A manganese iron lithium phosphate composite positive electrode material with mn ion self-compensation and multiple interception function and a preparation method thereof

Abstract

The application discloses a manganese iron lithium phosphate composite positive electrode material with Mn ion self-compensation and multiple interception functions and a preparation method thereof. The manganese iron lithium phosphate composite positive electrode material is composed of a manganese iron lithium phosphate positive electrode material and functional composite film micro-powder loaded on the surface and between the particles of the manganese iron lithium phosphate positive electrode material. The functional composite film of the functional composite film micro-powder is prepared from Na-merolite powder through Mn ion exchange, grafting and copolymerization, lithiumation and PVA composite film preparation. The manganese iron lithium phosphate composite positive electrode material is prepared from the manganese iron lithium phosphate positive electrode material and the functional composite film through mixing, crushing and freeze-drying. The manganese iron lithium phosphate composite positive electrode material realizes the inhibition of the dissolution of Mn 2+ in the manganese iron lithium phosphate positive electrode material from the source, improves the ion conduction performance and the cycle stability of the manganese iron lithium phosphate positive electrode material, and has the advantages of high capacity, long cycle life, high safety and the like.

Description

Technical Field

[0001] This invention relates to the field of lithium manganese iron phosphate cathode material technology, and in particular to a lithium manganese iron phosphate composite cathode material and its preparation method having Mn ion self-compensation and multiple interception functions. Background Technology

[0002] Among various lithium-ion battery cathode materials, lithium iron phosphate (LiFePO4) has gained widespread application due to its high safety and low cost, but its voltage plateau is relatively low (approximately 3.4 V), and its energy density is gradually approaching its theoretical limit. Lithium manganese iron phosphate (LiMn) is formed by introducing manganese. x Fe 1-x PO4, LMFP) material, with some Fe 2+ Replace with Mn 2+ Using Mn 2+ / Mn 3+ With a redox potential of approximately 4.1 V, the working voltage can be increased by about 0.7 V while maintaining the structural stability of olivine, thereby significantly improving the energy density of the material. This has become a research hotspot for high-energy-density cathode materials.

[0003] However, the industrial application of LMFP faces three major technical bottlenecks: First, its intrinsic electronic conductivity is extremely low, similar to lithium iron phosphate, resulting in poor electron mobility and severe polarization during high-rate charge and discharge; second, the migration rate of lithium ions in the one-dimensional diffusion channel is slow, and the intrinsic diffusion coefficient is typically only 10. -14 -10 -15 cm 2 The speed-to-second ( / s) range limits the material's fast-charging performance; thirdly, there is the issue of manganese ion dissolution, which causes Mn to leach out during cycling. 3+ It readily undergoes a disproportionation reaction to generate Mn 2+ It dissolves in the electrolyte, causing loss of active material and damage to the crystal structure (Jahn-Teller distortion), while the dissolved Mn... 2+ Manganese leaching can deposit on the surface of the negative electrode, damaging the SEI film and causing rapid capacity decay. Among these factors, manganese leaching is considered the most critical factor limiting the long-term cycle stability of LMFPs.

[0004] Therefore, developing an integrated modification technology that can simultaneously achieve synergistic effects of lattice manganese vacancy self-compensation, efficient lithium-ion conduction, physical barrier and chemical capture is of great practical significance for breaking through the cycle stability bottleneck of LMFP and promoting its commercial application. Summary of the Invention

[0005] Purpose of the invention: The purpose of this invention is to provide a lithium manganese iron phosphate composite cathode material and its preparation method that have Mn ion self-compensation and multiple interception functions, so as to suppress Mn ion in lithium manganese iron phosphate cathode materials from the source.2+ The dissolution of Mn ions improves the ion conductivity and cycle stability of the material. Technical solution: This invention relates to a lithium manganese iron phosphate composite cathode material with Mn ion self-compensation and multiple interception functions. This lithium manganese iron phosphate composite cathode material is composed of lithium manganese iron phosphate cathode material and functional composite film micropowder loaded on the surface and between particles of the lithium manganese iron phosphate cathode material.

[0006] The functional composite membrane of the functional composite membrane micropowder is prepared by the following steps:

[0007] (1) Preparation of Mn-M ...

[0008] (2) Preparation of γ-MPS-PEGMA-Mn ...

[0009] (3) Lithated γ-MPS-PEGMA-Mn-Mn-hydroxysilane: γ-MPS-PEGMA-Mn-Mn-hydroxysilane is placed in a lithium salt solution, and after reaction, filtration, washing and drying, γ-MPS-PEGMA-Mn-Mn-hydroxysilane powder is obtained.

[0010] (4) Preparation of functional composite membrane: Lithium-based γ-MPS-PEGMA-Mn-sodium hydroxyl silicate powder is placed in PVA solution, and a functional composite membrane is obtained by cross-linking reaction, degassing and casting.

[0011] Furthermore, the functional composite film powder of this composite cathode material accounts for 2-16 wt% of the mass of the lithium manganese iron phosphate cathode material.

[0012] Furthermore, in step (1) of preparing the functional composite membrane of the composite cathode material, the preparation of Mn-Na-malathion powder includes the following steps: adding Na-malathion powder to a 0.1-0.5 mol / L MnSO4 aqueous solution, stirring at 40-80℃ for 6-36 h, collecting by centrifugation, washing, and drying to obtain Mn-malathion powder; the mass ratio of Na-malathion powder to MnSO4 is (0.5-2):1. Preferably, the mass ratio of Na-malathion powder to MnSO4 is 1:1.

[0013] Furthermore, in step (2) of preparing the functional composite film of the composite cathode material, the preparation of γ-MPS-PEGMA-Mn-sodium hydroxyl silicate includes the following steps:

[0014] 1) Add Mn-M ...

[0015] 2) Disperse γ-MPS-Mn ...

[0016] Furthermore, in step (3) of the preparation of the functional composite membrane of the composite cathode material, the lithium-ionized γ-MPS-PEGMA-Mn-hydroxysilane is prepared by the following steps: γ-MPS-PEGMA-Mn-hydroxysilane powder is added to a 0.05-0.2 mol / L lithium acetate aqueous solution, the pH is adjusted to 7.5-8.0, and the mixture is stirred and reacted at 40-60℃ for 1-4 hours. After filtration, washing and drying, lithium-ionized γ-MPS-PEGMA-Mn-hydroxysilane powder is obtained; the solid-liquid ratio of the γ-MPS-PEGMA-Mn-hydroxysilane powder to the lithium acetate aqueous solution is 1:(20-50) g / mL.

[0017] Furthermore, in step (4) of preparing the functional composite film of the composite cathode material, the functional composite film is prepared by the following steps:

[0018] a) Lithium-based γ-MPS-PEGMA-Mn-methylsiliconized sodium powder is added to a PVA solution and stirred at room temperature for 1-3 hours. The mixture is then placed in an ice-water bath and ultrasonically dispersed at 200-400 W for 30-60 minutes to obtain a dispersion. The amount of lithium-based γ-MPS-PEGMA-Mn-methylsiliconized sodium powder added is 5-15 wt% of the PVA mass.

[0019] b) Add an aqueous solution of glutaraldehyde, a crosslinking agent, dropwise to the dispersion, and stir for 20-40 minutes to obtain a mixture; the amount of glutaraldehyde added is 2-5 wt% of the PVA mass.

[0020] C) The mixture is vacuum degassed at room temperature for 10-30 min, then cast into a mold, and naturally evaporated and dried at room temperature for 8-16 h, then transferred to an oven at 40-60℃ for 2-6 h. After drying, the mixture is peeled off to obtain a functional composite film with a thickness of 20-40 μm.

[0021] Furthermore, the PVA solution is prepared by the following steps: adding polyvinyl alcohol to deionized water, heating and stirring in a water bath at 80-90℃ to dissolve it, and preparing a 3-6 wt% PVA solution.

[0022] The present invention provides a method for preparing the above-mentioned lithium manganese iron phosphate composite cathode material with Mn ion self-compensation and multiple interception functions, comprising the following steps: mixing the functional composite film with lithium manganese iron phosphate cathode material powder to prepare a slurry with a solid content of 15-25 wt%, wet ball milling for 2-4 h, pre-freezing at -50~-30℃ for 2-6 h, and then freeze-drying at a cold trap temperature ≤-50℃ and a vacuum degree <10 Pa for 24-48 h to obtain composite powder; finally, grinding the composite powder for 3-10 min and passing it through a 200-400 mesh sieve to obtain the lithium manganese iron phosphate composite cathode material.

[0023] Furthermore, the lithium manganese iron phosphate cathode material powder of this composite cathode material is prepared by the following steps:

[0024] d) Prepare a grinding slurry with a solid content of 40-50% by mixing lithium, manganese, iron, phosphorus and carbon sources. After ball milling for 4-8 hours, vacuum dry at 120-180℃ for 8-16 hours to obtain precursor powder. The amount of carbon source added accounts for 6-10% of the total mass of the materials composed of lithium, manganese, iron, phosphorus and carbon sources.

[0025] e) The precursor powder is pre-calcined at 400-600℃ for 3-6 hours under an inert atmosphere, and then sintered at 650-800℃ for 6-12 hours to obtain LiMn with a carbon coating layer and 0.3≤x≤0.8. (1-x) Fe x PO4 powder.

[0026] Furthermore, in the preparation of the lithium manganese iron phosphate cathode material powder of this composite cathode material, the lithium source is selected from one or more of lithium carbonate, lithium hydroxide, lithium acetate, lithium phosphate, and lithium dihydrogen phosphate; the manganese source is selected from one or more of manganese sulfate, manganese oxalate, manganese nitrate, manganese tetroxide, manganese carbonate, and manganese dioxide; the iron source is selected from one or more of ferric chloride, ferrous oxalate, ferric nitrate, ferric oxide, and ferric oxide; the phosphorus source is selected from one or more of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and phosphoric acid; and the carbon source is selected from one or more of sucrose, glucose, polyethylene glycol, and polyvinyl alcohol.

[0027] Beneficial effects: Compared with the prior art, the significant advantages of the present invention are:

[0028] (1) The lithium manganese iron phosphate composite cathode material can effectively reduce the dissolution of Mn, with the dissolution being less than 30 ppm, or even less than 3 ppm;

[0029] (2) The lithium manganese iron phosphate composite cathode material has excellent structural stability and cycle stability. After 100 cycles at 1C, the capacity retention rate can still reach more than 94%, or even more than 98%.

[0030] (3) The lithium manganese iron phosphate composite cathode material has excellent ion conduction performance, with a resistivity of less than 61 Ω·cm, and even less than 5 Ω·cm; the first discharge specific capacity at 0.1C reaches more than 155 mAh / g, and even exceeds 168 mAh / g; the first discharge specific capacity at 1C reaches more than 145 mAh / g, and even exceeds 155 mAh / g.

[0031] This means that the lithium manganese iron phosphate composite cathode material achieves the goal of suppressing Mn content in lithium manganese iron phosphate cathode materials from the source. 2+ While dissolving, it improves its ion conductivity and cycle stability. Detailed Implementation

[0032] The technical solution of the present invention will be further described in detail below with reference to the embodiments.

[0033] Example 1

[0034] 1. Preparation of LiMn 0.6 Fe 0.4 PO4 cathode material

[0035] The lithium iron phosphate cathode material of Example 1 was prepared by the following steps:

[0036] (1) Prepare lithium carbonate, manganese sulfate, ferric chloride, and ammonium dihydrogen phosphate (these four components account for 92% of the total mass fraction of the material) according to the chemical molar ratio of n(Li):n(Mn):n(Fe):n(P)=1:0.6:0.4:1. Add them to a ball mill, then add 8% (by mass fraction of the total material) of sucrose, and use anhydrous ethanol as a solvent to form a grinding slurry (solid content of 45%). After ball milling for 7 hours, transfer it to a vacuum drying oven and dry it at 150℃ for 12 hours to obtain LiMn. 0.6 Fe 0.4 PO4 powder;

[0037] (2) LiMn 0.6 Fe 0.4 PO4 powder was transferred to a kiln and sintered at 500°C for 5 hours under nitrogen protection, followed by sintering at 740°C for 10 hours. After cooling to room temperature, it was ground into fine powder to obtain LiMn with a carbon layer coating. 0.6 Fe 0.4 PO4 powder.

[0038] 2. Preparation of functional composite membranes

[0039] (1) Dissolve 16g NaOH in 200mL of deionized water and stir until the solid is completely dissolved. Slowly add 270g silica sol (30% SiO2) while stirring, and stir further at room temperature for 2h to obtain a uniform gel. Transfer the obtained gel to a stainless steel autoclave and react it at 150℃ for 48h. After the reaction, wash the obtained solid product with deionized water until the pH of the filtrate is neutral. Place the washed filter cake in an oven and dry it at 60℃ for 12h to prepare pure phase Na-Mag powder (abbreviated as Na-Mag powder).

[0040] (2) Prepare a 0.3 mol / L MnSO4 aqueous solution by dissolving 5 g of MnSO4. Add 5 g of Na-Mag powder to the MnSO4 aqueous solution and stir at 60 °C for 20 h to allow manganese ions to replace sodium ions between the layers of Na-Mag powder, thereby obtaining Mn-Mag (abbreviated as Mn-Mag) with manganese ions between the layers. Collect the obtained material by centrifugation at 7000 rpm, wash it 8 times with deionized water, and dry it by freeze drying to obtain Mn-Mag powder.

[0041] (3) Mn-Mag powder was directly added to anhydrous toluene at a solid-liquid ratio of 1 g powder to 80 mL solvent. The mixture was ultrasonically dispersed for 15 min to obtain a dispersion. Under nitrogen protection, γ-methacryloxypropyltrimethoxysilane (γ-MPS) was added to the dispersion at 30% of the mass of Mn-Mag powder. The reaction system was heated to 110 °C and stirred under nitrogen atmosphere for reflux for 18 h to allow γ-MPS to enter the interlayer of Mn-Mn-Mag silicate and covalently bond with the silanol groups (Si-OH) on the surface of the layers. After the reaction was completed, the mixture was allowed to cool naturally and the product was separated by centrifugation. The product was washed twice with anhydrous toluene and three times with anhydrous ethanol to remove unreacted γ-MPS coupling agent. The product was dried in a vacuum drying oven at 50 °C for 12 h to obtain γ-MPS-Mn-Mag powder.

[0042] (4) Disperse γ-MPS-Mn-Mag powder in anhydrous toluene at a solid-liquid ratio of 1g powder to 50mL solvent, and separate the solution into the dispersion. Weigh polyethylene glycol methacrylate (PEGMA, molecular weight 300-500 Da) monomer and dissolve it in anhydrous toluene to prepare a 15 wt% solution. Add the monomer solution to the dispersion at a mass ratio of γ-MPS-Mn-Mag powder to PEGMA of 1:2. Add azobisisobutyronitrile (AIBN) as an initiator at a weight of 1.0% of the PEGMA monomer. After purging the reaction system with nitrogen for 15min to remove dissolved oxygen, heat the system to 70℃ under nitrogen protection and stir for 16h to induce free radical polymerization of PEGMA by the vinyl double bonds grafted onto the surface of the sodium silicate slab, forming a covalently bonded polyethylene glycol methacrylate network between the layers. After the reaction was completed, the product was centrifuged and washed three times with anhydrous ethanol to remove unreacted PEGMA monomers and homopolymers. The product was dried in a vacuum drying oven at 50°C for 12 hours to obtain γ-MPS-PEGMA-Mn-Mag powder.

[0043] (5) Add γ-MPS-PEGMA-Mn-Mag powder to a 0.1 mol / L CH3COOLi aqueous solution, adjust the pH to 7.5-8.0, and the solid-liquid ratio is 1 g powder to 30 mL solution. Place the mixture in a constant temperature water bath stirrer and stir gently at 50°C for 2 h for lithiation reaction. After lithiation exchange, filter and separate the product, and wash repeatedly with deionized water until the filtrate is neutral (pH=7). Dry the completely washed product in a vacuum drying oven at 50°C for 8 h to obtain lithiated γ-MPS-PEGMA-Mn-Mag powder.

[0044] (6) Weigh polyvinyl alcohol (PVA, degree of hydrolysis 98-99%, molecular weight approximately 89,000-98,000) and add it to deionized water. Heat and stir in a water bath at 85°C to dissolve and prepare a 5 wt% PVA solution. Cool to room temperature. Add lithium-ionized γ-MPS-PEGMA-Mn-Mag powder to the PVA solution at an amount of 8 wt% of the PVA mass. Stir at room temperature for 2 hours to ensure uniform dispersion. Place the mixture in an ice-water bath and ultrasonically disperse at 300 W for 45 minutes to ensure uniform dispersion of sodium silicate nanosheets in the PVA matrix, forming a "mortise and tenon interlocking" structure with chemical bonding and hydrogen bonding. Simultaneously add glutaraldehyde aqueous solution (25 wt%) as a crosslinking agent at an amount of 3 wt% of the PVA mass. Continue stirring for 30 minutes to further enhance the mechanical properties of the film. Place the mixed solution in a vacuum desiccator and degas under vacuum at room temperature for 20 minutes to remove air bubbles from the solution. After degassing, the solution was poured into a clean PTFE mold, placed horizontally, and allowed to evaporate naturally at room temperature for 12 hours, then transferred to an oven at 50°C for 4 hours. After drying and forming a film, the film was carefully peeled off the mold to obtain a functional sodium silicate / PVA composite film with a thickness of 20-40 μm.

[0045] 3. Preparation of lithium manganese iron phosphate composite cathode material

[0046] Weigh out functional sodium hydroxyl silicate / PVA composite membrane and LiMn 0.6 Fe 0.4 PO4 cathode material was mixed at 10% of the LMFP composite membrane mass. The mixed powder was added to anhydrous ethanol or deionized water to adjust the solid content to 20 wt%, and then wet-milled in a planetary ball mill at 250-350 rpm for 3 hours to obtain a homogeneous slurry. The slurry was transferred to a freeze-drying container and pre-frozen at -40℃ for 4 hours, followed by freeze-drying in a vacuum freeze dryer (cold trap temperature ≤ -50℃, vacuum degree <10Pa) for 24 hours. The resulting loose composite powder was gently ground in an agate mortar for 5 minutes and passed through a 300-mesh sieve to obtain the lithium manganese iron phosphate composite cathode material.

[0047] Examples 2 to 8

[0048] The steps in Examples 2 to 8 are the same as those in Example 1 for the lithium manganese iron phosphate cathode material and the functional composite film. The difference lies in the different mass ratio of the functional composite film to LMFP, as shown in Table 1 below:

[0049] Table 1. Percentage of functional composite membranes in LMFP mass in Examples 1-8

[0050]

[0051] Comparative Example 1

[0052] Comparative Example 1 served as a blank control group, which was directly the LiMn prepared in Example 1. 0.6 Fe 0.4 PO4 cathode material, without the addition of functional sodium silicate / PVA composite membrane.

[0053] Comparative Example 2

[0054] The LiMn used in Comparative Example 2 0.6 Fe 0.4 The PO4 cathode material is the same as in Example 1, except that the Na-Mag powder prepared in Example 1 is directly mixed with LiMn. 0.6 Fe 0.4 PO4 cathode materials are compounded to form composite cathode materials.

[0055] Comparative Example 3

[0056] The LiMn used in Comparative Example 3 0.6 Fe 0.4 The PO4 cathode material is the same as in Example 1, except that the Mn-Mag powder prepared in Example 1 is directly mixed with LiMn. 0.6 Fe 0.4 PO4 cathode materials are compounded to form composite cathode materials.

[0057] Comparative Example 4

[0058] The LiMn used in Comparative Example 4 0.6 Fe 0.4 The PO4 cathode material is the same as in Example 1, except that the γ-MPS-PEGMA-Mn-Mag powder prepared in Example 1 is directly mixed with LiMn. 0.6 Fe 0.4 PO4 cathode materials are compounded to form composite cathode materials.

[0059] Comparative Example 5

[0060] The LiMn used in Comparative Example 5 0.6 Fe 0.4 The PO4 cathode material is the same as in Example 1, except that the lithium-ionized γ-MPS-PEGMA-Mn-Mag powder prepared in Example 1 is directly mixed with LiMn. 0.6 Fe 0.4 PO4 cathode materials are compounded to form composite cathode materials.

[0061] Performance testing

[0062] The positive electrode materials of Examples 1-8 and Comparative Examples 1-5 were thoroughly mixed with the conductive agent acetylene black and the binder PVDF at a mass ratio of 90:5:5. An appropriate amount of N-methylpyrrolidone was added, and the mixture was thoroughly ground to form a positive electrode sheet. The sheet was then dried at 150 degrees Celsius in a vacuum for 12 hours for later use. A lithium metal sheet was used as the negative electrode, and a polyethylene composite membrane was used. The electrolyte was 1 mol / L and was a mixture of ethylene carbonate EC and dimethyl carbonate DMC. The half-cells were assembled in an argon-protected glove box and charged using constant current and constant voltage. The electrochemical test results are shown in Table 2.

[0063] Table 2. Physicochemical and Electrochemical Test Results

[0064]

[0065] As can be seen from the data in Table 2 of the embodiments and comparative examples, the lithium manganese iron phosphate composite cathode material prepared in the embodiments of the present invention can effectively reduce the dissolution of Mn, with a dissolution of less than 30 ppm, and even less than 3 ppm; it also has excellent structural stability and cycle stability, with a capacity retention rate of more than 94% after 100 cycles at 1C, and even more than 98%; in addition, it has excellent ion conduction performance, with a resistivity of less than 61 Ω·cm, and even less than 5 Ω·cm; the first discharge specific capacity at 0.1C reaches more than 155 mAh / g, and even exceeds 168 mAh / g, and the first discharge specific capacity at 1C reaches more than 145 mAh / g, and even exceeds 155 mAh / g.

[0066] Based on the above experimental results, combined with the preparation process of this composite cathode material, further mechanistic analysis reveals that:

[0067] First, this invention forms a "manganese ion compensation pool" by pre-embedding manganese ions between the layers of lithium manganese iron phosphate. When manganese vacancies appear in the lithium manganese iron phosphate lattice during long-term cycling, the pre-embedded Mn in the interlayer... 2+ It can replenish lattice defects in situ under the drive of concentration gradient and electric field, and repair the lattice damage that has been generated in real time, thereby suppressing Jahn-Teller distortion and structural collapse caused by the accumulation of manganese vacancies.

[0068] Secondly, this invention utilizes γ-MPS grafting and PEGMA copolymerization to form a covalently bonded polyethylene glycol methacrylate network between and on the surface of sodium silicate, which is then lithiated to obtain PEO-Li. + Ion conductor. This network establishes a continuous, fast lithium-ion transport channel between lithium manganese iron phosphate particles, where the ether oxygen atoms interact with Li... +The coordination / decoupling interaction provides lithium ions with an additional migration path besides the electrolyte. This results in a significant reduction in ion transport impedance at the interface, thereby effectively improving the rate performance and capacity utilization of the material.

[0069] Next, lithium-treated sodium silicate is composited with polyvinyl alcohol (PVA) to form a flexible film, which is then pulverized and freeze-dried to uniformly coat the surface of lithium manganese iron phosphate particles with composite film powder. The PVA molecular chains and the two-dimensional sheets of sodium silicate are cross-linked by hydrogen bonds and glutaraldehyde to form a three-dimensional network structure with interlocking tenon and mortise-and-tenon joints. This structure combines flexibility and rigidity, effectively absorbing and dispersing the stress generated by volume expansion during charging and discharging, preventing particle cracking and electrode structure damage, thereby further inhibiting manganese ion dissolution caused by cracks and extending battery cycle life.

[0070] Furthermore, by comparing the test results of Examples 1-5, it can be found that as the proportion of added composite film powder increases, the resistivity of lithium manganese iron phosphate cathode material decreases and the amount of manganese leached decreases rapidly.

[0071] Based on the decrease in resistivity, further mechanistic analysis reveals that it is the result of the synergistic effect of multiple factors: First, after lithiation treatment, the polyethylene oxide network formed by PEGMA grafting exhibits coordination between its ether oxygen atoms and lithium ions, constructing a continuous and efficient lithium-ion transport channel between LMFP particles, significantly reducing the interfacial ion transport impedance within the electrode. However, at low addition levels, this ion conduction network has not yet formed a continuous, interconnected structure, and lithium-ion transport between particles is mainly point-contact, resulting in higher impedance. With the increase of the composite membrane powder ratio, PEO-Li... + Chain segments gradually build a three-dimensional "bridging" structure on the particle surface and in the gaps, providing lithium ions with an additional fast migration path besides liquid-phase migration, effectively suppressing concentration polarization. Secondly, the bridging effect of the PVA matrix in the composite film powder improves the interfacial contact between LMFP particles, reducing contact resistance caused by loose particle packing. In addition, the existing carbon coating layer of the LMFP particles remains intact, continuing to provide the necessary electronic conduction pathways. The synergistic effect of the establishment of ion conduction channels, the improvement of interfacial contact, and the electronic conduction of the original carbon coating layer jointly contributes to the continuous decrease in powder resistivity.

[0072] The rapid decrease in manganese leaching also relies on a triple defense: ① Self-compensating pool: Mn pre-embedded in the Mag interlayer. 2+ The density increases with the amount added, and it can fill manganese vacancies in the LMFP lattice more promptly, reducing manganese leaching from the source; ② Physical barrier: Functional sodium silicate / PVA composite film powder uniformly coats the particles, forming a dense protective layer that mechanically blocks Mn. 2+③ Outward diffusion; chemical chelation: PVA hydroxyl groups, Mag silanol groups, and PEGMA ether bonds synergistically capture free Mn. 2+ .

[0073] The electrical properties are also the result of the combined effects of multiple factors: PEO-Li + The conduction network is Li + It provides a rapid auxiliary path, reducing irreversible overpotential during the first charge and allowing more lithium ions to re-intercalate during discharge, thereby reducing polarization. Functional sodium silicate / PVA forms a "mortise and tenon interlocking" structure, acting as an elastic adhesive between particles, further improving electrical contact. The porous network formed by freeze-drying allows the electrolyte to quickly penetrate to the particle surface, ensuring full activation of all active materials. These positive effects increase with increasing addition amount, resulting in a gradual increase in the specific capacity at 0.1C and 1C initial discharge. The improved capacity retention is the result of the synergistic effect of self-compensation, physical barriers, chelate trapping, and conductive networks.

[0074] Comparing the test results of Examples 1 and 6-8, it can be found that as the proportion of added composite film powder increases, the resistivity of the lithium manganese iron phosphate cathode material continues to decrease, and the manganese dissolution also decreases. This indicates that when the addition amount exceeds 10%, the conductive network has basically formed a three-dimensional interconnected structure. Further adding functional sodium hydroxylite / PVA powder mainly adds redundant conduction paths to the existing network, resulting in diminishing marginal benefits. In addition, excessive addition of functional sodium hydroxylite / PVA powder begins to occupy the volume between particles, hindering some direct contact and slightly increasing the electron jumping distance, thus significantly slowing down the rate of resistivity decrease. Regarding manganese dissolution, when the addition amount exceeds 10 wt%, a basically complete and dense composite film coating layer has been formed on the surface of the lithium manganese iron phosphate particles, and most of the manganese ion dissolution channels have been effectively blocked. At this point, further adding composite film powder can only supplement and strengthen the local weak areas of the coating layer. Therefore, although the manganese dissolution continues to decrease, the rate of decrease gradually slows down. This phenomenon indicates that an addition of 10 wt% has enabled the composite membrane to achieve a high level of particle coating, while excessive addition has not caused a rebound or deterioration in manganese leaching. This proves that the composite membrane material of the present invention has excellent process tolerance and can achieve continuous inhibition of manganese leaching within a wide range of addition amounts without any negative impact.

[0075] In terms of electrical performance, the excessive composite film powder forms a thick coating layer on the particle surface. Although this does not disrupt the ion conduction channels, it prolongs the effective diffusion distance of lithium ions from the electrolyte into the particle interior. Some surface active sites are physically blocked, resulting in a slight decrease in the lithium ion insertion / extraction efficiency during the first discharge. Similarly, the excessively thick composite film coating layer increases the resistance to lithium ion penetration on the particle surface. Some particles cannot complete sufficient lithium insertion / extraction within the limited polarization time at 1C rate, leading to a decrease in actual utilization. Nevertheless, within a wide addition range of 10 wt% to 16 wt%, the specific capacity remains above 146 mAh / g, significantly higher than the 144 mAh / g of the unmodified lithium manganese iron phosphate material, demonstrating that the composite material of this invention can maintain excellent electrochemical performance even at high addition levels. The slight decrease in capacity retention is mainly attributed to the thick coating layer formed by the excessive composite film powder on the particle surface. Although this does not impair the self-compensation and chelation functions, it slightly increases the diffusion tortuosity of lithium ions in the solid phase, leading to the accumulation of local concentration polarization during long-term cycling.

[0076] Example 2, with only 2 wt% of complete composite membrane powder added, was compared with the comparative examples (partially modified). As shown in the table, Example 2 exhibited significantly better powder resistivity (60.1 Ω·cm), manganese leaching (29.3 ppm), 0.1C / 1C initial discharge specific capacity (155.7 / 146.8 mAh / g), and capacity retention after 100 cycles at 1C (94.3%) compared to Comparative Examples 2 to 5 (all with 10% functional material). This demonstrates that even with only the minimum amount of complete functional composite membrane powder (2%), its overall electrochemical performance surpasses any partially modified scheme without PVA composite film. This proves that the "Mn" composite membrane constructed in this invention... 2+ The "self-compensating cell-PEGMA ion-conducting network-PVA physical barrier-chemical chelation" multi-synergistic system can only be achieved through a complete process chain (ion exchange → grafting → copolymerization → lithiation → PVA composite film preparation → pulverization → freeze drying), and cannot be replaced by any single or partial modification.

[0077] Therefore, the lithium manganese iron phosphate composite cathode material of the present invention achieves the suppression of Mn in lithium manganese iron phosphate cathode materials from the source. 2+ While dissolving, it also improves its ion conductivity and cycle stability. Furthermore, apart from the above embodiments, the technical effects claimed above can be achieved using the processes and parameters defined in this invention, therefore further experimental verification is not required.

Claims

1. A lithium iron phosphate composite cathode material with Mn ion self-compensation and multiple interception functions, characterized in that, The lithium manganese iron phosphate composite cathode material consists of lithium manganese iron phosphate cathode material and functional composite film powder loaded on the surface and between particles of lithium manganese iron phosphate cathode material. The functional composite membrane of the functional composite membrane micropowder is prepared by the following steps: (1) Preparation of Mn-M ... (2) Preparation of γ-MPS-PEGMA-Mn ... (3) Lithated γ-MPS-PEGMA-Mn-Mn-hydroxysilane: γ-MPS-PEGMA-Mn-Mn-hydroxysilane is placed in a lithium salt solution, and after reaction, filtration, washing and drying, γ-MPS-PEGMA-Mn-Mn-hydroxysilane powder is obtained. (4) Preparation of functional composite membrane: Lithium-based γ-MPS-PEGMA-Mn-sodium hydroxyl silicate powder is placed in PVA solution, and a functional composite membrane is obtained by cross-linking reaction, degassing and casting.

2. The lithium iron phosphate composite cathode material with Mn ion self-compensation and multiple interception functions according to claim 1, characterized in that, The functional composite film powder accounts for 2-16 wt% of the mass of the lithium manganese iron phosphate cathode material.

3. The lithium iron phosphate composite cathode material with Mn ion self-compensation and multiple interception functions according to claim 1, characterized in that, The steps for preparing the functional composite membrane (1) include the following steps for preparing Mn-Na ...

4. The lithium iron phosphate composite cathode material with Mn ion self-compensation and multiple interception functions according to claim 1, characterized in that, In step (2) of the preparation of the functional composite membrane, γ-MPS-PEGMA-Mn-sodium hydroxyl silicate includes the following steps: 1) Add Mn-M ... 2) Disperse γ-MPS-Mn ...

5. The lithium iron phosphate composite cathode material with Mn ion self-compensation and multiple interception functions according to claim 1, characterized in that, In step (3) of the preparation of the functional composite membrane, the lithium-ionized γ-MPS-PEGMA-Mn-hydroxysilane is prepared by the following steps: γ-MPS-PEGMA-Mn-hydroxysilane powder is added to a lithium acetate aqueous solution with a concentration of 0.05-0.2 mol / L, the pH is adjusted to 7.5-8.0, and the mixture is stirred at 40-60℃ for 1-4 h. After filtration, washing and drying, lithium-ionized γ-MPS-PEGMA-Mn-hydroxysilane powder is obtained; the solid-liquid ratio of the γ-MPS-PEGMA-Mn-hydroxysilane powder to the lithium acetate aqueous solution is 1:(20-50) g / mL.

6. The lithium iron phosphate composite cathode material with Mn ion self-compensation and multiple interception functions according to claim 1, characterized in that, In step (4) of the preparation of the functional composite membrane, the functional composite membrane is prepared by the following steps: a) Lithated γ-MPS-PEGMA-Mn-methylsiliconized sodium powder is added to a PVA solution and stirred at room temperature for 1-3 hours. The mixture is then placed in an ice-water bath and ultrasonically dispersed at 200-400 W for 30-60 minutes to obtain a dispersion. The amount of lithated γ-MPS-PEGMA-Mn-methylsiliconized sodium powder added is 5-15 wt% of the PVA mass. b) Add an aqueous solution of glutaraldehyde, a crosslinking agent, dropwise to the dispersion, and stir for 20-40 minutes to obtain a mixture; the amount of glutaraldehyde added is 2-5 wt% of the PVA mass. C) The mixture is vacuum degassed at room temperature for 10-30 min, then cast into a mold, and naturally evaporated and dried at room temperature for 8-16 h, then transferred to an oven at 40-60℃ for 2-6 h. After drying, the mixture is peeled off to obtain a functional composite film with a thickness of 20-40 μm.

7. The lithium iron phosphate composite cathode material with Mn ion self-compensation and multiple interception functions according to claim 6, characterized in that, The PVA solution is prepared by the following steps: polyvinyl alcohol is added to deionized water and heated and stirred in a water bath at 80-90℃ to dissolve it, thereby preparing a 3-6 wt% PVA solution.

8. A method for preparing the lithium manganese iron phosphate composite cathode material with Mn ion self-compensation and multiple interception functions as described in claim 2, characterized in that, The process includes the following steps: mixing the functional composite membrane with lithium manganese iron phosphate cathode material powder to prepare a slurry with a solid content of 15-25 wt%, wet ball milling for 2-4 hours, pre-freezing at -50~-30℃ for 2-6 hours, and then freeze-drying at a cold trap temperature ≤-50℃ and a vacuum degree <10 Pa for 24-48 hours to obtain the composite powder; finally, grinding the composite powder for 3-10 minutes and passing it through a 200-400 mesh sieve to obtain the lithium manganese iron phosphate composite cathode material.

9. The method for preparing a lithium iron phosphate composite cathode material with Mn ion self-compensation and multiple interception functions according to claim 8, characterized in that, The lithium iron phosphate cathode material powder is prepared by the following steps: d) Prepare a grinding slurry with a solid content of 40-50% by mixing lithium, manganese, iron, phosphorus and carbon sources. After ball milling for 4-8 hours, vacuum dry at 120-180℃ for 8-16 hours to obtain precursor powder. The amount of carbon source added accounts for 6-10% of the total mass of the materials composed of lithium, manganese, iron, phosphorus and carbon sources. e) The precursor powder is pre-calcined at 400-600℃ for 3-6 hours under an inert atmosphere, and then sintered at 650-800℃ for 6-12 hours to obtain LiMn with a carbon coating layer and 0.3≤x≤0.

8. (1-x) Fe x PO4 powder.

10. The method for preparing a lithium iron phosphate composite cathode material with Mn ion self-compensation and multiple interception functions according to claim 9, characterized in that, The lithium source is selected from one or more of lithium carbonate, lithium hydroxide, lithium acetate, lithium phosphate, and lithium dihydrogen phosphate. The manganese source is selected from one or more of manganese sulfate, manganese oxalate, manganese nitrate, manganese tetroxide, manganese carbonate, and manganese dioxide; The iron source is selected from one or more of ferric chloride, ferrous oxalate, ferric nitrate, ferric oxide, and ferric oxide; The phosphorus source is selected from one or more of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and phosphoric acid. The carbon source is selected from one or more of sucrose, glucose, polyethylene glycol, and polyvinyl alcohol.

Images