Lithium iron manganese phosphate cathode material based on multi-dimensional conductive network construction and grain boundary passivation and preparation method thereof

Abstract

The application discloses a kind of lithium iron manganese phosphate positive electrode materials based on multi-dimensional conductive network construction and grain boundary passivation and preparation method thereof, belong to lithium battery technical field.The application is by the synergistic process of "spray drying pore-forming-gas phase nitriding to construct conductive network-low temperature sintering passivation grain boundary", both greatly improve the electronic conductivity and ion diffusion efficiency of lithium iron manganese phosphate positive electrode material, effectively inhibit manganese elution and enhance cycle stability, also realize process low temperature energy saving, simple and controllable, with the potential of large-scale production, fundamentally solve the core defects of traditional technology.

Description

Technical Field

[0001] This invention relates to the field of lithium battery technology, specifically to a lithium manganese iron phosphate cathode material based on multi-dimensional conductive network construction and grain boundary passivation, and its preparation method. Background Technology

[0002] With the widespread application of lithium-ion batteries in new energy vehicles, energy storage systems, and other fields, the market has placed higher demands on the energy density, rate performance, and cycle life of batteries. Lithium manganese iron phosphate (LiMn) 0.6 Fe 0.4 PO4 (LMFP) cathode materials have become a research hotspot in the field of cathode materials in recent years because they combine the high safety of lithium iron phosphate (LFP) with the high voltage advantage of lithium manganese phosphate (LMP) (operating voltage 3.4-3.8V, energy density 15-20% higher than LFP).

[0003] However, the inherent defects of LMFP materials themselves severely restrict their industrial application: firstly, their electronic conductivity is extremely low (the electronic conductivity of native LMFP is only 10). -8 -10 -4 S / cm), far lower than commercial LFP (10 -3 The low S / cm ratio leads to hindered charge transport under high current, resulting in poor rate performance; secondly, the small lithium-ion diffusion coefficient (approximately 10) -9 -10 -8 cm 2 / s), the long lithium-ion insertion / extraction path during charging and discharging further exacerbates the problems of insufficient rate performance and cycle stability; thirdly, manganese is easily dissolved, and during charge-discharge cycles, Mn in the LMFP lattice... 2+ Easily oxidized to Mn by electrolyte 3+ / Mn 4+ It dissolves and not only destroys the crystal structure of the material, but also deposits on the surface of the negative electrode, causing the SEI film to rupture and the impedance to increase, resulting in a significant reduction in cycle life.

[0004] To address these issues, existing technologies primarily employ methods such as carbon coating, metal ion doping, and nano-sizing. While carbon coating can improve electronic conductivity, the carbon layers formed from traditional carbon sources like carbon black and graphite tend to agglomerate, making it impossible to construct a continuous conductive network. Metal ion doping (such as Mg...) 2+ Ni 2+ While nano-sizing can optimize the crystal structure, it can introduce lattice defects and reduce the crystallinity of the material. Although nano-sizing can shorten the lithium-ion diffusion path, nanoparticles are prone to agglomeration, and the large specific surface area leads to an increase in electrolyte adsorption, resulting in a decrease in the battery's initial coulombic efficiency.

[0005] Furthermore, current LMFP preparation processes still primarily rely on traditional solid-state methods, which require sintering at high temperatures of 750-800℃. This can easily lead to excessive grain growth, lithium volatilization, and increased grain boundary defects, further deteriorating the material's conductivity and structural stability. Although some studies have employed wet chemical methods such as sol-gel and hydrothermal methods to prepare LMFPs, these methods suffer from complex processes, high costs, and difficulties in large-scale production, making it difficult to meet industrial mass production demands. Therefore, developing an LMFP preparation technology that combines high conductivity, low manganese leaching, excellent cycle stability, and a simple, scalable process has become a key bottleneck that the industry urgently needs to overcome. Summary of the Invention

[0006] The technical problem to be solved by this invention is to overcome the shortcomings of the prior art and provide a lithium manganese iron phosphate cathode material and its preparation method based on multi-dimensional conductive network construction and grain boundary passivation. Through the synergistic process of "spray drying to create pores - vapor phase nitriding to build a conductive network - molten salt-assisted low-temperature sintering to passivate grain boundaries", the electronic conductivity and ion diffusion efficiency of the lithium manganese iron phosphate cathode material are greatly improved, manganese dissolution is effectively suppressed and cycle stability is enhanced, and it has the potential for large-scale production, fundamentally solving the core defects of traditional technology.

[0007] The technical solution of this invention is as follows:

[0008] On one hand, this invention provides a method for preparing lithium manganese iron phosphate cathode material based on multi-dimensional conductive network construction and grain boundary passivation, comprising the following steps:

[0009] S1 Preparation of precursor: Using lithium source, iron source, manganese source, phosphorus source, carbon source and pore-forming agent as raw materials, porous microsphere lithium manganese iron phosphate precursor is prepared by spray drying method;

[0010] S2 Pre-sintering and Vapor-phase Nitriding: The precursor is heated to 320-380℃ in an inert atmosphere at a rate of 4-6℃ / min (this range ensures sufficient decomposition of the pore-forming agent and avoids premature sintering and agglomeration of the precursor), and held for 1.5-2.5h (to balance the decomposition efficiency and energy consumption of the pore-forming agent and prevent insufficient holding time from causing residual impurities in the pores). The main purpose of this process is to decompose the pore-forming agent to form a porous structure and stabilize the carbon source, preparing for subsequent carbonization. The temperature is then further increased to 580-620℃ (this temperature range promotes the reaction between PAN and ammonia while avoiding excessive temperature leading to carbon layer ablation). Ammonia (NH3) is then introduced into the furnace tube for vapor-phase nitriding treatment, held for 0.8-1.2h, to obtain precursor powder treated with vapor-phase nitriding. NH3 reacts with PAN during pyrolysis to achieve nitrogen doping; the introduction of nitrogen can generate more defects in the carbon framework, significantly improving the electrochemical activity and electronic conductivity of the carbon layer.

[0011] S3 Molten Salt Assisted Low-Temperature Sintering and Simultaneous Coating: The precursor powder obtained in step S2 is uniformly mixed with LiCl-KCl molten salt, and heated to 620-680℃ at a rate of 2-4℃ / min under an inert atmosphere (far lower than the 750-800℃ of the traditional solid-state method, which can suppress excessive grain growth and retain the porous structure), and held at that temperature for 8-12h.

[0012] S4 Washing and Drying: The product was naturally cooled to room temperature, washed with deionized water, and vacuum dried to obtain lithium manganese iron phosphate cathode material based on multi-dimensional conductive network construction and grain boundary passivation.

[0013] Preferably, step S1 specifically includes the following steps:

[0014] S11 raw material preparation: according to the stoichiometric ratio Li : (Mn 0.6 Fe 0.4 Weigh the lithium source, iron source, manganese source and phosphorus source, and dissolve them together in deionized water to form a homogeneous mixed salt solution.

[0015] S12 Adding carbon source and pore-forming agent: Add carbon source and pore-forming agent to mixed salt solution to obtain slurry;

[0016] S13 Spray Drying: The above slurry is spray dried at an inlet temperature of 190-210℃ and an outlet temperature of 85-95℃. After instantaneous drying, a porous, hollow spherical precursor is obtained. This structure is beneficial for mass transfer in subsequent reactions and mitigates volume changes during charging and discharging.

[0017] Preferably, in step S11, the lithium source is lithium acetate, the iron source is ferrous sulfate, the manganese source is manganese acetate, and the phosphorus source is ammonium dihydrogen phosphate; the concentration of the mixed salt solution is 0.5-1.5 mol / L, which can ensure that the raw materials are fully dissolved, while avoiding the situation where the concentration is too low, resulting in low spray drying efficiency, or the concentration is too high, causing crystal precipitation and affecting the uniformity of the precursor.

[0018] Preferably, in step S12, the carbon source is an N-methylpyrrolidone (NMP) solution of polyacrylonitrile (PAN) with a concentration of 5-15 wt.% (this concentration can balance the carbon source dispersion and subsequent carbon coating amount, avoiding discontinuity of the carbon network due to too low a concentration or precursor aggregation due to too high a concentration); the pore-forming agent is polymethyl methacrylate (PMMA) microspheres; the amount of carbon source added is 5-12 wt.% of the total mass of metal ions in the mixed salt solution (to ensure the formation of a sufficient amount of nitrogen-doped carbon network and improve conductivity), and the amount of pore-forming agent added is 3-8 wt.% of the mass of the mixed salt solution (to construct a reasonable pore structure, ensuring the specific surface area of ​​the material while avoiding excessive pores that lead to a decrease in structural strength).

[0019] Preferably, in step S2, the ammonia gas flow rate is 50-150 mL / min. This flow rate ensures that the ammonia gas reacts fully with the precursor to achieve nitrogen doping, while avoiding insufficient nitriding due to excessively low flow rates or waste of raw materials and safety risks due to excessively high flow rates.

[0020] Preferably, in step S3, the LiCl content in the LiCl-KCl molten salt is 42-48 wt.% (close to the eutectic composition, which can lower the melting point of the molten salt to 350-380℃ and improve the ion conduction efficiency); the mass ratio of the precursor powder obtained in step S2 to the LiCl-KCl molten salt is 1:(2.5-3.5) (to ensure that the molten salt fully coats the precursor particles, while avoiding excessive molten salt that would increase the washing burden).

[0021] Preferably, in step S4, the vacuum drying temperature is 95-105℃.

[0022] On the other hand, the present invention provides a lithium manganese iron phosphate cathode material based on multi-dimensional conductive network construction and grain boundary passivation, which is prepared by the above-mentioned preparation method of lithium manganese iron phosphate cathode material based on multi-dimensional conductive network construction and grain boundary passivation.

[0023] This invention utilizes a synergistic process of "spray drying for pore formation, vapor-phase nitriding for conductive network construction, and molten salt-assisted low-temperature sintering for grain boundary passivation" to prepare lithium manganese iron phosphate cathode materials based on multi-dimensional conductive network construction and grain boundary passivation. Compared with existing technologies, this invention offers the following significant advantages:

[0024] (1) Significantly improves electronic conductivity and ion diffusion efficiency, resulting in significantly optimized rate performance.

[0025] Construction of a multi-dimensional conductive network: Through vapor-phase nitriding, a nitrogen-doped carbon layer is formed by reacting a carbon source with ammonia. Nitrogen introduces a large number of defect sites into the carbon framework, increasing the electronic conductivity to 10. -4 -10 -3 S / cm, compared to LMFP prepared by traditional solid-state method (10 -6 The S / cm ratio is increased by 2-3 orders of magnitude; at the same time, the nitrogen-doped carbon layer works synergistically with the porous structure formed by spray drying to construct a continuous conductive channel "within the particle and between particles", avoiding the problem of conductivity interruption caused by easy agglomeration of traditional carbon coating.

[0026] Shortened ion diffusion path: The hollow porous spherical precursor prepared by spray drying retains a certain porosity after low-temperature molten salt sintering at 620-680℃, providing sufficient diffusion channels for lithium ions; moreover, low-temperature sintering inhibits excessive grain growth, increasing the lithium ion diffusion coefficient to 3×10⁻⁶. -5 -6×10 -5 cm 2 / s, compared to the traditional solid-state method (9×10 -8cm 2 The capacity retention rate at 5C rate is 75-85%, which is three orders of magnitude higher than that of LMFP prepared by traditional methods.

[0027] (2) Inhibits manganese leaching and significantly improves cycle stability

[0028] Grain boundary passivation protective layer formation: During low-temperature sintering, LiCl-KCl molten salt can promote the formation of Li... + With PO4 3- In LMFP, a dense Li3PO4 protective layer is formed by enrichment at the grain boundaries. This protective layer not only prevents direct contact between the electrolyte and the grain boundaries but also inhibits Mn production. 2+ The oxidation and dissolution of manganese are controlled at 10-20 ppm, which is only 1 / 4-1 / 9 of that of the traditional solid-phase method (90 ppm), without hindering lithium ion migration, thus significantly improving interface stability, especially high-temperature cycling performance.

[0029] Enhanced structural stability: The elastic support of the nitrogen-doped carbon layer can mitigate the volume expansion of LMFP during charging and discharging, preventing material structure collapse; simultaneously, the Li3PO4 grain boundary protective layer reduces Mn 2+ The deposition on the negative electrode surface reduces the risk of SEI film rupture and impedance increase. The capacity retention rate reaches 88-96% after 2000 cycles at 1C rate, which is significantly improved compared with the traditional solid-state method (86%) and the comparison ratio of molten salt-free sintering (82%).

[0030] (3) The process is green and efficient, and has the potential for large-scale production.

[0031] Low-temperature energy saving: The molten salt-assisted sintering temperature (620-680℃) is significantly lower than that of the traditional solid-state method (750-800℃), resulting in reduced energy consumption and avoiding lithium volatilization and equipment wear caused by high-temperature sintering. Furthermore, low-temperature sintering produces nanocrystals, inhibiting excessive grain growth and manganese volatilization; the molten salt is easily washed away, without introducing impurities.

[0032] The process is simple and controllable: the steps of spray drying, gas phase nitriding, and molten salt sintering are all mature industrial processes that do not require complex equipment; parameters such as the concentration of mixed salt solution, ammonia gas flow rate, and molten salt ratio are easy to control, which can achieve stable control of the performance of precursors and final products, making it suitable for large-scale continuous production.

[0033] In summary, this invention fundamentally solves the core problems of poor conductivity, excessive manganese leaching, and insufficient cycle stability of LMFP materials through the synergistic design of multi-dimensional conductive networks and grain boundary passivation. At the same time, it takes into account both process economy and environmental protection, providing a practical and feasible technical solution for the industrial application of high-rate and long-life lithium-ion battery cathode materials. Detailed Implementation

[0034] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0035] Example 1

[0036] The method for preparing lithium manganese iron phosphate cathode material based on multi-dimensional conductive network construction and grain boundary passivation in this embodiment includes the following steps:

[0037] S1 Preparation of precursor

[0038] S11 raw material preparation: According to the stoichiometric ratio Li:(Mn) 0.6 Fe 0.4 ):P=1.05:1:1 (Lithium is slightly in excess to compensate for sintering loss). Weigh lithium acetate, ferrous sulfate, manganese acetate, and ammonium dihydrogen phosphate, and dissolve them together in deionized water to form a homogeneous 1 mol / L mixed salt solution.

[0039] S12 Addition of Carbon Source and Pore-Forming Agent: A carbon source and a pore-forming agent are added to a mixed salt solution to obtain a slurry; wherein, the carbon source is a 10 wt.% NMP solution of PAN, and the pore-forming agent is PMMA microspheres. The amount of carbon source added is 10 wt.% of the total mass of metal ions in the mixed salt solution, and the amount of pore-forming agent added is 5 wt.% of the mass of the mixed salt solution.

[0040] S13 Spray Drying: The above slurry is spray dried at an inlet temperature of 200°C and an outlet temperature of 90°C. After instantaneous drying, a porous, hollow spherical lithium manganese iron phosphate precursor composed of PAN, PMMA and metal salts is obtained.

[0041] S2 Pre-sintering and Vapor-phase Nitriding: The precursor was heated to 350°C in an argon atmosphere at a rate of 5°C / min and held for 2 hours; after further heating to 600°C, ammonia was introduced into the furnace tube at a flow rate of 100 mL / min for vapor-phase nitriding treatment and held for 1 hour to obtain precursor powder treated with vapor-phase nitriding.

[0042] S3 Molten Salt Assisted Low-Temperature Sintering and Simultaneous Coating

[0043] S31 Preparation of eutectic salt mixture: Mix LiCl and KCl at a LiCl content of 45 wt.% and grind them evenly to use as molten salt medium.

[0044] S32 Sintering and Coating: The precursor powder obtained in step S2 is uniformly mixed with LiCl-KCl molten salt and placed in an alumina crucible. The mass ratio of precursor powder to LiCl-KCl molten salt is 1:3. Under an inert atmosphere, the temperature is increased to 650℃ at a rate of 3℃ / min and held for 10h.

[0045] S4 Washing and Drying: The product was naturally cooled to room temperature, washed with deionized water to remove soluble LiCl and KCl molten salts, and then vacuum dried at 100°C to obtain porous microspheres, nitrogen-doped carbon-coated LMFP cathode material with a Li3PO4 protective layer on the surface.

[0046] Example 2

[0047] The method for preparing lithium manganese iron phosphate cathode material based on multi-dimensional conductive network construction and grain boundary passivation in this embodiment includes the following steps:

[0048] S1 Preparation of precursor

[0049] S11 raw material preparation: According to the stoichiometric ratio Li:(Mn) 0.6 Fe 0.4 ):P=1.05:1:1 (Lithium is slightly in excess to compensate for sintering loss). Weigh lithium acetate, ferrous sulfate, manganese acetate, and ammonium dihydrogen phosphate, and dissolve them together in deionized water to form a homogeneous 1.5 mol / L mixed salt solution.

[0050] S12 Addition of Carbon Source and Pore-Forming Agent: A carbon source and a pore-forming agent are added to a mixed salt solution to obtain a slurry; wherein the carbon source is a 15 wt.% NMP solution of PAN, and the pore-forming agent is PMMA microspheres. The amount of carbon source added is 12 wt.% of the total mass of metal ions in the mixed salt solution, and the amount of pore-forming agent added is 8 wt.% of the mass of the mixed salt solution.

[0051] S13 Spray Drying: The above slurry is spray dried at an inlet temperature of 210°C and an outlet temperature of 95°C. After instantaneous drying, a porous, hollow spherical lithium manganese iron phosphate precursor composed of PAN, PMMA and metal salts is obtained.

[0052] S2 Pre-sintering and Vapor-phase Nitriding: The precursor was heated to 380°C in an argon atmosphere at a rate of 6°C / min and held for 2.5 h; then the temperature was further increased to 620°C, and ammonia was introduced into the furnace tube at a flow rate of 150 mL / min for vapor-phase nitriding treatment, and held for 1.2 h to obtain precursor powder treated with vapor-phase nitriding.

[0053] S3 Molten Salt Assisted Low-Temperature Sintering and Simultaneous Coating

[0054] S31 Preparation of eutectic salt mixture: Mix LiCl and KCl at a LiCl content of 45 wt.% and grind them evenly to use as molten salt medium.

[0055] S32 Sintering and Coating: The precursor powder obtained in step S2 is uniformly mixed with LiCl-KCl molten salt and placed in an alumina crucible. The mass ratio of precursor powder to LiCl-KCl molten salt is 1:3. Under an inert atmosphere, the temperature is increased to 680℃ at a rate of 4℃ / min and held for 12h.

[0056] S4 Washing and Drying: The product was naturally cooled to room temperature, washed with deionized water to remove soluble LiCl and KCl molten salts, and then vacuum dried at 100°C to obtain porous microspheres, nitrogen-doped carbon-coated LMFP cathode material with a Li3PO4 protective layer on the surface.

[0057] Example 3

[0058] The method for preparing lithium manganese iron phosphate cathode material based on multi-dimensional conductive network construction and grain boundary passivation in this embodiment includes the following steps:

[0059] S1 Preparation of precursor

[0060] S11 raw material preparation: According to the stoichiometric ratio Li:(Mn) 0.6 Fe 0.4 ):P=1.05:1:1 (Lithium is slightly in excess to compensate for sintering loss). Weigh lithium acetate, ferrous sulfate, manganese acetate, and ammonium dihydrogen phosphate, and dissolve them together in deionized water to form a homogeneous 0.5 mol / L mixed salt solution.

[0061] S12 Addition of Carbon Source and Pore-Forming Agent: A carbon source and a pore-forming agent are added to a mixed salt solution to obtain a slurry; wherein, the carbon source is a 5 wt.% NMP solution of PAN, and the pore-forming agent is PMMA microspheres. The amount of carbon source added is 5 wt.% of the total mass of metal ions in the mixed salt solution, and the amount of pore-forming agent added is 3 wt.% of the mass of the mixed salt solution.

[0062] S13 Spray Drying: The above slurry is spray dried at an inlet temperature of 190°C and an outlet temperature of 85°C. After instantaneous drying, a porous, hollow spherical lithium manganese iron phosphate precursor composed of PAN, PMMA and metal salts is obtained.

[0063] S2 Pre-sintering and Vapor-phase Nitriding: The precursor was heated to 320°C in an argon atmosphere at a rate of 4°C / min and held for 1.5 h; then the temperature was further increased to 580°C, and ammonia was introduced into the furnace tube at a flow rate of 50 mL / min for vapor-phase nitriding treatment. The temperature was held for 0.8 h to obtain precursor powder treated with vapor-phase nitriding.

[0064] S3 Molten Salt Assisted Low-Temperature Sintering and Simultaneous Coating

[0065] S31 Preparation of eutectic salt mixture: Mix LiCl and KCl at a LiCl content of 45 wt.% and grind them evenly to use as molten salt medium.

[0066] S32 Sintering and Coating: The precursor powder obtained in step S2 is uniformly mixed with LiCl-KCl molten salt and placed in an alumina crucible. The mass ratio of precursor powder to LiCl-KCl molten salt is 1:3. Under an inert atmosphere, the temperature is increased to 620℃ at a rate of 2℃ / min and held for 8 hours.

[0067] S4 Washing and Drying: The product was naturally cooled to room temperature, washed with deionized water to remove soluble LiCl and KCl molten salts, and then vacuum dried at 100°C to obtain porous microspheres, nitrogen-doped carbon-coated LMFP cathode material with a Li3PO4 protective layer on the surface.

[0068] Comparative Example 1

[0069] Comparative Example 1 uses a traditional solid-state method to prepare lithium manganese iron phosphate cathode material, including the following steps:

[0070] S1 Raw Material Preparation and Weighing

[0071] According to the target product LiMn 0.6 Fe 0.4 The stoichiometric ratio of PO4 was calculated, and lithium acetate, ferrous sulfate, manganese acetate, and ammonium dihydrogen phosphate were accurately weighed. A 5% excess of lithium source was used (to compensate for lithium volatilization loss during high-temperature sintering) to ensure the lithium content in the final product met the standards.

[0072] S2 Raw Material Mixing and Grinding (Dry Mixing)

[0073] A planetary ball mill was used with agate balls as the grinding media at a ball-to-material ratio of 10:1. Weighed lithium acetate, ferrous sulfate, manganese acetate, and ammonium dihydrogen phosphate were added to the mill jar without the need for additional solvents (dry mixing). Milling was carried out continuously for 5 hours at 300 rpm. Through mechanical grinding, the solid raw materials were initially and uniformly mixed, while simultaneously breaking up particle agglomerates and increasing the particle contact area, thus creating conditions for subsequent high-temperature reactions.

[0074] S3 pre-sintering (low-temperature pretreatment)

[0075] The mixed powder is placed in an alumina crucible and then placed in a tube furnace. Argon gas is introduced to purge the air from the furnace and prevent oxidation of the raw materials during heating. The temperature is increased to 350°C at a rate of 3°C / min and held for 4 hours. This process decomposes the phosphorus source and initiates some of the raw materials' reactions (such as the pre-reaction of lithium and phosphorus sources), preventing powder agglomeration or crucible cracking due to rapid gas release during subsequent high-temperature sintering. It also removes adsorbed moisture and impurities from the raw materials.

[0076] S4 Secondary Grinding

[0077] The pre-sintered powder will form certain agglomerates due to the initial reaction. It needs to be removed from the tube furnace, cooled to room temperature, and then put back into the ball mill for 3 hours with the same ball-to-powder ratio and rotation speed. The purpose is to break up the agglomerates formed during pre-sintering, refine the particle size, further improve the uniformity of raw material mixing, compensate for the shortcomings of dry mixing, and lay the foundation for the full reaction of each component during subsequent high-temperature sintering.

[0078] S5 High-Temperature Sintering

[0079] The powder after secondary grinding was reloaded into the alumina crucible and placed back into the tube furnace. Argon gas was continuously introduced at a flow rate of 100 mL / min to ensure that the sintering process was carried out in an oxygen-free environment. The temperature was slowly increased to 800 °C at a rate of 3 °C / min and held for 15 h. Under high temperature conditions, the solid raw materials underwent chemical reactions through lattice diffusion, gradually forming LiMn. 0.6 Fe 0.4 PO4 crystal phase; at the same time, due to the high temperature, the generated LMFP grains will continue to grow, eventually forming micron-sized grains.

[0080] S6 Post-processing

[0081] The sintered product is a blocky or dense agglomerate, which is crushed by a jaw crusher and then screened to obtain LMFP powder with a relatively uniform particle size distribution.

[0082] Comparative Example 2

[0083] The difference from Example 1 is that step S2 pre-sintering and vapor phase nitriding are not performed.

[0084] Comparative Example 3

[0085] The difference from Example 1 is that step S3, molten salt-assisted low-temperature sintering and simultaneous coating, is not performed.

[0086] The cathode materials of Examples 1-3 and Comparative Examples 1-3 were assembled into CR2032 coin cells:

[0087] The active material lithium manganese iron phosphate, binder PVDF, and conductive agent Super P carbon black were mixed in an 8:1:1 ratio and stirred evenly with NMP solvent to form a homogeneous slurry. The prepared slurry was evenly coated onto the surface of aluminum foil. After coating, the electrode sheets were placed in a ventilated area for preliminary drying to remove some of the solvent. The preliminarily dried electrode sheets were placed in a vacuum oven to further remove residual solvent. The dried electrode sheets were cut into small round pieces with a diameter of 16 mm suitable for coin cell sizes. In an argon-filled glove box (water and oxygen content <0.1 ppm), the negative electrode shell, spring sheet, negative electrode (lithium metal sheet), separator Celgard 2325, positive electrode sheet, and positive electrode shell were stacked neatly in sequence. An appropriate amount of electrolyte (1 mol / L LiPF6 dissolved in a 1:1:1 volume ratio mixture of EC:DMC:EMC) was added to the separator to ensure full wetting of the positive and negative electrodes. The battery casing was pressurized and sealed using a coin cell packaging machine to ensure no leakage inside the battery. After sealing, the battery's appearance should be checked for damage, and electrochemical performance tests should be performed. The test results are shown in Table 1.

[0088] Table 1. Electrochemical performance test results of the cathode materials assembled in Examples 1-3 and Comparative Examples 1-3

[0089]

[0090] As can be seen from Table 1, Comparative Example 1, which uses the traditional solid-state method, suffers from multiple defects due to high temperature and non-functional design:

[0091] (1) Disadvantages of high-temperature sintering: 800℃ high temperature leads to excessive grain growth, lithium volatilization and increased grain boundary defects, while destroying the porous structure and doubly inhibiting electron and ion transport.

[0092] (2) No functional modification: No carbon coating, nitrogen doping and grain boundary passivation treatment, electrons rely solely on interparticle contact conduction, manganese leaching is unprotected, and the structure is prone to collapse during cycling.

[0093] Comparative Example 2 lacks vapor-phase nitriding, resulting in a sharp drop in conductivity due to the absence of a multi-dimensional conductive network: without a nitrogen-doped carbon network, the carbon layer is merely a product of the original PAN pyrolysis, without defect site modification, leading to a two-order-of-magnitude decrease in electronic conductivity and hindering charge transport under high current; the pore structure is unstable: the vapor-phase nitriding process can enhance the bonding force between the carbon network and the particles, but without it, the porous structure is prone to collapse during sintering, reducing ion diffusion channels, and at the same time, the grain boundaries are not nitrogen-passivated, increasing the amount of manganese dissolved.

[0094] Comparative Example 3, without molten salt sintering, loses its ability to control low temperatures and protect grain boundaries, leading to uncontrolled grain growth: Without the low-temperature sintering effect of LiCl-KCl molten salt (the traditional solid-state method uses high-temperature substitution), grain size increases, ion diffusion paths lengthen, and diffusion coefficient decreases; no protective layer exists at grain boundaries: a Li3PO4 grain boundary protective layer cannot be formed, and Mn... 2+ It is easily leached into the electrolyte, which damages the stability of the SEI film and leads to accelerated capacity decay during cycling; at the same time, without the densification effect of molten salt, the carbon layer is loosely bonded to the particles, resulting in impaired conductivity.

[0095] In summary, the embodiments of the present invention solve the core problems of poor conductivity, slow ion diffusion, and excessive manganese dissolution in traditional methods through the synergistic design of "porous structure + nitrogen-doped carbon network + molten salt grain boundary passivation". In contrast, the comparative examples have single or multiple performance defects due to the lack of key process steps, resulting in significantly inferior overall performance compared to the embodiments.

Claims

1. A method for preparing lithium manganese iron phosphate cathode material based on multi-dimensional conductive network construction and grain boundary passivation, characterized in that, Includes the following steps: S1 Preparation of precursor: Using lithium source, iron source, manganese source, phosphorus source, carbon source and pore-forming agent as raw materials, porous microsphere lithium manganese iron phosphate precursor is prepared by spray drying method; S2 pre-sintering and vapor phase nitriding: The precursor is heated to 320-380℃ in an inert atmosphere at a rate of 4-6℃ / min and held for 1.5-2.5h; after further heating to 580-620℃, ammonia gas is introduced into the furnace tube for vapor phase nitriding treatment and held for 0.8-1.2h to obtain precursor powder treated by vapor phase nitriding. S3 Molten Salt Assisted Low-Temperature Sintering and Synchronous Coating: The precursor powder obtained in step S2 is uniformly mixed with LiCl-KCl molten salt, and heated to 620-680℃ at a rate of 2-4℃ / min under an inert atmosphere, and held for 8-12h. S4 Washing and Drying: The product was naturally cooled to room temperature, washed with deionized water, and vacuum dried to obtain lithium manganese iron phosphate cathode material based on multi-dimensional conductive network construction and grain boundary passivation.

2. The preparation method of lithium manganese iron phosphate cathode material based on multi-dimensional conductive network construction and grain boundary passivation as described in claim 1, characterized in that, Step S1 specifically includes the following steps: S11 raw material preparation: according to the stoichiometric ratio Li : (Mn 0.6 Fe 0.4 Weigh the lithium source, iron source, manganese source and phosphorus source, and dissolve them together in deionized water to form a homogeneous mixed salt solution. S12 Adding carbon source and pore-forming agent: Add carbon source and pore-forming agent to mixed salt solution to obtain slurry; S13 Spray Drying: The above slurry is spray dried at an inlet temperature of 190-210℃ and an outlet temperature of 85-95℃; after instantaneous drying, a porous, hollow spherical precursor is obtained.

3. The preparation method of lithium manganese iron phosphate cathode material based on multi-dimensional conductive network construction and grain boundary passivation as described in claim 2, characterized in that, In step S11, the lithium source is lithium acetate, the iron source is ferrous sulfate, the manganese source is manganese acetate, and the phosphorus source is ammonium dihydrogen phosphate; the concentration of the mixed salt solution is 0.5-1.5 mol / L.

4. The preparation method of lithium manganese iron phosphate cathode material based on multi-dimensional conductive network construction and grain boundary passivation as described in claim 2, characterized in that, In step S12, the carbon source is an N-methylpyrrolidone solution of polyacrylonitrile with a concentration of 5-15 wt.%; the pore-forming agent is polymethyl methacrylate microspheres; the amount of carbon source added is 5-12 wt.% of the total mass of metal ions in the mixed salt solution, and the amount of pore-forming agent added is 3-8 wt.% of the mass of the mixed salt solution.

5. The method for preparing lithium manganese iron phosphate cathode material based on multi-dimensional conductive network construction and grain boundary passivation as described in claim 1, characterized in that, In step S2, the ammonia gas flow rate is 50-150 mL / min.

6. The method for preparing lithium manganese iron phosphate cathode material based on multi-dimensional conductive network construction and grain boundary passivation as described in claim 1, characterized in that, In step S3, the LiCl content in the LiCl-KCl molten salt is 42-48 wt.%; the mass ratio of the precursor powder obtained in step S2 to the LiCl-KCl molten salt is 1:(2.5-3.5).

7. The method for preparing lithium manganese iron phosphate cathode material based on multi-dimensional conductive network construction and grain boundary passivation as described in claim 1, characterized in that, In step S4, the vacuum drying temperature is 95-105℃.

8. A lithium manganese iron phosphate cathode material based on multi-dimensional conductive network construction and grain boundary passivation, characterized in that, The lithium manganese iron phosphate cathode material was prepared by the method described in any one of claims 1-7, which is based on the construction of a multi-dimensional conductive network and passivation of grain boundaries.

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