Gradient co-doped lithium iron manganese phosphate positive electrode material and preparation method thereof

Abstract

The invention discloses a gradient co-doped lithium manganese iron phosphate positive electrode material and a preparation method thereof, and belongs to the technical field of batteries. According to the invention, gradient co-doping is realized by injecting a solution B containing a V dopant step by step, rod-like particles are obtained by combining morphology regulation and control, and then the rod-like particles are subjected to multi-layer coating by Li3PO4, MgO and conductive carbon. A structure with a strong shell and a stable inner core is constructed by gradient distribution of V and Mg, ion transmission, interface stability and conductivity are synergistically improved by multi-layer coating, and the positive electrode material is excellent in rate capability, long in cycle life and suitable for the fields of power batteries and energy storage batteries.

Description

Technical Field

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

[0002] Lithium manganese iron phosphate (LiMn) 1-x Fe x PO4 (LMFP) is an important branch of cathode materials for lithium-ion batteries, boasting high theoretical specific capacity (170 mAh / g) and a stable charge-discharge platform (~3.4 V vs Li). + With its excellent thermal stability and cost advantages, LMFP (Li-FePO4) shows broad application prospects in power batteries, energy storage batteries, and other fields. Compared with lithium iron phosphate (LFP), the energy density of LMFP is significantly improved after the introduction of manganese; compared with ternary cathode materials (NCM / NCA), its cycle stability and safety are more competitive, making it one of the ideal choices for balancing energy density and safety performance.

[0003] However, LMFP materials still face many technical bottlenecks in practical applications, which limit their large-scale promotion: (1) Intrinsic electronic conductivity of LMFP (~10 -10 S / cm) and lithium-ion diffusion coefficient (~10) -14 cm 2 The capacity of the material is far below the commercial requirements, resulting in severe polarization during high-rate charge and discharge, insufficient capacity utilization, and poor rate performance. (2) During the charge-discharge cycle, Li + The insertion / deintercalation of manganese can cause changes in lattice volume, which can lead to lattice distortion, grain cracking, and other problems after long-term cycling, resulting in a decrease in cycle life. Simultaneously, the dissolution of manganese (Mn) 2+ (Dissolving and depositing on the surface of the negative electrode) will disrupt the stability of the electrode-electrolyte interface, further exacerbating the deterioration of battery performance; (3) The interface compatibility between the LMFP surface and the electrolyte is poor, and side reactions are prone to occur to generate an unstable solid electrolyte interphase (SEI) film, which increases the interfacial impedance. In addition, trace amounts of HF in the electrolyte will corrode the material surface and destroy the integrity of the crystal structure. (4) In the prior art, single-element doping is difficult to improve electronic conduction, ion diffusion and lattice stability at the same time; traditional coating layers (such as carbon layers and oxide layers) have problems such as uneven thickness and poor bonding with the substrate, which cannot effectively play the synergistic role of "ion conduction-structure protection-electronic conduction"; in addition, the homogeneous doping mode is difficult to adapt to the performance requirements of different regions inside the material, and cannot achieve the functional zoning of "surface strengthening-core stability".

[0004] To address these issues, the industry has implemented improvements through elemental doping, surface coating, and crystal morphology control. However, existing solutions still suffer from limitations such as limited functionality, poor synergistic effects, and complex processes. Therefore, developing a LMFP cathode material preparation technology that combines high electronic conductivity, rapid lithium-ion diffusion channels, stable lattice structure, and excellent interfacial compatibility is crucial for its large-scale application in high-power, long-cycle lithium-ion batteries. Summary of the Invention

[0005] The technical problem this invention aims to solve is to overcome the shortcomings of existing technologies and provide a gradient co-doped lithium manganese iron phosphate cathode material and its preparation method. Gradient co-doping is achieved through stepwise injection of a B solution containing V dopant, combined with morphology control to obtain rod-shaped particles, which are then coated with multiple layers of Li3PO4, MgO, and conductive carbon. The gradient distribution of V and Mg constructs a "strong outer shell and stable inner core" structure. The multi-layer coating synergistically enhances ion transport, interface stability, and conductivity. This cathode material exhibits excellent rate performance and long cycle life, making it suitable for power batteries and energy storage batteries.

[0006] The technical solution of this invention is as follows: On one hand, the present invention provides a method for preparing a gradient co-doped lithium manganese iron phosphate cathode material, comprising the following steps: S1 Preparation of gradient co-doped precursors S11 Preparation of Solution A: Dissolve manganese sulfate, ferrous sulfate and the first dopant magnesium sulfate in a mixed solvent of water and ethylene glycol (ethylene glycol is used to regulate crystal growth kinetics). S12 Preparation of Solution B: Dissolve phosphoric acid and the second dopant ammonium metavanadate in water, and adjust the pH to 7.5 with ammonia. S13 gradient doping: First, add solution A and half the mass of solution B to a high-pressure reactor, then add a morphology directing agent, and heat to 180-220℃ under stirring, and hold for 1-4 hours; then, inject the remaining solution B into the high-pressure reactor at a uniform rate over 2-5 hours using a high-pressure metering pump; after the addition is complete, continue the reaction at 160-200℃ for 12-24 hours; after the reaction is completed and cooled, collect the precipitate by centrifugation, wash and dry to obtain the precursor; S2 pre-lithiation and crystallization: The precursor, lithium carbonate, and carbon source are ball-milled and mixed in deionized water; after spray drying, they are calcined at 400-500℃ for 3-5 hours in an Ar / H2 mixed gas (95 / 5) to achieve pre-lithiation and preliminary carbonization of the carbon source; the pre-lithiation product is heated to 700-800℃ in a pure Ar atmosphere and held for 10-12 hours to completely crystallize the material and obtain the LMFP primary material; S3 Multi-layer Coating S31 Deposition of Lithium Phosphate Layer: The primary LMFP material is dispersed in an aqueous solution containing lithium dihydrogen phosphate (LiH2PO4) and urea (for pH adjustment by hydrolysis), and stirred at 75-85℃ for 1-3 hours; after washing and drying, an intermediate product coated with a 2-3 nm thick Li3PO4 layer is obtained. S32 Atomic Layer Deposition (ALD) Magnesium Oxide Layer: The above intermediate product is placed in the ALD reaction chamber, and magnesia pyrocene (MgCp2) and water are used as precursors. The mixture is subjected to 50 cycles at 140-160℃ to deposit a dense MgO film with a thickness of 2.5 nm. S33 Chemical Vapor Deposition (CVD) Conductive Carbon Layer: The ALD-treated material is placed in a fluidized bed CVD, and a mixture of acetylene (C2H2) and Ar is introduced at 550-650℃ to deposit a 5 nm thick continuous graphitized carbon layer, thus obtaining the gradient co-doped lithium manganese iron phosphate cathode material.

[0007] Preferably, in step S11, the concentration of manganese sulfate in solution A is 0.25-0.3 mol / L, the concentration of ferrous sulfate is 0.05-0.1 mol / L, and the concentration of the first dopant magnesium sulfate is 0.005-0.01 mol / L; in step S12, the concentration of phosphoric acid in solution B is 1-2 mol / L, and the concentration of the second dopant ammonium metavanadate is 0.05-0.1 mol / L; the volume ratio of solution A to solution B is 6:1.

[0008] Preferably, in step S13, the morphology guiding agent is hexadecyltrimethylammonium bromide (CTAB), and the amount added is 0.1-0.5% of the mass of the gradient co-doped lithium manganese iron phosphate cathode material.

[0009] Preferably, in step S2, lithium carbonate is added at a molar ratio of Li / (Mn+Fe+Mg)=(1-1.1):1; the carbon source is citric acid, which is added at a carbon content of 2wt.% in the final prepared cathode material.

[0010] Preferably, in step S31, the concentration of lithium dihydrogen phosphate in the aqueous solution is 0.1-0.2 mol / L, and the concentration of urea is 0.05-0.1 mol / L.

[0011] On the other hand, the present invention provides a gradient co-doped lithium manganese iron phosphate cathode material, which is prepared by the above-described method for preparing gradient co-doped lithium manganese iron phosphate cathode material.

[0012] Compared with the prior art, the present invention has the following advantages: This invention employs gradient co-doping: utilizing the gradient distribution of V (substituting P sites) and Mg (substituting Li sites), a synergistically enhanced structure of "strong outer shell and stable inner core" is constructed within the particle. High V doping on the surface enhances the covalent nature of the PO bonds, stabilizes the surface structure, and broadens the Li bond width. + Channels; incrementally increasing Mg doping within provides more Li + Vacancies are created and lattice distortion is suppressed. Simultaneously, this invention, through crystal growth regulation, obtains rod-shaped particles preferentially grown along the one-dimensional lithium-ion diffusion channel (

[010] direction), greatly shortening the Li... + The diffusion distance. Furthermore, this invention also designs three coating layers with different functions: the inner layer Li3PO4 is a fast ion conductor, promoting the diffusion of Li... + Interface transport; the middle layer MgO / Al2O3 acts as a physical and chemical barrier, effectively isolating HF in the electrolyte and inhibiting the dissolution of transition metals; the outer carbon layer ensures electron conduction. Detailed Implementation

[0013] 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.

[0014] Example 1 The method for preparing the gradient co-doped lithium manganese iron phosphate cathode material in this embodiment is characterized by comprising the following steps: S1 Preparation of gradient co-doped precursors S11 Preparation of Solution A: Dissolve 8 mol manganese sulfate, 2 mol ferrous sulfate and 0.17 mol magnesium sulfate (the first dopant) in a mixed solvent of 20 L deionized water and 10 L ethylene glycol.

[0015] S12 Preparation of Solution B: Dissolve 6.8 mol of phosphoric acid and 0.34 mol of the second dopant ammonium metavanadate in 5 L of deionized water, and adjust the pH to 7.5 with ammonia.

[0016] S13 Gradient Doping: First, solution A and half the mass of solution B are added to a 50L high-pressure reactor, followed by 20g of morphology-directing agent CTAB. The mixture is heated to 180℃ under stirring and held for 4 hours, at which point Mg-rich and V-poor crystal nuclei are generated. Then, the remaining solution B is injected into the high-pressure reactor at a uniform rate over 3 hours using a high-pressure metering pump. This step is crucial for forming the V concentration gradient: the V source added later is mainly enriched on the surface of the growing crystal, thus forming a gradient distribution where the V concentration decreases from the surface to the interior, and the Mg concentration increases from the surface to the interior. After the addition is complete, the reaction continues at 200℃ for 12 hours. After the reaction is completed and cooled, the precipitate is collected by centrifugation, washed, and dried to obtain a rod-shaped precursor.

[0017] S2 Pre-lithiation and Crystallization: The precursor, lithium carbonate (added at a ratio of Li / (Mn+Fe+Mg)=1:1), and citric acid (added at a final carbon content of 2wt.%) were ball-milled in deionized water for 4 h. After spray drying, the mixture was calcined at 400 °C for 5 h in an Ar / H2 (95 / 5) mixed gas to achieve pre-lithiation and preliminary carbonization of the carbon source. The pre-lithiation product was heated to 700 °C at a evaporation rate of 2 °C / min under a pure Ar atmosphere and held at that temperature for 12 h to allow the material to fully crystallize, yielding the primary LMFP material.

[0018] S3 Multi-layer Coating S31 Lithium Phosphate Deposition: 100g of LMFP primary material was dispersed in 1L of an aqueous solution containing lithium dihydrogen phosphate (0.1mol / L) and urea (0.05mol / L), and the mixture was stirred at 80℃ for 2h. Urea decomposed to release OH-. - The pH was locally increased to promote the uniform deposition of amorphous Li3PO4 on the surface of LMFP particles. After washing and drying, an intermediate product coated with a 2.5 nm thick Li3PO4 layer was obtained.

[0019] S32 atomic layer deposition of magnesium oxide layer: The above intermediate product is placed in the ALD reaction chamber, and magnesia and water are used as precursors. The reaction is carried out at 150°C for 50 cycles to deposit a dense MgO film with a thickness of 2.5 nm. S33 Chemical Vapor Deposition of Conductive Carbon Layer: The ALD-treated material is placed in a fluidized bed CVD, and a mixture of acetylene and Ar gas (volume ratio of acetylene and Ar is 5:95) is introduced at 600℃ to deposit a 5nm thick continuous graphitized carbon layer, thus obtaining the gradient co-doped lithium manganese iron phosphate cathode material.

[0020] Example 2 The method for preparing the gradient co-doped lithium manganese iron phosphate cathode material in this embodiment is characterized by comprising the following steps: S1 Preparation of gradient co-doped precursors S11 Preparation of Solution A: Dissolve 7.5 mol manganese sulfate, 1.5 mol ferrous sulfate and 0.15 mol magnesium sulfate (the first dopant) in a mixed solvent of 20 L deionized water and 10 L ethylene glycol.

[0021] S12 Preparation of Solution B: Dissolve 5 mol of phosphoric acid and 0.25 mol of the second dopant ammonium metavanadate in 5 L of deionized water, and adjust the pH to 7.5 with ammonia.

[0022] S13 Gradient Doping: First, solution A and half the mass of solution B are added to a 50L high-pressure reactor, followed by 20g of morphology-directing agent CTAB. The mixture is heated to 200℃ under stirring and held for 2 hours, at which point Mg-rich and V-poor crystal nuclei are generated. Then, the remaining solution B is injected into the high-pressure reactor at a uniform rate over 2 hours using a high-pressure metering pump. This step is crucial for forming the V concentration gradient: the V source added later is mainly enriched on the surface of the growing crystal, thus forming a gradient distribution where the V concentration decreases from the surface to the interior, and the Mg concentration increases from the surface to the interior. After the addition is complete, the reaction continues at 180℃ for 18 hours. After the reaction is completed and cooled, the precipitate is collected by centrifugation, washed, and dried to obtain a rod-shaped precursor.

[0023] S2 Pre-lithiation and Crystallization: The precursor, lithium carbonate (added at a ratio of Li / (Mn+Fe+Mg)=1:1), and citric acid (added at a final carbon content of 2 wt.%) were ball-milled in deionized water for 4 h. After spray drying, the mixture was calcined at 450 °C for 4 h in an Ar / H2 (95 / 5) mixed gas to achieve pre-lithiation and preliminary carbonization of the carbon source. The pre-lithiation product was heated to 750 °C at a rate of 2 °C / min under a pure Ar atmosphere and held for 11 h to completely crystallize the material, obtaining the primary LMFP material.

[0024] S3 Multi-layer Coating S31 Lithium Phosphate Deposition: 100g of LMFP primary material was dispersed in 1L of an aqueous solution containing lithium dihydrogen phosphate (0.1mol / L) and urea (0.05mol / L), and the mixture was stirred at 75℃ for 3h. Urea decomposed to release OH-. - The pH was locally increased to promote the uniform deposition of amorphous Li3PO4 on the surface of LMFP particles. After washing and drying, an intermediate product coated with a 3 nm thick Li3PO4 layer was obtained.

[0025] S32 atomic layer deposition of magnesium oxide layer: The above intermediate product is placed in the ALD reaction chamber, and magnesia and water are used as precursors. The reaction is carried out at 140°C for 50 cycles to deposit a dense MgO film with a thickness of 2.5 nm. S33 Chemical Vapor Deposition of Conductive Carbon Layer: The ALD-treated material is placed in a fluidized bed CVD, and a mixture of acetylene and Ar gas (volume ratio of acetylene and Ar is 5:95) is introduced at 550℃ to deposit a 5nm thick continuous graphitized carbon layer, thus obtaining the gradient co-doped lithium manganese iron phosphate cathode material.

[0026] Example 3 The method for preparing the gradient co-doped lithium manganese iron phosphate cathode material in this embodiment is characterized by comprising the following steps: S1 Preparation of gradient co-doped precursors S11 Preparation of Solution A: Dissolve 8 mol manganese sulfate, 2 mol ferrous sulfate and 0.17 mol magnesium sulfate (the first dopant) in a mixed solvent of 20 L deionized water and 10 L ethylene glycol.

[0027] S12 Preparation of Solution B: Dissolve 6.8 mol of phosphoric acid and 0.34 mol of the second dopant ammonium metavanadate in 5 L of deionized water, and adjust the pH to 7.5 with ammonia.

[0028] S13 Gradient Doping: First, solution A and half the mass of solution B are added to a 50L high-pressure reactor, followed by 20g of morphology-directing agent CTAB. The mixture is heated to 220℃ under stirring and held for 1 hour, at which point Mg-rich and V-poor crystal nuclei are generated. Then, the remaining solution B is injected into the high-pressure reactor at a uniform rate over 5 hours using a high-pressure metering pump. This step is crucial for forming the V concentration gradient: the V source added later is mainly enriched on the surface of the growing crystal, thus forming a gradient distribution where the V concentration decreases from the surface to the interior, and the Mg concentration increases from the surface to the interior. After the addition is complete, the reaction continues at 160℃ for 24 hours. After the reaction is completed and cooled, the precipitate is collected by centrifugation, washed, and dried to obtain a rod-shaped precursor.

[0029] S2 Pre-lithiation and Crystallization: The precursor, lithium carbonate (added at a ratio of Li / (Mn+Fe+Mg)=1:1), and citric acid (added at a final carbon content of 2wt.%) were ball-milled in deionized water for 4 h. After spray drying, the mixture was calcined at 500 °C for 3 h in an Ar / H2 (95 / 5) mixed gas to achieve pre-lithiation and preliminary carbonization of the carbon source. The pre-lithiation product was heated to 800 °C at a evaporation rate of 2 °C / min under a pure Ar atmosphere and held at that temperature for 10 h to allow the material to fully crystallize, thus obtaining the primary LMFP material.

[0030] S3 Multi-layer Coating S31 Lithium Phosphate Deposition: 100g of LMFP primary material was dispersed in 1L of an aqueous solution containing lithium dihydrogen phosphate (0.1mol / L) and urea (0.05mol / L), and the mixture was stirred at 85℃ for 1h. Urea decomposed to release OH-. -The pH was locally increased to promote the uniform deposition of amorphous Li3PO4 on the surface of LMFP particles. After washing and drying, an intermediate product coated with a 2 nm thick Li3PO4 layer was obtained.

[0031] S32 atomic layer deposition of magnesium oxide layer: The above intermediate product is placed in the ALD reaction chamber, and magnesia and water are used as precursors. The reaction is carried out at 160°C for 50 cycles to deposit a dense MgO film with a thickness of 2.5 nm. S33 Chemical Vapor Deposition of Conductive Carbon Layer: The ALD-treated material is placed in a fluidized bed CVD, and a mixture of acetylene and Ar gas (volume ratio of acetylene and Ar is 5:95) is introduced at 650℃ to deposit a 5nm thick continuous graphitized carbon layer, thus obtaining the gradient co-doped lithium manganese iron phosphate cathode material.

[0032] Comparative Example 1 The difference from Example 1 is that in step S13, all of liquid B is added to the high-pressure reactor together with liquid A at once.

[0033] Comparative Example 2 The difference from Example 1 is that step S31 is not performed.

[0034] Comparative Example 3 The difference from Example 1 is that step S32 is not performed.

[0035] Comparative Example 4 The difference from Example 1 is that step S33 is not performed.

[0036] The positive electrode materials of Examples 1-3 and Comparative Examples 1-4 were assembled into coin cells: the positive electrode material, conductive carbon black, and polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 8:1:1, and NMP was added to prepare a uniform slurry. The slurry was coated onto an aluminum foil current collector and dried under vacuum at 80°C for 12 hours. The slurry was then cut into positive electrode sheets with a diameter of 14 mm. In an argon glove box (water and oxygen content <0.1 ppm), the coin cells were stacked in the following order: positive electrode sheet → electrolyte → Celgard 2400 separator → electrolyte → lithium metal sheet (negative electrode, diameter 16 mm) → gasket → spring sheet → coin cell casing. The cells were then sealed using a battery sealing machine. The assembled coin cells were left to stand at room temperature for 12 hours to allow the electrolyte to fully wet the electrodes and separator, and were ready for testing.

[0037] 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 1-4 In Comparative Example 1, all of the B solution was added at once in step S13, failing to achieve a gradient distribution of V and Mg, resulting in a decrease in the performance of the prepared cathode material. This is because Example 1 achieved a "high V on the surface, high Mg inside" structure through gradient doping: surface V doping enhances the covalent nature of PO bonds and broadens the Li bond... + Transport channels; internal Mg doping provides Li + Vacancies and suppression of lattice distortion are achieved. Comparative Example 1, however, is homogeneous doped with uniform V and Mg distribution, failing to achieve the functional partitioning of "surface-enhanced ion transport + internally stable lattice." Although the electronic conductivity is close to that of Example 1, the lithium-ion diffusion coefficient is only 40% of that of Example 1, resulting in a 14% lower specific capacity at 5C high-rate discharge compared to Example 1. Furthermore, homogeneous doping cannot effectively suppress lattice distortion and Mn during cycling. 2+ Dissolution. The capacity retention of Comparative Example 1 after 200 cycles at 1C was 15.5 percentage points lower than that of Example 1. Mn 2+ The dissolution rate was 1.9 times that of Example 1. Mn 2+ Deposition on the negative electrode surface disrupts the electrode-electrolyte interface, resulting in a significantly higher interfacial impedance than in Example 1, further exacerbating capacity decay. Furthermore, homogeneous doping has limited effect on optimizing the crystal structure; the initial 1C discharge specific capacity is 12.7 mAh / g lower than in Example 1, reflecting that homogeneous doping cannot fully realize the material's capacity potential.

[0038] Comparative Example 2 did not undergo step S31, which involved depositing a lithium phosphate layer; only MgO and conductive carbon coating were retained, resulting in a performance degradation. This is because Li3PO4 is a fast ion conductor, and the 2-3 nm Li3PO4 inner layer in Example 1 can construct a continuous Li... + The Li3PO4 layer acts as a transport channel, reducing interfacial ion transport resistance. In Comparative Example 2, lacking this layer, the lithium-ion diffusion coefficient was only 3.7% of that in Example 1, resulting in a 17.7% lower 5C discharge specific capacity and a significant degradation in high-rate performance. The Li3PO4 layer buffers direct contact between the electrolyte and the LMFP substrate, inhibiting HF erosion of the material surface. In Comparative Example 2, without this protective layer, HF in the electrolyte directly attacks the LMFP surface, leading to Mn... 2+The dissolution rate was 3.2 times that of Example 1; simultaneously, an unstable SEI film easily formed on the surface, and the interfacial impedance was 75% higher than that of Example 1, further deteriorating the ion transport efficiency. The synergistic effect of lattice erosion and increased interfacial impedance resulted in a capacity retention rate of Comparative Example 2 after 200 cycles at 1C that was 23.3 percentage points lower than that of Example 1, making it one of the worst-performing samples in terms of cycling performance. Although the Li3PO4 layer is non-conductive, it can serve as a transition layer between the MgO and LMFP substrates, ensuring the continuity of the subsequent MgO and carbon layer coating. In Comparative Example 2, the lack of this transition layer resulted in a weak bond between MgO and the LMFP surface, easy detachment of the carbon layer, and an electronic conductivity of only 6.25% of that of Example 1, further exacerbating the insufficient capacity utilization.

[0039] Comparative Example 3 did not undergo step S32 atomic layer deposition of magnesium oxide, retaining only Li3PO4 and conductive carbon coating, resulting in a performance degradation. This is because MgO is a dense chemical barrier that effectively isolates HF in the electrolyte and inhibits transition metal dissolution. Without this layer in Comparative Example 3, the chemical protection capability of the Li3PO4 layer is limited, allowing HF to easily penetrate the carbon layer and Li3PO4 layer, eroding the LMFP substrate and leading to Mn... 2+ The dissolution rate was 3.4 times that of Example 1. A large amount of Mn... 2+ Deposited on the negative electrode surface, the SEI film integrity is compromised, resulting in an interfacial impedance 2.5 times that of Example 1, severely hindering electron and ion transport. The lattice structure is eroded by HF, and the increased interfacial impedance leads to a 23.7 mAh / g lower 1C initial discharge specific capacity compared to Example 1, and a 16.5 percentage point lower capacity retention rate after 200 1C cycles compared to Example 1. Although the lithium-ion diffusion coefficient is similar to Comparative Example 2, the higher interfacial impedance results in slightly better high-rate performance, but still significantly lower than Example 1. Furthermore, the dense structure of the MgO layer supports the uniform deposition of the outer carbon layer, preventing carbon agglomeration or detachment. In Comparative Example 3, the absence of the MgO layer weakens the bonding between the carbon layer and the Li3PO4 layer, resulting in a discontinuous conductive network and an electronic conductivity only 4.2% of that of Example 1, further exacerbating capacity decay.

[0040] Comparative Example 4 omitted step S33 (chemical vapor deposition of a conductive carbon layer), retaining only Li3PO4 and MgO coatings, resulting in decreased performance. This is because the conductive carbon layer is the core channel for electron transport. In Example 1, the 5nm graphitized carbon layer constructs a continuous conductive network, ensuring rapid electron migration. In Comparative Example 4, lacking this layer, electrons must transport through the insulating Li3PO4 and MgO layers, significantly reducing electronic conductivity and resulting in a 11% lower 5C discharge specific capacity compared to Example 1, leading to insufficient high-rate discharge capability. The conductive carbon layer further isolates the electrolyte from the inner coating layer, reducing HF erosion of the MgO and Li3PO4 layers. In Comparative Example 4, lacking this layer, the MgO layer is directly exposed to the electrolyte, making it susceptible to HF corrosion, leading to decreased Mn...2+ The dissolution rate was 3.9 times that of Example 1, the highest among all samples. Simultaneously, the interfacial impedance reached 3.1 times that of Example 1, severely hindering the electrochemical reaction. Furthermore, the interruption of electron transport and severe interfacial side reactions resulted in a 17.9 mAh / g lower initial discharge specific capacity at 1C compared to Example 1, and a 14.4 percentage point lower capacity retention rate after 200 1C cycles compared to Example 1, indicating a significant deterioration in both cycle stability and capacity performance.

Claims

1. A method for preparing a gradient co-doped lithium manganese iron phosphate cathode material, characterized in that, Includes the following steps: S1 Preparation of gradient co-doped precursors S11 Preparation of Solution A: Dissolve manganese sulfate, ferrous sulfate and the first dopant magnesium sulfate in a mixed solvent of water and ethylene glycol; S12 Preparation of Solution B: Dissolve phosphoric acid and the second dopant ammonium metavanadate in water, and adjust the pH to 7.5 with ammonia. S13 gradient doping: First, add solution A and half the mass of solution B to a high-pressure reactor, then add a morphology directing agent, and heat to 180-220℃ under stirring, and hold for 1-4 hours; then, inject the remaining solution B into the high-pressure reactor at a uniform rate over 2-5 hours using a high-pressure metering pump; after the addition is complete, continue the reaction at 160-200℃ for 12-24 hours; after the reaction is completed and cooled, collect the precipitate by centrifugation, wash and dry to obtain the precursor; S2 pre-lithiation and crystallization: The precursor, lithium carbonate, and carbon source are ball-milled and mixed in deionized water; after spray drying, they are calcined at 400-500℃ for 3-5 hours in an Ar / H2 mixed gas to achieve pre-lithiation and preliminary carbonization of the carbon source; the pre-lithiation product is heated to 700-800℃ in a pure Ar atmosphere and held for 10-12 hours to completely crystallize the material and obtain the LMFP primary material; S3 Multi-layer Coating S31 Deposition of Lithium Phosphate Layer: The primary LMFP material is dispersed in an aqueous solution containing lithium dihydrogen phosphate and urea, and stirred at 75-85℃ for 1-3 hours; after washing and drying, an intermediate product coated with a 2-3 nm thick Li3PO4 layer is obtained. S32 atomic layer deposition of magnesium oxide layer: The above intermediate product is placed in the ALD reaction chamber, and magnesia and water are used as precursors. The reaction is carried out at 140-160℃ for 50 cycles to deposit a dense MgO film with a thickness of 2.5 nm. S33 Chemical Vapor Deposition of Conductive Carbon Layer: The ALD-treated material is placed in a fluidized bed CVD, and a mixture of acetylene and Ar gas is introduced at 550-650℃ to deposit a 5nm thick continuous graphitized carbon layer, thus obtaining the gradient co-doped lithium manganese iron phosphate cathode material.

2. The method for preparing gradient co-doped lithium manganese iron phosphate cathode material as described in claim 1, characterized in that, In step S11, the concentration of manganese sulfate in solution A is 0.25-0.3 mol / L, the concentration of ferrous sulfate is 0.05-0.1 mol / L, and the concentration of the first dopant magnesium sulfate is 0.005-0.01 mol / L; in step S12, the concentration of phosphoric acid in solution B is 1-2 mol / L, and the concentration of the second dopant ammonium metavanadate is 0.05-0.1 mol / L; the volume ratio of solution A to solution B is 6:

1.

3. The method for preparing gradient co-doped lithium manganese iron phosphate cathode material as described in claim 1, characterized in that, In step S13, the morphology guiding agent is hexadecyltrimethylammonium bromide, and the amount added is 0.1-0.5% of the mass of the gradient co-doped lithium manganese iron phosphate cathode material.

4. The method for preparing gradient co-doped lithium manganese iron phosphate cathode material as described in claim 1, characterized in that, In step S2, lithium carbonate is added at a molar ratio of Li / (Mn+Fe+Mg)=(1-1.1):1; the carbon source is citric acid, which is added at a carbon content of 2wt.% in the final cathode material.

5. The method for preparing gradient co-doped lithium manganese iron phosphate cathode material as described in claim 1, characterized in that, In step S31, the concentration of lithium dihydrogen phosphate in the aqueous solution is 0.1-0.2 mol / L, and the concentration of urea is 0.05-0.1 mol / L.

6. A gradient co-doped lithium manganese iron phosphate cathode material, characterized in that, It was prepared by the method for preparing gradient co-doped lithium manganese iron phosphate cathode material as described in any one of claims 1-5.

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