A low specific surface area, high compaction and high capacity lithium iron manganese phosphate positive electrode material

Abstract

The application discloses a kind of low specific surface area, high compaction and high capacity lithium manganese iron phosphate positive electrode material, by the following steps: manganese source, iron source, phosphorus source and lithium source are placed in deionized water, by stirring, grinding, drying lithium manganese iron phosphate precursor powder is prepared;Lithium manganese iron phosphate precursor powder is calcined under the condition of low temperature 450-550 DEG C in inert atmosphere, and small particle size lithium manganese iron phosphate crystal nucleus is prepared;Lithium manganese iron phosphate precursor powder is calcined under the condition of high temperature 650-750 DEG C in inert atmosphere, and large particle size lithium manganese iron phosphate crystal nucleus is prepared;The temperature difference of high temperature condition and low temperature condition is not less than 200 DEG C;Large particle size lithium manganese iron phosphate crystal nucleus and small particle size lithium manganese iron phosphate crystal nucleus are mixed according to specific mass ratio, and lithium manganese iron phosphate preburning material is prepared;Lithium manganese iron phosphate preburning material and carbon source are placed in 750-800 DEG C, inert atmosphere and calcined, and lithium manganese iron phosphate positive electrode material is prepared;The lithium manganese iron phosphate positive electrode material of the application can guarantee electrochemical performance while reaching high compaction and low specific surface area, to improve the overall structural stability of battery.

Description

Technical Field

[0001] The present invention belongs to the field of battery cathode materials, 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 3 , , 2 , + , 5 , 3 ,

[0004] , 2 , 4+ Fe 1-x PO4(0 < x < 1) (LMFP) has a high voltage discharge platform of 4.1V, which is much higher than the 3.4V discharge platform of LiFePO4 (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 electrochemical performance being inferior to that of LFP.

[0003] In order to solve the problem of poor electrochemical performance, at present, mainly three improvement measures of grain nanocrystallization, carbon coating and ion doping are adopted. The essence of grain nanocrystallization of LMFP is to shorten the Li + ion diffusion path by reducing the grain size, and improve the migration of Li + ions for efficient insertion and extraction, so as to improve the rate performance of the battery; at the same time, grain nanocrystallization will also increase the specific surface area of LMFP, increase the reaction area with the electrolyte, and improve the electrochemical performance. The principle of carbon coating is to construct a conductive network, and use conductive carbon and manganese iron lithium to construct a fast conductive network, so that electrons can migrate rapidly between active substances during charge and discharge, reduce the internal resistance and charge-discharge polarization of the battery, and also increase the specific surface area of LMFP. Ion doping is to introduce defects, increase active sites, and improve conductivity and ion diffusion performance. Mg 2+ 、V 5+ 、Ti 4+ These three elements are the most widely used, and the introduction of V 5 + and 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 - 30m 2 / g, which is much higher than the specific surface area of LFP at 9 - 13m 2 / g. The nanocrystallization of grains results in its powder tap density being 1.8 - 2.0g / cm 3 , which is much lower than the powder tap density of LFP at 2.5 - 2.6g / cm 3, while the cathode material with a large BET specific surface area 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 battery 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 effectively solve the problem of poor electrochemical performance of lithium iron manganese phosphate, and further 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 battery 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 battery 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) and the small-particle-size lithium iron manganese phosphate crystal nuclei prepared in step (2) according to a mass ratio of x:(1 - x), 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] Among them, 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] This invention employs a two-stage calcination process to prepare lithium manganese iron phosphate (LMP) cathode materials with low specific surface area, high compaction, and high capacity. The first stage uses high and low temperature conditions (with a temperature difference of at least 200°C) to obtain large, uniform, smooth, and rounded LMP nuclei and small, non-uniform, rough-surfaced LMP nuclei. These two types of nuclei are then mixed according to specific ratios to form a size-graded nucleus. During the second high-temperature calcination, both types of LMP nuclei grow simultaneously. The large LMP nuclei are formed through a first sintering followed by a second sintering, and the small LMP nuclei are also formed through a first sintering followed by a second sintering. This results in a LMP cathode material with a size-graded particle size distribution, improving the compaction and electrical properties of the prepared cathode material, reducing the specific surface area, solving the problems of poor homogenization and coating processability in the front-end cell fabrication, and increasing the volumetric energy density. The temperature difference between the two is 200℃ or more, which makes the particle size of the small lithium manganese iron phosphate crystal nucleus significantly different from that of the large lithium manganese iron phosphate crystal nucleus. Then, after the large and small lithium manganese iron phosphate crystal nuclei are mixed and then sintered at a high temperature for the second time, the crystal nuclei grow at the same time and form the most compact arrangement.

[0015] Furthermore, during the second high-temperature calcination, the calcination temperature is close to the temperature at which large-particle-size lithium manganese iron phosphate (LMP) crystal nuclei are formed. The volume growth rate of large-particle-size LMP crystal nuclei is small, while the growth rate of small-particle-size LMP crystal nuclei is large. This allows small-particle-size LMP crystal nuclei to continuously fill the gaps between large-particle-size LMP crystal nuclei. Based on this, a carbon source is introduced as a reducing agent. Within the entire system, the addition of the carbon source can reduce the Fe content in the LMP crystal nuclei. 3+ (Ionic radius approximately 0.064 nm) Ions reduced to Fe 2+ (Ionic radius approximately 0.078 nm) This further promotes the overall cell volume of small-diameter lithium manganese iron phosphate nuclei, which are far more numerous than large-diameter lithium manganese iron phosphate nuclei, to be greater than the overall cell volume of large-diameter nuclei. On the basis of simultaneous growth of both, the small-diameter lithium manganese iron phosphate nuclei with a larger overall crystal volume can further fill the gaps between large and small particles, thereby obtaining a low specific surface area and high-compact lithium manganese iron phosphate cathode material.

[0016] Furthermore, the molar ratio of manganese source, iron source, phosphorus source and lithium source used in the lithium manganese iron 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 manganese iron phosphate cathode material of the present invention includes at least one of manganese carbonate, manganese tetroxide, manganese salt, ferromanganese oxide, or ferromanganese precursor. The iron source includes at least one of iron phosphate, iron tetroxide, iron salt, ferromanganese oxide, or ferromanganese precursor. The phosphorus source includes at least one of phosphoric acid, ammonium dihydrogen phosphate, lithium phosphate, or monoammonium phosphate. The lithium source includes at least one of lithium carbonate, lithium hydroxide, lithium sulfate, lithium nitrate, or lithium chloride precursor.

[0018] Furthermore, the amount of carbon source added to the lithium manganese iron phosphate cathode material of the present invention is 5-15% of the theoretical mass of lithium manganese iron phosphate. Preferably, the carbon source may 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 manganese iron phosphate cathode material of the present invention is carried out at 0-25°C and 20-50Hz for 0.5-2 hours. The grinding time is 0.5-6 hours, and the drying temperature is 150-250°C.

[0020] Beneficial Effects: Compared with existing technologies, the significant advantages of this invention are as follows: The lithium manganese iron phosphate cathode material of this invention, through the gradation of large and small particle sizes, forms a compact lithium manganese iron phosphate cathode material. Different particle size gradations form high-compacted lithium manganese iron phosphate, achieving suitable compaction and specific surface area while ensuring electrochemical performance. This not only solves the problems of poor homogenization and coating processability and low volumetric energy density in the front-end of cell preparation, but also improves the overall structural stability of the battery. Specifically, the prepared lithium manganese iron phosphate cathode material can achieve a 0.1C charging capacity of 164.78 mAh / g, a 0.1C discharging capacity of 163.56 mAh / g, an initial efficiency of over 99%, and a specific surface area between 10-11 m². 2 / g, the compacted density can reach about 2.55 g / cm³. 3 . Attached Figure Description

[0021] Figure 1 This is a SEM image of small-particle-size lithium manganese iron phosphate crystal nuclei burned under nitrogen at 450°C in Example 1 of the present invention.

[0022] Figure 2 This is a SEM image of the large-particle lithium manganese iron phosphate crystal nuclei burned under nitrogen at 650°C in Example 1 of the present invention.

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

[0024] Figure 4SEM primary particle size statistics of lithium manganese iron phosphate obtained by secondary calcination of large and small particle size lithium manganese iron phosphate crystal nuclei prepared by nitrogen calcination in Example 1 of the present invention.

[0025] Figure 5 SEM image of lithium manganese iron phosphate obtained by secondary calcination of small-particle lithium manganese iron phosphate nuclei prepared by nitrogen calcination at 450°C in Comparative Example 1 of this invention.

[0026] Figure 6 The first SEM particle size distribution of lithium manganese iron phosphate obtained by second calcination of small-particle-size lithium manganese iron phosphate crystal nuclei prepared by nitrogen calcination at 450℃ in Comparative Example 1 of this invention.

[0027] Figure 7 This is a SEM image of the small-particle-size lithium manganese iron phosphate crystal nuclei prepared by nitrogen combustion at 550℃ in Example 2 of the present invention;

[0028] Figure 8 This is a SEM image of the large-particle-size lithium manganese iron phosphate crystal nuclei prepared by nitrogen combustion at 750℃ in Example 2 of the present invention.

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

[0030] Figure 10 SEM primary particle size distribution of lithium manganese iron phosphate obtained by secondary calcination of large and small particle size lithium manganese iron phosphate nuclei prepared by nitrogen calcination in Example 2 of the present invention.

[0031] Figure 11 SEM image of lithium manganese iron phosphate obtained by secondary calcination of large-particle lithium manganese iron phosphate nuclei prepared by nitrogen calcination at 750°C in Comparative Example 2 of this invention.

[0032] Figure 12 The first SEM particle size distribution of lithium manganese iron phosphate obtained by second calcination of large-particle lithium manganese iron phosphate crystal nuclei prepared by nitrogen calcination at 750℃ in Comparative Example 2 of this invention.

[0033] Figure 13 SEM images of lithium manganese iron phosphate obtained by secondary calcination of large and small particle size lithium manganese iron phosphate nuclei prepared by nitrogen calcination at 750℃ and 450℃ respectively in Example 3 of the present invention.

[0034] Figure 14 SEM images of lithium manganese iron phosphate obtained by secondary calcination of large and small particle size lithium manganese iron phosphate nuclei prepared by nitrogen calcination at 650℃ and 550℃ respectively in Comparative Example 3 of the present invention.

[0035] Figure 15The images show SEM images of lithium manganese iron phosphate obtained by secondary calcination of large and small particle size lithium manganese iron phosphate nuclei prepared by nitrogen combustion at 750℃ and 450℃ respectively with the addition of a carbon source in Example 4 of the present invention. Detailed Implementation

[0036] The technical solution 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 are commercially available. In this solution, when preparing small-particle-size and large-particle-size lithium manganese iron phosphate crystal nuclei using lithium manganese iron phosphate precursor powder, the preparation steps are not sequential.

[0037] Example 1

[0038] The lithium iron phosphate cathode material of 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 at a molar ratio of 0.6:0.4:0.6:1 and stirred for 0.5 h at a stirring temperature of 25℃ and a stirring 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℃ to obtain lithium iron manganese phosphate precursor powder;

[0040] (2) The precursor powder of lithium iron manganese phosphate was calcined at a low temperature of 450°C for 10 hours under a nitrogen atmosphere and then naturally cooled to obtain small-particle-size lithium iron manganese phosphate crystal nuclei II.

[0041] (3) The lithium iron manganese phosphate precursor powder was calcined at a high temperature of 650°C for 10 hours under a nitrogen atmosphere and then naturally cooled to obtain large-particle lithium iron manganese phosphate crystal nuclei I.

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

[0043] (5) The pre-calcined lithium manganese iron phosphate material and glucose were placed in deionized water. The amount of glucose added was 10% of the theoretical mass of lithium manganese iron phosphate. The mixture was stirred for 0.5 h at a stirring temperature of 25℃ and a frequency of 20Hz to obtain a uniformly mixed slurry III. The uniformly mixed slurry III was ground for 2 h to obtain lithium manganese iron phosphate pre-calcined slurry IV. The lithium manganese iron phosphate pre-calcined slurry IV was dried at 150℃ to obtain lithium manganese iron phosphate pre-calcined material V. Finally, the lithium manganese iron phosphate pre-calcined material V was calcined at 750℃ under a nitrogen atmosphere for 14 h and then naturally cooled to obtain lithium manganese iron phosphate cathode material.

[0044] Comparative Example 1

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

[0046] (1) Manganese carbonate, iron phosphate, ammonium dihydrogen phosphate and lithium carbonate were placed in excess deionized water at a molar ratio of 0.6:0.4:0.6:1 and stirred for 0.5 h at a stirring temperature of 25℃ and a stirring 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℃ to obtain lithium iron manganese phosphate precursor powder;

[0047] (2) The precursor powder of lithium iron manganese phosphate was calcined at a low temperature of 450°C for 10 hours under a nitrogen atmosphere and then naturally cooled to obtain small-particle-size lithium iron manganese phosphate crystal nuclei II.

[0048] (3) Lithium manganese iron phosphate nucleus II and glucose were placed in deionized water. The amount of glucose added was 10% of the theoretical mass of lithium manganese iron phosphate. The mixture was stirred for 0.5 h at a stirring temperature of 25℃ and a frequency of 20Hz to obtain a uniformly mixed slurry III. The uniformly mixed slurry III was ground for 2 h to obtain lithium manganese iron phosphate pre-calcined slurry IV. The lithium manganese iron phosphate pre-calcined slurry IV was dried at 150℃ to obtain lithium manganese iron phosphate pre-calcined material V. Finally, the lithium manganese iron phosphate pre-calcined material V was calcined at 750℃ under a nitrogen atmosphere for 14 h and then naturally cooled to obtain lithium manganese iron phosphate cathode material.

[0049] Performance testing: Structural characterization

[0050] SEM images of the small-particle-size lithium manganese iron phosphate crystal nuclei prepared in Example 1 and Comparative Example 1 of this invention are shown below. Figure 1 As shown (from the same batch), it can be seen that the small-particle-size lithium manganese iron phosphate crystal nuclei obtained by low-temperature calcination have uneven particle size and rough surfaces. The SEM image of the large-particle-size lithium manganese iron phosphate crystal nuclei prepared in Example 1 of this invention is shown below. Figure 2 As shown, it can be seen that the large-particle-size lithium manganese iron phosphate crystal nuclei have relatively regular shapes at high temperatures, and most of them are uniform spherical particles with smooth surfaces.

[0051] The SEM image of LMFP obtained by calcining large and small particle sizes of lithium manganese iron phosphate in Example 1 is shown below. Figure 3 As shown, the particle size of the LMFP was statistically analyzed using SEM. Figure 4 As shown; from Figure 3It can be seen that in the LMFP material, lithium manganese iron phosphate particles of different sizes are closely packed, and the gaps between the large-sized lithium manganese iron phosphate particles are filled with many small-sized lithium manganese iron phosphate particles; through Figure 4 The statistics show that the D50 value of small-particle-size lithium manganese iron phosphate is 135nm, while the D50 value of large-particle-size lithium manganese iron phosphate is 273nm.

[0052] SEM images of lithium manganese iron phosphate prepared by secondary sintering of small-particle-size lithium manganese iron phosphate crystal nuclei in Comparative Example 1 are shown below. Figure 5 As shown, the particle size was statistically analyzed using SEM, as follows: Figure 6 The D50 value shown is 134.68 nm. It can be seen that most of the lithium manganese iron phosphate materials formed by high-temperature calcination of small-diameter lithium manganese iron phosphate are small-diameter particles, and different particle size distributions are not formed. Moreover, the gaps between particles are large. The surface shape of small-diameter lithium manganese iron phosphate is irregular, and it is difficult to stack them tightly together in the second calcination. Therefore, the cathode material prepared by Comparative Example 1 has the characteristics of high specific surface area, low compaction, and high capacity.

[0053] Example 2

[0054] The lithium iron phosphate cathode material of Example 2 was prepared by the following steps:

[0055] (1) Manganese carbonate, iron phosphate, ammonium dihydrogen phosphate and lithium carbonate were placed in excess deionized water at a molar ratio of 0.6:0.4:0.6:1 and stirred for 0.5 h at a stirring temperature of 25℃ and a stirring 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℃ to obtain lithium iron manganese phosphate precursor powder;

[0056] (2) The precursor powder of lithium iron manganese phosphate was calcined at a low temperature of 550°C for 10 hours under a nitrogen atmosphere and then naturally cooled to obtain small-particle-size lithium iron manganese phosphate crystal nuclei II.

[0057] (3) The lithium iron manganese phosphate precursor powder was calcined at a high temperature of 750°C for 10 hours under a nitrogen atmosphere and then naturally cooled to obtain large-particle-size lithium iron manganese phosphate crystal nuclei I.

[0058] (4) The large-particle-size lithium manganese iron phosphate crystal nuclei prepared in step (3) and the small-particle-size lithium manganese iron phosphate crystal nuclei prepared in step (2) are mixed at a mass ratio of 2:8 to obtain lithium manganese iron phosphate pre-calcined material.

[0059] (5) The pre-calcined lithium manganese iron phosphate material and glucose were placed in deionized water. The amount of glucose added was 15% of the theoretical mass of lithium manganese iron phosphate. The mixture was stirred for 0.5 h at a stirring temperature of 25℃ and a frequency of 20Hz to obtain a uniformly mixed slurry III. The uniformly mixed slurry III was ground for 2 h to obtain lithium manganese iron phosphate pre-calcined slurry IV. The lithium manganese iron phosphate pre-calcined slurry IV was dried at 150℃ to obtain lithium manganese iron phosphate pre-calcined material V. Finally, the lithium manganese iron phosphate pre-calcined material V was calcined at 780℃ under a nitrogen atmosphere for 14 h and then naturally cooled to obtain lithium manganese iron phosphate cathode material.

[0060] Comparative Example 2

[0061] The basic steps are the same as in Example 2, except that all the lithium manganese iron phosphate precursor powder is calcined at 750°C under a nitrogen atmosphere for 10 hours to obtain lithium manganese iron phosphate nuclei with large particle size. The final lithium iron manganese phosphate cathode material includes the following steps:

[0062] (1) Manganese carbonate, iron phosphate, ammonium dihydrogen phosphate and lithium carbonate were placed in excess deionized water at a molar ratio of 0.6:0.4:0.6:1 and stirred for 0.5 h at a stirring temperature of 25℃ and a stirring 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℃ to obtain lithium iron manganese phosphate precursor powder;

[0063] (2) The lithium iron manganese phosphate precursor powder was calcined at a high temperature of 750°C for 10 hours under a nitrogen atmosphere and then naturally cooled to obtain large-particle lithium iron manganese phosphate crystal nuclei I.

[0064] (3) Large-particle-size lithium manganese iron phosphate nuclei I and glucose were placed in deionized water. The amount of glucose added was 15% of the theoretical mass of lithium manganese iron phosphate. The mixture was stirred for 0.5 h at a stirring temperature of 25℃ and a frequency of 20Hz to obtain a uniformly mixed slurry III. The uniformly mixed slurry III was ground for 2 h to obtain lithium manganese iron phosphate pre-calcined slurry IV. The lithium manganese iron phosphate pre-calcined slurry IV was dried at 150℃ to obtain lithium manganese iron phosphate pre-calcined material V. Finally, the lithium manganese iron phosphate pre-calcined material V was calcined at 780℃ under a nitrogen atmosphere for 14 h and then naturally cooled to obtain lithium manganese iron phosphate cathode material.

[0065] Performance testing: Structural characterization

[0066] SEM images of the large-particle-size lithium manganese iron phosphate crystal nuclei prepared in Example 2 and Comparative Example 2 of this invention are shown below. Figure 8 As shown in the image (obtained from the same batch), it can be seen that the large-particle-size lithium manganese iron phosphate crystal nuclei still exhibit the characteristics of large particle size, smooth surface, and basically uniform size, compared with... Figure 2 In comparison, the particle size is larger than Figure 2 The two are large, and their shapes are not significantly different; the SEM image of the small-particle-size lithium manganese iron phosphate crystal nuclei prepared in Example 2 of this invention is shown below. Figure 7 As shown, small-particle-size lithium manganese iron phosphate crystal nuclei still exhibit characteristics such as small particle size, uneven size, and rough surface, but compared with... Figure 1 Compared to Figure 1 The crystal nuclei are larger and the surface is smoother. It can be seen from the appearance that temperature has a certain influence on the formation of small-sized lithium manganese iron phosphate crystal nuclei.

[0067] The SEM image of LMFP obtained by calcining large and small particle sizes of lithium manganese iron phosphate in Example 2 is shown below. Figure 9 As shown, the particle size of the LMFP was statistically analyzed using SEM. Figure 10 As shown; from Figure 9 It can be seen that in the LMFP material, the final LMFP is still composed of many small-sized lithium manganese iron phosphate particles filling the gaps between large-sized lithium manganese iron phosphate particles, and the overall particle size of LMFP is slightly larger than that of lithium manganese iron phosphate. Figure 3 It can be seen that the particle size difference between the large and small diameter lithium manganese iron phosphate in this embodiment is small, while Figure 10 The particle size distribution also shows that, in this embodiment, the D50 value of small-diameter lithium manganese iron phosphate particles is 139 nm, and the D50 value of large-diameter lithium manganese iron phosphate particles is 267 nm.

[0068] SEM images of lithium manganese iron phosphate prepared by secondary sintering of large-particle-size lithium manganese iron phosphate crystal nuclei in Comparative Example 2 are shown below. Figure 11 As shown, the particle size was statistically analyzed using SEM, as follows: Figure 12 The D50 value shown is 276.48 nm. The large-diameter lithium manganese iron phosphate has a relatively regular appearance, mostly spherical. Under secondary high-temperature calcination, some of the large-diameter lithium manganese iron phosphate nuclei will form small-diameter lithium manganese iron phosphate nuclei and fill the gaps between the large-diameter lithium manganese iron phosphate nuclei. However, comparing the SEM images of the cathode materials of Examples 1 and 2, it can be seen that the number of small-diameter lithium manganese iron phosphate nuclei is obviously insufficient and cannot completely fill the gaps between the large-diameter nuclei, resulting in a slightly lower compaction degree. However, because there are more large-diameter lithium manganese iron phosphate nuclei, 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 in Example 1, except that the high temperature in step (3) is 750℃ and the low temperature in step (2) is 450℃; the SEM image of the prepared lithium manganese iron phosphate cathode material is shown below. Figure 13As shown, under a temperature difference of 300℃, distinct large and small particle size lithium manganese iron phosphate crystal nuclei are formed. During the second sintering process, the small particle size lithium manganese iron phosphate fills the gaps between the large particle size lithium manganese iron phosphate, and they are tightly stacked to form the final lithium manganese iron phosphate cathode material.

[0071] Comparative Example 3

[0072] The basic steps are the same as 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 lithium manganese iron phosphate cathode material is shown below. Figure 14 As shown, under a temperature difference of 100℃, there was no obvious large-size or small-size lithium manganese iron phosphate crystal nuclei. The cathode material obtained by the final two-firing process had a relatively uniform particle size. Compared with Example 1, it can be seen that the appearance and size of the lithium manganese iron phosphate crystal nuclei in Comparative Example 3 are more consistent with the large-size lithium manganese iron phosphate crystal nuclei in Example 1. Furthermore, the gap between the lithium manganese iron phosphate crystal nuclei in Comparative Example 3 is slightly larger, and the compaction degree of the cathode material is lower.

[0073] Comparative Example 4

[0074] The basic steps are the same as in Example 3, except that the carbon source is added during the preparation of the lithium manganese iron phosphate precursor powder in step (1). After the carbon source is introduced, it is carbonized and coated on the crystal nucleus, which is not conducive to the growth of the crystal nucleus. The SEM image of the obtained lithium manganese iron phosphate cathode material is shown in Figure 3. Figure 15 As shown, from Figure 15 It can be clearly seen that the lithium manganese iron phosphate particles of all sizes are irregular in shape; and compared with the lithium manganese iron phosphate crystal nuclei after calcination in Examples 1-3, the growth of the crystal nuclei is not obvious, the stacking between the crystal nuclei is relatively loose, the gaps are large, there is no particle filling, and the compaction of the cathode material is low.

[0075] Performance testing: Electrical performance

[0076] The lithium manganese iron phosphate cathode materials obtained in Examples 1-3 and Comparative Examples 1-4 were used to prepare coin cells and their electrochemical performance was evaluated using the following method: Lithium manganese iron phosphate cathode material, Super P, and PVDF were uniformly mixed in an NMP solution at a ratio of 8:1:1 to obtain a slurry. The slurry was then applied to bright aluminum foil by hand and dried at 100°C. After the NMP had completely evaporated, the electrode was cut into 13mm diameter electrode sheets. The electrode sheets were then dried overnight at 105°C in a vacuum oven. After weighing, the electrode sheets were quickly transferred to a glove box. Lithium metal was used as the counter electrode, Celgard 2400 as the separator, and the electrolyte was 1 mol / L. LiPF6 was dissolved in a mixed solvent of EC / DMC / EMC (volume ratio 1:1:1), and the electrochemical performance of the assembled battery was tested using a Xinwei 4008 constant current test cabinet with a test voltage range of 2.0-4.35V. The electrochemical performance results are shown in Table 1.

[0077] Table 1 Compaction, Specific Gravity, and Electrochemical Performance of Examples and Comparative Examples

[0078]

[0079]

[0080] In conjunction with Example 1 and Comparative Example 1 in Table 1, lithium manganese iron phosphate cathode materials were prepared using small-particle-size lithium manganese iron phosphate nuclei. The small-particle-size lithium manganese iron phosphate cathode material can reduce Li... + Ion diffusion pathway, improving Li + The migration of ions improves its electrochemical performance, but at the same time, the high specific surface area and low compaction of the cathode material have an adverse effect on the overall structural stability of the battery subsequently prepared.

[0081] In both Example 2 and Comparative Example 2, lithium manganese iron phosphate cathode materials were prepared using large-particle-size lithium manganese iron phosphate nuclei. Although the compaction degree was increased and the specific surface area decreased, the large-particle-size lithium manganese iron phosphate increased the Li... + Ion diffusion pathway, reducing Li + The migration of ions leads to a significant decrease in its electrochemical performance.

[0082] Compared to Example 3 and Comparative Example 3, the temperature difference between the large and small particle sizes of lithium manganese iron phosphate prepared in Comparative Example 3 was 100°C, resulting in a smaller difference in particle size and limited interparticle filling. Consequently, the LMFP cathode material prepared by the second calcination process had a specific surface area higher than 11 m². 2 / g has a negative impact on the subsequent cell manufacturing process.

[0083] Compared with Example 3 and Comparative Example 4, when the carbon source is added first during the preparation of the 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 compaction degree 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 a size grading, it is possible to improve the compaction degree 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 compaction and high capacity.

[0085] In addition to the above embodiments, 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 trioxide, manganese salts, manganese iron oxides or manganese iron precursors. The iron source can also be at least one of iron trioxide, iron salts, manganese iron oxides or manganese iron precursors. 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 precursors. 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 generated lithium iron manganese phosphate.

[0086] The mass ratio of the large-particle-size lithium iron manganese phosphate crystal nuclei and the small-particle-size 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 will be given for verification.

Claims

1. A method for preparing a low specific surface area, high compacted and high capacity lithium iron manganese phosphate cathode 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 for 4 - 15 h under low-temperature conditions of 450 - 550 °C in an inert atmosphere to obtain lithium iron manganese phosphate crystal nuclei with small particle sizes; (3) Calcinate the lithium iron manganese phosphate precursor powder for 4 - 15 h under high-temperature conditions of 650 - 750 °C in an inert atmosphere 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, stir, grind and dry, and then calcine at 750 - 800 °C in 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 method for preparing the lithium manganese iron phosphate cathode 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 method for preparing lithium manganese iron phosphate cathode material according to claim 1 or 2, characterized in that, The manganese source includes at least one of manganese carbonate, manganese tetroxide, manganese iron oxide or manganese iron precursor.

4. The method for preparing lithium manganese iron phosphate cathode material according to claim 1 or 2, characterized in that, The iron source includes at least one of iron phosphate, iron tetroxide, manganese iron oxide or manganese iron precursor.

5. The method for preparing lithium manganese iron phosphate cathode 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 method for preparing lithium manganese iron phosphate cathode 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 precursor.

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

9. The method for preparing lithium manganese iron phosphate cathode 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 method for preparing lithium manganese iron phosphate cathode material according to claim 1, characterized in that, In steps (1) and step (5), the grinding time is 0.5 - 6 h, and the drying temperature is 150 - 250 °C.

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