Gradient-structure high-conductivity lithium manganese iron phosphate positive electrode material and preparation method thereof

Abstract

The invention discloses a high-conductivity lithium manganese iron phosphate positive electrode material with a gradient structure and a preparation method thereof, and belongs to the technical field of batteries. According to the technical scheme, a'manganese-rich core-uniform layer-iron-rich shell 'gradient precursor is prepared through a coprecipitation reaction, a lithium source, polyacrylonitrile and Ti3C2Tx are compounded to construct a three-dimensional conductive network, and a finished product is prepared through in-situ polymerization-carbonization-crystallization integrated treatment. The electronic conductivity of the positive electrode material reaches 10 <-2 > S / cm magnitude, the 5C capacity retention rate is 92%, and the 1C 1000-time cycle retention rate gt is achieved; and the capacity retention rate at-20 DEG C is 85%, so that the problems of poor conductivity, phase separation and manganese dissolution of the traditional material are effectively solved.

Description

Technical Field

[0001] This invention relates to the field of battery technology, specifically to a gradient structure high conductivity lithium manganese iron phosphate cathode material and its preparation method. Background Technology

[0002] Lithium-ion batteries, with their advantages of high energy density, long cycle life, and environmental friendliness, have been widely used in new energy vehicles, portable electronic devices, and energy storage systems. As downstream applications continue to demand higher battery performance, the performance upgrade of cathode materials, as the core component determining battery energy density, power density, and cycle life, has become a key focus of industry research and development.

[0003] Lithium manganese iron phosphate (LiMn) x Fe 1-x PO4 (LMFP) is used as a solid solution cathode material for lithium iron phosphate (LFP) and lithium manganese phosphate (LMP), combining the high safety, low cost, and excellent cycle stability of LFP with the high voltage (Mn) of LMP. 2+ / Mn 3+ Redox potential approximately 4.1V vs. Li + With its superior 3.4V (Li+) and theoretical specific capacity of up to 170mAh / g, and an energy density 15-20% higher than traditional LFP, it is considered one of the core candidates for next-generation lithium-ion battery cathode materials and has broad application prospects in mid-to-high-end new energy vehicles, high-power energy storage and other scenarios.

[0004] However, the industrialization of LMFP is still limited by three core technological bottlenecks caused by its intrinsic properties and manufacturing process, which seriously restrict its performance: (1) In the crystal structure of LMFP, Mn 4+ The presence of these elements leads to lattice distortion, and the strong covalent bonds formed between Fe and Mn ions and oxygen atoms significantly hinder electron transfer, resulting in an intrinsic electronic conductivity of only 10. -14 -10 -12 S / cm, much lower than LFP (electronic conductivity 10⁻⁶). -10 The slow electron transport rate (S / cm) causes severe polarization and drastic capacity decay in LMFPs during high-rate charging and discharging, making it difficult to meet the requirements of fast charging scenarios.

[0005] (2) Mn in LMFP 2+ / Mn 3+ with Fe 2+ / Fe 3+The redox potentials of the two ions differ significantly (approximately 0.7V). During the lithium insertion / extraction process, the redox reactions of the two ions in the homogeneous solid solution structure obtained by traditional preparation processes are asynchronous, which easily induces a chemical potential gradient within the crystal lattice, causing the solid solution to decompose into a Mn-rich phase and a Fe-rich phase (i.e., phase separation). Phase separation generates significant structural stress, causing particle cracking and electrolyte wetting failure, which in turn leads to rapid capacity decay and deterioration of cycle stability.

[0006] (3) Under a high-voltage operating platform of 4.1V, the Mn on the surface of LMFP 3+ Jahn-Teller distortion is prone to occur, causing Mn ions to dissolve from the crystal lattice and deposit on the negative electrode surface. This not only destroys the integrity of the positive electrode crystal structure, but also catalyzes the decomposition of the electrolyte to produce gases and byproducts, and exacerbates the irreversible growth of the SEI film (solid electrolyte interface film). At the same time, the high-voltage environment accelerates the interfacial reaction between the electrolyte and the positive electrode material, resulting in uneven thickness and poor stability of the CEI film (cathode electrolyte interface film), further deteriorating cycle performance and storage stability, becoming a key obstacle restricting the long-term service of LMFP.

[0007] Therefore, developing an LMFP fabrication technology that can simultaneously address the three core issues of "slow transport dynamics, phase separation, and interface instability," and achieving a comprehensive improvement in material conductivity, structural stability, and interface compatibility through synergistic innovation in structural design and performance control, is of great significance for promoting the industrial application of LMFP. Summary of the Invention

[0008] The technical problem to be solved by this invention is to overcome the shortcomings of the prior art and provide a gradient structure high conductivity lithium manganese iron phosphate cathode material and its preparation method. A gradient precursor of "manganese-rich core-uniform layer-iron-rich shell" is prepared through a co-precipitation reaction, and a lithium source, polyacrylonitrile, and Ti3C2T are compounded. x A three-dimensional conductive network is constructed, and the finished product is obtained through in-situ polymerization-carbonization-crystallization integrated processing, which effectively solves the problems of poor conductivity, phase separation and manganese leaching of traditional materials.

[0009] The technical solution of this invention is as follows: On one hand, the present invention provides a method for preparing a gradient-structured, highly conductive lithium manganese iron phosphate cathode material, comprising the following steps: Preparation of S1 gradient structure precursor S11 Preparation of Solution A: Prepare Solution A containing manganese salt and phosphorus source; S12 Preparation of Solution B: Prepare Solution B containing iron salt and phosphorus source; S13 coprecipitation reaction: Solution A and solution B are injected into the reactor, and the flow rates of the two solutions are controlled so that the molar ratio of manganese and iron changes radially during the reaction. (1) Initial stage of reaction (particle core formation stage): First, control the flow rate ratio of solution A to solution B to be 8:2 to form manganese-rich cores (Mn:Fe≈0.8:0.2). (2) Mid-reaction stage (intermediate layer formation stage): When the average particle size D50 > 1 μm, control the flow rate ratio of solution A to solution B to be 5:5 to form a uniform manganese-iron mixture layer. (3) Later stage of reaction (shell formation stage): When the average particle size D50 > 2.5 μm, control the flow rate ratio of solution A to solution B to be 2:8 until the average particle size D50 = 3-4 μm, forming an iron-rich shell (Mn:Fe≈0.2:0.8). The reaction was carried out under an inert atmosphere, with the temperature controlled at 60-80℃ and the pH value maintained at 3-4, ultimately yielding a gradient structure precursor with a "manganese-rich core-uniform layer-iron-rich shell" gradient structure. Construction of the S2 three-dimensional conductive network: The precursor was mixed with a lithium source, a carbon source, and a conductive agent, wherein the carbon source was polyacrylonitrile (PAN), and the conductive agent was a two-dimensional MXene material (Ti3C2T). x ); S3 in-situ polymerization-carbonization-crystallization integrated processing The mixture obtained in step S2 is placed in a tube furnace for the following operations: (1) In-situ polymerization stage: First, calcined at 200-300℃ for 1.5-2.5h in air atmosphere, PAN undergoes cyclization reaction, forming a cross-linked polymer network on the particle surface. At the same time, MXene is slightly oxidized, and oxygen-containing functional groups are generated on the surface. (2) Gradient carbonization-crystallization stage: In the first stage, the temperature is raised to 350-450℃ in Ar / H2 mixed atmosphere and held for 1.5-2.5h. PAN is carbonized into nitrogen-doped carbon, and MXene is partially converted into TiC to form a primary conductive network. In the second stage, the temperature is raised to 550-650℃ and held for 3-5h. The precursor begins to decompose and generate an intermediate phase, and the conductive network is further strengthened. In the third stage, the temperature is raised to 680-720℃ and held for 5-9h. Lithium manganese iron phosphate crystals grow. The iron-rich shell crystallizes first to form a stable framework, and the manganese-rich core crystallizes subsequently to obtain the lithium manganese iron phosphate cathode material.

[0010] Preferably, in steps S11 and S12, the manganese salt is MnSO4, the phosphorus source is H3PO4, and the iron salt is FeSO4; in solution A, the concentration of manganese salt is 0.5 mol / L and the concentration of phosphorus source is 0.5 mol / L; in solution B, the concentration of iron salt is 0.5 mol / L and the concentration of phosphorus source is 0.5 mol / L.

[0011] Preferably, in steps S11 and S12, 0.8-1.2 wt.% ascorbic acid is added to solution A and solution B respectively as an antioxidant.

[0012] Preferably, in step S2, the lithium source is Li2CO3.

[0013] Preferably, in step S2, the lithium source is added in a molar ratio of Li:(Mn+Fe):P=1.05:1:1, the carbon source accounts for 5-10% of the total mass of the mixture, and the conductive agent accounts for 3-5% of the total mass of the mixture.

[0014] Preferably, in step S2, the two-dimensional MXene material is Ti3C2T. x .

[0015] On the other hand, the present invention provides a gradient structure high conductivity lithium manganese iron phosphate cathode material, which is prepared by the above-described method for preparing a gradient structure high conductivity lithium manganese iron phosphate cathode material.

[0016] This invention, through synergistic innovation of "gradient structure design + three-dimensional conductive network construction + in-situ polymerization-carbonization-crystallization integrated process," breaks through the technical bottlenecks of traditional lithium manganese iron phosphate (LMFP) cathode materials from three dimensions: structural regulation, performance enhancement, and process optimization. Compared with existing technologies, it has the following significant advantages: 1. This invention innovatively constructs a three-dimensional conductive network of "nitrogen-doped carbon-TiC-MXene": A nitrogen-doped carbon layer, formed by in-situ polymerization and carbonization of the carbon source polyacrylonitrile (PAN), tightly coats the material surface, providing not only a continuous electron transport channel, but also enhancing the affinity with the electrolyte and accelerating lithium-ion diffusion through its nitrogen-containing functional groups; the two-dimensional MXene material (Ti3C2T...)... x The cathode material itself possesses extremely high electronic conductivity. During carbonization, it partially transforms into TiC, forming a synergistic conductive system with nitrogen-doped carbon, thus completely solving the problem of low intrinsic conductivity in LMFPs. Test results show that the electronic conductivity of the cathode material prepared by this invention is increased to 10. -2 With a capacity retention rate of 92% at 5C high-rate conditions, it is on the order of S / cm and far superior to traditional materials, meeting the high-power requirements of scenarios such as fast charging of new energy vehicles and high-power energy storage.

[0017] 2. This invention, by controlling the flow rate ratio of solutions A and B, prepares a radial gradient structure of "manganese-rich core-uniform layer-iron-rich shell," thus solving the phase separation problem from a thermodynamic perspective: the manganese-rich core ensures the energy density advantage of the high-voltage platform (4.1V), and the iron-rich shell (Fe... 2+ / Fe 3+The redox potential (3.4V) forms a smooth compositional transition band with the intermediate homogeneous layer, eliminating the chemical potential gradient caused by the difference in redox potentials of Mn and Fe in traditional homogeneous solid solutions. During charge and discharge, lithium ions migrate along the continuous diffusion channels formed by the gradient composition, and reaction regions at different potentials separate in an orderly manner, avoiding the independent precipitation of Mn-rich and Fe-rich phases, significantly reducing structural stress, and effectively preventing particle cracking. The cathode material of this invention retains >95% capacity after 1000 cycles at 1C.

[0018] 3. Addressing the industry challenges of Mn dissolution and electrolyte decomposition under high voltage, this invention achieves dual protection: Firstly, the Fe-O bond bonding energy of the iron-rich outer shell is higher than that of the Mn-O bond, resulting in greater chemical stability and effectively blocking internal Mn. 3+ Contact with electrolyte inhibits Mn 3+ The Jahn-Teller distortion and dissolution of MXene-derived TiC prevent SEI film damage caused by Mn ion deposition at the negative electrode. Furthermore, MXene-derived TiC exhibits strong catalytic activity, accelerating the reaction of PF6 in the electrolyte. - The decomposition of Fe induces the formation of a thin, dense, and homogeneous CEI film (cathode electrolyte interface film), significantly reducing interfacial impedance and inhibiting the continued decomposition of the electrolyte. Furthermore, the ascorbic acid added to solutions A and B effectively prevents Fe... 2+ Mn 2+ Oxidation ensures the uniformity of precursor components and further enhances the cycling stability of the material.

[0019] 4. Traditional LMFPs suffer from severe capacity decay at low temperatures (e.g., -20°C) due to their high lithium-ion diffusion activation energy, limiting their application in cold regions. The gradient structure design of this invention eliminates concentrated lattice defect regions through compositional gradients, lowering the energy barrier for lithium-ion diffusion. Simultaneously, the three-dimensional conductive network reduces interfacial polarization during electron transport, maintaining efficient electron / ion transport even at low temperatures. Test results show that the material of this invention achieves 85% capacity retention at -20°C, successfully addressing the industry's pain point of poor low-temperature performance and broadening the application scope of the material in high-latitude regions for new energy vehicles, outdoor energy storage, and other scenarios. Detailed Implementation

[0020] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] Example 1 The preparation method of the gradient structure highly conductive lithium manganese iron phosphate cathode material in this embodiment includes the following steps: Preparation of S1 gradient structure precursor S11 Preparation of Solution A: Prepare an acidic solution A containing 0.5M MnSO4 and 0.5M H3PO4, and add 1 wt.% ascorbic acid to solution A.

[0022] S12 Preparation of Solution B: Prepare an acidic solution B containing 0.5M FeSO4 and 0.5M H3PO4, and add 1 wt.% ascorbic acid to the solution B.

[0023] S13 coprecipitation reaction: Solution A and solution B are injected into the reactor, and the flow rates of the two solutions are controlled so that the molar ratio of manganese and iron changes radially during the reaction: First, the flow rate ratio of solution A to solution B is controlled at 8:2. When the average particle size D50 > 1 μm, the flow rate ratio of solution A to solution B is controlled at 5:5. Finally, when the average particle size D50 > 2.5 μm, the flow rate ratio of solution A to solution B is controlled at 2:8, until the average particle size D50 = 3-4 μm. The reaction is carried out under Ar protection, the temperature is controlled at 70℃, and the pH value is maintained at 3-4. The gradient structure precursor is finally collected.

[0024] Construction of S2 three-dimensional conductive network: The precursor was combined with Li2CO3, 8 wt.% PAN and 4 wt.% Ti3C2T x Ball milling for 4 hours.

[0025] S3 in-situ polymerization-carbonization-crystallization integrated processing The mixture obtained in step S2 was placed in a tube furnace and subjected to the following operations: first, it was calcined at 250°C for 2 hours in an air atmosphere; then, it was heated to 400°C and held for 2 hours in an Ar / H2 (95:5) mixed atmosphere; then, it was heated to 600°C and held for 4 hours; finally, it was heated to 700°C and held for 7 hours; after natural cooling, it was sieved to obtain the lithium manganese iron phosphate cathode material.

[0026] Example 2 The preparation method of the gradient structure highly conductive lithium manganese iron phosphate cathode material in this embodiment includes the following steps: Preparation of S1 gradient structure precursor S11 Preparation of Solution A: Prepare an acidic solution A containing 0.5M MnSO4 and 0.5M H3PO4, and add 1 wt.% ascorbic acid to solution A.

[0027] S12 Preparation of Solution B: Prepare an acidic solution B containing 0.5M FeSO4 and 0.5M H3PO4, and add 1 wt.% ascorbic acid to the solution B.

[0028] S13 coprecipitation reaction: Solution A and solution B are injected into the reactor, and the flow rates of the two solutions are controlled so that the molar ratio of manganese and iron changes radially during the reaction: first, the flow rate ratio of solution A to solution B is controlled at 8:2. When the average particle size D50 > 1 μm, the flow rate ratio of solution A to solution B is controlled at 5:5. Finally, when the average particle size D50 > 2.5 μm, the flow rate ratio of solution A to solution B is controlled at 2:8, until the average particle size D50 = 3-4 μm. The reaction is carried out under Ar protection, the temperature is controlled at 60℃, and the pH value is maintained at 3-4. The gradient structure precursor is finally collected.

[0029] Construction of S2 three-dimensional conductive network: The precursor was combined with Li2CO3, 5 wt.% PAN and 3 wt.% Ti3C2T x Ball milling for 4 hours.

[0030] S3 in-situ polymerization-carbonization-crystallization integrated processing The mixture obtained in step S2 was placed in a tube furnace and subjected to the following operations: first, it was calcined at 200°C for 2.5 hours in an air atmosphere; then, it was heated to 350°C and held for 2.5 hours in an Ar / H2 (95:5) mixed atmosphere; then, it was heated to 550°C and held for 5 hours; finally, it was heated to 680°C and held for 9 hours; after natural cooling, it was sieved to obtain the lithium manganese iron phosphate cathode material.

[0031] Example 3 The preparation method of the gradient structure highly conductive lithium manganese iron phosphate cathode material in this embodiment includes the following steps: Preparation of S1 gradient structure precursor S11 Preparation of Solution A: Prepare an acidic solution A containing 0.5M MnSO4 and 0.5M H3PO4, and add 1 wt.% ascorbic acid to solution A.

[0032] S12 Preparation of Solution B: Prepare an acidic solution B containing 0.5M FeSO4 and 0.5M H3PO4, and add 1 wt.% ascorbic acid to the solution B.

[0033] S13 coprecipitation reaction: Solution A and solution B are injected into the reactor, and the flow rates of the two solutions are controlled so that the molar ratio of manganese and iron changes radially during the reaction: first, the flow rate ratio of solution A to solution B is controlled at 8:2. When the average particle size D50 > 1 μm, the flow rate ratio of solution A to solution B is controlled at 5:5. Finally, when the average particle size D50 > 2.5 μm, the flow rate ratio of solution A to solution B is controlled at 2:8, until the average particle size D50 = 3-4 μm. The reaction is carried out under Ar protection, the temperature is controlled at 80℃, and the pH value is maintained at 3-4. The gradient structure precursor is finally collected.

[0034] Construction of S2 three-dimensional conductive network: The precursor was combined with Li2CO3, 10 wt.% PAN and 5 wt.% Ti3C2T x Ball milling for 4 hours.

[0035] S3 in-situ polymerization-carbonization-crystallization integrated processing The mixture obtained in step S2 was placed in a tube furnace and subjected to the following operations: first, it was calcined at 300°C for 1.5 h in an air atmosphere; then, it was heated to 450°C and held for 1.5 h in an Ar / H2 (95:5) mixed atmosphere, then heated to 650°C and held for 3 h, and finally heated to 720°C and held for 5 h; after natural cooling, the lithium manganese iron phosphate cathode material was obtained by sieving.

[0036] Comparative Example 1 The difference from Example 1 is that in step S13, the flow rate ratio of solution A to solution B is controlled to be 5:5 for coprecipitation reaction.

[0037] Comparative Example 2 The difference from Example 1 is that PAN is not added in step S2.

[0038] Comparative Example 3 The difference from Example 1 is that Ti3C2T is not added in step S2. x .

[0039] The positive electrode materials prepared in Examples 1-3 and Comparative Examples 1-3 were assembled into coin cells: the positive electrode material, acetylene black (conductive agent), and polyvinylidene fluoride (binder) were mixed in a mass ratio of 8:1:1, N-methylpyrrolidone was added to form a slurry, which was coated onto an aluminum foil current collector. After vacuum drying at 80°C for 12 hours, the slurry was cut into positive electrode sheets with a diameter of 14 mm and compacted for later use; lithium metal sheets were cut into negative electrode sheets with a diameter of 16 mm, and Celgard was selected. A 2400 polypropylene membrane (18 mm in diameter) was used as the separator. In an argon glove box (water and oxygen content < 1 ppm), the cells were stacked in the following order: positive electrode shell → positive electrode sheet → separator → electrolyte (1 mol / L LiPF6 / EC-DMC-EMC, approximately 100 μL added) → lithium sheet → gasket → spring sheet → negative electrode shell. The cells were then pressed and sealed using a button cell sealing machine. After assembly, the button cells were left to stand at room temperature for 24 hours to allow the electrolyte to fully wet the electrodes and separator, ready for testing.

[0040] Performance tests were conducted on it, and the results are shown in Table 1: Table 1 Performance test results of the assembled batteries in Examples 1-3 and Comparative Examples 1-3 As shown in Table 1, Comparative Example 1 did not adopt the "manganese-rich core-uniform layer-iron-rich shell" gradient structure design of this invention. Instead, it prepared a uniform solid solution precursor by maintaining a constant flow rate ratio of A to B solutions of 5:5 throughout the process, resulting in its performance being comprehensively inferior to Examples 1-3. This is because Mn and Fe are randomly distributed in the uniform solid solution, resulting in severe lattice distortion and the absence of continuous diffusion channels provided by the gradient structure, leading to an electronic conductivity of only 5.6 × 10⁻⁶. -8 S / cm, lithium-ion diffusion is hindered, directly resulting in a capacity retention of only 68% at 5C high rate and only 55% at -20℃ low temperature. Mn in a homogeneous solid solution 2+ / Mn 3+ (4.1V) and Fe 2+ / Fe 3+ The (3.4V) redox potential difference induces a significant chemical potential gradient, which easily decomposes into independent Mn-rich and Fe-rich phases during charge and discharge, generating severe structural stress, leading to particle cracking and electrolyte wetting failure. The 0.1C discharge specific capacity drops to 155mAh / g, and the capacity retention rate after 1000 cycles at 25℃ and 1C is only 82%. In addition, due to the lack of a physical barrier of iron-rich shell, Mn under high voltage... 3+ Jahn-Teller distortion and dissolution are prone to occur, which catalyzes electrolyte decomposition, exacerbates CEI membrane instability, and further deteriorates cycle stability.

[0041] Comparative Example 2 did not add the carbon source polyacrylonitrile (PAN) during the three-dimensional conductive network construction stage, only retaining Ti3C2T. x The conductive agent caused a significant decrease in performance compared to the previous example. This is because the nitrogen-doped carbon layer formed after in-situ polymerization and carbonization of PAN is a key component of the three-dimensional conductive network. It not only tightly coats the material particles to form continuous electron channels, but also interacts with Ti3C2T. x The derived TiC forms a synergistic conductive system. Without PAN, it relies solely on Ti3C2T. x It is difficult to form a fully covered conductive network, and the electronic conductivity drops to 3.8 × 10⁻⁶. -4 S / cm, 5C rate capacity retention drops to 75%. Nitrogen-doped carbon layers with nitrogen-containing functional groups enhance the affinity between the material and the electrolyte, accelerating lithium-ion migration. The absence of PAN leads to decreased electrolyte wettability, increased lithium-ion diffusion activation energy, and a capacity retention of only 62% at -20℃. Nitrogen-doped carbon generated by PAN carbonization can modulate the composition and structure of the CEI film, improving interfacial stability. Without PAN, only Ti3C2T... x The derived TiC catalysis was insufficient to form a dense and stable CEI film, and the electrolyte decomposition intensified, with the capacity retention dropping to 88% after 1000 cycles at 25°C and 1C.

[0042] Comparative Example 3 did not add two-dimensional MXene material (Ti3C2T) during the construction stage of the three-dimensional conductive network. x Only the PAN carbon source is retained. Ti3C2T x It inherently possesses extremely high electronic conductivity, and the conductive phase of TiC formed after carbonization exhibits superior conductivity compared to nitrogen-doped carbon. (Ti3C2T is missing.) x Subsequently, relying solely on nitrogen-doped carbon derived from PAN to form a single conductive network, the electron transport paths are limited, and the electronic conductivity drops to 1.2 × 10⁻⁶. -3 S / cm, 5C rate capacity retention drops to 78% Ti3C2T x The two-dimensional layered structure of Ti3C2T can reduce lithium-ion diffusion resistance and synergistically optimize electron / ion transport at low temperatures with nitrogen-doped carbon. However, the absence of Ti3C2T results in an increased lithium-ion diffusion activation energy and a capacity retention of only 65% ​​at -20°C. x Derivative TiC against PF6 - The decomposition exhibits strong catalytic activity, inducing the formation of a thin and dense CEI film. Nitrogen-doped carbon alone cannot achieve efficient interfacial catalysis; the CEI film stability is insufficient. Although the presence of PAN improves the cycling performance compared to Comparative Examples 1-2, it is still significantly lower than the 96% of Example 1.

Claims

1. A method for preparing a gradient-structured, highly conductive lithium manganese iron phosphate cathode material, characterized in that, Includes the following steps: Preparation of S1 gradient structure precursor S11 Preparation of Solution A: Prepare Solution A containing manganese salt and phosphorus source; S12 Preparation of Solution B: Prepare Solution B containing iron salt and phosphorus source; S13 coprecipitation reaction: Solution A and solution B are injected into the reactor, and the flow rates of the two solutions are controlled so that the molar ratio of manganese and iron changes radially during the reaction: first, the flow rate ratio of solution A to solution B is controlled at 8:2; when the average particle size D50 > 1 μm, the flow rate ratio of solution A to solution B is controlled at 5:5; finally, when the average particle size D50 > 2.5 μm, the flow rate ratio of solution A to solution B is controlled at 2:8, until the average particle size D50 = 3-4 μm; the reaction is carried out under an inert atmosphere, the temperature is controlled at 60-80℃, and the pH value is maintained at 3-4, finally obtaining a gradient structure precursor; Construction of S2 three-dimensional conductive network: The precursor is mixed with lithium source, carbon source and conductive agent, wherein the carbon source is polyacrylonitrile and the conductive agent is two-dimensional MXene material; S3 in-situ polymerization-carbonization-crystallization integrated processing The mixture obtained in step S2 is placed in a tube furnace and subjected to the following operations: first, calcined at 200-300℃ for 1.5-2.5h in an air atmosphere; then heated to 350-450℃ in an Ar / H2 mixed atmosphere and held for 1.5-2.5h; then heated to 550-650℃ and held for 3-5h; finally heated to 680-720℃ and held for 5-9h to obtain lithium manganese iron phosphate cathode material.

2. The method for preparing the gradient structure high conductivity lithium manganese iron phosphate cathode material as described in claim 1, characterized in that, In steps S11 and S12, the manganese salt is MnSO4, the phosphorus source is H3PO4, and the iron salt is FeSO4; in solution A, the concentration of manganese salt is 0.5 mol / L and the concentration of phosphorus source is 0.5 mol / L; in solution B, the concentration of iron salt is 0.5 mol / L and the concentration of phosphorus source is 0.5 mol / L.

3. The method for preparing the gradient structure high conductivity lithium manganese iron phosphate cathode material as described in claim 1, characterized in that, In steps S11 and S12, 0.8-1.2 wt.% ascorbic acid is added to solution A and solution B, respectively.

4. The method for preparing the gradient structure high conductivity lithium manganese iron phosphate cathode material as described in claim 1, characterized in that, In step S2, the lithium source is Li2CO3.

5. The method for preparing the gradient structure high conductivity lithium manganese iron phosphate cathode material as described in claim 1, characterized in that, In step S2, the lithium source is added in a molar ratio of Li:(Mn+Fe):P=1.05:1:1, the carbon source accounts for 5-10% of the total mass of the mixture, and the conductive agent accounts for 3-5% of the total mass of the mixture.

6. The method for preparing the gradient structure high conductivity lithium manganese iron phosphate cathode material as described in claim 1, characterized in that, In step S2, the two-dimensional MXene material is Ti3C2T. x .

7. A gradient-structured, highly conductive lithium manganese iron phosphate cathode material, characterized in that, It was prepared by the method for preparing the gradient structure high conductivity lithium manganese iron phosphate cathode material as described in any one of claims 1-6.

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