Modified lithium iron manganese phosphate positive electrode material, preparation method thereof, positive plate and lithium ion battery

Abstract

The invention provides a modified lithium iron manganese phosphate positive electrode material and a preparation method thereof, a positive plate and a lithium ion battery. The modified lithium manganese iron phosphate positive electrode material comprises transition metal element-doped lithium manganese iron phosphate and a silicon hydroxyl-containing carbon coating layer, wherein the surface of the transition metal element-doped lithium manganese iron phosphate is coated with the silicon hydroxyl-containing carbon coating layer. And the transition metal element replaces the position of an iron element in the lithium manganese iron phosphate, so that the ion diffusion coefficient of the lithium manganese iron phosphate and the compaction density of the modified lithium manganese iron phosphate positive electrode material can be improved. The existence of the silicon hydroxyl-containing carbon coating layer is beneficial for improving the electronic conductivity and the structural stability of the lithium manganese iron phosphate positive electrode material, so that the manganese dissolution rate of the lithium manganese iron phosphate positive electrode material in the high-temperature cycle process of the battery is reduced. Therefore, the modified lithium manganese iron phosphate positive electrode material disclosed by the invention has relatively high ion diffusion coefficient, electronic conductivity, compaction density, rate capability, energy density and cycling stability.

Description

Technical Field

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

[0002] The market demand for high energy density and high power density lithium-ion batteries is increasing. In the era of continuous updates and iterations of lithium-ion battery materials, compared with lithium iron phosphate, lithium manganese iron phosphate has gradually attracted attention due to its high voltage, high energy density and better low-temperature performance. x Mn 1-x The conductivity of PO4 is higher than that of LiMnPO4, but the improvement is limited, and it is difficult to make LiFe x Mn 1-x The electrochemical properties of PO4 materials are fully utilized. Simple carbon surface coating can only improve the electronic conductivity between lithium manganese iron phosphate particles, and it has little effect on the improvement of lithium manganese iron phosphate bulk conductivity. At the same time, its improvement on the lithium ion diffusion coefficient is also quite limited. Therefore, lithium manganese iron phosphate has problems such as low ion conductivity and electronic conductivity, low compaction density, poor cycle performance, and easy dissolution of manganese in high-temperature cycles. Summary of the invention

[0003] The main purpose of the present invention is to provide a modified lithium iron manganese phosphate positive electrode material and a preparation method thereof, a positive electrode sheet and a lithium ion battery, so as to solve the problems of lithium iron manganese phosphate in the prior art, such as low ion diffusion coefficient, low compaction density and poor cycle performance.

[0004] In order to achieve the above-mentioned purpose, according to one aspect of the present invention, a modified lithium iron manganese phosphate positive electrode material is provided, wherein the modified lithium iron manganese phosphate positive electrode material comprises lithium iron manganese phosphate doped with transition metal elements and a carbon coating layer containing silicon hydroxyl groups, wherein the carbon coating layer containing silicon hydroxyl groups is coated on the surface of the lithium iron manganese phosphate doped with transition metal elements.

[0005] The replacement of the iron element in lithium iron manganese phosphate by transition metal elements helps to improve the ion diffusion coefficient of lithium iron manganese phosphate and the compaction density of the modified lithium iron manganese phosphate positive electrode material. The presence of the carbon coating layer containing silanol helps to improve the electronic conductivity and structural stability of the lithium iron manganese phosphate positive electrode material, thereby helping to reduce the rate of manganese dissolution of the lithium iron manganese phosphate positive electrode material during the high-temperature cycle of the battery. Therefore, the modified lithium iron manganese phosphate positive electrode material of the present application has a higher ion diffusion coefficient, electronic conductivity, compaction density, rate performance, energy density and cycle stability.

[0006] Furthermore, the mass content of carbon element in the above-mentioned modified lithium iron manganese phosphate positive electrode material is 1.0-3.0wt%; and / or, the molar ratio of the doping element to the phosphorus element in the transition metal element-doped lithium iron manganese phosphate is 0.12-0.25:2; and / or, the mass content of silicon element in the modified lithium iron manganese phosphate positive electrode material is 0.01-1.0wt%; and / or, the transition metal element is selected from any one or more of tin, titanium and vanadium.

[0007] The modified lithium iron manganese phosphate positive electrode material having the above carbon content helps to improve the electronic conductivity and cycle stability of the modified lithium iron manganese phosphate positive electrode material. The molar ratio of the doping element to the phosphorus element within the above range helps to further improve the ion diffusion coefficient and compaction density of the modified lithium iron manganese phosphate positive electrode material. The modified lithium iron manganese phosphate positive electrode material having the above silicon content helps to further reduce the rate of manganese dissolution of the modified lithium iron manganese phosphate positive electrode material during high temperature cycling.

[0008] Furthermore, the compaction density of the modified lithium manganese iron phosphate positive electrode material is 2.58-2.67 g / cm 2 ; and / or, the ion diffusion coefficient of the modified lithium manganese iron phosphate positive electrode material is 7.0×10 -12 ~7.6×10 -12 cm 2 / s; and / or, the average particle size of the modified lithium manganese iron phosphate positive electrode material is 500-1500nm; and / or, the shape of the modified lithium manganese iron phosphate positive electrode material is spherical.

[0009] The modified lithium manganese iron phosphate positive electrode material having the above compaction density, ion diffusion coefficient, size and shape has high rate performance and cycle stability.

[0010] According to another aspect of the present invention, there is provided a method for preparing the aforementioned modified lithium iron manganese phosphate positive electrode material, the preparation method comprising the following steps: step S1, sequentially subjecting a raw material comprising a first phosphorus source, a first iron source, a first manganese source, a first transition metal source and water to a first mixing treatment, a first pH adjustment and a first hydrothermal reaction to obtain a precursor A; step S2, sequentially subjecting a raw material comprising a second phosphorus source, a second iron source, a second manganese source, a second transition metal source and water to a second mixing treatment, a second pH adjustment and a second hydrothermal reaction to obtain a precursor B; step S3, sequentially subjecting a raw material comprising precursor A, a precursor B, a lithium source and a siloxane compound to a third mixing treatment and a sintering treatment to obtain a modified lithium iron manganese phosphate positive electrode material; wherein the molar ratio of the transition metal element in the first transition metal source to the transition metal element in the second transition metal source is 1:0.1 to 0.5, and the sintering treatment is carried out in an inert atmosphere.

[0011] In step S1 and step S2, by in-situ doping of transition metal elements in the precursor, on the one hand, it helps to solve the problem of difficulty in doping transition metal elements in lithium iron manganese phosphate, and on the other hand, it helps to improve the ion diffusion coefficient of lithium iron manganese phosphate and the compaction density of the modified lithium iron manganese phosphate positive electrode material. The ratio of the amount of transition metal elements in the first transition metal source to the amount of transition metal elements in the second transition metal source is within the above range, which helps to reduce the particle size of lithium iron manganese phosphate, and then helps to improve the compaction density of the modified lithium iron manganese phosphate positive electrode material. The carbon network structure coating layer containing silicon hydroxyl groups formed in step S3 helps to reduce the rate of manganese dissolution of the lithium iron manganese phosphate positive electrode material during the high-temperature cycle of the battery. Therefore, the modified lithium iron manganese phosphate positive electrode material of the present application has a higher ion diffusion coefficient, electronic conductivity, compaction density and cycle stability.

[0012] Furthermore, the molar ratio of the phosphorus element in the above-mentioned first phosphorus source, the iron element in the first iron source, the manganese element in the first manganese source, and the transition metal element in the first transition metal source is 1: 0.2-0.5: 0.3-0.7: 0.1-0.2; and / or, the molar ratio of the phosphorus element in the second phosphorus source, the iron element in the second iron source, the manganese element in the second manganese source, and the transition metal element in the second transition metal source is 1: 0.2-0.5: 0.45-0.78: 0.02-0.05; and / or, the mass of the siloxane compound accounts for 0.3-2.5wt% of the total mass of the first phosphorus source, the first iron source, the first manganese source, the second phosphorus source, the second iron source, the second manganese source and the lithium source; and / or, the ratio of the total molar ratio of the phosphorus element in the first phosphorus source and the second phosphorus source to the molar ratio of the lithium element in the lithium source is 1: 1-3.

[0013] The ratio of the amount of phosphorus in the first phosphorus source, the iron in the first iron source, the manganese in the first manganese source, and the transition metal in the first transition metal source to the amount of phosphorus in the second phosphorus source, the iron in the second iron source, the manganese in the second manganese source, and the transition metal in the second transition metal source is within the above range, which helps to further improve the compaction density of the modified lithium manganese iron phosphate positive electrode material. The mass of the siloxane compound accounts for the total mass of the first phosphorus source, the first iron source, the first manganese source, the second phosphorus source, the second iron source, the second manganese source and the lithium source within the above range, which helps to further reduce the rate of manganese dissolution of the modified lithium manganese iron phosphate positive electrode material during high-temperature cycling. The ratio of the total amount of phosphorus in the first phosphorus source and the second phosphorus source in the lithium source to the amount of lithium is within the above range, which helps to promote the formation of lithium manganese iron phosphate.

[0014] Further, an acidic solution is used to perform the above-mentioned first pH adjustment and second pH adjustment; and / or, the pH values ​​of the solution obtained after the first pH adjustment and the solution obtained after the second pH adjustment are each independently 6.5 to 7.5; and / or, the acidic solution is selected from any one or more of hydrochloric acid, sulfuric acid and phytic acid; and / or, the first phosphorus source and the second phosphorus source are each independently selected from any one or more of phytic acid, NH4H2PO4 and phosphoric acid; and / or, the first iron source and the second iron source are each independently selected from any one or more of FeSO4·7H2O, Fe3(PO4)2 and Fe2O3; and / or, the first manganese source and the second manganese source are each independently selected from any one or more of FeSO4·7H2O, Fe3(PO4)2 and Fe2O3; The sources are each independently selected from any one or more of MnSO4, Mn2O3 and manganese acetate; and / or the first transition metal source and the second transition metal source are each independently selected from any one or more of SnCl4·5H2O, SnO2, Sn(OH)4, TiCl4, TiO2, Ti(SO4)2, V2O5, VOCl3 and ammonium metavanadate; and / or the lithium source is selected from any one or more of LiOH·H2O, Li2CO3 and CH3COOLi; and / or the siloxane compound is selected from any one or more of aminopropylsiloxane, aminovinylsiloxane and aminoethylsiloxane.

[0015] The pH values ​​of the solution obtained after the first pH adjustment and the solution obtained after the second pH adjustment are within the above range, which is helpful to obtain lithium iron manganese phosphate with regular morphology, rounded shape, uniform particle size and excellent electrical properties. The types of the first transition metal source and the second transition metal source are within the above range, which is helpful to further improve the ion diffusion coefficient of the modified lithium iron manganese phosphate positive electrode material. The types of siloxane compounds are within the above range, which is helpful to further improve the electronic conductivity and structural stability of the lithium iron manganese phosphate positive electrode material, and at the same time reduce the rate of manganese dissolution of the lithium iron manganese phosphate positive electrode material during the high-temperature cycle of the battery.

[0016] Furthermore, the temperature of the first hydrothermal reaction is 80-120°C; and / or the time of the first hydrothermal reaction is 2-5h; and / or the temperature of the second hydrothermal reaction is 150-190°C; and / or the time of the second hydrothermal reaction is 4-8h.

[0017] Controlling the temperature and time of the first hydrothermal reaction within the above range is conducive to the formation of small-particle pure phase precursor A. Too high a temperature and too long a time of the second hydrothermal reaction are not conducive to the normal growth of crystals; controlling the temperature and time of the second hydrothermal reaction within the above range is conducive to the generation of a pure phase precursor B of the target particle size.

[0018] Furthermore, the heating rate of the sintering treatment is 3-5°C / min; and / or the temperature of the sintering treatment is 700-780°C; and / or the time of the sintering treatment is 6-12h.

[0019] Controlling the temperature, time and heating rate of the sintering process within the above range is conducive to the production of a modified lithium manganese iron phosphate positive electrode material with a moderate particle size, good crystallinity and high electrochemical activity.

[0020] According to another aspect of the present invention, a positive electrode sheet is provided, which contains the aforementioned modified lithium iron manganese phosphate positive electrode material or the modified lithium iron manganese phosphate positive electrode material prepared by the aforementioned preparation method.

[0021] The positive electrode sheet has high ion diffusion coefficient, electronic conductivity, compaction density and cycle stability.

[0022] According to another aspect of the present invention, a lithium-ion battery is provided, comprising a positive electrode sheet, an electrolyte and a negative electrode sheet, wherein the positive electrode sheet is the positive electrode sheet mentioned above.

[0023] The above lithium-ion battery has high rate performance and cycle stability.

[0024] By applying the technical solution of the present invention, the present application disposes the transition metal element in lithium manganese iron phosphate, and the transition metal element replaces the position of the iron element in lithium manganese iron phosphate, so that the lattice of lithium manganese iron phosphate is distorted, thereby helping to improve the ion diffusion coefficient of lithium manganese iron phosphate. And because the doping of transition metal elements causes non-uniform nucleation of lithium manganese iron phosphate, it helps to increase the nucleation rate of lithium manganese iron phosphate and reduce the growth rate of crystal nuclei, thereby helping to reduce the particle size of lithium manganese iron phosphate, and then help to improve the compaction density of the modified lithium manganese iron phosphate positive electrode material. The presence of the carbon coating layer containing silicon hydroxyl groups helps to improve the electronic conductivity and structural stability of the lithium manganese iron phosphate positive electrode material, especially the silicon of silicon hydroxyl groups in the modified lithium manganese iron phosphate positive electrode material has a high affinity for fluoride ions, can preferentially combine with fluoride ions, and prevent fluoride ions from reacting with manganese ions, thereby helping to reduce the rate of manganese dissolution of the lithium manganese iron phosphate positive electrode material during the high-temperature cycle of the battery. Therefore, the modified lithium manganese iron phosphate positive electrode material of the present application has higher ion diffusion coefficient, electronic conductivity, compaction density, rate performance, energy density and cycle stability. DETAILED DESCRIPTION

[0025] It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present application can be combined with each other. The present invention will be described in detail below in conjunction with the embodiments.

[0026] As analyzed in the background technology of this application, lithium manganese iron phosphate in the prior art has problems such as low ion diffusion coefficient, low compaction density and poor cycle performance. In order to solve the above problems, this application provides a modified lithium manganese iron phosphate positive electrode material and a preparation method thereof, a positive electrode sheet and a lithium ion battery.

[0027] In a typical embodiment of the present application, a modified lithium iron manganese phosphate positive electrode material is provided, which includes lithium iron manganese phosphate doped with transition metal elements and a carbon coating layer containing silicon hydroxyl groups, and the carbon coating layer containing silicon hydroxyl groups is coated on the surface of the lithium iron manganese phosphate doped with transition metal elements.

[0028] The present application, by doping transition metal elements in lithium manganese iron phosphate, replaces the position of iron elements in lithium manganese iron phosphate with transition metal elements, so that the lattice of lithium manganese iron phosphate is distorted, thereby helping to improve the ion diffusion coefficient of lithium manganese iron phosphate. And because the doping of transition metal elements causes non-uniform nucleation of lithium manganese iron phosphate, it helps to increase the nucleation rate of lithium manganese iron phosphate and reduce the growth rate of crystal nuclei, thereby helping to reduce the particle size of lithium manganese iron phosphate, and then help to improve the compaction density of the modified lithium manganese iron phosphate positive electrode material. The presence of a carbon coating layer containing silicon hydroxyls helps to improve the electronic conductivity and structural stability of the lithium manganese iron phosphate positive electrode material, especially the silicon of silicon hydroxyls in the modified lithium manganese iron phosphate positive electrode material has a high affinity for fluoride ions, can preferentially combine with fluoride ions, prevent fluoride ions from reacting with manganese ions, thereby helping to reduce the rate of manganese dissolution of the lithium manganese iron phosphate positive electrode material during the high temperature cycle of the battery. Therefore, the modified lithium manganese iron phosphate positive electrode material of the present application has a high ion diffusion coefficient, electronic conductivity, compaction density, rate performance, energy density and cycle stability.

[0029] In one embodiment of the present application, the mass content of carbon element in the above-mentioned modified lithium iron manganese phosphate positive electrode material is 1.0-3.0wt%; and / or, the molar ratio of the doping element to the phosphorus element in the transition metal element-doped lithium iron manganese phosphate is 0.12-0.25:2; and / or, the mass content of silicon element in the modified lithium iron manganese phosphate positive electrode material is 0.01-1.0wt%; and / or, the transition metal element is selected from any one or more of tin, titanium and vanadium.

[0030] It is preferred to control the mass content of carbon in the modified lithium iron manganese phosphate positive electrode material within the above range, which helps to make the modified lithium iron manganese phosphate positive electrode material have a higher rate performance while further improving the electronic conductivity and cycle stability of the modified lithium iron manganese phosphate positive electrode material. It is preferred to control the molar ratio of the doping element to the phosphorus element in the transition metal element-doped lithium iron manganese phosphate within the above range, which helps to further improve the ion diffusion coefficient and compaction density of the modified lithium iron manganese phosphate positive electrode material. It is preferred to control the mass content of silicon in the modified lithium iron manganese phosphate positive electrode material within the above range, which helps to further reduce the rate of manganese dissolution of the modified lithium iron manganese phosphate positive electrode material during high-temperature cycling. It is preferred to control the type of transition metal elements within the above range, which helps to further improve the ion diffusion coefficient and compaction density of the modified lithium iron manganese phosphate positive electrode material.

[0031] In one embodiment of the present application, the compaction density of the modified lithium manganese iron phosphate positive electrode material is 2.58-2.67 g / cm 2 ; and / or, the ion diffusion coefficient of the modified lithium manganese iron phosphate positive electrode material is 7×10 -12 ~7.6×10 -12 cm 2 / s; and / or, the average particle size of the modified lithium manganese iron phosphate positive electrode material is 500-1500nm; and / or, the shape of the modified lithium manganese iron phosphate positive electrode material is spherical.

[0032] Since the modified lithium manganese iron phosphate positive electrode material has the above-mentioned compaction density, ion diffusion coefficient, size and shape, the modified lithium manganese iron phosphate positive electrode material has higher rate performance and cycle stability.

[0033] In another typical embodiment of the present application, a method for preparing the aforementioned modified lithium iron manganese phosphate positive electrode material is provided, and the preparation method comprises the following steps: step S1, sequentially performing a first mixing treatment, a first pH adjustment and a first hydrothermal reaction on a raw material including a first phosphorus source, a first iron source, a first manganese source, a first transition metal source and water to obtain a precursor A; step S2, sequentially performing a second mixing treatment, a second pH adjustment and a second hydrothermal reaction on a raw material including a second phosphorus source, a second iron source, a second manganese source, a second transition metal source and water to obtain a precursor B; step S3, sequentially performing a third mixing treatment and a sintering treatment on a raw material including precursor A, a precursor B, a lithium source and a siloxane compound to obtain a modified lithium iron manganese phosphate positive electrode material; wherein the molar ratio of the transition metal element in the first transition metal source to the transition metal element in the second transition metal source is 1:0.1 to 0.5, and the sintering treatment is performed in an inert atmosphere.

[0034] In step S1 and step S2, by in-situ doping of transition metal elements in the precursor, the transition metal elements in the precursor directly replace the position of the Fe element, which helps to solve the problem of difficulty in doping transition metal elements in lithium iron manganese phosphate, and the doped transition metal elements induce the lattice distortion of lithium iron manganese phosphate, thereby helping to improve the ion diffusion coefficient of the modified lithium iron manganese phosphate positive electrode material. By controlling the amount of transition metal doping elements, different precursors are obtained to adjust the particle size distribution of lithium iron manganese phosphate particles, and preferably the ratio of the amount of transition metal elements in the first transition metal source to the amount of transition metal elements in the second transition metal source is controlled within the above range. When the transition metal doping elements are added, the system undergoes non-uniform nucleation, which helps to increase the nucleation rate of lithium iron manganese phosphate and reduce the growth rate of the crystal nucleus, thereby helping to reduce the particle size of lithium iron manganese phosphate, and then helping to improve the compaction density of the modified lithium iron manganese phosphate positive electrode material. In step S3, during the high-temperature sintering process of the siloxane compound, the siloxane in the organic matter is cracked to form silanol groups, and the siloxane compound is dehydrated, condensed, and carbonized in turn to form a cross-linked carbonized structure, and finally a carbon network structure coating containing silanol groups is formed. The silicon in the surface silanol groups has a high affinity for fluoride ions, and can preferentially bind to fluoride ions to prevent fluoride ions from reacting with manganese ions, thereby helping to reduce the rate of manganese dissolution from the lithium iron manganese phosphate positive electrode material during the high-temperature cycle of the battery. Therefore, the modified lithium iron manganese phosphate positive electrode material of the present application has a high ion diffusion coefficient, electronic conductivity, compaction density and cycle stability.

[0035] In addition, in order to fully mix the materials, it is preferred that the first mixing treatment and the second mixing treatment are stirred and mixed, and the stirring and mixing time is 1 to 2 hours. After the first pH adjustment and the second pH adjustment are completed, stirring is continued for 30 to 60 minutes, and the stirring time is, the first hydrothermal reaction and the second hydrothermal reaction are carried out in a polytetrafluoroethylene-lined reactor. After the first hydrothermal reaction and the second hydrothermal reaction are completed, the reaction products are filtered, washed and dried in sequence to obtain a precursor, and the drying conditions are vacuum dried at 60 to 100 ° C for 10 to 20 hours. The third mixing treatment is ball milling, the speed of the ball mill is 10000 to 15000 rpm, and the time of the ball mill is 5 to 10 hours. In order to save costs, it is preferred that the gas in the inert atmosphere is selected from any one or more of nitrogen, argon and helium.

[0036] In one embodiment of the present application, the molar ratio of the phosphorus element in the above-mentioned first phosphorus source, the iron element in the first iron source, the manganese element in the first manganese source, and the transition metal element in the first transition metal source is 1: 0.2-0.5: 0.3-0.7: 0.1-0.2; and / or, the molar ratio of the phosphorus element in the second phosphorus source, the iron element in the second iron source, the manganese element in the second manganese source, and the transition metal element in the second transition metal source is 1: 0.2-0.5: 0.45-0.78: 0.02-0.05; and / or, the mass of the siloxane compound accounts for 0.3-2.5wt% of the total mass of the first phosphorus source, the first iron source, the first manganese source, the second phosphorus source, the second iron source, the second manganese source and the lithium source; and / or, the ratio of the total molar ratio of the phosphorus element in the first phosphorus source and the second phosphorus source to the molar ratio of the lithium element in the lithium source is 1: 1-3.

[0037] It is preferred to control the ratio of the amount of phosphorus in the first phosphorus source, the iron in the first iron source, the manganese in the first manganese source, the transition metal in the first transition metal source, and the ratio of the amount of phosphorus in the second phosphorus source, the iron in the second iron source, the manganese in the second manganese source, and the transition metal in the second transition metal source within the above range, which helps to further regulate the size of the modified lithium manganese iron phosphate positive electrode material, thereby helping to further improve the compaction density of the modified lithium manganese iron phosphate positive electrode material. It is preferred to control the mass of the siloxane compound to the total mass of the first phosphorus source, the first iron source, the first manganese source, the second phosphorus source, the second iron source, the second manganese source and the lithium source within the above range, which helps to control the mass content of silicon in the modified lithium manganese iron phosphate positive electrode material, thereby helping to further reduce the rate of manganese dissolution of the modified lithium manganese iron phosphate positive electrode material during high temperature cycling. It is preferred to control the ratio of the total amount of phosphorus in the first phosphorus source and the second phosphorus source in the lithium source to the amount of lithium in the above range, which helps to promote the formation of lithium manganese iron phosphate.

[0038] In one embodiment of the present application, an acidic solution is used to perform the above-mentioned first pH adjustment and the second pH adjustment; and / or, the pH values ​​of the solution obtained after the first pH adjustment and the solution obtained after the second pH adjustment are each independently 6.5 to 7.5; and / or, the acidic solution is selected from any one or more of hydrochloric acid, sulfuric acid and phytic acid; and / or, the first phosphorus source and the second phosphorus source are each independently selected from any one or more of phytic acid, NH4H2PO4 and phosphoric acid; and / or, the first iron source and the second iron source are each independently selected from any one or more of FeSO4·7H2O, Fe3(PO4)2 and Fe2O3; and / or, the first manganese source and the second manganese source is each independently selected from any one or more of MnSO4, Mn2O3 and manganese acetate; and / or, the first transition metal source and the second transition metal source are each independently selected from any one or more of SnCl4·5H2O, SnO2, Sn(OH)4, TiCl4, TiO2, Ti(SO4)2, V2O5, VOCl3 and ammonium metavanadate; and / or, the lithium source is selected from any one or more of LiOH·H2O, Li2CO3 and CH3COOLi; and / or, the siloxane compound is selected from any one or more of aminopropylsiloxane, aminovinylsiloxane and aminoethylsiloxane.

[0039] It is preferred to control the pH values ​​of the solution obtained after the first pH adjustment and the solution obtained after the second pH adjustment within the above range, which helps to obtain lithium iron manganese phosphate with regular morphology, uniform particle size and excellent electrical properties. It is preferred to control the types of the acidic solution, the first phosphorus source and the second phosphorus source, the first iron source and the second iron source, the first manganese source and the second manganese source and the lithium source within the above range, which helps to enrich the selectivity of the material. It is preferred to control the types of the first transition metal source and the second transition metal source within the above range, which helps to further improve the ion diffusion coefficient of the modified lithium iron manganese phosphate positive electrode material. It is preferred to control the types of siloxane compounds within the above range, and the siloxane in the organic matter is cracked during the high-temperature sintering process to form silanol groups, while the amino group and the hydroxyl group are dehydrated, condensed, and carbonized to form a cross-linked carbonized structure, and finally a carbon coating layer containing silanol groups on the surface is formed, which helps to further improve the electronic conductivity and structural stability of the lithium iron manganese phosphate positive electrode material, and at the same time reduce the rate of manganese dissolution of the lithium iron manganese phosphate positive electrode material during the high-temperature cycle of the battery.

[0040] In one embodiment of the present application, the temperature of the first hydrothermal reaction is 80-120°C; and / or, the time of the first hydrothermal reaction is 2-5 hours; and / or, the temperature of the second hydrothermal reaction is 150-190°C; and / or, the time of the second hydrothermal reaction is 4-8 hours.

[0041] If the temperature of the first hydrothermal reaction is too high and the time is too long, it is not conducive to reducing the particle size of the precursor A. If the temperature of the first hydrothermal reaction is too low and the time is too short, it is not conducive to improving the purity of the precursor A. It is preferred to control the temperature and time of the first hydrothermal reaction within the above range, which is conducive to the formation of small particles of pure phase precursor A. If the temperature of the second hydrothermal reaction is too high and the time is too long, it is not conducive to the normal growth of the crystal. If the temperature of the second hydrothermal reaction is too low and the time is too short, it is not conducive to the generation of the precursor B of the target particle size and the purity is not high. It is preferred to control the temperature and time of the second hydrothermal reaction within the above range, which is conducive to the generation of the pure phase precursor B of the target particle size.

[0042] In one embodiment of the present application, the heating rate of the sintering treatment is 3-5°C / min; and / or the temperature of the sintering treatment is 700-780°C; and / or the time of the sintering treatment is 6-12h.

[0043] Sintering treatment with too high temperature, too long time and too high heating rate will lead to excessive crystal growth and increase of particle size, thereby reducing the specific surface area and conductivity of the modified lithium iron manganese phosphate positive electrode material, which is not conducive to the diffusion of lithium ions and affects its charge and discharge performance. Sintering treatment with too low temperature, too short time and too low heating rate will lead to insufficient crystallinity and phase purity of the modified lithium iron manganese phosphate positive electrode material, affecting its electrochemical activity. It is preferred to control the temperature, time and heating rate of the sintering treatment within the above range, which is conducive to the generation of a modified lithium iron manganese phosphate positive electrode material with moderate particle size, good crystallinity and high electrochemical activity.

[0044] In another typical embodiment of the present application, a positive electrode sheet is provided, which contains the aforementioned modified lithium iron manganese phosphate positive electrode material or the modified lithium iron manganese phosphate positive electrode material prepared by the aforementioned preparation method.

[0045] Since the above-mentioned positive electrode sheet contains the modified lithium manganese iron phosphate positive electrode material of the present application, the positive electrode sheet has a higher ion diffusion coefficient, electronic conductivity, compaction density and cycle stability.

[0046] In another typical embodiment of the present application, a lithium-ion battery is provided, including a positive electrode sheet, an electrolyte and a negative electrode sheet, wherein the positive electrode sheet is the positive electrode sheet mentioned above.

[0047] Since the positive electrode sheet of the above-mentioned lithium ion battery contains the modified lithium iron manganese phosphate positive electrode material of the present application, the lithium ion battery has higher rate performance and cycle stability.

[0048] The beneficial effects of the present application will be further illustrated below in conjunction with embodiments.

[0049] Example 1

[0050] Step S1, according to the molar ratio of phosphorus, iron, manganese and tin being 1:0.2:0.7:0.1, phytic acid, FeSO4·7H2O, MnSO4, SnCl4·5H2O and deionized water are mixed and stirred for 1 hour to obtain a mixed solution, wherein the amount of deionized water added accounts for 70wt% of the mixed solution; the pH value of the mixed solution is adjusted to 6.5 with hydrochloric acid solution and stirred for 30 minutes, and then the above reaction system is transferred to a reactor for a first hydrothermal reaction, the hydrothermal reaction temperature is 120°C, the reaction time is 4 hours, and then naturally cooled to room temperature after completion. After filtering and washing, it is vacuum dried at 60°C for 10 hours to obtain a precursor A.

[0051] Step S2, according to the molar ratio of phosphorus, iron, manganese and tin being 1:0.2:0.78:0.02, phytic acid, FeSO4·7H2O, MnSO4, SnCl4·5H2O and deionized water are mixed and stirred for 2 hours to obtain a mixed solution, wherein the amount of deionized water added accounts for 70wt% of the mixed solution; the pH value of the mixed solution is adjusted to 6.5 with hydrochloric acid solution and stirred for 30 minutes, and then the above reaction system is transferred to a reactor for a second hydrothermal reaction, the hydrothermal reaction temperature is 150°C, the reaction time is 4 hours, and then naturally cooled to room temperature after completion. After filtering and washing, it is vacuum dried at 60°C for 10 hours to obtain precursor B.

[0052] Step S3, ball-milling and mixing precursor A, precursor B, LiOH·H2O and aminopropylsiloxane, wherein the molar ratio of tin element in precursor A to tin element in precursor B is 1:0.2, the molar ratio of lithium element to the total phosphorus element in precursor A and precursor B is 3:1, the added amount of aminopropylsiloxane is 0.3wt% of the total mass of phosphorus source, iron source, manganese source and lithium source, the ball milling rotation speed is 10000rpm, and the ball milling time is 5h to obtain a mixed precursor. The mixed precursor was sintered in a tubular furnace with N2 as a protective atmosphere, the temperature was increased to 730°C at a heating rate of 3°C / min, and sintered at 730°C for 8 hours to obtain a modified lithium iron manganese phosphate positive electrode material, wherein the mass content of carbon element in the modified lithium iron manganese phosphate positive electrode material is 1.0wt%, the molar ratio of tin element to phosphorus element is 0.12:2, and the mass content of silicon element in the modified lithium iron manganese phosphate positive electrode material is 0.01wt%.

[0053] Example 2

[0054] The difference from Example 1 is that, in step S1, phosphoric acid, FeSO4·7H2O, MnSO4, SnCl4·5H2O and deionized water are mixed and stirred for 1 hour according to the molar ratio of phosphorus, iron, manganese and tin being 1:0.3:0.6:0.1 to obtain a mixed solution, in which the amount of deionized water added accounts for 70wt% of the mixed solution; the pH value of the mixed solution is adjusted to 7 with hydrochloric acid solution and stirred for 40 minutes, and then the above reaction system is transferred to a reactor for a first hydrothermal reaction, the hydrothermal reaction temperature is 100°C, the reaction time is 4 hours, and then it is naturally cooled to room temperature after completion. After filtering and washing, it is vacuum dried at 70°C for 10 hours to obtain precursor A.

[0055] Step S2, according to the molar ratio of phosphorus, iron, manganese and tin being 1:0.3:0.67:0.03, phosphoric acid, FeSO4·7H2O, MnSO4, SnCl4·5H2O and deionized water are mixed and stirred for 1 hour to obtain a mixed solution, wherein the amount of deionized water added accounts for 70wt% of the mixed solution; the pH value of the mixed solution is adjusted to 7 with a hydrochloric acid solution and stirred for 40 minutes, and then the above reaction system is transferred to a reactor for a second hydrothermal reaction, the hydrothermal reaction temperature is 160°C, the reaction time is 4 hours, and then naturally cooled to room temperature after the end. After filtering and washing, it is vacuum dried at 70°C for 10 hours to obtain a precursor B.

[0056] Step S3, ball-milling and mixing precursor A, precursor B, LiOH·H2O and aminovinylsiloxane, wherein the molar ratio of tin element in precursor A to tin element in precursor B is 1:0.3, the molar ratio of lithium element to the total phosphorus element in precursor A and precursor B is 3:1, the amount of aminovinylsiloxane added is 0.6wt% of the total mass of phosphorus source, iron source, manganese source and lithium source, the ball milling speed is 11000rpm, and the ball milling time is 5h to obtain a mixed precursor. The mixed precursor is sintered in a tubular furnace with N2 as a protective atmosphere, heated to 750°C at a heating rate of 3°C / min, and sintered at 750°C for 10h to obtain a modified lithium manganese iron phosphate positive electrode material.

[0057] Example 3

[0058] The difference from Example 1 is that, in step S1, phosphoric acid, Fe2O3, MnSO4, SnO2 and deionized water are mixed and stirred for 1 hour according to the molar ratio of phosphorus, iron, manganese and tin being 1:0.4:0.5:0.1 to obtain a mixed solution, in which the amount of deionized water added accounts for 70wt% of the mixed solution; the pH value of the mixed solution is adjusted to 7 with hydrochloric acid solution and stirred for 40 minutes, and then the above reaction system is transferred to a reactor for a first hydrothermal reaction, the hydrothermal reaction temperature is 120°C, the reaction time is 4 hours, and then naturally cooled to room temperature after the end. After filtering and washing, it is vacuum dried at 70°C for 12 hours to obtain precursor A.

[0059] Step S2, according to the molar ratio of phosphorus, iron, manganese and tin being 1:0.4:0.56:0.04, phosphoric acid, Fe2O3, MnSO4, SnO2 and deionized water are mixed and stirred for 1 hour to obtain a mixed solution, in which the amount of deionized water added accounts for 70wt% of the mixed solution; the pH value of the mixed solution is adjusted to 7 with hydrochloric acid solution and stirred for 40 minutes, and then the above reaction system is transferred to a reactor for a second hydrothermal reaction. The hydrothermal reaction temperature is 170°C and the reaction time is 4 hours. After the end, it is naturally cooled to room temperature. After filtering and washing, it is vacuum dried at 70°C for 12 hours to obtain precursor B.

[0060] Step S3, ball-milling and mixing precursor A, precursor B, Li2CO3 and aminoethylsiloxane, wherein the molar ratio of tin element in precursor A to tin element in precursor B is 1:0.4, the molar ratio of lithium element to the total phosphorus element in precursor A and precursor B is 3:1, the amount of aminoethylsiloxane added is 0.9wt% of the total mass of phosphorus source, iron source, manganese source and lithium source, the ball milling speed is 12000rpm, and the ball milling time is 6h to obtain a mixed precursor. The mixed precursor is sintered in a tubular furnace with N2 as a protective atmosphere, heated to 760°C at a heating rate of 4°C / min, and sintered at 760°C for 8h to obtain a modified lithium manganese iron phosphate positive electrode material.

[0061] Example 4

[0062] The difference from Example 1 is that, in step S1, NH4H2PO4, Fe2O3, Mn2O3, SnO2 and deionized water are mixed and stirred for 2 hours according to the molar ratio of phosphorus, iron, manganese and tin being 1:0.5:0.3:0.2 to obtain a mixed solution, in which the amount of deionized water added accounts for 70wt% of the mixed solution; the pH value of the mixed solution is adjusted to 7.5 with hydrochloric acid solution and stirred for 50 minutes, and then the above reaction system is transferred to a reactor for a first hydrothermal reaction, the hydrothermal reaction temperature is 120°C, the reaction time is 5 hours, and then it is naturally cooled to room temperature after completion. After filtering and washing, it is vacuum dried at 100°C for 15 hours to obtain precursor A.

[0063] Step S2, according to the molar ratio of phosphorus, iron, manganese and tin being 1:0.5:0.45:0.05, NH4H2PO4, Fe2O3, Mn2O3, SnO2 and deionized water are mixed and stirred for 2 hours to obtain a mixed solution, in which the amount of deionized water added accounts for 70wt% of the mixed solution; the pH value of the mixed solution is adjusted to 7.5 with hydrochloric acid solution and stirred for 50 minutes, and then the above reaction system is transferred to a reactor for a second hydrothermal reaction. The hydrothermal reaction temperature is 180°C and the reaction time is 5 hours. After the end, it is naturally cooled to room temperature. After filtering and washing, it is vacuum dried at 100°C for 15 hours to obtain precursor B.

[0064] Step S3, ball-milling and mixing precursor A, precursor B, Li2CO3 and aminoethylsiloxane, wherein the molar ratio of tin element in precursor A to tin element in precursor B is 1:0.25, the molar ratio of lithium element to the total phosphorus element in precursor A and precursor B is 3:1, the amount of aminoethylsiloxane added is 1.5wt% of the total mass of phosphorus source, iron source, manganese source and lithium source, the ball milling speed is 13000rpm, and the ball milling time is 8h to obtain a mixed precursor. The mixed precursor is sintered in a tubular furnace with N2 as a protective atmosphere, heated to 780°C at a heating rate of 5°C / min, and sintered at 780°C for 8h to obtain a modified lithium manganese iron phosphate positive electrode material.

[0065] Example 5

[0066] The difference from Example 1 is that, in step S1, NH4H2PO4, Fe3(PO4)2, manganese acetate, Sn(OH)4 and deionized water are mixed and stirred for 1 hour according to the molar ratio of phosphorus, iron, manganese and tin being 1:0.5:0.3:0.2 to obtain a mixed solution, in which the amount of deionized water added accounts for 70wt% of the mixed solution; the pH value of the mixed solution is adjusted to 7 with hydrochloric acid solution and stirred for 50 minutes, and then the above reaction system is transferred to a reactor for a first hydrothermal reaction, the hydrothermal reaction temperature is 100°C, the reaction time is 3 hours, and then it is naturally cooled to room temperature after completion. After filtering and washing, it is vacuum dried at 100°C for 15 hours to obtain precursor A.

[0067] Step S2, according to the molar ratio of phosphorus, iron, manganese and tin being 1:0.5:0.45:0.05, NH4H2PO4, Fe3(PO4)2, manganese acetate, Sn(OH)4 and deionized water are mixed and stirred for 1 hour to obtain a mixed solution, in which the amount of deionized water added accounts for 70wt% of the mixed solution; the pH value of the mixed solution is adjusted to 7 with hydrochloric acid solution and stirred for 50 minutes, and then the above reaction system is transferred to a reactor for a second hydrothermal reaction. The hydrothermal reaction temperature is 190°C and the reaction time is 4 hours. After the reaction is completed, it is naturally cooled to room temperature. After filtering and washing, it is vacuum dried at 100°C for 15 hours to obtain precursor B.

[0068] Step S3, ball-milling and mixing precursor A, precursor B, Li2CO3 and aminoethylsiloxane, wherein the molar ratio of tin element in precursor A to tin element in precursor B is 1:0.25, the molar ratio of lithium element to the total phosphorus element in precursor A and precursor B is 3:1, the amount of aminoethylsiloxane added is 1.8wt% of the total mass of phosphorus source, iron source, manganese source and lithium source, the ball milling speed is 14000rpm, and the ball milling time is 8h to obtain a mixed precursor. The mixed precursor is sintered in a tubular furnace with N2 as a protective atmosphere, heated to 780°C at a heating rate of 3°C / min, and sintered at 780°C for 8h to obtain a modified lithium manganese iron phosphate positive electrode material.

[0069] Example 6

[0070] The difference from Example 1 is that, in step S1, phytic acid, FeSO4·7H2O, manganese acetate, Sn(OH)4 and deionized water are mixed and stirred for 1 hour according to the molar ratio of phosphorus, iron, manganese and tin being 1:0.3:0.5:0.2 to obtain a mixed solution, wherein the amount of deionized water added accounts for 70wt% of the mixed solution; the pH value of the mixed solution is adjusted to 6.5 with hydrochloric acid solution and stirred for 60 minutes, and then the above reaction system is transferred to a reactor for a first hydrothermal reaction, the hydrothermal reaction temperature is 100°C, the reaction time is 4 hours, and then naturally cooled to room temperature after the end. After filtering and washing, it is vacuum dried at 90°C for 20 hours to obtain a precursor A.

[0071] Step S2, according to the molar ratio of phosphorus, iron, manganese and tin being 1:0.3:0.65:0.05, phytic acid, FeSO4·7H2O, manganese acetate, Sn(OH)4 and deionized water are mixed and stirred for 1 hour to obtain a mixed solution, wherein the amount of deionized water added accounts for 70wt% of the mixed solution; the pH value of the mixed solution is adjusted to 6.5 with a hydrochloric acid solution and stirred for 60 minutes, and then the above reaction system is transferred to a reactor for a second hydrothermal reaction, the hydrothermal reaction temperature is 170°C, the reaction time is 4 hours, and then naturally cooled to room temperature after the end. After filtering and washing, it is vacuum dried at 90°C for 20 hours to obtain a precursor B.

[0072] Step S3, ball-milling and mixing precursor A, precursor B, Li2CO3 and aminopropylsiloxane, wherein the molar ratio of tin element in precursor A to tin element in precursor B is 1:0.25, the molar ratio of lithium element to the total phosphorus element in precursor A and precursor B is 3:1, the amount of aminopropylsiloxane added is 2.1wt% of the total mass of phosphorus source, iron source, manganese source and lithium source, the ball milling speed is 15000rpm, and the ball milling time is 10h to obtain a mixed precursor. The mixed precursor is sintered in a tubular furnace with N2 as a protective atmosphere, heated to 750°C at a heating rate of 4°C / min, and sintered at 750°C for 10h to obtain a modified lithium manganese iron phosphate positive electrode material.

[0073] Example 7

[0074] The difference from Example 1 is that, in step S1, phosphoric acid, FeSO4·7H2O, MnSO4, SnCl4·5H2O and deionized water are mixed and stirred for 1 hour according to the molar ratio of phosphorus, iron, manganese and tin being 1:0.2:0.6:0.2 to obtain a mixed solution, in which the amount of deionized water added accounts for 70wt% of the mixed solution; the pH value of the mixed solution is adjusted to 7 with hydrochloric acid solution and stirred for 60 minutes, and then the above reaction system is transferred to a reactor for a first hydrothermal reaction, the hydrothermal reaction temperature is 110°C, the reaction time is 5 hours, and then naturally cooled to room temperature after the end. After filtering and washing, it is vacuum dried at 80°C for 20 hours to obtain precursor A.

[0075] Step S2, according to the molar ratio of phosphorus, iron, manganese and tin being 1:0.2:0.75:0.05, phosphoric acid, FeSO4·7H2O, MnSO4, SnCl4·5H2O and deionized water are mixed and stirred for 1 hour to obtain a mixed solution, wherein the amount of deionized water added accounts for 70wt% of the mixed solution; the pH value of the mixed solution is adjusted to 7 with a hydrochloric acid solution and stirred for 60 minutes, and then the above reaction system is transferred to a reactor for a second hydrothermal reaction, the hydrothermal reaction temperature is 150°C, the reaction time is 5 hours, and then naturally cooled to room temperature after completion. After filtering and washing, it is vacuum dried at 80°C for 20 hours to obtain a precursor B.

[0076] Step S3, ball-milling and mixing precursor A, precursor B, Li2CO3 and aminopropylsiloxane, wherein the molar ratio of tin element in precursor A to tin element in precursor B is 1:0.25, the molar ratio of lithium element to the total phosphorus element in precursor A and precursor B is 3:1, the amount of aminopropylsiloxane added is 2.5wt% of the total mass of phosphorus source, iron source, manganese source and lithium source, the ball milling speed is 12000rpm, and the ball milling time is 10h to obtain a mixed precursor. The mixed precursor is sintered in a tubular furnace with N2 as a protective atmosphere, heated to 760°C at a heating rate of 3°C / min, and sintered at 760°C for 12h to obtain a modified lithium manganese iron phosphate positive electrode material.

[0077] Example 8

[0078] The difference from Example 1 is that the amount of aminopropylsiloxane added is 2.5wt% of the total mass of the phosphorus source, iron source, manganese source and lithium source, and a modified lithium iron manganese phosphate positive electrode material is finally obtained, wherein the mass content of carbon element in the modified lithium iron manganese phosphate positive electrode material is 3.0wt%, and the mass content of silicon element in the modified lithium iron manganese phosphate positive electrode material is 1wt%.

[0079] Example 9

[0080] The difference from Example 1 is that the amount of aminopropylsiloxane added is 3wt% of the total mass of the phosphorus source, iron source, manganese source and lithium source, and a modified lithium iron manganese phosphate positive electrode material is finally obtained, wherein the mass content of carbon element in the modified lithium iron manganese phosphate positive electrode material is 3.3wt%, and the mass content of silicon element in the modified lithium iron manganese phosphate positive electrode material is 1.5wt%.

[0081] Example 10

[0082] The difference from Example 1 is that in step S1, the molar ratio of phosphorus, iron, manganese and tin is 1:0.5:0.73:0.2; in step S2, the molar ratio of phosphorus, iron, manganese and tin is 1:0.5:0.45:0.05, and finally a modified lithium manganese iron phosphate positive electrode material is obtained, in which the molar ratio of tin to phosphorus is 0.25:2.

[0083] Embodiment 11

[0084] The difference from Example 1 is that in step S1, the molar ratio of phosphorus, iron, manganese and tin is 1:0.1:0.8:0.05; in step S2, the molar ratio of phosphorus, iron, manganese and tin is 1:0.1:0.80:0.01, and finally a modified lithium manganese iron phosphate positive electrode material is obtained, in which the molar ratio of tin to phosphorus is 0.06:2.

[0085] Example 12

[0086] The difference from Example 1 is that titanium element is used to replace tin element, the titanium source is TiO2, and finally a modified lithium manganese iron phosphate positive electrode material is obtained.

[0087] Embodiment 13

[0088] The difference from Example 1 is that the temperature of the first hydrothermal reaction is 80° C., the time of the first hydrothermal reaction is 5 hours, the temperature of the second hydrothermal reaction is 150° C., the time of the second hydrothermal reaction is 8 hours, and finally a modified lithium manganese iron phosphate positive electrode material is obtained.

[0089] Embodiment 14

[0090] The difference from Example 1 is that the temperature of the first hydrothermal reaction is 120° C., the time of the first hydrothermal reaction is 2 h, the temperature of the second hydrothermal reaction is 190° C., the time of the second hydrothermal reaction is 4 h, and finally a modified lithium manganese iron phosphate positive electrode material is obtained.

[0091] Embodiment 15

[0092] The difference from Example 1 is that the temperature of the first hydrothermal reaction is 130° C., the time of the first hydrothermal reaction is 1 hour, the temperature of the second hydrothermal reaction is 200° C., the time of the second hydrothermal reaction is 3 hours, and finally a modified lithium manganese iron phosphate positive electrode material is obtained.

[0093] Example 16

[0094] The difference from Example 1 is that the heating rate of the sintering treatment is 3° C. / min, the sintering temperature is 700° C., and the sintering time is 12 h, and finally a modified lithium manganese iron phosphate positive electrode material is obtained.

[0095] Embodiment 17

[0096] The difference from Example 1 is that the heating rate of the sintering treatment is 5° C. / min, the sintering temperature is 780° C., and the sintering time is 6 h, and finally a modified lithium manganese iron phosphate positive electrode material is obtained.

[0097] Embodiment 18

[0098] The difference from Example 1 is that the heating rate of the sintering treatment is 6° C. / min, the temperature of the sintering treatment is 800° C., and the sintering treatment time is 5 h, and finally a modified lithium manganese iron phosphate positive electrode material is obtained.

[0099] Comparative Example 1

[0100] The difference from Example 1 is that no tin element is added in step S1 and step S2, and the molar ratio of phosphorus element, iron element and manganese element is 1:0.2:0.8, and finally a lithium manganese iron phosphate positive electrode material is obtained.

[0101] Comparative Example 2

[0102] The difference from Example 1 is that aminopropylsiloxane is not added in step S3, and lithium manganese iron phosphate positive electrode material is finally obtained.

[0103] Battery Assembly

[0104] The above-mentioned lithium manganese iron phosphate positive electrode material: conductive carbon black superP: PVDF: NMP = 1: 0.2: 0.2: 1.5 were mixed evenly in a slurry mixer to obtain a lithium manganese iron phosphate mixed slurry. The battery was prepared using artificial graphite as the negative electrode material, LiPF6 / EC+DEC (volume ratio 1: 1) as the electrolyte, and Celgard2400 membrane as the separator.

[0105] Performance Testing

[0106] 1C discharge specific capacity test conditions: 1C constant current discharge, voltage range is 2.0~4.5V.

[0107] Capacity retention rate test conditions: 55°C, voltage range 2-4.5V, cycle 200 cycles.

[0108] The lithium manganese iron phosphate positive electrode materials prepared in the above examples and comparative examples were subjected to a compaction density test, and the assembled batteries were tested for 200-cycle capacity retention, 1C discharge specific capacity and ion diffusion coefficient. The test results are shown in Table 1.

[0109] Table 1

[0110]

[0111]

[0112] As can be seen from Table 1 above, the lithium ion battery prepared using the modified nano-spherical lithium manganese iron phosphate composite positive electrode material and the positive electrode sheet of the lithium ion battery provided in the embodiment of the present invention has excellent compaction density, high ion diffusion coefficient and good cycle capacity retention rate, which are significantly higher than the relevant performance of the lithium ion batteries in Comparative Examples 1 and 2. Therefore, the positive electrode material provided in the above embodiment can significantly improve the rate performance of the battery and increase the energy density of the battery.

[0113] From the above description, it can be seen that the above embodiments of the present invention achieve the following technical effects:

[0114] The present application, by doping transition metal elements in lithium manganese iron phosphate, replaces the position of iron elements in lithium manganese iron phosphate with transition metal elements, so that the lattice of lithium manganese iron phosphate is distorted, thereby helping to improve the ion diffusion coefficient of lithium manganese iron phosphate. And because the doping of transition metal elements causes non-uniform nucleation of lithium manganese iron phosphate, it helps to increase the nucleation rate of lithium manganese iron phosphate and reduce the growth rate of crystal nuclei, thereby helping to reduce the particle size of lithium manganese iron phosphate, and then help to improve the compaction density of the modified lithium manganese iron phosphate positive electrode material. The presence of a carbon coating layer containing silicon hydroxyls helps to improve the electronic conductivity and structural stability of the lithium manganese iron phosphate positive electrode material, especially the silicon of silicon hydroxyls in the modified lithium manganese iron phosphate positive electrode material has a high affinity for fluoride ions, can preferentially combine with fluoride ions, prevent fluoride ions from reacting with manganese ions, thereby helping to reduce the rate of manganese dissolution of the lithium manganese iron phosphate positive electrode material during the high temperature cycle of the battery. Therefore, the modified lithium manganese iron phosphate positive electrode material of the present application has a high ion diffusion coefficient, electronic conductivity, compaction density, rate performance, energy density and cycle stability.

[0115] The above are only embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A modified lithium manganese iron phosphate positive electrode material, characterized in that: The modified lithium iron manganese phosphate positive electrode material comprises lithium iron manganese phosphate doped with transition metal elements and a carbon coating layer containing silanol groups, wherein the carbon coating layer containing silanol groups is coated on the surface of the lithium iron manganese phosphate doped with transition metal elements.

2. The modified lithium iron manganese phosphate positive electrode material according to claim 1, characterized in that: The mass content of carbon element in the modified lithium iron manganese phosphate positive electrode material is 1.0-3.0wt%; and / or, the molar ratio of the doping element to the phosphorus element in the transition metal element-doped lithium iron manganese phosphate is 0.12-0.25:2; and / or, the mass content of silicon element in the modified lithium iron manganese phosphate positive electrode material is 0.01-1.0wt%; and / or, the transition metal element is selected from any one or more of tin, titanium and vanadium.

3. The modified lithium iron manganese phosphate positive electrode material according to claim 1 or 2, characterized in that: The compaction density of the modified lithium manganese iron phosphate positive electrode material is 2.58-2.67 g / cm 2 ; and / or, the ion diffusion coefficient of the modified lithium manganese iron phosphate positive electrode material is 7.0×10 -12 ~7.6×10 -12 cm 2 / s; and / or, the average particle size of the modified lithium manganese iron phosphate positive electrode material is 500-1500nm; and / or, the shape of the modified lithium manganese iron phosphate positive electrode material is spherical.

4. A method for preparing the modified lithium manganese iron phosphate positive electrode material according to any one of claims 1 to 3, characterized in that: The preparation method comprises the following steps: Step S1, subjecting raw materials including a first phosphorus source, a first iron source, a first manganese source, a first transition metal source and water to a first mixing treatment, a first pH adjustment and a first hydrothermal reaction in sequence to obtain a precursor A; Step S2, subjecting the raw materials including a second phosphorus source, a second iron source, a second manganese source, a second transition metal source and water to a second mixing treatment, a second pH adjustment and a second hydrothermal reaction in sequence to obtain a precursor B; Step S3, subjecting the raw materials including the precursor A, the precursor B, the lithium source and the siloxane compound to a third mixing treatment and a sintering treatment in sequence to obtain the modified lithium manganese iron phosphate positive electrode material; The molar ratio of the transition metal element in the first transition metal source to the transition metal element in the second transition metal source is 1:0.1-0.5, and the sintering process is performed in an inert atmosphere.

5. The preparation method according to claim 4, characterized in that: The molar ratio of the phosphorus element in the first phosphorus source, the iron element in the first iron source, the manganese element in the first manganese source, and the transition metal element in the first transition metal source is 1:0.2-0.5:0.3-0.7:0.1-0.2; and / or, the molar ratio of phosphorus in the second phosphorus source, iron in the second iron source, manganese in the second manganese source, and transition metal in the second transition metal source is 1:0.2-0.5:0.45-0.78:0.02-0.05; And / or, the mass of the siloxane compound accounts for 0.3-2.5wt% of the total mass of the first phosphorus source, the first iron source, the first manganese source, the second phosphorus source, the second iron source, the second manganese source and the lithium source; And / or, the ratio of the total amount of phosphorus in the first phosphorus source and the second phosphorus source to the amount of lithium in the lithium source is 1:1-3.

6. The preparation method according to claim 4 or 5, characterized in that: Using an acidic solution to perform the first pH adjustment and the second pH adjustment; and / or, the pH values ​​of the solution obtained after the first pH adjustment and the solution obtained after the second pH adjustment are each independently 6.5 to 7.5; And / or, the acidic solution is selected from any one or more of hydrochloric acid, sulfuric acid and phytic acid; And / or, the first phosphorus source and the second phosphorus source are each independently selected from any one or more of phytic acid, NH4H2PO4 and phosphoric acid; and / or, the first iron source and the second iron source are each independently selected from any one or more of FeSO4·7H2O, Fe3(PO4)2 and Fe2O3; And / or, the first manganese source and the second manganese source are independently selected from any one or more of MnSO4, Mn2O3 and manganese acetate; and / or, the first transition metal source and the second transition metal source are each independently selected from any one or more of SnCl4·5H2O, SnO2, Sn(OH)4, TiCl4, TiO2, Ti(SO4)2, V2O5, VOCl3 and ammonium metavanadate; And / or, the lithium source is selected from any one or more of LiOH·H2O, Li2CO3 and CH3COOLi; And / or, the silicone compound is selected from any one or more of aminopropyl silicone, aminovinyl silicone and aminoethyl silicone.

7. The preparation method according to any one of claims 4 to 6, characterized in that: The temperature of the first hydrothermal reaction is 80 to 120° C.; and / or the time of the first hydrothermal reaction is 2 to 5 hours; And / or, the temperature of the second hydrothermal reaction is 150-190° C.; and / or, the time of the second hydrothermal reaction is 4-8 hours.

8. The preparation method according to any one of claims 4 to 7, characterized in that The heating rate of the sintering treatment is 3 to 5° C. / min; and / or, the temperature of the sintering treatment is 700 to 780° C.; and / or, the time of the sintering treatment is 6 to 12 hours.

9. A positive electrode sheet, characterized in that: The positive electrode sheet contains the modified lithium iron manganese phosphate positive electrode material according to any one of claims 1 to 3 or the modified lithium iron manganese phosphate positive electrode material prepared by the preparation method according to any one of claims 4 to 8.

10. A lithium ion battery comprising a positive electrode, an electrolyte and a negative electrode, characterized in that: The positive electrode sheet is the positive electrode sheet according to claim 9.

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