Lithium manganese iron phosphate material and preparation method therefor, positive electrode sheet and preparation method therefor, and battery

Abstract

A lithium manganese iron phosphate material and a preparation method therefor, a positive electrode sheet and a preparation method therefor, and a battery. The structural formula of the lithium manganese iron phosphate material is LiFexMnyA(1-x-y)PO4, wherein 0<x<1, 0<y<1, 0≤1-x-y≤0.2, and a doping element A is selected from at least one of Be, Ca, Mg, Ba, and Sr. The surface alkalinity value of the lithium manganese iron phosphate material is 10-150 μmol / g.

Description

Lithium manganese iron phosphate material and preparation method thereof, positive electrode sheet and preparation method thereof, battery

[0001] This application claims priority to the Chinese patent application filed with the China Patent Office on September 27, 2023, with application number 202311265390.0. The entire contents of the above application are incorporated by reference into this application. Technical Field

[0002] The present application relates to the field of lithium batteries, and in particular to a lithium iron manganese phosphate material and a preparation method thereof, a positive electrode sheet and a preparation method thereof, and a battery. Background Art

[0003] Currently, the lithium-ion batteries used in new energy vehicles primarily include lithium iron phosphate (LFP) batteries, ternary batteries, and lithium manganese iron phosphate (LMP) batteries. LMP batteries use lithium manganese iron phosphate (LMFP) as the cathode material. LMP, an upgraded version of LMP, is a novel cathode material obtained by adding manganese to LMP. Compared to LMP, LMP offers a higher voltage platform and 20% higher energy density, while retaining the safety and low-cost characteristics of LMP cells. SUMMARY OF THE INVENTION

[0004] However, the lithium iron manganese phosphate material in the related art has the problem of poor processing performance. That is, when the lithium iron manganese phosphate material is mixed with components such as a binder and a solvent to prepare a positive electrode slurry, the impurities in the lithium iron manganese phosphate material will react with the binder, resulting in an increase in the viscosity of the binder, which makes it difficult for the binder to be evenly dispersed in the solvent. Therefore, a large amount of solvent needs to be added to effectively disperse the binder, resulting in a low solid content of the positive electrode slurry, which in turn leads to poor electrical performance of the positive electrode sheet prepared using the positive electrode slurry, and further leads to poor cycle performance of the lithium-ion battery.

[0005] In the first aspect, the present invention provides a lithium iron manganese phosphate material, the structural formula of the lithium iron manganese phosphate material is LiFe x Mn y A (1-x-y) PO4, wherein 0<x<1, 0<y<1, 0≤1-xy≤0.2, and the doping element A is selected from at least one of Be, Ca, Mg, Ba and Sr; and the surface basicity value of the lithium manganese iron phosphate material is 10μmol / g~150μmol / g.

[0006] In a second aspect, an embodiment of the present application provides a method for preparing a lithium manganese iron phosphate material, comprising:

[0007] The first raw materials including a manganese source, an iron source, a chelating agent, a reducing agent and an alkaline solution are mixed to obtain a first reaction system. The pH value of the first reaction system is controlled to be 9.0-11.0. After the components in the first reaction system react, the precursor Fe x Mn (1-x) (OH)2, where 0<x<1;

[0008] The precursor Fe x Mn (1-x) A second raw material including (OH)2, a lithium source, a phosphorus source, a dopant, and a carbon source is mixed to obtain a second reaction system, the pH value of the second reaction system is controlled to be 8.0-9.5, the second reaction system is spray-granulated to obtain a sintered precursor, and the sintered precursor is sintered to obtain a lithium manganese iron phosphate material, the structural formula of the lithium manganese iron phosphate material is LiFe x Mn y A (1-x-y) PO4, wherein 0<x<1, 0<y<1, 0≤1-xy≤0.2, and the doping element A is selected from at least one of Be, Ca, Mg, Ba and Sr.

[0009] In a third aspect, an embodiment of the present application provides a positive electrode sheet, comprising a positive electrode current collector and a positive electrode active material layer arranged on the positive electrode current collector, wherein the positive electrode active material layer comprises the lithium manganese iron phosphate material as described above or the lithium manganese iron phosphate material prepared according to the preparation method of the lithium manganese iron phosphate material as described above.

[0010] In a fourth aspect, an embodiment of the present application provides a method for preparing a positive electrode sheet, comprising:

[0011] Providing a solid raw material and a solvent, and uniformly mixing the solid raw material and the solvent to obtain a positive electrode slurry, wherein the solid raw material includes a lithium iron manganese phosphate material, a binder, and a conductive agent, and the lithium iron manganese phosphate material is the lithium iron manganese phosphate material described above or is a lithium iron manganese phosphate material prepared according to the preparation method of the lithium iron manganese phosphate material described above;

[0012] A positive electrode current collector is provided, the positive electrode slurry is applied on the positive electrode current collector, and the positive electrode slurry is dried to form a positive electrode active material layer to obtain a positive electrode sheet.

[0013] In a fifth aspect, an embodiment of the present application provides a battery, comprising a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, wherein the positive electrode sheet is the positive electrode sheet as described above or is a positive electrode sheet prepared according to the preparation method of the positive electrode sheet as described above. Beneficial effects

[0014] The beneficial effects of the present application are as follows: the surface alkalinity value of the lithium iron manganese phosphate material provided in the embodiment of the present application is 10μmol / g~150μmol / g, and the surface alkalinity test can avoid the large test error caused by the alkalinity of the material itself and the alkalinity of distilled water itself in the conventional wet test; therefore, the surface alkalinity value more truly reflects the surface alkalinity caused by excess residual lithium on the surface of the material in a dry state, and more directly reflects the influence of the subsequent processing of the material on the slurry properties; the surface alkalinity value of the lithium iron manganese phosphate material is within a reasonable range, which means that the excess Li content in the lithium iron manganese phosphate material is low, It also shows that the content of lithium carbonate impurities and lithium hydroxide impurities is low. Therefore, when the lithium manganese iron phosphate material is used to prepare the positive electrode slurry, the problem of lithium carbonate impurities and lithium hydroxide impurities reacting with the binder to increase the viscosity of the binder and making it difficult to disperse can be avoided or alleviated. The lithium manganese iron phosphate material and the binder can be dispersed while reducing the amount of solvent added, thereby increasing the solid content of the positive electrode slurry. After the positive electrode slurry is coated on the current collector and rolled, the compaction density of the positive electrode slurry layer on the surface of the current collector can be increased, thereby improving the electrical properties of the positive electrode sheet and further improving the cycle performance of the lithium-ion battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] FIG1 is a flow chart of a method for preparing lithium manganese iron phosphate material provided in an embodiment of the present application.

[0016] FIG2 is a flow chart of a method for preparing a positive electrode sheet provided in an embodiment of the present application. Modes for Carrying Out the Invention

[0017] The following will be combined with the drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the embodiments described are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without making creative efforts are within the scope of protection of this application.

[0018] It should be noted that in order to solve the problem of poor processing performance of lithium manganese iron phosphate materials in the related art, the inventors of the present application conducted a large number of experimental studies and finally found that the reason for the poor processing performance of lithium manganese iron phosphate materials in the related art is that the surface alkalinity value of the lithium manganese iron phosphate materials in the related art is relatively high, which means that the excess Li content in the lithium manganese iron phosphate material is relatively high, and also that the content of lithium carbonate impurities and lithium hydroxide impurities is relatively high. Since lithium carbonate and lithium hydroxide will destroy the molecular structure of the binder, resulting in the breakage of the polymer chains in the binder molecules, the viscosity of the binder will increase, and the binder will be difficult to be evenly dispersed in the solvent. Therefore, a large amount of solvent needs to be added to effectively disperse the binder, resulting in a low solid content of the positive electrode slurry, which in turn leads to poor electrical performance of the positive electrode sheet prepared using the positive electrode slurry, and thus poor cycle performance of the lithium-ion battery.

[0019] Please refer to FIG1 . The present embodiment provides a lithium manganese iron phosphate material. The structural formula of the lithium manganese iron phosphate material is LiFe x Mn y A (1-x-y) PO4, wherein 0<x<1, 0<y<1, 0≤1-xy≤0.2, and the doping element A is selected from at least one of Be (beryllium), Ca (calcium), Mg (magnesium), Ba (barium) and Sr (strontium); the surface basicity value of the lithium manganese iron phosphate material is 10μmol / g~150μmol / g.

[0020] Illustratively, x can be 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, etc., y can be 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, etc., and 1-xy can be 0, 0.01, 0.02, 0.03, 0.05, 0.08, 0.1, 0.12, 0.15, 0.18, 0.2, etc.

[0021] Illustratively, the surface alkalinity value of the lithium manganese iron phosphate material can be 10 μmol / g, 15 μmol / g, 30 μmol / g, 50 μmol / g, 70 μmol / g, 90 μmol / g, 110 μmol / g 120 μmol / g, 150 μmol / g, etc.; preferably, the surface alkalinity value of the lithium manganese iron phosphate material is 15 μmol / g~110 μmol / g.

[0022] Illustratively, the particle size D50 of the lithium manganese iron phosphate material is 0.1 μm to 0.6 μm, such as 0.1 μm, 0.2 μm, 0.3 μm, 0.4 μm, 0.5 μm, 0.6 μm, etc.

[0023] Illustratively, the carbon content in the lithium manganese iron phosphate material is 1.0 wt% to 2.5 wt%, for example, 1.0 wt%, 1.2 wt%, 1.5 wt%, 1.8 wt%, 2.0 wt%, 2.3 wt%, 2.5 wt%, etc.

[0024] Illustratively, the moisture content in the lithium manganese iron phosphate material is 50 ppm to 300 ppm, for example, 50 ppm, 80 ppm, 100 ppm, 150 ppm, 180 ppm, 200 ppm, 250 ppm, 280 ppm, 300 ppm, etc.

[0025] Illustratively, the particle size D50 of the primary particles of the lithium manganese iron phosphate material is 0.1 μm to 0.6 μm, such as 0.1 μm, 0.2 μm, 0.3 μm, 0.4 μm, 0.5 μm, 0.6 μm, etc.

[0026] Illustratively, the lithium manganese iron phosphate material may have a single crystal structure or a quasi-single crystal structure.

[0027] The lithium iron manganese phosphate material provided in the embodiment of the present application has a surface alkalinity value of 10 μmol / g~150 μmol / g. The surface alkalinity value of the lithium iron manganese phosphate material is relatively low, which means that the content of lithium carbonate impurities and lithium hydroxide impurities in the lithium iron manganese phosphate material is low. Therefore, when the lithium iron manganese phosphate material is used to prepare a positive electrode slurry, the problem of lithium carbonate impurities and lithium hydroxide impurities reacting with the binder to increase the viscosity of the binder and making it difficult to disperse can be avoided or alleviated. The lithium iron manganese phosphate material and the binder can be dispersed while reducing the amount of solvent added, thereby increasing the solid content of the positive electrode slurry. After the positive electrode slurry is coated on the current collector and rolled, the compaction density of the positive electrode slurry layer on the surface of the current collector can be increased, thereby improving the electrical properties of the positive electrode sheet and further improving the cycle performance of the lithium-ion battery.

[0028] It should be noted that in the related art, when the lithium iron manganese phosphate material is used to prepare the positive electrode slurry, due to the high surface alkalinity value of the lithium iron manganese phosphate material and the poor processing performance, more solvent needs to be added to disperse the lithium iron manganese phosphate material and the binder, and the solid content of the obtained positive electrode slurry is usually 40%~55%. The lithium iron manganese phosphate material of the present application has a lower surface alkalinity value and better processing performance. When the lithium iron manganese phosphate material is used to prepare the positive electrode slurry, the solid content of the obtained positive electrode slurry can reach 60%~70%. That is to say, the lithium iron manganese phosphate material of the embodiment of the present application is conducive to improving the solid content of the positive electrode slurry. After the positive electrode slurry is coated on the current collector and rolled, the compaction density of the positive electrode slurry layer on the surface of the current collector can be improved, thereby improving the electrical properties of the positive electrode sheet, and then improving the cycle performance of the lithium-ion battery.

[0029] Referring to FIG. 1 , an embodiment of the present application further provides a method for preparing a lithium iron manganese phosphate material, which is used to prepare the lithium iron manganese phosphate material in any of the above embodiments. The method for preparing the lithium iron manganese phosphate material includes:

[0030] S110, mixing the first raw material including the manganese source, the iron source, the chelating agent, the reducing agent and the alkaline solution to obtain a first reaction system, controlling the pH value of the first reaction system to be 9.0-11.0, and reacting the components in the first reaction system to obtain the precursor Fe x Mn (1-x) (OH)2, where 0<x<1.

[0031] It should be noted that by controlling the pH value of the first reaction system to 9.0-11.0, that is, maintaining a high alkalinity of the first reaction system, the Mn and Fe ions can be fully precipitated to form the precursor Fe x Mn (1-x) (OH)2.

[0032] It is understood that the pH value of the first reaction system can be controlled by adjusting the amount of alkaline solution added. For example, the pH value of the first reaction system can be controlled to be 9.0, 9.2, 9.5, 9.8, 10.0, 10.2, 10.5, 10.8, 11.0, etc.

[0033] Illustratively, the alkaline solution includes water and an alkaline substance, the alkaline substance includes at least one of sodium hydroxide and potassium hydroxide, and the concentration of the alkaline substance in the alkaline solution is 0.1 mol / L~4 mol / L, for example, 0.1 mol / L, 0.2 mol / L, 0.3 mol / L, 1 mol / L, 2 mol / L, 3 mol / L, 4 mol / L, etc.

[0034] Exemplarily, the manganese source includes at least one of manganese sulfate, manganese phosphate, manganese oxalate, manganese trioxide, manganese tetraoxide, manganese acetate, manganese carbonate, manganese hydroxide, manganese chloride, manganese sulfate containing crystal water, methylcyclopentadienyl tricarbonyl manganese, manganese pyrophosphate, manganese trichloride, manganese metasilicate, decacarbonyl manganese, manganese titanate, manganese carbide, manganese nitrate, manganese monoxide, manganese heptoxide and manganese dioxide. Preferably, the manganese source includes at least one of divalent manganese sources such as manganese sulfate containing crystal water, manganese nitrate, manganese oxalate and manganese carbonate, because the cost of the divalent manganese source is low, which helps to achieve industrial production and helps to prepare Fe with stable valence. x Mn (1-x) (OH)2 precursor.

[0035] Illustratively, the iron source includes ferrous sulfate, ferrous sulfate containing crystal water, ferrous oxide, ferrous chloride, ferrous iodide, ferrous bromide, ferrous sulfide, ferrous hydroxide, ferrous oxalate, ferrous lactate, ferrous gluconate, ferrous acetate, ferrous nitrate, ferrous tannate, elemental iron, ferric oxide, ferric tetroxide, ferric phosphate, ferric phosphate containing crystal water, ferric sulfate, ferric sulfate containing crystal water, ferric nitrate, ferric chloride, ferric tannate and ferric hydroxide. Preferably, the iron source includes at least one of a divalent iron source such as ferric sulfate, ferric nitrate and ferrous oxalate containing crystal water, because the cost of the divalent iron source is low, which helps to achieve industrial production and helps to prepare Fe with stable valence. x Mn (1-x) (OH)2 precursor.

[0036] It is understood that the role of the complexing agent is to complex the Mn and Fe ions together so that the precursor Fe x Mn (1-x) In (OH)2, Mn and Fe ions are evenly distributed.

[0037] Exemplarily, the complexing agent includes a first metal complexing agent and a second metal complexing agent, the first metal complexing agent includes at least one of sodium citrate and sodium tartrate, the second metal complexing agent includes at least one of ethylenediamine and disodium ethylenediaminetetraacetate, and the mass ratio of the first metal complexing agent to the second metal complexing agent is (80~99.9):(0.1~20).

[0038] It should be noted that the first metal complexing agent (sodium citrate and sodium tartrate) is a weak Mn and Fe ion complexing agent, while the second metal complexing agent (ethylenediamine or disodium ethylenediaminetetraacetic acid) is a strong Mn and Fe ion complexing agent. The present application forms a metal ion complexing agent with strong comprehensive performance by compounding the first metal complexing agent and the second metal complexing agent. On the one hand, the complexing agent helps to completely complex the Mn and Fe ions, that is, to completely precipitate the Mn and Fe ions. On the other hand, it helps to control the reaction rate so that the complexing rate and precipitation rate match, so that the Mn and Fe ions are uniformly precipitated.

[0039] For example, the mass ratio of the first metal complexing agent to the second metal complexing agent is (80-99.9): (0.1-20), such as 80:20, 85:15, 90:10, 95:5, 99:1, 99.9:0.1, etc. Preferably, the mass ratio of the first metal complexing agent to the second metal complexing agent is (80-95): (5-20). Within this mass ratio range, the complexing agent formed by the combination of the first metal complexing agent and the second metal complexing agent has a strong complexing ability, which can completely precipitate the Mn and Fe ions in the solution, and the precipitation rates of the Mn and Fe ions match, and the obtained precursor Fex Mn (1-x) In (OH)2, Mn and Fe ions are evenly distributed, which can improve the precursor Fe x Mn (1-x) Stability of (OH)2.

[0040] Illustratively, the concentration of the complexing agent in the first reaction system is 0.05 mol / L to 1 mol / L, for example, 0.05 mol / L, 0.1 mol / L, 0.2 mol / L, 0.3 mol / L, 0.4 mol / L, 0.5 mol / L, 0.6 mol / L, 0.7 mol / L, 0.8 mol / L, 0.9 mol / L, 1 mol / L, etc.

[0041] Illustratively, the reducing agent includes at least one of sodium thiosulfate and sodium hypophosphite, and the concentration of the reducing agent in the first reaction system is 0.01 mol / L~0.5 mol / L, for example, 0.01 mol / L, 0.05 mol / L, 0.1 mol / L, 0.2 mol / L, 0.3 mol / L, 0.4 mol / L, 0.5 mol / L, etc.

[0042] It should be noted that the role of adding a reducing agent to the first reaction system is to reduce the trivalent iron ions (Fe 3+ ) is reduced to divalent iron ions (Fe 2+ ), and the trivalent or higher manganese ions (Mn 3+ 、Mn 4+ etc.) are reduced to divalent manganese ions (Mn 2+ ), this is because only divalent iron ions (Fe 2+ ) and divalent manganese ions (Mn 2+ ) can react to form a structurally stable precursor Fe x Mn (1-x) (OH)2, while the trivalent iron ion (Fe 3+ ) will react with hydroxide to generate unstable Fe(OH)3, trivalent or higher manganese ions (Mn 3+ 、Mn 4+ etc.) will react with hydroxide to generate unstable Mn(OH)3, Mn(OH)4, etc., thus failing to obtain the target product (Fe x Mn (1-x) (OH)2).

[0043] Illustratively, the first raw materials including a manganese source, an iron source, a complexing agent, a reducing agent, and an alkaline solution may be mixed in a first protective atmosphere, and the first protective atmosphere includes at least one of nitrogen and an inert gas.

[0044] For example, the inert gas may include at least one of helium (He), neon (Ne), argon (Ar), krypton (Kr), and xenon (Xe).

[0045] It should be noted that by mixing the first raw materials including the manganese source, the iron source, the chelating agent, the reducing agent and the alkaline solution in the first protective atmosphere, it is possible to prevent the divalent Mn and Fe ions from being oxidized by oxygen in the air to form trivalent Fe ions and manganese ions (Mn 3+ 、Mn 4+ etc.), which in turn leads to a decrease in the processing performance of lithium manganese iron phosphate materials.

[0046] For example, the precursor Fe x Mn (1-x) In (OH)2, x can be 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, etc.

[0047] For example, the precursor Fe x Mn (1-x) The particle size D50 of (OH)2 is 1μm~3μm, for example, 1μm, 1.2μm, 1.5μm, 1.8μm, 2μm, 2.2μm, 2.5μm, 2.8μm, 3μm, etc.

[0048] S120, will include the precursor Fe x Mn (1-x) The second raw materials including (OH)2, lithium source, phosphorus source, dopant and carbon source are mixed to obtain a second reaction system, the pH value of the second reaction system is controlled to be 8.0-9.5, the second reaction system is spray granulated to obtain a sintered precursor, and the sintered precursor is sintered to obtain a lithium manganese iron phosphate material. The structural formula of the lithium manganese iron phosphate material is LiFe x Mn y A (1-x-y) PO4, wherein 0<x<1, 0<y<1, 0≤1-xy≤0.2, and the doping element A is selected from at least one of Be, Ca, Mg, Ba and Sr.

[0049] For example, the precursor Fe x Mn (1-x) The second raw material including (OH)2, lithium source, phosphorus source, dopant and carbon source is mixed, so that the components in the second raw material can be mixed by a sand mill and the second raw material can also be ground by a sand mill to further reduce the particle size of the components in the second raw material.

[0050] Illustratively, the solid content of the second reaction system obtained after grinding and mixing with a sand mill is 25 wt% to 50 wt%, for example, 25 wt%, 30 wt%, 35 wt%, 40 wt%, 45 wt%, 50 wt%, etc.

[0051] Illustratively, the pH value of the second reaction system can be controlled to be 8.0, 8.2, 8.5, 8.8, 9.0, 9.2, 9.5, etc.

[0052] For example, the lithium source includes at least one of lithium carbonate, lithium oxide, lithium hydroxide, lithium chloride, lithium nitrate, lithium nitrite, lithium formate, lithium acetate, lithium oxalate, lithium phosphate, dilithium hydrogen phosphate, lithium dihydrogen phosphate and lithium citrate, and the precursor Fe x Mn (1-x) The molar ratio between (OH)2 and the lithium source is 1:(1.0~1.1), for example, 1:1.0, 1:1.02, 1:1.05, 1:1.08, 1:1.1, etc.

[0053] It should be noted that by x Mn (1-x) The molar ratio between (OH)2 and the lithium source is set to 1: (1.0~1.1), that is, the amount of lithium source added is kept at a slightly excessive level. On the one hand, this is to prevent the volatilization of lithium elements during the sintering process, resulting in insufficient lithium content in the prepared lithium iron manganese phosphate material. When the lithium content is too low, it will lead to structural lithium deficiency, so that the Mn ions in the lithium iron manganese phosphate material cannot remain stable at a valence of 2, which is not conducive to the later cycle stability. On the other hand, it can avoid the problem of excessive lithium content in the lithium iron manganese phosphate material. This is because when the lithium content in the lithium iron manganese phosphate material is too high, the pH value of the lithium iron manganese phosphate material will be too high. When the pH value of the lithium iron manganese phosphate material is too high, it will affect its processing performance, so that the stability of the positive electrode slurry formed by the lithium iron manganese phosphate material, solvent, binder, etc. is reduced, thereby affecting the electrical properties of the prepared positive electrode sheet.

[0054] For example, the phosphorus source includes at least one of phosphoric acid, ammonium dihydrogen phosphate, ammonium phosphate, lithium dihydrogen phosphate, manganese phosphate, sodium dihydrogen phosphate, magnesium phosphate, aluminum phosphate, diammonium hydrogen phosphate and phosphorus pentoxide, and the precursor Fe x Mn (1-x) The molar ratio between (OH)2 and the phosphorus source is 1:(1.0~1.2), for example, 1:1.0, 1:1.05, 1:1.1, 1:1.2, etc.

[0055] It is understandable that when the precursor Fe x Mn (1-x)When the molar ratio between (OH)2 and phosphorus source is 1: (1.0~1.2), it means that the amount of phosphorus source added is kept at a slightly excessive level, thereby preventing the loss of phosphorus during the reaction and sintering process, which leads to the problem of insufficient phosphate. In addition, when the precursor Fe x Mn (1-x) When the molar ratio between (OH)2 and the phosphorus source is greater than 1:1.2, it means that the amount of phosphorus source added is too much, which can easily lead to the pH value of the final product (lithium manganese iron phosphate material) being too low, and cause excessive phosphate to react with iron ions to generate impurities such as iron phosphate, thereby reducing the purity of the lithium manganese iron phosphate material.

[0056] Illustratively, the dopant includes at least one of oxalate containing doping element A, acetate containing doping element A, chloride containing doping element A, sulfate containing doping element A, and nitrate containing doping element A.

[0057] Illustratively, the addition amount of the doping element A in the dopant in the second reaction system is 500 ppm to 4000 ppm, for example, 500 ppm, 800 ppm, 1000 ppm, 1500 ppm, 2000 ppm, 2500 ppm, 3000 ppm, 3500 ppm, 4000 ppm, etc.

[0058] It can be understood that the purpose of introducing the dopant into the second reaction system is to introduce the doping element A into the lithium manganese iron phosphate material, and the doping element A is selected from at least one of the divalent metal elements such as Be, Ca, Mg, Ba and Sr. Since the valence state of the doping element A is consistent with the valence state of the divalent manganese ion and the divalent iron ion, and the valence state of the doping element A is stable, it is beneficial to improve the lithium manganese iron phosphate material (LiFe x Mn y A (1-x-y) PO4), when lithium manganese iron phosphate material is used in lithium-ion batteries, it is beneficial to maintain the stability of the lithium ion inlet and outlet channels, thereby improving the cycle performance of the battery.

[0059] Illustratively, the carbon source includes at least one of glucose, sucrose, furan resin, urea-formaldehyde resin, pyrimidine resin, phenolic resin, epoxy resin, polyvinyl alcohol, polystyrene, polymethyl methacrylate, polytetrafluoroethylene, polyvinylidene fluoride, polyacrylonitrile, styrene-butadiene rubber, carboxymethyl cellulose, fructose, lactose, starch, citric acid, carbon black, graphene, carbon nanotubes, carbon fiber, and activated carbon.

[0060] For example, the precursor Fe x Mn (1-x)The molar ratio between (OH)2 and the carbon source is 1:(1.5~3.5), for example, 1:1.5, 1:2, 1:2.5, 1:3, 1:3.5, etc.

[0061] It should be noted that the carbon source can act as a reducing agent, that is, it can reduce the trivalent iron ions (Fe 3+ ) is reduced to divalent iron ions (Fe 2+ ), and the trivalent or higher manganese ions (Mn 3+ 、Mn 4+ etc.) are reduced to divalent manganese ions (Mn 2+ ), this is because when the trivalent iron ions (Fe 3+ ) and trivalent or higher manganese ions (Mn 3+ 、Mn 4+ When the content of the carbon source is too high, more structural lithium (lithium carbonate, lithium hydroxide, etc.) will be present in the prepared lithium manganese iron phosphate material, which will lead to a decrease in the processing performance of the lithium manganese iron phosphate material, which is not conducive to the formation of a positive electrode slurry with a high solid content, and further reduce the cycle performance of the battery; on the other hand, the carbon source can form a carbon coating layer after sintering, and the carbon coating layer is wrapped on the surface of the primary particles of the lithium manganese iron phosphate material. Since the carbon coating layer has conductive properties, it can improve the conductive properties of the lithium manganese iron phosphate material, thereby improving the cycle performance of the lithium-ion battery containing the lithium manganese iron phosphate material.

[0062] Illustratively, the second raw material may further include a pH regulator, and the molar ratio of the pH regulator to the phosphorus source is (0.1-1):15, for example, 0.1:15, 0.2:15, 0.3:15, 0.4:15, 0.5:15, 0.6:15, 0.7:15, 0.8:15, 0.9:15, 1:15, etc.

[0063] It should be noted that, in the embodiment of the present application, the technical means for controlling the pH value of the second reaction system to be 8.0-9.5 is to control the precursor Fe x Mn (1-x) The addition ratio of (OH)2 and the phosphorus source (such as phosphoric acid, etc.) is another aspect of adjusting the pH value of the second reaction system by adding a pH adjuster to the second reaction system.

[0064] Exemplarily, the pH adjuster includes at least one of hydrogen peroxide and ammonia water, the concentration of hydrogen peroxide is 20wt%~40wt% (for example, 20wt%, 25wt%, 30wt%, 35wt%, 40wt%, etc.), and the concentration of ammonia water is 20wt%~30wt% (for example, 20wt%, 22wt%, 25wt%, 28wt%, 30wt%, etc.); It should be noted that the mechanism of hydrogen peroxide adjusting pH is: 2Fe 2+ +H2O2+2H + →2Fe 3+ +2H2O.

[0065] Exemplarily, the sintering precursor is sintered in a second protective atmosphere, the second protective atmosphere including a protective gas and an alkyl gas, wherein the protective gas includes at least one of nitrogen and an inert gas, the alkyl gas includes at least one of methane, propane, and butane, and the volume percentage of the alkyl gas in the second protective atmosphere is 0.1 vol% to 3 vol% (e.g., 0.1 vol%, 0.5 vol%, 1 vol%, 1.5 vol%, 2 vol%, 2.5 vol%, 3 vol%, etc.). Exemplarily, the volume percentage of the protective gas in the second protective atmosphere is 97 vol% to 99.9 vol% (e.g., 97 vol%, 97.5 vol%, 98 vol%, 98.5 vol%, 99 vol%, 99.5 vol%, 99.9 vol%, etc.).

[0066] For example, the second reaction system may be spray-granulated using a spray drying device.

[0067] For example, the sintering precursor may be sintered at a temperature of 650° C. to 800° C. (eg, 650° C., 680° C., 700° C., 720° C., 750° C., 780° C., 800° C., etc.).

[0068] It is understandable that after the sintering precursor is sintered, the lithium manganese iron phosphate material obtained by sintering is in the form of secondary particles. Therefore, after the sintering precursor is sintered, the lithium manganese iron phosphate material needs to be crushed to crush the secondary particles into primary particles.

[0069] Illustratively, the particle size D50 of the primary particles of the lithium manganese iron phosphate material is 0.1 μm to 0.6 μm, such as 0.1 μm, 0.2 μm, 0.3 μm, 0.4 μm, 0.5 μm, 0.6 μm, etc.

[0070] Illustratively, the carbon content in the lithium manganese iron phosphate material is 1.0wt%~2.5wt% (for example, 1.0wt%, 1.2wt%, 1.5wt%, 1.8wt%, 2.0wt%, 2.2wt%, 2.5wt%, etc.), and the carbon is coated on the surface of the primary particles in the form of a carbon coating layer. Since the carbon coating layer has conductive properties, the conductive properties of the lithium manganese iron phosphate material can be improved, thereby improving the cycle performance of the lithium-ion battery containing the lithium manganese iron phosphate material.

[0071] Illustratively, the moisture content in the lithium manganese iron phosphate material is controlled to be below 300 ppm. In some embodiments, the moisture content in the lithium manganese iron phosphate material prepared in the present application is measured by the Karl Fischer moisture test method (test temperature 190°C) and is 50 ppm~300 ppm (for example, 50 ppm, 100 ppm, 150 ppm, 200 ppm, 250 ppm, 300 ppm, etc.).

[0072] The surface alkalinity value of the lithium iron manganese phosphate material prepared in the present application is 10μmol / g~150μmol / g (for example, 10μmol / g, 30μmol / g, 50μmol / g, 70μmol / g, 90μmol / g, 110μmol / g, 130μmol / g, 150μmol / g, etc.). For example, the surface alkalinity value of the lithium iron manganese phosphate material can be measured by a heat flow micro-adsorption calorimeter. The specific test method is as follows: first, under a vacuum degree of 0.01pa, the lithium iron manganese phosphate material is heated to 1023K and then heat-treated for 2 hours to fully decompose Li2CO3 and LiOH on the surface of the lithium iron manganese phosphate material to generate Li2O; then, at 423K, dried CO2 is used as an acidic adsorption molecule, and the adsorption amount of CO2 is tested using a heat flow micro-adsorption calorimeter. The adsorption amount of CO2 is recorded as the surface alkalinity value of the lithium iron manganese phosphate material.

[0073] An embodiment of the present application provides a positive electrode sheet, including a positive electrode current collector and a positive electrode active material layer arranged on the positive electrode current collector, the positive electrode active material layer includes the lithium iron manganese phosphate material in any of the above embodiments or the lithium iron manganese phosphate material prepared according to the preparation method of the lithium iron manganese phosphate material in any of the above embodiments.

[0074] Illustratively, the positive electrode active material layer further includes a binder and a conductive agent, and the mass ratio of the lithium manganese iron phosphate material, the binder and the conductive agent is (90~99):(1~5):(1~5), for example, 90:1:1, 90:1:5, 90:5:1, 90:5:5, 94:3:3, 99:1:1, 99:1:5, 99:5:1, 99:5:5, etc.

[0075] Illustratively, the binder includes at least one of polyvinylidene fluoride, polymethyl methacrylate, and styrene-butadiene rubber.

[0076] Illustratively, the conductive agent includes at least one of activated carbon, conductive carbon black, carbon nanotubes, and graphene.

[0077] Illustratively, the positive electrode current collector may be aluminum foil, carbon-coated aluminum foil, or graphene aluminum foil.

[0078] For example, the compaction density of the positive electrode active material layer can be 2.1 g / m 3 ~2.6g / m 3 , for example 2.1g / m 3 , 2.2g / m 3 , 2.3g / m 3 , 2.4g / m 3 , 2.5g / m 3 , 2.6g / m 3 wait.

[0079] The present application may provide a method for preparing a positive electrode sheet, which is used to prepare the positive electrode sheet in any of the above embodiments. The method for preparing the positive electrode sheet includes:

[0080] S210, providing a solid raw material and a solvent, and uniformly mixing the solid raw material and the solvent to obtain a positive electrode slurry, wherein the solid raw material includes a lithium iron manganese phosphate material, a binder and a conductive agent, and the lithium iron manganese phosphate material is the lithium iron manganese phosphate material in any of the above embodiments or is a lithium iron manganese phosphate material prepared according to the preparation method of the lithium iron manganese phosphate material in any of the above embodiments.

[0081] Illustratively, in the solid raw material, the mass ratio of lithium manganese iron phosphate material, binder and conductive agent is (90~99):(1~5):(1~5), for example, 90:1:1, 90:1:5, 90:5:1, 90:5:5, 94:3:3, 99:1:1, 99:1:5, 99:5:1, 99:5:5, etc.

[0082] Illustratively, the mass percentage of the solid raw materials in the positive electrode slurry is 60 wt % to 70 wt %, for example, 60 wt %, 62 wt %, 65 wt %, 67 wt %, 70 wt %, etc.

[0083] Illustratively, the binder includes at least one of polyvinylidene fluoride (PVDF), polymethyl methacrylate, and styrene-butadiene rubber.

[0084] Illustratively, the conductive agent includes at least one of activated carbon, conductive carbon black, carbon nanotubes, and graphene.

[0085] Illustratively, the positive electrode current collector may be aluminum foil, carbon-coated aluminum foil, or graphene aluminum foil.

[0086] Illustratively, the solvent may include at least one of N-methylpyrrolidone (NMP), N,N-dimethylformamide (DMF), and N,N-dimethylacetamide (DMA).

[0087] S220 , providing a positive electrode current collector, applying a positive electrode slurry on the positive electrode current collector, drying the positive electrode slurry to form a positive electrode active material layer, and obtaining a positive electrode sheet.

[0088] For example, the positive electrode slurry may be applied to both sides of the positive electrode current collector by a coating method.

[0089] Illustratively, the positive electrode slurry can be dried by baking, with a baking temperature of 100°C to 120°C (e.g., 100°C, 105°C, 110°C, 115°C, 120°C, etc.), and a baking time of 1 second to 30 seconds (e.g., 1 second, 5 seconds, 10 seconds, 15 seconds, 20 seconds, 30 seconds, etc.).

[0090] For example, after the positive electrode slurry is dried, the surface density of the positive electrode active material layer on the surface of the positive electrode current collector is measured to be 160 g / m 2 ~250g / m 2 , for example 160g / m 2 , 180g / m 2 , 200g / m 2 , 230g / m 2 , 250g / m 2 wait.

[0091] For example, after the positive electrode slurry is dried, the positive electrode slurry can be rolled. After rolling, the compaction density of the positive electrode active material layer on the surface of the positive electrode current collector is measured to be 2.1 g / m 3 ~2.6g / m 3 For example, 2.1g / m 3 , 2.2g / m 3 , 2.3g / m 3 , 2.4g / m 3 , 2.5g / m 3 , 2.6g / m 3 wait.

[0092] The present application also provides a battery, comprising a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, wherein the positive electrode sheet is the positive electrode sheet in any of the above embodiments or a positive electrode sheet prepared according to the preparation method of the positive electrode sheet in any of the above embodiments.

[0093] The lithium manganese iron phosphate material and its preparation method, the positive electrode sheet and its preparation method, and the battery provided in this application are introduced in detail below in the form of specific embodiments.

[0094] Example 1

[0095] A method for preparing lithium manganese iron phosphate material, comprising:

[0096] Under nitrogen atmosphere, a manganese source, an iron source, a complexing agent, a reducing agent and an alkaline solution are mixed to obtain a first reaction system. The pH value of the first reaction system is controlled to be 9.0-11.0. After the components in the first reaction system react, the precursor Fe 0.37 Mn 0.63 (OH)2; wherein the manganese source is manganese sulfate, the iron source is ferrous sulfate, the complexing agent includes sodium citrate (90wt%) and ethylenediamine (10wt%), the reducing agent is sodium thiosulfate, and the pH value of the first reaction system is 9.0;

[0097] The precursor Fe 0.35 Mn 0.65 (OH)2, lithium source, phosphorus source, dopant, carbon source and pH regulator are added to a sand mill for grinding and mixing. 0.35 Mn 0.65 The molar ratio of (OH)2 to the phosphorus source is 1:1.05 to obtain a second reaction system. The pH value of the second reaction system is 9.0. The second reaction system is spray granulated to obtain a sintered precursor. The sintered precursor is sintered to obtain a lithium manganese iron phosphate material. The structural formula of the lithium manganese iron phosphate material is LiMn 0.6 Fe 0.35 Be 0.05 PO4, the surface alkalinity value of the obtained lithium manganese iron phosphate material is 80μmol / g; wherein, the lithium source is lithium carbonate, the phosphorus source is phosphoric acid, the dopant is beryllium oxalate (BeC2O4), the carbon source is glucose, and the pH regulator is ammonia water.

[0098] Example 2

[0099] A method for preparing a lithium manganese iron phosphate material, which differs from Example 1 in that: the dopant is different, the pH value of the second reaction system is different, and the structural formula of the obtained lithium manganese iron phosphate material is different;

[0100] In this embodiment 2, the added dopant is magnesium oxalate (MgC2O4); 0.37 Mn 0.63(OH)2, lithium source, phosphorus source, dopant, carbon source and pH regulator were added to a sand mill for grinding and mixing. The pH value of the obtained second reaction system was 9.0, and the surface basicity of the obtained lithium manganese iron phosphate material was 90 μmol / g. The structural formula of the obtained lithium manganese iron phosphate material was LiMn 0.6 Fe 0.35 Mg 0.05 PO4.

[0101] Example 3

[0102] A method for preparing a lithium manganese iron phosphate material, which differs from Example 1 in that the structural formula of the precursor obtained after the reaction of the first reaction system is different, and the structural formula of the obtained lithium manganese iron phosphate material is different;

[0103] In Example 3, the structural formula of the precursor is Fe 0.39 Mn 0.61 (OH)2, the structural formula of the obtained lithium manganese iron phosphate material is LiMn 0.6 Fe 0.38 Be 0.02 PO4.

[0104] Example 4

[0105] A method for preparing a lithium manganese iron phosphate material, which differs from Example 1 in that: the dopant is different, the pH value of the second reaction system is different, and the structural formula of the obtained lithium manganese iron phosphate material is different;

[0106] In this embodiment 4, the added dopant is barium oxalate (BaC2O4); 0.37 Mn 0.63 (OH)2, lithium source, phosphorus source, dopant, carbon source and pH regulator were added to a sand mill for grinding and mixing. The pH value of the obtained second reaction system was 8.0. The surface alkalinity of the obtained lithium manganese iron phosphate material was 10 μmol / g. The structural formula of the obtained lithium manganese iron phosphate material was LiMn 0.6 Fe 0.35 Ba 0.05 PO4.

[0107] Example 5

[0108] A preparation method of lithium manganese iron phosphate material, which is different from Example 1 in that: the precursor Fe 0.35 Mn 0.65 The molar ratio of (OH)2 and phosphorus source was different, and the pH value of the second reaction system was different;

[0109] In Example 5, the precursor Fe 0.35 Mn 0.65The molar ratio of (OH)2 to the phosphorus source is 1:1.0, the pH value of the second reaction system is 9.5, and the surface alkalinity of the obtained lithium manganese iron phosphate material is 110 μmol / g.

[0110] Example 6

[0111] A preparation method of lithium manganese iron phosphate material, which is different from Example 1 in that: the precursor Fe 0.35 Mn 0.65 The molar ratio of (OH)2 and phosphorus source was different, and the pH value of the second reaction system was different;

[0112] In Example 6, the precursor Fe 0.35 Mn 0.65 The molar ratio of (OH)2 to the phosphorus source is 1:1.2, the pH value of the second reaction system is 8.5, and the surface alkalinity of the obtained lithium manganese iron phosphate material is 30 μmol / g.

[0113] Example 7

[0114] A method for preparing a lithium-ion battery, comprising:

[0115] 1. Preparation of positive electrode slurry: LiMn prepared in Example 1 0.6 Fe 0.35 Be 0.05 PO4 material, binder polyvinylidene fluoride (PVDF), and conductive agent (conductive carbon black) were mixed at a ratio of 94:3:3, and solvent N-methylpyrrolidone (NMP) was added to control the viscosity of the positive electrode slurry to 20000mPa.s and the solid content of the positive electrode slurry to 65wt%;

[0116] 2. Prepare the positive electrode slurry: Coat the positive electrode slurry on both surfaces of the current collector aluminum foil with a coating density of 200 g / m 2 , and roller compacted, the compaction density is 2.35g / m 3 ;

[0117] 3. Preparation of negative electrode sheet: Mix the negative electrode material graphite, conductive agent acetylene black and binder sodium carboxymethyl cellulose (CMC) and styrene butadiene rubber (SBR) in a mass percentage of 94:1:2:3, then add solvent deionized water and stir evenly to prepare negative electrode slurry; evenly apply the above negative electrode slurry on both surfaces of the negative electrode current collector copper foil, and then dry, roll and slice to obtain a surface density of 90g / cm 3 , compacted density is 1.6g / cm 3 The negative electrode;

[0118] 4. Prepare the electrolyte: mix ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) in a volume ratio of 1:1:1 to obtain an organic solvent, and then dissolve the fully dried lithium salt LiPF6 in the organic solvent to prepare an electrolyte with a concentration of 1 mol / L;

[0119] 5. Battery assembly: stack the positive electrode sheet, diaphragm, and negative electrode sheet in sequence, then weld the tabs, bake, inject an appropriate amount of electrolyte after the moisture content is qualified, package, then age, form, evacuate and seal to prepare a lithium-ion battery.

[0120] Example 8

[0121] A method for preparing a lithium-ion battery, which differs from Example 7 in that: when preparing the positive electrode slurry, the lithium manganese iron phosphate material used is the lithium manganese iron phosphate material prepared in Example 2, and the solid content of the positive electrode slurry is 68wt%.

[0122] Example 9

[0123] A method for preparing a lithium-ion battery, which differs from Example 7 in that: when preparing the positive electrode slurry, the lithium manganese iron phosphate material used is the lithium manganese iron phosphate material prepared in Example 3, and the solid content of the positive electrode slurry is 65wt%.

[0124] Example 10

[0125] A method for preparing a lithium-ion battery, which differs from Example 7 in that: when preparing the positive electrode slurry, the lithium manganese iron phosphate material used is the lithium manganese iron phosphate material prepared in Example 4, and the solid content of the positive electrode slurry is 67wt%.

[0126] Example 11

[0127] A method for preparing a lithium-ion battery, which differs from Example 7 in that: when preparing the positive electrode slurry, the lithium manganese iron phosphate material used is the lithium manganese iron phosphate material prepared in Example 5, and the solid content of the positive electrode slurry is 63wt%.

[0128] Example 12

[0129] A method for preparing a lithium-ion battery, which differs from Example 9 in that: when preparing the positive electrode slurry, the lithium manganese iron phosphate material used is the lithium manganese iron phosphate material prepared in Example 6, and the solid content of the positive electrode slurry is 70wt%.

[0130] Comparative Example 1

[0131] A method for preparing a lithium manganese iron phosphate material is different from Example 1 in that the pH value of the second reaction system is different. In Comparative Example 1, the pH value of the second reaction system is 7.5, that is, the surface alkalinity value of the lithium manganese iron phosphate material is 8 μmol / g.

[0132] Comparative Example 2

[0133] A method for preparing a lithium manganese iron phosphate material is different from Example 1 in that the pH value of the second reaction system is different. In Comparative Example 1, the pH value of the second reaction system is 10, that is, the surface alkalinity value of the lithium manganese iron phosphate material is 180 μmol / g.

[0134] Comparative Example 3

[0135] A method for preparing a lithium iron manganese phosphate material, which differs from Example 1 in that no dopant is added when preparing the second reaction system, and the structural formula of the obtained lithium iron manganese phosphate material is different; in this comparative example 3, the structural formula of the obtained lithium iron manganese phosphate material is LiMn 0.65 Fe 0.35 PO4.

[0136] Comparative Example 4

[0137] A method for preparing a lithium-ion battery, which differs from Example 9 in that: the lithium manganese iron phosphate material used in preparing the positive electrode slurry is different, and the solid content of the positive electrode slurry is different;

[0138] In Comparative Example 4, the lithium manganese iron phosphate material used is the lithium manganese iron phosphate material prepared in Comparative Example 1, and the solid content of the positive electrode slurry is 50 wt %.

[0139] Comparative Example 5

[0140] A method for preparing a lithium-ion battery, which differs from Example 9 in that: the lithium manganese iron phosphate material used in preparing the positive electrode slurry is different, and the solid content of the positive electrode slurry is different;

[0141] In Comparative Example 5, the lithium manganese iron phosphate material used is the lithium manganese iron phosphate material prepared in Comparative Example 2, and the solid content of the positive electrode slurry is 45 wt %.

[0142] Comparative Example 6

[0143] A method for preparing a lithium-ion battery, which differs from Example 9 in that: when preparing the positive electrode slurry, the lithium manganese iron phosphate material used is the lithium manganese iron phosphate material (LiMn) prepared in Comparative Example 3. 0.65 Fe 0.35 PO4).

[0144] By comparing Examples 7-12 with Comparative Examples 4-5, it can be seen that in the preparation method of the lithium ion battery recorded in Examples 7-12, the solid content of the positive electrode slurry can reach 70wt%, while in the preparation method of the lithium ion battery recorded in Comparative Example 4, the solid content of the positive electrode slurry can only reach 50wt%, and in the preparation method of the lithium ion battery recorded in Comparative Example 5, the solid content of the positive electrode slurry can only reach 45wt%; it is known that the surface alkalinity value of the lithium iron manganese phosphate material used in Examples 7-12 ranges from 10μmol / g to 150μmol / g, while the surface alkalinity value of the lithium iron manganese phosphate material used in Comparative Example 4 is 8μmol / g, and the surface alkalinity value of the lithium iron manganese phosphate material used in Comparative Example 4 is 180μmol / g, which shows that when the surface alkalinity value of the lithium iron manganese phosphate material is 10μmol / g to 150μmol / g, the lithium iron manganese phosphate material has better processing performance, and the dispersion effect between the lithium iron manganese phosphate material and the binder and solvent is better, which helps to reduce the amount of solvent added, thereby increasing the solid content of the positive electrode slurry.

[0145] The electrical performance of the lithium-ion batteries prepared in Examples 7-12 and Comparative Examples 4-6 was tested. The testing method included placing the lithium-ion batteries in a constant temperature box at 45°C, charging them at a constant current and constant voltage of 1C to 4.2V, cutting off at 0.02C, and then discharging them at 1C for a cycle. The cycle capacity and average voltage were recorded. The number of cycles at which the capacity retention rate was 80% was obtained, and the voltage drop was recorded. The test results are shown in the following table.

[0146] Cycle number Voltage drop Example 7 4000 cls 60 mV Example 8 3870 cls 80 mV Example 9 3800 cls 90 mV Example 10 3920 cls 85 mV Example 11 3680 cls 95 mV Example 12 3750 cls 85 mV Comparative example 4 2250 cls 180 mV Comparative example 5 2570 cls 150 mV Comparative example 6 3160 cls 120 mV

[0147] From the table above we can see that:

[0148] By comparing Examples 7-12 with Comparative Examples 4-5, it can be seen that when the capacity retention rate is 80%, the number of cycles of the lithium ion batteries prepared in Examples 7-12 is greater than the number of cycles of the lithium ion batteries prepared in Comparative Examples 4-5, and the voltage drop of the lithium ion batteries prepared in Examples 7-12 is less than the voltage drop of the lithium ion batteries prepared in Comparative Examples 4-5. This shows that the lithium ion batteries prepared in Examples 7-12 of the present application have better cycle performance. The difference between the batteries of Examples 7-12 and Comparative Examples 4-5 is that: The surface alkalinity value of the lithium manganese iron phosphate material used is in the range of 10 μmol / g to 150 μmol / g, while the surface alkalinity value of the lithium manganese iron phosphate material used in Comparative Example 4 is 8 μmol / g, and the surface alkalinity value of the lithium manganese iron phosphate material used in Comparative Example 4 is 180 μmol / g. This shows that when the surface alkalinity value of the lithium manganese iron phosphate material is 10 μmol / g to 150 μmol / g, the positive electrode sheet prepared using the lithium manganese iron phosphate material has better electrical properties, thereby being able to improve the cycle performance of the lithium ion battery containing the positive electrode sheet.

[0149] By comparing Examples 7-12 with Comparative Example 6, it can be seen that when the capacity retention rate is 80%, the number of cycles of the lithium ion batteries prepared in Examples 7-12 is greater than the number of cycles of the lithium ion batteries prepared in Comparative Example 6, and the voltage drop of the lithium ion batteries prepared in Examples 7-12 is less than the voltage drop of the lithium ion battery prepared in Comparative Example 6. This shows that the lithium ion batteries prepared in Examples 7-12 of the present application have better cycle performance. This is because the lithium iron manganese phosphate material used in the preparation of the lithium ion battery in Comparative Example 6 is an undoped lithium iron manganese phosphate material (LiMn 0.65 Fe 0.35 PO4), while the lithium iron manganese phosphate materials used in the preparation of lithium ion batteries in Examples 7-12 of the present application are all doped lithium iron manganese phosphate materials (LiMn 0.6 Fe 0.35 Be 0.05 PO4、LiMn 0.6 Fe 0.35 Mg 0.05 PO4、LiMn 0.6 Fe 0.38 Be 0.02 PO4、LiMn 0.6 Fe 0.35 Ba 0.05PO4), it can be understood that, in Examples 7-12 of the present application, by adding doping element A (Be, Mg, Ba) to the lithium manganese iron phosphate material, since the valence state of the doping element A is consistent with the valence state of divalent manganese ions and divalent iron ions, and the valence state of the doping element A is stable, it is beneficial to improve the structural stability of the lithium manganese iron phosphate material. When the lithium manganese iron phosphate material is used in a lithium-ion battery, it is beneficial to maintain the stability of the lithium ion entry and exit channels, thereby improving the cycle performance of the battery.

Claims

1. A lithium iron manganese phosphate material, wherein the structural formula of the lithium iron manganese phosphate material is LiFe x Mn y A (1-x-y) PO4, where 0<x<1, 0<y<1, 0≤1-xy≤0.2, the doping element A is selected from at least one of Be, Ca, Mg, Ba and Sr; the surface basicity value of the lithium manganese iron phosphate material is 10μmol / g~150μmol / g.

2. The lithium iron manganese phosphate material according to claim 1, wherein: The particle size D50 of the lithium manganese iron phosphate material is 0.1 μm to 0.6 μm; and / or The carbon content in the lithium manganese iron phosphate material is 1.0wt% to 2.5wt%; and / or The water content in the lithium manganese iron phosphate material is 50ppm~300ppm.

3. A method for preparing lithium manganese iron phosphate material, wherein: include: A first raw material including a manganese source, an iron source, a complexing agent, a reducing agent and an alkaline solution is mixed to obtain a first reaction system, and the pH value of the first reaction system is controlled to be 9.0-11.

0. After the components in the first reaction system react, a precursor Fe x Mn (1-x) (OH)2, where 0<x<1; The precursor Fe x Mn (1-x) (OH)2, a lithium source, a phosphorus source, a dopant, and a carbon source are mixed to obtain a second reaction system, the pH value of the second reaction system is controlled to be 8.0-9.5, the second reaction system is spray granulated to obtain a sintered precursor, and the sintered precursor is sintered to obtain a lithium manganese iron phosphate material, wherein the structural formula of the lithium manganese iron phosphate material is LiFe x Mn y A (1-x-y) PO4, wherein 0<x<1, 0<y<1, 0≤1-xy≤0.2, and the doping element A is selected from at least one of Be, Ca, Mg, Ba and Sr.

4. The method for preparing lithium iron manganese phosphate material according to claim 3, wherein: The manganese source comprises at least one of manganese sulfate, manganese phosphate, manganese oxalate, manganese trioxide, manganese tetraoxide, manganese acetate, manganese carbonate, manganese hydroxide, manganese chloride, manganese sulfate containing crystal water, methylcyclopentadienyl tricarbonyl manganese, manganese pyrophosphate, manganese trichloride, manganese metasilicate, manganese decacarbonyl, manganese titanate, manganese carbide, manganese nitrate, manganese monoxide, manganese heptoxide and manganese dioxide; and / or The iron source comprises at least one of ferrous sulfate, ferrous sulfate containing crystal water, ferrous oxide, ferrous chloride, ferrous iodide, ferrous bromide, ferrous sulfide, ferrous hydroxide, ferrous oxalate, ferrous lactate, ferrous gluconate, ferrous acetate, ferrous nitrate, ferrous tannate, elemental iron, ferrous oxide, ferrous tetroxide, ferric phosphate, ferric phosphate containing crystal water, ferric sulfate, ferric sulfate containing crystal water, ferric nitrate, ferric chloride, ferric tannate and ferric hydroxide; and / or The complexing agent includes a first metal complexing agent and a second metal complexing agent, the first metal complexing agent includes at least one of sodium citrate and sodium tartrate, the second metal complexing agent includes at least one of ethylenediamine and disodium ethylenediaminetetraacetate, the mass ratio of the first metal complexing agent to the second metal complexing agent is (80-99.9): (0.1-20); the concentration of the complexing agent in the first reaction system is 0.05 mol / L-1 mol / L; and / or The reducing agent comprises at least one of sodium thiosulfate and sodium hypophosphite, and the concentration of the reducing agent in the first reaction system is 0.01 mol / L to 0.5 mol / L; and / or The alkaline solution includes water and an alkaline substance, the alkaline substance includes at least one of sodium hydroxide and potassium hydroxide, and the concentration of the alkaline substance in the alkaline solution is 0.1 mol / L to 4 mol / L.

5. The method for preparing lithium iron manganese phosphate material according to claim 3, wherein: The lithium source includes at least one of lithium carbonate, lithium oxide, lithium hydroxide, lithium chloride, lithium nitrate, lithium nitrite, lithium formate, lithium acetate, lithium oxalate, lithium phosphate, dilithium hydrogen phosphate, lithium dihydrogen phosphate and lithium citrate. x Mn (1-x) The molar ratio between (OH)2 and the lithium source is 1:(1.0-1.1); and / or The phosphorus source includes at least one of phosphoric acid, ammonium dihydrogen phosphate, ammonium phosphate, lithium dihydrogen phosphate, manganese phosphate, sodium dihydrogen phosphate, magnesium phosphate, aluminum phosphate, diammonium hydrogen phosphate and phosphorus pentoxide. x Mn (1-x) The molar ratio between (OH)2 and the phosphorus source is 1:(1.0-1.2); and / or The dopant comprises at least one of oxalate containing doping element A, acetate containing doping element A, chloride containing doping element A, sulfate containing doping element A and nitrate containing doping element A, and the doping element A in the dopant is added in an amount of 500ppm to 4000ppm in the second reaction system; and / or The carbon source comprises at least one of glucose, sucrose, furan resin, urea-formaldehyde resin, pyrimidine resin, phenolic resin, epoxy resin, polyvinyl alcohol, polystyrene, polymethyl methacrylate, polytetrafluoroethylene, polyvinylidene fluoride, polyacrylonitrile, styrene-butadiene rubber, carboxymethyl cellulose, fructose, lactose, starch, citric acid, carbon black, graphene, carbon nanotubes, carbon fiber, and activated carbon. The precursor Fe x Mn (1-x) The molar ratio between (OH)2 and the carbon source is 1:(1.5~3.5).

6. The method for preparing lithium iron manganese phosphate material according to claim 3, wherein: The second raw material further includes a pH regulator, and the molar ratio of the pH regulator to the phosphorus source is (0.1-1):15; The pH adjuster includes at least one of hydrogen peroxide and ammonia water, the concentration of the hydrogen peroxide is 20wt%-40wt%, and the concentration of the ammonia water is 20wt%-30wt%.

7. The method for preparing lithium iron manganese phosphate material according to claim 3, wherein: mixing a first raw material including a manganese source, an iron source, a complexing agent, a reducing agent and an alkaline solution in a first protective atmosphere, wherein the first protective atmosphere includes at least one of nitrogen and an inert gas; and / or The sintering precursor is sintered in a second protective atmosphere, wherein the second protective atmosphere comprises a protective gas and an alkyl gas, wherein the volume percentage of the alkyl gas is 0.1 vol% to 3 vol%.

8. The method for preparing lithium iron manganese phosphate material according to claim 7, wherein: The alkyl gas comprises at least one of methane, propane and butane; and / or The sintering precursor is sintered at a temperature of 650° C. to 800° C. for a sintering time of 24 to 36 hours.

9. The method for preparing lithium iron manganese phosphate material according to claim 3, wherein: The precursor Fe x Mn (1-x) The particle size D50 of (OH)2 is 1 μm to 3 μm; and / or The particle size D50 of the primary particles of the lithium manganese iron phosphate material is 0.1 μm to 0.6 μm.

10. A positive electrode sheet, comprising a positive electrode current collector and a positive electrode active material layer arranged on the positive electrode current collector, wherein the positive electrode active material layer comprises the lithium iron manganese phosphate material according to any one of claims 1-2 or the lithium iron manganese phosphate material prepared according to the preparation method of the lithium iron manganese phosphate material according to any one of claims 3-9.

11. The positive electrode sheet according to claim 10, wherein: The positive electrode active material layer also includes a binder and a conductive agent. The mass ratio of the lithium manganese iron phosphate material, the binder and the conductive agent is (90~99): (1~5): (1~5). The binder includes at least one of polyvinylidene fluoride, polymethyl methacrylate and styrene-butadiene rubber. The conductive agent includes at least one of activated carbon, conductive carbon black, carbon nanotubes and graphene.

12. A method for preparing a positive electrode sheet, comprising: Providing a solid raw material and a solvent, and mixing the solid raw material and the solvent evenly to obtain a positive electrode slurry, wherein the solid raw material comprises a lithium iron manganese phosphate material, a binder and a conductive agent, and the lithium iron manganese phosphate material is the lithium iron manganese phosphate material according to any one of claims 1 to 2 or the lithium iron manganese phosphate material prepared according to the preparation method of the lithium iron manganese phosphate material according to any one of claims 3 to 9; A positive electrode current collector is provided, the positive electrode slurry is applied on the positive electrode current collector, the positive electrode slurry is dried to form a positive electrode active material layer, and a positive electrode sheet is obtained.

13. The method for preparing a positive electrode sheet according to claim 12, wherein: In the solid raw material, the mass ratio of lithium manganese iron phosphate material, binder and conductive agent is (90-99): (1-5): (1-5); the mass percentage of the solid raw material in the positive electrode slurry is 60wt%-70wt%; After the positive electrode slurry is dried, the positive electrode slurry is rolled; the surface density of the positive electrode active material layer is 160g / m 2 ~250g / m 2 The compaction density of the positive electrode active material layer is 2.1 g / m 3 ~2.6g / m 3 .

14. A battery comprising a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, wherein the positive electrode sheet is the positive electrode sheet according to any one of claims 10 to 11 or is a positive electrode sheet prepared by the method for preparing a positive electrode sheet according to any one of claims 12 to 13.

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