A core-shell structure lithium manganese iron phosphate positive electrode material with double carbon layers and gradient interfaces and a preparation method thereof

Abstract

The application relates to a core-shell structure manganese iron lithium phosphate positive electrode material with double carbon layers and a gradient interface and a preparation method thereof, and belongs to the technical field of lithium ion battery positive electrode materials. x Fe 1‑x PO4(0.5<=x<=0.8) inner core, an inner carbon layer, a gradient transition zone, iron-rich LiMn y Fe 1‑y PO4(0.1<=y<=0.4) outer shell and an outer carbon layer, and presents a five-layer composite structure. The preparation method comprises the following steps: (1) taking an oxide manganese source as raw material, preparing an inner carbon layer coated manganese-rich body through low-temperature pre-sintering and dry ball-milling carbon coating; (2) uniformly coating an iron-rich outer layer precursor on the surface of the body by using a sol-gel assisted evaporation method; and (3) forming a gradient transition zone by inducing element diffusion through secondary sintering and in-situ forming an outer carbon layer. The application solves the technical problems of low reactivity of the oxide manganese source and the ion transmission hindered by the traditional coating layer, and the obtained material has a first discharge specific capacity of 150-160 mAh g –1 , a 5C rate capacity retention rate of >80%, a capacity retention rate of >95% after 1C cycle for 100 times, and excellent rate performance and cycle stability.

Description

Technical Field

[0001] This invention relates to a core-shell structure lithium manganese iron phosphate cathode material with a double carbon layer and a gradient interface, and its preparation method, belonging to the technical field of lithium-ion battery cathode materials. Background Technology

[0002] Lithium manganese iron phosphate (LiMn) x Fe 1-x LiFePO4 (LiMnPO4), as an olivine-type cathode material, combines the high safety and good cycle stability of lithium iron phosphate (LiFePO4) with the high operating voltage characteristics of lithium manganese phosphate (LiMnPO4) (approximately 4.1V vs. Li / Li). + The theoretical specific capacity reaches 170 mAh g. –1 Energy density can reach 700 Wh kg –1 It is considered one of the ideal cathode materials for next-generation high-energy-density power batteries and energy storage batteries.

[0003] However, LMFP materials face the following major technical challenges: (1) Extremely low intrinsic conductivity: The electronic conductivity of LiMnPO4 is approximately 10. –10 S cm –1 The lithium-ion diffusion coefficient is approximately 10. –16 cm 2 s –1 It is much lower than that of LiFePO4 (electronic conductivity 10). –9 S cm –1 Lithium-ion diffusion coefficient 10 –14 cm 2 s –1 This results in poor rate performance of the material, with severely insufficient capacity utilization at high current densities.

[0004] (2) The Jahn-Teller effect leads to structural instability: During charging and discharging, Mn 3+ The presence of Mn induces a strong Jahn-Teller effect, leading to lattice distortion and volume changes, resulting in poor cyclic stability of the material. Meanwhile, Mn... 2+ It is easily dissolved in the electrolyte, which further exacerbates capacity decay.

[0005] (3) Difficulty in applying manganese oxide sources: In industrial production, battery-grade manganese oxide sources (such as Mn3O4 and MnO) have the advantages of low cost and wide availability, but their reactivity is much lower than that of soluble manganese salts. The thermal decomposition temperature of Mn3O4 is as high as 900℃ or more. When synthesizing directly using the traditional solid-state method, it often requires high temperature above 800℃ and long sintering time (>15 hours), which leads to excessive grain growth (particle size >5 μm), serious lithium volatilization loss (excess lithium needs to reach 10-15%), and a significant increase in energy consumption.

[0006] To address these issues, researchers have developed various modification strategies, including: Carbon coating technology: This technology improves electronic conductivity by coating the surface of materials with a conductive carbon layer. However, traditional carbon coating usually employs simple physical mixing or wet coating methods, which have the following drawbacks: (i) the carbon layer is unevenly distributed, resulting in a discontinuous conductive network; (ii) the carbon layer often forms a dense coating layer (20-50 nm thick) on the particle surface, which, while improving electronic conductivity, also hinders lithium-ion transport, especially during high-current charging and discharging (≥3C), where ion transport impedance becomes the main limiting factor, leading to increased polarization and capacity loss; (iii) a single carbon layer is difficult to establish an effective electron transport channel within micron-sized particles.

[0007] Core-shell structure design: By constructing a "manganese-rich core-iron-rich shell" structure, the manganese-rich core provides high capacity and high voltage, while the iron-rich shell provides structural stability. Existing core-shell structure materials are usually prepared by liquid-phase co-precipitation or hydrothermal methods, which have the following problems: ① The process is complex, requiring precise control of multiple parameters such as pH, temperature, and reaction time; ② It relies on expensive spray drying or freeze-drying equipment, resulting in large equipment investment; ③ It generates a large amount of wastewater containing manganese and iron ions, resulting in high treatment costs and significant environmental pressure; ④ Simple core-shell structures often exhibit significant lattice mismatch and stress concentration at the interface, resulting in insufficient bonding and easy shell peeling during cycling; ⑤ If there is no compositional gradient transition at the interface, lithium ions will encounter significant transport resistance when crossing the core-shell interface.

[0008] Nanotechnology: shortens ion diffusion distance by reducing particle size. However, nanomaterials suffer from low tap density (<0.8 g cm⁻¹). –3 Its large specific surface area leads to increased side reactions and poor processing performance, making it difficult to meet the requirements of high energy density batteries.

[0009] Element doping: Doping with elements such as Ti, V, and Mg can improve electrical conductivity and structural stability. However, doping elements often occupy lithium sites or transition metal sites, reducing the theoretical capacity of the material. Moreover, the doping process is complex and costly.

[0010] In summary, existing technologies still face numerous challenges in preparing high-performance LMFP materials using low-cost manganese oxide sources. In particular, key technical issues that urgently need to be addressed include how to ensure high material capacity while simultaneously considering electronic conductivity and ion diffusion kinetics, constructing a composite coating structure that neither hinders ion transport nor hinders efficient electron channels, and how to achieve tight bonding and gradient transition at the core-shell interface through simple and controllable processes.

[0011] Therefore, developing a simple, low-cost, and environmentally friendly technology that can fully utilize manganese oxide sources and achieve high-performance LMFP cathode materials through innovative structural design and preparation processes is of great practical significance and application value for promoting the industrialization of lithium-ion batteries and reducing production costs. Summary of the Invention

[0012] To address the technical problems of existing lithium manganese iron phosphate materials, such as low electronic conductivity, small ion diffusion coefficient, poor cycle stability, low reactivity of manganese oxide source, traditional carbon coating hindering ion transport, and loose core-shell interface bonding, this invention provides a core-shell structured lithium manganese iron phosphate cathode material with a double carbon layer and gradient interface, and its preparation method.

[0013] This invention utilizes a combined process of "stepwise sintering - dry carbon coating - sol-gel evaporation - gradient construction" to prepare lithium manganese iron phosphate cathode materials with a five-layer composite structure consisting of a "manganese-rich core - inner carbon layer - gradient transition region - iron-rich shell - outer carbon layer". Specifically, low-temperature pre-sintering activates the manganese oxide source, dry ball milling forms the inner carbon layer, the sol-gel method precisely coats the outer layer, and secondary sintering induces interfacial gradient formation, achieving synergistic optimization of electron conduction and ion transport.

[0014] The present invention is achieved through the following technical solution.

[0015] A method for preparing a core-shell structured lithium manganese iron phosphate cathode material with a double carbon layer and a gradient interface includes the following steps: Step 1, Preparation of manganese-rich body coated with inner carbon layer: Lithium source, phosphorus source, manganese oxide source and iron source are mixed in stoichiometric ratio, ball milling media are added and wet ball milling is performed, and vacuum drying is performed to obtain mixture A; Mixture A was subjected to low-temperature pre-sintering at 350–450°C for 2–6 hours under an inert atmosphere to obtain precursor B; The first carbon source is added to the precursor B and dry ball milling is performed. Then, high-temperature sintering is carried out at 600-750℃ for 6-12 hours under an inert or reducing atmosphere to obtain a manganese-rich bulk material with an inner carbon layer. Step 2, Coating of iron-rich outer layer precursor: Dissolve lithium source, manganese source, iron source and phosphorus source in deionized water, adjust the pH value to 4-7, and prepare coating solution; The manganese-rich bulk material prepared in step 1 was added to the coating solution, ultrasonically dispersed, and stirred at 60-90°C to evaporate the solvent until a gel-like slurry was formed. After further drying, it was treated in a vacuum oven to obtain the core-shell precursor powder. Step 3: Secondary sintering to build a gradient structure: The core-shell precursor powder obtained in step 2 is sintered at 600-700℃ for 4-8 hours under an inert or reducing atmosphere to induce the diffusion of interfacial elements to form a gradient transition region. At the same time, the outer layer crystallizes and carbonizes in situ to obtain a core-shell structure lithium manganese iron phosphate cathode material with a double carbon layer and a gradient interface.

[0016] In step 1, the manganese oxide source is at least one of Mn3O4, MnO2, and MnO.

[0017] In step 1, the molar ratio of the lithium source, phosphorus source, manganese oxide source, and iron source, Li:Mn:Fe:P, is 1:(0.5~0.8):(0.2~0.5):1; In step 1, the lithium source is one or a mixture of several of lithium carbonate, lithium hydroxide, lithium dihydrogen phosphate, and lithium acetate in any proportion; The phosphorus source is one or a mixture of several of the following in any proportion: ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and lithium dihydrogen phosphate; The iron source is one or a mixture of several of ferrous oxalate, ferrous acetate, ferric oxide, and ferric phosphate in any proportion.

[0018] In step 1, the wet ball milling speed is 300-500 rpm, the ball milling time is 4-10 hours, and the ball-to-material ratio is 8:1-15:1.

[0019] In step 1, the heating rate of the low-temperature pre-sintering is 5-10℃ / min.

[0020] In step 1, the first carbon source is one or more of sucrose, glucose, starch, and acetylene black, and the amount added is 5 to 10 wt% of the mass of precursor B.

[0021] In step 1, the dry ball milling does not add liquid media, the ball milling speed is ≥300 rpm, and the ball milling time is 6 to 12 hours.

[0022] In step 1, the heating rate of the high-temperature sintering is 5-10℃ / min; the reducing atmosphere is an Ar / H2 mixture with an H2 volume fraction of 2-8%.

[0023] In step 2, the molar ratio of lithium source, manganese source, iron source, and phosphorus source, Li:Mn:Fe:P, is 1:(0.1~0.4):(0.6~0.9):1; The lithium source in step 2 is one or a mixture of lithium acetate, lithium hydroxide, and lithium nitrate in any proportion. The manganese source is one or a mixture of manganese acetate, manganese nitrate, manganese sulfate, and manganese chloride in any proportion; The iron source is one or a mixture of several of ferric citrate, ferric nitrate, and ferrous acetate in any proportion; The phosphorus source is one or a mixture of several of phosphoric acid, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate in any proportion.

[0024] In step 2, the iron source is ferric citrate, and citric acid also serves as a chelating agent and a second carbon source. If the iron source is other, citric acid, EDTA, and polyacrylic acid need to be added as chelating agents and second carbon sources.

[0025] In step 2, the gel-like slurry is formed by heating in an oil bath or by stirring and evaporating on a heating plate, with an evaporation temperature of 60–90°C.

[0026] In step 2, the temperature of the vacuum oven treatment is 100-120°C, and the time is 8-15 hours.

[0027] In step 3, the heating rate of the secondary sintering is 3 to 10 °C / min.

[0028] In step 3, the secondary sintering temperature is lower than or equal to the high-temperature sintering temperature in step 1, and the temperature difference is controlled within the range of 0 to 100°C.

[0029] In step 3, the inert or reducing atmosphere is at least one of nitrogen, argon, or a mixture of argon and hydrogen. When a mixture of argon and hydrogen is used, the volume fraction of H2 is 3 to 6%.

[0030] A core-shell structure lithium manganese iron phosphate cathode material with a double carbon layer and gradient interface includes a manganese-rich core, an inner carbon layer, a gradient transition region, an iron-rich outer shell, and an outer carbon layer.

[0031] The manganese-rich core is composed of LiMn x Fe 1-x PO4 (0.5≤x≤0.8), with an average particle size of 0.5–2.0 μm; the inner carbon layer thickness is 5–15 nm; the gradient transition region thickness is 20–50 nm; the iron-rich outer shell is composed of LiMn. y Fe 1-y PO4 (0.1≤y≤0.4), with a thickness of 50–200 nm; the outer carbon layer has a thickness of 10–30 nm; The manganese-rich core comprises 75wt% to 90wt%, and the iron-rich outer shell includes an outer carbon layer comprising 10wt% to 25wt%.

[0032] A lithium-ion battery is made using the above-mentioned positive electrode material as the positive electrode active material.

[0033] The beneficial effects of this invention are: The lithium manganese iron phosphate cathode material with a double carbon layer gradient structure prepared in this invention has the following advantages: First, the double carbon layer structure (inner carbon layer + outer carbon layer) provides a continuous electron conduction network, significantly improving the material's electronic conductivity. Second, the gradient transition region eliminates abrupt changes at the core-shell interface, reducing ion transport impedance and achieving synergistic electron-ion transport. The iron-rich shell inhibits Mn dissolution and enhances structural stability. The material achieves a specific capacity of 150-160 mAh g⁻¹ at its first discharge at 0.1C. –1 The capacity retention rate at 5C rate is >80%, and the capacity retention rate after 100 cycles at 1C is >95%. Furthermore, this invention uses a manganese oxide source, which is simple, environmentally friendly, low-cost, and easy to scale up for production. Attached Figure Description

[0034] Figure 1 This is a schematic diagram of the process flow for preparing a core-shell structured lithium manganese iron phosphate cathode material with a double carbon layer and a gradient interface in an embodiment of the present invention.

[0035] Figure 2 This is a schematic diagram of the microstructure model of the cathode material described in this invention, showing the five-layer distribution relationship of the manganese-rich core, inner carbon layer, gradient transition region, iron-rich outer shell, and outer carbon layer. Detailed Implementation

[0036] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0037] A method for preparing a core-shell structured lithium manganese iron phosphate cathode material with a double carbon layer and a gradient interface includes the following steps: Step 1, Preparation of manganese-rich body coated with inner carbon layer: Lithium source, phosphorus source, manganese oxide source and iron source are mixed in stoichiometric ratio, ball milling media are added and wet ball milling is performed, and vacuum drying is performed to obtain mixture A; Mixture A was subjected to low-temperature pre-sintering at 350–450°C for 2–6 hours under an inert atmosphere to obtain precursor B; The first carbon source is added to the precursor B and dry ball milling is performed. Then, high-temperature sintering is carried out at 600-750℃ for 6-12 hours under an inert or reducing atmosphere to obtain a manganese-rich bulk material with an inner carbon layer. Step 2, Coating of iron-rich outer layer precursor: Dissolve lithium source, manganese source, iron source and phosphorus source in deionized water, adjust the pH value to 4-7, and prepare coating solution; The manganese-rich bulk material prepared in step 1 was added to the coating solution, ultrasonically dispersed, and stirred at 60-90°C to evaporate the solvent until a gel-like slurry was formed. After further drying, it was treated in a vacuum oven to obtain the core-shell precursor powder. Step 3: Secondary sintering to build a gradient structure: The core-shell precursor powder obtained in step 2 is sintered at 600-700℃ for 4-8 hours under an inert or reducing atmosphere to induce the diffusion of interfacial elements to form a gradient transition region. At the same time, the outer layer crystallizes and carbonizes in situ to obtain a core-shell structure lithium manganese iron phosphate cathode material with a double carbon layer and a gradient interface.

[0038] In some embodiments, in step 1, the manganese oxide source is at least one of Mn3O4, MnO2, and MnO.

[0039] In some embodiments, in step 1, the molar ratio of the lithium source, phosphorus source, manganese oxide source, and iron source, Li:Mn:Fe:P, is 1:(0.5-0.8):(0.2-0.5):1; for example, in some embodiments, the molar ratio of the lithium source, phosphorus source, manganese oxide source, and iron source, Li:Mn:Fe:P, is 1:0.5:0.2:1; the molar ratio of the lithium source, phosphorus source, manganese oxide source, and iron source, Li:Mn:Fe:P, is 1:0.6:0.4:1; or the molar ratio of the lithium source, phosphorus source, manganese oxide source, and iron source, Li:Mn:Fe:P, is 1:0.8:0.5:1. In step 1, the lithium source is one or a mixture of several of lithium carbonate, lithium hydroxide, lithium dihydrogen phosphate, and lithium acetate in any proportion; The phosphorus source is one or a mixture of several of the following in any proportion: ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and lithium dihydrogen phosphate; The iron source is one or a mixture of several of ferrous oxalate, ferrous acetate, ferric oxide, and ferric phosphate in any proportion.

[0040] In some embodiments, in step 1, the rotational speed of the wet ball mill is 300 to 500 rpm. For example, in some embodiments, the rotational speed of the wet ball mill is 300 rpm, 400 rpm, or 500 rpm, the milling time is 4 hours, 6 hours, 8 hours, or 10 hours, and the ball-to-material ratio is 8:1 to 15:1. In some embodiments, for example, the ball-to-material ratio is 8:1, 10:1, or 15:1.

[0041] In some embodiments, the heating rate of the low-temperature pre-sintering in step 1 is 5 to 10 °C / min. In some embodiments, for example, the heating rate of the low-temperature pre-sintering is 5 °C / min, 8 °C / min, or 10 °C / min.

[0042] In some embodiments, in step 1, the first carbon source is one or more of sucrose, glucose, starch, and acetylene black, and the amount added is 5 to 10 wt% of the mass of precursor B. In some embodiments, for example, the amount added is 5 wt%, 8 wt%, or 10 wt% of the mass of precursor B.

[0043] In some implementations, in step 1, the dry ball milling does not involve the addition of a liquid medium, the ball milling speed is ≥300 rpm, and the ball milling time is 6 to 12 hours. In some implementations, for example, the ball milling time is 6 hours, 7 hours, or 12 hours.

[0044] In some embodiments, in step 1, the heating rate of the high-temperature sintering is 5 to 10 °C / min. In some embodiments, for example, the heating rate of the high-temperature sintering is 5 °C / min, 8 °C / min, or 10 °C / min. The reducing atmosphere is an Ar / H2 mixture with an H2 volume fraction of 2 to 8%. In some embodiments, for example, the H2 volume fraction is 2%, 4%, 6%, or 8%.

[0045] In some implementation schemes, in step 2, the molar ratio of lithium source, manganese source, iron source, and phosphorus source is Li:Mn:Fe:P = 1:(0.1~0.4):(0.6~0.9):1; The lithium source in step 2 is one or a mixture of lithium acetate, lithium hydroxide, and lithium nitrate in any proportion. The manganese source is one or a mixture of manganese acetate, manganese nitrate, manganese sulfate, and manganese chloride in any proportion; The iron source is one or a mixture of several of ferric citrate, ferric nitrate, and ferrous acetate in any proportion; The phosphorus source is one or a mixture of several of phosphoric acid, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate in any proportion.

[0046] In some implementations, in step 2, the iron source is ferric citrate, with citric acid also serving as a chelating agent and a second carbon source; if the iron source is other, citric acid, EDTA, and polyacrylic acid need to be added as chelating agents and second carbon sources.

[0047] In some implementations, in step 2, the gel-like slurry is formed by heating in an oil bath or by stirring and evaporating on a heating plate at a temperature of 60–90°C, such as 60°C, 70°C, 80°C, or 90°C.

[0048] In some implementations, in step 2, the temperature of the vacuum oven treatment is 100-120°C and the time is 8-15 hours; for example, the temperature of the vacuum oven treatment is 100°C, 110°C or 120°C and the time is 8 hours, 10 hours or 15 hours.

[0049] In some embodiments, in step 3, the heating rate of the secondary sintering is 3 to 10 °C / min; the heating rate is 3 °C / min, 5 °C / min, 8 °C / min or 10 °C / min.

[0050] In some implementations, in step 3, the secondary sintering temperature is lower than or equal to the high-temperature sintering temperature in step 1, and the temperature difference is controlled within the range of 0 to 100°C.

[0051] In some embodiments, in step 3, the inert or reducing atmosphere is at least one of nitrogen, argon, or an argon-hydrogen mixture, and when an argon-hydrogen mixture is used, the H2 volume fraction is 3-6%. Example 1

[0052] Preparation target: Core LiMn 0.6 Fe 0.4 PO4, outer LiMn 0.2 Fe 0.8 PO4 (10% coating) like Figure 1 As shown, the preparation method of the core-shell structure lithium manganese iron phosphate cathode material with a double carbon layer and gradient interface includes the following specific steps: (1) Preparation of manganese-rich body coated with inner carbon layer Weigh out 4.16 g of lithium dihydrogen phosphate (LiH2PO4), 3.66 g of manganese trioxide (Mn3O4), and 4.32 g of ferrous oxalate dihydrate (FeC2O4·2H2O) according to a molar ratio of Li:Mn:Fe:P = 1:0.6:0.4:1. Add 30 mL of anhydrous ethanol and ball mill in a planetary ball mill at 300 rpm for 6 hours, with a ball-to-material ratio of 10:1. Vacuum dry at 80 °C for 12 hours to obtain mixture A. Place mixture A in a tube furnace and heat to 400 °C at 5 °C / min under an argon atmosphere, hold at that temperature for 4 hours for low-temperature pre-sintering, and allow to cool naturally to room temperature to obtain precursor B. Add 5 wt% glucose (approximately 0.5 g) to precursor B and dry ball mill for 12 hours (300 rpm). The dry-ball-milled powder was placed in a tube furnace and sintered at 680°C at a rate of 10°C / min under an argon atmosphere for 8 hours. After natural cooling to room temperature, the powder was ground and sieved to obtain a manganese-rich bulk material with an inner carbon layer (Core / C). in ).

[0053] (2) Coating of iron-rich outer precursor According to LiMn 0.2 Fe 0.8The stoichiometric ratio of PO4 (coating amount 10%) was determined by weighing 0.22 g of lithium acetate dihydrate (LiCH3COO·2H2O), 0.11 g of manganese acetate tetrahydrate (Mn(CH3COO)2·4H2O), 0.42 g of ferric citrate (FeC6H5O7), and 0.21 g of phosphoric acid (H3PO4, 85%), dissolving them in 50 mL of deionized water, and adjusting the pH to 6.0 with ammonia to prepare the coating solution. 3 g of the manganese-rich bulk material prepared in step (1) was added to the coating solution and ultrasonically dispersed for 30 minutes to ensure uniform dispersion of the bulk particles. The mixture was placed in an 80°C oil bath and mechanically stirred (400 rpm) to continuously evaporate the solvent until a viscous gel-like slurry was formed. The mixture was then transferred to an oven and dried at 110°C for 12 hours, followed by grinding and sieving to obtain the core-shell precursor powder.

[0054] (3) Secondary firing structure to build gradient structure The core-shell precursor powder was placed in a tube furnace and heated to 650°C at a heating rate of 5°C / min under an argon atmosphere, and held at that temperature for 5 hours for secondary sintering. After natural cooling to room temperature, it was ground and sieved to obtain LiMn, a lithium manganese iron phosphate cathode material with a double carbon layer and manganese iron gradient structure. 0.6 Fe 0.4 PO4@LiMn 0.2 Fe 0.8 PO4 / C, with a manganese-rich core of approximately 87 wt%, an iron-rich outer shell of approximately 9 wt%, and a carbon content of approximately 4 wt%, the specific structure is as follows: Figure 2 As shown. Example 2

[0055] Preparation target: Core LiMn 0.7 Fe 0.3 PO4, outer LiMn 0.3 Fe 0.7 PO4 (15% coating) like Figure 1 As shown, the preparation method of the core-shell structure lithium manganese iron phosphate cathode material with a double carbon layer and gradient interface includes the following specific steps: (1) Preparation of manganese-rich body coated with inner carbon layer Weigh out 4.16 g of lithium dihydrogen phosphate, 6.41 g of manganese tetroxide, and 2.156 g of ferrous oxalate dihydrate according to a molar ratio of Li:Mn:Fe:P = 1:0.7:0.3:1. Add 35 mL of anhydrous ethanol and ball mill in a planetary ball mill at 350 rpm for 8 hours, with a ball-to-material ratio of 12:1. Vacuum dry at 90 °C for 10 hours to obtain mixture A. Place mixture A in a tube furnace and heat to 420 °C at 10 °C / min under a nitrogen atmosphere, hold for 5 hours for low-temperature pre-sintering, and allow to cool naturally to room temperature to obtain precursor B. Add 6 wt% sucrose (approximately 0.6 g) to precursor B and dry ball mill for 10 hours (350 rpm). Place the dry-ball-milled powder in a tube furnace and heat to 680 °C at 10 °C / min under an argon atmosphere, hold for 10 hours for high-temperature sintering. After naturally cooling to room temperature, the material is ground and sieved to obtain a manganese-rich bulk material with an inner carbon layer (Core / C). in ).

[0056] (2) Coating of iron-rich outer precursor According to LiMn 0.3 Fe 0.7 The stoichiometric ratio of PO4 (coating amount 15%) was determined by weighing 0.34 g lithium acetate dihydrate, 0.25 g manganese acetate tetrahydrate, 0.58 g ferric citrate, and 0.33 g phosphoric acid, dissolving them in 60 mL of deionized water, and adjusting the pH to 6.5 with ammonia to prepare the coating solution. 3 g of the manganese-rich bulk material prepared in step (1) was added to the coating solution and ultrasonically dispersed for 35 minutes to ensure uniform dispersion of the bulk particles. The mixture was placed in an 85°C oil bath and mechanically stirred (400 rpm) to continuously evaporate the solvent until a viscous gel-like slurry was formed. The mixture was then transferred to an oven and dried at 110°C for 14 hours, followed by grinding and sieving to obtain the core-shell precursor powder.

[0057] (3) Secondary firing structure to build gradient structure The core-shell precursor powder was placed in a tube furnace and heated to 660°C at a heating rate of 5°C / min under an argon atmosphere, and held at that temperature for 6 hours for secondary sintering. After natural cooling to room temperature, it was ground and sieved to obtain LiMn, a lithium manganese iron phosphate cathode material with a double carbon layer and manganese iron gradient structure. 0.7 Fe 0.3 PO4@ LiMn 0.3 Fe 0.7 PO4 / C, with a manganese-rich core of approximately 82 wt%, an iron-rich outer shell of approximately 13 wt%, and a carbon content of approximately 5 wt%, the specific structure is as follows: Figure 2 As shown. Example 3

[0058] Preparation target: Core LiMn 0.5 Fe0.5 PO4, outer LiMn 0.1 Fe 0.9 PO4 (20% coating) like Figure 1 As shown, the preparation method of the core-shell structure lithium manganese iron phosphate cathode material with a double carbon layer and gradient interface includes the following specific steps: (1) Preparation of manganese-rich bulk material coated with inner carbon layer Weigh out 4.16 g of lithium dihydrogen phosphate, 4.58 g of manganese tetroxide, and 3.60 g of ferrous oxalate dihydrate according to a molar ratio of Li:Mn:Fe:P = 1:0.5:0.5:1. Add 40 mL of anhydrous ethanol and ball mill in a planetary ball mill at 400 rpm for 5 hours, with a ball-to-material ratio of 8:1. Vacuum dry at 85 °C for 12 hours to obtain mixture A. Place mixture A in a tube furnace and heat to 380 °C at 10 °C / min under an argon atmosphere for 6 hours for low-temperature pre-sintering. Allow to cool naturally to room temperature to obtain precursor B. Add 5 wt% glucose (approximately 0.5 g) to precursor B and dry ball mill for 8 hours (400 rpm). Place the dry-ball-milled powder in a tube furnace and heat to 650 °C at 10 °C / min under an argon-hydrogen mixed atmosphere (5% H2 volume fraction) for 12 hours for high-temperature sintering. After naturally cooling to room temperature, the material is ground and sieved to obtain a manganese-rich bulk material with an inner carbon layer (Core / C). in ).

[0059] (2) Coating of iron-rich outer precursor According to LiMn 0.1 Fe 0.9 The stoichiometric ratio of PO4 (coating amount 20%) was determined by weighing 0.49 g lithium acetate dihydrate, 0.12 g manganese acetate tetrahydrate, 1.05 g ferric citrate, and 0.47 g phosphoric acid, dissolving them in 70 mL of deionized water, and adjusting the pH to 5.5 with ammonia to prepare the coating solution. 3 g of the manganese-rich bulk material prepared in step (1) was added to the coating solution and ultrasonically dispersed for 40 minutes to ensure uniform dispersion of the bulk particles. The mixture was placed in a 75°C oil bath and mechanically stirred (350 rpm) to continuously evaporate the solvent until a viscous gel-like slurry was formed. The mixture was then transferred to an oven and dried at 100°C for 15 hours, followed by grinding and sieving to obtain the core-shell precursor powder.

[0060] (3) Secondary firing structure to build gradient structure The core-shell precursor powder was placed in a tube furnace and heated to 620°C at a heating rate of 5°C / min under an argon atmosphere, and held at that temperature for 7 hours for secondary sintering. After natural cooling to room temperature, it was ground and sieved to obtain LiMn, a lithium manganese iron phosphate cathode material with a double carbon layer and manganese iron gradient structure. 0.5 Fe 0.5PO4@ LiMn 0.1 Fe 0.9 PO4 / C, with a manganese-rich core of approximately 78 wt%, an iron-rich outer shell of approximately 17 wt%, and a carbon content of approximately 5 wt%, the specific structure is as follows: Figure 2 As shown. Example 4

[0061] Preparation target: Core LiMn 0.8 Fe 0.2 PO4, outer LiMn 0.2 Fe 0.8 PO4 (12% coating content) like Figure 1 As shown, the preparation method of the core-shell structure lithium manganese iron phosphate cathode material with a double carbon layer and gradient interface includes the following specific steps: (1) Preparation of manganese-rich bulk material coated with inner carbon layer Weigh out 4.16 g of lithium dihydrogen phosphate, 7.32 g of manganese tetroxide, and 1.44 g of ferrous oxalate dihydrate according to a molar ratio of Li:Mn:Fe:P = 1:0.8:0.2:1. Add 30 mL of anhydrous ethanol and ball mill in a planetary ball mill at 300 rpm for 7 hours, with a ball-to-material ratio of 10:1. Vacuum dry at 80 °C for 12 hours to obtain mixture A. Place mixture A in a tube furnace and heat to 400 °C at 10 °C / min under an argon atmosphere for low-temperature pre-sintering, holding for 4 hours. Allow to cool naturally to room temperature to obtain precursor B. Add 6 wt% sucrose (approximately 0.6 g) to precursor B and dry ball mill for 12 hours (300 rpm). Place the dry-ball-milled powder in a tube furnace and heat to 670 °C at 10 °C / min under an argon atmosphere for high-temperature sintering, holding for 9 hours. After naturally cooling to room temperature, the material is ground and sieved to obtain a manganese-rich bulk material with an inner carbon layer (Core / C). in ).

[0062] (2) Coating of iron-rich outer precursor According to LiMn 0.2 Fe 0.8 To prepare the coating solution, weigh 0.26 g of lithium acetate dihydrate, 0.13 g of manganese acetate tetrahydrate, 0.50 g of ferric citrate, and 0.25 g of phosphoric acid, and dissolve them in 55 mL of deionized water. Adjust the pH to 6.0 with ammonia water. Add 3 g of the manganese-rich bulk material prepared in step (1) to the coating solution and ultrasonically disperse for 30 minutes to ensure uniform dispersion of the bulk particles. Place the mixture in an 80°C oil bath and mechanically stir (400 rpm) to continuously evaporate the solvent until a viscous gel-like slurry is formed (approximately 4-5 hours). Transfer to an oven and dry at 110°C for 12 hours, then grind and sieve to obtain the core-shell precursor powder.

[0063] (3) Secondary firing structure to build gradient structure The core-shell precursor powder was placed in a tube furnace and heated to 640°C at a heating rate of 5°C / min under an argon atmosphere, and held at that temperature for 6 hours for secondary sintering. After natural cooling to room temperature, it was ground and sieved to obtain LiMn, a lithium manganese iron phosphate cathode material with a double carbon layer and manganese iron gradient structure. 0.8 Fe 0.2 PO4@ LiMn 0.2 Fe 0.8 PO4 / C, with a manganese-rich core of approximately 84 wt%, an iron-rich outer shell of approximately 11 wt%, and a carbon content of approximately 5 wt%, the specific structure is as follows: Figure 2 As shown. Example 5

[0064] Preparation objective: Using MnO as the manganese oxide source, with LiMn as the core. 0.6 Fe 0.4 PO4, outer LiMn 0.2 Fe 0.8 PO4 (10% coating) like Figure 1 As shown, the preparation method of the core-shell structure lithium manganese iron phosphate cathode material with a double carbon layer and gradient interface includes the following specific steps: (1) Preparation of manganese-rich body coated with inner carbon layer Weigh out 4.16 g of lithium dihydrogen phosphate, 1.7 g of manganese monoxide, and 2.88 g of ferrous oxalate dihydrate according to a molar ratio of Li:Mn:Fe:P = 1:0.6:0.4:1. Add 30 mL of anhydrous ethanol and ball mill in a planetary ball mill at 300 rpm for 6 hours, with a ball-to-material ratio of 10:1. Vacuum dry at 80 °C for 12 hours to obtain mixture A. Place mixture A in a tube furnace and heat to 350 °C at 10 °C / min under an argon atmosphere for low-temperature pre-sintering, holding for 5 hours. Allow to cool naturally to room temperature to obtain precursor B. Add 5 wt% glucose (approximately 0.5 g) to precursor B and dry ball mill for 12 hours (300 rpm). Place the dry-ball-milled powder in a tube furnace and heat to 630 °C at 10 °C / min under an argon atmosphere for high-temperature sintering, holding for 8 hours. After naturally cooling to room temperature, the material is ground and sieved to obtain a manganese-rich bulk material with an inner carbon layer (Core / C). in ).

[0065] (2) Coating of iron-rich outer precursor According to LiMn 0.2 Fe 0.8The stoichiometric ratio of PO4 (coating amount 10%) was determined by weighing 0.22 g lithium acetate dihydrate, 0.11 g manganese acetate tetrahydrate, 0.42 g ferric citrate, and 0.21 g phosphoric acid, dissolving them in 50 mL of deionized water, and adjusting the pH to 6.0 with ammonia to prepare the coating solution. 3 g of the manganese-rich bulk material prepared in step (1) was added to the coating solution and ultrasonically dispersed for 30 minutes to ensure uniform dispersion of the bulk particles. The mixture was placed in an 80°C oil bath and mechanically stirred (400 rpm) to continuously evaporate the solvent until a viscous gel-like slurry was formed. The mixture was then transferred to an oven and dried at 110°C for 12 hours, followed by grinding and sieving to obtain the core-shell precursor powder.

[0066] (3) Secondary firing structure to build gradient structure The core-shell precursor powder was placed in a tube furnace and heated to 610°C at a heating rate of 5°C / min under an argon atmosphere, and held at that temperature for 5 hours for secondary sintering. After natural cooling to room temperature, it was ground and sieved to obtain LiMn, a lithium manganese iron phosphate cathode material with a double carbon layer and manganese iron gradient structure. 0.6 Fe 0.4 PO4@ LiMn 0.2 Fe 0.8 PO4 / C, with a manganese-rich core of approximately 87 wt%, an iron-rich outer shell of approximately 9 wt%, and a carbon content of approximately 4 wt%, the specific structure is as follows: Figure 2 As shown.

[0067] Test case Electrochemical performance testing methods: The materials prepared in Examples 1-5 were thoroughly mixed with conductive agent Super P and binder polyvinylidene fluoride (PVDF) at a mass ratio of 8:1:1 in N-methylpyrrolidone (NMP) solvent to form a homogeneous slurry. The slurry was coated onto an aluminum foil current collector, vacuum dried at 80°C for 12 hours, and then die-cut into circular electrode sheets with a diameter of 12 mm. The active material loading was approximately 2.5–3.0 mg / cm³. 2 A CR2032 coin cell was assembled in an argon-filled glove box using a lithium foil as the counter electrode, a Celgard 2500 polypropylene membrane as the separator, and a 1 mol / L LiPF6 solution of ethylene carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate (DEC) (volume ratio 1:1:1) as the electrolyte. The cells were allowed to stand at 30°C for 12 hours before electrochemical performance testing. The LAND CT2001A electrochemical testing system was used within the voltage window of 2.5–4.5 V (vs. Li). + Charge and discharge tests were performed under / Li) conditions.

[0068] Table 1 summarizes the electrochemical performance test results for each embodiment. First, the battery's first-cycle charge-discharge capacity and coulombic efficiency were tested at 0.1C. Subsequently, the battery was cycled 5 times each at 0.2C, 0.5C, 1C, 3C, and 5C rates to evaluate rate performance. The 5C rate capacity retention rate was the percentage of the 5C discharge capacity relative to the 0.1C discharge capacity. Finally, the battery was cycled 100 times at 1C, and the capacity retention rate was the percentage of the discharge capacity on the 100th cycle relative to the capacity on the 1st cycle.

[0069] Table 1. Electrochemical performance of the double carbon layer gradient interface core-shell structured LMFP cathode materials prepared in each embodiment.

[0070] The data in the table shows that: (1) The initial discharge capacity at 0.1C in all embodiments was between 153 and 158 mAh g. –1 Within the range, the initial coulombic efficiency exceeded 96%, indicating that the double carbon layer coating structure effectively suppressed side reactions and ensured the high reversibility of the material.

[0071] (2) Examples 1 and 2, by employing moderate core manganese content (x = 0.6~0.7) and shell manganese content (y = 0.2~0.3), achieved a good balance between high-rate performance and cycle stability, with a 1C discharge capacity of 148~149 mAh g⁻¹. –1 The 5C rate capacity retention exceeds 82%, and after 100 cycles, the capacity retention remains above 95%. This is attributed to the gradient interface structure effectively mitigating lattice strain during charge and discharge, while the double carbon layer structure simultaneously enhances electronic conductivity and ion transport kinetics.

[0072] (3) Although the core layer manganese content in Example 3 is low (x=0.5), which limits the theoretical capacity of the material, the shell layer manganese content is the lowest (y=0.1), resulting in the best structural stability and thus exhibiting the highest cycle stability (96.8%).

[0073] (4) The core layer of Example 4 has the highest manganese content (x=0.8), which provides a higher initial capacity, but the structural strain caused by the phase transition is large at high rates, resulting in relatively low 5C rate performance (79.9%).

[0074] (5) A comparison of the various embodiments reveals that the synergistic optimization of the manganese-iron ratio in the core composition, the iron content in the shell, and the thickness of the gradient transition layer plays a decisive role in the final electrochemical performance. Among them, the process parameters determined in Example 1 (x=0.6, y=0.2) achieve the best overall performance in terms of capacity, rate performance, and cycle stability, which is attributed to the reasonable gradient distribution of the core and shell components combined with the synergistic effect of the double carbon layer.

[0075] In summary, the core-shell structured lithium manganese iron phosphate cathode material with a double carbon layer and gradient interface prepared by this invention has successfully achieved a balance between high capacity, excellent rate performance, and long cycle life by precisely controlling the core-shell composition, optimizing the gradient interface structure, and constructing an inner and outer double carbon coating system. This provides a new technical path for the design of high-performance lithium-ion battery cathode materials.

[0076] The specific embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited to the above embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of the present invention.

Claims

1. A method for preparing a core-shell structured lithium manganese iron phosphate cathode material with a double carbon layer and a gradient interface, characterized in that, Includes the following steps: Step 1, Preparation of manganese-rich body coated with inner carbon layer: Lithium source, phosphorus source, manganese oxide source and iron source are mixed in stoichiometric ratio, ball milling media are added and wet ball milling is performed, and vacuum drying is performed to obtain mixture A; Mixture A was subjected to low-temperature pre-sintering at 350–450°C for 2–6 hours under an inert atmosphere to obtain precursor B; The first carbon source is added to the precursor B and dry ball milling is performed. Then, high-temperature sintering is carried out at 600-750℃ for 6-12 hours under an inert or reducing atmosphere to obtain a manganese-rich bulk material with an inner carbon layer. Step 2, Coating of iron-rich outer layer precursor: Dissolve lithium source, manganese source, iron source and phosphorus source in deionized water, adjust the pH value to 4-7, and prepare coating solution; The manganese-rich bulk material prepared in step 1 was added to the coating solution, ultrasonically dispersed, and stirred at 60-90°C to evaporate the solvent until a gel-like slurry was formed. After further drying, it was treated in a vacuum oven to obtain the core-shell precursor powder. Step 3: Secondary sintering to build a gradient structure: The core-shell precursor powder obtained in step 2 is sintered at 600-700℃ for 4-8 hours under an inert or reducing atmosphere to induce the diffusion of interfacial elements to form a gradient transition region. At the same time, the outer layer crystallizes and carbonizes in situ to obtain a core-shell structure lithium manganese iron phosphate cathode material with a double carbon layer and a gradient interface.

2. The method for preparing the core-shell structured lithium manganese iron phosphate cathode material with a double carbon layer and gradient interface according to claim 1, characterized in that: In step 1, the manganese oxide source is at least one of Mn3O4, MnO2, and MnO.

3. The method for preparing the core-shell structured lithium manganese iron phosphate cathode material with a double carbon layer and gradient interface according to claim 1, characterized in that: In step 1, the molar ratio of the lithium source, phosphorus source, manganese oxide source, and iron source, Li:Mn:Fe:P, is 1:(0.5~0.8):(0.2~0.5):1; In step 1, the lithium source is one or a mixture of several of lithium carbonate, lithium hydroxide, lithium dihydrogen phosphate, and lithium acetate in any proportion; The phosphorus source is one or a mixture of several of the following in any proportion: ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and lithium dihydrogen phosphate; The iron source is one or a mixture of several of ferrous oxalate, ferrous acetate, ferric oxide, and ferric phosphate in any proportion.

4. The method for preparing the core-shell structured lithium manganese iron phosphate cathode material with a double carbon layer and gradient interface according to claim 1, characterized in that: In step 1, the first carbon source is one or more of sucrose, glucose, starch, and acetylene black in any proportion, and the amount added is 5 to 10 wt% of the mass of precursor B.

5. The method for preparing the core-shell structured lithium manganese iron phosphate cathode material with a double carbon layer and gradient interface according to claim 1, characterized in that: In step 2, the molar ratio of lithium source, manganese source, iron source, and phosphorus source, Li:Mn:Fe:P, is 1:(0.1~0.4):(0.6~0.9):1; The lithium source in step 2 is one or a mixture of lithium acetate, lithium hydroxide, and lithium nitrate in any proportion. The manganese source is one or a mixture of manganese acetate, manganese nitrate, manganese sulfate, and manganese chloride in any proportion; The iron source is one or a mixture of several of ferric citrate, ferric nitrate, and ferrous acetate in any proportion; The phosphorus source is one or a mixture of several of phosphoric acid, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate in any proportion.

6. The method for preparing the core-shell structured lithium manganese iron phosphate cathode material with a double carbon layer and gradient interface according to claim 5, characterized in that: In step 2, the iron source is ferric citrate, and citric acid also serves as a chelating agent and a second carbon source. If the iron source is other, citric acid, EDTA, and polyacrylic acid need to be added as chelating agents and second carbon sources.

7. The method for preparing the core-shell structured lithium manganese iron phosphate cathode material with a double carbon layer and gradient interface according to claim 1, characterized in that: In step 3, the secondary sintering temperature is lower than or equal to the high-temperature sintering temperature in step 1, and the temperature difference is controlled within the range of 0 to 100°C.

8. A core-shell structured lithium manganese iron phosphate cathode material with a double carbon layer and a gradient interface, characterized in that: The lithium manganese iron phosphate cathode material prepared according to any one of claims 1 to 7 includes a manganese-rich core, an inner carbon layer, a gradient transition region, an iron-rich outer shell, and an outer carbon layer.

9. The core-shell structured lithium manganese iron phosphate cathode material with a double carbon layer and a gradient interface according to claim 8, characterized in that: The manganese-rich core is composed of LiMn x Fe 1-x PO4 (0.5≤x≤0.8), with an average particle size of 0.5–2.0 μm; the inner carbon layer thickness is 5–15 nm; the gradient transition region thickness is 20–50 nm; the iron-rich outer shell is composed of LiMn. y Fe 1-y PO4 (0.1≤y≤0.4), with a thickness of 50–200 nm; the outer carbon layer has a thickness of 10–30 nm; The manganese-rich core comprises 75wt% to 90wt%, and the iron-rich outer shell includes an outer carbon layer comprising 10wt% to 25wt%.

10. A lithium-ion battery, characterized in that: It is made using the positive electrode material according to any one of claims 1-9 as the positive electrode active material.

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