Lithium manganese iron phosphate composite material, preparation method thereof and secondary battery containing lithium manganese iron phosphate composite material

Abstract

The invention belongs to the field of batteries, and particularly discloses a lithium manganese iron phosphate composite material, a preparation method thereof and a secondary battery containing the lithium manganese iron phosphate composite material. The preparation method comprises the following steps: firstly preparing ferromanganese phosphate containing a special pore-forming agent, and then mixing the ferromanganese phosphate serving as a precursor with raw materials such as a lithium source, a doped metal element M source and a carbon source; the preparation method comprises the following steps: respectively grinding the mixture into large and small particle mixtures with different particle sizes, grinding and mixing the large and small particle mixtures, and carrying out spray drying to obtain an unsintered precursor mixture, and finally carrying out heat treatment sintering on the unsintered precursor mixture twice to obtain the lithium manganese iron phosphate composite material. According to the method disclosed by the invention, the lithium manganese iron phosphate composite material has the advantages of high compaction density, high rate capability, low cost and high gram capacity, and the application prospect of the material is widened.

Description

Technical Field

[0001] This invention belongs to the field of batteries, specifically relating to a lithium manganese iron phosphate composite material and its preparation method, and a secondary battery containing the same. Background Technology

[0002] Lithium manganese iron phosphate (LMFP) possesses excellent thermodynamic and kinetic stability and safety performance, making it widely used in commercial batteries. LMFP is mainly produced through two methods: liquid-phase and solid-phase methods. Liquid-phase methods can be further divided into hydrothermal, solvothermal, sol-gel, and co-precipitation methods. The hydrothermal method is the most widely used, with lower energy consumption and cost, and allows for control over particle size. However, due to the high-temperature and high-pressure environment, it requires sophisticated equipment and operational control, hindering large-scale adoption and resulting in poor batch-to-batch consistency, making industrialization difficult. Solid-phase methods for LMFP preparation suffer from uneven reaction, large particle size, difficulty in morphology control, and poor batch-to-batch consistency. While liquid-phase processes produce LMFP with good consistency, they are more expensive and still suffer from low compaction density, low electronic conductivity, slow lithium-ion diffusion rate, poor rate performance, and low specific capacity. These limitations prevent effective improvement in LMFP compaction density and specific capacity, thus restricting its application scenarios.

[0003] Manganese iron phosphate (MFP) itself possesses good chemical stability, is not easily degraded during storage and transportation, and has multiple synthesis methods available, allowing for the selection of a suitable process route based on specific circumstances. However, as a precursor to lithium iron phosphate (LMFP) produced via the liquid-phase method, MFP requires high purity; impurities in MFP will affect LMFP performance. Furthermore, its Me / P and Mn / Fe ratios significantly influence LMFP performance. In addition, the particle size and microstructure of MFP have a substantial impact on LMFP performance; currently, the most common microstructure of MFP is plate-like. When preparing lithium iron phosphate using MFP as a precursor, MFP and other raw materials generally need to be mixed through grinding or sand milling. However, plate-like MFP is difficult to disperse into primary particles, hindering the formation of suitable gradations in subsequent stages, making it difficult to improve the electrical properties of LMFP by controlling MFP particle size. Therefore, pore-forming agents are typically added during the precipitation process of MFP preparation, followed by sintering of the precipitate. The pore-forming agent decomposes at high temperatures to generate gas, thereby increasing the porosity of MFP and improving its particle performance. However, the high-temperature sintering required for MFP hole creation will further increase its production cost significantly. Summary of the Invention

[0004] To address the issues of low compaction density, specific capacity, and rate performance of LMFP prepared using MFP as a precursor in the prior art, this invention provides a lithium manganese iron phosphate composite material, its preparation method, and a secondary battery containing the same.

[0005] To achieve the above objectives, the following technical solutions are specifically included: In a first aspect, a method for preparing a lithium manganese iron phosphate composite material includes the following steps: (1) Dissolve the first pore-forming agent in a solvent, add the second pore-forming agent and solid electrolyte, and then successively pass through ball milling, spray drying, heat treatment and crushing to obtain the third pore-forming agent; the decomposition temperature of the first pore-forming agent is T1, and the decomposition temperature of the second pore-forming agent is T2. (2) Under inert gas protection, a solution containing ferrous and manganese sources and a solution containing phosphorus sources are mixed to obtain a first mixture; under inert gas protection, an alkaline solution is added dropwise to the first mixture to carry out a precipitation reaction and a third pore-forming agent is added, followed by washing, filtration and drying to obtain a second mixture; the pH value of the precipitation reaction is 5.5-8; (3) The second mixture, lithium source, doped metal element M source, carbon source and solvent are mixed to obtain a third mixture; Take one portion of the third mixture and grind it into a slurry 1 with an average particle size of d1; Take another portion of the third mixture and grind it into a slurry 2 with an average particle size of d2; the d1 and the d2 satisfy the following relationship: d1 < d2; (4) The slurry 1 and the slurry 2 are sequentially ball-milled and spray-dried to obtain a fourth mixture; the average particle size of the fourth mixture is ≤10μm; (5) The fourth mixture is subjected to a first sintering, a second sintering and cooling in sequence to obtain the lithium manganese iron phosphate composite material; The sintering temperature for the first sintering is T3, and the sintering temperature for the second sintering is T4, satisfying the condition: T1 < T3 < T2 < T4.

[0006] First, a first pore-forming agent is dissolved in a solvent to form a solution containing the first pore-forming agent. Then, a second pore-forming agent and a solid electrolyte are added. The two pore-forming agents and the solid electrolyte are then sequentially ball-milled, spray-dried, heat-treated, and crushed to obtain a special novel third pore-forming agent. The first pore-forming agent is pre-formed into a solution, which helps it to encapsulate the second pore-forming agent and the solid electrolyte. The decomposition temperature of the first pore-forming agent is lower than that of the second pore-forming agent. The two pore-forming agents with different decomposition temperatures play their roles at different temperatures during subsequent heat treatment.

[0007] Secondly, under inert gas protection and with controlled pH of the reaction system, an alkaline solution is used to react with ferrous, manganese, and phosphorus sources to form ferromanganese phosphate containing multiple water molecules (water of crystallization). The inert gas protection is maintained until the precipitation reaction is complete, preventing the precipitate from being oxidized to higher valence states, which would affect the proportions of elements in the ferromanganese phosphate. Simultaneously, a third pore-forming agent is added during the precipitation reaction to form ferromanganese phosphate containing multiple water molecules, facilitating its incorporation into the ferromanganese phosphate and resulting in more uniform mixing. Furthermore, the ferromanganese phosphate incorporating the third pore-forming agent does not require further sintering; it only needs to be dried before use as a precursor for LMFP production, thus reducing the cost of LMFP production.

[0008] Then, complex water-molecule ferromanganese phosphate with a third pore-forming agent is used as a precursor for LMFP and mixed with raw materials such as lithium source, doped metal element M source, and carbon source. The mixture is ground in two parts: one part is ground into small particle mixture and the other part is ground into large particle mixture. The two parts are then ground, mixed, and spray-dried to obtain an unsintered precursor mixture. The particle size distribution is adjusted by mixing large and small particles, which improves the compaction of the finished product and offsets the effect of the pore-forming agent on compaction during the sintering stage. This improves the compaction performance of sintered ferromanganese phosphate. The particle size change of the unsintered precursor mixture before and after sintering is small. By controlling the particle size of the unsintered precursor mixture powder to ≤10μm, the particle size of the finished product can be indirectly controlled.

[0009] Finally, the unsintered precursor mixture was subjected to two heat treatments to obtain the final lithium manganese iron phosphate composite material. During the sintering stage, at a relatively low temperature, the low-temperature pore-forming agent in the lithium manganese iron phosphate began to generate gas and create pores. The pore-forming agent can accelerate the dehydration rate of the sintering process, reduce the sintering time, improve the morphology of LMFP particles, and form large pores on the particle surface. At the same time, the high-temperature pore-forming agent and solid electrolyte encapsulated by the low-temperature pore-forming agent are in-situ coated on the inner walls of these large pores. This allows the small LMFP particles and small solid electrolyte particles to embed into the pores of the large LMFP particles, resulting in better conductivity between them during the sintering and fusion process, and effectively promoting the intercalation of lithium ions during solid-state sintering. The high-temperature pore-forming agent plays a role at a higher temperature, decomposing to generate nanoscale cracks. The conductive material remaining after decomposition is left in these cracks, improving conductivity and providing more pathways for lithium ion intercalation and deintercalation, thereby achieving the goal of improving the rate performance and specific capacity of the lithium manganese iron phosphate composite material.

[0010] Therefore, this invention first prepares manganese iron phosphate containing a special pore-forming agent, then uses it as a precursor to mix with a lithium source, the doped metal element M source, a carbon source, and other raw materials; the mixture is then ground into large and small particles of different sizes, and the large and small particles are ground, mixed, and spray-dried to obtain an unsintered precursor mixture; finally, the unsintered precursor mixture is sintered through two heat treatments to obtain a lithium manganese iron phosphate composite material. The method of this invention enables the lithium manganese iron phosphate composite material to possess the advantages of high rate capability, low cost, and high specific capacity, thus broadening the application prospects of the material.

[0011] Furthermore, the unsintered precursor mixture prepared by the method of the present invention exhibits minimal particle size change before and after sintering. The particle size of the finished product can be indirectly controlled by controlling the particle size of the unsintered precursor mixture, achieving higher precision than conventional processes. This aspect can be used to prepare small-particle unsintered precursor mixtures, which can then be used to prepare small-particle lithium manganese iron phosphate composite materials. When these small-particle lithium manganese iron phosphate composite materials are used as the positive electrode active material, the positive electrode can achieve denser filling, which helps to improve the contact degree of the solid-solid interface on the positive electrode side of the all-solid battery and improves the battery performance.

[0012] Preferably, T1 is 250-400℃.

[0013] More preferably, in step (1), the first pore-forming agent includes at least one of nylon, ammonium carbonate, ammonium bicarbonate, ammonium oxalate, polymethyl methacrylate, methyl methacrylate, methyl cellulose, polyvinyl alcohol (PVA), carboxymethyl cellulose (CMC), ascorbic acid, polyfurfuryl alcohol, or polymethyl methacrylate.

[0014] Preferably, T2 is 500-650℃.

[0015] More preferably, in step (1), the second pore-forming agent includes at least one of polystyrene, polyvinyl butyral, modified graphene, or modified wood particles; the modified graphite includes graphene oxide.

[0016] Preferably, in step (1), the solid electrolyte includes at least one of lithium aluminum titanium phosphate (LATP), lithium aluminum germanium phosphate (LAGP), lithium lanthanum titanate (LLTO), or lithium lanthanum zirconium oxide (LLZO). Embedding the solid electrolyte into the pore-forming agent facilitates the placement of the solid electrolyte within the pores of the LMFP, further improving the conductivity and electrical performance of the LMFP.

[0017] More preferably, the lithium titanium aluminum phosphate has the chemical formula Li. 1+a Al a Ti 2-a (PO4)3, where 0 ≤ a ≤ 0.5; Further preferably, the chemical formula of the lithium germanium aluminum phosphate is Li 1+b Al b Ge 2-b (PO4)3, where 0 ≤ b ≤ 0.5; the chemical formula of the lithium lanthanum titanate is Li 3c La 2 / 3-c Ti d T 1-d O3, where 0 < c < 2 / 3, 0 < d ≤ 1, and T is at least one of Nb, W, Ti, Hf, Ru, Mo, Nd, Ba, Ga, In, Ge, Sn, Sb, Se.

[0018] Further preferably, the chemical formula of the lithium lanthanum zirconium oxide type is Li 7-x La3Zr 2-x M x O 12 , 0 ≤ x ≤ 1; where the doping elements of M include at least one of Nb, Ta, Ti, Ga, Ge, Y, Gd, W, Mo, Sn, Sb, Se or Ru. <00000�8>Preferably, in step (1), the solvent includes at least one of ethanol or water.

[0020] Preferably, in step (1), the average particle size of the solid electrolyte is 0.1 - 0.3 μm.

[0021] Preferably, in step (1), the mass ratio of the first pore-forming agent, the second pore-forming agent and the solid electrolyte is (1 - 9):(1 - 9):1, and more preferably (4 - 6):(3 - 5):1.

[0022] Preferably, in step (1), the ball milling time is 1 - 3 h. <0000l06>Preferably, in step (1), the inlet air temperature of the spray drying is 150 - 220 °C, and the outlet air temperature of the spray drying is 65 - 110 °C.

[0024] Preferably, in step (1), the heat treatment temperature is 95 - 110 °C, and the heat treatment time is 0.5 - 3 h. Performing low-temperature heat treatment on the mixture of the first pore-forming agent, the second pore-forming agent and the solid electrolyte at the above temperature can improve its strength, avoid the fragmentation and dispersion of its particles, improve the integrity of the three, and is conducive to its functioning as a whole in the subsequent sintering process.

[0025] Preferably, in step (1), the average particle size of the third pore-forming agent is 10 - 100 nm. Controlling the particle size of the third pore-forming agent is conducive to its uniform dispersion inside the iron manganese phosphate containing multiple water molecules formed by the precipitation reaction.

[0026] Preferably, in step (2), the ferrous source in the solution containing the ferrous source and the manganese source includes at least one of ferrous chloride, ferrous sulfate or ferrous nitrate.

[0027] Preferably, in step (2), the manganese source in the solution containing ferrous and manganese sources includes at least one of manganese chloride, manganese sulfate, or manganese nitrate.

[0028] Preferably, in step (2), the mass percentages of Fe and Mn in the solution containing ferrous and manganese sources are 3%-20% and 3%-20%, respectively.

[0029] Preferably, in step (2), the phosphorus source in the phosphorus-containing solution includes at least one of ammonium phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, or phosphoric acid.

[0030] Preferably, in step (2), the pH value of the phosphorus source solution is 6-8, and the mass percentage of P in the phosphorus source solution is 3%-20%.

[0031] Preferably, in step (2), the molar ratio of Fe in the ferrous source, Mn in the manganese source, and P in the phosphorus source in the solution containing the ferrous source and the solution containing the phosphorus source is (1-9):(1-9):(10-13).

[0032] Preferably, in step (2), the alkali in the alkaline solution includes at least one of ammonia, sodium hydroxide, or sodium carbonate.

[0033] Preferably, in step (2), the precipitation reaction takes 0.5-8 hours.

[0034] Preferably, in step (2), the dropping time is <6h. By controlling the dropping time of the alkaline solution to control the reaction rate of the precipitation reaction process, the inventors of this invention have found that at the above-mentioned dropping time, a precipitate with more uniform particle size and morphology can be obtained, which is beneficial for its use as a precursor to prepare LMFP with better electrical properties.

[0035] More preferably, in step (2), the dripping time is 60-180 min.

[0036] Preferably, in step (2), the drying temperature is 100-220°C.

[0037] More preferably, in step (2), the drying is flash drying.

[0038] Preferably, in step (2), the filtration is pressure filtration.

[0039] Preferably, in step (2), the average particle size of the second mixture is less than 200 μm. By controlling the pH value, dropping time, and raw material concentration during the precipitation process, the particle size of the ferric phosphate precipitate containing the third pore-forming agent and rehydration can be controlled to be less than 200 μm, which is more conducive to uniform mixing with other raw materials and control of the particle size of the mixture.

[0040] Preferably, in step (2), the mass percentage of the third pore-forming agent is 1%-30% based on the total mass of the ferrous source, manganese source and phosphorus source in the solution containing the ferrous source and the solution containing the phosphorus source.

[0041] Preferably, in step (2), the time when the alkaline solution starts to be added is recorded as h0, the time when the third pore-forming agent starts to be added is recorded as h1, and the time when the alkaline solution stops to be added is recorded as h3; satisfying h0 < h1 < h3.

[0042] More preferably, the time interval between h0 and h1 is h2, which is 5-30 min. Adding the third pore-forming agent after the precipitation reaction begins with the addition of alkaline solution and before the addition of alkaline solution ends facilitates the mixing of the third pore-forming agent with its precipitate, which is beneficial for the third pore-forming agent to exert its pore-forming effect subsequently.

[0043] Preferably, in step (3), the lithium source includes at least one of lithium hydroxide, lithium carbonate, lithium dihydrogen phosphate, lithium oxalate, lithium acetate, or lithium phosphate.

[0044] Preferably, in step (3), the metal element M in the doping metal element M source includes at least one of Ti, Mg, Al, V, Nb, or Zn; the doping metal element M source includes at least one of an oxide of metal element M or a salt thereof. Doping the above-mentioned metal elements in LMFP can further improve the ionic conductivity and enhance the specific capacity and rate performance of the battery.

[0045] Preferably, in step (3), the doped metal element M source includes at least one of titanium source, magnesium source, aluminum source, vanadium source, and niobium source.

[0046] More preferably, the titanium source includes titanium dioxide, the magnesium source includes at least one of magnesium oxide and magnesium chloride, the aluminum source includes aluminum phosphate, the vanadium source includes at least one of ammonium metavanadate and vanadium oxalate, and the niobium source includes at least one of niobium pentoxide, niobium oxalate, and ammonium niobium oxalate.

[0047] Preferably, in step (3), the carbon source includes at least one of starch, sucrose, glucose, citric acid, polyethylene glycol (PEG), polyvinylpyrrolidone (PVP), polypropylene, conductive carbon black, acetylene black, carbon nanotubes, or graphene.

[0048] Preferably, in step (3), the solvent includes water.

[0049] Preferably, in step (3), the third mixture further includes a solid electrolyte More preferably, in step (3), the solid electrolyte includes at least one of lithium aluminum titanium phosphate (LATP), lithium aluminum germanium phosphate (LAGP), lithium lanthanum titanate (LLTO), or lithium lanthanum zirconium oxide type (LLZO).

[0050] More preferably, the chemical formula of the lithium aluminum titanium phosphate is Li 1+a Al a Ti 2-a (PO4)3, where 0 ≤ a ≤ 0.5; More preferably, the chemical formula of the lithium aluminum germanium phosphate is Li 1+b Al b Ge 2-b (PO4)3, where 0 ≤ b ≤ 0.5; the chemical formula of the lithium lanthanum titanate is Li 3c La 2 / 3-c Ti d T 1-d O3, where 0 < c < 2 / 3, 0 < d ≤ 1, and T is at least one of Nb, W, Ti, Hf, Ru, Mo, Nd, Ba, Ga, In, Ge, Sn, Sb, Se.

[0051] More preferably, the chemical formula of the lithium lanthanum zirconium oxide type is Li 7-x La3Zr 2-x M x O 12 , 0 ≤ x ≤ 1; where the dopable elements of M include at least one of Nb, Ta, Ti, Ga, Ge, Y, Gd, W, Mo, Sn, Sb, Se, or Ru.

[0052] Preferably, in step (3), the second mixture contains Fe element, Mn element, and P element, and the molar ratio of Li element in the lithium source, Fe element in the second mixture, and metal element M in the metal element M source of the dopant is (1 - 1.06):(0.25 - 0.45):(0.001 - 0.03).

[0053] More preferably, in step (3), the second mixture contains Fe element, Mn element, and P element, and the molar ratio of Fe element, Mn element, P element in the second mixture, Li element in the lithium source, and metal element M in the metal element M source of the dopant is (0.25 - 0.45):(0.55 - 0.75):(1 - 1.06):(0.55 - 0.9):(0.001 - 0.03).

[0054] Preferably, in step (3), the carbon source has a mass percentage of 1%-8% based on the total mass of the second mixture, the lithium source, and the doped metal element M source.

[0055] Preferably, in step (3), the mass percentage of the solid electrolyte is 0%-10% based on the total mass of the second mixture, the lithium source, and the doped metal element M source.

[0056] Preferably, in step (3), the average particle size of the solid electrolyte is 0.1-0.3 μm.

[0057] Preferably, in step (3), the solvent content in the third mixture is 40%-80% by mass, based on the mass of the third mixture.

[0058] Preferably, in step (3), the solid content of the third mixture is 10wt%-90wt%, d1 is 0.1-0.4μm, and d2 is 0.3-1μm.

[0059] The term "slurry" as used in this article refers to a mixture containing solid particles and liquid, wherein the average particle size is obtained by testing with a laser particle size analyzer.

[0060] Preferably, in step (4), the mass ratio of slurry 1 to slurry 2 is (2-9):(1-8).

[0061] Preferably, in step (4), the inlet air temperature of the spray dryer is 150-220℃, and the outlet air temperature of the spray dryer is 65-110℃.

[0062] Preferably, in step (4), the average particle size of the fourth mixture is 2-9 μm. The inventors have verified through experiments that it is feasible to control the D50 of the fourth mixture within 10 μm, and it is further preferred to be 2-9 μm. At this point, the particle size of the fourth mixture is better controlled, and the electrical properties of the lithium manganese iron phosphate composite material are better obtained.

[0063] Preferably, in step (4), the ball milling time is 10-30 min.

[0064] Preferably, in step (5), T3 is 250-400℃, the rate of heating to T3 is 0.5-15℃ / min, and the time for the first sintering is 0.5-6h.

[0065] Preferably, in step (5), the temperature of T4 is 580-700℃, the rate of heating to T4 is 0.5-15℃ / min, and the time of the second sintering is 4-26h.

[0066] The first sintering is carried out at a low temperature, causing the low-temperature pore-forming agent to decompose; the second sintering is carried out at a relatively high temperature, causing the high-temperature pore-forming agent to decompose, thus achieving staged pore formation. The inventors of this invention have discovered that the sintering temperature and time affect the compaction density and specific capacity of the final LMFP particles. Within the aforementioned sintering temperature and time range, LMFP exhibits superior compaction density and specific capacity.

[0067] More preferably, the first and second sintering are performed consecutively and in three stages, specifically including the following process: heating from room temperature to T3 at a heating rate of 0.5-15℃ / min and holding at that temperature for (first) sintering; then heating from T3 to T5 at a heating rate of 0.5-1.5℃ / min; and finally heating to T4 at a heating rate of 1.6-3℃ / min and holding at that temperature for (second) sintering; wherein T3 < T5 < T4. The first sintering is performed at a low temperature, allowing the low-temperature pore-forming agent to decompose. When heating from the first sintering temperature to the second sintering temperature, the heating rate gradually increases, allowing the low-temperature pore-forming agent sufficient time and a sufficiently high temperature to completely decompose, while also minimizing the heating time and improving sintering efficiency.

[0068] More preferably, the T5 temperature is 400-500℃.

[0069] Secondly, the present invention provides a method for preparing the aforementioned lithium manganese iron phosphate composite material, which yields a lithium manganese iron phosphate composite material.

[0070] The lithium manganese iron phosphate composite material of the present invention has a high compaction density and specific capacity, wherein the compaction density reaches 2.20-2.40 g / cm³. 3 Its capacity is as high as 140-160mAh / g.

[0071] Preferably, the D50 of the lithium manganese iron phosphate composite material is 2-10 μm, and more preferably 5-8.5 μm.

[0072] Thirdly, the present invention provides a secondary battery, comprising a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode comprises the aforementioned lithium manganese iron phosphate composite material.

[0073] In some embodiments, the electrolyte includes an organic solvent, a lithium salt, and additives. The present invention does not impose specific limitations on the types of organic solvents and lithium salts, which can be selected according to actual needs.

[0074] In some embodiments, the organic solvent may be at least one of ethylene carbonate (EC), fluoroethylene carbonate (FEC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propylene carbonate (PC), methyl ethyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butenyl carbonate (BC), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), dimethyl sulfone (MSM), or diethyl sulfone (ESE).

[0075] In some embodiments, the lithium salt may be at least one of lithium hexafluorophosphate (LiPF6), lithium bisfluorosulfonylimide (LiFSI), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium bis(trifluoromethanesulfonylimide) (LiTFSI), lithium difluorophosphate (LiPO2F2), lithium difluorodioxophosphate (LiDFOP), lithium tetrafluorooxophosphate (LiTFOP), lithium trifluoromethanesulfonate (LiTFS), lithium difluorooxophosphate borate (LiDFOB), or lithium dioxophosphate borate (LiBOB).

[0076] In some embodiments, the positive electrode sheet includes a positive electrode active material.

[0077] In some embodiments, the positive electrode active material layer includes a positive electrode active material, and the positive electrode active material includes the lithium manganese iron phosphate composite material.

[0078] In some embodiments, the mass percentage of the positive electrode active material is 50%-99% based on the total mass of the positive electrode sheet.

[0079] In some embodiments, the positive electrode sheet further includes a positive electrode binder and a positive electrode conductive agent.

[0080] In some embodiments, the positive electrode adhesive includes at least one of styrene-butadiene rubber (SBR), waterborne acrylic resin, carboxymethyl cellulose (CMC), polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyvinyl butyral (PVB), ethylene-vinyl acetate copolymer (EVA), or polyvinyl alcohol (PVA), and is not limited to the above types. The adhesive can be selected according to actual needs.

[0081] In some embodiments, the mass percentage of the positive electrode adhesive is less than or equal to 5% based on the total mass of the positive electrode sheet.

[0082] In some embodiments, the positive electrode conductive agent includes at least one of graphite, acetylene black, carbon black, Ketjen black, carbon nanotubes, graphene, or carbon nanofibers, but is not limited to the above-mentioned types. The conductive agent can be selected according to actual needs.

[0083] In some embodiments, the mass percentage of the positive electrode conductive agent is 1% to 3% based on the total mass of the positive electrode sheet.

[0084] The positive electrode sheet can be prepared by conventional methods in the art. For example, the preparation method of the positive electrode sheet includes the following steps: mixing a solvent, a conductive agent, a binder and a positive electrode active material to obtain a positive electrode slurry; coating the positive electrode slurry onto the surface of the current collector side, and then drying and cold pressing to obtain the positive electrode sheet.

[0085] In some embodiments, the solvent may include N-methylpyrrolidone (NMP), but is not limited thereto.

[0086] In some embodiments, the negative electrode sheet includes a negative electrode active material.

[0087] In some embodiments, the negative electrode active material includes at least one of graphite, hard carbon, soft carbon, silicon, silicon-carbon materials, silicon-oxygen materials, or metallic lithium, but is not limited thereto.

[0088] In some embodiments, the mass percentage of the negative electrode active material is 50%-99% based on the total mass of the negative electrode sheet.

[0089] In some embodiments, the negative electrode sheet also includes a negative electrode adhesive and a negative electrode conductive agent.

[0090] In some embodiments, the negative electrode conductive agent includes at least one of graphite, acetylene black, carbon black (e.g., Super-P, i.e. SP), Ketjen black, carbon nanotubes, graphene, or carbon nanofibers, but is not limited to the above-mentioned types. The conductive agent can be selected according to actual needs.

[0091] In some embodiments, the negative electrode adhesive includes at least one of styrene-butadiene rubber (SBR), waterborne acrylic resin, carboxymethyl cellulose (CMC), polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyvinyl butyral (PVB), ethylene-vinyl acetate copolymer (EVA), or polyvinyl alcohol (PVA), and is not limited to the above types. The adhesive can be selected according to actual needs.

[0092] In some embodiments, the mass percentage of the negative electrode adhesive is less than or equal to 5% based on the total mass of the negative electrode sheet.

[0093] In some embodiments, the mass percentage of the negative electrode conductive agent is 1% to 3% based on the total mass of the negative electrode sheet.

[0094] The negative electrode sheet can be prepared by conventional methods in the art. For example, the preparation method of the negative electrode sheet includes the following steps: mixing a solvent, a conductive agent, a binder and a negative electrode active material to obtain a negative electrode slurry; coating the negative electrode slurry onto the surface of the current collector, and then drying and cold pressing to obtain the negative electrode sheet.

[0095] The secondary battery of the present invention can be prepared according to conventional methods in the art; such preparation methods may specifically include the following steps: a structure containing a positive electrode, a separator and a negative electrode stacked together is wound to obtain an electrode assembly; the electrode assembly is placed in a packaging shell, an electrolyte is injected and the shell is sealed to obtain a secondary battery.

[0096] Compared with existing technologies, this invention has the following advantages: This invention first prepares manganese iron phosphate containing a special pore-forming agent, then uses it as a precursor to mix with raw materials such as a lithium source, a doped metal element M source, and a carbon source; the mixture is then ground into large and small particles of different sizes, and the large and small particles are further ground, mixed, and spray-dried to obtain an unsintered precursor mixture. Finally, the unsintered precursor mixture undergoes two heat treatments for sintering to obtain a lithium manganese iron phosphate composite material. The method of this invention allows the lithium manganese iron phosphate composite material to possess the advantages of high compaction density, high rate capability, low cost, and high specific capacity, thus broadening the application prospects of the material. Attached Figure Description

[0097] Figure 1 The constant current charging ratio curve of the LMFP prepared in Example 1 after being fabricated into a pouch cell is shown.

[0098] Figure 2 The discharge capacity ratio curve of the LMFP prepared in Example 1 after being fabricated into a soft-pack battery. Detailed Implementation

[0099] To better illustrate the purpose, technical solution, and advantages of this invention, specific embodiments will be used to further explain the invention below. Unless otherwise specified, the test methods used in the embodiments and / or comparative examples are conventional methods; the materials and reagents used, unless otherwise specified, are commercially available.

[0100] The solid electrolyte powders mentioned below have a particle size of D50 = 0.2 μm.

[0101] Example 1 A method for preparing a lithium manganese iron phosphate composite material includes the following steps: S1. Dissolve nylon 6 in an ethanol / water (volume ratio 1:1) solution, with a nylon 6 concentration of 10 wt%; then add graphene oxide and Li. 1.3 Al 0.3 Ti 1.7 (PO4)3 nanoparticles, nylon 6, graphene oxide and LATP in a mass ratio of 0.5:0.4:0.1, were then ball-milled for 2 hours, spray-dried and heat-treated at 100°C for 1 hour, and then crushed to D50=50nm to obtain a special pore-forming agent; wherein the inlet air temperature of the spray drying was 180°C and the outlet air temperature was 80°C.

[0102] S2. Weigh the raw materials according to the molar ratio of Fe, Mn, and P in ferrous sulfate, manganese sulfate, ammonium dihydrogen phosphate, and solid phosphoric acid: Fe:Mn:P:P = 0.4:0.62:1.08:0.1. Based on the total mass of ferrous sulfate, manganese sulfate, solid phosphoric acid, and ammonium dihydrogen phosphate, weigh 3 wt% of the special pore-forming agent and a certain amount of ammonia water.

[0103] Ferrous sulfate and manganese sulfate were dissolved in pure water with a Fe content of 4 wt%, and all phosphoric acid was added before dissolution to control the pH of the solution to 1.4. Ammonium dihydrogen phosphate was dissolved in pure water with a phosphorus content of 8 wt%, and after dissolution, all of it was added to the above solution to obtain the first mixture, the pH of which was 2.1. All of the above processes were carried out under nitrogen protection.

[0104] S3. Add ammonia water dropwise to the first mixture to adjust the pH of the mixed solution to 7.0±0.1, and carry out a precipitation reaction. The addition should be completed within 180 minutes, and a special pore-forming agent should be added at the 15th minute of the addition process. After the addition is completed, continue the precipitation for 1 hour. All the above processes are carried out under nitrogen protection. Then, the material is transferred to a filter press for washing with pure water and filtration. The filter cake is flash-dried at 180°C to obtain manganese iron phosphate containing a special pore-forming agent and multiple water molecules. For convenience, this is called the second mixture.

[0105] S4. Weigh out the raw material lithium carbonate, the above-mentioned second mixture, and the dopant source magnesium oxide according to the molar ratio of Li, Fe, and dopant metal element M (Mg) of 1.05:0.38:0.02; and based on the total mass of lithium carbonate, the second mixture, and the dopant source magnesium oxide, weigh out 1.8 wt% of glucose as the carbon source, and weigh out 4 wt% of the solid electrolyte powder LATP (Li 1.1 Al 0.1 Ti 1.9 (PO4)3), and deionized water with a solid content of 35% was prepared from the above raw materials. The above raw materials were then mixed with water to obtain a third mixture.

[0106] Two portions of the third mixture were taken and ball-milled separately in two sand mills to obtain slurry 1 and slurry 2, respectively. The particle size of slurry 1 was D50=0.2μm and the particle size of slurry 2 was D50=0.5μm. Slurry 1 and slurry 2 were poured into a ball mill at a mass ratio of 7:3 and ball-milled for 15 minutes. The mixed slurry was then spray-dried to obtain a powdered fourth mixture. The particle size D50 of the fourth mixture was controlled to be 6μm. The inlet air temperature of the spray drying was set to 220℃ and the outlet air temperature was set to 100℃. S5. The fourth mixture is then sintered twice under a nitrogen atmosphere. The temperature is increased from room temperature to the first sintering plateau of 300°C at a rate of 2°C / min, and sintered at this temperature for 1 hour. Subsequently, the temperature is increased to 450°C at a rate of 1°C / min, and then to the second sintering plateau of 660°C at a rate of 2°C / min, and sintered at this temperature for 8 hours. After cooling, the material is not broken, and the particle size D is obtained. 50 =8μm lithium manganese iron phosphate composite material.

[0107] Example 2 The difference between this embodiment and Embodiment 1 is that in step S5, the temperature of the second sintering platform is 700℃ and the sintering time is 6h, while the rest are the same.

[0108] Example 3 The difference between this embodiment and Embodiment 1 is that in step S1, nylon 6 is replaced with an equal amount of polyvinyl alcohol (PVA), while the rest are the same.

[0109] Example 4 The difference between this embodiment and Embodiment 1 is that in step S2, while the volume of ammonia water added is the same, the ammonia water addition time is controlled to be 60 minutes, and the rest are the same.

[0110] Example 5 The difference between this embodiment and Embodiment 1 is that, in step S4, solid electrolyte powder LATP (Li) is not added. 1.1 Al 0.1 Ti 1.9 (PO4)3), the rest are the same.

[0111] Example 6 The difference between this embodiment and embodiment 1 is that in step S2, 8 wt% of the special pore-forming agent is weighed out, while the rest are the same.

[0112] Example 7 The difference between this embodiment and Embodiment 1 is that in step S1, the mass ratio of nylon 6, graphene oxide and LATP is changed to 0.1:0.1:0.1, while the rest are the same.

[0113] Example 8 The difference between this embodiment and Embodiment 1 is that in step S1, the mass ratio of nylon 6, graphene oxide and LATP is changed to 0.9:0.9:0.1, while the rest are the same.

[0114] Example 9 The difference between this embodiment and Embodiment 1 is that in step S5, the temperature of the second sintering platform is 580℃ and the sintering time is 6h, while the rest are the same.

[0115] Example 10 The difference between this embodiment and Embodiment 1 is that in step S5, the temperature of the first sintering platform is 400℃ and the sintering time is 1 hour, while the rest are the same.

[0116] Example 11 The difference between this embodiment and Embodiment 1 is that in step S1, graphene oxide is replaced with an equal amount of polystyrene, while the rest remains the same.

[0117] Example 12 The difference between this embodiment and Embodiment 1 is that in step S3, ammonia water is added dropwise to the first mixture to adjust the pH of the mixed solution to 8.0±0.1, while the rest are the same.

[0118] Example 13 The difference between this embodiment and Embodiment 1 is that in step S3, ammonia water is added dropwise to the first mixture to adjust the pH of the mixed solution to 5.5±0.1, while the rest are the same.

[0119] Comparative Example 1 The difference between this comparative example and Example 1 is that in step S1, only solid electrolyte and solvent are used, and nylon and graphene oxide are not added, while the rest are the same.

[0120] Comparative Example 2 The difference between this comparative example and Example 1 is that, based on step S3, a sintering process is added to the second mixture, with a sintering temperature of 550°C and a sintering time of 2 hours, while the rest are the same.

[0121] Comparative Example 3 The difference between this comparative example and Example 1 is that in step S3, the third mixture is directly milled to D50=0.3μm and then spray-dried to obtain the fourth mixture, which is not divided into two parts and ground into slurries with different particle sizes. The rest is the same.

[0122] Comparative Example 4 The difference between this comparative example and Example 1 is that in step S4, the spray particle size is not controlled, the D50 of the fourth mixture powder after spray drying is 18μm, and the actual finished product lithium manganese iron phosphate D50 is 23μm, the rest are the same.

[0123] Comparative Example 5 The difference between this comparative example and Example 1 is that in step S3, ammonia water is added dropwise to the first mixture to adjust the pH of the mixed solution to 8.8±0.1, while the rest are the same.

[0124] Comparative Example 6 The difference between this comparative example and Example 1 is that nylon 6 is not added in step S1, but the rest are the same.

[0125] Comparative Example 7 The difference between this comparative example and Example 1 is that graphene oxide is not added in step S1, while the rest are the same.

[0126] Comparative Example 8 The difference between this comparative example and Example 1 is that in step S5, a sintering process is performed. Specifically, the temperature is increased from room temperature to the sintering platform of 660°C at a heating rate of 2°C / min, and sintered at this sintering temperature for 9 hours. The rest are the same.

[0127] (1) Compacted density: When the powder material to be tested is compressed under the action of external force, the voids between the powders are filled during the compression process, the contact area between particles increases, the attraction between atoms is generated and the mechanical fit between particles is enhanced, thus forming a compact with a certain density and strength. By measuring the thickness of the compact and calculating its volume in combination with the mold diameter, the compacted density of the powder material under a certain pressure can be determined. For lithium manganese iron phosphate compacted density test parameters, it is recommended that the test pressure should not be less than 80MPa, the test mold diameter should be selected in the range of 10-20mm, and the sampling amount should be appropriately adjusted in combination with the mold diameter. After the sample test is completed, the pressure, resistance, thickness, resistivity and powder conductivity data are obtained. Among them, the compacted density and powder conductivity are taken as values ​​under a test pressure of 200MPa, and the corresponding compacted density is calculated in combination with the following formula: ; In the formula, Compacted density (g / cm³) 3 m is the sample mass (g); S is the area of ​​the test electrode (cm²). 2 L represents the thickness of the test sample (cm) after being compressed.

[0128] (2) The specific capacity and rate performance of the lithium manganese iron phosphate composite materials of each embodiment and comparative example were tested, as follows: Fabrication of button-type lithium-ion batteries: A1. Place the lithium manganese iron phosphate composite material and conductive agent (acetylene black) of each embodiment or comparative example into an oven and bake at 120°C for 4 hours, then transfer to a desiccator to cool; then, weigh the corresponding proportion of lithium manganese iron phosphate composite material and conductive agent (acetylene black) in a mass ratio of 80:10:10 and add them to a 50mL beaker, then add the binder solution (N-methylpyrrolidone solution of polyvinylidene fluoride with a mass fraction of 5%), and stir with a stirrer to form a paste; A2. Apply the paste evenly to aluminum foil to obtain a single-sided surface density of 60 g / m². 2 The positive electrode sheet was then placed in a forced-air drying oven and dried at 120℃ for 2 hours, followed by compression (the compression density of the positive electrode sheet was 2.0 g / cm³). 3 Cut into circular positive electrode sheets with a diameter of 12mm; A3. In an argon atmosphere glove box, a button-type lithium-ion battery is assembled using a circular positive electrode as the positive electrode, a lithium sheet as the negative electrode, a nickel mesh as the current collector, and a mixed solution of 1 mol / L LiPF6 ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (EC to EMC volume ratio of 3:7) as the electrolyte.

[0129] Fabrication of stacked soft-pack lithium-ion batteries: A1. Positive Electrode Homogenization: The lithium manganese iron phosphate composite material and conductive agent (acetylene black) of each embodiment or comparative example were placed in an oven and baked at 120°C for 14 hours, then transferred to a desiccator for cooling; then, the corresponding proportions of lithium manganese iron phosphate composite material, binder PVDF (polyvinylidene fluoride), conductive agent 1 (acetylene black), and conductive agent 2 (CNT) were weighed according to a mass ratio of 94:3:2.5:0.5. Acetylene black), weigh NMP (5% N-methylpyrrolidone solution of polyvinylidene fluoride by mass) according to 48% solid content. First, pour the lithium manganese iron phosphate composite material, PVDF (polyvinylidene fluoride) binder, and conductive agent 1 (acetylene black) into a 5L planetary homogenizer and dry mix for 25 minutes. Then add NMP and stir for 2.5 hours. Next, add CNT and stir for 15 minutes, controlling the viscosity to 7000~10000. If the viscosity is not up to standard, add a small amount of NMP to adjust the viscosity. Finally, defoam under vacuum and sieve the material. A2. Negative Electrode Homogenization: Weigh the corresponding raw materials according to the ratio of graphite:CMC:SP:SBR=95.5:1.4:1.5:1.6, and weigh deionized water according to 50% solid content. First, add 85% deionized water and CMC to a 5L planetary homogenizer, stir for 2 hours, and then remove the slurry for later use. Add graphite and SP to another 5L planetary homogenizer, dry mix for 10 minutes, then add half of the slurry and stir for 30 minutes. Then add the remaining slurry and stir for 60 minutes. Finally, add the remaining deionized water and SBR and stir for 30 minutes. Control the viscosity to 3000~5000. If the viscosity is not up to standard, add a small amount of deionized water to adjust the viscosity. Finally, defoam under vacuum and sieve the material. A3. Positive electrode coating to die-cutting: Qualified positive electrode slurry material is evenly coated onto aluminum foil using a coating machine to obtain a single-sided surface density of 144 g / m². 2 The positive electrode sheet is dried and then rolled (the compressed density of the positive electrode sheet is 2.2 g / cm³). 3 (), and die-cut into rectangular positive electrode sheets of 157x117mm; A4. Negative electrode coating to die-cutting: Qualified negative electrode slurry material is evenly coated onto copper foil using a coating machine to obtain a single-sided surface density of 62 g / m². 2 The negative electrode sheet is dried and then rolled (the density of the negative electrode sheet is 1.45 g / cm³). 3 (), die-cut into rectangular negative electrode sheets of 159x119mm; A5. Through processes such as stacking, baking, liquid injection, formation, and capacity testing, stacked soft-pack lithium-ion batteries are assembled.

[0130] Capacity test: Under conditions of 25℃±2℃, the assembled button lithium-ion batteries were placed on a battery testing system (Blue Electric series battery testing system) for charge-discharge cycle testing. The test conditions were as follows: charge-discharge rate of 0.1C, voltage range of 2.5V-4.5V, and the 0.1C discharge specific capacity of each battery was recorded. The number of button lithium-ion batteries in each example or comparative example was 5, that is, the experiment was repeated 5 times. The 0.1C discharge specific capacity data was averaged, and the specific capacity of the lithium manganese iron phosphate composite material was calculated according to the following formula: The specific capacity (mAh / g) of the lithium manganese iron phosphate composite material = the average value of the 0.1C discharge specific capacity (in mAh) / the mass of the lithium manganese iron phosphate composite material (in g).

[0131] Ratio performance test: Under conditions of 25℃±2℃, the assembled stacked soft-pack lithium-ion batteries were placed on a battery testing system (Xinwei series battery testing system) for multiple charge and discharge tests. The test conditions were as follows: multiple rate charging (constant current constant voltage charging, discharge at 0.33C) were 0.33C, 1C, 3C, and 4C; multiple rate discharging (charging at 0.33C) were 0.33C, 1C, 5C, and 8C, with a voltage range of 2.5V-4.3V. The charge and discharge capacity of each battery was recorded. Fresh batteries were used for each example or comparative example of the stacked soft-pack lithium-ion battery at different rates. The constant current charging ratio and discharge capacity ratio of the lithium manganese iron phosphate composite material were calculated according to the following formula: The constant current charging ratio (%) of lithium manganese iron phosphate composite material = capacity during constant current charging (in Ah) / total capacity charged by constant current and constant voltage charging (in Ah). The discharge capacity ratio (%) of lithium manganese iron phosphate composite material = discharge capacity at different rates (in Ah) / discharge capacity at 0.33C discharge (in Ah).

[0132] Example 1: Test results of constant current charge-discharge ratio of lithium manganese iron phosphate composite material at different rates (0.33C-4C) are as follows: Figure 1 and Figure 2 As shown in the figure, the specific test results for compaction density, specific capacity, and partial charge-discharge capacity ratio are shown in Table 1.

[0133] Table 1 As shown in the examples, the lithium manganese iron phosphate composite material prepared by the present invention has high compaction density and specific capacity, wherein the compaction density reaches 2.2-2.4 g / cm³. 3 It has a capacity of 147-161mAh / g, a 4C constant current charging ratio of 80%-89%, and an 8C discharge capacity ratio of 93.2%-99.1%.

[0134] As can be seen from Example 1 and Comparative Examples 1 and 6-7, the special pore-forming agent must include both low-temperature and high-temperature pore-forming agents in order to effectively improve the particle porosity of MFP, thereby improving the compaction density, specific capacity and rate performance of LMFP.

[0135] As can be seen from Example 1 and Comparative Example 2, in the preparation method of the present invention, it is not necessary to pre-sinter the MFP containing the pore-forming agent and the complex water molecules to form pores. Instead, it can be directly used as a precursor for preparing LMFP. The compaction density, specific capacity and rate performance of LMFP can be improved by utilizing the sintering process for preparing LMFP, which can reduce production costs.

[0136] As shown in Example 1 and Comparative Examples 3-4, the present invention uses complex water-molecule manganese iron phosphate with a special pore-forming agent as a precursor for LMFP, and mixes it with raw materials such as lithium source, doped metal element, and carbon source. The mixture is ground in two parts: one part is ground into small particle mixture, and the other part is ground into large particle mixture. The two parts are then ground, mixed, and spray-dried to obtain an unsintered precursor mixture. The particle size distribution is adjusted by mixing large and small particles, which improves the compaction of the finished product and offsets the influence of the pore-forming agent on compaction during the sintering stage. This improves the compaction performance of sintered lithium manganese iron phosphate. The particle size change of the unsintered precursor mixture before and after sintering is small. By controlling the particle size of the unsintered precursor mixture powder to ≤10μm, the particle size of the finished product can be indirectly controlled, thereby improving the compaction density, specific capacity, and rate performance of LMFP. If the particle size is not controlled as in Comparative Example 4, the particle size of LMFP is too large. When preparing the negative electrode sheet, the LMFP particles are easy to fall off during the coating of the negative electrode slurry, which makes it impossible to form a uniform negative electrode sheet.

[0137] As can be seen from Examples 1, 12-13 and Comparative Example 5, when the pH value of the precipitation reaction is increased to 8.8, the local pH value is too high during the process of adding alkaline solution, and the particles agglomerate, resulting in subsequent gradation imbalance and a significant decrease in LMFP compaction density. However, due to the decrease in LMFP compaction density, the battery capacity increases slightly.

[0138] As can be seen from Example 1 and Comparative Example 8, Comparative Example 8 only underwent one sintering process. The rapid temperature rise caused the low-temperature pore-forming agent to decompose more violently, which did not effectively coat the pores and also had an adverse effect on the compaction density of LMFP.

[0139] As can be seen from Examples 1-2 and 9, changes in the temperature and time of the second sintering have a slight impact on the compaction density, specific capacity and rate performance of LMFP. The preferred temperature for the second sintering is 580-700℃.

[0140] As can be seen from Examples 1 and 10, the temperature and time of the first sintering have a slight effect on the compaction density, specific capacity and rate performance of LMFP. The preferred temperature for the first sintering is 250-400℃.

[0141] As can be seen from Examples 1 and 3, various types of low-temperature pore-forming agents can be used to achieve the purpose of improving the compaction density, specific capacity and rate performance of LMFP.

[0142] As can be seen from Examples 1 and 11, various types of high-temperature pore-forming agents can be used to achieve the purpose of improving the compaction density, specific capacity and rate performance of LMFP.

[0143] As can be seen from Examples 1 and 4, the rate of adding alkaline solution affects the particle properties of MFP, and thus affects the compaction density, specific capacity and rate performance of LMFP.

[0144] As can be seen from Examples 1 and 5, adding a solid electrolyte during the preparation of LMFP will further improve the compaction density, specific capacity and rate performance of LMFP.

[0145] As can be seen from Examples 1 and 6, the amount of special pore-forming agent has a certain impact on the sintering process. The preferred content of pore-forming agent is 3wt%-8wt%, at which point the compaction density, specific capacity and rate performance of LMFP are better.

[0146] As can be seen from Examples 1 and 7-8, the addition ratio of the two pore-forming agents has a slight effect on the particle properties and in-situ coating effect of LMFP, which in turn affects the compaction density, specific capacity and rate performance of LMFP.

[0147] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A method for preparing a lithium manganese iron phosphate composite material, characterized in that, Includes the following steps: (1) Dissolve the first pore-forming agent in a solvent, add the second pore-forming agent and solid electrolyte, and then successively pass through ball milling, spray drying, heat treatment and crushing to obtain the third pore-forming agent; The decomposition temperature of the first pore-forming agent is T1, and the decomposition temperature of the second pore-forming agent is T2. (2) Under inert gas protection, a solution containing ferrous and manganese sources and a solution containing phosphorus sources are mixed to obtain a first mixture; under inert gas protection, an alkaline solution is added dropwise to the first mixture to carry out a precipitation reaction and a third pore-forming agent is added, followed by washing, filtration and drying to obtain a second mixture; the pH value of the precipitation reaction is 5.5-8; (3) The second mixture, lithium source, doped metal element M source, carbon source and solvent are mixed to obtain a third mixture; Take one portion of the third mixture and grind it into a slurry 1 with an average particle size of d1; Take another portion of the third mixture and grind it into a slurry 2 with an average particle size of d2; the d1 and the d2 satisfy the following relationship: d1 < d2; (4) The slurry 1 and the slurry 2 are sequentially ball-milled and spray-dried to obtain a fourth mixture; the average particle size of the fourth mixture is ≤10μm; (5) The fourth mixture is subjected to a first sintering, a second sintering and cooling in sequence to obtain the lithium manganese iron phosphate composite material; The sintering temperature for the first sintering is T3, and the sintering temperature for the second sintering is T4, satisfying the condition: T1 < T3 < T2 < T4.

2. The preparation method of the lithium manganese iron phosphate composite material as described in claim 1, characterized in that, Includes at least one of the following A and B: A. In step (1), the first pore-forming agent includes at least one of nylon, ammonium carbonate, ammonium bicarbonate, ammonium oxalate, polymethyl methacrylate, methyl methacrylate, methyl cellulose, polyvinyl alcohol, carboxymethyl cellulose, ascorbic acid, polyfurfuryl alcohol or polymethyl methacrylate; B. In step (1), the second pore-forming agent includes at least one of polystyrene, polyvinyl butyral, modified graphene, or modified wood particles; the modified graphite includes graphene oxide.

3. The preparation method of the lithium manganese iron phosphate composite material as described in claim 1, characterized in that, In step (2), based on the total mass of the ferrous source, manganese source and phosphorus source in the solution containing the ferrous source and manganese source and the solution containing the phosphorus source, the mass percentage of the third pore-forming agent is 1%-30%.

4. The preparation method of the lithium manganese iron phosphate composite material as described in claim 1, characterized in that, Includes at least one of the following CDs: C. In step (5), T3 is 250-400℃, the rate of heating to T3 is 0.5-15℃ / min, and the time for the first sintering is 0.5-6h; D. In step (5), T4 is 580-700℃, the rate of heating to T4 is 0.5-15℃ / min, and the time for the second sintering is 4-26h.

5. The preparation method of the lithium manganese iron phosphate composite material as described in claim 1, characterized in that, Includes at least one of the following E to K: E. In step (1), the solid electrolyte includes at least one of lithium aluminum titanium phosphate, lithium aluminum germanium phosphate, lithium lanthanum titanate, or lithium lanthanum zirconium oxide. F. In step (1), the solvent includes at least one of ethanol or water; G. In step (1), the mass ratio of the first pore-forming agent, the second pore-forming agent, and the solid electrolyte is (1-9):(1-9):1; H. In step (1), the ball milling time is 1-3 hours; I. In step (1), the inlet air temperature of the spray drying is 150-220℃, and the outlet air temperature of the spray drying is 65-110℃. J. In step (1), the temperature of the heat treatment is 95-110℃ and the time of the heat treatment is 0.5-3h; K. In step (1), the average particle size of the third pore-forming agent is 10-100 nm.

6. The method for preparing the lithium manganese iron phosphate composite material as described in claim 1, characterized in that, Includes at least one of the following LVs: L. In step (2), the solution containing ferrous and manganese sources includes at least one of ferrous chloride, ferrous sulfate, or ferrous nitrate. M. In step (2), the solution containing ferrous and manganese sources includes at least one of manganese chloride, manganese sulfate, or manganese nitrate. N. In step (2), the mass percentages of Fe and Mn in the solution containing ferrous and manganese sources are 3%-20% and 3%-20%, respectively. O. In step (2), the phosphorus source in the solution contains at least one of ammonium phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, or phosphoric acid. P. In step (2), the pH value of the phosphorus source solution is 6-8, and the mass percentage of P in the phosphorus source solution is 3%-20%; Q. In step (2), the molar ratio of Fe in the ferrous source, Mn in the manganese source, and P in the phosphorus source in the solution containing the ferrous source and the solution containing the phosphorus source is (1-9):(1-9):(10-13). R. In step (2), the alkali in the alkaline solution includes at least one of ammonia, sodium hydroxide, or sodium carbonate; S. In step (2), the precipitation reaction takes 0.5-8 hours; T. In step (2), the dripping time is <6h; U. In step (2), the drying temperature is 100-220℃; V. In step (2), the average particle size of the second mixture is less than 200 μm.

7. The method for preparing the lithium manganese iron phosphate composite material as described in claim 1, characterized in that, Includes at least one of the following: I to X I. In step (3), the lithium source includes at least one of lithium hydroxide, lithium carbonate, lithium dihydrogen phosphate, lithium oxalate, lithium acetate, or lithium phosphate; II. In step (3), the metal element M in the doped metal element M source includes at least one of Ti, Mg, Al, V, Nb or Zn; the doped metal element M source includes at least one of the oxide of metal element M or its salt. Ⅲ. In step (3), the carbon source includes at least one of starch, sucrose, glucose, citric acid, polyethylene glycol, polyvinylpyrrolidone, polypropylene, conductive carbon black, acetylene black, carbon nanotubes or graphene. IV. In step (3), the solvent includes water; V. In step (3), the third mixture further includes a solid electrolyte, which includes at least one of lithium aluminum titanium phosphate, lithium aluminum germanium phosphate, lithium lanthanum titanate, or lithium lanthanum zirconium oxide. VI. In step (3), the second mixture contains Fe, Mn and P elements, and the molar ratio of Li element in the lithium source, Fe element in the second mixture and metal element M in the doped metal element M source is (1-1.06):(0.25-0.45):(0.001-0.03). VII. In step (3), based on the total mass of the second mixture, the lithium source, and the doped metal element M source, the mass percentage of the carbon source is 1%-8%; VIII. In step (3), based on the total mass of the second mixture, the lithium source, and the doped metal element M source, the mass percentage of the solid electrolyte is 0%-10%; IX. In step (3), based on the mass of the third mixture, the mass percentage of the solvent in the third mixture is 40%-80%; X. In step (3), the solid content of the third mixture is 10wt%-90wt%, d1 is 0.1-0.4μm, and d2 is 0.3-1μm.

8. The method for preparing the lithium manganese iron phosphate composite material as described in claim 1, characterized in that, Includes at least one of the following a to d: a. In step (4), the mass ratio of slurry 1 to slurry 2 is (2-9):(1-8); b. In step (4), the inlet air temperature of the spray drying is 150-220℃, and the outlet air temperature of the spray drying is 65-110℃. c. In step (4), the average particle size of the fourth mixture is 2-9 μm; d. In step (4), the ball milling time is 10-30 min.

9. A lithium manganese iron phosphate composite material prepared by the preparation method of any one of claims 1-8.

10. A secondary battery, comprising a positive electrode, a negative electrode, and an electrolyte, characterized in that, The positive electrode comprises the lithium manganese iron phosphate composite material as described in claim 9.

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