A lithium iron manganese phosphate composite material, a preparation method and application thereof

Abstract

This invention discloses a lithium manganese iron phosphate composite material, its preparation method, and its applications. The preparation method of the lithium manganese iron phosphate composite material of this invention includes: 1) mixing polyvinylpyrrolidone, a manganese source, an iron source, and a solvent to obtain a metal ion solution; 2) mixing the metal ion solution and a potassium ferricyanide solution, and then co-precipitating them to obtain a Prussian blue precursor; 3) mixing the Prussian blue precursor, a phosphorus source, and a lithium source, followed by pre-calcination and calcination to obtain the lithium manganese iron phosphate composite material. This preparation method not only enables the preparation of a core-shell structured spherical precursor via co-precipitation, but also allows for the simple preparation of a cavity-structured, nitrogen-doped carbon-coated lithium manganese iron phosphate composite material using this precursor as a template. Furthermore, it offers advantages such as simple process, strong controllability, low cost, and suitability for practical production. Simultaneously, the lithium manganese iron phosphate composite material prepared by this invention exhibits good stability, high-temperature resistance, and excellent electrochemical performance.

Description

Technical Field

[0001] This invention relates to the technical field of electrode materials, specifically to a lithium manganese iron phosphate composite material, its preparation method, and its application. Background Technology

[0002] In recent years, lithium iron phosphate has become increasingly popular due to its high theoretical capacity (170 mAh g / L). -1 With its suitable operating voltage platform (3.45V), good electrochemical safety, and thermal stability, it has become a commercially viable cathode material for large-scale applications. However, its relatively low energy density (578Wh / kg) is a limiting factor. -1 Lithium iron phosphate (LiFePO4) can no longer meet the growing demands of electric vehicles. In contrast, another olivine-based LiMnPO4 cathode material offers advantages such as a higher operating voltage (4.1V) and a higher theoretical energy density (700Wh / kg). -1 This provides complementary advantages. However, LiMnPO4 exhibits inherently poor electronic conductivity (<10⁻⁶). -10 S cm -1 ), ion diffusion (<10) -16 cm 2 s -1 ), and Mn 2+ Transform into Mn 3+ The process involves the Jahn-Teller effect, which causes changes in volume, leading to structural instability and resulting in suboptimal capacity retention.

[0003] Although, using Mn 2+ Replacing part of the Fe in LiFePO4 2+ LiMn was prepared x Fe 1-x Theoretically, lithium iron manganese phosphate (LMFP) cathode materials possess the advantages of both the thermal safety and structural stability of LiFePO4 and the high operating voltage and high energy density of LiMnPO4. However, the following technical challenges still exist in the preparation of lithium iron manganese phosphate cathode materials:

[0004] (1) Existing solid-phase synthesis methods generally involve mixing LiFePO4 with small molecule organic substances (such as glucose, sucrose and citric acid) or polymers (such as phenolic resins), and then performing heat treatment to convert the organic molecules into an amorphous carbon matrix to obtain carbon-coated lithium iron phosphate. However, this method suffers from problems such as uneven distribution of product elements and poor electrochemical performance.

[0005] (2) The solvothermal method has problems such as high equipment requirements, complex process, great safety hazards and poor atom economy, and is not suitable for large-scale production.

[0006] (3) During use, the primary nanoparticles of lithium iron manganese phosphate as positive electrode active materials are prone to agglomeration, forming secondary microspheres that increase the lithium-ion transport distance and easily lead to poor rate performance. In response, relevant technicians will design positive electrode materials with special structures to improve their rate performance and energy density; however, the synthesis of these special structure positive electrode materials often requires a large amount of metal raw materials and additional carbon sources, more complex processes and higher energy consumption, which will also bring problems such as uneven doping elements, easy carbon element shedding, poor electrochemical stability of lithium iron manganese phosphate positive electrode materials themselves, and low energy density, making it difficult to apply to actual production.

[0007] Therefore, there is an urgent need to develop a method that is simple, highly controllable, low-cost, suitable for actual production, and capable of producing lithium manganese iron phosphate composite materials with good stability and excellent electrochemical performance. Summary of the Invention

[0008] To overcome the problems of uneven dopant distribution, easy detachment of dopants and coating layers, easy collapse of material structure under high temperature and long-term use, and unsuitability of preparation processes for practical production in existing technologies, and considering that LiFePO4, as the mainstream cathode material for lithium-ion batteries (LIBs), still suffers from poor electronic conductivity, slow lithium-ion diffusion kinetics, and a two-phase transition mechanism involving a large amount of structural rearrangement, resulting in unsatisfactory rate performance, and the need to incorporate large amounts of conductive carbon to improve electronic conductivity during electrode preparation, leading to a low volumetric energy density, cation doping and morphology control have been widely used to address these issues as the performance requirements of electrical devices continue to increase.

[0009] One of the objectives of this invention is to provide a method for preparing lithium manganese iron phosphate composite materials.

[0010] The second objective of this invention is to provide a lithium manganese iron phosphate composite material with a simple synthesis process, uniform element distribution, and excellent electrochemical stability, as well as its applications.

[0011] The inventive concept of this invention is as follows: First, a Prussian blue precursor with uniform metal element distribution and a core-shell structure is synthesized via a co-precipitation method. Using the Prussian blue precursor as a self-sacrificing template, nitrogen source, and carbon source, the precursor is then thoroughly mixed with a Li source and a P source. Finally, a cavity-containing lithium manganese iron phosphate composite material (i.e., LiMn) is synthesized via a solid-state sintering method. x Fe 1-x PO4 / NC cathode material, where 0.2≤x≤0.4). This preparation method is simple, low-cost, and requires minimal equipment, making it suitable for large-scale production.

[0012] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0013] In a first aspect, the present invention provides a method for preparing a lithium manganese iron phosphate composite material, comprising the following steps:

[0014] 1) Mix polyvinylpyrrolidone, manganese source, iron source and solvent to obtain a metal ion solution;

[0015] 2) The metal ion solution described in step 1) and the K3[Fe(CN)6] solution are mixed and subjected to a co-precipitation reaction to obtain the Prussian blue precursor;

[0016] 3) After mixing the Prussian blue precursor, phosphorus source and lithium source described in step 2), the mixture is pre-calcined and calcined to obtain lithium manganese iron phosphate composite material.

[0017] In step 3), both the pre-calcination and calcination are carried out under a protective atmosphere. The pre-calcination temperature is 300℃~400℃, and the calcination temperature is 600℃~700℃.

[0018] Preferably, the solvent in step 1) is selected from at least one of water, ethanol, methanol, ethylene glycol, and glycerol.

[0019] Preferably, the preparation method of the lithium manganese iron phosphate composite material includes the following steps:

[0020] 1) Dissolve polyvinylpyrrolidone in ethanol and water to obtain a PVP solution;

[0021] A metal ion solution is obtained by mixing a PVP solution, a manganese source, and an iron source.

[0022] 2) The metal ion solution described in step 1) is added to K3[Fe(CN)6] solution and mixed, and then subjected to a co-precipitation reaction to obtain the Prussian blue precursor;

[0023] 3) The Prussian blue precursor, phosphorus source, lithium source and dispersant described in step 2) are mixed by wet grinding, and then pre-calcined and calcined to obtain lithium manganese iron phosphate composite material.

[0024] In step 3), both the pre-calcination and calcination are carried out under a protective atmosphere. The pre-calcination temperature is 300℃~400℃, and the calcination temperature is 600℃~700℃.

[0025] Specifically, in step 1), a PVP solution is first prepared, and then mixed with manganese and iron sources. This facilitates uniform dispersion of the reactants in the co-precipitation reaction, enabling the controllable production of relatively uniform, core-shell structured spherical Prussian blue precursor particles. If the solvent in the PVP solution is only water, significant aggregation will occur during the co-precipitation reaction, which is detrimental to the controllable production of relatively uniform spherical Prussian blue precursor particles (see...). Figure 2 and Figure 3 ).

[0026] Preferably, the average molecular weight of the polyvinylpyrrolidone in step 1) is 15,000-17,000.

[0027] Preferably, the volume ratio of ethanol to water in step 1) is (0.5-2):1.

[0028] Preferably, the concentration of the PVP solution in step 1) is 7.5–10 g / L.

[0029] Preferably, the manganese source in step 1) is one or more of manganese(II) sulfate, manganese(II) chloride, and manganese(II) nitrate.

[0030] Preferably, the iron source in step 1) is one or more of ferric chloride (III), ferric sulfate (III), and ferric nitrate (III).

[0031] Specifically, in step 1), a soluble divalent manganese salt is selected as the manganese source, and a soluble trivalent ferric salt is selected as the iron source. This is so that after reacting with the K3[Fe(CN)6] solution in step 3), relatively uniform particle size, core-shell structure, and nanoscale Prussian blue precursor spherical particles can be controllably obtained (see...). Figure 2 and Figure 3 If a soluble ferrous salt is selected in step 1), the resulting precursor particles will have various particle shapes (including cuboid, cube, and spherical particles), making it difficult to guarantee the repeatability and controllability of the process.

[0032] Preferably, the molar ratio of the manganese source and the iron source in step 1) is (0.6 to 1): 1.

[0033] More preferably, the molar ratio of the manganese source and the iron source in step 1) is (0.8 to 0.9): 1.

[0034] Preferably, the molar ratio of the amount of polyvinylpyrrolidone used in step 1) to the manganese source is (1300-1500) g: 1 mol.

[0035] Preferably, the ratio of the total molar amount of manganese and iron sources in step 1) to the molar amount of K3[Fe(CN)6] in the K3[Fe(CN)6] solution in step 2) is (1.0~1.2):1.

[0036] Preferably, the solvent in the K3[Fe(CN)6] solution in step 2) is selected from at least one of ethanol and water.

[0037] Preferably, the concentration of the K3[Fe(CN)6] solution in step 2) is 15–25 mmol / L. -1 .

[0038] Preferably, the volume ratio of the metal ion solution in step 1) to the K3[Fe(CN)6] solution in step 2) is (1-3):1.

[0039] Preferably, the temperature of the coprecipitation reaction in step 2) is 50℃~80℃.

[0040] Preferably, the coprecipitation reaction in step 2) takes 4 to 8 hours.

[0041] More preferably, the coprecipitation reaction in step 2) takes 5 to 7 hours.

[0042] Preferably, the Prussian blue precursor in step 3) is a spherical particle with a particle size of 150-350 nm.

[0043] More preferably, the Prussian blue precursor in step 3) is a spherical particle with a particle size of 180-220 nm.

[0044] Preferably, the Prussian blue precursor in step 3) is a spherical particle with a core-shell structure.

[0045] More preferably, the core of the Prussian blue precursor in step 3) includes a Mn(II)-CN-Fe(III) characteristic structure, and the shell includes a Mn(III)-CN-Fe(II) characteristic structure.

[0046] Specifically, this structure is advantageous for producing the final lithium manganese iron phosphate composite material.

[0047] Preferably, the phosphorus source in step 3) is selected from at least one of sodium phosphate, potassium phosphate, triammonium phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium dihydrogen phosphate, dipotassium hydrogen phosphate, potassium dihydrogen phosphate, disodium hydrogen phosphate, iron phosphate, manganese phosphate, lithium phosphate, lithium dihydrogen phosphate, and phosphoric acid.

[0048] More preferably, the phosphorus source in step 3) is selected from at least one of ammonium dihydrogen phosphate and diammonium hydrogen phosphate.

[0049] Preferably, the lithium source in step 3) is selected from at least one of lithium carbonate, lithium hydroxide, lithium acetate, lithium oxalate, lithium formate, lithium benzoate, lithium citrate, lithium chloride, lithium bromide, lithium phosphate, lithium dihydrogen phosphate, lithium hydrogen phosphate, lithium sulfate, lithium nitrate, and lithium tert-butoxide.

[0050] More preferably, the lithium source in step 3) is selected from at least one of lithium carbonate, lithium hydroxide, lithium acetate, lithium oxalate, and lithium formate.

[0051] Preferably, the mass ratio of the MnFe-PBA precursor particles, phosphorus source, and lithium source in step 3) is (1.1-2):1:(0.2-0.8).

[0052] More preferably, the mass ratio of the MnFe-PBA precursor particles, phosphorus source, and lithium source in step 3) is (1.4-1.6):1:(0.4-0.6).

[0053] Preferably, the dispersant in step 3) is selected from at least one of alcohol, anhydrous ethanol, methanol, ethylene glycol, and glycerol.

[0054] Preferably, the wet grinding time in step 3) is 20 to 40 minutes.

[0055] Preferably, the protective gas in step 3) is selected from at least one of nitrogen, argon, helium, and neon.

[0056] Preferably, the pre-calcination time in step 3) is 4 to 8 hours, and the calcination time is 4 to 8 hours.

[0057] Preferably, the heating rate for the pre-calcination in step 3) is 3–6 °C / min. -1 The heating rate for calcination is 3–6 °C / min. -1 .

[0058] In a second aspect, the present invention provides a lithium manganese iron phosphate composite material prepared by the preparation method described in the first aspect.

[0059] Preferably, the lithium manganese iron phosphate composite material comprises spherical particles assembled from lithium manganese iron phosphate particles and a nitrogen-doped carbon layer covering the spherical particles.

[0060] More preferably, the lithium manganese iron phosphate composite material includes LiMn x Fe 1-x The spherical particles assembled from PO4 particles and the nitrogen-doped carbon layer covering the spherical particles; wherein the value of x is in the range of 0.2≤x≤0.4.

[0061] Preferably, the value of x is in the range of 0.2 ≤ x ≤ 0.3. More preferably, the value of x is in the range of 0.25.

[0062] Preferably, the particle size of the spherical particles is 150–300 nm.

[0063] Preferably, the LiMn x Fe 1-x The particle size of PO4 is 150–300 nm.

[0064] More preferably, the LiMn x Fe 1-x The particle size of PO4 is 180–220 nm.

[0065] Preferably, the lithium manganese iron phosphate composite material has a cavitation structure inside.

[0066] Preferably, the outer surface of the lithium manganese iron phosphate composite material is composed of LiMn. x Fe 1-x The composite material consists of a PO4 particle layer (0.2 ≤ x ≤ 0.4) and a nitrogen-doped carbon layer. Preferably, the outer thickness of the lithium manganese iron phosphate composite material is 20–40 nm.

[0067] Preferably, the carbon in the nitrogen-doped carbon layer includes graphitic carbon.

[0068] Preferably, the thickness of the nitrogen-doped carbon layer is 1 to 3 nm.

[0069] Preferably, the mass percentage of the nitrogen-doped carbon layer in the lithium manganese iron phosphate composite material is 3% to 5%.

[0070] Preferably, the lithium manganese iron phosphate particles are of the olivine phase. Specifically, the lithium manganese iron phosphate particles are a pure phase material with good stability, and the Mn element is successfully introduced into the Fe position in LiFePO4 in the form of doping.

[0071] Preferably, the lithium manganese iron phosphate composite material includes divalent manganese, trivalent manganese, divalent iron, and trivalent iron.

[0072] Preferably, the specific surface area of ​​the lithium manganese iron phosphate composite material is 120-130 m². 2 g –1 .

[0073] Preferably, the pore size of the lithium manganese iron phosphate composite material is 2-30 nm.

[0074] Thirdly, the present invention provides a positive electrode comprising the lithium manganese iron phosphate composite material described in the second aspect.

[0075] Fourthly, the present invention provides a battery comprising the lithium manganese iron phosphate composite material described in the second aspect.

[0076] The beneficial effects of the present invention are: the preparation method of the lithium manganese iron phosphate composite material of the present invention can not only obtain a core-shell structured spherical precursor by co-precipitation reaction, but also simply rely on the precursor as a template to obtain a cavity structure and nitrogen-doped carbon coating lithium manganese iron phosphate composite material, and has the advantages of simple process, strong controllability, low cost and suitability for actual production.

[0077] Meanwhile, the lithium manganese iron phosphate composite material prepared by this invention has the advantages of good stability, high temperature resistance, and excellent electrochemical performance.

[0078] Specifically:

[0079] (1) The preparation method of the lithium manganese iron phosphate composite material of the present invention is to prepare a spherical precursor with a core-shell structure, and then to control the amount of raw materials, the mixing method and the calcination conditions so that the metal can move qualitatively to the outside, thereby forming a lithium manganese iron phosphate composite material with a loose porous structure, a stable internal cavity structure and an external structure composed of fine nanoparticles and a nitrogen-doped carbon layer. This not only enables the preparation of the nitrogen-doped carbon layer without the introduction of additional carbon and nitrogen sources, which is suitable for actual large-scale production, but also solves the problems of uneven distribution of C and N in the composite material, easy shedding of the nitrogen-doped carbon layer, high temperature resistance and stability of the lithium manganese iron phosphate composite material.

[0080] (2) When the lithium manganese iron phosphate composite material of the present invention is used as a positive electrode material in lithium-ion batteries, it has a special delithiation mechanism and excellent electrochemical performance.

[0081] (3) This invention proposes a synthetic route using Prussian blue as a self-sacrificing template, simultaneously achieving morphology control, Mn doping, and N doping carbon coating to improve electrochemical performance. The material structure and microstructure are analyzed using characterization techniques such as XRD, FT-IR, SEM, and TEM. Subsequently, the cathode material is assembled into a coin cell, and the electrochemical performance, including cyclic voltammetry, electrochemical impedance spectroscopy, cycle performance, and rate performance, is tested and analyzed.

[0082] (4) This invention also investigated the influence of material particle morphology, phase transition mechanism during (delithiation) process, and electrochemical performance. Results showed that the lithium manganese iron phosphate composite material exhibits excellent cycle stability, good lithium-ion diffusion kinetics, and outstanding rate performance. This is likely due to the appropriate Mn doping amount, uniform local distribution of Fe / Mn elements, N-rich carbon coating, and the formation of a non-equilibrium single-phase solid solution, which avoids large volume changes and additional structural rearrangements. Density functional theory (DFT) calculations further confirmed that, compared with LFP and LM… 0.25 F0.75 Compared to P, due to LM 0.25 F 0.75 The smaller band gap and lower delithiation energy of P indicate enhanced electronic conductivity and good lithium-ion diffusion, providing theoretical support.

[0083] (5) LiMn synthesized in this invention 0.25 Fe 0.75 PO4 / NC exhibits excellent cycling performance (retaining 164.7 mAh g⁻¹ after 200 cycles at a 0.5C current density). –1 Specific capacity) and rate performance (96.1 mAh g at 10C) –1 Meanwhile, off-site XRD revealed LiMn 0.25 Fe 0.75 The primary mechanism of PO4 / NC delithiation is a single-phase solid solution mechanism, which avoids major structural reconstruction, thus explaining its superior rate performance. Furthermore, density functional theory (DFT) calculations verified the effect of Mn doping, demonstrating the superior performance of LiMn. 0.25 Fe 0.75 The superiority of PO4 / NC as a LIB cathode provides good support for the experimental results. Attached Figure Description

[0084] Figure 1 LiMn in Example 1 0.25 Fe 0.75 Refined XRD pattern of PO4 / NC material.

[0085] Figure 2 The images show the infrared spectrum and transmission electron microscope (TEM) image of the MnFe-PBA precursor particles in Example 1.

[0086] Figure 3 This is a SEM image of the MnFe-PBA precursor particles in Example 1.

[0087] Figure 4 LiMn in Example 1 0.25 Fe 0.75 SEM images and particle size distribution of PO4 / NC materials.

[0088] Figure 5 LiMn in Example 1 0.25 Fe 0.75 TEM and HRTEM images of PO4 / NC material.

[0089] Figure 6 LiMn in Example 1 0.25 Fe 0.75A schematic diagram of the morphology and structure of PO4 / NC material and an elemental mapping diagram of Mn, Fe, N, P and O.

[0090] Figure 7 LiMn in Example 1 0.25 Fe 0.75 N2 adsorption / desorption isotherms and pore size distribution of PO4 / NC material.

[0091] Figure 8 LiMn in Example 1 0.25 Fe 0.75 UV-Raman and HRTEM spectra of PO4 / NC materials.

[0092] Figure 9 LiMn in Example 1 0.25 Fe 0.75 XPS spectra of PO4 / NC materials.

[0093] Figure 10 LiMn in Example 1 0.25 Fe 0.75 The PO4 / NC material was tested at a voltage of 2.3-4.8V and a scan rate of 0.2mV / s. -1 The CV curve obtained from the second lap measurement.

[0094] Figure 11 LiMn in Example 1 0.25 Fe 0.75 The charge-discharge curves of PO4 / NC material after 150 cycles were measured under the conditions of voltage 2.3-4.8V and current 0.5C.

[0095] Figure 12 LiMn in Example 1 0.25 Fe 0.75 Cyclic stability test results of PO4 / NC material under conditions of voltage 2.3-4.8V and current 0.5C.

[0096] Figure 13 LM of Example 1 0.25 F 0.75 P / NC, LFP / NC of Comparative Example 1, and LM of Comparative Example 2 0.5 F 0.5 The graph shows the rate performance results of P / NC measured at different current densities (0.5~10C), where LM 0.25 F 0.75 P / NC indicates LiMn 0.25 Fe 0.75 PO4 / NC and LFP / NC represent LiFePO4 / NC, LM 0.5 F0.5 P / NC indicates LiMn 0.5 Fe 0.5 PO4 / NC.

[0097] Figure 14 LiMn in Example 1 0.25 Fe 0.75 ex-XRD patterns and schematic diagrams of the delithiation mechanism of PO4 / NC and LiMnFePO4 / NC in Comparative Example 1 during the delithiation process.

[0098] Figure 15 LiMn in Example 1 0.25 Fe 0.75 PO4 / NC materials at different scan rates (0.2-1.0 mV / s) -1 The CV curves and analysis results are shown below; where (a) is LiMn 0.25 Fe 0.75 CV curves of PO4 / NC materials at different scan rates, (b) is the calculated b-value of the redox peak after processing the data in (a), and (c) is the LiMn peak. 0.25 Fe 0.75 PO4 / NC material at a scan rate of 1.0 mV s -1 CV curves at time; (d) is LiMn 0.25 Fe 0.75 A comparison of the pseudocapacitive contribution rates of PO4 / NC materials measured at different scan rates.

[0099] Figure 16 LiMn in Example 1 0.25 Fe 0.75 Figures showing the results of PO4 / NC material tests using the electrostatic intermittent titration procedure (GITT); where (a) is the voltage response curve, (b) is the result of the constant current intermittent titration test, and (c) is the change in lithium-ion diffusion coefficient during charging and discharging.

[0100] Figure 17 For example, LiFePO4 / NC in Comparative Example 1 and LiMn in Comparative Example 2 0.5 Fe 0.5 XRD pattern of PO4 / NC.

[0101] Figure 18 The images show SEM images of the precursors in Example 1, Comparative Example 1, and Comparative Example 2, as well as SEM and TEM images of the final composite materials. Detailed Implementation

[0102] The present invention will be further described in detail below through specific embodiments.

[0103] Example 1

[0104] This embodiment provides a method for preparing a lithium manganese iron phosphate composite material, including the following steps:

[0105] 1) Weigh 3g of polyvinylpyrrolidone (PVP, average molecular weight: 16000) and dissolve it in a mixed solution of 200mL anhydrous ethanol and 200mL deionized water to obtain a 7.5-10g / L PVP solution.

[0106] 2) Add 2.18 mmol MnSO4 and 2.55 mmol FeCl3 to the PVP solution in step 1), dissolve them completely, stir vigorously with a magnetic stirrer for 30 minutes, and then sonicate for 10 minutes to obtain a homogeneous metal ion solution.

[0107] 3) Measure 200 mL of 20 mmol L -1 K3[Fe(CN)6] solution (solvent: water) was added to the metal ion solution in step 2), and after thorough mixing, it was heated to 60°C and stirred continuously for 6 hours to obtain the reacted liquid.

[0108] 4) Collect the precipitate in the liquid after the above reaction by centrifugation, wash the precipitate three times each with deionized water and ethanol, and then dry the precipitate in an oven at 70°C for 12 hours to obtain MnFe-PBA precursor particles.

[0109] 5) Add 0.5g of MnFe-PBA precursor particles from step 4), 0.33g of ammonium dihydrogen phosphate (NH4H2PO4, 2.86mmol), 0.15g of lithium carbonate (Li2CO3, 2.0mmol) and a small amount of alcohol to a mortar and grind for 30 minutes to obtain a mixture.

[0110] The above mixture was placed in a tube furnace, and nitrogen gas was introduced. Under the nitrogen atmosphere, the mixture was first heated to 5°C for 5 minutes. -1 The temperature was increased to 350°C and pre-calcined for 6 hours; then, the temperature was increased to 5°C per minute. -1 The heating rate was continued to be increased to 650℃ and calcined for 6 hours to obtain lithium manganese iron phosphate composite material (i.e., LiMn). 0.25 Fe 0.75 PO4 / NC material, marked as LMFP / NC, form: powder.

[0111] Application Example 1

[0112] This application example demonstrates the use of lithium manganese iron phosphate composite materials, specifically including the following steps:

[0113] 1) The LiMn in Example 1 0.25 Fe 0.75PO4 / NC material, as the active material, is mixed and ground for 30 minutes with conductive agent (acetylene black, Super P Li) and binder (polyvinylidene fluoride, PVDF) at a mass ratio of 8:1:1. The mixture is then dissolved in N-methylpyrrolidone (NMP) solvent and stirred for 12 hours to obtain a slurry.

[0114] 2) The slurry from step 1) is evenly coated onto aluminum foil, dried in a vacuum oven at 110°C for 10 hours, and then cut into circular electrode sheets with a diameter of 12 mm (the mass loading of the active material on the circular electrode sheet is approximately 1.6 mg / cm³). -2 );

[0115] 3) Using the circular electrode sheet from step 2) as the positive electrode, the Celgard 2400 film (PP material) as the separator, and the lithium foil as the negative electrode;

[0116] LiPF6 was dissolved in a mixed solvent (the volume ratio of ethylene carbonate EC, diethyl carbonate DC, and methyl carbonate EMC was 1:1:1) to prepare a LiPF6 electrolyte with a concentration of 1 mol / L.

[0117] Inside an argon-filled glove box, the entire battery assembly process is completed in the order of placing the positive electrode (electrode plate), separator, electrolyte, negative electrode, gasket, and spring. Finally, the battery is assembled using a battery sealing machine.

[0118] It should be noted that the examples and application examples are based on this battery, and a series of electrochemical tests were conducted to obtain the performance test results.

[0119] Characterization and performance testing:

[0120] 1. LiMn in Example 1 0.25 Fe 0.75 The refined X-ray diffraction (XRD) pattern of PO4 / NC material, as shown below. Figure 1 As shown.

[0121] Depend on Figure 1 It can be known that: LiMn 0.25 Fe 0.75 All diffraction peaks of the PO4 / NC material (i.e., lithium manganese iron phosphate composite material) are consistent with those of LiMnPO4 (JCPDS card No. 74-0375) and LiFePO4 (JCPDS card No. 81-1173) in the orthorhombic Pnmb space group, and no impurity peaks were found, indicating that LiMn in the lithium manganese iron phosphate composite material is consistent with these peaks. 0.25 Fe 0.75The PO4 exhibits high crystallinity and a pure olivine phase, with Mn successfully introduced as a dopant into the Fe sites within LiFePO4. Furthermore, based on the refinement results, we obtained LiMn... 0.25 Fe 0.75 The cell parameters of PO4 / NC are as follows: a = 10.34994, b = 6.02515, c = 4.70320, volume = 293.291. It can be seen that the cell is slightly larger than that of traditional LiFePO4, which also proves the successful doping of Mn.

[0122] 2. Infrared spectroscopy and transmission electron microscopy images of the MnFe-PBA precursor in Example 1, as shown below. Figure 2 As shown, where, Figure 2 (a) Infrared characterization spectrum of the MnFe-PBA precursor; Figure 2 Transmission electron microscopy image of (b) MnFe-PBA precursor.

[0123] Depend on Figure 2 It can be seen that the infrared spectrum of the MnFe-PBA precursor is at a wavelength of 2149 cm⁻¹. -1 and 2080cm -1 The presence of two characteristic peaks representing -CN- bonds at the position indicates the presence of a mixed phase in the precursor material. These peaks are attributed to Mn(II)-CN-Fe(III) and Mn(III)-CN-Fe(II), respectively (II and III represent the valence states of the elements, indicating the presence of a mixed phase in the precursor material). Furthermore, transmission electron microscopy (TEM) images of the MnFe-PBA precursor particles confirm that the MnFe-PBA precursor (i.e., the Prussian blue precursor) is a core-shell structured nanoparticle.

[0124] The phase composition of the core and shell can be determined by etching. Generally, Fe(III) has a higher affinity for -NC- groups than Fe(II), therefore Mn(III)-CN-Fe(II) and Mn(II)-CN-Fe(III) can be distinguished by their chemical etching activity. The spatial distribution of the phase can be studied by etching MnFe-PBA with mercaptoacetic acid (TGA). TGA preferentially dissolves the core while leaving the shell unaffected, resulting in a hollow cage morphology. This hollow cage still matches PBA (Prussian blue) in the XRD pattern, but in the infrared spectrum, it is located at 2150 cm⁻¹. -1 The disappearance of the peak confirms that the core composition is Mn(II)-CN-Fe(III) and the shell composition is Mn(III)-CN-Fe(II).

[0125] In summary, it can be concluded that the core of the MnFe-PBA precursor has a characteristic structure of "Mn(II)-CN-Fe(III)" and the shell has a characteristic structure of "Mn(III)-CN-Fe(II)".

[0126] 3. Scanning Electron Microscope (SEM) image of the MnFe-PBA precursor particles in Example 1, as shown. Figure 3 As shown. LiMn in Example 1 0.25 Fe 0.75 SEM images of PO4 / NC materials, such as Figure 4 As shown in (a) in the figure, Figure 4 The illustration in (a) is a LiMn sample with a higher magnification. 0.25 Fe 0.75 SEM images of PO4 / NC materials. Statistical analysis showed that LiMn in Example 1... 0.25 Fe 0.75 Particle size distribution diagram of PO4 / NC bulk material, as shown below Figure 4 As shown in (b) of the diagram.

[0127] Depend on Figure 3 and Figure 4 It can be known that: Figure 3 The MnFe-PBA precursor is a spherical particle with a core-shell structure and a particle size of 180–220 nm, which is composed of a large number of fine particles with a particle size of 10–30 nm.

[0128] LiMn derived from this precursor 0.25 Fe 0.75 PO4 / NC materials retain the spherical particle characteristics of the precursor very well (see...) Figure 4 Similarly, the surface is covered with many fine particles with a diameter of 10 to 30 nm, thus having a rough, loose and porous structure.

[0129] Combined with other characterizations, it can be explained that LiMn 0.25 Fe 0.75 PO4 / NC material consists of spherical particles with a hole structure and a nitrogen-doped carbon layer. The particle size of the spherical particles ranges from 150 to 280 nm, with an average particle size of 211.9 nm.

[0130] LiMn in Example 1 0.25 Fe 0.75 Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images of PO4 / NC materials, as shown below. Figure 5 As shown; Figure 5 (a) and (b) in the figure are LiMn 0.25 Fe 0.75TEM image of PO4 / NC material. Figure 5 (c) in the text represents LiMn 0.25 Fe 0.75 HRTEM images of PO4 / NC materials. Figure 5 (d) in the text refers to... Figure 5 Selected area electron diffraction pattern of (c).

[0131] Depend on Figure 5 It can be seen that the TEM image shows LiMn 0.25 Fe 0.75 Internal microstructure of PO4 / NC materials; LiMn 0.25 Fe 0.75 The lighter internal color of PO4 / NC material confirms that it contains a large number of loose cavities. These cavities are caused by the directional diffusion of metal ions. The internal ions have higher surface energy than the external ions, leading to their migration to the outside and the formation of internal cavities (i.e., void structures). LiMn... 0.25 Fe 0.75 The exterior of the PO4 / NC material (thickness: 35nm) consists of fine particles and a nitrogen-doped carbon layer.

[0132] The HRTEM images clearly show continuous lattice fringes with interspaces of 0.52 nm, 0.35 nm, and 0.25 nm, which correspond to LiMn, respectively. 0.25 Fe 0.75 The (200), (111), and (311) crystal planes of the olivine phase in PO4 / NC materials correspond (see...). Figure 5 (c)). For LiMn 0.25 Fe 0.75 Selected area electron diffraction (SEEG) tests were performed on the interior of the PO4 / NC particle material. Figure 5 (d) has a significant effect on LiMn 0.25 Fe 0.75 The characteristic diffraction rings of the olivine phase in the (200), (101), and (211) planes in PO4 / NC materials indicate that LiMn 0.25 Fe 0.75 PO4 / NC material is olivine phase (pure phase).

[0133] LiMn in Example 1 0.25 Fe 0.75 The UV-Raman and HRTEM spectra of the PO4 / NC material are shown below. Figure 8 (a) and Figure 8 As shown in (b) of the diagram.

[0134] Depend on Figure 8 It can be known that: LiMn 0.25Fe 0.75 Raman spectra of PO4 / NC materials are as follows Figure 3-4 (a) shows 1335cm -1 and 1590cm -1 The two peaks centered on the center correspond to the disorder-induced D bands (sp). 3 Hybridization) and G-bands vibrating in the plane (sp) 2 Hybridization). The intensities of these two characteristic peaks (represented by I) D and I G The intensity ratio of the D band to the G band (Ig) can represent crystallinity and graphitization. Raman spectroscopy is used to determine the intensity ratio of the D band to the G band (Ig). D / I G The value of 0.92 indicates that LiMn x Fe 1-x The carbon in PO4 / C exists in the form of graphitic carbon, which helps to enhance electrical conductivity. Furthermore, at 960 cm⁻¹... –1 The nearby peaks can be attributed to PO4. 3- It is related to intermolecular motion and asymmetric stretching vibrations.

[0135] Combination Figure 5 , Figure 6 , Figure 8 and Figure 9 Analysis shows that LiMn 0.25 Fe 0.75 The outer layer of the PO4 / NC nanoparticles is a nitrogen-doped carbon layer with a thickness of about 2 nm. This carbon layer covers "spherical particles with internal vacancy structures and external fine particles with a particle size of 10-30 nm". Raman and XPS results show that the carbon in this carbon layer is graphitic carbon.

[0136] LiMn in Example 1 0.25 Fe 0.75 The morphological structure diagram and elemental mapping diagram of PO4 / NC material are shown below. Figure 6 (a) and Figure 6 As shown in (b) of the diagram.

[0137] Depend on Figure 6 It can be concluded that, combining the above conclusions and the high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) image, the LiMn in Example 1 can be explained. 0.25 Fe 0.75 PO4 / NC materials, from the inside out, have a hole structure, an outer shell assembled from tiny nanoparticles, and a nitrogen-doped carbon layer (see...). Figure 6 (a) in the figure, and these cavities are caused by the directional outward diffusion of metal ions during the calcination process.

[0138] Meanwhile, the elemental mapping corresponding to the HAADF-STEM diagram illustrates the presence of Mn, Fe, P, O, C, and N elements in LiMn. 0.25 Fe 0.75 The carbon layer on the surface of the PO4 / NC material is doped with N, and the distribution of N is achieved. This indicates that the preparation method of the present invention can be used to prepare LiMn. 0.25 Fe 0.75 PO4 was successfully doped with nitrogen.

[0139] 4. LiMn in Example 1 0.25 Fe 0.75 N2 adsorption / desorption isotherms and pore size distribution diagram (internal inset) of PO4 / NC material, such as... Figure 7 As shown.

[0140] Depend on Figure 7 It can be known that: LiMn 0.25 Fe 0.75 The isothermal adsorption curves of PO4 / NC materials not only exhibit type IV isotherm characteristics but also show a distinct hysteresis loop, confirming the adsorption properties of LiMn. 0.25 Fe 0.75 The presence of micropores and mesopores (pore size: ≤30nm, specifically 2~30nm) in PO4 / NC materials can explain the presence of LiMn. 0.25 Fe 0.75 The PO4 / NC nanoparticles exhibit a rich porous structure, which is consistent with the characterization results of SEM and TEM.

[0141] LiMn 0.25 Fe 0.75 The specific surface area of ​​PO4 / NC material can reach 122.30 m². 2 g –1 This is related to LiMn 0.25 Fe 0.75 It is related to the specific structure and composition of PO4 / NC materials themselves.

[0142] 5. The LiMn from Example 1 0.25 Fe 0.75 The PO4 / NC material was tested by inductively coupled plasma mass spectrometry (ICP-MS), and the results are shown in Table 1.

[0143] Table 1 LiMn 0.25 Fe 0.75 ICP-MS test results of PO4 / NC materials

[0144]

[0145] As shown in Table 1, the lithium iron phosphate composite material of the present invention can achieve co-doping of C, N, and Mn elements, and the molar ratios of Mn, Fe, and P are 0.251:0.755:1, indicating that it is a LiMn composite material with an external N-doped carbon layer and a large number of vacancy structures inside. 0.25 Fe 0.75 PO4 nanosphere material. Further analysis of thermogravimetric analysis and mass loss results showed that the nitrogen-doped carbon layer in the lithium manganese iron phosphate composite material in Example 1 accounted for 3% to 5% of the total mass.

[0146] LiMn in Example 1 0.25 Fe 0.75 XPS spectra of PO4 / NC materials, such as Figure 9 As shown; where, Figure 9 (a) in the image is the high-resolution XPS spectrum of N1s. Figure 9 (b) in the image is the high-resolution XPS spectrum of Fe 2p. Figure 9 (c) in the image is the high-resolution XPS spectrum of P 2p. Figure 9 (d) in the image is the high-resolution XPS spectrum of Mn 2p. Figure 9 (e) in the image represents the high-resolution XPS spectrum of O1s. Figure 9 (f) in the figure is the high-resolution XPS spectrum of C1s; "sat" in the XPS figure means satellite peak.

[0147] LiMn 0.25 Fe 0.75 XPS full spectrum and elemental distribution map of PO4 / NC material show that the elements in this composite material include Li, O, C, N, P, Fe and Mn.

[0148] Depend on Figure 9 It can be seen that in the high-resolution XPS spectrum of N1s, the three peaks at binding energies of 394.1 eV, 399.2 eV and 401.1 eV correspond to the characteristic peaks of nitrogen in pyridine, nitrogen in pyrrole and nitrogen in graphite, respectively. This proves the successful doping of nitrogen in the lithium iron phosphate composite material of Example 1, and also shows that the outermost nitrogen-doped carbon layer of the lithium iron phosphate composite material includes graphite.

[0149] In the fitted XPS spectrum of Fe 2p, characteristic peaks were found near 712.4 eV and 726.3 eV, which correspond to Fe 2p, respectively. 3 / 2 and Fe 2p 1 / 2 The energy difference is 13.9 eV, reflecting the energy difference in LiMn. 0.25 Fe 0.75 Fe in PO4 / NC materials 2+The existence of the state; the peaks at 713.8 eV and 726.8 eV correspond to Fe. 3+ This indicates that Fe has a higher oxidation state. 3+ The generation of.

[0150] The XPS spectrum of P 2p has a broad peak at 134.2 eV, corresponding to PO4. 3- part.

[0151] The XPS spectrum of Mn 2p exhibits two main peaks at 642.5 eV and 654.1 eV, corresponding to spin-orbit coupling of Mn 2p, respectively. 3 / 2 and Mn 2p 1 / 2 The peaks centered at 642.5 eV and 654.5 eV correspond to Mn. 2+ The peaks at 644.4 eV and 655.6 eV correspond to Mn 3+ This indicates that Mn is in the preparation process 2+ Partial oxidation occurs. It is worth noting that in composite materials, the formation of a small number of heterovalent ions (e.g., trivalent manganese ions and trivalent iron ions) is beneficial to improving electronic conductivity, enhancing overall kinetics, and thus optimizing electrochemical performance.

[0152] The O 1s spectrum confirmed the presence of CO, C=O, PO, and Fe / Mn-O bonds, with centers at 533.1 eV, 532.2 eV, 531.5 eV, and 530.9 eV, respectively. The C 1s spectrum had a sp. of 284.3 eV. 2 The three peaks are hybrid carbon (C=C), epoxy group (CO) at 286.1 eV, and carbonyl group at 287.8 eV.

[0153] 6. Using an electrochemical workstation (CHI-760E, China), at a voltage of 2.3-4.8V and a scan rate of 0.2mV / s... -1 The LiMn content of Example 1 was measured below. 0.25 Fe 0.75 The cyclic voltammetry (CV) curve of the PO4 / NC material in the second cycle is shown below. Figure 10 As shown. The LiMn content of Example 1 was measured using a blue electrode tester (CT 2001A, China) under conditions of 2.3-4.8V and 0.5C. 0.25 Fe 0.75 The charge-discharge curve of the PO4 / NC material after 150 cycles, as shown below. Figure 11 As shown.

[0154] Depend on Figure 10 The cyclic voltammetry curves show that: LiMn 0.25 Fe 0.75During the charge-discharge process, the redox reactions of Fe and Mn in PO4 / NC materials are relatively independent. The redox peak of Fe is around 3.4V, and the redox peak of Mn is around 4.2V. The reaction process is as follows:

[0155] LiMn 2+ x Fe 2+ 1–x PO4 → LiMn 2+ x Fe 3+ 1–x PO4 + (1–x)Li + + (1–x)e - (1)

[0156] LiMn 2+ x Fe 3+ 1–x PO4 → Mn 3+ x Fe 3+ 1–x PO4+ xLi + + xe - (2)

[0157] at the same time, Figure 11 The charge-discharge curves show plateaus near 3.45V and 4.1V, corresponding to the redox reactions of Fe and Mn, respectively, which are consistent with the CV curve results.

[0158] 7. Using a blue electrode tester (CT 2001A, China), the LiMn in Example 1 was tested under conditions of 2.3-4.8V and 0.5C. 0.25 Fe 0.75 The long-term cycling stability of PO4 / NC materials is shown in the test results as follows: Figure 12 As shown. The LiMn in Example 1 was tested using a blue electric current tester (CT 2001A, China) under different current densities (0.5–10C). 0.25 Fe 0.75 PO4 / NC (i.e., LM) 0.25 F 0.75 P / NC), LiFePO4 / NC (LFP / NC) in Comparative Example 1 and LiMn in Comparative Example 2 0.5 Fe 0.5 PO4 / NC(LM 0.5 F 0.5 The rate performance of P / NC is as follows: Figure 13 As shown.

[0159] Depend on Figure 12 and Figure 13 It can be seen that after 200 cycles at a 0.5C rate, LiMn 0.25 Fe 0.75 The reversible capacity of the PO4 / NC material remains at 164.7 mAh g. -1 The coulombic efficiency remains above 98%, indicating that the lithium iron manganese phosphate composite material of this invention exhibits excellent cycle stability as a cathode material. This result demonstrates that the LiMn in Example 1... 0.25 Fe 0.75 PO4 / NC materials exhibit good structural and electrochemical stability.

[0160] Furthermore, based on the rate performance at different current densities ( Figure 13 The results showed that, compared with Comparative Example 1 and Comparative Example 2, under the same test conditions, the LiMn in Example 1... 0.25 Fe 0.75 PO4 / NC exhibits high specific capacity and stable rate performance. This is due to the high specific capacity of LiMn. 0.25 Fe 0.75 The hole structure inside the PO4 / NC material and the coating structure of the N-doped carbon layer can act as a buffer, reducing the adverse effects of charging and discharging at different current densities on the cathode material structure (specifically, volume change), thus contributing to the improvement of LiMn. 0.25 Fe 0.75 Structural stability and electrochemical performance stability of PO4 / NC materials.

[0161] LiMn in Example 1 0.25 Fe 0.75 The X-ray diffraction (ex-XRD) spectra of the PO4 / NC material and the LiMnFePO4 / NC material in Comparative Example 1 during the delithiation process are shown in the figures below. Figure 14 As shown in (a) and (c); where, Figure 14 The right side of (a) is a magnified view of 2θ = 16 to 18; Figure 14 The left side of (c) is a magnified view of 2θ = 16 to 18; Figure 14 The word "fresh" in this context indicates a sample that has not undergone any treatment (delithiation process). Figure 14 The voltage in the figure represents the charging voltage, indicating different degrees of lithium delithiation; a higher charging voltage indicates a greater degree of lithium delithiation. The LiMn in Example 1... 0.25 Fe 0.75 A schematic diagram of the delithiation mechanism of PO4 / NC materials, as shown below. Figure 14 As shown in (b) of the diagram.

[0162] Depend on Figure 14 It can be seen that: LiMn in Example 10.25 Fe 0.75 The in-situ XRD pattern of the PO4 / NC material shows that the positions of the characteristic peaks change continuously with the delithiation process, indicating a delithiation mechanism dominated by solid solution behavior. A slight two-phase structure can be observed at 3.4V, suggesting that LiMn... 0.25 Fe 0.75 The delithiation process of PO4 / NC materials is carried out through the formation of a non-equilibrium solid solution, accompanied by a slight hysteresis behavior.

[0163] Pure lithium iron phosphate (without manganese doping) follows a two-phase lithium intercalation / deintercalation mechanism during charging and discharging. As the deintercalation process proceeds, it separates into two phases (see...). Figure 14 (c) and T (Triphylite) represent lithium iron phosphate, and H (Heterosite) represent lithium iron phosphate. Traditional lithium iron phosphate cathode materials follow a two-phase nucleation and deintercalation mechanism, a process involving significant volume changes and substantial structural rearrangements, severely limiting high-rate performance. The establishment of a non-equilibrium single-phase solid solution avoids structural rearrangements, allowing for more flexible and efficient lithium-ion insertion and extraction at high current densities, thus contributing to excellent high-rate performance.

[0164] Compared with existing lithium iron phosphate materials and two-phase delithiation mechanisms, its difference lies in LiMn 0.25 Fe 0.75 The solid solution delithiation mechanism of PO4 / NC materials avoids structural rearrangement and compositional recombination. A small amount of manganese ion doping is beneficial for the establishment of a non-equilibrium single-phase solid solution, achieving excellent high-rate performance. However, the amount of manganese ion doping and the calcination in an inert atmosphere are crucial in this invention. Excessive manganese ion doping hinders the establishment of a non-equilibrium single-phase solid solution; if the precursor is first calcined in air to consume carbon components before being mixed with the lithium and phosphorus sources and calcined in an inert atmosphere, the carbon content will be too low, leading to particle aggregation, relatively poor conductivity, and unsatisfactory rate performance.

[0165] 8. Using an electrochemical workstation (CHI-760E, China), the LiMn in Example 1... 0.25 Fe 0.75 PO4 / NC materials at different scan rates (0.2-1.0 mV / s) -1 The CV curves under ( ), and the analysis results based on these CV curves, such as Figure 15 As shown; where, Figure 15 (a) is LiMn 0.25 Fe 0.75 CV curves of PO4 / NC material at different scan rates Figure 15 (b) is for Figure 15 (a) The calculated b-values ​​of the redox peaks after data processing. Figure 15 (c) is LiMn 0.25 Fe 0.75 PO4 / NC material at a scan rate of 1.0 mV s -1 CV curve at time; Figure 15 (d) is LiMn 0.25 Fe 0.75 A comparison of the pseudocapacitive contribution rates of PO4 / NC materials measured at different scan rates.

[0166] Existing technology shows that pseudocapacitive behavior can be evaluated using the following formula, where i and v represent peak current and scan rate, respectively.

[0167] i = av b (3)

[0168] i(V) = k1v + k2v 1 / 2 (4)

[0169] Figure 15 The results show: Figure 15 (b) By fitting the slopes of Log(i) and Log(v), the value of b in formula (3) can be obtained. When the value of b is close to 0.5, the process is mainly diffusion-based, while when the value of b is close to 1.0, it represents a process mainly based on capacitance. In formula (4), k1v symbolizes the pseudocapacitive characteristic, and k2v 1 / 2 Corresponding to diffusion characteristics, Figure 15 (c) indicates that the scan rate is 1.0 mV / s. -1 The contribution rate of pseudocapacitance is 92.9%, indicating that capacitance is dominant. Figure 15 (d) indicates that the pseudocapacitive contribution gradually increases with increasing cyclic voltammetry scan rate. Using LiMn... 0.25 Fe 0.75 The cathode material prepared by PO4 / NC has a high pseudocapacitance contribution, indicating a faster kinetic reaction, which promotes excellent rate performance. This is related to the improved electronic conductivity brought about by appropriate manganese doping, as well as the abundant active sites provided by the relatively large specific surface area.

[0170] 9. LiMn in Example 1 0.25 Fe 0.75 The battery, assembled from PO4 / NC materials, was tested using the electrostatic intermittent titration (GITT) procedure of Blue Electric Test (CT 2001A, China), with a voltage range of 2.3-4.8V and a resting time of 1 hour between each charge / discharge cycle. The results are as follows. Figure 16 As shown; where, Figure 16 (a) is the voltage response curve. Figure 16(b) is a graph showing the results of a constant current intermittent titration test. Figure 16 (c) is a graph showing the change in lithium-ion diffusion coefficient during charging and discharging.

[0171] Figure 16 Results analysis: To investigate the lithium storage advantage of this electrode, its electrochemical kinetics were studied using an electrostatic intermittent titration procedure (GITT). Figure 16 (a) illustrates the voltage response of the electrode, which matches the charge-discharge curve and has the same operating voltage plateau at 3.45V and 4.1V. Figure 16 (b) shows the complete spectrum of the delithiation and lithiation processes; Figure 16 (c) indicates that the lithium-ion diffusion coefficient of this material remains at 10 during charging and discharging. –12 cm 2 s –1 Up to 10 -6 cm 2 s –1 The cathode material exhibits rapid lithium-ion diffusion; these results confirm the superior electrochemical kinetics of the material, which is attributed to the increased number of additional active sites that accelerate electron transport and improve reaction kinetics.

[0172] Comparative Example 1

[0173] This comparative example provides a method for preparing a lithium iron phosphate composite material. The difference between this method and Example 1 is that the 2.18 mmol MnSO4 and 2.55 mmol FeCl3 in step 2) are replaced with 4 mmol FeCl3, and the final sample obtained is LiFePO4 / NC, denoted as LFP / NC.

[0174] Analysis revealed that the sample possessed a pure-phase structure of lithium iron phosphate (see...). Figure 17 In (a) of the sample, the final product has the morphology of solid spherical particles.

[0175] Comparative Example 2

[0176] This comparative example provides a method for preparing a lithium manganese iron phosphate composite material. The difference between this method and Example 1 is that in step 2), 2.18 mmol MnSO4 and 2.55 mmol FeCl3 are replaced with 4.0 mmol MnSO4 and 0.8 mmol FeCl3, resulting in a final sample of LiMn... 0.5 Fe 0.5 PO4 / NC, denoted as LM 0.5 F 0.5 P / NC.

[0177] Analysis revealed that the sample also exhibited an olivine structure. Figure 17 In (b) of the article, the morphology of the final product is similar to that of LiMn.0.25 Fe 0.75 PO4 and NC materials are similar, consisting of spherical particles.

[0178] Material characterization:

[0179] exist Figure 18 In this diagram, x represents the doping amount of Mn (i.e., the molar ratio of Mn to P). SEM images of the precursor particles in Example 1 and LiMn are shown. 0.25 Fe 0.75 The SEM and TEM images of PO4 / NC are as follows: Figure 18 As shown in (b), (e), and (h) in Figure 1; the SEM images, LiFePO4 / NC SEM images, and TEM images of the precursor particles in Comparative Example 1 are shown in Figure 2. Figure 18 The images are shown in (a), (d), and (g); SEM images of the precursor particles in Comparative Example 2, and LiMn. 0.5 Fe 0.5 The SEM and TEM images of PO4 / NC are as follows: Figure 18 As shown in (c), (f), and (i).

[0180] Depend on Figure 18 It can be seen that the formation and morphology of the materials in Example 1, Comparative Example 1, and Comparative Example 2 are different. With increasing manganese content, the precursor morphology evolves from a smooth cube to a sphere, and then to an irregular octahedron. The final product roughly inherits the morphology of the precursor, although the particle size difference is not significant. Therefore, the morphology should not have a major impact on performance. The most significant difference is the amount of manganese doped (LM). 0.25 F 0.75 P / NC exhibits the best rate performance (see...) Figure 13 This is related to its solid solution delithiation mechanism; LFP / NC follows the traditional two-phase delithiation mechanism (see...). Figure 13 and Figure 14 The significant volume change and structural reorganization prevent it from flexibly adapting to high current densities, resulting in relatively poor rate performance. Furthermore, when the manganese doping level reaches 0.5%, the material exhibits more severe polarization during cycling, while Mn... 2+ Transform into Mn 3+ The process involves the Jahn-Teller effect, which causes changes in volume and results in suboptimal rate performance.

[0181] It is also important to emphasize that this invention presents a simple atomic control strategy to prepare MOF-derived lithium manganese iron phosphate composite materials with special structures through a simple and feasible solid-state sintering method. This material, as a cathode material, exhibits excellent cycle life and higher rate performance, thanks to its suitable Mn doping amount, high specific surface area, abundant internal hole structure, loose porous structure, N-doped conductive carbon coating, and favorable lithium insertion / extraction mechanism. Compared to the two-phase transition process of lithium iron phosphate / carbon materials, the single-phase solid solution transition of the lithium manganese iron phosphate composite material of this invention avoids significant structural rearrangement and volume changes, thereby effectively reducing lithium-ion transport barriers and improving electronic conductivity. This MOF self-sacrificing strategy of adjusting morphological characteristics and controlling the number of dopant ions provides a new approach for designing transition metal phosphate-based cathode materials.

[0182] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A method for preparing a lithium manganese iron phosphate composite material, characterized in that, Includes the following steps: 1) Mix polyvinylpyrrolidone, manganese source, iron source and solvent to obtain a metal ion solution; 2) The metal ion solution described in step 1) and the K3[Fe(CN)6] solution are mixed and subjected to a co-precipitation reaction to obtain the Prussian blue precursor; 3) After mixing the Prussian blue precursor, phosphorus source and lithium source described in step 2), the mixture is pre-calcined and calcined to obtain lithium manganese iron phosphate composite material. In step 3), both the pre-calcination and calcination are carried out under a protective atmosphere, the pre-calcination temperature is 300℃~400℃, and the calcination temperature is 600℃~700℃. The lithium manganese iron phosphate composite material comprises spherical particles assembled from lithium manganese iron phosphate particles and a nitrogen-doped carbon layer coating the spherical particles; the chemical formula of the lithium manganese iron phosphate particles is LiMn. x Fe 1-x PO4, where the range of x is 0.2≤x≤0.

4.

2. The method for preparing the lithium manganese iron phosphate composite material according to claim 1, characterized in that: The preparation method of the lithium manganese iron phosphate composite material includes the following steps: 1) Dissolve polyvinylpyrrolidone in ethanol and water to obtain a PVP solution; A metal ion solution is obtained by mixing a PVP solution, a manganese source, and an iron source. 2) The metal ion solution described in step 1) is added to K3[Fe(CN)6] solution and mixed, and then subjected to a co-precipitation reaction to obtain the Prussian blue precursor; 3) The Prussian blue precursor, phosphorus source, lithium source and dispersant described in step 2) are mixed by wet grinding, and then pre-calcined and calcined to obtain lithium manganese iron phosphate composite material. In step 3), both the pre-calcination and calcination are carried out under a protective atmosphere. The pre-calcination temperature is 300℃~400℃, and the calcination temperature is 600℃~700℃.

3. The method for preparing the lithium manganese iron phosphate composite material according to claim 1 or 2, characterized in that: Step 3) The pre-calcination time is 4-8 hours, and the calcination time is 4-8 hours; And / or, in step 3), the heating rate for pre-calcination is 3~6℃ / min, and the heating rate for calcination is 3~6℃ / min.

4. The method for preparing the lithium manganese iron phosphate composite material according to claim 1 or 2, characterized in that: Step 1) The manganese source is one or more of manganese sulfate, manganese chloride, and manganese nitrate; the manganese in the manganese source is divalent. And / or, the iron source in step 1) is one or more of ferric chloride, ferric sulfate, and ferric nitrate; the iron in the iron source is in the valence trivalent (+3). And / or, the molar ratio of the manganese source and the iron source in step 1) is (0.6~1.0):

1.

5. The method for preparing the lithium manganese iron phosphate composite material according to claim 1 or 2, characterized in that: The ratio of the total molar amount of manganese and iron sources in step 1) to the molar amount of K3[Fe(CN)6] in the K3[Fe(CN)6] solution in step 2) is (1.0~1.2):1; And / or, the temperature of the coprecipitation reaction in step 2) is 50℃~80℃; And / or, the coprecipitation reaction in step 2) takes 4 to 8 hours.

6. The method for preparing the lithium manganese iron phosphate composite material according to claim 2, characterized in that: Step 3) The dispersant is selected from at least one of anhydrous ethanol, methanol, ethylene glycol, and glycerol; And / or, in step 3), the mass ratio of the Prussian blue precursor, phosphorus source, and lithium source is (1.1~2):1:(0.2~0.8).

7. A lithium manganese iron phosphate composite material prepared by the preparation method according to any one of claims 1 to 6.

8. A positive electrode material, characterized in that, It includes the lithium manganese iron phosphate composite material as described in claim 7.

9. A battery, characterized in that, It includes the lithium manganese iron phosphate composite material as described in claim 7.

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