A kind of lithium iron manganese phosphate and its preparation method and application

Abstract

The present invention belongs to the technical field of lithium batteries, and specifically relates to a lithium iron manganese phosphate and its preparation method and application. The preparation method of the lithium iron manganese phosphate provided by the present invention comprises the following steps: mixing manganese dioxide, a lithium source, an iron source and a carbon source to obtain a mixed material; sintering the mixed material to obtain the lithium iron manganese phosphate; the sintering comprises low-temperature sintering, medium-temperature sintering and high-temperature sintering in sequence; the temperature of the low-temperature sintering is greater than or equal to 450°C and less than 550°C, the temperature of the medium-temperature sintering is greater than or equal to 550°C and less than 625°C, and the temperature of the high-temperature sintering is greater than or equal to 625°C and less than 750°C. The present invention adopts a "step-by-step continuous temperature control" roasting method to realize a solid-liquid mixing method, and self-constructs a lithium iron manganese phosphate material with a three-dimensional structure, small and uniform particle size through a specific sintering process.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium batteries, and in particular relates to lithium manganese iron phosphate and a preparation method and application thereof. Background Art

[0002] Lithium batteries have become the mainstream power products for electric tools due to their high energy, high power and high environmental protection. At present, the main power material of lithium batteries is lithium iron phosphate material, which has the advantages of good safety, long service life and low cost. Compared with lithium iron phosphate material, lithium iron manganese phosphate has the advantages of high specific capacity, high energy density, long cycle life and high voltage platform, which is more suitable for lithium battery power materials; however, the particle size of lithium iron manganese phosphate prepared by traditional solid phase method and liquid phase method is relatively large, and D50 is mostly 1.5~4μm. Due to its large particle size, lithium iron manganese phosphate has poor conductivity, which limits its gram capacity, thereby limiting its application in lithium batteries. Summary of the invention

[0003] In view of this, the present invention provides a lithium manganese iron phosphate and a preparation method and application thereof. The lithium manganese iron phosphate prepared according to the preparation method provided by the present invention has a smaller particle size and a uniform particle size distribution, which improves its conductivity; using it in a lithium battery can increase the gram capacity of the lithium battery.

[0004] In order to solve the above technical problems, the present invention provides a method for preparing lithium manganese iron phosphate, comprising the following steps:

[0005] Mixing manganese dioxide, a lithium source, an iron source and a carbon source to obtain a mixed material;

[0006] The mixed material is sintered to obtain the lithium manganese iron phosphate; the sintering includes low-temperature sintering, medium-temperature sintering and high-temperature sintering performed in sequence; the temperature of the low-temperature sintering is greater than or equal to 450°C and less than 550°C, the temperature of the medium-temperature sintering is greater than or equal to 550°C and less than 625°C, and the temperature of the high-temperature sintering is greater than or equal to 625°C and less than 750°C.

[0007] Preferably, the holding time of the low-temperature sintering is 1 to 3 hours.

[0008] Preferably, the holding time of the medium-temperature sintering is 3 to 5 hours.

[0009] Preferably, the holding time of the high temperature sintering is 8 to 10 hours.

[0010] Preferably, the lithium source includes lithium carbonate, lithium hydroxide or lithium nitrate, the iron source includes iron phosphate, the manganese dioxide is electrolytic manganese dioxide or chemical manganese dioxide; the carbon source includes bean dregs, sweet potato dregs, distiller's grains, kudzu vine dregs or betel nut dregs.

[0011] The molar ratio of the manganese dioxide, the iron element in the iron source and the lithium element in the lithium source is 0.8-0.2:0.2-0.8:1;

[0012] The mass ratio of the total mass of the manganese dioxide, the lithium source and the iron source to the carbon source is 100:2-20.

[0013] Preferably, the mixing is carried out under stirring conditions, the stirring speed is 200 to 600 rpm, and the stirring time is 2 to 3 hours.

[0014] Preferably, after the sintering, the method further comprises: screening the sintered product to obtain the lithium manganese iron phosphate.

[0015] Preferably, the sieve has an aperture of 100 to 325 meshes.

[0016] The present invention also provides lithium manganese iron phosphate prepared according to the preparation method described in the above technical scheme, and the D50 of the lithium manganese iron phosphate is 0.5-1.5 μm.

[0017] The present invention also provides the application of the lithium iron manganese phosphate described in the above technical solution in a lithium battery.

[0018] The present invention provides a method for preparing lithium manganese iron phosphate, comprising the following steps: mixing manganese dioxide, a lithium source, an iron source and a carbon source to obtain a mixed material; sintering the mixed material to obtain the lithium manganese iron phosphate; the sintering includes low-temperature sintering, medium-temperature sintering and high-temperature sintering in sequence; the temperature of the low-temperature sintering is greater than or equal to 450°C and less than 550°C, the temperature of the medium-temperature sintering is greater than or equal to 550°C and less than 625°C, and the temperature of the high-temperature sintering is greater than or equal to 625°C and less than 750°C. The present invention adopts a "step-by-step continuous temperature control" roasting method to achieve a solid-liquid mixing method, and self-constructs a lithium manganese iron phosphate material with a three-dimensional structure, small and uniform particle size through a specific sintering process. Manganese dioxide melts during the low-temperature sintering process, and the lithium source is dispersed in the molten manganese dioxide to form a solid-liquid mixed phase. At the same time, after the temperature is raised to the temperature required for medium-temperature sintering based on the low temperature of low-temperature sintering, the iron source particles undergo initial splitting due to uneven expansion of the particles caused by uneven heating, resulting in stress imbalance, and split into particles of uneven size; during the medium-temperature sintering process, the lithium source is in a solid-liquid mixed phase, thereby lowering the melting point, forming a uniform liquid phase of the lithium source and manganese dioxide, and at the same time, the larger iron source particles formed by the initial splitting (small iron source particles can resist uneven expansion of particles caused by uneven heating to produce stress imbalance, and large particles cannot resist uneven expansion of particles caused by uneven heating to produce stress imbalance, resulting in secondary splitting) undergo secondary splitting to form particles of uniform size; high-temperature sintering increases the generation rate and composition uniformity of lithium iron phosphate nanocrystal seeds, and further accelerates the nanocrystal seeds-induced uniform crystallization and growth of grains, and finally generates a preparation of lithium iron manganese phosphate material with a three-dimensional structure, small and uniform particle size. The preparation method provided by the present invention breaks through the technical bottleneck of the difficulty in homogenizing microscopic components in the traditional solid phase method, has simple operation, easy process control and low production cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 This is a SEM image of the lithium manganese iron phosphate prepared in Example 1;

[0020] Figure 2 This is a SEM image of the lithium manganese iron phosphate prepared in Example 2;

[0021] Figure 3 This is a SEM image of the lithium manganese iron phosphate prepared in Example 3;

[0022] Figure 4 This is a SEM image of the lithium manganese iron phosphate prepared in Comparative Example 1;

[0023] Figure 5 This is the SEM image of the lithium manganese iron phosphate prepared in Comparative Example 2. DETAILED DESCRIPTION

[0024] The present invention provides a method for preparing lithium manganese iron phosphate, comprising the following steps:

[0025] Mixing manganese dioxide, a lithium source, an iron source and a carbon source to obtain a mixed material;

[0026] The mixed material is sintered to obtain the lithium manganese iron phosphate; the sintering includes low-temperature sintering, medium-temperature sintering and high-temperature sintering performed in sequence; the temperature of the low-temperature sintering is greater than or equal to 450°C and less than 550°C, the temperature of the medium-temperature sintering is greater than or equal to 550°C and less than 625°C, and the temperature of the high-temperature sintering is greater than or equal to 625°C and less than 750°C.

[0027] The present invention mixes manganese dioxide, a lithium source, an iron source and a carbon source to obtain a mixed material. In the present invention, the manganese dioxide is preferably electrolytic manganese dioxide or chemical manganese dioxide, more preferably electrolytic manganese dioxide. In the present invention, the lithium source preferably includes lithium carbonate, lithium hydroxide or lithium nitrate, more preferably lithium carbonate. In the present invention, the iron source preferably includes iron phosphate. In the present invention, the carbon source is preferably a solid waste carbon source; the carbon source preferably includes bean dregs, sweet potato dregs, wine lees, kudzu dregs or betel nut dregs, more preferably bean dregs, sweet potato dregs, kudzu dregs, and further preferably bean dregs. In the present invention, the carbon source can reduce manganese in manganese dioxide and iron in the iron source, and the carbon source remaining in the product lithium manganese iron phosphate can further improve the conductivity of lithium manganese iron phosphate.

[0028] In the present invention, the molar ratio of the manganese dioxide, the iron element in the iron source and the lithium element in the lithium source is preferably 0.8-0.2: 0.2-0.8: 1, more preferably 0.6: 0.4: 1. In the present invention, the mass ratio of the total mass of the manganese dioxide, the lithium source and the iron source to the carbon source is preferably 100: 2-20, more preferably 100: 5-15, and further preferably 100: 8-10.

[0029] In the present invention, the mixing is preferably carried out under stirring conditions, and the stirring speed is preferably 200-600 rpm, more preferably 300-500 rpm, and further preferably 350-400 rpm; the stirring time is preferably 2-3 h, and more preferably 2.3-2.5 h.

[0030] After obtaining the mixed material, the present invention sintered the mixed material to obtain the lithium manganese iron phosphate. In the present invention, the sintering includes low-temperature sintering, medium-temperature sintering and high-temperature sintering performed in sequence. In the present invention, the temperature of the low-temperature sintering is greater than or equal to 450°C and less than 550°C, preferably greater than or equal to 500°C and less than 550°C; the temperature of the low-temperature sintering is specifically 520-530°C, 540-550°C or 510-520°C; the holding time of the low-temperature sintering is preferably 1-3h, more preferably 2h. In the present invention, the temperature of the medium-temperature sintering is greater than or equal to 550°C and less than 625°C, preferably 560-600°C; the temperature of the medium-temperature sintering is specifically 590-600°C, 615-625°C or 560-570°C; the holding time of the medium-temperature sintering is preferably 3-5h, more preferably 4h. In the present invention, the temperature of the high temperature sintering is greater than or equal to 625°C and less than 750°C, preferably 650-700°C; the temperature of the high temperature sintering is specifically 650-660°C, 690-700°C or 680-690°C; the holding time of the high temperature sintering is preferably 8-10h, more preferably 9h. In order to facilitate operation, the present invention limits the low sintering temperature, the medium temperature sintering temperature and the high temperature sintering temperature within a certain range, and the difference between the maximum and minimum values ​​of the low sintering temperature, the medium temperature sintering temperature and the high temperature sintering temperature is preferably not more than 30°C, more preferably 10-20°C. The present invention preferably heats up to the required temperature for medium temperature sintering on the basis of the low temperature sintering temperature, and the present invention has no special limitation on the heating rate; the present invention preferably heats up to the required temperature for high temperature sintering on the basis of the medium temperature sintering temperature, and the present invention has no special limitation on the heating rate.

[0031] In the present invention, after the sintering, the process preferably further comprises: sieving the sintered product to obtain the lithium manganese iron phosphate. In the present invention, the aperture of the sieve for sieving is preferably 100 to 325 meshes, more preferably 200 to 300 meshes.

[0032] The present invention also provides lithium manganese iron phosphate prepared according to the preparation method described in the above technical scheme, wherein the D50 of the lithium manganese iron phosphate is 0.5-1.5 μm, preferably 0.8-1.5 μm, 0.5-1.2 μm or 0.9-1.4 μm.

[0033] The present invention also provides the use of the lithium iron manganese phosphate described in the above technical solution in a lithium battery. In the present invention, the lithium battery prepared by using the lithium iron manganese phosphate as a power material has a higher gram capacity.

[0034] In order to further illustrate the present invention, the technical solutions provided by the present invention are described in detail below in conjunction with embodiments, but they should not be construed as limiting the protection scope of the present invention.

[0035] The embodiment has no special requirements on the weight of the material, as long as it can meet the dosage ratio.

[0036] Example 1

[0037] Using electrolytic manganese dioxide, lithium carbonate, iron phosphate and soybean dregs as raw materials, the molar ratio of manganese element, iron element and lithium element is 0.6:0.4:1, the total mass of manganese dioxide, lithium carbonate and iron phosphate and the mass ratio of carbon source soybean dregs is 100:8, stirring for 3 hours at a rotation speed of 350 rpm to obtain a mixed material;

[0038] The mixed material is placed in a roasting furnace for low-temperature sintering, medium-temperature sintering and high-temperature sintering in sequence. The temperature of the low-temperature sintering is 520-530°C, the insulation time of the low-temperature sintering is 2h, and then the temperature is raised to 590-600°C for medium-temperature sintering for 4h, and finally the temperature is raised to 650-660°C for high-temperature sintering for 9h; the product after high-temperature sintering is passed through a 200-mesh standard sieve to obtain lithium manganese iron phosphate.

[0039] Example 2

[0040] Lithium manganese iron phosphate is prepared according to the method of Example 1, except that the temperature of low-temperature sintering is 540-550°C, and the holding time of low-temperature sintering is 2.5 hours; the temperature of medium-temperature sintering is 615-625°C, and the holding time of medium-temperature sintering is 3.5 hours; the temperature of high-temperature sintering is 690-700°C, and the holding time of high-temperature sintering is 8 hours.

[0041] Example 3

[0042] Lithium manganese iron phosphate was prepared according to the method of Example 1, except that the temperature of low-temperature sintering was 510-520°C, and the holding time of low-temperature sintering was 3 h; the temperature of medium-temperature sintering was 560-570°C, and the holding time of medium-temperature sintering was 5 h; the temperature of high-temperature sintering was 680-690°C, and the holding time of high-temperature sintering was 10 h.

[0043] Comparative Example 1

[0044] Prepare a mixed material according to the method of Example 1;

[0045] The mixed material was pre-sintered at 300-500° C. for 3 h in a nitrogen atmosphere and then calcined at 600-800° C. for 7 h to obtain lithium manganese iron phosphate.

[0046] Comparative Example 2

[0047] Prepare a mixed material according to the method of Example 1;

[0048] The mixed material is sintered at 600-700°C for 6 hours to obtain lithium manganese iron phosphate.

[0049] The lithium manganese iron phosphate prepared in Examples 1 to 3 and Comparative Examples 1 to 2 was examined by scanning electron microscopy to obtain SEM images, as shown in FIG. Figures 1 to 5 shown; Figure 1 This is a SEM image of the lithium manganese iron phosphate prepared in Example 1. Figure 2 This is a SEM image of the lithium manganese iron phosphate prepared in Example 2. Figure 3 This is a SEM image of the lithium manganese iron phosphate prepared in Example 3. Figure 4 This is a SEM image of the lithium manganese iron phosphate prepared in Comparative Example 1; Figure 5 This is a SEM image of the lithium manganese iron phosphate prepared in Comparative Example 2. The average particle size of the lithium manganese iron phosphate prepared in Examples 1 to 3 and Comparative Examples 1 to 2 was obtained by analyzing the SEM images, and the results are listed in Table 1.

[0050] Table 1 Particle size of lithium manganese iron phosphate prepared in Examples 1 to 3 and Comparative Examples 1 to 2

[0051] Example D50(μm) Example 1 0.8～1.5 Example 2 0.5～1.2 Example 3 0.9～1.4 Comparative Example 1 2～4 Comparative Example 2 1～5

[0052] Combining Table 1 and Figures 1 to 5 It can be seen that the lithium manganese iron phosphate prepared according to the preparation method provided by the present invention has a smaller particle size and a uniform particle size distribution.

[0053] The lithium manganese iron phosphate prepared in Examples 1 to 3 and Comparative Examples 1 to 2 was used as a power material to prepare a lithium battery, and a charge and discharge cycle test was performed to obtain the gram capacity, capacity retention rate and cycle performance. The results are listed in Table 2.

[0054] Table 2 Electrochemical properties of lithium manganese iron phosphate prepared in Examples 1 to 3 and Comparative Examples 1 to 2

[0055]

[0056] As can be seen from Table 2, the 0.1C discharge specific capacity of Example 1 is 17.9 mAh / g higher than that of Comparative Example 2, the 5C / 0.1C capacity retention rate is 7.8% higher, and the 0.5C cycle retention rate is 7.6% higher, indicating that the lithium manganese iron phosphate prepared in the embodiment of the present invention has a higher gram capacity and good rate performance, and improves the cycle performance.

[0057] Although the above embodiment describes the present invention in detail, it is only a part of the embodiments of the present invention, not all of the embodiments. People can also obtain other embodiments based on this embodiment without creativity, and these embodiments all fall within the protection scope of the present invention.

Claims

1. A method for preparing lithium manganese iron phosphate, characterized in that: The following steps are involved: Mixing manganese dioxide, a lithium source, an iron source and a carbon source to obtain a mixed material; the iron source is iron phosphate; The mixed material is sintered to obtain the lithium manganese iron phosphate; the sintering includes low-temperature sintering, medium-temperature sintering and high-temperature sintering performed in sequence; the temperature of the low-temperature sintering is greater than or equal to 450°C and less than 550°C, the temperature of the medium-temperature sintering is greater than or equal to 550°C and less than 625°C, and the temperature of the high-temperature sintering is greater than or equal to 625°C and less than 750°C.

2. The method for preparing lithium manganese iron phosphate according to claim 1, characterized in that: The holding time of the low temperature sintering is 1 to 3 hours.

3. The method for preparing lithium manganese iron phosphate according to claim 1, characterized in that: The holding time of the medium temperature sintering is 3 to 5 hours.

4. The method for preparing lithium manganese iron phosphate according to claim 1, characterized in that: The high temperature sintering holding time is 8 to 10 hours.

5. The method for preparing lithium iron manganese phosphate according to claim 1, characterized in that: The lithium source includes lithium carbonate, lithium hydroxide or lithium nitrate; the manganese dioxide is electrolytic manganese dioxide or chemical manganese dioxide; the carbon source includes bean dregs, sweet potato dregs, wine lees, kudzu root dregs or betel nut dregs; The molar ratio of the manganese dioxide, the iron element in the iron source and the lithium element in the lithium source is 0.8-0.2:0.2-0.8:1; The mass ratio of the total mass of the manganese dioxide, the lithium source and the iron source to the carbon source is 100:2-20.

6. The method for preparing lithium manganese iron phosphate according to claim 1 or 5, characterized in that: The mixing is carried out under stirring conditions, the stirring speed is 200-600 rpm, and the stirring time is 2-3 hours.

7. The method for preparing lithium manganese iron phosphate according to claim 1, characterized in that: After the sintering, the method further comprises: screening the sintered product to obtain the lithium manganese iron phosphate.

8. The method for preparing lithium iron manganese phosphate according to claim 7, characterized in that: The aperture of the sieve for screening is 100 to 325 meshes.

9. Lithium manganese iron phosphate prepared according to the preparation method according to any one of claims 1 to 8, characterized in that: The D50 of the lithium manganese iron phosphate is 0.5-1.5 μm.

10. Use of the lithium iron manganese phosphate according to claim 9 in lithium batteries.

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