A dual-gradient lithium manganese iron phosphate material and its preparation method

Abstract

This invention discloses a dual-gradient lithium manganese iron phosphate material and its preparation method. A radially dual-gradient distribution of manganese iron pyrophosphate precursor is prepared by a stepwise continuous co-precipitation method, strictly achieving the structural design of high manganese in the inner layer and low manganese in the outer layer, and low doping of Mg-Al composite in the inner layer and high doping of Mg-Al composite in the outer layer. Subsequently, the precursor is uniformly mixed with lithium source and organic carbon source, and then calcined at high temperature. The resulting dual-gradient cathode material can not only rely on the high manganese composition of the inner layer to ensure the high-voltage energy density of the material, but also precisely suppress the surface Jan Taylor effect, disproportionation reaction and manganese dissolution through the synergistic effect of low manganese and high doping in the outer layer, thus alleviating the high-voltage side reactions above 4.2V, while taking into account both bulk lithium-ion transport and structural stability.

Description

Technical Field

[0001] This invention relates to the field of cathode materials for lithium-ion secondary batteries, and more particularly to a dual-gradient lithium manganese iron phosphate material and its preparation method. Background Technology

[0002] In the current era of rapid development in the new energy power battery and energy storage battery industry, the performance of lithium-ion battery cathode materials directly determines the overall energy density, cycle life, safety, and cost of the battery. Traditional lithium iron phosphate materials have advantages such as structural stability, long cycle life, excellent safety, and low cost, and have been widely used in new energy vehicles, energy storage power stations, and other fields. However, their discharge platform is only about 3.4V, their theoretical specific capacity is limited, and their energy density improvement has encountered a bottleneck, making it difficult to meet the high energy density requirements of high-end power batteries and long-term energy storage scenarios. Lithium manganese iron phosphate, as a core modified and upgraded material of lithium iron phosphate, retains the high stability and high safety of the original olivine structure of lithium iron phosphate by introducing manganese into the olivine lattice, while increasing the discharge voltage platform to about 4.1V. The energy density is 15%-20% higher than that of traditional lithium iron phosphate, making it a new type of cathode material that balances safety, cost, and energy density. It has great industrialization potential in passenger vehicle power batteries, residential energy storage, and industrial and commercial energy storage. Current research and industrialization of lithium manganese iron phosphate mainly focus on material synthesis, ion doping, surface coating, and morphology control. The mainstream technical routes include: using Mg... ²+ Al ³+ Zr 4+ Doping with metal ions improves the ionic conductivity of materials; coating with carbon materials or inorganic oxides enhances surface stability; and optimizing the sintering process controls grain size and improves rate performance. However, none of the aforementioned technologies can completely solve the intrinsic defects of lithium manganese iron phosphate materials, especially in high-voltage charging and discharging scenarios, where material performance deteriorates significantly, severely restricting its large-scale application in high-voltage, high-energy-density scenarios.

[0003] Research has revealed that the core technological flaws of lithium manganese iron phosphate stem from a series of intrinsic problems caused by the introduction of manganese: Firstly, when the material is charged to above 4.0V, Mn... 2+ Oxidation reaction occurs to produce Mn 3+ Firstly, it triggers a strong Jan Taylor effect, inducing axial tetragonal distortion of the MnO6 octahedron, weakening the Mn-O bond energy, and disrupting the integrity of the crystal structure. Secondly, Jan Taylor distortion accelerates the transformation of MnO6 octahedrons. ³+ Disproportionation reaction occurs (2Mn) 3+ →Mn 2+ +Mn 4+ ), generating Mn 2+It readily dissolves in the electrolyte, migrates to the negative electrode surface and deposits, repeatedly damaging the negative electrode SEI film, consuming the active lithium inside the battery, leading to irreversible capacity loss, and at the same time, residual Mn 4+ This can lead to lattice oxygen defects, further exacerbating structural collapse; third, when the charging voltage rises above 4.2V, the coupling effect of the Jan Taylor distortion and disproportionation reaction causes the battery impedance to increase, the average voltage to decrease, and the cycle stability to drop significantly; fourth, lithium manganese iron phosphate has low intrinsic electronic conductivity and lithium-ion diffusion coefficient, and under high voltage, lattice distortion further blocks the one-dimensional diffusion channel of lithium ions, resulting in significant degradation of rate performance and low-temperature performance.

[0004] Further analysis of the shortcomings of existing homogeneous materials and conventional modification methods reveals a natural spatial contradiction between the performance requirements and intrinsic defects of lithium manganese iron phosphate: the inner bulk phase of the material requires a higher proportion of manganese to maintain a high voltage plateau of around 4.0V, ensuring that the overall energy density meets the target. However, under high voltage, moderate lattice distortion in the inner layer can optimize lithium-ion diffusion kinetics, eliminating the need for excessively high doping concentrations to avoid disrupting lattice integrity. The material surface, which is in direct contact with the electrolyte, is the core site for the Jan Taylor effect, disproportionation reaction, and manganese dissolution, necessitating a reduction in manganese content to decrease Mn content. 3+ To reduce the generation of MnO6 and weaken the distortion driving force at its source, it is necessary to increase the concentration of doping elements and stabilize the surface MnO6 octahedral structure through high-density doping to block MnO6 formation. 3+ Disproportionation and electrolyte erosion channels. Existing conventional uniformly doped lithium iron phosphate materials with homogeneous manganese-iron ratios cannot simultaneously meet the requirements of high energy density in the inner layer and high stability in the surface layer. Uniform high doping leads to an increase in bulk lattice defects and obstruction of lithium-ion transport, while uniform low doping cannot effectively suppress surface defects. High-manganese homogenized materials are difficult to control manganese dissolution and high-voltage side reactions in the surface layer, while low-manganese homogenized materials cannot give full play to the core advantages of high voltage and high energy density.

[0005] Therefore, developing a lithium manganese iron phosphate cathode material with a gradient structure of high manganese in the inner layer and low manganese in the outer layer, and low doping in the inner layer and high doping in the outer layer, and with a controllable preparation process, to solve the industry pain points of poor high-voltage adaptability, rapid cycle degradation, and inability to balance energy density and stability of existing homogeneous materials, has become a technical problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0006] The technical solution of this invention to solve the above-mentioned technical problems and achieve the technical effects is as follows: A method for preparing a dual-gradient lithium manganese iron phosphate material includes the following steps: S1. Preparation of core-mixed metal solution: according to LiMn x Fe 1-x-y Al y / 2 Mg y / 2Weigh out the corresponding manganese, iron, aluminum and magnesium sources with stoichiometric ratios of PO4 and 0.7≤x≤0.9, 0.01≤y≤0.02, and add 0.1%~0.3% ascorbic acid of total metal salt mass. Add deionized water and stir to prepare a core mixed metal solution with a concentration of 1mol / L-3mol / L. S2. Preparation of the outer shell mixed metal solution: according to LiMn x Fe 1-x-y Al y / 2 Mg y / 2 Weigh out the corresponding manganese, iron, aluminum and magnesium sources with stoichiometric ratios of PO4 and 0.1≤x≤0.3, 0.03≤y≤0.05, and add 0.1%~0.3% ascorbic acid of the total metal salt mass. Add deionized water and stir to prepare a mixed metal solution with a concentration of 1mol / L-3mol / L. S3. Preparation of manganese ferropyrophosphate precursor: The core mixed metal solution, sodium pyrophosphate solution and ammonia water are titrated for 5h-8h under the conditions of stirring speed of 300 rpm-600 rpm, temperature of 45℃-55℃, nitrogen atmosphere protection and control of solution pH=6-7. After aging for 1h-2h, the outer shell mixed metal solution is titrated for 0.5h-1h under the same conditions, and then aged for another 1h-2h. Then, the mixture is filtered, washed and dried to obtain manganese ferropyrophosphate precursor. S4. Manganese iron pyrophosphate precursor, lithium source, organic carbon source and deionized water are prepared into a slurry with a solid content of 30wt%-60wt% at a ratio of Li / (Mn+Fe+Al+Mg)=1.02-1.05. The slurry is spray-dried at a temperature of 150℃-250℃, and then calcined at 650℃-750℃ in an inert atmosphere for 6h-16h. After demagnetization, a dual-gradient lithium manganese iron phosphate material is obtained.

[0007] This invention, following the steps described above, first prepares a radially dual-gradient manganese-iron pyrophosphate precursor using a stepwise continuous co-precipitation method. This strictly achieves a structural design where the inner layer of the particle is high-manganese, the outer layer is low-manganese, the inner layer is a Mg-Al composite with low doping, and the outer layer is a Mg-Al composite with high doping. Subsequently, the precursor is uniformly mixed with a lithium source and an organic carbon source, and then calcined at high temperature to obtain the target dual-gradient cathode material. The key to this invention is the construction of a heterogeneous gradient material with radially distributed manganese and dopant elements, achieving a gradient structure of high manganese in the inner layer and low manganese in the outer layer, and low doping in the inner layer and high doping in the outer layer. This provides an effective solution to the pain points of existing technologies. It can ensure the high-voltage energy density of the material by relying on the high-manganese composition of the inner layer, and precisely suppress the surface Jan Taylor effect, disproportionation reaction, and manganese dissolution through the synergistic effect of low manganese and high doping in the outer layer, mitigating high-voltage side reactions above 4.2V, while simultaneously considering bulk lithium-ion transport and structural stability.

[0008] According to a preferred embodiment, the manganese source is one or more combinations of manganese sulfate, manganese chloride, manganese nitrate, and manganese acetate.

[0009] According to a preferred embodiment, the iron source is one or more combinations of ferrous sulfate, ferrous chloride, and ferrous nitrate.

[0010] According to a preferred embodiment, the aluminum source is one or more combinations of aluminum sulfate, aluminum chloride, aluminum nitrate, and aluminum acetate.

[0011] According to a preferred embodiment, the magnesium source is one or more combinations of magnesium sulfate, magnesium chloride, magnesium nitrate, and magnesium acetate.

[0012] According to a preferred embodiment, the lithium source is one or more combinations of lithium carbonate, lithium hydroxide, and lithium acetate.

[0013] According to a preferred embodiment, the organic carbon source is one or more combinations of glucose, citric acid, and sucrose.

[0014] According to a preferred embodiment, the concentration of the sodium pyrophosphate solution is 1 mol / L to 3 mol / L.

[0015] A dual-gradient lithium manganese iron phosphate material, prepared according to the above method, comprises a core and a shell with gradient transitions in manganese content and doping amount; wherein the core is a high-manganese / low-doped lithium manganese iron phosphate material with the molecular formula LiMn. x Fe 1-x-y Al y / 2 Mg y / 2 PO4, and 0.7≤x≤0.9, 0.01≤y≤0.02; the outer shell is made of low-manganese / high-doped lithium manganese iron phosphate material, with the molecular formula LiMn. x Fe 1-x-y Al y / 2 Mg y / 2 PO4, and 0.1≤x≤0.3, 0.03≤y≤0.05.

[0016] According to a preferred embodiment, the carbon content of the above-mentioned dual-gradient lithium manganese iron phosphate material is 1%-3%, and the specific surface area is 10m². 2 / g-30 m 2 / g, particle size D 50 =3μm-6μm.

[0017] Preferably, the carbon content of the dual-gradient lithium manganese iron phosphate material is 1.5%-2.5%, and the specific surface area is 15m². 2 / g-25 m 2 / g, particle size D 50=4μm-6μm. More preferably, the dual-gradient lithium manganese iron phosphate material has a carbon content of 2% and a specific surface area of ​​20 m². 2 / g, particle size D 50 =4μm-5μm.

[0018] The present invention has the following beneficial effects: This invention constructs a heterogeneous gradient material with radial gradient distributions of manganese and dopant elements. This allows for high energy density through a high-manganese inner layer, while the synergistic effect of a low-manganese outer layer with high doping precisely suppresses the surface Jan Taylor effect, disproportionation reaction, and manganese dissolution, mitigating high-voltage side reactions above 4.2V. Simultaneously, it balances bulk lithium-ion transport and structural stability. The material structure of this invention, characterized by "high manganese and low iron + low element doping internally, and low manganese and high iron + high element doping externally," not only relies on elemental doping to reduce manganese dissolution but also incorporates a design where the low manganese content in the outer layer makes dissolution less likely. This balances providing high capacity in the inner layer with high stability in the outer layer. Compared to material structures with unchanged LMFP composition in both inner and outer layers, this design better suppresses the surface Jan Taylor effect, disproportionation reaction, and manganese dissolution.

[0019] This invention first synthesizes a ferromanganese pyrophosphate precursor. Compared to directly precipitating ferromanganese phosphate precursors, manganese and iron elements can form an atomically homogeneous solid solution in the ferromanganese pyrophosphate lattice, effectively avoiding elemental segregation, phase separation, and impurity phase formation caused by differences in manganese and iron precipitation kinetics. Compared to conventional ferromanganese carbonate precursors, the ferromanganese pyrophosphate precursor has a stable structure, is porous, and has high reactivity, which can significantly improve batch consistency and process stability. This invention uses Mg-Al composite doping, with Mg and Al in an equimolar ratio, which, while satisfying doping requirements, also achieves a synergistic effect of "1+1>2", i.e., Mg²⁺ > ... + It helps soften the crystal lattice, provides ion transport channels, and suppresses Mn³ + Disproportionation; Al³ + This helps strengthen the framework, provide rigid support, lock the crystal lattice, and suppress the polarization and distortion of Mn-O bonds. The combination of the two forms a "flexible channel + rigid framework," which can most thoroughly block the structural collapse caused by the Jamneller effect.

[0020] This invention uses a "two-step precipitation + one-time sintering" method, which requires only one sintering. Compared with the multiple sintering of existing technologies, the preparation method of this invention has low energy consumption, can reduce costs and increase efficiency in the industry, and has good economic benefits. Attached Figure Description

[0021] Figure 1 This is a process flow diagram of the preparation method of the dual-gradient lithium manganese iron phosphate material according to an embodiment of the present invention; Figure 2 This is a schematic diagram of the structure of the dual-gradient lithium manganese iron phosphate material according to an embodiment of the present invention; Figure 3 This is a SEM image of Embodiment 1 of the present invention; Figure 4 This is the XRD pattern of Embodiment 1 of the present invention.

[0022] In the diagram: 1. Kernel, 2. Outer layer. Detailed Implementation

[0023] The principles and features of the present invention are described below with reference to the accompanying drawings. The examples given are for illustrative purposes only and are not intended to limit the scope of the invention. Unless otherwise specified in the examples, conventional conditions or conditions recommended by the manufacturer should be followed. Reagents or instruments whose manufacturers are not specified are all commercially available products.

[0024] Please refer to Figure 1 The process flow diagram of the preparation method of the dual-gradient lithium manganese iron phosphate material of the present invention includes the following steps: First, the prepared core mixed metal solution and shell mixed metal solution are titrated together with ammonium pyrophosphate solution and ammonia water to obtain a high-manganese, low-doped manganese iron pyrophosphate precursor.

[0025] Secondly, the high-manganese, low-doped manganese iron pyrophosphate precursor was mixed with deionized water, an organic carbon source, and a lithium source.

[0026] Finally, after spray drying, calcination and demagnetization, a dual-gradient lithium manganese iron phosphate material was obtained.

[0027] The present invention will be further described in detail below with reference to the embodiments.

[0028] Example 1 The preparation method of the dual-gradient lithium manganese iron phosphate material in this embodiment includes the following steps: 1) Prepare the core mixed metal solution A: according to LiMn x Fe 1-x-y Al y / 2 Mg y / 2 To determine the stoichiometric ratio of PO4, weigh out the corresponding manganese, iron, aluminum, and magnesium sources, and simultaneously add 0.1%~0.3% of ascorbic acid as a reducing agent, which accounts for 0.1%~0.3% of the total metal salt mass. Dissolve the solutions in deionized water and stir until completely dissolved to prepare a mixed metal solution with a total molar concentration of Mn+Fe+Al+Mg of 2mol / L, where x=0.7 and y=0.02. 2) Prepare shell-side mixed solution B: according to LiMn x Fe 1-x-y Al y / 2 Mg y / 2To determine the stoichiometric ratio of PO4, weigh out the corresponding manganese, iron, aluminum, and magnesium sources, and simultaneously add 0.1%~0.3% of ascorbic acid as a reducing agent, which accounts for 0.1%~0.3% of the total metal salt mass. Dissolve the solutions in deionized water and stir until completely dissolved to prepare a mixed metal solution with a total molar concentration of Mn+Fe+Al+Mg of 2mol / L, where x=0.1 and y=0.05. 3) Prepare the precipitant solution: Prepare a 2 mol / L sodium pyrophosphate solution; 4) Under the conditions of stirring speed of 600 rpm and temperature of 55℃, simultaneously and uniformly add solution A, sodium pyrophosphate precipitant solution and ammonia water, control pH=6.0~7.0, and titrate for 8 hours; 5) Aging for 2 hours; 6) While maintaining constant stirring speed, nitrogen flow rate, and temperature, simultaneously and uniformly add solution B, sodium pyrophosphate precipitant solution, and ammonia water, controlling pH = 6.0~7.0, with a total titration time of 1 hour; 7) After aging for 2 hours, the product was filtered, washed, and dried to obtain a dual-gradient manganese iron pyrophosphate precursor; 8) Mix manganese iron pyrophosphate precursor, lithium carbonate, glucose and deionized water in a certain proportion until the solid content is 40% and Li / (Mn+Fe+Al+Mg)=1.02. 10. The preparation method according to claim 2, characterized in that: the carbon content of the final product in step 11) is 1~3%, and the specific surface area is 10~30 m². 2 / g, particle size D 50 =3~6μm.

[0029] 9) Spray dry the above slurry at a temperature of 150°C; 10) The above materials are roasted at a temperature of 650°C, in a N2 atmosphere, for 16 hours. 11) Demagnetize and package the above materials to obtain the final product.

[0030] Example 2 The preparation method of the dual-gradient lithium manganese iron phosphate material in this embodiment includes the following steps: 1) Prepare the core mixed metal solution A: according to LiMn x Fe 1-x-y Al y / 2 Mg y / 2 To determine the stoichiometric ratio of PO4, weigh out the corresponding manganese, iron, aluminum, and magnesium sources, and simultaneously add 0.1%~0.3% of ascorbic acid as a reducing agent, which accounts for 0.1%~0.3% of the total metal salt mass. Dissolve the solutions in deionized water and stir until completely dissolved to prepare a mixed metal solution with a total molar concentration of Mn+Fe+Al+Mg of 2mol / L, where x=0.8 and y=0.02. 2) Prepare shell-side mixed solution B: according to LiMn x Fe 1-x-y Al y / 2 Mg y / 2 To determine the stoichiometric ratio of PO4, weigh out the corresponding manganese, iron, aluminum, and magnesium sources, and simultaneously add 0.1%~0.3% of ascorbic acid as a reducing agent, which accounts for 0.1%~0.3% of the total metal salt mass. Dissolve the solutions in deionized water and stir until completely dissolved to prepare a mixed metal solution with a total molar concentration of Mn+Fe+Al+Mg of 2mol / L, where x=0.1 and y=0.05. 3) Prepare the precipitant solution: Prepare a 2 mol / L sodium pyrophosphate solution; 4) Under the conditions of stirring speed of 600 rpm and temperature of 45℃, simultaneously and uniformly add solution A, sodium pyrophosphate precipitant solution and ammonia water, control pH=6.0~7.0, and titrate for 5 hours; 5) Aging for 2 hours; 6) While maintaining constant stirring speed, nitrogen flow rate, and temperature, simultaneously and uniformly add solution B, sodium pyrophosphate precipitant solution, and ammonia water, controlling pH = 6.0~7.0, with a total titration time of 1 hour; 7) After aging for 2 hours, the product was filtered, washed, and dried to obtain a dual-gradient manganese iron pyrophosphate precursor; 8) Mix manganese iron pyrophosphate precursor, lithium carbonate, glucose and deionized water in a certain proportion until the solid content is 40% and Li / (Mn+Fe+Al+Mg)=1.02. 9) Spray dry the above slurry at a temperature of 200℃; 10) The above materials are roasted at a temperature of 700℃, in a N2 atmosphere, for 16 hours. 11) Demagnetize and package the above materials to obtain the final product.

[0031] Example 3 The preparation method of the dual-gradient lithium manganese iron phosphate material in this embodiment includes the following steps: 1) Prepare the core mixed metal solution A: according to LiMn x Fe 1-x-y Al y / 2 Mg y / 2 To determine the stoichiometric ratio of PO4, weigh out the corresponding manganese, iron, aluminum, and magnesium sources, and simultaneously add 0.1%~0.3% of ascorbic acid as a reducing agent, which accounts for 0.1%~0.3% of the total metal salt mass. Dissolve the solutions in deionized water and stir until completely dissolved to prepare a mixed metal solution with a total molar concentration of Mn+Fe+Al+Mg of 2mol / L, where x=0.8 and y=0.01. 2) Prepare shell-side mixed solution B: according to LiMn x Fe 1-x-y Aly / 2 Mg y / 2 To determine the stoichiometric ratio of PO4, weigh out the corresponding manganese, iron, aluminum, and magnesium sources, and simultaneously add 0.1%~0.3% of ascorbic acid as a reducing agent, which accounts for 0.1%~0.3% of the total metal salt mass. Dissolve the solutions in deionized water and stir until completely dissolved to prepare a mixed metal solution with a total molar concentration of Mn+Fe+Al+Mg of 2mol / L, where x=0.2 and y=0.05. 3) Prepare the precipitant solution: Prepare a 2 mol / L sodium pyrophosphate solution; 4) Under the conditions of stirring speed of 600 rpm and temperature of 55℃, solution A, sodium pyrophosphate precipitant solution and ammonia water are added dropwise at the same rate, and the pH is controlled at 6.0~7.0. The titration time is 6h. 5) Aging for 2 hours; 6) While maintaining constant stirring speed, nitrogen flow rate, and temperature, simultaneously and uniformly add solution B, sodium pyrophosphate precipitant solution, and ammonia water, controlling the pH to 6.0~7.0, with a total titration time of 0.5h; 7) After aging for 2 hours, the product was filtered, washed, and dried to obtain a dual-gradient manganese iron pyrophosphate precursor; 8) Mix manganese ferric pyrophosphate precursor, lithium hydroxide, sucrose and deionized water in a certain proportion until the solid content is 50% and Li / (Mn+Fe+Al+Mg)=1.05. 9) Spray dry the above slurry at a temperature of 250°C; 10) The above materials are roasted at a temperature of 700℃, in a N2 atmosphere, for 16 hours. 11) Demagnetize and package the above materials to obtain the final product.

[0032] Example 4 The preparation method of the dual-gradient lithium manganese iron phosphate material in this embodiment includes the following steps: 1) Prepare the core mixed metal solution A: according to LiMn x Fe 1-x-y Al y / 2 Mg y / 2 To determine the stoichiometric ratio of PO4, weigh out the corresponding manganese, iron, aluminum, and magnesium sources, and simultaneously add 0.1%~0.3% of ascorbic acid as a reducing agent, which accounts for 0.1%~0.3% of the total metal salt mass. Dissolve the solutions in deionized water and stir until completely dissolved to prepare a mixed metal solution with a total molar concentration of Mn+Fe+Al+Mg of 2mol / L, where x=0.9 and y=0.01. 2) Prepare shell-side mixed solution B: according to LiMn x Fe 1-x-y Al y / 2 Mg y / 2To determine the stoichiometric ratio of PO4, weigh out the corresponding manganese, iron, aluminum, and magnesium sources, and simultaneously add 0.1%~0.3% of ascorbic acid as a reducing agent, which accounts for 0.1%~0.3% of the total metal salt mass. Dissolve the solutions in deionized water and stir until completely dissolved to prepare a mixed metal solution with a total molar concentration of Mn+Fe+Al+Mg of 2mol / L, where x=0.2 and y=0.05. 3) Prepare the precipitant solution: Prepare a 2 mol / L sodium pyrophosphate solution; 4) Under the conditions of stirring speed of 600 rpm and temperature of 55℃, simultaneously and uniformly add solution A, sodium pyrophosphate precipitant solution and ammonia water, control pH=6.0~7.0, and titrate for 8 hours; 5) Aging for 2 hours; 6) While maintaining constant stirring speed, nitrogen flow rate, and temperature, simultaneously and uniformly add solution B, sodium pyrophosphate precipitant solution, and ammonia water, controlling pH = 6.0~7.0, with a total titration time of 1 hour; 7) After aging for 2 hours, the product was filtered, washed, and dried to obtain a dual-gradient manganese iron pyrophosphate precursor; 8) Mix manganese ferric pyrophosphate precursor, lithium hydroxide, sucrose and deionized water in a certain proportion until the solid content is 40% and Li / (Mn+Fe+Al+Mg)=1.05. 9) Spray dry the above slurry at a temperature of 250°C; 10) The above materials are roasted at a temperature of 750°C, in a N2 atmosphere, for 16 hours. 11) Demagnetize and package the above materials to obtain the final product.

[0033] Example 5 The preparation method of the dual-gradient lithium manganese iron phosphate material in this embodiment includes the following steps: S1. Preparation of core-mixed metal solution: according to LiMn x Fe 1-x-y Al y / 2 Mg y / 2 Weigh out the corresponding manganese, iron, aluminum and magnesium sources with stoichiometric ratios of PO4 and x=0.7, y=0.016, add 0.1% ascorbic acid of total metal salt mass, add deionized water and stir to prepare a core mixed metal solution with a concentration of 1 mol / L. S2. Preparation of the outer shell mixed metal solution: according to LiMn x Fe 1-x-y Al y / 2 Mg y / 2Weigh out the corresponding manganese, iron, aluminum and magnesium sources with stoichiometric ratios of PO4 and x=0.2, y=0.04, add 0.1% ascorbic acid of the total metal salt mass, add deionized water and stir to prepare a shell mixed metal solution with a concentration of 1 mol / L. S3. Preparation of manganese ferropyrophosphate precursor: The core mixed metal solution, sodium pyrophosphate solution and ammonia water were titrated for 8 hours under the conditions of stirring speed of 300 rpm, temperature of 55℃, nitrogen atmosphere protection and control of solution pH=6-7. After aging for 1 hour, the outer shell mixed metal solution was titrated for 0.5 hours under the same conditions and then aged for another 1 hour. Then, the solution was filtered, washed and dried to obtain manganese ferropyrophosphate precursor. S4. The manganese iron pyrophosphate precursor, lithium source, organic carbon source and deionized water are prepared into a slurry with a solid content of 30wt% in the ratio of Li / (Mn+Fe+Al+Mg)=1.02. The slurry is spray-dried at 250°C and then calcined at 650°C in an inert atmosphere for 16h. After demagnetization, a dual-gradient lithium manganese iron phosphate material is obtained.

[0034] Example 6 The preparation method of the dual-gradient lithium manganese iron phosphate material in this embodiment includes the following steps: S1. Preparation of core-mixed metal solution: according to LiMn x Fe 1-x-y Al y / 2 Mg y / 2 Weigh out the corresponding manganese, iron, aluminum and magnesium sources with stoichiometric ratios of PO4 and x=0.9, y=0.02, add ascorbic acid accounting for 0.3% of the total metal salt mass, add deionized water and stir to prepare a core mixed metal solution with a concentration of 3mol / L. S2. Preparation of the outer shell mixed metal solution: according to LiMn x Fe 1-x-y Al y / 2 Mg y / 2 Weigh out the corresponding manganese, iron, aluminum and magnesium sources with stoichiometric ratios of PO4 and x=0.3, y=0.05, add ascorbic acid accounting for 0.3% of the total metal salt mass, add deionized water and stir to prepare a shell mixed metal solution with a concentration of 3mol / L. S3. Preparation of manganese ferropyrophosphate precursor: The core mixed metal solution, sodium pyrophosphate solution and ammonia water were titrated for 5 hours under the conditions of stirring speed of 600 rpm, temperature of 45℃, nitrogen atmosphere protection and control of solution pH=6-7. After aging for 2 hours, the outer shell mixed metal solution was titrated for 1 hour under the same conditions and then aged for 2 hours. Then, the solution was filtered, washed and dried to obtain manganese ferropyrophosphate precursor. S4. The manganese iron pyrophosphate precursor, lithium source, organic carbon source and deionized water are prepared into a slurry with a solid content of 60wt% in the ratio of Li / (Mn+Fe+Al+Mg)=1.05. The slurry is spray-dried at 150°C and then calcined at 750°C in an inert atmosphere for 6 hours. After demagnetization, a dual-gradient lithium manganese iron phosphate material is obtained.

[0035] Example 7 The preparation method of the dual-gradient lithium manganese iron phosphate material in this embodiment includes the following steps: S1. Preparation of core-mixed metal solution: according to LiMn x Fe 1-x-y Al y / 2 Mg y / 2 Weigh out the corresponding manganese, iron, aluminum and magnesium sources with stoichiometric ratios of PO4 and x=0.8, y=0.015, add ascorbic acid accounting for 2% of the total metal salt mass, add deionized water and stir to prepare a core mixed metal solution with a concentration of 2mol / L. S2. Preparation of the outer shell mixed metal solution: according to LiMn x Fe 1-x-y Al y / 2 Mg y / 2 Weigh out the corresponding manganese, iron, aluminum and magnesium sources with stoichiometric ratios of PO4 and x=0.2, y=0.04, add 0.2% ascorbic acid of the total metal salt mass, add deionized water and stir to prepare a shell mixed metal solution with a concentration of 2mol / L. S3. Preparation of manganese ferropyrophosphate precursor: The core mixed metal solution, sodium pyrophosphate solution and ammonia water were titrated for 7 h under the conditions of stirring speed of 400 rpm, temperature of 50℃, nitrogen atmosphere protection and control of solution pH=6-7. After aging for 1.5 h, the outer shell mixed metal solution was titrated for 0.75 h under the same conditions, and then aged for another 1.5 h. Then, the solution was filtered, washed and dried to obtain manganese ferropyrophosphate precursor. S4. The manganese iron pyrophosphate precursor, lithium source, organic carbon source and deionized water are prepared into a slurry with a solid content of 50wt% in the ratio of Li / (Mn+Fe+Al+Mg)=1.03. The slurry is spray-dried at 200℃ and then calcined at 700℃ in an inert atmosphere for 10h. After demagnetization, a dual-gradient lithium manganese iron phosphate material is obtained.

[0036] Comparative Example 1 This comparative example structure does not contain external low-manganese / high-doped lithium manganese iron phosphate material; other processes are the same as in Example 1, specifically including: 1) Prepare the core mixed metal solution A: according to LiMn x Fe 1-x-y Aly / 2 Mg y / 2 To determine the stoichiometric ratio of PO4, weigh out the corresponding manganese, iron, aluminum, and magnesium sources, and simultaneously add 0.1%~0.3% of ascorbic acid as a reducing agent, which accounts for 0.1%~0.3% of the total metal salt mass. Dissolve the solutions in deionized water and stir until completely dissolved to prepare a mixed metal solution with a total molar concentration of Mn+Fe+Al+Mg of 2mol / L, where x=0.7 and y=0.02. 2) Preparation of precipitant solution: Prepare a 2 mol / L sodium pyrophosphate solution; 3) Under the conditions of stirring speed of 600 rpm and temperature of 55℃, solution A, sodium pyrophosphate precipitant solution and ammonia water are added dropwise at a uniform rate, and the pH is controlled at 6.0~7.0. The titration time is 8h. 4) Aging for 2 hours; 5) Mix manganese iron pyrophosphate precursor, lithium carbonate, glucose and deionized water in a certain proportion until the solid content is 40% and Li / (Mn+Fe+Al+Mg)=1.02. 6) Spray dry the above slurry at a temperature of 150°C; 7) The above materials are roasted at a temperature of 650℃, in a N2 atmosphere, for 16 hours. 8) Demagnetize and package the above materials to obtain the final product.

[0037] Comparative Example 2 This comparative example uses the same internal and external doping elements, all with low doping levels. Other processes are the same as in Example 1, specifically including: 1) Prepare the core mixed metal solution A: according to LiMn x Fe 1-x-y Al y / 2 Mg y / 2 To determine the stoichiometric ratio of PO4, weigh out the corresponding manganese, iron, aluminum, and magnesium sources, and simultaneously add 0.1%~0.3% of ascorbic acid as a reducing agent, which accounts for 0.1%~0.3% of the total metal salt mass. Dissolve the solutions in deionized water and stir until completely dissolved to prepare a mixed metal solution with a total molar concentration of Mn+Fe+Al+Mg of 2mol / L, where x=0.7 and y=0.02. 2) Prepare shell-side mixed solution B: according to LiMn x Fe 1-x-y Al y / 2 Mg y / 2 To determine the stoichiometric ratio of PO4, weigh out the corresponding manganese, iron, aluminum, and magnesium sources, and simultaneously add 0.1%~0.3% of ascorbic acid as a reducing agent, which accounts for 0.1%~0.3% of the total metal salt mass. Dissolve the solutions in deionized water and stir until completely dissolved to prepare a mixed metal solution with a total molar concentration of Mn+Fe+Al+Mg of 2mol / L, where x=0.1 and y=0.02. 3) Prepare the precipitant solution: Prepare a 2 mol / L sodium pyrophosphate solution; 4) Under the conditions of stirring speed of 600 rpm and temperature of 55℃, simultaneously and uniformly add solution A, sodium pyrophosphate precipitant solution and ammonia water, control pH=6.0~7.0, and titrate for 8 hours; 5) Aging for 2 hours; 6) While maintaining constant stirring speed, nitrogen flow rate, and temperature, simultaneously and uniformly add solution B, sodium pyrophosphate precipitant solution, and ammonia water, controlling pH = 6.0~7.0, with a total titration time of 1 hour; 7) After aging for 2 hours, the product was filtered, washed, and dried to obtain a dual-gradient manganese iron pyrophosphate precursor; 8) Mix the manganese iron pyrophosphate precursor, lithium source, organic carbon source and deionized water in a certain proportion until the solid content is 40% and Li / (Mn+Fe+Al+Mg)=1.02. 9) Spray dry the above slurry at a temperature of 150°C; 10) The above materials are roasted at a temperature of 650°C, in a N2 atmosphere, for 16 hours. 11) Demagnetize and package the above materials to obtain the final product.

[0038] Comparative Example 3 This comparative example has the same internal and external doping elements, both with high doping levels. Other processes are the same as in Example 1, specifically including: 1) Prepare the core mixed metal solution A: according to LiMn x Fe 1-x-y Al y / 2 Mg y / 2 To determine the stoichiometric ratio of PO4, weigh out the corresponding manganese, iron, aluminum, and magnesium sources, and simultaneously add 0.1%–0.3% ascorbic acid as a reducing agent (based on the total mass of the metal salts). Dissolve the solutions in deionized water and stir until completely dissolved to prepare a mixed metal solution with a total molar concentration of Mn + Fe + Al + Mg of 2 mol / L. (x = 0.7, y = 0.05) 2) Prepare shell-side mixed solution B: according to LiMn x Fe 1-x-y Al y / 2 Mg y / 2 To determine the stoichiometric ratio of PO4, weigh out the corresponding manganese, iron, aluminum, and magnesium sources, and simultaneously add 0.1%~0.3% of ascorbic acid as a reducing agent, which accounts for 0.1%~0.3% of the total metal salt mass. Dissolve the solutions in deionized water and stir until completely dissolved to prepare a mixed metal solution with a total molar concentration of Mn+Fe+Al+Mg of 2mol / L, where x=0.1 and y=0.05. 3) Prepare the precipitant solution: Prepare a 2 mol / L sodium pyrophosphate solution; 4) Under the conditions of stirring speed of 600 rpm and temperature of 55℃, simultaneously and uniformly add solution A, sodium pyrophosphate precipitant solution and ammonia water, control pH=6.0~7.0, and titrate for 8 hours; 5) Aging for 2 hours; 6) While maintaining constant stirring speed, nitrogen flow rate, and temperature, simultaneously and uniformly add solution B, sodium pyrophosphate precipitant solution, and ammonia water, controlling pH = 6.0~7.0, with a total titration time of 1 hour; 7) After aging for 2 hours, the product was filtered, washed, and dried to obtain a dual-gradient manganese iron pyrophosphate precursor; 8) Mix manganese iron pyrophosphate precursor, lithium carbonate, glucose and deionized water in a certain proportion until the solid content is 40% and Li / (Mn+Fe+Al+Mg)=1.02. 9) Spray dry the above slurry at a temperature of 150°C; 10) The above materials are roasted at a temperature of 650°C, in a N2 atmosphere, for 16 hours. 11) Demagnetize and package the above materials to obtain the final product.

[0039] Test case The positive electrode active materials prepared in each embodiment and each comparative example were used to make lithium-ion secondary batteries (button batteries), and the specific capacity of the first cycle discharge and the retention rate after 50 cycles of each battery were tested. The results are shown in Table 1.

[0040] The method for preparing the coin cell includes: using the positive electrode active materials obtained in Examples 1-7 and Comparative Examples 1-3 as the positive electrode active materials, weighing the positive electrode active materials, conductive SP and binder PVDF in a mass ratio of 90:5:5, adding NMP and dispersing and stirring for 2 hours to mix evenly, forming a slurry, coating it on aluminum foil, and then cutting it into electrode sheets with a diameter of 14 mm and a coating amount of 12 mg / cm². 2 The compacted density is 2.1 g / cm³. 3 A lithium metal sheet is used as the negative electrode, Celgard 2400 as the separator, and the electrolyte solute is 1.0 mol / L LiPF6 with a solution ratio of EC:DMC:EMC of 1:1:1 (volume ratio). A CR2032 coin cell is assembled in the following order: negative electrode shell, positive electrode sheet, electrolyte, separator, electrolyte, lithium sheet, and positive electrode shell. The coin cell is then sealed using a coin cell sealing machine to complete the fabrication of the composite positive electrode active material lithium-ion battery.

[0041] The initial discharge specific capacity test conditions for the button cell were LR 2032, 0.1C discharge capacity, 2.5~4.3V, vs. Li+ / Li; the cycle test conditions were 45℃, 2.5~4.3V, 0.5C / 0.5C, and the charging and discharging equipment used was a Landian charge and discharge meter.

[0042] Table 1. Comparison of electrochemical performance of samples prepared in the examples and comparative examples.

[0043] As can be seen from the data in Examples 1-7 in the table, the dual-gradient lithium manganese iron phosphate material prepared by the present invention has high specific energy and good energy retention rate.

[0044] Comparative Example 1, which does not contain external low-manganese / high-doped lithium manganese iron phosphate material, exhibits a significantly worse energy retention rate. The main reason is that after charging, Mn... 3+ Excessive content cannot suppress Mn³ during charging and discharging. + The Jiang-Taylor distortion and manganese dissolution exacerbate electrolyte side reactions, leading to continuous destruction of the crystal structure and a continuous increase in interfacial impedance, ultimately resulting in a significant deterioration in energy retention.

[0045] Comparative Example 2 uses the same low-doping elements both internally and externally, resulting in a relatively constant energy density but a lower capacity retention. This is mainly because the low doping content of the material fails to effectively suppress Mn³⁺. + The Ginger-Taylor distortion results in insufficient lattice stability and significant manganese dissolution. At the same time, the interface structure is susceptible to electrolyte erosion and continuous deterioration, leading to accelerated capacity decay during cycling and ultimately a significant deterioration in the material's cycling performance. In addition, the low surface doping level results in a low ion diffusion coefficient, which affects the material's capacity utilization.

[0046] Comparative Example 3 has the same internal and external doping elements, both with high doping levels, but the discharge specific energy is low. The main reason is that the excessively inert doping elements crowd out the effective proportion of active materials in the material, reducing the number of active sites that can participate in lithium insertion / extraction reactions per unit mass, which leads to a decrease in the material's reversible specific capacity and ultimately manifests as a low specific energy.

[0047] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a dual-gradient lithium manganese iron phosphate material, characterized in that, Includes the following steps: S1, preparing the inner core mixed metal solution: taking the stoichiometric ratio of manganese source, iron source, aluminum source and magnesium source according to LiMn x Fe 1-x-y Al y / 2 Mg y / 2 PO4 and 0.7≤x≤0.9, 0.01≤y≤0.02, and adding 0.1%-0.3% of ascorbic acid based on the total mass of metal salt, adding deionized water and stirring to obtain an inner core mixed metal solution with a concentration of 1mol / L-3mol / L; S2. Preparation of the outer shell mixed metal solution: according to LiMn x Fe 1-x-y Al y / 2 Mg y / 2 Weigh out the corresponding manganese, iron, aluminum and magnesium sources with stoichiometric ratios of PO4 and 0.1≤x≤0.3, 0.03≤y≤0.05, and add 0.1%~0.3% ascorbic acid of the total metal salt mass. Add deionized water and stir to prepare a mixed metal solution with a concentration of 1mol / L-3mol / L. S3. Preparation of manganese ferropyrophosphate precursor: The core mixed metal solution, sodium pyrophosphate solution and ammonia water are titrated for 5h-8h under the conditions of stirring speed of 300 rpm-600 rpm, temperature of 45℃-55℃, nitrogen atmosphere protection and control of solution pH=6-7. After aging for 1h-2h, the outer shell mixed metal solution is titrated for 0.5h-1h under the same conditions, and then aged for another 1h-2h. Then, the mixture is filtered, washed and dried to obtain manganese ferropyrophosphate precursor. S4. The manganese iron pyrophosphate precursor, lithium source, organic carbon source and deionized water are prepared into a slurry with a solid content of 30wt%-60wt% in a ratio of Li / (Mn+Fe+Al+Mg)=1.02-1.

05. The slurry is spray-dried at a temperature of 150℃-250℃, and then calcined at 650℃-750℃ in an inert atmosphere for 6h-16h. After demagnetization, a dual-gradient lithium manganese iron phosphate material is obtained.

2. The method for preparing the dual-gradient lithium manganese iron phosphate material according to claim 1, characterized in that, The manganese source is one or a combination of manganese sulfate, manganese chloride, manganese nitrate, and manganese acetate.

3. The method for preparing the dual-gradient lithium manganese iron phosphate material according to claim 1, characterized in that, The iron source is one or a combination of ferrous sulfate, ferrous chloride, and ferrous nitrate.

4. The method for preparing the dual-gradient lithium manganese iron phosphate material according to claim 1, characterized in that, The aluminum source is one or a combination of aluminum sulfate, aluminum chloride, aluminum nitrate, and aluminum acetate.

5. The method for preparing the dual-gradient lithium manganese iron phosphate material according to claim 1, characterized in that, The magnesium source is one or more combinations of magnesium sulfate, magnesium chloride, magnesium nitrate, and magnesium acetate.

6. The method for preparing the dual-gradient lithium manganese iron phosphate material according to claim 1, characterized in that, The lithium source is one or a combination of lithium carbonate, lithium hydroxide, and lithium acetate.

7. The method for preparing the dual-gradient lithium manganese iron phosphate material according to claim 1, characterized in that, The organic carbon source is one or more combinations of glucose, citric acid, and sucrose.

8. The method for preparing the dual-gradient lithium manganese iron phosphate material according to claim 1, characterized in that, The concentration of the sodium pyrophosphate solution is 1 mol / L to 3 mol / L.

9. A dual-gradient lithium manganese iron phosphate material, characterized in that, The preparation method according to any one of claims 1 to 8 comprises a core and a shell with a gradient transition structure in both manganese content and doping amount; wherein the core is a high-manganese / low-doped lithium manganese iron phosphate material with the molecular formula LiMn x Fe 1-x-y Al y / 2 Mg y / 2 PO4, and 0.7≤x≤0.9, 0.01≤y≤0.02; the outer shell is made of low-manganese / high-doped lithium manganese iron phosphate material, with the molecular formula LiMn. x Fe 1-x-y Al y / 2 Mg y / 2 PO4, and 0.1≤x≤0.3, 0.03≤y≤0.

05.

10. The dual-gradient lithium manganese iron phosphate material according to claim 9, characterized in that, The dual-gradient lithium manganese iron phosphate material has a carbon content of 1%-3% and a specific surface area of ​​10m². 2 / g-30m 2 / g, particle size D 50 =3μm-6μm.

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