A lithium manganese iron phosphate composite material and its preparation method and application

Abstract

The present invention relates to the field of lithium-ion battery positive electrode materials, and more specifically, to a lithium iron manganese phosphate composite material, and its preparation method and application. The method comprises: performing a first washing on the precipitated slurry; aging the first washed filter cake; performing a second washing on the reaction system after the aging; mixing a first clear liquid with an iron source; mixing the first mixed system with ammonia water and performing a first reaction; mixing the reaction system after the first reaction with the second clear liquid and performing a second reaction; performing a third washing on the precursor suspension to obtain a lithium iron manganese phosphate precursor; mixing a lithium source, a carbon source, a lithium iron manganese phosphate precursor and the first washed filter cake and performing a hydrothermal reaction; performing a fourth washing, drying and sintering on the reaction system after the hydrothermal reaction. This method not only solves the problem of wastewater recycling in the iron phosphate production process, but also reduces the production cost of lithium iron manganese phosphate. The prepared material is small in size and has good electrochemical properties.

Description

Technical Field

[0001] The present invention relates to the field of lithium-ion battery positive electrode materials, and in particular to a lithium manganese iron phosphate composite material and a preparation method and application thereof. Background Art

[0002] Lithium iron phosphate is the most commonly used cathode material in lithium batteries currently in use, but it also has inevitable drawbacks, such as low energy density. The high voltage of the manganese element in lithium iron phosphate (LFP) gives LFP a higher voltage platform (4.1V). Therefore, LFP has an energy density approximately 15% higher than LFP, and its cycle performance and safety performance are comparable to LFP. This balance of high safety and high energy density holds great promise for future applications.

[0003] Iron phosphate is a precursor of lithium iron phosphate and is in high demand. However, when rutile mother liquor, a by-product of titanium dioxide, is used as the iron source, the wastewater generated in the precipitation section of the two-step iron phosphate production process has a high manganese ion content, and the wastewater generated in the aging section has a high phosphate content. If treated as wastewater, the treatment cost is high and the process is complicated. Therefore, these two parts of wastewater can be used as manganese sources and phosphorus sources respectively to prepare lithium manganese iron phosphate, which greatly saves production costs. However, there are few applications for this part.

[0004] In view of this, the present invention is proposed. Summary of the Invention

[0005] In response to the problems existing in the prior art, the purpose of the present invention is to provide a method for preparing a lithium iron manganese phosphate composite material using iron phosphate wastewater from different sections as a manganese source and a phosphorus source, respectively; in addition, the amorphous iron phosphate obtained in the precipitation section is hydrothermally treated with a lithium iron manganese phosphate precursor to obtain a lithium iron phosphate-coated lithium iron manganese phosphate composite material.

[0006] In order to achieve the above-mentioned purpose of the present invention, the following technical solutions are adopted:

[0007] One aspect of the present invention relates to a method for preparing a lithium manganese iron phosphate composite material, comprising the following steps:

[0008] (a) performing a first washing on the precipitated slurry to obtain a first clear liquid and a first washed filter cake; aging the first washed filter cake; and performing a second washing on the reaction system after the aging to obtain a second clear liquid;

[0009] (b) mixing the first clear liquid and an iron source to obtain a first mixed system; and mixing the first mixed system with aqueous ammonia to perform a first reaction;

[0010] (c) mixing the reaction system after the first reaction and the second clear liquid, and then performing a second reaction to obtain a precursor suspension; and performing a third washing on the precursor suspension to obtain a lithium manganese iron phosphate precursor;

[0011] (d) mixing a lithium source, a carbon source, the lithium iron manganese phosphate precursor and the first washed filter cake and subjecting the mixture to a hydrothermal reaction; and subjecting the reaction system after the hydrothermal reaction to a fourth washing, drying and sintering to obtain the lithium iron manganese phosphate composite material.

[0012] The present invention provides a method for preparing a lithium iron manganese phosphate composite material by using iron phosphate wastewater from different work sections as a manganese source and a phosphorus source, respectively. The method not only solves the problem of wastewater recycling in the iron phosphate production process, but also reduces the production cost of the lithium iron manganese phosphate. The prepared lithium iron manganese phosphate composite material is small in size and has good electrochemical properties.

[0013] Another aspect of the present invention also relates to the lithium manganese phosphate composite material prepared by the preparation method of the lithium manganese phosphate composite material.

[0014] Another aspect of the present invention also relates to a positive electrode plate, which is mainly made of the lithium manganese iron phosphate composite material prepared by the preparation method of the lithium manganese iron phosphate composite material or the lithium manganese iron phosphate composite material.

[0015] Another aspect of the present invention also relates to a lithium-ion battery, comprising the above-mentioned positive electrode sheet.

[0016] Compared with the prior art, the present invention has the following beneficial effects:

[0017] (1) The present invention provides a method for preparing lithium iron manganese phosphate material using iron phosphate wastewater from different sections as a manganese source and a phosphorus source, respectively. This method not only solves the problem of wastewater recycling in the iron phosphate production process, but also reduces the production cost of lithium iron manganese phosphate;

[0018] (2) In addition, the concentrations of manganese ions and phosphate radicals in the iron phosphate wastewater are low. When participating in the reaction, the growth momentum of the crystal after nucleation is small. At this time, the growth of the crystal is mainly affected by thermodynamics, which is conducive to reducing the particle size. The advantages of high-speed mixing, efficient mass transfer and heat transfer, and short residence time of the reactants in the microreactor are utilized to further reduce the particle size of the lithium manganese iron phosphate precursor.

[0019] (3) The amorphous iron phosphate obtained in the precipitation process is mixed with a small particle lithium manganese iron phosphate precursor and subjected to hydrothermal treatment. During the process of the amorphous iron phosphate particles being disrupted and reorganized into a crystalline form, the iron phosphate is coated with the small particle lithium manganese iron phosphate precursor. The prepared lithium manganese iron phosphate composite material has a smaller size and better electrochemical properties. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the specific embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0021] Figure 1 This is a scanning electron microscope image of the lithium manganese iron phosphate composite material prepared in Example 1;

[0022] Figure 2 This is a scanning electron microscope image of the lithium manganese iron phosphate composite material prepared in Comparative Example 1. DETAILED DESCRIPTION

[0023] The technical scheme of the present invention will be clearly and completely described below in conjunction with the accompanying drawings and specific embodiments, but it will be understood by those skilled in the art that the following described embodiments are part of embodiments of the present invention, rather than all embodiments, and are only used to illustrate the present invention, and should not be considered as limiting the scope of the present invention. Based on the embodiments in the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative work are within the scope of protection of the present invention. Those who do not specify specific conditions in the embodiments are carried out according to normal conditions or the conditions recommended by the manufacturer. Those whose reagents or instruments are not specified by the manufacturer are conventional products that can be purchased commercially.

[0024] One aspect of the present invention relates to a method for preparing a lithium manganese iron phosphate composite material, comprising the following steps:

[0025] (a) performing a first washing on the precipitated slurry obtained in the ferric phosphate precipitation step to obtain a first clear liquid and a first-wash filter cake; aging the first-wash filter cake; and performing a second washing on the reaction system after the aging to obtain a second clear liquid;

[0026] (b) mixing the first clear liquid and an iron source to obtain a first mixed system; mixing the first mixed system with aqueous ammonia and then performing a first reaction to combine manganese and iron with hydroxide to generate hydroxide;

[0027] (c) mixing the reaction system after the first reaction and the second clear liquid and performing a second reaction, wherein the hydroxide generated in the above reaction reacts with the phosphate in the clear liquid to generate a lithium iron manganese phosphate precursor, ammonium iron manganese phosphate, to obtain a precursor suspension; and performing a third washing on the precursor suspension to obtain a lithium iron manganese phosphate precursor;

[0028] (d) mixing a lithium source, a carbon source, the lithium iron manganese phosphate precursor and the first washed filter cake and subjecting the mixture to a hydrothermal reaction; and subjecting the reaction system after the hydrothermal reaction to a fourth washing, drying and sintering to obtain the lithium iron manganese phosphate composite material.

[0029] The present invention provides a method for preparing a lithium iron manganese phosphate composite material by using iron phosphate wastewater from different work sections as a manganese source and a phosphorus source, respectively. The method not only solves the problem of wastewater recycling in the iron phosphate production process, but also reduces the production cost of the lithium iron manganese phosphate. The prepared lithium iron manganese phosphate composite material is small in size and has good electrochemical properties.

[0030] The concentrations of manganese ions and phosphate radicals in the filtrate and wash water from the filter press washing after precipitation and the filtrate and wash water from the filter press washing after aging are low. When participating in the reaction, the growth kinetics of the crystal after nucleation is relatively small. At this time, the growth of the crystal is mainly affected by thermodynamics, which is conducive to reducing the particle size. The particle size of the lithium manganese iron phosphate precursor is further reduced by utilizing the advantages of high-speed mixing, efficient mass transfer and heat transfer, and short residence time of reactants in the microreactor.

[0031] The amorphous iron phosphate obtained in the precipitation section is mixed with a small-particle lithium manganese iron phosphate precursor and subjected to hydrothermal treatment. During the process of disrupting and reorganizing the crystal form of the amorphous iron phosphate particles, the iron phosphate coats the small-particle lithium manganese iron phosphate precursor and combines with the lithium source to obtain the LFP@LMFP composite material.

[0032] Furthermore, in step (b), the molar ratio of iron to manganese in the first mixed system is 1:(0.9-1.1) (eg, 1:0.9, 1:1.0 or 1:1.1).

[0033] Furthermore, the iron source is mixed with the first clear liquid in the form of a ferrous salt solution.

[0034] Furthermore, the concentration of the ferrous salt solution is 0.1 to 1 mol / L (eg, 0.1 mol / L, 0.2 mol / L, 0.4 mol / L, 0.6 mol / L, 0.8 mol / L or 1 mol / L).

[0035] Furthermore, in step (b), the mixing volume ratio of the ammonia water and the first mixed system is 1:(0.9-1.1) (eg, 1:0.9, 1:1.0 or 1:1.1).

[0036] Furthermore, the concentration of the ammonia water is 10 wt% to 15 wt% (eg, 10%, 11%, 12%, 13%, 14% or 15%).

[0037] Furthermore, in step (c), the mixing volume ratio of the reaction system after the completion of the first reaction and the second clear liquid is 1:(0.9-1.1) (for example, 1:0.9, 1:1.0 or 1:1.1).

[0038] Furthermore, the manganese content in the first clear solution is 0.05 wt% to 0.2 wt% (eg, 0.05 wt%, 0.1 wt%, 0.15 wt% or 0.2 wt%).

[0039] Furthermore, the phosphorus content in the second clear liquid is 0.3 wt% to 0.6 wt% (eg, 0.3 wt%, 0.4 wt%, 0.5 wt% or 0.6 wt%).

[0040] Furthermore, in step (d), the molar ratio of Li and P in the mixed system after mixing the lithium source, the carbon source, the lithium iron manganese phosphate precursor and the washed filter cake is (1.05-1):1 (for example, 1.05:1, 1.06:1, 1.07:1, 1.08:1, 1.09:1 or 1:1).

[0041] Preferably, in step (d), the mass ratio of the lithium manganese iron phosphate precursor to the washed filter cake is 100:(20-50) (for example, 100:20, 100:25, 100:30, 100:35, 100:40, 100:45 or 100:50).

[0042] Preferably, in step (d), the mass of the carbon source is 10% to 30% (e.g., 10%, 15%, 20%, 25% or 30%) of the mass of the mixed system after mixing the lithium source, the carbon source, the lithium iron manganese phosphate precursor and the washed filter cake.

[0043] Furthermore, the temperature of the hydrothermal reaction is 90-160°C (eg, 90°C, 100°C, 110°C, 120°C, 130°C, 140°C, 150°C or 160°C).

[0044] Furthermore, the hydrothermal reaction time is 4 to 24 hours (eg, 4 hours, 8 hours, 12 hours, 16 hours, 20 hours or 24 hours).

[0045] Furthermore, the sintering temperature is 600-800°C (eg, 600°C, 630°C, 650°C, 680°C, 700°C, 730°C, 750°C, 780°C or 800°C).

[0046] Furthermore, the heating rate of the sintering is 1-3°C / min (eg, 1°C / min, 2°C / min, or 3°C / min).

[0047] Furthermore, the sintering time is 4 to 10 hours (for example, 4 hours, 5 hours, 6 hours, 7 hours, 8 hours, 9 hours or 10 hours).

[0048] Furthermore, the aging temperature is 75-98°C (e.g., 75°C, 78°C, 80°C, 83°C, 85°C, 88°C, 90°C, 93°C, 95°C or 98°C).

[0049] Furthermore, the aging time is 0.5 to 4 hours (eg, 0.5 hours, 1 hour, 2 hours, 3 hours or 4 hours).

[0050] Furthermore, the iron source includes at least one of ferrous chloride, ferrous nitrate, ferrous sulfate or ferrous oxalate.

[0051] Furthermore, the lithium source includes at least one of lithium hydroxide, lithium oxalate, lithium carbonate, lithium nitrate or lithium sulfate.

[0052] Furthermore, the carbon source includes at least one of sucrose, glucose, PEG6000 or PEG2000.

[0053] Furthermore, the first mixed system and the ammonia water are pumped into the microreactor from both ends through a peristaltic pump, and the first reaction is carried out by contacting in the middle of the microreactor.

[0054] Furthermore, the flow rate ratio of the first mixing system and the ammonia water into the microreactor is 1:1.

[0055] Furthermore, after the first reaction is completed, the reaction system and the second clear liquid are pumped into the unreacted ends through a peristaltic pump, and the second reaction is carried out by contacting in the middle of the microreactor to obtain a precursor suspension.

[0056] Furthermore, the flow rate ratio of the reaction system after the first reaction and the second clear liquid entering the microreactor is 1:1.

[0057] Furthermore, the volume flow rate of the peristaltic pump is 20 to 80 mL / min (eg, 20 mL / min, 30 mL / min, 40 mL / min, 50 mL / min, 60 mL / min, 70 mL / min, or 80 mL / min).

[0058] Another aspect of the present invention also relates to the lithium manganese phosphate composite material prepared by the preparation method of the lithium manganese phosphate composite material.

[0059] Another aspect of the present invention also relates to a positive electrode plate, which is mainly made of the lithium manganese iron phosphate composite material prepared by the preparation method of the lithium manganese iron phosphate composite material or the lithium manganese iron phosphate composite material.

[0060] Another aspect of the present invention also relates to a lithium-ion battery, comprising the above-mentioned positive electrode sheet.

[0061] The embodiments of the present invention will be described in detail below with reference to specific examples and comparative examples.

[0062] Example 1

[0063] The preparation method of the lithium manganese iron phosphate composite material provided in this embodiment includes the following steps:

[0064] (1) The precipitated slurry obtained in the ferric phosphate precipitation process is filtered and washed to obtain a clear solution 1 and a first-wash filter cake, wherein the manganese content in the clear solution 1 is 0.1 wt %. The first-wash filter cake is aged and then filtered and washed to obtain a clear solution 2, wherein the phosphorus content in the clear solution 2 is 0.5%. The aging temperature is 86° C. and the time is 2 h.

[0065] (2) 0.5 mol / L ferrous sulfate solution and clear solution 1 are mixed to obtain a first mixed system, wherein the molar ratio of iron to manganese in the first mixed system is 1:1; the first mixed system and 12% ammonia water are pumped into the microreactor from both ends through a peristaltic pump, wherein the mixing volume ratio of ammonia water to the first mixed system is 1:1, and the volume flow rate of the peristaltic pump is 50 mL / min, and a first reaction is carried out by contact in the middle to obtain a hydroxide suspension;

[0066] (3) The hydroxide suspension and the clear solution 2 are pumped into the microreactor from both ends through a peristaltic pump, the mixed volume ratio of the hydroxide suspension and the clear solution 2 is 1:1, the volume flow rate of the peristaltic pump is 50 mL / min, and a second reaction is carried out through contact in the middle to form a precursor suspension, which is then washed by filter press to obtain a lithium manganese iron phosphate precursor with a diameter of 30 to 50 nm;

[0067] (4) Lithium carbonate, PEG6000, lithium iron manganese phosphate precursor and a washed filter cake were mixed in deionized water and transferred to a hydrothermal kettle for hydrothermal reaction; the molar ratio of Li and P in the mixed system after the lithium source, carbon source, lithium iron manganese phosphate precursor and the washed filter cake was 1.05:1; the mass ratio of the lithium iron manganese phosphate precursor and the washed filter cake was 100:30; the mass of the carbon source was 20% of the mass of the mixed system after the lithium source, carbon source, lithium iron manganese phosphate precursor and the washed filter cake were mixed; the hydrothermal temperature was 120°C, the heating rate was 2°C / min, and the time was 8h;

[0068] (5) The obtained precipitate was washed, dried, and sintered at 700°C for 6 hours to obtain a lithium manganese iron phosphate composite material; its scanning electron microscopy showed that Figure 1 shown.

[0069] Example 2

[0070] The preparation method of the lithium manganese iron phosphate composite material provided in this embodiment includes the following steps:

[0071] (1) The precipitated slurry obtained in the ferric phosphate precipitation stage is filtered and washed to obtain a clear liquid 1 and a first-wash filter cake, wherein the manganese content in the clear liquid 1 is 0.2 wt %. The first-wash filter cake is aged and then filtered and washed to obtain a clear liquid 2, wherein the phosphorus content in the clear liquid 2 is 0.3%. The aging temperature is 98° C. and the time is 0.5 h.

[0072] (2) 0.5 mol / L ferrous chloride solution and clear solution 1 are mixed to obtain a first mixed system, wherein the molar ratio of iron to manganese in the first mixed system is 1:1.1; the first mixed system and 12% ammonia water are pumped into the microreactor from both ends through a peristaltic pump, wherein the mixing volume ratio of ammonia water to the first mixed system is 1:1.1, the volume flow rate of the peristaltic pump is 50 mL / min, and a first reaction is carried out through contact in the middle to obtain a hydroxide suspension;

[0073] (3) The hydroxide suspension and the clear solution 2 are pumped into the microreactor from both ends through a peristaltic pump, the mixed volume ratio of the hydroxide suspension and the clear solution 2 is 1:0.9, the volume flow rate of the peristaltic pump is 50 mL / min, and a second reaction is carried out through contact in the middle to form a precursor suspension, which is then filtered and washed to obtain a lithium manganese iron phosphate precursor with a particle size of 50 to 70 nm;

[0074] (4) Lithium hydroxide, sucrose, lithium iron manganese phosphate precursor and a washed filter cake are mixed in deionized water and transferred to a hydrothermal kettle for hydrothermal reaction; the molar ratio of Li and P in the mixed system after the lithium source, carbon source, lithium iron manganese phosphate precursor and the washed filter cake is 1.08:1; the mass ratio of the lithium iron manganese phosphate precursor and the washed filter cake is 100:20; the mass of the carbon source is 10% of the mass of the mixed system after the lithium source, carbon source, lithium iron manganese phosphate precursor and the washed filter cake are mixed; the hydrothermal temperature is 160°C and the time is 4 hours;

[0075] (5) The obtained precipitate was washed and dried, and sintered at 800°C for 4 h at a heating rate of 3°C / min to obtain a lithium manganese iron phosphate composite material; its scanning electron microscopy was as follows Figure 1 shown.

[0076] Example 3

[0077] The preparation method of the lithium manganese iron phosphate composite material provided in this embodiment includes the following steps:

[0078] (1) The precipitated slurry obtained in the ferric phosphate precipitation process is filtered and washed to obtain a clear solution 1 and a first-wash filter cake, wherein the manganese content in the clear solution 1 is 0.05 wt %. The first-wash filter cake is aged and then filtered and washed to obtain a clear solution 2, wherein the phosphorus content in the clear solution 2 is 0.6%. The aging temperature is 75° C. and the time is 4 h.

[0079] (2) 0.5 mol / L ferrous nitrate solution and clear solution 1 are mixed to obtain a first mixed system, wherein the molar ratio of iron to manganese in the first mixed system is 1:0.9; the first mixed system and 12% ammonia water are pumped into the microreactor from both ends through a peristaltic pump, wherein the mixing volume ratio of ammonia water to the first mixed system is 1:0.9, and the volume flow rate of the peristaltic pump is 50 mL / min, and a first reaction is carried out by contact in the middle to obtain a hydroxide suspension;

[0080] (3) The hydroxide suspension and the clear solution 2 are pumped into the microreactor from both ends through a peristaltic pump, the mixed volume ratio of the hydroxide suspension and the clear solution 2 is 1:1.1, the volume flow rate of the peristaltic pump is 50 mL / min, and a second reaction is carried out through contact in the middle to form a precursor suspension, which is then filtered and washed to obtain a lithium manganese iron phosphate precursor with a particle size of 70 to 100 nm;

[0081] (4) lithium oxalate, glucose, lithium iron manganese phosphate precursor and a washed filter cake are mixed in deionized water and transferred to a hydrothermal kettle for hydrothermal reaction; the molar ratio of Li and P in the mixed system after the lithium source, carbon source, lithium iron manganese phosphate precursor and the washed filter cake is 1:1; the mass ratio of the lithium iron manganese phosphate precursor and the washed filter cake is 100:50; the mass of the carbon source is 30% of the mass of the mixed system after the lithium source, carbon source, lithium iron manganese phosphate precursor and the washed filter cake are mixed; the hydrothermal temperature is 90°C and the time is 24h;

[0082] (5) The obtained precipitate was washed and dried, and sintered at 600°C for 10 h at a heating rate of 1°C / min to obtain a lithium manganese iron phosphate composite material; its scanning electron microscopy was as follows Figure 1 shown.

[0083] Comparative Example 1

[0084] In this comparative example, no microreactor was used, and only a beaker was used for the reaction. Manganese sulfate was used as the manganese source, and ammonium dihydrogen phosphate was used as the phosphorus source. Other steps were the same as in Example 1 to obtain lithium manganese iron phosphate. The scanning electron microscope showed that Figure 2 shown.

[0085] Comparative Example 2

[0086] The difference between this comparative example and Example 1 is that the amorphous iron phosphate is not mixed, that is, only the carbon-coated lithium manganese iron phosphate material is generated.

[0087] Experimental example

[0088] The particle sizes of the lithium manganese iron phosphate composite materials prepared in the examples and comparative examples were measured, and the results are shown in Table 1.

[0089] Table 1

[0090] Particle size (nm) Example 1 50～200 Example 2 60～300 Example 3 70～400 Comparative Example 1 100～800 Comparative Example 2 200～800

[0091] Lithium manganese iron phosphate composite material, PVDF, and carbon black were stirred in a mass ratio of 94:3:3 to form a uniformly dispersed slurry. After coating and vacuum drying, the slurry was rolled and punched into 14 mm positive electrode discs. The lithium sheet was used as the negative electrode, and a 16 mm Celgard 2400 was used as the separator. The electrolyte was a 1 mol / L mixture of LiPF6, dimethyl carbonate, and ethyl methyl carbonate (volume ratio of 1:1:1). CR2025 button cells were assembled in a dry and argon-filled glove box. The test voltage was controlled in the range of 2.5-4.5 V. The test results are shown in Table 2.

[0092] Table 2 Performance test results

[0093]

[0094] The lithium manganese iron phosphate composite material prepared by the specific method of the present invention has a more excellent 0.1C first discharge specific capacity and first efficiency. Comparative Example 1 does not use a microreactor and does not use iron phosphate wastewater containing low manganese and phosphorus content, and Comparative Example 2 does not mix with iron phosphate. The 0.1C first discharge specific capacity and first efficiency of the lithium manganese iron phosphate composite materials prepared in Comparative Examples 1 and 2 are both lower than those in Example 1.

[0095] from Figure 1 and Figure 2 It can be seen that the lithium manganese iron phosphate composite material particles prepared by the specific method of the present invention are smaller, ranging from 50 to 200 nm, and the particle size distribution is relatively uniform, while the lithium manganese iron phosphate composite material particles prepared in the comparative example are larger, ranging from 100 to 800 nm, and the particle size distribution is uneven.

[0096] Although the present invention has been illustrated and described using specific embodiments, it should be appreciated that the above embodiments are merely intended to illustrate the technical solutions of the present invention rather than to limit them. Those skilled in the art should understand that the technical solutions described in the above embodiments may be modified, or some or all of the technical features thereof may be replaced by equivalents, without departing from the spirit and scope of the present invention. However, these modifications or replacements do not deviate from the essence of the corresponding technical solutions within the scope of the technical solutions of the embodiments of the present invention. Therefore, this means that all such replacements and modifications within the scope of the present invention are included in the appended claims.

Claims

1. A method for preparing a lithium manganese iron phosphate composite material, characterized in that: The following steps are involved: (a) washing the precipitated slurry obtained in the ferric phosphate precipitation stage to obtain a first clear liquid and a first washed filter cake; Aging the washed filter cake; and performing a second washing on the reaction system after aging to obtain a second clear liquid; The manganese content in the first clear solution is 0.05wt%~0.2wt%; The phosphorus content in the second clear liquid is 0.3wt%~0.6wt%; The aging temperature is 75-98°C; The aging time is 0.5 to 4 hours; (b) mixing the first clear liquid and an iron source to obtain a first mixed system; and mixing the first mixed system with aqueous ammonia to perform a first reaction; (c) mixing the reaction system after the first reaction and the second clear liquid, and then performing a second reaction to obtain a precursor suspension; and performing a third washing on the precursor suspension to obtain a lithium manganese iron phosphate precursor; (d) mixing a lithium source, a carbon source, the lithium iron manganese phosphate precursor and the first washed filter cake and subjecting the mixture to a hydrothermal reaction; and subjecting the reaction system after the hydrothermal reaction to a fourth washing, drying and sintering to obtain the lithium iron manganese phosphate composite material.

2. The method for preparing the lithium iron manganese phosphate composite material according to claim 1, wherein: In step (b), the molar ratio of iron to manganese in the first mixed system is 1:(0.9-1.1).

3. The method for preparing the lithium manganese iron phosphate composite material according to claim 1, wherein: The iron source includes at least one of ferrous chloride, ferrous nitrate, ferrous sulfate or ferrous oxalate.

4. The method for preparing the lithium manganese iron phosphate composite material according to claim 1, wherein: In step (b), the mixing volume ratio of the ammonia water to the first mixed system is 1:(0.9-1.1).

5. The method for preparing the lithium iron manganese phosphate composite material according to claim 1, wherein: In step (c), the volume ratio of the reaction system after the first reaction and the second clear liquid is 1:(0.9-1.1).

6. The method for preparing the lithium manganese iron phosphate composite material according to claim 1, characterized in that: In step (d), the molar ratio of Li to P in the mixed system after the lithium source, the carbon source, the lithium iron manganese phosphate precursor and the washed filter cake are mixed is (1.05-1):

1.

7. The method for preparing the lithium iron manganese phosphate composite material according to claim 1, wherein: In step (d), the mass ratio of the lithium manganese iron phosphate precursor to the washed filter cake is 100:(20-50).

8. The method for preparing the lithium manganese iron phosphate composite material according to claim 1, characterized in that: In step (d), the mass of the carbon source is 10% to 30% of the mass of the mixed system of the lithium source, the carbon source, the lithium iron manganese phosphate precursor and the washed filter cake.

9. The method for preparing the lithium manganese iron phosphate composite material according to claim 1, characterized in that: The lithium source includes at least one of lithium hydroxide, lithium oxalate, lithium carbonate, lithium nitrate or lithium sulfate.

10. The method for preparing the lithium manganese iron phosphate composite material according to claim 1, characterized in that: The carbon source includes at least one of sucrose, glucose, PEG6000 or PEG2000.

11. The method for preparing the lithium manganese iron phosphate composite material according to claim 1, characterized in that: The temperature of the hydrothermal reaction is 90-160°C; The hydrothermal reaction time is 4 to 24 hours.

12. The method for preparing the lithium iron manganese phosphate composite material according to claim 1, wherein: The sintering temperature is 600-800°C; The sintering heating rate is 1-3°C / min; The sintering time is 4 to 10 hours.

13. The lithium iron manganese phosphate composite material prepared by the method for preparing the lithium iron manganese phosphate composite material according to any one of claims 1 to 12.

14. A positive electrode plate, characterized in that: The lithium iron phosphate composite material is mainly prepared by the lithium iron phosphate composite material prepared by the preparation method of the lithium iron phosphate composite material according to any one of claims 1 to 12 or the lithium iron phosphate composite material according to claim 13.

15. A lithium ion battery, characterized in that: Including the positive electrode sheet according to claim 14.

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