Compound lithium ferric manganese phosphate material as well as preparation method and application thereof

Abstract

The invention relates to the technical field of lithium ion battery positive electrode materials, in particular to a compound lithium ferric manganese phosphate material and a preparation method and application thereof, by controlling the manganese-phosphorus ratio of manganese phosphate and the iron-phosphorus ratio of iron phosphate during solid-phase synthesis, the compaction density of the material is improved, and finally the rate capability and the energy density of the material are improved. The preparation method of the compound lithium ferric manganese phosphate material specifically comprises the following steps: mixing a lithium source, manganese phosphate with a manganese-phosphorus ratio of more than or equal to 0.985 and iron phosphate with an iron-phosphorus ratio of less than or equal to 0.96 according to a required stoichiometric ratio, then adding a carbon source which accounts for 8-10% of the total mass of the lithium source, the manganese phosphate and the iron phosphate, adding a solvent, grinding to obtain precursor slurry, and drying to obtain the compound lithium ferric manganese phosphate material. The carbon source is polyacrylic acid or a mixture of polyacrylic acid and other carbon-containing substances; performing spray drying on the precursor slurry to obtain a precursor; carrying out heat treatment on the precursor at 600-800 DEG C in an inert atmosphere for 2-20 hours to obtain a grey black product; and crushing the grey black product to obtain a final product.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of lithium ion battery positive electrode materials, in particular to a compound type lithium iron manganese phosphate material and a preparation method and application thereof. BACKGROUND

[0002] As a new type of energy storage technology, lithium ion batteries are widely used in many fields including electric vehicles. Lithium iron manganese phosphate is an important lithium ion battery positive electrode material, which is called an upgraded version of lithium iron phosphate. It has a similar crystal structure to lithium iron phosphate, and has a higher voltage platform, higher energy density and better low temperature performance than lithium iron phosphate, and thus is widely concerned.

[0003] In the prior art, lithium iron manganese phosphate is mainly prepared based on a solid phase method: lithium source, iron manganese source and carbon source are used as main raw materials, the raw materials are mixed according to the stoichiometric ratio and are uniformly ground, then nitrogen, argon or other protective gas is introduced for pretreatment at a low temperature to pre-decompose the raw materials, and then high-temperature calcination is performed to obtain the finished product. Lithium iron manganese phosphate is essentially a solid solution of lithium manganese phosphate and lithium iron phosphate. In order to realize the atomic-level mixing of the two substances, the above preparation method has a high requirement for the grinding effect of the raw materials, and a poor grinding effect will directly lead to poor electrochemical performance of the product. In addition, since the electronic conductivity and ionic conductivity of lithium manganese phosphate are poor, the obtained product has poor rate performance and energy density.

[0004] Therefore, the present application is proposed. SUMMARY

[0005] In view of the defects of low material compaction density and poor rate performance in the prior art, the application provides a compound type lithium iron manganese phosphate material and a preparation method and application thereof.

[0006] Firstly, the application provides a preparation method of a compound type lithium iron manganese phosphate material, which comprises the following operation steps:

[0007] S1, mixing lithium source, manganese phosphate and iron phosphate according to the required stoichiometric ratio, then adding 8%-10% of the total mass of the lithium source, the manganese phosphate and the iron phosphate of a carbon source, and adding a solvent for grinding to obtain a precursor slurry, the manganese-phosphorus ratio of the manganese phosphate is greater than or equal to 0.985, the iron-phosphorus ratio of the iron phosphate is less than or equal to 0.96, and the carbon source is polyacrylic acid or a mixture of polyacrylic acid and other carbon-containing substances; the manganese-phosphorus ratio refers to the molar ratio of manganese to phosphorus, and the iron-phosphorus ratio refers to the molar ratio of iron to phosphorus;

[0008] S2, spray drying the precursor slurry to obtain a precursor;

[0009] S3, heat-treating the precursor at 600-800 DEG C under inert atmosphere for 2-20h to obtain a gray-black product;

[0010] S4, crushing the gray-black product to obtain a final product.

[0011] The present application unexpectedly found that: when lithium source, manganese phosphate, and iron phosphate are used as raw materials, when the manganese phosphate has a manganese-phosphorus ratio of greater than or equal to 0.985, the high manganese-phosphorus ratio has a higher melting point than the low manganese-phosphorus ratio due to the fluxing effect of phosphorus, and the particles of the high manganese-phosphorus ratio are not easy to grow, and the above manganese-phosphorus ratio range helps to limit the growth of lithium manganese phosphate particles; when the iron phosphate has an iron-phosphorus ratio of less than or equal to 0.96, the low iron-phosphorus ratio has a lower melting point than the high iron-phosphorus ratio due to the fluxing effect of phosphorus, and the particles of the low iron-phosphorus ratio are easy to grow, and the above iron-phosphorus ratio range helps to promote the growth of lithium iron phosphate particles; under the above high manganese-phosphorus ratio and the above low iron-phosphorus ratio, the growth of lithium manganese phosphate particles is moderately inhibited, which helps to nanoize the lithium manganese phosphate particles, thereby shortening the lithium ion diffusion path, improving the lithium ion conductivity and the lithium ion diffusion rate, while the growth of lithium iron phosphate particles is moderately promoted, which helps to micronize the lithium iron phosphate particles, thereby providing a more continuous electron conduction network to improve the electronic conductivity; at the same time, the size of the lithium manganese phosphate particles and the lithium iron phosphate particles is properly matched, which promotes each other and optimizes the material on the basis of not affecting the formation of a lithium manganese iron phosphate solid solution, thereby improving the compaction density of the material and ultimately improving the rate performance and energy density of the material.

[0012] Further, the manganese phosphate is manganese phosphate monohydrate, and the manganese-phosphorus ratio of the manganese phosphate monohydrate is greater than or equal to 0.989.

[0013] In the present embodiment, the manganese phosphate used in the present application is preferably manganese phosphate monohydrate, and the manganese-phosphorus ratio thereof is preferably greater than or equal to 0.989.

[0014] Further, the iron-phosphorus ratio of the iron phosphate is greater than or equal to 0.955, i.e., the iron-phosphorus ratio is preferably in the range of 0.955-0.96, such as 0.955, 0.956, 0.957, 0.958, 0.959, or 0.96.

[0015] Further, the molecular weight of the polyacrylic acid is 2000-4000.

[0016] In the present embodiment, the molecular weight of the polyacrylic acid is preferably in the range of 2000-4000, and the polyacrylic acid with the above molecular weight as a carbon source or in combination with other carbon-containing substances to form the carbon source required by the present application can not only ensure a moderate residual carbon rate in the final product, but also reduce the cost of the carbon source.

[0017] Further, the mass percentage of polyacrylic acid in the carbon source is 30%-75%, for example, the mass percentage of polyacrylic acid can be 30%, 31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%, 39%, 40%, 41%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, 50%, 51%, 52%, 53%, 54%, 55%, 56%, 57%, 58%, 59%, 60%, 61%, 62%, 63%, 64%, 65%, 66%, 67%, 68%, 69%, 70%, 71%, 72%, 73%, 74%, 75%, etc.

[0018] Polyacrylic acid is a high-quality organic carbon source, which has high carbon yield, good residual carbon conductivity, and is easy to disperse and form a thin and dense, high-strength graphitized carbon coating layer. The obtained graphitized carbon coating layer not only has good conductivity, but also can inhibit the dissolution of manganese ions and improve the interface stability. However, when polyacrylic acid is used alone, the high-temperature viscosity increases, and excessive melting can easily cause particle adhesion. Therefore, it is preferred to be used in combination with other carbon-containing substances. The high-temperature decomposition of other carbon-containing substances produces micropores, which can not only offset the melting adhesion of polyacrylic acid, improve the fluidity, allow a higher sintering temperature to ensure complete crystallization, and improve the uniformity of the carbon coating layer, but also can reduce the total carbon content and improve the material compaction density.

[0019] In the present embodiment, the preferred mass ratio of polyacrylic acid to other carbon-containing substances is 1:(0.5-2), for example, it can be 1:0.5, 1:0.6, 1:0.7, 1:0.8, 1:0.9, 1:1, 1:1.1, 1:1.2, 1:1.3, 1:1.4, 1:1.5, 1:1.6, 1:1.7, 1:1.8, 1:1.9 or 1:2.

[0020] In the present embodiment, the other carbon-containing substances are preferably at least one of sodium citrate, citric acid, sodium oleate, oleic acid, polyethylene glycol, polyvinylpyrrolidone, glucose, ascorbic acid, sucrose, dopamine hydrochloride, starch, graphene oxide, reduced graphene, carbon nanotubes, and ketjen black.

[0021] In the embodiment, the other carbon-containing substance is most preferably glucose, and the carbon formed from glucose has a porous structure that is easy to compress, which on the one hand introduces a porous framework structure on the basis of the thin and dense carbon coating layer formed by the polyacrylic acid, helps to build a hierarchical conductive network, further improves the conductivity and enhances the mechanical strength of the carbon coating layer, and on the other hand can further improve the compaction density while also shortening the lithium ion diffusion path, so that the lithium ion diffusion capacity is enhanced; therefore, the polyacrylic acid and glucose compound as a carbon source, while maintaining the appropriate residual carbon rate, helps to realize the leap in conductivity, ion diffusion optimization and cycle stability improvement, and is a better choice that takes into account high performance, low cost and process friendliness.

[0022] In the embodiment, the lithium source is selected from at least one of lithium hydroxide, lithium carbonate, lithium dihydrogen phosphate and lithium acetate.

[0023] Further, the solvent is deionized water, and the solid content of the precursor slurry is 30wt%-50wt%, such as specifically 30wt%, 31wt%, 32wt%, 33wt%, 34wt%, 35wt%, 36wt%, 37wt%, 38wt%, 39wt%, 40wt%, 41wt%, 42wt%, 43wt%, 44wt%, 45wt%, 46wt%, 47wt%, 48wt%, 49wt%, 50wt%.

[0024] Further, in step S1, the grinding is performed until the D50 of the iron phosphate and the manganese phosphate is not greater than 300nm; in step S3, the precursor is heat-treated at 750℃ under an inert atmosphere for 10h, and sintering under this condition can avoid oxidation of the material and excessive particle growth while ensuring complete crystallization; in step S4, the crushing treatment is specifically air jet crushing.

[0025] Compared with the prior art, the preparation method proposed by the present application is conventional and simple to operate, and only by controlling the manganese-phosphorus ratio of the raw material manganese phosphate and the iron-phosphorus ratio of the iron phosphate can the compaction density of the lithium iron manganese phosphate material be significantly improved, the rate performance and energy density can be improved, and the preparation method proposed by the present application is less affected by other operating conditions such as mixing uniformity and grinding effect, has strong feasibility, and has great prospects for production.

[0026] Secondly, the present application proposes a compound type lithium iron manganese phosphate material.

[0027] Further, it is found through detection that the residual carbon rate in the compound type lithium iron manganese phosphate material is 1.39wt%-1.75wt%.

[0028] Finally, the present application proposes an application of a compound type lithium iron manganese phosphate material: for a lithium ion battery positive electrode material.

[0029] The complex iron manganese lithium phosphate material is prepared as a positive electrode active material to prepare a positive electrode slurry and coat on a positive electrode current collector, dry, roll to obtain a positive electrode sheet, the positive electrode slurry further contains a conductive agent, a binder and a solvent, the conductive agent, the binder and the solvent are all conventional technical means in the art, for example, the conductive agent is selected from at least one of carbon black SP, acetylene black, carbon nanotube CNTs and graphene, the binder is selected from at least one of polytetrafluoroethylene, polyvinylidene fluoride, nitrile rubber, styrene ethylene butylene styrene copolymer, polyacrylonitrile, styrene butadiene styrene copolymer, polyether ether ketone, lithium polyacrylate and sodium polyacrylate, and the solvent is selected from one of deionized water, N-methyl pyrrolidone NMP, dimethyl sulfoxide DMSO, tetrahydrofuran THF and N,N-dimethylformamide DMF, and deionized water and NMP are the most common.

[0030] In specific use, the positive electrode sheet, a negative electrode sheet, an electrolyte, a separator and the like are assembled into a lithium battery, and the rest except the positive electrode sheet are all the prior art. For example, the negative electrode sheet is selected from a lithium sheet. For example, the electrolyte includes a solvent, an electrolyte salt and an additive, the solvent is selected from one or more of ethylene carbonate, fluoroethylene carbonate, propylene carbonate, fluoropropylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate and ethyl butyrate, the electrolyte salt is selected from one or more of lithium hexafluorophosphate, lithium hexafluoroarsenate, lithium perchlorate, lithium bisfluorosulfonimide and lithium bis-trifluorosulfonimide, and the additive is selected from one or more of carbonate-based additives, nitrile-based additives, sulfur-containing additives, fluorine-containing additives, boron-containing additives and phosphorus-containing additives. For example, the separator can be selected from any one of a PP film, a PE film, an aluminum oxide coated separator, a polymer coated separator and a solid electrolyte coated separator. BRIEF DESCRIPTION OF DRAWINGS

[0031] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings needed in the embodiment description will be briefly introduced as follows. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can also be obtained by those skilled in the art without any creative effort on the basis of these drawings.

[0032] Figure 1 SEM diagram of the product obtained in Example 1;

[0033] Figure 2 SEM comparison diagram of the product obtained in Example 1 and the products obtained in Comparative Examples 1, 2 and 3;

[0034] Figure 3 0.1C, 1C charge-discharge curve diagram for Example 1;

[0035] Figure 4 0.1C, 1C charge-discharge curve diagram for Comparative Example 1;

[0036] Figure 5 0.1C, 1C charge-discharge curve diagram for Comparative Example 2;

[0037] Figure 6 0.1C, 1C charge-discharge curve diagram for Comparative Example 3. DETAILED DESCRIPTION

[0038] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative work fall within the scope of the present application.

[0039] The important raw materials involved in the following examples and comparative examples are as follows: iron phosphate and manganese phosphate monohydrate are purchased from Zhejiang Sicheng Energy New Material Co., Ltd., polyacrylic acid (molecular weight 3000) is purchased from Jinjile (Hunan) Chemical Co., Ltd., and the rest of the raw materials not specifically mentioned are commercially available. The devices involved in the following examples and comparative examples are all conventional devices in the art.

[0040] Example 1

[0041] The compound type lithium iron manganese phosphate material is prepared according to the following operation steps:

[0042] S1, 0.5 mol Li2CO3, 0.4 mol FePO4 (iron to phosphorus ratio of 0.955), 0.6 mol MnPO4·H2O (manganese to phosphorus ratio of 0.985) and 8% of the total mass of the above-mentioned substances carbon source are mixed, the carbon source is a mixture of polyacrylic acid and glucose with a mass ratio of 1:1 (i.e. the mass percentage of polyacrylic acid in the carbon source is 50%), an appropriate amount of deionized water is added and placed in a sand mill for grinding, taking FePO4 particle size as the criterion, grinding to D50 less than 300 nm, obtaining a precursor slurry with a solid content of 45wt%;

[0043] S2, the precursor slurry is spray dried to obtain a precursor;

[0044] S3, the precursor is placed in an atmosphere furnace and heat treated at 750℃ in a nitrogen atmosphere for 10 hours to obtain a gray-black product;

[0045] S4, the obtained gray-black product is subjected to jet milling to obtain the final product LiMn 0.6Fe 0.4 PO4material.

[0046] Example 2

[0047] Compared with Example 1, the iron-phosphorus ratio of FePO4 is adjusted from 0.955 to 0.957, and the rest is consistent with Example 1.

[0048] Example 3

[0049] Compared with Example 1, the iron-phosphorus ratio of FePO4 is adjusted from 0.955 to 0.959, and the rest is consistent with Example 1.

[0050] Example 4

[0051] Compared with Example 1, the iron-phosphorus ratio of FePO4 is adjusted from 0.955 to 0.96, and the rest is consistent with Example 1.

[0052] Example 5

[0053] Compared with Example 1, the manganese-phosphorus ratio of MnPO4·H2O is adjusted from 0.985 to 0.987, and the rest is consistent with Example 1.

[0054] Example 6

[0055] Compared with Example 1, the manganese-phosphorus ratio of MnPO4·H2O is adjusted from 0.985 to 0.989, and the rest is consistent with Example 1.

[0056] Example 7

[0057] Compared with Example 1, the manganese-phosphorus ratio of MnPO4·H2O is adjusted from 0.985 to 0.99, and the rest is consistent with Example 1.

[0058] Example 8

[0059] Compared with Example 1, the amount of carbon source added is unchanged and also consists of polyacrylic acid and glucose, but the mass percentage of polyacrylic acid in the carbon source is adjusted from 50% to 66.7%, and the rest is consistent with Example 1.

[0060] Example 9

[0061] Compared with Example 1, the amount of carbon source added is unchanged and also consists of polyacrylic acid and glucose, but the mass percentage of polyacrylic acid in the carbon source is adjusted from 50% to 75%, and the rest is consistent with Example 1.

[0062] Example 10

[0063] Compared with Example 1, the amount of the carbon source added is unchanged and also consists of polyacrylic acid and glucose, but the mass percentage of polyacrylic acid in the carbon source is adjusted from 50% to 30%, and the rest is consistent with Example 1.

[0064] Example 11

[0065] Compared with Example 1, the amount of the carbon source added is unchanged and also consists of polyacrylic acid and glucose, but the mass percentage of polyacrylic acid in the carbon source is adjusted from 50% to 33.3%, and the rest is consistent with Example 1.

[0066] Example 12

[0067] Compared with Example 1, the amount of the carbon source added is unchanged, but the composition is adjusted from a mixture of polyacrylic acid and glucose with a mass ratio of 1:1 to a mixture of polyacrylic acid and sucrose with a mass ratio of 1:1, and the rest is consistent with Example 1.

[0068] Example 13

[0069] Compared with Example 1, the amount of the carbon source added is unchanged, but the composition is adjusted from a mixture of polyacrylic acid and glucose with a mass ratio of 1:1 to a mixture of polyacrylic acid and citric acid with a mass ratio of 1:1, and the rest is consistent with Example 1.

[0070] Example 14

[0071] Compared with Example 1, the composition of the carbon source is unchanged, but the addition ratio is adjusted from 8% to 9%, and the rest is consistent with Example 1.

[0072] Example 15

[0073] Compared with Example 1, the composition of the carbon source is unchanged, but the addition ratio is adjusted from 8% to 10%, and the rest is consistent with Example 1.

[0074] Comparative Example 1

[0075] Compared with Example 1, FePO4 with an iron-phosphorus ratio of 0.955 is adjusted to an equal amount of FePO4 with an iron-phosphorus ratio of 0.954, and the rest is consistent with Example 1.

[0076] Comparative Example 2

[0077] Compared with Example 1, FePO4 with an iron-phosphorus ratio of 0.955 is adjusted to an equal amount of FePO4 with an iron-phosphorus ratio of 0.961, and the rest is consistent with Example 1.

[0078] Comparative Example 3

[0079] Compared with Example 1, the MnPO4·H2O with a manganese-phosphorus ratio of 0.985 is adjusted to the same amount of MnPO4·H2O with a manganese-phosphorus ratio of 0.984, and the rest is consistent with Example 1.

[0080] Comparative Example 4

[0081] Compared with Example 1, the carbon source is adjusted from a mixture of polyacrylic acid and glucose with a mass ratio of 1:1 to a mixture of polyethylene glycol and glucose with a mass ratio of 1:1 in the same amount, and the rest is consistent with Example 1.

[0082] Comparative Example 5

[0083] Compared with Example 1, the amount of carbon source is unchanged and also consists of polyacrylic acid and glucose, but the mass percentage of polyacrylic acid in the carbon source is adjusted from 50% to 25%, and the rest is consistent with Example 1.

[0084] Comparative Example 6

[0085] Compared with Example 1, the composition of the carbon source is unchanged, but the addition ratio is adjusted from 8% to 6%, and the rest is consistent with Example 1.

[0086] Comparative Example 7

[0087] Compared with Example 1, the composition of the carbon source is unchanged, but the addition ratio is adjusted from 8% to 12%, and the rest is consistent with Example 1.

[0088] Comparative Example 8

[0089] Compared with Example 1, the amount of carbon source is unchanged and also consists of polyacrylic acid and glucose, but the mass percentage of polyacrylic acid in the carbon source is adjusted from 50% to 80%, and the rest is consistent with Example 1.

[0090] The specific experimental conditions of the above examples and comparative examples are shown in Table 1 below:

[0091] Table 1

[0092]

[0093]

[0094] In addition, the products obtained in each of the examples and the comparative examples are used for the positive electrode of a lithium ion battery and assembled into a button cell to determine the influence on the performance of the battery. Specifically, the product of each of the examples and each of the comparative examples is mixed with conductive carbon SP and adhesive PVDF at 90wt%:5wt%:5wt% and uniformly dispersed in NMP solvent to prepare a positive electrode slurry; the positive electrode slurry is coated on a positive electrode current collector, dried to form a positive electrode sheet, and cut into a round sheet with a diameter of 16 mm; in an argon glove box, the positive electrode sheet obtained above is used as the positive electrode, lithium sheet is used as the counter electrode, 1M LiPF6 electrolyte (LiPF6 is dissolved in a mixed solvent of EC / EMC at a volume ratio of 3:7 to obtain, and 2v% of VC is added to the mixed solvent) is used as the electrolyte, and a commercial polypropylene separator is used as the separator, the test voltage range is 2.5-4.3V, constant current and constant voltage charging is performed, and the constant voltage current cutoff is 0.05C. The specific assembly operation is performed according to the conventional preparation of lithium ion batteries. The test results are shown in Table 1, and the 0.1C and 1C charge-discharge curves of Example 1, Comparative Example 1, Comparative Example 2, Comparative Example 3 are shown in Figures Figures 3-6

[0095] As shown in Table 1, Figures 1-6

[0096] (1) From the test results of Examples 1-14, it can be seen that the compaction density of the lithium iron manganese phosphate material prepared by the preparation method of the present application is 2.40-2.45 mg / cm 3 , the 0.1C charge capacity is 150-154 mAh / g, the 0.1C discharge capacity is 147-150 mAh / g, and the 1C discharge capacity is 140-143 mAh / g, and the capacity attenuation from 0.1C to 1C is small. Obviously, the lithium iron manganese phosphate material obtained by the preparation method of the present application has high compaction density, large specific capacity and good rate performance. As shown in Figure Figure 1 , the SEM image of the product obtained in Example 1 shows that the size of the particles inside the material is relatively uniform, and the micro-morphology of the products of the remaining examples is similar to Figure 1 , and therefore is not attached separately.

[0097] (2) From the comparison of the test results of Example 1, Example 2, Example 3, Example 4, Comparative Example 1 and Comparative Example 2, it can be seen that the iron-phosphorus ratio of FePO4 has a significant influence on the performance of the product. When the iron-phosphorus ratio is between 0.955-0.96, the specific capacity and rate performance of the product are better than those of the iron-phosphorus ratio of 0.954 and the iron-phosphorus ratio of 0.961. It can be seen that a lower iron-phosphorus ratio within a proper range helps to improve the electrochemical performance of the material. As shown in Figure Figure 2 ​​As shown in the SEM images of the product of Comparative Example 1 and the SEM images of the product of Example 1, if the iron-phosphorus ratio is less than the range defined in the present application, the material will have a local particle growth phenomenon, as shown in Comparative Example 1 with an iron-phosphorus ratio of 0.954; if the iron-phosphorus ratio is greater than the range defined in the present application, the material will also have a local particle growth phenomenon, as shown in Comparative Example 2 with an iron-phosphorus ratio of 0.961.

[0098] (3) As shown by the test results of Example 1, Example 5, Example 6, Example 7 and Comparative Example 3, the manganese-phosphorus ratio of MnPO4·H2O has a significant effect on the performance of the product, and when the manganese-phosphorus ratio is greater than or equal to 0.985, the specific capacity and rate performance of the product are significantly better than when the manganese-phosphorus ratio is 0.984. It can be seen that a higher manganese-phosphorus ratio helps to improve the electrochemical performance of the material. As shown in the SEM images of the product of Comparative Example 3 and the SEM images of the product of Example 1, due to the low manganese-phosphorus ratio, the material obtained in Comparative Example 3 has a local particle growth phenomenon. Figure 2

[0099] (4) The components and proportions of the carbon source and the amount of addition have a significant effect on the electrochemical performance of the product.

[0100] As shown by the test results of Example 1, Example 12 and Example 13, under the same proportion of polyacrylic acid, the effect of using polyacrylic acid and glucose as a carbon source is better than using polyacrylic acid and sucrose or polyacrylic acid and citric acid. Further, if the polyacrylic acid is replaced by polyethylene glycol, the specific capacity and rate performance of the obtained product will be significantly worse. It can be seen that, compared with polyethylene glycol, polyacrylic acid is more helpful to form a carbon coating layer with good performance. The reason for this should be that the carbon coating layer formed by polyacrylic acid is a thin and dense graphitized carbon coating layer with high mechanical strength, which not only has good electrical conductivity, but also can inhibit the dissolution of manganese ions and improve the interface stability, so the electrochemical performance of the product is better.

[0101] As shown by the test results of Example 8, Example 9, Example 10, Example 11 and Comparative Example 4, Comparative Example 5 and Comparative Example 8, when polyacrylic acid and glucose are mixed in a proportion of 30%-75% of polyacrylic acid as a carbon source, the specific capacity and rate performance of the obtained product are both good; when the proportion of polyacrylic acid is less than 30%, such as 0 as shown in Comparative Example 4 and 25% as shown in Comparative Example 5, the content of polyacrylic acid is too low, resulting in poor quality of the carbon coating layer, which leads to unsatisfactory electrochemical performance; when the proportion of polyacrylic acid is higher than 75%, such as 80% as shown in Comparative Example 8, the content of polyacrylic acid is too high, the residual carbon rate is low, and the material compaction density is reduced, which ultimately leads to unsatisfactory electrochemical performance. Therefore, the mass percentage of polyacrylic acid in the carbon source of the present application is 30%-75%, and further preferably the mass percentage of polyacrylic acid in the mixed carbon source is 33.3%-66.7%.

[0102] ​From the test results of Example 1, Example 13, Example 14 and Comparative Example 6, Comparative Example 7, it can be seen that the addition amount of the carbon source has a significant influence on the electrochemical performance of the product. The suitable addition amount of the carbon source is 8%-10% of the total mass of the lithium source, manganese phosphate and iron phosphate. Within this addition range, the product has high compaction density, large specific capacity and good rate performance. If the addition amount of the carbon source is lower or higher than the limited range, the compaction density, specific capacity and rate performance of the obtained product are all deteriorated. It is speculated that when the addition amount of the carbon source is not within the limited range, the uniformity, continuity and integrity of the carbon coating layer of the obtained lithium manganese iron phosphate material are poor, and the poor coating effect leads to that the carbon coating layer cannot play a good conductive role, thus being not conducive to the specific capacity and rate performance.

[0103] In summary: the polyacrylic acid or the mixture of polyacrylic acid and other carbon-containing substances is used as the carbon source in the present application, and the manganese phosphate with a high manganese-phosphorus ratio and the iron phosphate with a low iron-phosphorus ratio are compounded, which helps to solve the problems of small compaction density, poor rate performance and poor energy density of the lithium manganese iron phosphate prepared by the solid-phase method, and finally the lithium manganese iron phosphate material with large specific capacity, high compaction density and good rate performance is obtained.

[0104] Although the embodiments of the present application have been shown and described above, it should be understood that the above embodiments are exemplary and should not be construed as limiting the present application, and those skilled in the art can modify, modify, replace and deform the above embodiments within the scope of the present application. In addition, those skilled in the art can combine and combine the different embodiments or examples described in the present specification and the features of different embodiments or examples without contradiction.

Claims

1. A method for preparing a compound lithium iron manganese phosphate material, characterized in that: The following steps are included: S1. Lithium source, manganese phosphate, and iron phosphate are mixed in the required stoichiometric ratio. Then, 8%-10% of the total mass of the lithium source, manganese phosphate, and iron phosphate is added as a carbon source, and solvent is added for grinding to obtain a precursor slurry. The manganese-to-phosphorus ratio of the manganese phosphate is ≥0.985, and the iron-to-phosphorus ratio of the iron phosphate is ≤0.

96. The carbon source is polyacrylic acid or a mixture of polyacrylic acid and other carbon-containing substances. The manganese-to-phosphorus ratio refers to the molar ratio of manganese to phosphorus, and the iron-to-phosphorus ratio refers to the molar ratio of iron to phosphorus. S2. Spray dry the precursor slurry to obtain the precursor; S3. The precursor is heat-treated at 600-800℃ under an inert atmosphere for 2-20 hours to obtain a gray-black product; S4. The gray-black product is pulverized to obtain the final product.

2. The preparation method of the compound lithium iron manganese phosphate material according to claim 1, characterized in that: The manganese phosphate is manganese phosphate monohydrate, and the manganese-to-phosphorus ratio of the manganese phosphate monohydrate is ≥0.

989.

3. The preparation method of the compound lithium iron manganese phosphate material according to claim 1, characterized in that: The iron-to-phosphorus ratio of the iron phosphate is ≥0.

955.

4. The preparation method of the compound lithium iron manganese phosphate material according to claim 1, characterized in that: The molecular weight of the polyacrylic acid is 2000-4000.

5. The preparation method of the compounded lithium iron manganese phosphate material according to claim 1, characterized in that: The carbon source contains 30%-75% polyacrylic acid by mass. Preferably, the mass ratio of polyacrylic acid to other carbon-containing substances is 1:(0.5-2); Preferably, the other carbon-containing substances are selected from at least one of sodium citrate, citric acid, sodium oleate, oleic acid, polyethylene glycol, polyvinylpyrrolidone, glucose, ascorbic acid, sucrose, dopamine hydrochloride, starch, graphene oxide, reduced graphene, carbon nanotubes, and Ketjen black.

6. The method for preparing the compounded lithium iron manganese phosphate material according to claim 1, characterized in that: The solvent is deionized water, and the solid content of the precursor slurry is 30wt%-50wt%.

7. The preparation method of the compounded lithium iron manganese phosphate material according to claim 1, characterized in that: In step S1, the grinding is carried out until the D50 of the iron phosphate and the manganese phosphate is no greater than 300 nm; In step S3, the precursor is heat-treated at 750°C in an inert atmosphere for 10 hours. In step S4, the pulverization process is specifically airflow pulverization.

8. A compound lithium iron manganese phosphate material prepared according to the preparation method of any one of claims 1-7.

9. The compounded lithium iron manganese phosphate material according to claim 8, characterized in that: Its residual carbon content is 1.39wt%-1.75wt%.

10. An application of the compounded lithium iron manganese phosphate material as described in claim 8, characterized in that: Used in cathode materials for lithium-ion batteries.

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