A high-rate lithium iron manganese phosphate material and its preparation method and application

Abstract

The present invention relates to the technical field of lithium ion battery positive electrode materials, and specifically discloses a high-rate type manganese iron phosphate lithium material and its preparation method and application, the manganese iron phosphate lithium material has high compaction density, large specific capacity and good rate performance, and the preparation method is as follows: a lithium source, a manganese source, an iron source and a phosphorus source are mixed in the required stoichiometric ratio, and then a carbon source of 6%-10% of the total mass of the raw material is added, and a solvent is added for grinding to obtain a precursor slurry, wherein the lithium source is lithium phytate or a mixture of lithium phytate and a traditional lithium source, and the carbon source is polyacrylic acid or a mixture of polyacrylic acid and other carbonaceous substances; the precursor slurry is dried to obtain a precursor; the precursor is subjected to heat treatment and pulverization. The present invention uses lithium phytate or a mixture of lithium phytate and a traditional lithium source as a lithium source, and uses polyacrylic acid or a mixture of polyacrylic acid and other carbonaceous substances as a carbon source, thereby solving the problems of low grinding efficiency, poor carbon coating effect and low material compaction density when preparing manganese iron phosphate lithium by solid phase method.

Description

Technical Field

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

[0002] Lithium-ion batteries, a highly anticipated new energy storage technology, are being used in a variety of fields, including electric vehicles. Lithium iron manganese phosphate (LiMnFePO4) is a key cathode material for lithium-ion batteries. It not only offers the performance advantages of LiFePO4, but also boasts a higher voltage platform, higher energy density, and improved low-temperature performance, making it a popular material for researchers.

[0003] In the prior art, lithium iron manganese phosphate is mainly prepared based on the solid phase method. The lithium sources used are lithium carbonate and lithium dihydrogen phosphate, the phosphorus sources used are ammonium dihydrogen phosphate, iron phosphate, phosphoric acid, etc., and the carbon sources used are carbohydrate carbon sources, polyethylene glycol, etc. First, due to the high hardness of lithium carbonate and iron phosphate, the hard particles collide and rub against each other during solid phase mixing and grinding, making it difficult to refine and easily leading to uneven mixing; although ammonium dihydrogen phosphate is relatively soft, it will decompose during the grinding and sintering process to produce ammonia and water vapor, affecting the powder state; phosphoric acid is a corrosive liquid, which increases the difficulty of mixing; polyethylene glycol may become sticky during the grinding process, causing powder agglomeration, adhesion to grinding balls and tank walls; therefore, when the above raw materials are mixed and ground, the grinding efficiency is low. Secondly, the carbon layer formed after high-temperature cracking and carbonization of the carbohydrate carbon source is usually amorphous carbon with medium conductivity. The carbon layer formed by the cracking of polyethylene glycol is also limited in continuity, uniformity and conductivity, so the carbon coating effect is poor. Thirdly, the solid-phase method makes it difficult to precisely control the product morphology. Furthermore, uneven mixing and poor grinding efficiency all lead to a wide particle size distribution. Furthermore, in order to improve the carbon coating effect, a high amount of low-density carbon is added to occupy more spaces between particles, hindering the close packing of particles and resulting in a low compaction density of the material. In summary, the current raw material combination for producing lithium iron manganese phosphate by the solid-phase method has the defects of low grinding efficiency, poor carbon coating effect, and low material compaction density, which is not conducive to improving the rate performance of lithium iron manganese phosphate materials.

[0004] For this purpose, this application is filed. Summary of the Invention

[0005] In view of the defects of the existing technology such as low grinding efficiency, poor carbon coating effect and low material compaction density, the present invention provides a high-rate lithium iron manganese phosphate material and its preparation method and application.

[0006] First, the present invention provides a method for preparing a high-rate lithium iron manganese phosphate material, comprising the following steps:

[0007] S1. A lithium source, a manganese source, an iron source, and a phosphorus source are mixed in a desired stoichiometric ratio, and then a carbon source is added that accounts for 6% to 10% of the total mass of the lithium source, the manganese source, the iron source, and the phosphorus source, and a certain amount of solvent is added for grinding to obtain a precursor slurry, wherein the lithium source is lithium phytate or a mixture of lithium phytate and a traditional lithium source, and the carbon source is polyacrylic acid or a mixture of polyacrylic acid and other carbon-containing substances;

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

[0009] S3, heat treating the precursor at 600-800°C under an inert atmosphere (N2 or Ar) for 2-20h to obtain a gray-black product;

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

[0011] The lithium source of the present invention is lithium phytate (chemical formula: C6H6Li 12 O 42 P6) or a mixture of lithium phytate and a traditional lithium source. Lithium phytate contains six phosphate groups, each with strong coordination ability. The manganese and iron sources are generally the corresponding metal oxides. The phosphates can form stable multidentate chelates with iron and manganese ions, which in turn weaken the lattice energy of the metal oxides, making it easier for mechanical forces to break the particles, resulting in finer and more uniform dispersion of the iron and manganese source particles. Ultimately, using this multidentate chelate as a liquid medium, the iron and manganese sources are molecularly dispersed. During the heating stage of heat treatment, this multidentate chelate decomposes into amorphous FePO4 / MnPO4 at around 300°C, significantly increasing the specific surface area of ​​the precursor and enhancing its activity. This accelerates the solid-phase reaction during heat treatment. Combined with polyacrylic acid or a mixture of polyacrylic acid and other carbon-containing substances, which have high cracking carbon yields, good residual carbon conductivity, and are easily dispersed, as a carbon source, a uniform, continuous, and complete low-content, highly conductive carbon layer can be formed. The resulting lithium iron manganese phosphate material has a large specific capacity, high compaction density, and excellent rate performance.

[0012] Furthermore, when the lithium source is a mixture of lithium phytate and a traditional lithium source, the lithium element provided by the lithium phytate accounts for at least 50% of the total lithium element, that is, based on the molar amount of the Li element, at least half or more of the Li element is provided by lithium phytate. In this range of added amount, the lithium phytate can provide sufficient phytate radicals to fully chelate and coordinate with iron ions and manganese ions, thereby weakening the lattice energy of the iron source and manganese source to the greatest extent, making the iron source and manganese source particles easier to be destroyed.

[0013] Furthermore, when the carbon source is a mixture of polyacrylic acid and other carbon-containing substances, the mass percentage of polyacrylic acid is 30%-75%.

[0014] Polyacrylic acid is a high-quality organic carbon source, and its cracked carbon yield is high, residual carbon has good electrical conductivity, is easy to disperse and form a thin and dense, mechanically strong graphitized carbon coating, the resulting graphitized carbon coating not only has good electrical conductivity, but also can suppress the dissolution of manganese ions, thereby improving interfacial stability. However, when polyacrylic acid is used alone, high-temperature viscosity increases and excessive melting easily causes particle adhesion. Therefore, it is preferably used in combination with other carbonaceous materials, and other carbonaceous materials pyrolysis produces micropores, which can offset the melt bonding force of polyacrylic acid, allowing higher sintering temperatures to ensure that crystallization is complete.

[0015] Furthermore, the molecular weight of the polyacrylic acid is 2000-4000. Polyacrylic acid with a molecular weight within this range is used as a carbon source or is compounded with other carbon-containing substances to form the carbon source required by the present invention, which not only ensures a moderate residual carbon rate in the final product, but also has a low cost of the carbon source.

[0016] Furthermore, the other carbon-containing substances are selected from at least one of sodium citrate, citric acid, sodium oleate, oleic acid, polyvinyl pyrrolidone, polyethylene glycol, glucose, ascorbic acid, sucrose, dopamine hydrochloride, starch, graphene oxide, reduced graphene, carbon nanotubes, and Ketjen black, and the other carbon-containing substances are preferably glucose;

[0017] The carbon source is a mixture of polyacrylic acid and other carbon-containing substances in a mass ratio of 1:(0.5-2). The present invention has found that when polyacrylic acid and glucose are compounded as a carbon source, compared with a single pure polyacrylic acid carbon source, it not only has a cost advantage, but also can achieve the following effects:

[0018] (1) Optimizing the carbon layer structure. The carbon formed by glucose has a porous structure. Therefore, introducing a porous skeleton structure on the basis of the thin and dense carbon coating formed by polyacrylic acid helps to construct a hierarchical conductive network, further improve the conductivity and enhance the mechanical strength of the carbon coating;

[0019] (2) Reduce the melt viscosity of polyacrylic acid, improve its fluidity, and enhance the uniformity of the carbon coating layer;

[0020] (3) Reduce the total carbon content and increase the compaction density of the material;

[0021] (4) The glucose carbon layer has a porous structure that is easily compressed, which can further increase the compaction density while shortening the lithium ion diffusion path, thereby enhancing the lithium ion expansion capacity;

[0022] In summary, the combination of polyacrylic acid and glucose as a carbon source helps achieve a jump in conductivity, optimize ion diffusion, and improve cycle stability while maintaining an appropriate residual carbon rate. It is an optimal choice for balancing high performance, low cost, and process friendliness.

[0023] The traditional lithium source is selected from at least one of lithium hydroxide, lithium carbonate, lithium dihydrogen phosphate, and lithium acetate.

[0024] Furthermore, the solvent is deionized water, and the multidentate chelate formed by phosphate and iron ions and manganese ions is easily soluble in water. After dissolving in water, it can prevent fine particles from agglomerating again. Therefore, using deionized water as the solvent is more helpful to improve the grinding efficiency and improve the grinding effect; the solid content of the precursor slurry is 30wt%-50wt%; further, the preferred solid content of the precursor slurry is 45wt%.

[0025] Furthermore, in step S1, the manganese source is manganese trioxide, the iron source is iron oxide, and the phosphorus source is phosphoric acid, and they are ground until the D50 of the iron oxide is not greater than 300 nm; in step S3, the precursor obtained in S2 is heat treated at 750°C in an inert atmosphere for 10 hours and sintered under these conditions, which can not only prevent the material from being oxidized, but also prevent excessive particle growth while ensuring complete crystallization; in step S4, the pulverization treatment is specifically airflow pulverization.

[0026] Secondly, the present invention proposes a high-rate lithium iron manganese phosphate material.

[0027] Furthermore, the residual carbon rate in the high-rate lithium iron manganese phosphate material is 1.55wt%-2.3wt%, and the residual carbon rate in the high-rate lithium iron manganese phosphate material is preferably 1.7wt%-2.3wt%. At this time, the high-rate lithium iron manganese phosphate material has excellent specific capacity and rate performance.

[0028] Thirdly, the present invention proposes an application of the high-rate lithium manganese iron phosphate material: for use as a positive electrode material for lithium-ion batteries.

[0029] Compared with the existing technology, the present invention uses lithium phytate or a mixture of lithium phytate and a traditional lithium source as the lithium source, and polyacrylic acid or a mixture of polyacrylic acid and other carbon-containing substances as the carbon source, which helps to solve the problems of low grinding efficiency, poor carbon coating effect, and low material compaction density in the solid-phase method for preparing lithium iron manganese phosphate, and ultimately obtains a lithium iron manganese phosphate material with high compaction density, large specific capacity, and good rate performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0031] Figure 1 0.1C and 1C charge-discharge curves of the product obtained in Example 1.

[0032] Figure 2 0.1C and 1C charge-discharge curves of the product obtained in Example 2.

[0033] Figure 3 0.1C and 1C charge-discharge curves of the product obtained in Example 3.

[0034] Figure 4 0.1C and 1C charge-discharge curves of the product obtained in Example 4.

[0035] Figure 5 0.1C and 1C charge-discharge curves of the product obtained in Example 5.

[0036] Figure 6 The 0.1C and 1C charge-discharge curves of the product obtained in Comparative Example 1 are shown.

[0037] Figure 7 The 0.1C and 1C charge-discharge curves of the product obtained in Comparative Example 2 are shown.

[0038] Figure 8 The 0.1C and 1C charge-discharge curves of the product obtained in Comparative Example 3 are shown. DETAILED DESCRIPTION

[0039] The following will be combined with the drawings in the embodiments of this application to clearly and completely describe the technical solutions in the embodiments of this application. Obviously, the embodiments described are only part of the embodiments of this application, and are not all embodiments. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0040] The polyacrylic acid involved in the following examples and comparative examples was purchased from Jinjinle (Hunan) Chemical Co., Ltd., with a molecular weight of 3000. The remaining raw materials involved in the following examples were all commercially available, and the devices involved in the following examples were all conventional devices in the art.

[0041] Example 1

[0042] The lithium manganese iron phosphate material was prepared according to the following steps:

[0043] S1. Mix 0.083 mol of lithium phytate, 0.196 mol of Fe2O3, 0.294 mol of Mn2O3, 0.5 mol of H3PO4 and a carbon source accounting for 7% of the total mass of lithium phytate, Fe2O3, Mn2O3, and H3PO4, where the carbon source is a mixture of polyacrylic acid and glucose in a mass ratio of 1:1. Add an appropriate amount of deionized water and grind in a sand mill. Grind until its D50 is less than 300 nm based on the Fe2O3 particle size to obtain a precursor slurry with a solid content of 45 wt%.

[0044] S2, spray drying the precursor slurry to obtain a precursor;

[0045] S3, placing the precursor in an atmosphere furnace and heat treating it at 750°C in a nitrogen atmosphere for 10 hours to obtain a gray-black product;

[0046] S4, the obtained gray-black product is subjected to air flow pulverization to obtain the final product LiMn 0.6 Fe 0.4 PO4 material.

[0047] Example 2

[0048] Compared with Example 1, the lithium source was adjusted from 0.083 mol of lithium phytate to a mixture of 0.042 mol of lithium phytate and 0.5 mol of lithium hydroxide, and the amount of H3PO4 added was adjusted from 0.5 mol to 0.75 mol. The rest remained the same as Example 1.

[0049] Example 3

[0050] Compared with Example 1, the carbon source was adjusted from a mixture of polyacrylic acid and glucose in a mass ratio of 1:1 to a mixture of polyacrylic acid and glucose in a mass ratio of 1:0.5 with equal addition amounts, that is, the proportion of polyacrylic acid in the carbon source was 66.7%, and the rest remained consistent with Example 1.

[0051] Example 4

[0052] Compared with Example 1, the carbon source was adjusted from a mixture of polyacrylic acid and glucose in a mass ratio of 1:1 to a mixture of polyacrylic acid and glucose in a mass ratio of 3:1 with equal addition amounts, that is, the proportion of polyacrylic acid in the carbon source was 75%, and the rest remained consistent with Example 1.

[0053] Example 5

[0054] Compared with Example 1, the amount of carbon source added remained unchanged, but the composition of the carbon source: the percentage of polyacrylic acid in the mixture of polyacrylic acid and glucose was adjusted to 30%, and the rest remained consistent with Example 1.

[0055] Example 6

[0056] Compared with Example 1, the carbon source was adjusted from a mixture of polyacrylic acid and glucose in a mass ratio of 1:1 to a mixture of polyacrylic acid and glucose in a mass ratio of 1:2 with equal addition amounts, that is, the proportion of polyacrylic acid in the carbon source was 33.3%, and the rest remained consistent with Example 1.

[0057] Example 7

[0058] Compared with Example 1, the amount of carbon source added remained unchanged, but the composition was adjusted from a mixture of polyacrylic acid and glucose in a mass ratio of 1:1 to a mixture of polyacrylic acid and sucrose in a mass ratio of 1:1, and the rest remained consistent with Example 1.

[0059] Example 8

[0060] Compared with Example 1, the amount of carbon source added remained unchanged, but the composition was adjusted from a mixture of polyacrylic acid and glucose in a mass ratio of 1:1 to a mixture of polyacrylic acid and citric acid in a mass ratio of 1:1, and the rest remained the same as Example 1.

[0061] Example 9

[0062] Compared with Example 1, the composition of the carbon source remains unchanged, but the addition is adjusted from 7% to 6%, and the rest remains the same as Example 1.

[0063] Example 10

[0064] Compared with Example 1, the composition of the carbon source remains unchanged, but the addition is adjusted from 7% to 10%, and the rest remains the same as Example 1.

[0065] Comparative Example 1

[0066] Compared with Example 1, 0.083 mol of lithium phytate was adjusted to 1 mol of lithium hydroxide, and the added amount of H3PO4 was adjusted from 0.5 mol to 1 mol. The rest remained the same as Example 1.

[0067] Comparative Example 2

[0068] Compared with Example 1, 0.083 mol of lithium phytate was adjusted to a mixture of 0.028 mol of lithium phytate and 0.67 mol of lithium hydroxide, and the added amount of H3PO4 was adjusted from 0.5 mol to 0.83 mol. The rest remained the same as Example 1.

[0069] Comparative Example 3

[0070] Compared with Example 1, the carbon source was adjusted from a mixture of polyacrylic acid and glucose in a mass ratio of 1:1 to a mixture of polyethylene glycol and glucose in a mass ratio of 1:1 with equal addition amounts, and the rest remained the same as Example 1.

[0071] Comparative Example 4

[0072] Compared with Example 1, the amount of carbon source added remained unchanged, but the composition of the carbon source: the percentage of polyacrylic acid in the mixture of polyacrylic acid and glucose was adjusted to 25%, and the rest remained the same as in Example 1.

[0073] Comparative Example 5

[0074] Compared with Example 1, 0.083 mol of lithium phytate was adjusted to 0.17 mol of lithium hexametaphosphate, and the amount of H3PO4 added was adjusted from 0.5 mol to 0. The rest remained the same as Example 1.

[0075] Comparative Example 6

[0076] Compared with Example 1, the composition of the carbon source remains unchanged, but the addition is adjusted from 7% to 5%, and the rest remains the same as Example 1.

[0077] Comparative Example 7

[0078] Compared with Example 1, the composition of the carbon source remains unchanged, but the addition is adjusted from 7% to 12%, and the rest remains the same as Example 1.

[0079] Comparative Example 8

[0080] Compared with Example 1, the amount of carbon source added remained unchanged, but the composition of the carbon source: the percentage of polyacrylic acid in the mixture of polyacrylic acid and glucose was adjusted to 80%, and the rest remained consistent with Example 1.

[0081] The specific experimental conditions of the above examples and comparative examples are shown in Table 1 below:

[0082] Table 1

[0083]

[0084] In addition, the products obtained in each example and comparative example were used as lithium-ion battery positive electrodes and assembled into button cells to measure their impact on battery performance. Specifically, the products from each example and comparative example were mixed with conductive carbon SP and binder PVDF at a ratio of 90 wt%:5 wt%:5 wt% and uniformly dispersed in NMP solvent to form a positive electrode slurry. The slurry was then applied to a positive electrode current collector, dried, and formed into positive electrode sheets, which were then cut into 16 mm diameter discs. In an argon glove box, the resulting positive electrode sheets served as the positive electrode, a lithium sheet was used as the counter electrode, and a 1 M LiPF6 electrolyte (LiPF6 dissolved in a 3:7 (volume ratio) EC / EMC mixture containing 2% (volume percentage) VC was used as the electrolyte. A commercial polypropylene separator was used as the separator. The test voltage range was 2.5-4.3 V, and constant current and constant voltage charging was performed with a constant voltage current cutoff of 0.05 C. The specific assembly procedures were performed according to conventional lithium-ion battery manufacturing procedures. The test results are shown in Table 1.

[0085] As shown in Table 1, Figures 1-8 Commonly shown:

[0086] (1) From the test results of Examples 1 to 10, it can be seen that the compacted density of the lithium manganese iron phosphate material prepared according to the preparation method proposed in the present invention is 2.40-2.49 mg / cm 3 , 0.1C charging capacity is 151-153 mAh / g, 0.1C discharge capacity is 147-149 mAh / g, 1C discharge capacity is 140-143 mAh / g; Obviously, compared with Comparative Example 1, the lithium manganese iron phosphate material obtained by the preparation method of the present invention has a high compaction density and a large specific capacity; Figure 1-Figure 5 、 Figure 6 As shown, compared with Comparative Example 1, the products obtained in Examples 1-5 exhibit less capacity decay from 0.1C to 1C, indicating better rate performance. The products obtained in Examples 6-10 also exhibit similar and good rate performance to that of the product in Example 1, and the corresponding charge-discharge curves are not further described.

[0087] (2) Compared with traditional lithium sources, using lithium phytate as a lithium source can significantly improve the electrochemical performance of the product. From the test results of Example 1, Example 2, Comparative Example 1, and Comparative Example 2, it can be seen that when the lithium source is adjusted from lithium hydroxide to lithium phytate or a mixture of lithium phytate and lithium hydroxide (the lithium supply ratio of lithium phytate is 50%), the 0.1C charge capacity, 0.1C discharge capacity, and 1C discharge capacity of the obtained product are significantly improved, and the rate performance is relatively good; if the lithium phytate content in the mixture of lithium phytate and lithium hydroxide is low (as shown in Comparative Example 2, the lithium supply ratio of lithium phytate is 33.3%), the charge capacity and discharge capacity of the obtained product are improved, but the improvement effect is very limited. Therefore, when a mixture of lithium phytate and a traditional lithium source is used as a lithium source, the lithium supply ratio of lithium phytate is preferably 50% or more. Otherwise, the chelation effect will be insignificant, and the effect of improving the grinding effect and efficiency will be limited, resulting in an unsatisfactory effect on improving the electrochemical performance of the product. In addition, the present invention also compares the product performance of other phosphorus-containing lithium salts with chelating effects as lithium sources. As shown in Comparative Example 5, under the premise that other conditions are the same as those in Example 1, if lithium phytate is replaced with an equal amount of lithium hexametaphosphate, the relevant electrochemical properties of the resulting product are significantly deteriorated. It is speculated that the reason may be that lithium hexametaphosphate is hydrolyzed into lithium dihydrogen phosphate in water, thereby significantly reducing the chelating effect, which is not conducive to weakening the lattice energy of iron and manganese oxides, and therefore has no obvious promoting effect on the particle refinement and uniform dispersion of iron and manganese sources; and the hydrolysis product of lithium phytate is phytic acid root, which has a significant chelating effect and can effectively promote the mechanical force to destroy the iron and manganese source particles during grinding, thereby ultimately improving the grinding efficiency and the grinding effect.

[0088] (3) The components and proportions of the carbon source have a significant impact on the electrochemical performance of the product. From the comparison of the test results of Example 1, Example 7, and Example 8, it can be seen that, at the same proportion of polyacrylic acid, the effect of polyacrylic acid and glucose as a carbon source is relatively better than that of acrylic acid and sucrose or acrylic acid and citric acid. From the comparison of the test results of Example 1 and Comparative Example 3, it can be seen that if polyacrylic acid is adjusted to an equal amount of polyethylene glycol, the specific capacity and rate performance of the obtained product will be significantly deteriorated. Obviously, compared with polyethylene glycol, polyacrylic acid is more conducive to forming a carbon coating with good performance. The reason for this should be that the carbon coating formed by polyacrylic acid is a thin, dense, and mechanically strong graphitized carbon coating, which not only has good conductivity, but also can inhibit the dissolution of manganese ions and improve interface stability, so the product has better electrochemical performance. From the test results of Example 1, Example 3-Example 6, Comparative Example 4, and Comparative Example 8, it can be seen that: when polyacrylic acid and glucose are mixed as a carbon source according to polyacrylic acid accounting for 30%-75%, the specific capacity and rate performance of the resulting product are all excellent; and when polyacrylic acid accounts for less than 30%, such as 25% as shown in Comparative Example 4, due to the low content of polyacrylic acid, the quality of the resulting carbon coating is poor, resulting in unsatisfactory electrochemical performance; when polyacrylic acid accounts for more than 75%, such as 80% as shown in Comparative Example 8, due to the excessively high content of polyacrylic acid, the residual carbon rate increases, the compacted density of the material decreases, and the excessively high content of polyacrylic acid easily leads to excessive melt viscosity and poor fluidity, which in turn makes the carbon coating uniformity in the resulting product poor, ultimately resulting in unsatisfactory electrochemical performance. Therefore, the carbon source of the present invention is preferably a mixed carbon source composed of polyacrylic acid and glucose, and wherein the mass percentage of polyacrylic acid is 30%-75%, and further preferably the mass percentage of polyacrylic acid in the mixed carbon source is 33.3%-66.7%.

[0089] (4) The amount of carbon source added has a significant effect on the electrochemical performance of the product. From the comparison of the test results of Example 1 (7% carbon source addition), Example 9 (6% carbon source addition), Example 10 (10% carbon source addition), Comparative Example 6 (5% carbon source addition), and Comparative Example 7 (12% carbon source addition), it can be seen that when the carbon source is a mixture of polyacrylic acid and glucose in a mass ratio of 1:1, the amount of carbon source added is appropriately controlled at 6%-10% of the total mass of lithium phytate, Fe2O3, Mn2O3, and H3PO4. At this time, the residual carbon rate in the obtained product is 1.55wt%-2.3wt%, and it has a high compaction density, large specific capacity, and good rate performance. In particular, when the residual carbon rate is 1.7wt%-2.3wt%, the various properties are particularly good. If the amount of carbon source added is lower or higher than the specified range, the obtained product will deteriorate in terms of compaction density, specific capacity, and rate performance. It is speculated that when the amount of carbon source added is not within the limited range, the uniformity, continuity and integrity of the carbon coating layer of the obtained lithium iron manganese phosphate material are poor. The poor coating effect causes the carbon coating layer to be unable to play a good conductive role, which is not conducive to the specific capacity and rate performance.

[0090] In summary, the present invention uses lithium phytate or a mixture of lithium phytate and a traditional lithium source as a lithium source, and polyacrylic acid or a mixture of polyacrylic acid and other carbon-containing substances as a carbon source, which helps to solve the problems of low grinding efficiency, poor carbon coating effect, and low material compaction density in the solid-phase method for preparing lithium iron manganese phosphate, and ultimately obtains a lithium iron manganese phosphate material with large specific capacity, high compaction density, and good rate performance.

[0091] Although embodiments of the present invention have been shown and described above, it should be understood that the above embodiments are illustrative and are not to be construed as limiting the present invention. Those skilled in the art may alter, modify, replace, and modify the above embodiments within the scope of the present invention. Furthermore, those skilled in the art may combine and incorporate the various embodiments or examples described in this specification, as well as features thereof, without conflicting requirements.

Claims

1. A method for preparing a high-rate lithium iron manganese phosphate material, characterized by: The steps are as follows: S1. A lithium source, a manganese source, an iron source, and a phosphorus source are mixed in a desired stoichiometric ratio, and then a carbon source is added that accounts for 6% to 10% of the total mass of the lithium source, the manganese source, the iron source, and the phosphorus source, and a solvent is added for grinding to obtain a precursor slurry, wherein the lithium source is lithium phytate or a mixture of lithium phytate and a traditional lithium source. When the lithium source is a mixture of lithium phytate and a traditional lithium source, the molar percentage of lithium element provided by the lithium phytate to the total lithium element is at least 50%; and the carbon source is a mixture of polyacrylic acid and other carbon-containing substances, and the mass percentage of polyacrylic acid is 30% to 75%. S2, spray drying the precursor slurry to obtain a precursor; S3, heat treating the precursor at 600-800° C. under an inert atmosphere for 2-20 h to obtain a gray-black product; S4, crushing the gray-black product to obtain a final product.

2. The method for preparing the high-rate lithium iron manganese phosphate material according to claim 1, characterized in that: The molecular weight of the polyacrylic acid is 2000-4000.

3. The method for preparing the high-rate lithium iron manganese phosphate material according to claim 1, wherein: The other carbon-containing substances are selected from at least one of sodium citrate, citric acid, sodium oleate, oleic acid, polyvinyl pyrrolidone, polyethylene glycol, glucose, ascorbic acid, sucrose, dopamine hydrochloride, starch, graphene oxide, reduced graphene, carbon nanotubes, and Ketjen black; The traditional lithium source is selected from at least one of lithium hydroxide, lithium carbonate, lithium dihydrogen phosphate, and lithium acetate; The carbon source is a mixture of polyacrylic acid and other carbon-containing substances in a mass ratio of 1:(0.5-2).

4. The method for preparing the high-rate 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 30 wt %-50 wt %.

5. The method for preparing the high-rate lithium iron manganese phosphate material according to claim 1, characterized in that: In step S1, the manganese source is manganese trioxide, the iron source is iron oxide, and the phosphorus source is phosphoric acid, and the grinding is performed until the D50 of the iron oxide is no more than 300 nm; In step S3, the precursor is heat treated at 750° C. under an inert atmosphere for 10 hours; In step S4, the pulverization process is specifically air flow pulverization.

6. A high-rate lithium manganese iron phosphate material prepared according to the method for preparing a high-rate lithium manganese iron phosphate material according to any one of claims 1 to 5.

7. The high-rate lithium iron manganese phosphate material according to claim 6, characterized in that: The residual carbon rate is 1.55wt%-2.3wt%.

8. An application of the high-rate lithium iron manganese phosphate material according to claim 6, characterized in that: Used as positive electrode material for lithium-ion batteries.

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