitrate or lithium chloride precursors.

[0018] Furthermore, the addition amount of the carbon source used in the lithium iron manganese phosphate cathode material of the present invention is 5-15% of the mass of the theoretically generated lithium iron manganese phosphate. Preferably, the carbon source can include at least one of glucose, sucrose, starch, cellulose, polyvinyl alcohol, soluble starch, cellulose, ascorbic acid or phenolic resin.

[0019] Furthermore, the stirring during the preparation of the lithium iron manganese phosphate cathode material of the present invention is carried out at 0-25°C and a frequency of 20-50 Hz for 0.5-2 h. The grinding time is 0.5-6 h, and the drying temperature is 150-250°C.

[0020] Beneficial effects: Compared with the prior art, the significant advantages of the present invention are as follows: The lithium iron manganese phosphate cathode material of the present invention forms a lithium iron manganese phosphate cathode material with a compact structure through the grading of large and small particle sizes. The formation of different particle size gradings results in a high-compactness lithium iron manganese phosphate, which, while ensuring electrochemical performance, achieves an appropriate compactness and specific surface area, thereby not only solving the problems of poor homogenization and coating processability at the front end of cell preparation and low volume energy density, but also improving the overall structural stability of the battery. Specifically, the 0.1C charge capacity of the prepared lithium iron manganese phosphate cathode material can reach 164.78 mAh / g, the 0.1C discharge capacity can reach 163.56 mAh / g, the first efficiency reaches more than 99%, and the specific surface area is between 10-11 m 2 / g, and the tap density can reach about 2.55 g / cm 3 . Description of the Drawings

[0021] Figure 1 SEM image of the small particle size lithium iron manganese phosphate crystal nuclei sintered in nitrogen at 450°C in Example 1 of the present invention;

[0022] Figure 2 SEM image of the large particle size lithium iron manganese phosphate crystal nuclei sintered in nitrogen at 650°C in Example 1 of the present invention;

[0023] Figure 3 SEM image of the lithium iron manganese phosphate obtained by secondary sintering of the large and small particle size lithium iron manganese phosphate crystal nuclei prepared by nitrogen sintering in Example 1 of the present invention;

[0024] Figure 4Statistical chart of the primary particle size of lithium iron manganese phosphate obtained by secondary firing of large and small particle size lithium iron manganese phosphate crystal nuclei prepared by nitrogen firing in Example 1 of the present invention;

[0025] Figure 5 SEM image of lithium iron manganese phosphate obtained by secondary firing of small particle size lithium iron manganese phosphate crystal nuclei prepared by nitrogen firing at 450 °C in Comparative Example 1 of the present invention;

[0026] Figure 6 Statistical chart of the primary particle size of lithium iron manganese phosphate obtained by secondary firing of small particle size lithium iron manganese phosphate crystal nuclei prepared by nitrogen firing at 450 °C in Comparative Example 1 of the present invention;

[0027] Figure 7 SEM image of small particle size lithium iron manganese phosphate crystal nuclei prepared by nitrogen firing at 550 °C in Example 2 of the present invention;

[0028] Figure 8 SEM image of large particle size lithium iron manganese phosphate crystal nuclei prepared by nitrogen firing at 750 °C in Example 2 of the present invention;

[0029] Figure 9 SEM image of lithium iron manganese phosphate obtained by secondary firing of large and small particle size lithium iron manganese phosphate crystal nuclei prepared by nitrogen firing in Example 2 of the present invention;

[0030] Figure 10 Statistical chart of the primary particle size of lithium iron manganese phosphate obtained by secondary firing of large and small particle size lithium iron manganese phosphate crystal nuclei prepared by nitrogen firing in Example 2 of the present invention;

[0031] Figure 11 SEM image of lithium iron manganese phosphate obtained by secondary firing of large particle size lithium iron manganese phosphate crystal nuclei prepared by nitrogen firing at 750 °C in Comparative Example 2 of the present invention;

[0032] Figure 12 Statistical chart of the primary particle size of lithium iron manganese phosphate obtained by secondary firing of large particle size lithium iron manganese phosphate crystal nuclei prepared by nitrogen firing at 750 °C in Comparative Example 2 of the present invention;

[0033] Figure 13 SEM image of lithium iron manganese phosphate obtained by secondary firing of large and small particle size lithium iron manganese phosphate crystal nuclei prepared by nitrogen firing at 750 °C and 450 °C respectively in Example 3 of the present invention;

[0034] Figure 14 SEM image of lithium iron manganese phosphate obtained by secondary firing of large and small particle size lithium iron manganese phosphate crystal nuclei prepared by nitrogen firing at 650 °C and 550 °C respectively in Comparative Example 3 of the present invention;

[0035] Figure 15SEM images of lithium iron manganese phosphate obtained by second firing of large and small particle size lithium iron manganese phosphate crystal nuclei prepared by first firing in nitrogen at 750 °C and 450 °C respectively and adding a carbon source in Example 4 of the present invention. Detailed implementation manners

[0036] The technical solutions of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be noted that all raw materials used in the present invention can be purchased commercially. In this solution, when preparing small particle size lithium iron manganese phosphate crystal nuclei and large particle size lithium iron manganese phosphate crystal nuclei from lithium iron manganese phosphate precursor powder, there is no sequence preference for the preparation steps of the two.

[0037] Example 1

[0038] The lithium iron manganese phosphate cathode material of this Example 1 was prepared by the following steps:

[0039] (1) Manganese carbonate, iron phosphate, ammonium dihydrogen phosphate and lithium carbonate were placed in excess deionized water according to a molar ratio of 0.6:0.4:0.6:1, and stirred and reacted for 0.5 h at a stirring temperature of 25 °C and a frequency of 20 Hz to obtain a uniformly mixed slurry; the uniformly mixed slurry was ground for 2 h to obtain a lithium iron manganese phosphate precursor slurry; the lithium iron manganese phosphate precursor slurry was dried at 150 °C to obtain a lithium iron manganese phosphate precursor powder;

[0040] (2) The lithium iron manganese phosphate precursor powder was calcined at a low temperature of 450 °C in a nitrogen atmosphere for 10 h, and after natural cooling, small particle size lithium iron manganese phosphate crystal nuclei II were obtained;

[0041] (3) The lithium iron manganese phosphate precursor powder was calcined at a high temperature of 650 °C in a nitrogen atmosphere for 10 h, and after natural cooling, large particle size lithium iron manganese phosphate crystal nuclei I were obtained;

[0042] (4) The large particle size lithium iron manganese phosphate crystal nuclei prepared in step (3) and the small particle size lithium iron manganese phosphate crystal nuclei prepared in step (2) were mixed according to a mass ratio of 3:7 to obtain a lithium iron manganese phosphate pre-calcined material;

[0043] (5) The lithium iron manganese phosphate pre-calcined material and glucose were placed in deionized water. The addition amount of this glucose was 10% of the mass of theoretically generated lithium iron manganese phosphate. Stirred and reacted for 0.5 h at a stirring temperature of 25 °C and a frequency of 20 Hz to obtain a uniformly mixed slurry III; the uniformly mixed slurry III was ground for 2 h to obtain a lithium iron manganese phosphate pre-calcined material slurry IV; the lithium iron manganese phosphate pre-calcined material slurry IV was dried at 150 °C to obtain a lithium iron manganese phosphate pre-calcined material V; finally, the lithium iron manganese phosphate pre-calcined material V was calcined at 750 °C in a nitrogen atmosphere for 14 h, and after natural cooling, the lithium iron manganese phosphate cathode material was obtained.

[0044] Comparative Example 1

[0045] The basic steps are the same as those in Example 1, except that in step (4), all the lithium iron manganese phosphate precursor powders are calcined at a low temperature of 450 °C in a nitrogen atmosphere for 10 h to obtain lithium iron manganese phosphate crystal nuclei with all small particle sizes. Specifically, the steps are as follows:

[0046] (1) Manganese carbonate, iron phosphate, ammonium dihydrogen phosphate and lithium carbonate are placed in excessive deionized water according to a molar ratio of 0.6:0.4:0.6:1, and stirred and reacted for 0.5 h at a stirring temperature of 25 °C and a frequency of 20 Hz to obtain a uniformly mixed slurry; the uniformly mixed slurry is ground for 2 h to obtain a lithium iron manganese phosphate precursor slurry; the lithium iron manganese phosphate precursor slurry is dried at 150 °C to obtain lithium iron manganese phosphate precursor powders;

[0047] (2) The lithium iron manganese phosphate precursor powders are calcined at a low temperature of 450 °C in a nitrogen atmosphere for 10 h, and after natural cooling, small-particle-size lithium iron manganese phosphate crystal nuclei II are obtained;

[0048] (3) The lithium iron manganese phosphate crystal nuclei II and glucose are placed in deionized water, and the added amount of the glucose is 10% of the mass of the theoretically generated lithium iron manganese phosphate. Stirred and reacted for 0.5 h at a stirring temperature of 25 °C and a frequency of 20 Hz to obtain a uniformly mixed slurry III; the uniformly mixed slurry III is ground for 2 h to obtain a lithium iron manganese phosphate pre-calcined material slurry IV; the lithium iron manganese phosphate pre-calcined material slurry IV is dried at 150 °C to obtain a lithium iron manganese phosphate pre-calcined material V; finally, the lithium iron manganese phosphate pre-calcined material V is calcined at 750 °C in a nitrogen atmosphere for 14 h, and after natural cooling, a lithium iron manganese phosphate cathode material is obtained.

[0049] Performance detection: Structure characterization

[0050] The SEM diagrams of the small-particle-size lithium iron manganese phosphate crystal nuclei prepared in Example 1 and Comparative Example 1 of the present invention are as Figure 1 shown (prepared from the same batch). It can be seen that the crystal nucleus particle sizes of the small-particle-size lithium iron manganese phosphate crystal nuclei obtained by low-temperature calcination are uneven and the surface is rough. The SEM diagram of the large-particle-size lithium iron manganese phosphate crystal nuclei prepared in Example 1 of the present invention is as Figure 2 shown. It can be seen that the shape of the large-particle-size lithium iron manganese phosphate crystal nuclei at high temperature is relatively regular, mostly spherical particles with uniform particles and a smooth surface.

[0051] The SEM diagram of the LMFP obtained by secondary calcination after mixing large- and small-particle-size lithium iron manganese phosphates in Example 1 is as Figure 3 shown, and the primary particle sizes of the SEM of the LMFP are statistically analyzed, as Figure 4 shown; from Figure 3It can be seen that in the LMFP material, lithium iron manganese phosphate with different particle sizes is closely arranged, and the gaps between large-sized lithium iron manganese phosphate are filled with many small-sized lithium iron manganese phosphate; through Figure 4 statistics, it can be seen that the D50 value of the small-sized lithium iron manganese phosphate is 135 nm, and the D50 value of the large-sized lithium iron manganese phosphate is 273 nm.

[0052] The SEM image of lithium iron manganese phosphate prepared by the second calcination of the small-sized lithium iron manganese phosphate crystal nuclei in Comparative Example 1 is as shown in Figure 5 shown, and the statistics of its SEM primary particle size are as shown in Figure 6 shown, and the D50 value is 134.68 nm; it can be seen that most of the particles in the lithium iron manganese phosphate material formed by high-temperature calcination of small-sized lithium iron manganese phosphate are relatively small in particle size, and different particle size gradations are not formed, and the gaps between the particles are large. The surface shape of the small-sized lithium iron manganese phosphate is irregular and it is difficult to stack tightly together during the second calcination. Therefore, the positive electrode material prepared in Comparative Example 1 has the characteristics of high specific surface area, low tap density, and high capacity.

[0053] Example 2

[0054] The lithium iron manganese phosphate positive electrode material of this Example 2 is prepared by the following steps:

[0055] (1) Manganese carbonate, iron phosphate, ammonium dihydrogen phosphate and lithium carbonate are placed in excessive deionized water according to a molar ratio of 0.6:0.4:0.6:1, and stirred and reacted for 0.5 h at a stirring temperature of 25 °C and a frequency of 20 Hz to obtain a uniformly mixed slurry; the uniformly mixed slurry is ground for 2 h to obtain a lithium iron manganese phosphate precursor slurry; the lithium iron manganese phosphate precursor slurry is dried at 150 °C to obtain a lithium iron manganese phosphate precursor powder;

[0056] (2) The lithium iron manganese phosphate precursor powder is calcined at a low temperature of 550 °C in a nitrogen atmosphere for 10 h, and after natural cooling, small-sized lithium iron manganese phosphate crystal nuclei II are obtained;

[0057] (3) The lithium iron manganese phosphate precursor powder is calcined at a high temperature of 750 °C in a nitrogen atmosphere for 10 h, and after natural cooling, large-sized lithium iron manganese phosphate crystal nuclei I are obtained;

[0058] (4) The large-sized lithium iron manganese phosphate crystal nuclei prepared in step (3) are mixed with the small-sized lithium iron manganese phosphate crystal nuclei prepared in step (2) according to a mass ratio of 2:8 to obtain a lithium iron manganese phosphate pre-calcined material;

[0059] (5) Put the pre - calcined lithium iron manganese phosphate material and glucose into deionized water. The addition amount of this glucose is 15% of the mass of lithium iron manganese phosphate theoretically generated. Under the conditions of a stirring temperature of 25 °C and a frequency of 20 Hz, stir - react for 0.5 h to obtain a uniformly - mixed slurry III; grind the uniformly - mixed slurry III for 2 h to obtain a lithium iron manganese phosphate pre - calcined material slurry IV; dry the lithium iron manganese phosphate pre - calcined material slurry IV at 150 °C to obtain a pre - calcined lithium iron manganese phosphate material V; finally, calcine the pre - calcined lithium iron manganese phosphate material V at 780 °C under a nitrogen atmosphere for 14 h, and after natural cooling, obtain the lithium iron manganese phosphate cathode material.

[0060] Comparative Example 2

[0061] The basic steps are the same as those in Example 2, except that all the lithium iron manganese phosphate precursor powders are calcined at 750 °C under a nitrogen atmosphere for 10 h to obtain all large - particle - size lithium iron manganese phosphate crystal nuclei. The finally obtained lithium iron manganese phosphate cathode material specifically includes the following steps:

[0062] (1) Put manganese carbonate, iron phosphate, ammonium dihydrogen phosphate, and lithium carbonate in a molar ratio of 0.6:0.4:0.6:1 into excessive deionized water. Under the conditions of a stirring temperature of 25 °C and a frequency of 20 Hz, stir - react for 0.5 h to obtain a uniformly - mixed slurry; grind the uniformly - mixed slurry for 2 h to obtain a lithium iron manganese phosphate precursor slurry; dry the lithium iron manganese phosphate precursor slurry at 150 °C to obtain a lithium iron manganese phosphate precursor powder;

[0063] (2) Calcine the lithium iron manganese phosphate precursor powder at 750 °C under a nitrogen atmosphere for 10 h, and after natural cooling, obtain large - particle - size lithium iron manganese phosphate crystal nuclei I;

[0064] (3) Put the large - particle - size lithium iron manganese phosphate crystal nuclei I and glucose into deionized water. The addition amount of this glucose is 15% of the mass of lithium iron manganese phosphate theoretically generated. Under the conditions of a stirring temperature of 25 °C and a frequency of 20 Hz, stir - react for 0.5 h to obtain a uniformly - mixed slurry III; grind the uniformly - mixed slurry III for 2 h to obtain a lithium iron manganese phosphate pre - calcined material slurry IV; dry the lithium iron manganese phosphate pre - calcined material slurry IV at 150 °C to obtain a pre - calcined lithium iron manganese phosphate material V; finally, calcine the pre - calcined lithium iron manganese phosphate material V at 780 °C under a nitrogen atmosphere for 14 h, and after natural cooling, obtain the lithium iron manganese phosphate cathode material.

[0065] Performance detection: Structure characterization

[0066] The SEM images of the large - particle - size lithium iron manganese phosphate crystal nuclei prepared in Example 2 and Comparative Example 2 of the present invention are as Figure 8 shown (prepared from the same batch). It can be seen that the large - particle - size lithium iron manganese phosphate crystal nuclei still show the characteristics of larger particle size, smooth surface, and basically uniform size, which is the same asFigure 2 In comparison, the particle size is larger than that of Figure 2 , and there is little difference in their shapes; the SEM image of the small-particle-size lithium iron manganese phosphate crystal nuclei prepared in Example 2 of the present invention is as shown in Figure 7 . It can be seen that the small-particle-size lithium iron manganese phosphate crystal nuclei still show characteristics such as small particle size, uneven size, and rough surface. However, compared with Figure 1 , the crystal nucleus particle size of Figure 1 is a little larger, and the surface is smoother. It can be seen from the appearance that temperature has a certain influence on the formation of small-particle-size lithium iron manganese phosphate crystal nuclei.

[0067] The SEM image of the LMFP obtained by the second calcination after mixing large- and small-particle-size lithium iron manganese phosphates in Example 2 is as shown in Figure 9 , and the SEM primary particle size of the LMFP is statistically analyzed, as shown in Figure 10 ; it can be seen from Figure 9 that in the LMFP material, the finally formed LMFP is still that many small-particle-size lithium iron manganese phosphates are filled in the gaps between the large-particle-size lithium iron manganese phosphates, and the overall particle size of the LMFP is slightly larger than that of Figure 3 . It can be seen that the particle size difference between the large- and small-particle-size lithium iron manganese phosphates in the LMFP of this example is small. And the primary particle size diagram of Figure 10 also shows that in this example, the D50 value of the small-particle-size lithium iron manganese phosphate is 139 nm, and the D50 value of the large-particle-size lithium iron manganese phosphate is 267 nm.

[0068] The SEM image of the lithium iron manganese phosphate prepared by the second calcination of the large-particle-size lithium iron manganese phosphate crystal nuclei in Comparative Example 2 is as shown in Figure 11 , and its SEM primary particle size is statistically analyzed, as shown in Figure 12 . The D50 value is 276.48 nm; the appearance of the large-particle-size lithium iron manganese phosphate is relatively regular, and most of them are spherical. Some of the large-particle-size lithium iron manganese phosphate crystal nuclei will form small-particle-size lithium iron manganese phosphate crystal nuclei and fill between the large-particle-size lithium iron manganese phosphate crystal nuclei during the secondary high-temperature calcination. However, by comparing the SEM images of the positive electrode materials in Examples 1 and 2, it can be seen that the number of small-particle-size lithium iron manganese phosphate crystal nuclei is significantly insufficient and cannot completely fill the gaps between the large-particle-size ones, resulting in a slightly lower compaction degree. However, due to the large amount of large-particle-size lithium iron manganese phosphate, its specific surface area is slightly lower, and its electrochemical performance is significantly lower than that of other examples.

[0069] Example 3

[0070] The basic steps are the same as those in Example 1, except that the high-temperature temperature in step (3) is 750 °C and the low-temperature temperature in step (2) is 450 °C; the SEM image of the prepared lithium iron manganese phosphate positive electrode material is as shown in Figure 13As shown, at a temperature difference of 300 °C, obvious lithium iron manganese phosphate crystal nuclei with large and small particle sizes are formed. During the second sintering process, the lithium iron manganese phosphate with small particle sizes fills the gaps between the lithium iron manganese phosphate with large particle sizes and tightly stacks to form the final lithium iron manganese phosphate cathode material.

[0071] Comparative Example 3

[0072] The basic steps are the same as those in Example 1, except that the high-temperature in step (3) is 650 °C and the low-temperature in step (2) is 550 °C. The SEM image of the prepared lithium iron manganese phosphate cathode material is as Figure 14 shown. At a temperature difference of 100 °C, obvious lithium iron manganese phosphate crystal nuclei with large and small particle sizes are not shown. For the finally second-sintered cathode material, its particle size is relatively uniform. By comparing with Example 1, it can be seen that the appearance and size of the lithium iron manganese phosphate crystal nuclei in Comparative Example 3 are relatively consistent with those of the large-particle-size lithium iron manganese phosphate crystal nuclei in Example 1, and the gaps between the lithium iron manganese phosphate crystal nuclei in Comparative Example 3 are slightly larger, and the compaction degree of the cathode material is lower.

[0073] Comparative Example 4

[0074] The basic steps are the same as those in Example 3, except that: the carbon source is added when preparing the lithium iron manganese phosphate precursor powder in step (1). After the introduction of the carbon source, it is carbonized and coated on the crystal nuclei, which is not conducive to the growth of the crystal nuclei. The SEM image of the prepared lithium iron manganese phosphate cathode material is as Figure 15 shown. It can be clearly seen from Figure 15 that the lithium iron manganese phosphate particles with large and small particle sizes are all irregular in shape; and by comparing with Examples 1-3, the volume growth of the lithium iron manganese phosphate crystal nuclei after the second sintering is not obvious, the stacking between the crystal nuclei is relatively loose, the gaps are large, there is no particle filling, and the compaction degree of the cathode material is lower.

[0075] Performance detection: Electrical performance

[0076] The lithium iron manganese phosphate cathode materials obtained in the above Examples 1-3 and Comparative Examples 1-4 were prepared into coin cells by the following method and their electrochemical performance was evaluated: The lithium iron manganese phosphate cathode material, Super P and PVDF were uniformly mixed in an NMP solution at a ratio of 8:1:1 to obtain a mixed slurry. The slurry was coated on a shiny aluminum foil by hand coating, and then dried at 100 °C. After the NMP was completely volatilized, the electrode sheet was punched into an electrode sheet with a diameter of 13 mm, and then the electrode sheet was placed in a vacuum oven and dried overnight at 105 °C. After weighing the electrode sheet, it was quickly transferred to a glove box. Using metallic lithium as the counter electrode, Celgard 2400 as the separator, and the electrolyte being 1 mol / L LiPF6 dissolved in a mixed solvent of EC / DMC / EMC (volume ratio 1:1:1), the assembled battery was subjected to electrochemical performance testing. The testing equipment was a Neware 4008 constant current testing cabinet, and the testing voltage range was 2.0 - 4.35 V. The electrochemical performance results are shown in Table 1.

[0077] Table 1 Compaction, specific surface area, and electrochemical performance of examples and comparative examples

[0078]

[0079]

[0080] Combining Example 1 and Comparative Example 1 in Table 1, both used lithium iron manganese phosphate crystal nuclei with small particle sizes to prepare lithium iron manganese phosphate cathode materials. The lithium iron manganese phosphate cathode material with small particle sizes can reduce the Li + ion diffusion path, improve the migration of Li + ions, and thus improve its electrochemical performance. However, at the same time, this cathode material has a high specific surface area and a low compaction degree, which has an adverse effect on the overall structural stability of the subsequent prepared battery.

[0081] Combining Example 2 and Comparative Example 2, both used lithium iron manganese phosphate crystal nuclei with large particle sizes to prepare lithium iron manganese phosphate cathode materials. Although the compaction degree increased and the specific surface area decreased, the large particle size of lithium iron manganese phosphate would increase the Li + ion diffusion path, reduce the migration of Li + ions, and thus cause a significant decline in its electrochemical performance.

[0082] Compared with Example 3 and Comparative Example 3, the temperature difference for preparing lithium iron manganese phosphate with large and small particle sizes in Comparative Example 3 was 100 °C, resulting in a small difference in its large and small particle sizes and limited filling of particle gaps. Finally, the specific surface area of the LMFP cathode material prepared by the second sintering was higher than 11 m 2 / g, which had a negative impact on the subsequent process of preparing the battery core.

[0083] Compared with Example 3 and Comparative Example 4, when the carbon source is added first in the preparation of lithium iron manganese phosphate precursor, coating will be formed in the early stage, thereby inhibiting particle growth. Although the electrochemical performance can be improved, the tap density is relatively low and the specific surface area is relatively large.

[0084] It can be seen from this that by using the preparation process of the present invention for the LMFP cathode material with size grading, it is possible to improve the tap density on the basis of excellent electrochemical performance, reaching about 2.55 g / cm 3 and making its specific surface area around 10 m 2 / g, that is, meeting the lithium iron manganese phosphate cathode material with a low specific surface area, high tap density and high capacity.

[0085] In addition to the above examples, it should be noted that the molar ratio of the manganese source, iron source, phosphorus source, and lithium source used in the preparation of the lithium iron manganese phosphate cathode material by the present invention can be a:(1 - a):b:c, where 0.05 ≤ a ≤ 0.95; 1 ≤ b ≤ 1.05; 1 ≤ c ≤ 1.08. Among them, the manganese source can also be at least one of manganese tetroxide, manganese salt, manganese iron oxide or manganese iron precursor. The iron source can also be at least one of iron tetroxide, iron salt, manganese iron oxide or manganese iron precursor. The phosphorus source can also be at least one of phosphoric acid, lithium phosphate or monoammonium phosphate. The lithium source can also be at least one of lithium hydroxide, lithium sulfate, lithium nitrate or lithium chloride precursor. The carbon source can also include at least one of sucrose, starch, cellulose, polyvinyl alcohol, soluble starch, cellulose, ascorbic acid or phenolic resin. The addition amount of the carbon source can be 5 - 15% of the mass of the theoretically produced lithium iron manganese phosphate.

[0086] The mass ratio of the large-sized lithium iron manganese phosphate crystal nuclei and the small-sized lithium iron manganese phosphate crystal nuclei prepared respectively can be x:(1 - x), where 0.05 < x < 0.45. The temperature range of the secondary calcination is 750 - 800 °C and the calcination time is 10 - 25 h.

[0087] That is, by using the preparation process of the present invention and the defined parameter range, the technical effects claimed by the present invention can be achieved, and thus no further separate examples are listed for verification.

Claims

1. A low specific surface area, high compaction and high capacity lithium manganese iron phosphate positive electrode material, characterized in that: It is prepared by the following steps: (1) Put the manganese source, iron source, phosphorus source and lithium source into deionized water, and prepare the lithium iron manganese phosphate precursor powder through stirring, grinding and drying; (2) Calcinate the lithium iron manganese phosphate precursor powder under an inert atmosphere at a low temperature of 450 - 550 °C for 4 - 15 h to obtain lithium iron manganese phosphate crystal nuclei with small particle sizes; (3) Calcinate the lithium iron manganese phosphate precursor powder under an inert atmosphere at a high temperature of 650 - 750 °C for 4 - 15 h to obtain lithium iron manganese phosphate crystal nuclei with large particle sizes; (4) Mix the lithium iron manganese phosphate crystal nuclei with large particle sizes prepared in step (3) and the lithium iron manganese phosphate crystal nuclei with small particle sizes prepared in step (2) according to the mass ratio of x:(1 - x), where 0.05 < x < 0.45, to obtain the lithium iron manganese phosphate pre - calcined material; (5) Put the lithium iron manganese phosphate pre - calcined material and the carbon source into deionized water, after stirring, grinding and drying, and calcine at 750 - 800 °C under an inert atmosphere for 10 - 25 h to obtain the lithium iron manganese phosphate cathode material; Among them, in steps (2) and (3), the temperature difference between the high - temperature condition and the low - temperature condition is not less than 200 °C.

2. The lithium iron manganese phosphate positive electrode material according to claim 1, characterized in that: In step (1), the molar ratio of the manganese source, iron source, phosphorus source and lithium source is a:(1 - a):b:c, where 0.05 ≤ a ≤ 0.95, 1 ≤ b ≤ 1.05, 1 ≤ c ≤ 1.

08.

3. The lithium iron manganese phosphate positive electrode material according to claim 1 or 2, characterized in that: The manganese source includes at least one of manganese carbonate, manganese tetroxide, manganese salts, manganese iron oxides or manganese iron precursors.

4. The lithium iron manganese phosphate positive electrode material according to claim 1 or 2, characterized in that: The iron source includes at least one of iron phosphate, iron tetroxide, iron salts, manganese iron oxides or manganese iron precursors.

5. The lithium iron manganese phosphate positive electrode material according to claim 1 or 2, characterized in that: The phosphorus source includes at least one of phosphoric acid, ammonium dihydrogen phosphate, lithium phosphate or ammonium monophosphate.

6. The lithium iron manganese phosphate positive electrode material according to claim 1 or 2, characterized in that: The lithium source includes at least one of lithium carbonate, lithium hydroxide, lithium sulfate, lithium nitrate or lithium chloride precursors.

7. The lithium iron manganese phosphate positive electrode material according to claim 1 or 2, characterized in that: In step (5), the addition amount of the carbon source is 5 - 15% of the mass of the theoretically generated lithium iron manganese phosphate.

8. The lithium iron manganese phosphate positive electrode material according to claim 1 or 7, characterized in that: In step (5), the carbon source includes at least one of glucose, sucrose, starch, cellulose, polyvinyl alcohol, soluble starch, cellulose, ascorbic acid or phenolic resin.

9. The lithium iron manganese phosphate positive electrode material according to claim 1, characterized in that: In steps (1) and (5), the stirring is carried out at 0 - 25 °C and a frequency of 20 - 50 Hz for 0.5 - 2 h.

10. The lithium iron manganese phosphate positive electrode material according to claim 1, characterized in that: In steps (1) and (5), the grinding time is 0.5 - 6 h, and the drying temperature is 150 - 250 °C.

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