Preparation method of lithium iron manganese phosphate, positive electrode material and lithium ion battery

Abstract

This application discloses a method for preparing lithium manganese iron phosphate, a cathode material, and a lithium-ion battery, relating to the field of battery technology. This application uses a manganese source and an iron source to obtain manganese iron oxide through solid-state sintering, wherein at least one of the manganese source and the iron source is an oxide. The manganese iron oxide is then mixed with a lithium source and a phosphorus source in a solid-state mixture, and solid-state sintering is performed at 350–900°C to obtain lithium manganese iron phosphate. The preparation method provided by this application can reduce the use of sulfate, thereby reducing the generation of toxic sulfur dioxide gas. Furthermore, it facilitates the synthesis of high-purity, high-tap-density lithium manganese iron phosphate materials, reduces process costs, and improves the performance of the obtained lithium manganese iron phosphate materials. The cathode material and lithium-ion battery provided by this application include lithium manganese iron phosphate prepared by the above method.

Description

Technical Field

[0001] This application relates to the field of battery technology, and more specifically, to a method for preparing lithium manganese iron phosphate, cathode materials, and lithium-ion batteries. Background Technology

[0002] Currently, the synthesis methods for lithium manganese iron phosphate are mainly divided into solid-state methods and co-precipitation methods. The co-precipitation method uses a manganese source, an iron source, and a complexing agent to co-precipitate and generate a precursor. The precursor then reacts with a phosphorus source and a lithium source in a solid or liquid phase to generate lithium manganese iron phosphate, as illustrated in Chinese invention patent CN105047922A. The traditional solid-state method uses a manganese source, an iron source, a phosphorus source, and a lithium source for sintering; this process is the simplest, but the resulting material has the worst performance. Therefore, the co-precipitation method is the mainstream approach. It can be seen that LiMn... x Fe 1-x Whether it's lithium iron phosphate (LiFePO4) with x=0 or lithium manganese iron phosphate (LiFePO4) with x≠0, the synthesis of these materials is divided into solid-phase and liquid-phase methods. The traditional solid-phase method is simple, but the resulting material has the worst performance. The liquid-phase method produces better performance, but the corrosion requirements of the equipment lead to high costs and greater environmental pressure. There are also related technologies that have designed a novel solid-phase process for synthesizing LiMn. x Fe 1-x PO4 materials offer improved performance and low cost. However, they suffer from several drawbacks. The use of manganese and iron sulfates as manganese and iron sources leads to the generation of toxic sulfur dioxide during synthesis, requiring treatment. Furthermore, the resulting lithium iron manganese oxide is highly hard, making ball milling difficult and hindering the production of high-purity lithium iron manganese phosphate. The different thermal decomposition temperatures of the two sulfates cause agglomeration during the formation of iron manganese oxide, increasing the difficulty of ball milling and reducing the chance of obtaining pure-phase iron manganese oxide. Additionally, the synthesis process in these technologies is lengthy, involves high temperatures, and results in a low purity of the iron manganese oxide synthesized from the reaction of manganese and iron salts, along with impurities such as iron oxide and manganese oxide as byproducts.

[0003] Therefore, this application is hereby submitted. Summary of the Invention

[0004] The purpose of this application includes providing a method for preparing lithium manganese iron phosphate, which can reduce the emission of toxic sulfur dioxide, improve material performance, and reduce process costs. The purpose of this application also includes providing a cathode material and a lithium-ion battery.

[0005] This application is implemented as follows:

[0006] In a first aspect, this application provides a method for preparing lithium manganese iron phosphate, comprising:

[0007] Manganese iron oxide is obtained by solid-state sintering using manganese and iron sources, wherein at least one of the manganese and iron sources is an oxide.

[0008] Manganese iron oxide is mixed with lithium and phosphorus sources in a solid phase and then sintered in a solid phase at 350–900 °C to obtain lithium manganese iron phosphate.

[0009] In an optional embodiment, the step of obtaining manganese iron oxide by solid-state sintering using a manganese source and an iron source includes:

[0010] A first mixture containing manganese and iron sources is subjected to solid-state sintering at 300–1200 °C to obtain a first manganese iron oxide (Mn). a Fe 1-a ) m O n ;

[0011] A second mixture containing manganese and iron sources is subjected to solid-state sintering at 300–1200 °C to obtain a second manganese iron oxide (Mn). b Fe 1-b ) m O n Among them, the second manganese iron oxide (Mn b Fe 1-b ) m O n The median particle size is smaller than that of the first manganese iron oxide (Mn a Fe 1-a ) m O n The median particle size, 0 < b < a < 1;

[0012] The first manganese iron oxide (Mn) a Fe 1-a ) m O n Second manganese iron oxide (Mn b Fe 1-b ) m O n Solid-phase mixing is carried out, and solid-phase sintering is performed at 300–1200℃ to obtain manganese-iron oxide with a manganese-rich inner layer and an iron-rich outer layer.

[0013] In an optional embodiment, when the first manganese iron oxide (Mn) is... a Fe 1-a ) m O n Second manganese iron oxide (Mn b Fe 1-b ) m O n In the solid-phase mixing step, and in the solid-phase mixing step of manganese iron oxide with lithium source and phosphorus source, one or more of carbon source, M source and N source are also added;

[0014] Among them, the M source is a doped cation source, and the N source is a doped anion source.

[0015] In an optional embodiment, the carbon source is one or more of sucrose, glucose, fructose, citric acid, phenolic resin, polyvinyl alcohol, polyethylene glycol, starch, carbon black, acetylene black, graphite, graphene, and conductive carbon nanotubes.

[0016] In optional embodiments, the doped cation source includes one or more of aluminum, magnesium, nickel, cobalt, titanium, copper, calcium, niobium, chromium, zinc, lanthanum, antimony, tellurium, strontium, tungsten, indium, and yttrium, and the doped anion source includes fluorine and / or sulfur.

[0017] In an optional embodiment, the manganese source is one or more of manganese tetroxide, manganese trioxide, manganese sulfide, manganese dioxide, manganese monohydroxymonooxide MnO(OH), hydrated manganese dioxide MnO(OH)2, manganese hydroxide Mn(OH)2, manganese sulfate, manganese carbonate, manganese oxalate, and manganese acetate.

[0018] In an optional embodiment, the iron source is one or more of the following: iron(II,III) oxide, ferric oxide, ferrous oxide, basic iron oxide FeO(OH), ferric hydroxide, ferrous hydroxide, ferrous sulfate, ferric sulfate, ferric carbonate, ferric oxalate, ferrous oxalate, ferric acetate, and ferric citrate.

[0019] In an optional embodiment, the lithium source is one or more of lithium carbonate, lithium hydroxide, lithium phosphate, lithium dihydrogen phosphate, lithium hydrogen phosphate, lithium oxalate, lithium acetate, lithium sulfate, lithium nitrate, and lithium chloride.

[0020] The phosphorus source is one or more of the following: ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium tripolyphosphate, phosphoric acid, calcium phosphate, phosphate ester, lithium dihydrogen phosphate, iron phosphate, lithium phosphate, lithium dihydrogen phosphate, and manganese phosphate.

[0021] Secondly, this application provides a cathode material comprising lithium manganese iron phosphate prepared by any of the preparation methods described in the foregoing embodiments.

[0022] Thirdly, this application provides a lithium-ion battery, including a positive electrode, a negative electrode, an electrolyte, and a separator, wherein the positive electrode includes the positive electrode material described in the aforementioned embodiments.

[0023] This application has the following beneficial effects:

[0024] This application uses manganese and iron sources to obtain manganese iron oxide through solid-state sintering, wherein at least one of the manganese and iron sources is an oxide. The manganese iron oxide is then mixed with lithium and phosphorus sources in a solid-state mixture and sintered at 350–900°C to obtain lithium manganese iron phosphate. Using a solid-state mixture of manganese and iron sources to produce manganese iron oxide reduces the reaction temperature and time. Since at least one of the manganese and iron sources is an oxide, the use of sulfates can be reduced, thereby reducing the generation of toxic sulfur dioxide. Furthermore, it avoids the problem of different thermal decomposition temperatures when the two sulfates are used as iron and manganese sources. Different thermal decomposition temperatures lead to lower purity of the resulting manganese iron oxide and the presence of impurity byproducts such as iron oxide and manganese oxide. In such cases, the material's tap density is insufficient, making it difficult to synthesize high-purity, high-tap-density lithium manganese iron phosphate material, ultimately affecting battery performance. Simultaneously, the preparation method of this application results in manganese iron oxide with reduced hardness, decreasing ball milling energy consumption and facilitating the synthesis of lithium manganese iron phosphate. The lithium manganese iron phosphate synthesized from high-purity manganese iron oxide materials possesses a stable manganese plateau, ensuring battery cycle life and enabling more accurate estimation of the state of charge (SOC). The method described in this application improves the conductivity of the lithium manganese iron phosphate material, allowing for a reduction in carbon coating content and specific surface area.

[0025] The cathode material and lithium-ion battery provided in this application include lithium manganese iron phosphate prepared by the above-mentioned method, which has better performance. Attached Figure Description

[0026] To more clearly illustrate the technical solutions of the embodiments of this application, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of this application and should not be regarded as a limitation of the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0027] Figure 1 The (Mn) prepared in Example 1 and Comparative Example 1 of this application 0.9 Fe 0.6 XRD pattern of 2O4;

[0028] Figure 2 The (Mn) prepared in Example 1 and Comparative Example 1 of this application 0.9 Fe 0.6 SEM image of 2O4;

[0029] Figure 3 The LiMn prepared in Example 1 and Comparative Examples 1 and 2 of this application are 0.6 Fe 0.4 XRD pattern of PO4;

[0030] Figure 4The LiMn prepared in Example 1 and Comparative Examples 1 and 2 of this application are 0.6 Fe 0.4 SEM image of PO4;

[0031] Figure 5 This is the LiMn prepared in Example 1 of this application. 0.6 Fe 0.4 Discharge curve of PO4;

[0032] Figure 6 This is the LiMn prepared in Comparative Example 1 of this application. 0.6 Fe 0.4 Discharge curve of PO4;

[0033] Figure 7 This is the LiMn prepared in Comparative Example 2 of this application. 0.6 Fe 0.4 Discharge curve of PO4. Detailed Implementation

[0034] In existing technologies, the liquid-phase method for lithium manganese iron phosphate (LFP) involves co-precipitating manganese iron carbonate or manganese iron hydroxide, resulting in a porous and loose morphology with low density. The LFP material synthesized using the co-precipitated manganese iron, lithium, and phosphorus sources also exhibits low tap density, large specific surface area, low compaction density, low energy density, high self-discharge, and a very rapid decline in the cycle manganese voltage plateau, leading to poor cycling performance. The traditional solid-phase method for LFP involves mixing and sintering a manganese, iron, phosphorus, and lithium source. While simple, this method produces materials with poor performance. Based on this, a new preparation process has been developed. First, the manganese and iron sources (both sulfates) are mixed and then solid-phase sintered, causing thermal decomposition of the manganese and iron sources to obtain manganese iron oxide. Then, the manganese iron oxide is mixed with the lithium and phosphorus sources and subjected to a second solid-phase sintering to obtain LFP. This preparation method is also simple, yielding low-priced lithium manganese iron phosphate (LMP) with superior characteristics compared to traditional solid-phase and liquid-phase methods. It also exhibits higher tap and compaction densities, higher energy density, smaller specific surface area, lower self-discharge, and longer cycle life. However, this process has drawbacks. The use of manganese and iron sulfates as manganese and iron sources leads to the generation of toxic sulfur dioxide gas during the synthesis reaction, requiring treatment. Furthermore, the synthesized LMP oxide has high hardness, making it difficult to ball mill and hindering the production of high-purity LMP. The different thermal decomposition temperatures of the two sulfates cause agglomeration during LMP oxide formation, increasing the difficulty of ball milling and hindering the formation of pure LMP oxide. Additionally, the synthesis reaction time and temperature in this technology are relatively long, resulting in a low purity of the LMP oxide synthesized from the reaction of manganese and iron salts, and the presence of impurity byproducts such as iron oxide and manganese oxide.

[0035] To address the shortcomings of the aforementioned related technologies, this application provides a method for preparing lithium manganese iron phosphate, a cathode material, and a lithium-ion battery, which can reduce the manufacturing process cost of lithium manganese iron phosphate, reduce toxic gas emissions, and improve product performance.

[0036] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0037] The method for preparing lithium manganese iron phosphate provided in this application includes the following steps:

[0038] Step S100: Using a manganese source and an iron source, manganese iron oxide is obtained by solid-state sintering, wherein at least one of the manganese source and the iron source is an oxide.

[0039] At least one of the manganese and iron sources is an oxide, which can reduce the use of sulfates and thus reduce the generation of toxic sulfur dioxide. In one case, the iron source of sulfate can be uniformly mixed with manganese oxide after melting, resulting in a complete reaction and high purity of the product. Furthermore, this embodiment avoids the problem of different thermal decomposition temperatures when the two sulfates are used as iron and manganese sources. Different thermal decomposition temperatures lead to lower purity of the resulting iron-manganese oxide and the presence of impurity byproducts such as iron oxide and manganese oxide, resulting in insufficient tap density of the material and making it difficult to synthesize high-purity, high-tap-density lithium manganese iron phosphate material, ultimately affecting battery performance. Different thermal decomposition temperatures of the two sulfates also cause the iron-manganese oxide to easily agglomerate, increasing the difficulty of ball milling and hindering the formation of pure-phase iron-manganese oxide. The iron-manganese oxide synthesized in this embodiment has reduced hardness, which can reduce ball milling energy consumption, facilitating the synthesis of lithium manganese iron phosphate, thus reducing process costs. Moreover, the preparation method of this embodiment can obtain high-purity iron-manganese oxide (Mn... x Fe 1-x-y ) m O n Where, 0 < x < 1, 0 < y < 1, 0 < 1 - xy < 1, 1:1 ≤ m:n ≤ 1:2, 0 <n≤4。

[0040] Optionally, the manganese source is one or more of manganese tetroxide, manganese trioxide, manganese sulfide, manganese dioxide, manganese monohydroxymonooxide MnO(OH), hydrated manganese dioxide MnO(OH)2, manganese hydroxide Mn(OH)2, manganese sulfate, manganese carbonate, manganese oxalate, and manganese acetate.

[0041] Optionally, the iron source is one or more of the following: iron(II,III) oxide, ferric oxide, ferrous oxide, basic iron oxide FeO(OH), ferric hydroxide, ferrous hydroxide, ferrous sulfate, ferric sulfate, ferric carbonate, ferric oxalate, ferrous oxalate, ferric acetate, and ferric citrate.

[0042] Specifically, step S100 may include:

[0043] Step S110: The first mixture containing manganese and iron sources is subjected to solid-state sintering at 300–1200 °C to obtain the first manganese iron oxide (Mn). a Fe 1-a ) m O n ;

[0044] Step S120: The second mixture containing manganese and iron sources is subjected to solid-state sintering at 300–1200 °C to obtain the second manganese iron oxide (Mn). b Fe 1-b ) m O n Among them, the second manganese iron oxide (Mn b Fe 1-b ) m O n The median particle size is smaller than that of the first manganese iron oxide (Mn a Fe 1-a ) m O n The median particle size, 0 < b < a < 1;

[0045] Step S130, the first manganese iron oxide (Mn a Fe 1-a ) m O n Second manganese iron oxide (Mn b Fe 1-b ) m O n Solid-phase mixing is carried out, and solid-phase sintering is performed at 300–1200℃ to obtain manganese-iron oxide with a manganese-rich inner layer and an iron-rich outer layer.

[0046] In this embodiment, a first manganese iron lithium oxide (Mn) with a high manganese content is used. a Fe 1-a ) m O n and low-manganese-content second manganese iron oxide (Mn b Fe 1-b ) m O n And the first manganese iron oxide (Mn a Fe 1-a ) m On The median particle size (D50) is greater than that of the second manganese iron oxide (Mn b Fe 1-b ) m O n The median particle size of the two results in a smaller particle size of the second manganese iron oxide (Mn) after mixing. b Fe 1-b ) m O n Uniformly distributed large-particle-size first manganese iron oxide (Mn a Fe 1-a ) m O n Surrounding the material, a sintering reaction forms a manganese-iron oxide layer with a manganese-rich inner layer and an iron-rich outer layer. This design, with a manganese-rich inner layer and an iron-rich outer layer, keeps the manganese-rich portion of the material away from the electrolyte, reducing the likelihood of manganese dissolution and improving the material's stability and cycle performance.

[0047] Optionally, the first manganese iron oxide (Mn a Fe 1-a ) m O n The median particle size is 1–6 μm, and the second manganese iron oxide (Mn) b Fe 1-b ) m O n The median particle size is 0.1–0.3 μm.

[0048] In step S200, manganese iron oxide is mixed with lithium source and phosphorus source in a solid phase and sintered in a solid phase at 350-900℃ to obtain lithium manganese iron phosphate.

[0049] Because step S100 can yield high-purity manganese iron oxide (Mn x Fe 1-x-y ) m O n Lithium manganese iron phosphate (LiMn) synthesized from high-purity manganese iron oxide materials x Fe 1-x-y PO4 has a stable manganese plateau, which can ensure the cycle life of the battery and make the state of charge (SOC) estimation more accurate. The lithium manganese iron phosphate material prepared by the method provided in this application has high conductivity, can reduce the carbon coating content of the material, and reduce the specific surface area of ​​the material.

[0050] Optionally, during the solid-phase mixing process in steps S130 and S200, one or more of a carbon source, an M source, and an N source may be added; wherein the M source is a doped cation source and the N source is a doped anion source.

[0051] Adding a carbon source can form a carbon-coated lithium manganese iron phosphate material; adding an M source can obtain a cation-doped lithium manganese iron phosphate material; adding an N source can obtain an anion-doped lithium manganese iron phosphate material. When a carbon source is added in steps S130 and S200, and at least one of an M source and an N source is added, step S130 can obtain manganese iron oxide (Mn... x Fe 1-x- y M y ) m O n N z / C, Step S200 yields lithium manganese iron phosphate (LiMn). x Fe 1-x-y M y PO 4-z N z / C. Where, 0 < x < 1, 0 ≤ y < 1, 0 ≤ z ≤ 0.1, 1:1 ≤ m:(n+z) ≤ 1:2, 0 <n≤4。

[0052] Optionally, the carbon source is one or more of sucrose, glucose, fructose, citric acid, phenolic resin, polyvinyl alcohol, polyethylene glycol, starch, carbon black, acetylene black, graphite, graphene, and conductive carbon nanotubes.

[0053] Optionally, the doped cation source includes one or more of aluminum, magnesium, nickel, cobalt, titanium, copper, calcium, niobium, chromium, zinc, lanthanum, antimony, tellurium, strontium, tungsten, indium, and yttrium, and the doped anion source includes fluorine and / or sulfur.

[0054] In step S200, optionally, the lithium source is one or more of lithium carbonate, lithium hydroxide, lithium phosphate, lithium dihydrogen phosphate, lithium hydrogen phosphate, lithium oxalate, lithium acetate, lithium sulfate, lithium nitrate, and lithium chloride; and the phosphorus source is one or more of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium tripolyphosphate, phosphoric acid, calcium phosphate, phosphate ester, lithium dihydrogen phosphate, iron phosphate, lithium phosphate, lithium dihydrogen phosphate, and manganese phosphate.

[0055] Optionally, both lithium and phosphorus sources can be lithium dihydrogen phosphate. Using lithium dihydrogen phosphate is a cheaper synthesis method, as it can be directly synthesized from manganese iron oxide to lithium manganese iron phosphate. Furthermore, the stoichiometric ratio of phosphate to lithium in lithium dihydrogen phosphate to lithium manganese iron phosphate is consistent, eliminating the need to use two compounds that contain phosphorus and lithium sources respectively.

[0056] The above preparation method can be used to prepare lithium manganese iron phosphate materials with olivine structure.

[0057] The positive electrode material provided in this application includes the lithium manganese iron phosphate material prepared by the above-described preparation method. The positive electrode material may also include one or more of layered lithium polycarbonate, spinel-type lithium manganese oxide, and layered lithium manganese-rich base materials. A lithium-ion battery provided in this application includes a positive electrode sheet, a negative electrode sheet, an electrolyte, and a separator, wherein the positive electrode sheet includes the above-described positive electrode material.

[0058] The following detailed description of the features and product performance of the preparation method of lithium manganese iron phosphate of this application is provided in conjunction with the embodiments.

[0059] Example 1

[0060] MnO and FeSO4·7H2O were mixed at a molar ratio of 8:1, followed by solid-state mixing. The homogeneous mixture was then heated to 500℃ for high-temperature solid-state sintering to obtain the lithium manganese iron phosphate precursor (MnO·7H2 ... 0.8 Fe 0.1 )2O4, i.e. Mn 1.6 Fe 0.2 O4 or Mn(Mn) 0.3 Fe 0.1 )2O4. Because manganese and iron are adjacent elements in the periodic table, manganese-iron exists as a solid solution in this precursor. (Mn 0.8 Fe 0.1 The median particle size D50 of 2O4 is 6 μm. The reaction equation is:

[0061] 5O₂ + 8MnO + FeSO₄·7H₂O → 5(MnO) 0.8 Fe 0.1 )2O4+SO2+7H2O.

[0062] MnO and FeSO4·7H2O were mixed at a molar ratio of 1:5, followed by solid-state mixing. The homogeneous mixture was then heated to 500℃ for high-temperature solid-state sintering to obtain the lithium manganese iron phosphate precursor (MnO·7H2 ... 0.1 Fe 0.5 )2O4, i.e. Mn 0.2 Fe 1.0 O4. Because manganese and iron are adjacent elements in the periodic table, manganese-iron exists as a solid solution in this precursor. (Mn 0.1 Fe 0.5 The median particle size D50 of 2O4 is 0.1 μm. The reaction equation is:

[0063] 9O2+2MnO+10FeSO4·7H2O→10(Mn 0.1 Fe 0.5 )2O4+10SO2+70H2O.

[0064] The uniformly mixed material was heated to 500℃ for high-temperature solid-state sintering to obtain lithium manganese iron phosphate precursor (Mn). 0.9 Fe 0.6 )2O4, i.e. Mn 1.8 Fe 1.2 O4 or Mn(Mn) 0.4 Fe 0.6 )2O4. Because manganese and iron are adjacent elements in the periodic table, manganese-iron exists in the precursor as a solid solution, although the chemical formula is (Mn2O4). 0.9 Fe 0.6 )2O4, but the inner layer is basically manganese-rich (Mn) 0.8 Fe 0.1 The outer layer is primarily iron-rich (Mn2O4), while the inner layer is mainly composed of iron-rich (Mn2O4). 0.1 Fe 0.5 )2O4. Because manganese and iron are adjacent elements in the periodic table, the manganese-iron precursor exists in solid solution form. The reaction equation is:

[0065] (Mn 0.8 Fe 0.1 )2O4+(Mn 0.1 Fe 0.5 )2O4→(Mn 0.9 Fe 0.6 )2O4.

[0066] The obtained (Mn) 0.9 Fe 0.6 Lithium manganese iron phosphate (LiMn) was obtained by solid-state mixing and milling of 2O4 and lithium dihydrogen phosphate, followed by high-temperature solid-state sintering at 500℃. 0.6 Fe 0.4 PO4. Although the chemical formula is LiMn 0.6 Fe 0.4 It contains PO4, but it is a lithium manganese iron phosphate with a manganese-rich inner layer and an iron-rich outer layer. The reaction equation is:

[0067] 2(Mn 0.9 Fe 0.6 )2O4 + 6LiH2PO4 → 6LiMn 0.6 Fe 0.4 PO4 + 6H2O↑ + O2↑.

[0068] The prepared lithium manganese iron phosphate LiMn 0.6 Fe 0.4PO4 was mixed with a conductive agent and a binder to prepare a positive electrode slurry. The slurry contained 97.2% active material, 1.7% conductive agent (conductive carbon black, conductive graphite, conductive carbon nanotubes, graphene), and 1.1% binder (polyvinylidene fluoride). The content of the solvent N-methylpyrrolidone was adjusted to achieve a solid content of approximately 60%. The thoroughly mixed slurry was coated onto the surface of current collector aluminum foil, dried, and then rolled and sliced ​​to obtain the positive electrode sheet. A square full cell assembled using the above positive electrode sheet was subjected to 1C charge-1C discharge cycle performance testing. The square battery had a capacity of 20 Ah, a thickness of 15 mm, a width of 119 mm, and a height of 208 mm.

[0069] Figure 1 The black line in the middle is LiMn. 0.6 Fe 0.4 PO4 precursor (Mn) 0.9 Fe 0.6 The X-ray diffraction (XRD) pattern of Mn2O4 shows that the substance synthesized in this embodiment (Mn2O4) 0.9 Fe 0.6 )2O4, i.e., Mn(Mn) 0.4 Fe 0.6 The light gray MnFe2O4 peak corresponds well with the colorimetric card PDF#38-0430.

[0070] Figure 2 The middle left figure is (Mn) 0.9 Fe 0.6 The SEM image of 2O4 shows that the material is homogeneous with good morphology. The particle size and tap density of the tested material are as follows: D50 is 7 μm, and the tap density is as high as 2.5 g / cm³. 3 Therefore, the product synthesized in this embodiment is manganese iron oxide, rather than a simple mixture of manganese oxide and iron oxide.

[0071] Figure 3 The black line represents lithium manganese iron phosphate (LiMn). 0.6 Fe 0.4 X-ray diffraction (XRD) pattern of PO4. Figure 4 The middle left image shows lithium manganese iron phosphate (LiMn). 0.6 Fe 0.4 The scanning electron microscope (SEM) image of PO4 shows good material morphology. The electrical conductivity and compaction density of the material were tested; the electrical conductivity is 0.038 S / cm, and the powder compaction density is as high as 2.7 g / cm³. 3 .

[0072] Figure 5The discharge curves of the lithium manganese iron phosphate battery at room temperature and 1C charge-discharge cycle are shown. As can be seen, the discharge curves overlap well after 250 cycles. The initial discharge capacity is 148.39 mAh / g, and after 250 cycles, the discharge capacity is 145.86 mAh / g, with a capacity retention of 98.30%. It is also noted that the manganese plateau potential is 4.00V. The constant current charge ratio at room temperature and 1C is 86%.

[0073] Example 2

[0074] MnSO4·H2O and FeO were mixed at a molar ratio of 8:1, followed by solid-state mixing. The homogeneous mixture was then heated to 500℃ for high-temperature solid-state sintering to obtain the lithium manganese iron phosphate precursor (MnSO4·H2O·Fe ... 0.8 Fe 0.1 )2O4, i.e. Mn 1.6 Fe 0.2 O4 or Mn(Mn) 0.3 Fe 0.1 )2O4. Because manganese and iron are adjacent elements in the periodic table, manganese-iron exists as a solid solution in this precursor. (Mn 0.8 Fe 0.1 The median particle size D50 of 2O4 is 1 μm. The reaction equation is:

[0075] 3O2 + 16MnSO4·H2O + 2FeO → 10(Mn 0.8 Fe 0.1 )2O4+16SO2+16H2O.

[0076] MnSO4·H2O and FeO were mixed at a molar ratio of 1:5, followed by solid-state mixing. The homogeneous mixture was then heated to 500℃ for high-temperature solid-state sintering to obtain the lithium manganese iron phosphate precursor (MnSO4·H2O·Fe ... 0.1 Fe 0.5 )2O4, i.e. Mn 0.2 Fe 1.0 O4. Because manganese and iron are adjacent elements in the periodic table, manganese-iron exists as a solid solution in this precursor. (Mn 0.1 Fe 0.5 The median particle size D50 of 2O4 is 0.2 μm. The reaction equation is:

[0077] 13O2+2MnSO4·H2O+10FeO→10(Mn 0.1 Fe 0.5 )2O4+2SO2+2H2O.

[0078] The uniformly mixed material was heated to 500℃ for high-temperature solid-state sintering to obtain lithium manganese iron phosphate precursor (Mn).0.9 Fe 0.6 )2O4, i.e. Mn 1.8 Fe 1.2 O4 or Mn(Mn) 0.4 Fe 0.6 )2O4. Because manganese and iron are adjacent elements in the periodic table, manganese-iron exists in the precursor as a solid solution, although the chemical formula is (Mn2O4). 0.9 Fe 0.6 )2O4, but the inner layer is basically manganese-rich (Mn) 0.8 Fe 0.1 The outer layer is primarily iron-rich (Mn2O4), while the inner layer is mainly composed of iron-rich (Mn2O4). 0.1 Fe 0.5 )2O4. Because manganese and iron are adjacent elements in the periodic table, the manganese-iron precursor exists in solid solution form. The reaction equation is:

[0079] (Mn 0.8 Fe 0.1 )2O4+(Mn 0.1 Fe 0.5 )2O4→(Mn 0.9 Fe 0.6 )2O4.

[0080] The obtained (Mn) 0.9 Fe 0.6 Lithium manganese iron phosphate (LiMn) was obtained by solid-phase mixing and milling of 2O4 and lithium dihydrogen phosphate, followed by high-temperature solid-phase sintering at 500℃. 0.6 Fe 0.4 PO4. Although the chemical formula is LiMn 0.6 Fe 0.4 It contains PO4, but it is a lithium manganese iron phosphate with a manganese-rich inner layer and an iron-rich outer layer. The reaction equation is:

[0081] 2(Mn 0.9 Fe 0.6 )2O4 + 6LiH2PO4 → 6LiMn 0.6 Fe 0.4 PO4 + 6H2O↑ + O2↑.

[0082] The particle size, specific surface area, and tap density of the material were tested. The results showed that the material's D50 was 1.6 μm and its specific surface area was 15 m². 2 / g, with a tap density as high as 1.3g / cm³. 3 The compacted density of the material can reach 2.8 g / cm³. 3 This is significantly higher than the commonly used 0.8–1.0 g / cm³ in commercial products. 3 The tap density and 2.3 g / cm³ 3 The compaction density is far lower than the commonly used 20m³ in commercial applications.2 The specific surface area is / g. Higher compaction density allows for higher electrode roll compaction density, and thinner electrodes can accommodate more electrodes within a given battery casing, ultimately resulting in higher energy density. Simultaneously, a lower specific surface area reduces binder content, allowing for a higher proportion of active material, further improving battery energy density; moreover, a lower specific surface area reduces side reactions between the material and electrolyte, improving battery shelf life and cycle life.

[0083] Example 3

[0084] MnO and FeO were mixed at a molar ratio of 8:1, followed by solid-state mixing. The homogeneous mixture was then heated to 500℃ for high-temperature solid-state sintering to obtain the lithium manganese iron phosphate precursor (MnO). 0.8 Fe 0.1 )2O4, i.e. Mn 1.6 Fe 0.2 O4 or Mn(Mn) 0.3 Fe 0.1 )2O4. Because manganese and iron are adjacent elements in the periodic table, manganese-iron exists as a solid solution in this precursor. (Mn 0.8 Fe 0.1 The median particle size D50 of 2O4 is 4 μm. The reaction equation is:

[0085] 11O2 + 16MnO + 2FeO → 10(Mn 0.8 Fe 0.1 )2O4.

[0086] MnO and FeO were mixed at a molar ratio of 1:5, followed by solid-state mixing. The homogeneous mixture was then heated to 500℃ for high-temperature solid-state sintering to obtain the lithium manganese iron phosphate precursor (MnO). 0.1 Fe 0.5 )2O4, i.e. Mn 0.2 Fe 1.0 O4. Because manganese and iron are adjacent elements in the periodic table, manganese-iron exists as a solid solution in this precursor. (Mn 0.1 Fe 0.5 The median particle size D50 of 2O4 is 0.3 μm. The reaction equation is:

[0087] 7O2 + MnO + 5FeO → 5(Mn 0.1 Fe 0.5 )2O4.

[0088] The uniformly mixed material was heated to 500℃ for high-temperature solid-state sintering to obtain lithium manganese iron phosphate precursor (Mn). 0.9 Fe 0.6 )2O4, i.e. Mn1.8 Fe 1.2 O4 or Mn(Mn) 0.4 Fe 0.6 )2O4. Because manganese and iron are adjacent elements in the periodic table, manganese-iron exists in the precursor as a solid solution, although the chemical formula is (Mn2O4). 0.9 Fe 0.6 )2O4, but the inner layer is basically manganese-rich (Mn) 0.8 Fe 0.1 The outer layer is primarily iron-rich (Mn2O4), while the inner layer is mainly composed of iron-rich (Mn2O4). 0.1 Fe 0.5 )2O4. Because manganese and iron are adjacent elements in the periodic table, the manganese-iron precursor exists in solid solution form. The reaction equation is:

[0089] (Mn 0.8 Fe 0.1 )2O4+(Mn 0.1 Fe 0.5 )2O4→(Mn 0.9 Fe 0.6 )2O4.

[0090] Manganese oxides can also be generated through the thermal decomposition of substances such as monohydroxy manganese oxide (MnO(OH)), hydrated manganese dioxide (MnO(OH)2), manganese hydroxide (Mn(OH)2), manganese sulfate, manganese carbonate, manganese oxalate, and manganese acetate. For example, manganese hydroxide (Mn(OH)2) decomposes to produce MnO and H2O, while manganese carbonate decomposes to produce MnO and CO2. Similarly, iron oxides can be generated through the thermal decomposition of substances such as basic ferric oxide (FeO(OH)), ferric hydroxide, ferrous hydroxide, ferrous sulfate, ferric sulfate, ferric carbonate, ferric oxalate, ferrous oxalate, ferric acetate, and ferric citrate. For example, FeO(OH) decomposes to produce FeO and H2O, while ferric carbonate decomposes to produce FeO and CO2.

[0091] The obtained (Mn) 0.9 Fe 0.6 Lithium manganese iron phosphate (LiMn) was obtained by solid-state mixing and milling of 2O4 and lithium carbonate, followed by high-temperature solid-state sintering at 500℃. 0.6 Fe 0.4 PO4. The reaction equation is:

[0092] 2(Mn 0.9 Fe 0.6 )2O4 + 3Li2CO3 + 6H3PO4 → 6LiMn 0.6 Fe 0.4 PO4 + 9H2O↑ + 3CO2↑ + O2↑.

[0093] Currently, the price of lithium carbonate is 310,000 RMB / ton, lithium dihydrogen phosphate is 110,000 RMB / ton, and phosphoric acid is 6,000 RMB / ton. 74 tons of lithium carbonate and 196 tons of phosphoric acid are used to synthesize 314.8 tons of lithium manganese iron phosphate (LiMn). 0.6 Fe 0.4 PO4, therefore, LiMn synthesized by lithium carbonate and lithium dihydrogen phosphate processes 0.6 Fe 0.4 The cost of PO4 is above 76,600 RMB / ton. 104 tons of lithium dihydrogen phosphate can be synthesized into 157.4 tons of lithium manganese iron phosphate (LiMn). 0.6 Fe 0.4 PO4, therefore, the lithium dihydrogen phosphate process is used to synthesize LiMn. 0.6 Fe 0.4 The cost of PO4 is above 72,700 RMB / ton. The lithium manganese iron phosphate (LFP) synthesis process using lithium dihydrogen phosphate is the most cost-effective, being more than 5% cheaper than the lithium carbonate and lithium dihydrogen phosphate processes.

[0094] Example 4

[0095] Mn3O4 and Fe2O3 were mixed in a molar ratio of 2:1, followed by solid-state mixing. The homogeneous mixture was then heated to 500℃ for high-temperature solid-state sintering to obtain the lithium manganese iron phosphate precursor Mn3O4. 0.6 Fe 0.2 O. Mn 0.6 Fe 0.2 The median particle size D50 of O is 4 μm. The reaction equation is:

[0096] 4Mn3O4 + 2Fe2O3 → 20Mn 0.6 Fe 0.2 O+O2↑.

[0097] Mn3O4 and Fe2O3 were mixed in a molar ratio of 1:2, followed by solid-state mixing. The homogeneous mixture was then heated to 500℃ for high-temperature solid-state sintering to obtain the lithium manganese iron phosphate precursor Mn3O4. 0.3 Fe 0.4 O. Mn 0.3 Fe 0.4 The median particle size D50 of O is 0.2 μm. The reaction equation is:

[0098] Mn3O4 + 2Fe2O3 → 10Mn 0.3 Fe 0.4 O.

[0099] The uniformly mixed material was heated to 500℃ for high-temperature solid-state sintering to obtain the lithium manganese iron phosphate precursor Mn. 0.9 Fe 0.6O. Because manganese and iron are adjacent elements in the periodic table, manganese-iron exists in the precursor as a solid solution, although its chemical formula is Mn. 0.9 Fe 0.6 O, but the inner layer is mainly composed of manganese-rich Mn. 0.6 Fe 0.2 O, while the outer layer is mainly iron-rich Mn. 0.3 Fe 0.4 O. Because manganese and iron are adjacent elements in the periodic table, the manganese-iron precursor exists in solid solution form. The reaction equation is:

[0100] 2Mn 0.6 Fe 0.2 O+2Mn 0.3 Fe 0.4 O→2Mn 0.9 Fe 0.6 O+O2↑.

[0101] The obtained Mn 0.9 Fe 0.6 O and lithium dihydrogen phosphate were mixed in a solid phase, milled, and then sintered at 500℃ to obtain lithium manganese iron phosphate (LiMn). 0.6 Fe 0.4 PO4. Although the chemical formula is LiMn 0.6 Fe 0.4 It contains PO4, but it is a lithium manganese iron phosphate with a manganese-rich inner layer and an iron-rich outer layer. The reaction equation is:

[0102] 4Mn 0.9 Fe 0.6 O + 6LiH₂PO₄ + O₂ → 6LiMn 0.6 Fe 0.4 PO4 + 6H2O.

[0103] Comparative Example 1

[0104] MnSO4·H2O was used as the manganese source, and FeSO4·7H2O was used as the iron source, with a molar ratio of MnSO4·H2O to FeSO4·7H2O of 3:2. Solid-state mixing was then performed. The homogeneous mixture was heated to 600℃ for high-temperature solid-state sintering to obtain the lithium manganese iron phosphate precursor (MnSO4·H2O·FeSO4·7 ... 0.9 Fe 0.6 )2O4, i.e. Mn 1.8 Fe 1.2 O4 or Mn(Mn) 0.4 Fe 0.6 )2O4. Because manganese and iron are adjacent elements in the periodic table, the manganese-iron precursor exists in solid solution form. The reaction equation is:

[0105] 9MnSO4·H2O+6FeSO4·7H2O→5(Mn 0.9 Fe 0.6 )2O4+15SO2↑+51H2O↑+5O2↑.

[0106] (Mn) 0.9 Fe 0.6 Lithium manganese iron phosphate (LiMn) was obtained by solid-phase mixing and milling of 2O4, lithium carbonate, and ammonium dihydrogen phosphate, followed by high-temperature solid-phase sintering at 500℃. 0.6 Fe 0.4 PO4. Although the chemical formula is LiMn 0.6 Fe 0.4 It contains PO4, but it is a lithium manganese iron phosphate with a manganese-rich inner layer and an iron-rich outer layer. The reaction equation is:

[0107] 2(Mn 0.9 Fe 0.6 )2O4+3Li2CO3+6NH4H2PO4→6LiMn 0.6 Fe 0.4 PO4+9H2O↑+3CO2↑+O2↑+6NH3↑.

[0108] The prepared lithium manganese iron phosphate LiMn 0.6 Fe 0.4 PO4 was mixed with a conductive agent and a binder to prepare a positive electrode slurry. The slurry contained 97.2% active material, 1.7% conductive agent (conductive carbon black, conductive graphite, conductive carbon nanotubes, graphene), and 1.1% binder (polyvinylidene fluoride). The content of the solvent N-methylpyrrolidone was adjusted to achieve a solid content of approximately 60%. The thoroughly mixed slurry was coated onto the surface of current collector aluminum foil, dried, and then rolled and sliced ​​to obtain the positive electrode sheet. A square full cell assembled using the above positive electrode sheet was subjected to 1C charge-1C discharge cycle performance testing. The square battery had a capacity of 20 Ah, a thickness of 15 mm, a width of 119 mm, and a height of 208 mm.

[0109] Figure 1 The dark gray line represents the comparative example 1LiMn. 0.6 Fe 0.4 PO4 precursor (Mn) 0.9 Fe 0.6 The X-ray diffraction (XRD) pattern of Mn2O4 shows that the substance synthesized in Comparative Example 1 (Mn2O4) 0.9 Fe 0.6 )2O4, i.e., Mn(Mn) 0.4 Fe 0.6The light gray MnFe2O4 peak corresponds relatively well to the colorimetric chart PDF#38-0430, but the dark gray line has many extraneous peaks compared to the light gray line. For example, there are two extraneous peaks near 25°, two extraneous peaks near 40°, and two extraneous peaks near 65°.

[0110] Figure 2 The right-middle figure is Comparative Example 1 (Mn) 0.9 Fe 0.6 The SEM image of 2O4 shows that the material is homogeneous with good morphology. The particle size and tap density of the tested material are as follows: D50 is 6 μm, and the tap density is as high as 2.4 g / cm³. 3 All were lower than those in Example 1.

[0111] Figure 3 The dark gray line represents Comparative Example 1: Lithium manganese iron phosphate (LiMn). 0.6 Fe 0.4 X-ray diffraction (XRD) pattern of PO4. Figure 4 The middle figure shows Comparative Example 1: Lithium manganese iron phosphate (LiMn). 0.6 Fe 0.4 Scanning electron microscope (SEM) image of PO4. The electrical conductivity and compaction density of the material were tested; the electrical conductivity was 0.030 S / cm, and the powder compaction density was 2.5 g / cm³. 3 .

[0112] Figure 6 The discharge curve of a comparative example, a lithium manganese iron phosphate battery, is shown after 1C charge-discharge cycles at room temperature at comparison example 1. It can be seen that after 250 cycles, the discharge curves overlap relatively well, although they change as the cycles progress. The initial discharge capacity is 141.18 mAh / g, and after 250 cycles, the discharge capacity is 137.96 mAh / g, with a capacity retention of 97.72%. It is also noted that the manganese plateau potential is 3.95V. The constant current charge ratio at 1C at room temperature is 75%.

[0113] Comparative Example 2

[0114] Using visual inspection of commercially available lithium manganese iron phosphate (LiMn) 0.6 Fe 0.4PO4 was mixed with a conductive agent and a binder to prepare a positive electrode slurry. The slurry contained 97.2% active material, 1.7% conductive agent (conductive carbon black, conductive graphite, conductive carbon nanotubes, graphene), and 1.1% binder (polyvinylidene fluoride). The content of the solvent N-methylpyrrolidone was adjusted to achieve a solid content of approximately 60%. The thoroughly mixed slurry was coated onto the surface of current collector aluminum foil, dried, and then rolled and sliced ​​to obtain the positive electrode sheet. A square full cell assembled using the above positive electrode sheet was subjected to 1C charge-1C discharge cycle performance testing. The square battery had a capacity of 20 Ah, a thickness of 15 mm, a width of 119 mm, and a height of 208 mm.

[0115] Figure 3 The light gray line represents the comparative example: lithium manganese iron phosphate (LiMn). 0.6 Fe 0.4 X-ray diffraction (XRD) pattern of PO4.

[0116] Figure 4 The right-middle figure shows the comparative example 2, lithium manganese iron phosphate (LiMn). 0.6 Fe 0.4 Scanning electron microscope (SEM) image of PO4. The electrical conductivity and compaction density of the material were tested; the electrical conductivity was 0.007 S / cm, and the powder compaction density was 2.0 g / cm³. 3 The lithium manganese iron phosphate material in Example 1 had the highest electrical conductivity and powder compaction, followed by Comparative Example 1, and then Comparative Example 2.

[0117] Figure 7 The discharge curves of a comparative example 2-manganese iron phosphate battery undergoing 1C charge-discharge cycles at room temperature at a comparative rate are shown. It can be seen that after 250 cycles, the discharge curves show poor overlap, with significant changes in curve consistency as cycling progresses. The initial discharge capacity is 131.80 mAh / g, and after 250 cycles, the discharge capacity is 120.39 mAh / g, with a capacity retention of 91.34%. It is also noted that the manganese plateau potential is 3.90 V. The constant current charge ratio at 1C at room temperature is 50%.

[0118] Example 1, Comparative Example 1, and Comparative Example 2 are compared as follows:

[0119]

[0120]

[0121] As can be seen, after adopting the preparation method of this embodiment, compared with Comparative Examples 1 and 2, the conductivity, powder compaction, initial specific capacity of the lithium manganese iron phosphate LiMn0.6Fe0.4PO4 material, capacity retention after 250 cycles, manganese plateau potential, and charging constant current ratio of Example 1 are all greatly improved.

[0122] In summary, compared with the prior art, the beneficial effects of the method for preparing lithium manganese iron phosphate provided in this application are as follows:

[0123] 1. By improving the synthesis process, a new solid-phase synthesis method for lithium manganese iron phosphate is provided. The lithium manganese iron phosphate material synthesized by this method has high purity, stable cycle, high conductivity, high tap density, small specific surface area, and high electrode compaction density. The lithium-ion battery prepared from it has the characteristics of high charging constant current ratio, long cycle life, high energy density, and low self-discharge.

[0124] 2. By selecting manganese and iron sources, the generation of toxic sulfur dioxide gas is reduced, ball milling energy consumption is lowered, synthesis reaction time is reduced, and reaction temperature is lowered, which has the advantages of being environmentally friendly and low-cost.

[0125] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A method for preparing lithium manganese iron phosphate, characterized in that, include: Manganese iron oxide is obtained by solid-state sintering using a manganese source and an iron source, wherein at least one of the manganese source and the iron source is an oxide; The manganese iron oxide is mixed with a lithium source and a phosphorus source in a solid phase, and then solid-phase sintered at 350~900℃ to obtain lithium manganese iron phosphate. The steps for obtaining manganese iron oxide using a manganese source and an iron source via solid-state sintering include: The first mixture containing the manganese source and the iron source is subjected to solid-state sintering at 300~1200℃ to obtain a first manganese iron oxide, wherein the first manganese iron oxide is (Mn 0.8 Fe 0.1 )2O4 or Mn 0.6 Fe 0.2 O; The second mixture containing the manganese source and the iron source is subjected to solid-state sintering at 300~1200℃ to obtain a second manganese iron oxide, wherein the second manganese iron oxide is (Mn 0.1 Fe 0.5 )2O4 or Mn 0.3 Fe 0.4 O, wherein the median particle size of the second manganese iron oxide is smaller than the median particle size of the first manganese iron oxide; The first manganese iron oxide and the second manganese iron oxide are mixed in a solid phase and sintered in a solid phase at 300~1200℃ to obtain the manganese iron oxide with a manganese-rich inner layer and an iron-rich outer layer.

2. The method for preparing lithium manganese iron phosphate according to claim 1, characterized in that, In the step of solid-phase mixing of the first manganese iron oxide and the second manganese iron oxide, and in the step of solid-phase mixing of the manganese iron oxide with a lithium source and a phosphorus source, one or more of a carbon source, an M source, and an N source are also added. Wherein, the M source is a doped cation source, and the N source is a doped anion source.

3. The method for preparing lithium manganese iron phosphate according to claim 2, characterized in that, The carbon source is one or more of the following: sucrose, glucose, fructose, citric acid, phenolic resin, polyvinyl alcohol, polyethylene glycol, starch, carbon black, acetylene black, graphite, graphene, and conductive carbon nanotubes.

4. The method for preparing lithium manganese iron phosphate according to claim 2, characterized in that, The doped cation source includes one or more of aluminum, magnesium, nickel, cobalt, titanium, copper, calcium, niobium, chromium, zinc, lanthanum, antimony, tellurium, strontium, tungsten, indium, and yttrium, and the doped anion source includes fluorine and / or sulfur.

5. The method for preparing lithium manganese iron phosphate according to claim 1, characterized in that, The manganese source is one or more of the following: manganese tetroxide, manganese trioxide, manganese sulfide, manganese dioxide, manganese monohydroxymonooxide (MnO(OH)), hydrated manganese dioxide (MnO(OH)2), manganese hydroxide (Mn(OH)2), manganese sulfate, manganese carbonate, manganese oxalate, and manganese acetate.

6. The method for preparing lithium manganese iron phosphate according to claim 1, characterized in that, The iron source is one or more of the following: iron(II,III) oxide, ferric oxide, ferrous oxide, basic iron oxide FeO(OH), ferric hydroxide, ferrous hydroxide, ferrous sulfate, ferric sulfate, ferric carbonate, ferric oxalate, ferrous oxalate, ferric acetate, and ferric citrate.

7. The method for preparing lithium manganese iron phosphate according to claim 1, characterized in that, The lithium source is one or more of lithium carbonate, lithium hydroxide, lithium phosphate, lithium dihydrogen phosphate, lithium hydrogen phosphate, lithium oxalate, lithium acetate, lithium sulfate, lithium nitrate, and lithium chloride. The phosphorus source is one or more of the following: ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium tripolyphosphate, phosphoric acid, calcium phosphate, phosphate ester, iron phosphate, lithium phosphate, lithium dihydrogen phosphate, and manganese phosphate.

8. A positive electrode material, characterized in that, Lithium manganese iron phosphate prepared by any one of claims 1 to 7.

9. A lithium-ion battery, comprising a positive electrode, a negative electrode, an electrolyte, and a separator, characterized in that, The positive electrode sheet comprises the positive electrode material as described in claim 8.

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