A lithium iron manganese phosphate material, a preparation method thereof, a pole piece and a battery

Abstract

The application provides a lithium manganese iron phosphate material, a preparation method thereof, a pole piece and a battery. The preparation method comprises the following steps: performing first sintering treatment on a lithium manganese iron phosphate precursor at a first temperature, and then performing second sintering treatment on the lithium manganese iron phosphate precursor at a second temperature to obtain the lithium manganese iron phosphate material; wherein the first temperature is lower than the second temperature; the lithium manganese iron phosphate precursor comprises a phosphorus source, a manganese source, an iron source, a lithium source, a carbon source and a complexing agent, the complexing agent comprises a first complexing agent and a second complexing agent; the thermal decomposition temperature of the first complexing agent is not higher than the first temperature, and the thermal decomposition temperature of the second complexing agent is higher than the first temperature and not higher than the second temperature. The method can prepare a lithium manganese iron phosphate positive electrode material which has a high compaction density and a good cycle performance.

Description

Technical Field

[0001] This invention relates to the field of energy storage technology, and in particular to a lithium manganese iron phosphate material, its preparation method, electrode sheet, and battery. Background Technology

[0002] Lithium manganese iron phosphate (LMFP) is an olivine-structured cathode material for lithium-ion batteries, a solid solution of lithium iron phosphate (LFP) and lithium manganese phosphate (LMP). By partially introducing manganese into the LFP lattice, LMFP inherits the high safety, thermal stability, and low cost advantages of LFP. Simultaneously, utilizing the higher redox potential of manganese, the voltage platform is increased from 3.4V of LFP to approximately 3.8-4.1V, resulting in a theoretical energy density increase of about 10%-20% compared to LFP. LMFP also exhibits better low-temperature performance, with significantly better capacity retention than LFP at -20℃. Therefore, it is considered an upgraded cathode material of LFP, showing broad application potential in power batteries, energy storage systems, and two-wheeled vehicles.

[0003] In existing technologies, the preparation processes of LMFP cathode materials mainly employ methods similar to LFP, such as high-temperature solid-state methods, co-precipitation methods, hydrothermal methods, or sol-gel methods. These typically require modification treatments such as carbon coating, ion doping, and nano-sizing to improve material properties. A typical process flow includes raw material mixing, pretreatment, calcination and sintering, and post-treatment steps, with the manganese-iron ratio controlled to balance voltage boost and structural stability. However, existing technologies result in materials with poor conductivity and rate performance; insufficient cycle stability, especially with significant capacity decay at high temperatures or high voltages; and low compaction density, affecting electrode energy density and processing performance.

[0004] Therefore, how to improve the compaction density of lithium manganese iron phosphate materials while taking into account their cycle life is an important problem that urgently needs to be solved in this field. Summary of the Invention

[0005] This invention provides a method for preparing lithium manganese iron phosphate material, which can prepare lithium manganese iron phosphate cathode material that takes into account both compaction density and cycle performance.

[0006] This invention provides a lithium manganese iron phosphate material that significantly improves compaction density while ensuring cycle life.

[0007] This invention provides an electrode sheet that significantly improves compaction density while ensuring cycle life.

[0008] This invention provides a battery that significantly improves compaction density while ensuring cycle life.

[0009] In a first aspect, the present invention provides a method for preparing lithium manganese iron phosphate material, comprising the following steps: subjecting a lithium manganese iron phosphate precursor to a first sintering treatment at a first temperature, and then subjecting it to a second sintering treatment at a second temperature to obtain the lithium manganese iron phosphate material; wherein...

[0010] The first temperature is lower than the second temperature;

[0011] The lithium manganese iron phosphate precursor includes a phosphorus source, a manganese source, an iron source, a lithium source, a carbon source, and a complexing agent, wherein the complexing agent includes a first complexing agent and a second complexing agent;

[0012] The thermal decomposition temperature of the first complexing agent is not higher than the first temperature, and the thermal decomposition temperature of the second complexing agent is higher than the first temperature but not higher than the second temperature.

[0013] In one possible implementation, as described above, the first temperature is 250-350°C and the second temperature is 650-800°C.

[0014] In one possible implementation, as described above, in the lithium manganese iron phosphate precursor, the Fe coordination stability constant of the first complexing agent is lower than the Fe coordination stability constant of the second complexing agent, and the Mn coordination stability constant of the first complexing agent is lower than the Mn coordination stability constant of the second complexing agent.

[0015] In one possible implementation, as described above, the first sintering treatment takes 1-2 hours, and the second sintering treatment takes 6-12 hours.

[0016] In one possible implementation, as described above, the first complexing agent comprises an amino group and has a thermal decomposition temperature of 200-300°C, and the second complexing agent comprises a multidentate chelate structure and has a thermal decomposition temperature of 400-600°C.

[0017] In one possible implementation, as described above, the ratio of the total molar amount of the complexing agent to the total molar amount of iron and manganese elements in the lithium manganese iron phosphate precursor is (0.05-0.3):1.

[0018] In one possible implementation, as described above, the molar ratio of the first complexing agent to the second complexing agent is 1:(0.25-4).

[0019] In a second aspect, the present invention provides a lithium manganese iron phosphate material, which is prepared by the method described in any one of the first aspects of the present invention.

[0020] Thirdly, the present invention provides an electrode comprising the lithium manganese iron phosphate material described in the second aspect of the present invention.

[0021] Fourthly, the present invention provides a battery comprising the electrode sheet described in the third aspect of the present invention.

[0022] The method for preparing lithium manganese iron phosphate (LFP) material provided by this invention achieves a balanced optimization between compaction density and cycle life through a synergistic design of two-stage sintering and stepwise decomposition of dual complexing agents. In the first sintering process, the first complexing agent rapidly pyrolyzes at a relatively low first temperature, initiating LFP nucleation and generating nanoscale particles. In the second sintering process, the second complexing agent is slowly released at a higher second temperature to drive stable crystal growth, forming micron-sized large particles. This in-situ synthesis yields LFP material with absolutely uniform manganese and iron element distribution and spontaneous particle size distribution, achieving both high compaction density and good cycle life. Attached Figure Description

[0023] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with the invention and, together with the description, serve to explain the principles of the invention.

[0024] Figure 1 The XRD pattern of Embodiment 1 provided by the present invention;

[0025] Figure 2 The SEM image of Embodiment 1 provided by the present invention;

[0026] Figure 3 The SEM image of Comparative Example 1 provided by this invention;

[0027] Figure 4 The SEM image of Comparative Example 2 provided by the present invention.

[0028] The accompanying drawings have illustrated specific embodiments of the invention, which will be described in more detail below. These drawings and descriptions are not intended to limit the scope of the invention in any way, but rather to illustrate the concept of the invention to those skilled in the art through reference to particular embodiments. Detailed Implementation

[0029] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below in conjunction with the embodiments of this invention. Obviously, the described embodiments are only some embodiments of this invention, not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0030] Lithium manganese iron phosphate (LFP) is a lithium-ion battery cathode material with an olivine structure, combining the structural stability of lithium iron phosphate with the high voltage platform of lithium manganese phosphate. Compared to traditional lithium iron phosphate, LFP offers higher operating voltage and energy density while retaining better thermal stability, safety, and cycle performance, making it highly valuable for applications in power batteries, energy storage batteries, and high-safety electrochemical systems.

[0031] In existing technologies, lithium manganese iron phosphate materials are typically prepared using a single high-temperature sintering process or a simple two-stage sintering process, and conductivity and particle dispersion are improved by adding carbon sources and complexing agents. However, in actual preparation processes, it is often difficult to simultaneously achieve the desired compaction density and cycle life.

[0032] The inventors explained that this is because when the sintering temperature is increased to enhance particle densification, it can easily lead to abnormal growth of primary particles, increased manganese dissolution, and decreased lithium-ion diffusion performance, thereby affecting cycle stability. On the other hand, while lowering the sintering temperature is beneficial for maintaining smaller particles and a faster diffusion rate, it can easily result in insufficient particle bonding, higher porosity, and lower compaction density.

[0033] To address the aforementioned problems, this invention proposes a method for preparing lithium manganese iron phosphate materials based on staged pyrolysis regulation using dual complexing agents. By introducing a first weak complexing agent and a second complexing agent with different thermal decomposition temperatures, and in conjunction with a two-stage sintering process, graded particle growth and in-situ gradation are achieved. Thus, the material can both utilize the gradation of micron- and nano-sized particles to improve compaction density and retain ion diffusion advantages and structural stability, thereby achieving a synergistic improvement in compaction density and cycle life.

[0034] In a first aspect, the present invention provides a method for preparing lithium manganese iron phosphate material, comprising the following steps: subjecting a lithium manganese iron phosphate precursor to a first sintering treatment at a first temperature, and then subjecting it to a second sintering treatment at a second temperature to obtain lithium manganese iron phosphate material; wherein, the first temperature is lower than the second temperature; the lithium manganese iron phosphate precursor comprises a phosphorus source, a manganese source, an iron source, a lithium source, a carbon source, and a complexing agent, the complexing agent comprising a first complexing agent and a second complexing agent; the thermal decomposition temperature of the first complexing agent is not higher than the first temperature, and the thermal decomposition temperature of the second complexing agent is higher than the first temperature but not higher than the second temperature.

[0035] Complexing agents are organic compounds that can form coordination structures with transition metal components, including manganese and iron. They can not only improve the uniformity of precursor mixing, but more importantly, they can regulate the reactivity and release rhythm of metal components.

[0036] In the first sintering process, due to the low thermal stability of the first complexing agent, its coordination structure preferentially undergoes pyrolysis and collapse after reaching the first temperature. This significantly enhances the diffusion capacity and reactivity of some of the complexed manganese and iron components, allowing them to rapidly participate in the formation of lithium manganese iron phosphate crystal nuclei. Therefore, a large number of crystal nuclei form in the system within a short time, making the nucleation rate significantly higher than the particle growth rate. When a large number of crystal nuclei form simultaneously in the system, the total mass is contested by multiple nuclei, resulting in the in-situ formation of small and uniformly distributed nanoscale primary particles. These nanoscale particles have shorter lithium-ion diffusion paths and larger reaction interfaces, which helps reduce polarization during charge and discharge processes and improve cycle stability.

[0037] In the second sintering process, as the temperature further increases, the second complexing agent, which has higher thermal stability, begins to degrade, and the remaining metal components it complexes are gradually activated. Since a large number of stable crystal nuclei have already formed in the system at this stage, the subsequent process no longer relies on rapid nucleation, but is mainly driven by the Oswald ripening mechanism to promote stable crystal growth.

[0038] Oswald curing refers to the gradual instability of small particles in a system due to their high curvature and surface energy. Some of these particles migrate to the surface of larger, more thermodynamically stable particles, causing the smaller particles to gradually shrink or even disappear, while the larger particles continue to grow. Because the second complexing agent does not decompose instantaneously but releases the metal components slowly, the curing process is relatively mild, avoiding the abnormal coarsening and severe agglomeration problems that occur in traditional high-temperature sintering processes.

[0039] In this process, stable crystal growth not only signifies a gradual increase in particle size but also a gradual improvement in the crystal lattice structure. Because atoms have more time for diffusion and arrangement during slow growth, lattice defects are reduced, cation mixing is minimized, and elemental uniformity of the particles is improved, thereby enhancing the material's structural stability and cycle life. Simultaneously, the micron-sized secondary particles formed under Oswald ripening effectively improve the packing efficiency between particles and reduce the porosity inside the electrode, thus contributing to increased compaction density. Ultimately, the material forms a gradation composed of nanoscale primary particles and micron-sized secondary particles, achieving synergistic optimization of compaction density and cycle life.

[0040] In one possible embodiment, the lithium manganese iron phosphate precursor includes a phosphorus source, a manganese source, an iron source, a lithium source, and a carbon source. For example, the lithium source may include at least one of lithium carbonate and lithium hydroxide; the phosphorus source may include at least one of ammonium dihydrogen phosphate and iron phosphate; the iron source may include ferrous oxalate; and the manganese source may include at least one of manganese trioxide, manganese oxide, and manganese carbonate. The carbon source may include at least one of glucose, sucrose, and starch.

[0041] In one possible embodiment, the lithium manganese iron phosphate precursor contains lithium, manganese, iron, phosphorus, and oxygen in a molar ratio of 1:x:y:1:4, where 0.3 ≤ x ≤ 0.7 and y = 1 - x; it also contains 10-15 wt.% carbon.

[0042] In one possible embodiment, the precursor can be a wet precursor obtained by ball milling or sand milling, or it can be a composite particle obtained by spray granulation, or a material formed by dry mixing followed by granulation. For example, a raw material system including phosphorus source, manganese source, iron source and lithium source can be ball-milled at a speed of 100-3000 r / min for 0.5-24 h to obtain lithium manganese iron phosphate precursor.

[0043] In one possible embodiment, the sintering process can be carried out using conventional sintering equipment such as tube furnaces, roller furnaces, box furnaces, pusher furnaces, or rotary furnaces. A protective gas can be introduced during the sintering process to reduce the risk of transition metal oxidation and lithium salt volatilization under high-temperature conditions. The protective gas may include nitrogen, argon, helium, or any combination thereof, preferably nitrogen or argon. The first and second sintering processes can be completed continuously in the same sintering equipment or independently in different temperature zones.

[0044] It should be understood that the choices in the above solutions are examples and do not represent a limitation on the present invention.

[0045] In one possible embodiment, as described above, the first temperature is 250-350°C and the second temperature is 650-800°C.

[0046] In this embodiment, the first temperature is relatively low overall, and the atomic diffusion ability of the system is limited. Therefore, the nucleation rate is significantly higher than the crystal growth rate, which is conducive to the formation of small and uniformly distributed nanoscale primary particles. The second temperature range can significantly increase the atomic diffusion rate, which is beneficial to reduce lattice defects, improve crystal integrity, and enhance the degree of fusion between particles, thereby forming a relatively dense micron-scale secondary particle structure and improving the tap density and compaction density of the material.

[0047] In one possible embodiment, as described above, in the lithium manganese iron phosphate precursor, the Fe coordination stability constant of the first complexing agent is lower than the Fe coordination stability constant of the second complexing agent, and the Mn coordination stability constant of the first complexing agent is lower than the Mn coordination stability constant of the second complexing agent.

[0048] In this embodiment, because the first complexing agent has a weak coordination ability with the Fe and Mn metal components, it is easier for the complexed Fe and Mn components to preferentially lose their coordination constraints and participate in solid-phase diffusion and crystal nucleation during the heating process. At this time, the activity of the local metal components in the system increases rapidly, thereby significantly increasing the crystal nucleation rate and enabling the system to preferentially form a large number of small and uniformly distributed nanoscale primary crystal nuclei. In addition, because the second complexing agent has a high coordination stability constant with Fe and Mn ions, its coordination structure can remain relatively stable during the first stage of heating. Therefore, some Fe and Mn components will not participate in the reaction immediately, but will gradually lose their coordination constraints and be slowly released at higher temperatures. The particle growth process is relatively stable, which can reduce the risk of abnormal coarsening and local agglomeration.

[0049] In one possible embodiment, as described above, the first sintering treatment takes 1-2 hours and the second sintering treatment takes 6-12 hours.

[0050] In this embodiment, since the primary goal of the first sintering treatment is to form a large number of uniform nanoscale primary crystal nuclei, rather than to promote continuous particle growth, a time of 1-2 hours is beneficial to ensure sufficient nucleation while suppressing premature coarsening of primary particles. The second sintering treatment, set to 6-12 hours, is mainly used to complete the slow decomposition of the second complexing agent, crystal maturation, and secondary particle densification. The longer high-temperature holding time can improve atomic diffusion, allowing some high-surface-energy small particles to gradually migrate to thermodynamically more stable large particles, thereby promoting the densification of the internal structure of the secondary particles.

[0051] In one possible embodiment, as described above, the first complexing agent comprises an amino group and has a thermal decomposition temperature of 200-300°C, and the second complexing agent comprises a multidentate chelate structure and has a thermal decomposition temperature of 400-600°C.

[0052] In this embodiment, the lone pair electrons of the amino group in the first complexing agent can form coordination bonds with Fe and Mn, allowing the metal components to be uniformly dispersed in the precursor. However, since amino coordination is usually a relatively weak monodentate or low-coordination-stability structure, the coordination structure is more prone to breakage during heating. The amino-containing structure can also release nitrogen-containing components during pyrolysis, thereby forming a preliminary nitrogen-doped or nitrogen-carbon composite coating structure on the particle surface, improving the material's electronic conductivity. For example, the first complexing agent can be one or more combinations of glycine, alanine, or serine.

[0053] The multidentate chelating structure of the second complexing agent can simultaneously form stable cyclic coordination structures with Fe and Mn metal components through multiple coordination sites. Its coordination stability is significantly higher than that of ordinary amino ligands. Therefore, it can maintain relative stability in the first sintering stage and will not immediately release all metal components, but will release them slowly in the second sintering process. This reduces the abnormal coarsening problem in traditional high-temperature sintering and improves particle densification and mechanical stability. For example, the second complexing agent can be one or more of ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA), or polyacrylamide (PAM).

[0054] In one possible embodiment, as described above, the ratio of the total molar amount of the complexing agent to the total molar amount of iron and manganese elements in the lithium manganese iron phosphate precursor is (0.05-0.3):1.

[0055] In this embodiment, by controlling the ratio of complexing agent to Fe and Mn elements, it is possible to ensure that most Fe and Mn components form a stable coordination structure, improve the uniformity of the precursor and the ability to control stage nucleation, and reduce the risk of increased porosity and loose particles caused by excessive organic components.

[0056] In one possible embodiment, as described above, the molar ratio of the first complexing agent to the second complexing agent is 1:(0.25-4).

[0057] In this embodiment, by controlling the molar ratio of the first complexing agent to the second complexing agent, the system can simultaneously possess sufficient low-temperature rapid nucleation capability and high-temperature stable maturation capability. The first complexing agent is responsible for establishing a large number of uniform nanocrystal nuclei, while the second complexing agent is responsible for continuously and slowly supplying Fe and Mn components during the subsequent high-temperature stage, promoting particle reconstruction, fusion, and internal densification. Ultimately, this achieves synergistic optimization of compaction density and cycle life.

[0058] Secondly, the present invention provides a lithium manganese iron phosphate material, which is prepared by any of the methods of the first aspect of the present invention.

[0059] The lithium manganese iron phosphate material provided by this invention has a dense micron-sized secondary particle structure composed of nanoscale primary particles. The fine and uniform primary particles effectively shorten the lithium-ion diffusion path, reduce electrochemical polarization during charge and discharge, and improve the structural stability of the material during cycling. Meanwhile, the densified secondary particles reduce the internal porosity of the particles, improve the mechanical strength and packing efficiency, and the in-situ gradation relationship between the two further enhances the compaction density of the material. Therefore, the resulting material possesses both high compaction density and excellent cycle life.

[0060] Thirdly, the present invention provides an electrode comprising the lithium manganese iron phosphate material of the second aspect of the present invention.

[0061] The electrode prepared using the aforementioned lithium manganese iron phosphate material exhibits high structural stability after compaction. Due to the uniform particle size distribution and dense internal structure of the secondary particles, severe particle breakage and structural collapse are less likely to occur during rolling, effectively increasing the amount of active material filling the electrode and thus improving its volumetric energy density. Simultaneously, the fine primary particle structure retained within the material maintains good lithium-ion diffusion channels and electron transport networks under high compaction conditions, reducing concentration polarization issues in thick electrodes and thereby improving the long-term cycling stability of the electrode.

[0062] The electrode of the present invention specifically includes a positive current collector and a positive active layer formed of lithium manganese iron phosphate positive electrode material disposed on the surface of the positive current collector.

[0063] In one possible embodiment, the electrode sheet described above can be prepared by the following steps: dispersing lithium manganese iron phosphate cathode material and binder in an appropriate amount of N-methylpyrrolidone (NMP) solvent, and may also add a conductive agent, and then thoroughly stirring and mixing to form a uniform cathode slurry; uniformly coating the cathode slurry onto the cathode current collector, and then drying, rolling and slitting to obtain the cathode sheet.

[0064] In one possible embodiment, the positive electrode active layer comprises, by weight percentage, 70-99 wt% of positive electrode active material, 0.5-15 wt% of conductive agent, and 0.5-15 wt% of binder; more specifically, it comprises 80-98 wt% of positive electrode active material, 1-10 wt% of conductive agent, and 1-10 wt% of binder.

[0065] The positive current collector can be made of at least one of aluminum foil or nickel foil; the conductive agent can be selected from at least one of carbon black, acetylene black, graphene, Ketjen black, and carbon fiber; and the binder can be selected from at least one of polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl fluoride, polyethylene, polypropylene, polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, ethylene oxide-containing polymers, polyvinylpyrrolidone, and polyurethane.

[0066] Fourthly, the present invention provides a battery comprising the electrode sheet of the third aspect of the present invention.

[0067] The lithium-ion battery assembled using the electrode provided by this invention exhibits both high energy density and long cycle life. On one hand, the high-density electrode can load more active material within a limited volume, thereby increasing the battery's volumetric energy density. On the other hand, the uniformly distributed elemental particle structure within the material reduces lithium-ion diffusion resistance and improves the uniformity of charge-discharge reactions, thus slowing down capacity decay during cycling. Therefore, the battery can maintain a high capacity retention rate and good cycle stability even under long-cycle, high-rate, and high-load conditions.

[0068] In addition to the aforementioned positive electrode, the battery of the present invention also includes a negative electrode, an electrolyte, and a separator. The present invention does not strictly limit the negative electrode active material in the negative electrode; it can be at least one of the negative electrode active materials commonly used in batteries, such as graphite, hard carbon, soft carbon, mesophase carbon microspheres, silicon-based negative electrode materials (mainly including silicon suboxide and silicon-carbon negative electrodes), and tin-based negative electrode materials (mainly including tin and tin alloys).

[0069] This invention does not strictly limit the choice of electrolyte, and may include one or more solvents commonly used in lithium-ion battery electrolytes, as well as lithium salts commonly used in lithium-ion electrolytes. For example, the solvent may be ethylene carbonate, propylene carbonate, butene carbonate, fluoroethylene carbonate (FEC), dimethyl carbonate (DMC), diethyl carbonate (DEC), difluoroethylene carbonate (DFEC), dipropyl carbonate, methyl ethyl carbonate (EMC), ethyl acetate, ethyl propionate, propyl acetate, propyl propionate, sulfolane, γ-butyrolactone, etc.; the lithium salt may be one or more of lithium hexafluorophosphate (LiPF6), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI).

[0070] This invention does not strictly limit the choice of separator material. It can be one of the separator materials commonly used in lithium-ion batteries, such as polypropylene separator (PP), polyethylene separator (PE), polypropylene / polyethylene double-layer composite membrane (PP / PE), polyimide electrospun separator (PI), polypropylene / polyethylene / polypropylene triple-layer composite membrane (PP / PE / PP), cellulose nonwoven separator, and separator with ceramic coating.

[0071] In one possible embodiment, the battery described above is prepared by the following steps: a bare cell is obtained by winding or stacking a positive electrode, a separator, and a negative electrode; the bare cell is then packaged into a pre-stamped aluminum-plastic film bag. After the packaged battery is dried at 85°C, electrolyte is injected into the dried battery. The battery undergoes resting, formation, and secondary sealing to complete the battery preparation.

[0072] The present invention will be described below through specific embodiments.

[0073] Example 1

[0074] The preparation method of lithium manganese iron phosphate material in this embodiment includes the following steps:

[0075] 1) Weigh 3.68g lithium acetate, 7.51g manganese acetate, 2.94g ferrous oxalate, 5.87g ammonium dihydrogen phosphate, and 3g sucrose, accounting for 15% of the total mass of the above raw materials. Weigh 0.19g glycine (first complexing agent) and 1.46g ethylenediaminetetraacetic acid (second complexing agent), such that the molar ratio of the first complexing agent to the second complexing agent is 1:2; and the ratio of the total molar amount of the complexing agent to the total molar amount of transition metal ions (the sum of Mn and Fe) in the system is 0.15:1. Add all the above materials to 500mL of deionized water, stir for 6h to mix, and dry at 100℃ for 12h to obtain the lithium manganese iron phosphate precursor.

[0076] 2) The lithium manganese iron phosphate precursor was placed in a tube furnace and argon gas was introduced as a protective atmosphere. The temperature was increased to the first temperature of 300℃ at 3℃ / min and held for 1.5h for the first sintering treatment. Then the temperature was increased to the second temperature of 750℃ at 3℃ / min and held for 8h for the second sintering treatment. The lithium manganese iron phosphate material was obtained by natural cooling.

[0077] Example 2

[0078] The preparation method of lithium manganese iron phosphate material in this embodiment includes the following steps:

[0079] 1) Weigh 1.85g lithium carbonate, 2.13g manganese oxide, 1.44g iron oxide, 5.75g ammonium dihydrogen phosphate, and 1.12g glucose (10% of the total mass of the above raw materials). Weigh 0.18g alanine (first complexing agent) and 0.20g diethylenetriaminepentaacetic acid (second complexing agent), such that the molar ratio of the first complexing agent to the second complexing agent is 4:1; and the ratio of the total molar amount of the complexing agent to the total molar amount of transition metal ions (Mn and Fe) in the system is 0.05:1. Add all the above materials to 500mL of deionized water, stir for 6h to mix, and dry at 100℃ for 12h to obtain the lithium manganese iron phosphate precursor.

[0080] 2) The lithium manganese iron phosphate precursor was placed in a tube furnace and nitrogen was introduced as a protective atmosphere. The temperature was increased to the first temperature of 250°C at 5°C / min and held for 1 hour for the first sintering treatment. Then the temperature was increased to the second temperature of 650°C at 5°C / min and held for 12 hours for the second sintering treatment. The lithium manganese iron phosphate material was obtained by natural cooling.

[0081] Example 3

[0082] 1) Weigh 5.10g lithium acetate, 6.69g manganese sulfate tetrahydrate, 8.08g ferric nitrate nonahydrate, 4.9g phosphoric acid, and 1.98g polyethylene glycol (8% of the total mass of the above raw materials). Weigh 0.32g serine (first complexing agent) and 0.85g polyacrylamide (second complexing agent), such that the molar ratio of the first complexing agent to the second complexing agent is 1:4; and the ratio of the total molar amount of the complexing agent to the total molar amount of transition metal ions (Mn and Fe) in the system is 0.3:1. Add all the above materials to 500mL deionized water, stir for 6h to mix, and dry at 100℃ for 12h to obtain the lithium manganese iron phosphate precursor.

[0083] 2) The lithium manganese iron phosphate precursor was placed in a tube furnace and nitrogen was introduced as a protective atmosphere. The temperature was increased to the first temperature of 350°C at 5°C / min and held for 2 hours for the first sintering treatment. Then the temperature was increased to the second temperature of 800°C at 5°C / min and held for 6 hours for the second sintering treatment. The lithium manganese iron phosphate material was obtained by natural cooling.

[0084] Example 4

[0085] This embodiment is basically the same as that of Embodiment 1, except that the molar ratio of the first complexing agent and the second complexing agent is 1:0.1.

[0086] Example 5

[0087] This embodiment is basically the same as Embodiment 1, except that the ratio of the total molar amount of the complexing agent to the total molar amount of iron and manganese elements in the lithium manganese iron phosphate precursor is 1:1.

[0088] Comparative Example 1

[0089] Comparative Example 1 is basically the same as Example 1, except that the complexing agent only includes 1.46g of ethylenediaminetetraacetic acid.

[0090] Comparative Example 2

[0091] Comparative Example 2 is basically the same as Example 1, except that the complexing agent only includes 0.19g of glycine.

[0092] Test Example 1

[0093] The lithium manganese iron phosphate material obtained in Example 1 was mixed in a ball mill for 30 min and then characterized by X-ray diffraction (XRD). The test conditions were as follows: Cu Kα rays (λ=1.5406 Å) were used as the radiation source, the working voltage was 40 kV, the working current was 40 mA, the scanning range was 10°-80°, and the scanning rate was 5° / min.

[0094] like Figure 1The XRD pattern of Example 1 provided by the present invention is shown. All peaks are characteristic peaks of lithium manganese iron phosphate, and no impurity phase peaks are produced, indicating that lithium manganese iron phosphate cathode material has been successfully synthesized.

[0095] Test Example 2

[0096] After mixing the lithium manganese iron phosphate materials obtained in Example 1, Comparative Examples 1 and 2 in a ball mill for 30 minutes, the microstructure of the materials was observed using a scanning electron microscope (SEM). The samples were sputter-coated with gold before testing.

[0097] like Figure 2 The image shown is an SEM image of Embodiment 1 provided by the present invention; as follows: Figure 3 The image shown is a SEM image of Comparative Example 1 provided by the present invention, as follows: Figure 4 The image shown is the SEM image of Comparative Example 2 provided by this invention. As can be seen from the image, in Example 1, large and small particles coexist, with the large particles having a size of 2 μm and the small particles having a size of about 500 nm. This indicates that the particles synthesized in situ simultaneously were successfully graded.

[0098] Test Example 3

[0099] The compacted density of the lithium manganese iron phosphate composite materials obtained in the above examples and comparative examples was measured using a Sansi compaction density meter. 1 mg was weighed and placed in a 13 mm diameter disc mold, and the pressure was maintained at 29.4 kN for 30 seconds before the data was read from the device.

[0100] The cycle life of the materials in Examples 1-5 and Comparative Examples 1-2 was tested using coin cells. Polyvinylidene fluoride (PVDF) was dissolved in N-methylpyrrolidone to prepare a 95% PVDF solution. The mass ratio of lithium manganese iron phosphate composite material, conductive carbon black, and PVDF was calculated to be 90:5:5, and the mixture was stirred into a slurry. The slurry was uniformly coated onto aluminum foil, dried at 100°C for 12 hours, cut into circular positive electrode sheets, and dried in a vacuum oven at 100°C for 12 hours. Constant current charge-discharge cycle tests were conducted at 25°C within a voltage range of 2.5-4.35 V, with a cycle rate of 1 C, and the battery capacity retention rate during the cycle was recorded. The results are shown in Table 1.

[0101]

[0102] As can be seen from the data in Table 1, the lithium manganese iron phosphate cathode material provided by the present invention improves the compaction density while maintaining cycle life, and there is no loss in discharge specific capacity.

[0103] Finally, it should be noted that other embodiments of the invention will readily occur to those skilled in the art upon consideration of the specification and practice of the invention disclosed herein. This invention is intended to cover any variations, uses, or adaptations of the invention that follow the general principles of the invention and include common knowledge or customary techniques in the art not disclosed herein, and is not limited to the precise structures described above and shown in the accompanying drawings, and various modifications and changes can be made without departing from its scope. The scope of the invention is limited only by the appended claims.

Claims

1. A method for preparing lithium manganese iron phosphate material, characterized in that, The process includes the following steps: subjecting the lithium manganese iron phosphate precursor to a first sintering treatment at a first temperature, and then subjecting it to a second sintering treatment at a second temperature to obtain the lithium manganese iron phosphate material; wherein, The first temperature is lower than the second temperature; The lithium manganese iron phosphate precursor includes a phosphorus source, a manganese source, an iron source, a lithium source, a carbon source, and a complexing agent, wherein the complexing agent includes a first complexing agent and a second complexing agent; The thermal decomposition temperature of the first complexing agent is not higher than the first temperature, and the thermal decomposition temperature of the second complexing agent is higher than the first temperature but not higher than the second temperature.

2. The method according to claim 1, characterized in that, The first temperature is 250-350℃, and the second temperature is 650-800℃.

3. The method according to claim 2, characterized in that, In the lithium manganese iron phosphate precursor, the Fe coordination stability constant of the first complexing agent is lower than that of the Fe coordination stability constant of the second complexing agent, and the Mn coordination stability constant of the first complexing agent is lower than that of the Mn coordination stability constant of the second complexing agent.

4. The method according to claim 3, characterized in that, The first sintering treatment takes 1-2 hours, and the second sintering treatment takes 6-12 hours.

5. The method according to claim 1, characterized in that, The first complexing agent comprises an amino group and has a thermal decomposition temperature of 200-300°C, and the second complexing agent comprises a multidentate chelate structure and has a thermal decomposition temperature of 400-600°C.

6. The method according to claim 1, characterized in that, The ratio of the total molar amount of the complexing agent to the total molar amount of iron and manganese elements in the lithium manganese iron phosphate precursor is (0.05-0.3):

1.

7. The method according to any one of claims 1-6, characterized in that, The molar ratio of the first complexing agent to the second complexing agent is 1:(0.25-4).

8. A lithium manganese iron phosphate material, characterized in that, Prepared by the method described in any one of claims 1-7.

9. An electrode sheet, characterized in that, Includes the lithium manganese iron phosphate material as described in claim 8.

10. A battery, characterized in that, Includes the electrode as described in claim 9.

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