Lithium manganese iron phosphate composite material as well as preparation method and application thereof

Abstract

The invention belongs to the field of lithium batteries, and particularly discloses a lithium manganese iron phosphate composite material as well as a preparation method and application thereof. According to the preparation method, the porous iron phosphate is prepared firstly, and then the porous iron phosphate, the lithium source, the manganese source, the phosphorus source, the doped metal element source, the carbon source and the solid electrolyte are subjected to one-time sintering to prepare the lithium iron manganese phosphate composite material, so that the lithium iron manganese phosphate composite material has high compaction density, lithium ion transmission rate and gram volume, and the application prospect of the material is widened.

Description

Technical Field

[0001] The present invention belongs to the field of lithium batteries, and in particular relates to a lithium manganese iron phosphate composite material and a preparation method and application thereof. Background Art

[0002] Lithium manganese iron phosphate (LMFP) boasts excellent thermodynamic and kinetic stability and safety performance, and is widely used in commercial batteries. LMFP is primarily produced through liquid-phase and solid-phase methods. Liquid-phase methods can be further categorized into hydrothermal, solvothermal, sol-gel, and coprecipitation methods. The hydrothermal method is the most widely used, offering low energy consumption and cost, and allows for controlled particle size. However, due to its high-temperature and high-pressure production environment, it requires high equipment and operational control, making it difficult to scale up. Furthermore, the consistency between different batches of product is poor, making industrialization challenging. While solid-phase synthesis offers low industrialization challenges, the preparation of LMFP by this method is subject to issues such as uneven reaction, large particle size, difficult morphology control, and poor batch consistency. Consequently, existing solid-phase processes for preparing LMFP still suffer from low compacted density, low electronic conductivity, slow lithium ion diffusion rate, and low specific capacity utilization. This inability to effectively improve the compacted density and specific capacity utilization of LMFP limits its application scenarios.

[0003] Ferric phosphate (FP) is one of the raw materials used in the preparation of LMFP. Its cost, however, influences the production of LMFP to a certain extent. FP is widely available and relatively stable in price. It is chemically stable and resistant to deterioration during storage and transportation. A variety of synthesis methods are available, allowing for the selection of an appropriate process route based on practical needs. Furthermore, as a raw material for LMFP, FP requires high purity. Impurities in FP can affect LMFP performance. The particle size and micromorphology of ferric phosphate significantly impact LMFP performance. Currently, the micromorphology of FP is often molten. When using FP as a raw material for the solid-phase preparation of lithium manganese iron phosphate (LMFP), FP and other raw materials are typically mixed by grinding or sand milling. However, molten FP is difficult to grind and disperse into primary particles, hindering the formation of a suitable gradation in subsequent steps. This makes it difficult to control the FP particle size and improve the electrical properties of LMFP. Summary of the Invention

[0004] In response to the problems of low compaction density and gram capacity of lithium manganese iron phosphate prepared by the process involved in the above-mentioned prior art, the present invention will provide a lithium manganese iron phosphate composite material and its preparation method and application.

[0005] To achieve the above objectives, the following technical solutions are specifically included:

[0006] In a first aspect, the present invention provides a method for preparing a lithium manganese iron phosphate composite material, comprising the following steps:

[0007] (1) adding a solution containing a ferrous source, a solution containing a phosphorus source, and an oxidant dropwise into a reaction vessel simultaneously to carry out a first-stage reaction to obtain a mixed system;

[0008] (2) adding a pore-forming agent to the mixed system, carrying out a second-stage reaction, and then washing, drying, and sintering for the first time to obtain porous iron phosphate;

[0009] (3) The porous iron phosphate, lithium source, manganese source, phosphorus source, doped metal element M source, carbon source, optional solid electrolyte and solvent are subjected to ball milling, spray drying and second sintering in sequence to obtain a manganese iron phosphate lithium composite material.

[0010] In the method of the present invention, in the first stage reaction, in the presence of a solution containing a phosphorus source and an oxidant, divalent iron in a solution containing a ferrous source is oxidized to trivalent iron while generating hydroxy ferric phosphate; a pore-forming agent is then added and the reaction is continued to obtain dihydrated ferric phosphate containing the pore-forming agent; the dihydrated ferric phosphate containing the pore-forming agent is sintered to obtain porous ferric phosphate with a suitable particle size distribution; and the porous ferric phosphate is then reacted with other raw materials (including a doping source and a carbon source) by a solid-phase method to obtain a manganese iron lithium phosphate composite material.

[0011] Among them, on the one hand, the method of preparing iron phosphate by a two-stage reaction in the present invention can make the size of the deposited part of the particles larger and the size of the other part of the particles smaller, especially obtaining some smaller primary particles, achieving the goal of obtaining two particles with different size distributions at one time, so that the iron phosphate forms a good particle size distribution. On the other hand, the pore-forming agent can physically create pores in the iron phosphate during the sintering process, so that the number of pore structures of the iron phosphate is significantly increased; at the same time, the pore-forming agent accelerates the water removal rate during the sintering process, reduces the sintering time, improves the morphology of the iron phosphate particles, and can further improve the particle size distribution effect of the iron phosphate, so that the prepared iron phosphate has both a porous structure and a good particle size distribution. Using porous iron phosphate as a raw material for preparing lithium manganese iron phosphate, its porous structure can effectively promote the embedding of lithium ions during the solid phase sintering process; and because the primary particles of porous iron phosphate are smaller, it is easier to form a good particle size distribution. A good particle size distribution can obtain a better dense packing effect, improve the microstructure of the powder raw material, such as pore distribution, etc. The particle size distribution of iron phosphate affects the effect of ball milling mixing of iron phosphate with other raw materials and the sintering behavior during solid phase sintering to prepare lithium manganese iron phosphate. When porous iron phosphate with good particle size distribution is ball-milled and sintered with other raw materials in sequence, the iron phosphate has the characteristics of good particle size distribution and porous structure, which not only enables it to be better mixed with other raw materials, but also improves the sintering performance of the solid-phase sintering process to prepare lithium manganese iron phosphate, thereby achieving the purpose of increasing the compaction density and specific capacity of the lithium manganese iron phosphate product. In addition, during the solid-phase sintering process to prepare lithium manganese iron phosphate, the present invention coats the lithium manganese iron phosphate with a solid electrolyte and a carbon source, which can further improve the lithium ion transmission rate of the lithium manganese iron phosphate composite material and greatly improve the specific capacity of the material.

[0012] Therefore, the present invention prepares a manganese iron phosphate lithium composite material by first preparing a porous iron phosphate, and then sintering it with a lithium source, a manganese source, a phosphorus source, a doped metal element M source, a carbon source, and a solid electrolyte. The manganese iron phosphate lithium composite material can have a high compaction density, a lithium ion transmission rate and a gram capacity, thereby broadening the application prospects of the material.

[0013] Preferably, in step (1), the pH value of the solution containing the ferrous source is 1.2-3. Controlling the pH value of the solution containing the ferrous source within the above range can better regulate the degree of oxidation of divalent iron to trivalent iron, which is conducive to the formation of porous ferric phosphate. Preferably, the pH value of the solution containing the ferrous source is regulated by at least one of phosphoric acid and sulfuric acid.

[0014] Preferably, in step (1), the concentration of divalent iron ions in the mixed system is ≤ 2 mg / L. The concentration of divalent iron ions in the mixed system can be detected by conventional methods, such as using potassium ferricyanide solution.

[0015] Preferably, in step (1), the mass percentage of Fe element in the solution containing the ferrous source is 3%-20%.

[0016] Preferably, in step (1), the iron source in the solution containing a ferrous source includes at least one of ferrous chloride, ferrous sulfate or ferrous nitrate.

[0017] Preferably, in step (1), the phosphorus source in the solution containing a phosphorus source and the phosphorus source in step (3) are each independently selected from at least one of ammonium phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate or phosphoric acid.

[0018] Preferably, in step (1), the oxidant comprises at least one of hydrogen peroxide, oxygen or chloric acid.

[0019] Preferably, in step (1), the molar ratio of the Fe element in the solution containing the ferrous source, the oxidant, and the P element in the solution containing the phosphorus source is 1:(1.3-1.8):(1-1.3).

[0020] Preferably, in step (1), the pH value of the solution containing the phosphorus source is 6-8.

[0021] Preferably, in step (1), the mass percentage of P element in the solution containing the phosphorus source is 3%-20%.

[0022] Preferably, in step (1), the temperature of the first stage reaction is 30-60° C., and the time of the first stage reaction is 0.5-8 h.

[0023] When a solution containing a ferrous source, a solution containing a phosphorus source, and an oxidant are simultaneously added dropwise to a reaction vessel, a first-stage reaction is carried out. In this process, the oxidant oxidizes the divalent iron to trivalent iron, while the trivalent iron reacts with phosphoric acid to form ferric phosphate. In this way, the rate of ferric phosphate formation can be controlled, which is conducive to the formation of porous ferric phosphate.

[0024] Preferably, in step (1), the dripping time is 60-180 minutes. Under the above dripping time, the degree of reaction can be controlled, which is conducive to increasing the probability of generating porous iron phosphate, and controlling the size of the iron phosphate particles within a relatively suitable range, which is more conducive to improving the particle size of the generated lithium manganese iron phosphate through the particle grading of the iron phosphate, thereby increasing the compaction density of the lithium manganese iron phosphate.

[0025] Preferably, in step (1), the dripping rate of the solution containing the ferrous source is 1-2.3 L / min, more preferably 1.5-2 L / min.

[0026] Preferably, in step (1), the droplet acceleration rate of the solution containing the phosphorus source is 0.2-0.3 L / min, more preferably 0.25-0.27 L / min.

[0027] Preferably, in step (1), when the oxidant is a hydrogen peroxide solution, the concentration of the hydrogen peroxide solution is 25 wt%-30 wt%, and the dripping rate of the hydrogen peroxide solution is 0.5-1.5 L / min, more preferably 1-1.2 / min.

[0028] Further preferably, in step (1), the volume ratio of the solution containing the ferrous source, the solution containing the phosphorus source and the hydrogen peroxide solution is (160-170):(20-25):(90-100).

[0029] Preferably, in step (2), the pore-forming agent includes at least one of ammonium carbonate, ammonium bicarbonate, ammonium oxalate, polymethacrylate, methyl methacrylate, methyl cellulose, polyvinyl alcohol (PVA), carboxymethyl cellulose (CMC), ascorbic acid or polystyrene.

[0030] A pore-forming agent is added to the mixed system obtained after the first-stage reaction. Here, the pore-forming agent can be better dispersed on the surface of the iron phosphate particles. Continuing the second-stage reaction can allow the unreacted raw materials to continue to react and the existing small iron phosphate particles to continue to grow into large particles, making the reaction more complete and ultimately forming stable porous iron phosphate particles; the iron phosphate particles containing the pore-forming agent are then sintered. The pore-forming agent decomposes and produces gas during the sintering process, and further physical pores are formed in it, which can significantly improve the pore structure of the iron phosphate material and improve its particle morphology characteristics, thereby facilitating the synthesis of subsequent manganese iron phosphate lithium composite materials with high compaction density and gram capacity.

[0031] Preferably, in step (2), the mass percentage of the pore-forming agent is 1%-30% based on the mass of the iron source in the solution containing the ferrous source and the mass of the phosphorus source in the solution containing the phosphorus source.

[0032] Preferably, in step (2), the temperature of the second stage reaction is 55-110° C., and the time of the second stage reaction is 0.5-8 h.

[0033] Preferably, in step (1) and step (2), the first-stage reaction and the second-stage reaction are each carried out under at least one of ultrasonic dispersion, microwave dispersion or stirring dispersion.

[0034] Preferably, in step (2), the drying temperature is 100-220° C., and the drying method is flash drying.

[0035] Preferably, in step (2), the washing method is water washing and filter pressing in a filter press.

[0036] Preferably, in step (2), the temperature of the first sintering is 500-900°C, the time of the first sintering is 2-6 hours, and the rate of heating to the first sintering temperature is 3-15°C / min. Within the above range of the first sintering temperature, time, and heating rate, the iron phosphate has a more porous structure.

[0037] Preferably, in step (2), the average particle size of the porous ferric phosphate is less than 30 μm, and further preferably, the average particle size of the porous ferric phosphate is 6-15 μm.

[0038] Preferably, in step (3), the molar ratio of the Fe element in the porous iron phosphate, the Li element in the lithium source, the Mn element in the manganese source, the P element in the phosphorus source and the M element in the doping metal element M source is (0.25-0.45):(1-1.06):(0.55-0.75):(0.55-0.9):(0.001-0.01).

[0039] Preferably, in step (3), based on the total mass of the porous iron phosphate, lithium source, manganese source, phosphorus source and doping metal element M source, the mass percentage of the carbon source is 1%-8%.

[0040] Preferably, in step (3), based on the total mass of the porous iron phosphate, lithium source, manganese source, phosphorus source and doping metal element M source, the mass percentage of the solid electrolyte is 0.1%-10%.

[0041] Preferably, in step (3), based on the total mass of the porous iron phosphate, lithium source, manganese source, phosphorus source, doped metal element M source, carbon source, solid electrolyte and solvent, the mass percentage of the solvent is 40%-80%.

[0042] Preferably, in step (3), the average particle size of the particles in the slurry obtained after the ball milling is 0.15-1 μm; further preferably, the average particle size of the particles in the slurry obtained after the ball milling is 0.2-0.35 μm.

[0043] Preferably, in step (3), the spray drying temperature is 80-300°C.

[0044] Preferably, in step (3), the temperature of the second sintering is 550-850°C, the time of the second sintering is 4-26 hours, and the rate of heating to the second sintering temperature is 3-15°C / min. Within the above-mentioned second sintering temperature and time range, the lithium manganese iron phosphate composite material has a better compacted density and gram capacity.

[0045] Preferably, in step (3), the average particle size of the lithium manganese iron phosphate composite material is 0.2-12 μm.

[0046] Preferably, in step (3), the metal element M in the doping metal element M source includes at least one of Ti, Mg, Al, V, and Nb.

[0047] Preferably, in step (3), the doping metal element M source includes at least one of a titanium source, a magnesium source, an aluminum source, a vanadium source, and a niobium source.

[0048] Further 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.

[0049] Preferably, in step (3), the solvent includes at least one of water and ethanol.

[0050] Preferably, in step (3), the carbon source includes at least one of starch, sucrose, glucose, citric acid, polyethylene glycol (PEG), polyvinyl pyrrolidone (PVP), polypropylene, conductive carbon black, acetylene black, carbon nanotubes or graphene.

[0051] Preferably, in step (3), the solid electrolyte comprises at least one of lithium aluminum titanium phosphate (LATP), lithium aluminum germanium phosphate (LAGP), lithium lanthanum titanate (LLTO), or lithium lanthanum zirconium oxide (LLZO). LATP, LAGP, and LLTO are NASICON structure ion conductors, and LLZO is a Garnet structure ion conductor.

[0052] More preferably, the chemical formula of the LATP is: Li 1+a Al a Ti 2-a (PO4)3,0≤a≤0.5.

[0053] More preferably, the chemical formula of the LAGP is: Li 1+b Al b Ge 2-b (PO4)3, where 0≤b≤0.5.

[0054] Further preferably, the chemical formula of the LLTO is: Li 3c La 2 / 3-c Ti d T 1-dO3, 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, or Se.

[0055] Further preferably, the chemical formula of the LLZO is: Li 7-x La3Zr 2-x M x O 12 , 0 ≤ x ≤ 1; where M is a dopable element, which may include at least one of Nb, Ta, Ti, Ga, Ge, Y, Gd, W, Mo, Sn, Sb, Se, or Ru.

[0056] In a second aspect, the present invention provides a lithium iron manganese phosphate composite material prepared by the method for preparing the lithium iron manganese phosphate composite material described above.

[0057] The lithium iron manganese phosphate composite material of the present invention has a high tap density and specific capacity. Among them, the tap density reaches 2.25 - 2.5 g / cm 3 , and the specific capacity is as high as 150 - 160 mAh / g.

[0058] In a third aspect, the present invention provides a secondary battery, including an electrolyte, a positive electrode sheet, and a negative electrode sheet, wherein the positive electrode sheet includes the lithium iron manganese phosphate composite material described above.

[0059] In some embodiments, the electrolyte includes an organic solvent, a lithium salt, and an additive. The types of the organic solvent and the lithium salt are not specifically limited in this application and can be selected according to actual needs.

[0060] 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), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butylene 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).

[0061] In some embodiments, the lithium salt can be at least one of lithium hexafluorophosphate (LiPF6), lithium bisfluorosulfonyl imide (LiFSI), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium bistrifluoromethanesulfonyl imide (LiTFSI), lithium difluorophosphate (LiPO2F2), lithium difluorobisoxalatophosphate (LiDFOP), lithium tetrafluorooxalatophosphate (LiTFOP), lithium trifluoromethanesulfonate (LiTFS), lithium difluorooxalatoborate (LiDFOB), or lithium dioxalatoborate (LiBOB).

[0062] In some embodiments, the positive electrode sheet includes a positive electrode active material.

[0063] 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.

[0064] In some embodiments, based on the total mass of the positive electrode sheet, the mass percentage of the positive electrode active material is 50%-99%.

[0065] In some embodiments, the positive electrode sheet further includes a binder and a conductive agent.

[0066] In some embodiments, the adhesive includes at least one of styrene-butadiene rubber (SBR), water-based acrylic resin, carboxymethyl cellulose (CMC), polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyvinyl butyral (PVB), ethylene-vinyl acetate copolymer (EVA) or polyvinyl alcohol (PVA), but is not limited to the above types, and the adhesive can be selected according to actual needs.

[0067] In some embodiments, based on the total mass of the positive electrode sheet, the binder has a mass percentage of less than or equal to 5%.

[0068] In some embodiments, the 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. The conductive agent can be selected according to actual needs.

[0069] In some embodiments, the conductive agent has a mass percentage of 1% to 3% based on the total mass of the positive electrode sheet.

[0070] The positive electrode sheet can be prepared using 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 on one surface of the composite film, drying, and cold pressing to obtain the positive electrode sheet.

[0071] In some embodiments, the solvent may include N-methylpyrrolidone (NMP), but is not limited thereto.

[0072] In some embodiments, the negative electrode sheet includes a negative electrode active material.

[0073] In some embodiments, the negative electrode active material includes at least one of graphite, hard carbon, soft carbon, silicon, silicon-carbon material, silicon-oxygen material, or metallic lithium, but is not limited thereto.

[0074] In some embodiments, based on the total mass of the negative electrode sheet, the mass percentage of the negative electrode active material is 50%-99%.

[0075] In some embodiments, the negative electrode sheet further includes a binder and a conductive agent.

[0076] The negative electrode sheet can be prepared using conventional methods in the art. For example, the negative electrode sheet preparation method 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 on one surface of the composite film, drying, and cold pressing to obtain the negative electrode sheet.

[0077] The secondary battery of the present application can be prepared according to conventional methods in the field; for example, the preparation method may specifically include the following steps: a structure containing a positive electrode sheet, a composite film and a negative electrode sheet is stacked and 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.

[0078] Compared with the prior art, the present invention has the following beneficial effects: the present invention prepares porous iron phosphate first, and then sinters it with a lithium source, a manganese source, a phosphorus source, a doped metal element source, a carbon source, and a solid electrolyte once to obtain a manganese iron phosphate lithium composite material. The manganese iron phosphate lithium composite material can have a high compaction density, a lithium ion transmission rate, and a gram capacity, thereby broadening the application prospects of the material. BRIEF DESCRIPTION OF THE DRAWINGS

[0079] Figure 1 This is the SEM image of the porous iron phosphate in Example 1.

[0080] Figure 2 This is a gram capacity curve of the deduction charge using the lithium manganese iron phosphate composite material of Example 1 as the positive electrode active material.

[0081] Figure 3 This is the SEM image of the molten iron phosphate in Comparative Example 4. DETAILED DESCRIPTION

[0082] To better illustrate the objectives, technical solutions, and advantages of the present invention, the present invention will be further described below with reference to specific examples. The experimental methods used in the examples and / or comparative examples are conventional methods unless otherwise specified; the materials and reagents used are commercially available unless otherwise specified.

[0083] Example 1

[0084] A method for preparing a lithium manganese iron phosphate composite material comprises the following steps:

[0085] S1. Weighing raw materials of ferrous sulfate, phosphoric acid, ammonium dihydrogen phosphate, and 27 wt% aqueous hydrogen peroxide solution in a molar ratio of Fe:P:P:O=1:0.35:0.77:1.6; based on the total mass of ferrous sulfate, solid phosphoric acid, and ammonium dihydrogen phosphate, taking 3% of the total mass of carboxymethyl cellulose (CMC) and a certain amount of sodium hydroxide; dissolving the ferrous sulfate in pure water at an iron content of 8 wt%, and then adding all the phosphoric acid to control the pH of the solution to obtain a mixed solution 1, wherein the pH value of the mixed solution 1 is 2; dissolving the ammonium dihydrogen phosphate in pure water at an 8 wt% phosphorus content, and then adding sodium hydroxide to adjust the pH of the solution to 7.0±0.1 to obtain a mixed solution 2;

[0086] S2. Mixed solution 1, hydrogen peroxide solution, and mixed solution 2 were slowly added dropwise to the reactor at a volume ratio of 165:23:95, and stirring and ultrasonication were turned on. The three solutions were added continuously for 90 minutes, and the addition rate of mixed solution 1 was 1.83 L / min. Under stirring and ultrasonication, oxidation reaction and ferric phosphate generation reaction were carried out simultaneously; after the addition was completed, the reaction was continued under stirring and ultrasonication for 60 minutes, and then potassium ferricyanide solution was used to detect whether the solution contained divalent iron ions. The results showed that no divalent iron ions were detected in the solution at this time, and the divalent iron ions were considered to be completely oxidized to trivalent iron ions (the detection limit was ≤2 mg / L);

[0087] S3, adding carboxymethyl cellulose (CMC) and heating to 90° C., keeping the temperature to continue the reaction until the slurry turns white (about 0.5 to 2 hours), and then keeping the temperature for 30 minutes; then cooling to 45° C., transferring the material to a filter press for water washing and filter pressing; flash drying the filter cake at 120° C., and sintering the dried powder in a rotary kiln for the first time at a heating rate of 5° C. / min, a first sintering temperature of 800° C., and a first sintering time of 4.5 hours; after sintering, the material is crushed to D50 = 13 ± 2 μm to obtain porous iron phosphate;

[0088] S4, according to the molar ratio of Li, Fe, Mn, P and doping metal element M of 1.05:0.38:0.6:0.66:0.02, weighing raw materials lithium carbonate, porous iron phosphate, manganese carbonate, ammonium dihydrogen phosphate and doping source ammonium metavanadate; and based on the total mass of lithium carbonate, porous iron phosphate, manganese carbonate, ammonium dihydrogen phosphate and doping source ammonium metavanadate, weighing 2% of the total mass of sucrose as a carbon source, weighing 5% of the total mass of solid electrolyte powder LATP (Li 1.1 Al 0.1 Ti 1.9 (PO4)3), and deionized water is added at a solid content of 40%, and the above raw materials are mixed by ball milling in a sand mill to obtain a mixed slurry, the particle size of the mixed slurry is D50 = 0.2 μm, and the mixed slurry is spray-dried to obtain a dry powder; wherein the inlet temperature of the sprayer is set to 220°C, and the outlet temperature is set to 100°C;

[0089] S5. The dried powder was sintered for the second time under nitrogen atmosphere protection, with a heating rate of 5°C / min, a second sintering temperature of 700°C, and a second sintering time of 8h. After cooling, the material was crushed by a jet mill to obtain a particle size of D 50 =1.0μm manganese iron phosphate lithium composite material.

[0090] Example 2

[0091] The difference between this embodiment and embodiment 1 is that:

[0092] In step S5, the second sintering temperature is 720° C., and the second sintering time is 6 h.

[0093] Example 3

[0094] The difference between this embodiment and embodiment 1 is that:

[0095] In steps S1 and S3, the special pore-forming additive was changed from CMC to an equal amount of polyvinyl alcohol (PVA).

[0096] Example 4

[0097] The difference between this embodiment and embodiment 1 is that:

[0098] In step S2, the time for dropping the mixed solution 1, hydrogen peroxide, and the mixed solution 2 is controlled to be 180 minutes.

[0099] Example 5

[0100] The difference between this embodiment and embodiment 1 is that:

[0101] No solid electrolyte is added in step S4.

[0102] Example 6

[0103] The difference between this embodiment and embodiment 1 is that:

[0104] In step S3, the heating rate is 8°C / min, and the first sintering temperature is 620°C.

[0105] Example 7

[0106] The difference between this embodiment and embodiment 1 is that:

[0107] In step S4, 3% of the total mass of glucose is used as a carbon source.

[0108] Example 8

[0109] The difference between this embodiment and embodiment 1 is that:

[0110] The raw materials weighed in step S4 are lithium carbonate, porous iron phosphate, manganese dioxide, ammonium dihydrogen phosphate and magnesium oxide as a doping source.

[0111] Example 9

[0112] The difference between this comparative example and Example 1 is:

[0113] In step S1, 8% of the total mass of carboxymethyl cellulose (CMC) is taken.

[0114] Example 10

[0115] The difference between this comparative example and Example 1 is:

[0116] In step S3, the sintered material is crushed to D50 = 8 ± 2 μm.

[0117] Example 11

[0118] The difference between this comparative example and Example 1 is:

[0119] The particle size of the mixed slurry in step S4 is D50=0.35 μm.

[0120] Example 12

[0121] The difference between this comparative example and Example 1 is:

[0122] In step S5, the second sintering temperature is 640° C., and the second sintering time is 10 h.

[0123] Comparative Example 1

[0124] The difference between this comparative example and Example 1 is:

[0125] In step S4, commercially available conventional molten iron phosphate is used.

[0126] Comparative Example 2

[0127] The difference between this comparative example and Example 1 is:

[0128] Steps S1 to S3 are not performed, and in step S4, commercially available ferric metaphosphate is used as the iron source instead of porous ferric phosphate.

[0129] Comparative Example 3

[0130] The difference between this comparative example and Example 1 is:

[0131] Steps S1 to S3 are not performed, and in step S4, commercially available ferric pyrophosphate is used as the iron source instead of porous ferric phosphate.

[0132] Comparative Example 4

[0133] The difference between this comparative example and Example 1 is that CMC is not added in step S3.

[0134] Comparative Example 5

[0135] The difference between this embodiment and embodiment 1 is that:

[0136] In step S2, mixed solution 1, hydrogen peroxide solution, and mixed solution 2 are added to the reactor in sequence. Because the three are not added at the same time, the hydrogen peroxide solution can quickly oxidize the divalent iron in mixed solution 1 to trivalent iron. When mixed solution 2 is added, the trivalent iron and phosphate ions quickly form ferric phosphate. The reaction rate is too fast, resulting in the formed ferric phosphate being molten, and ultimately making it impossible to obtain porous ferric phosphate.

[0137] Performance testing:

[0138] (1) Compaction density: The powder material to be tested is compressed under the action of external force. During the compression process, the gaps between the powders are filled, the contact area between the particles increases, the atoms are attracted to each other, and the mechanical fit between the particles is enhanced, thereby 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 compaction density of the powder material under a certain pressure can be determined. The test parameters for the compaction density of lithium manganese iron phosphate recommend that the test pressure should be no less than 80MPa, and the test mold diameter can be selected in the range of 10-20mm. The sampling volume needs to be appropriately adjusted in combination with the mold diameter. After the sample test is completed, the pressure, pressure, resistance, thickness, resistivity, and powder conductivity data are obtained. Among them, the compaction density and powder conductivity are taken as the values under the test pressure of 200MPa, and the corresponding compaction density is calculated in combination with the following formula:

[0139]

[0140] Where D is the compacted density (g / cm 3 ); m is the sample mass (g); S is the test electrode area (cm 2); L is the thickness of the test sample after compression (cm).

[0141] (2) The lithium manganese iron phosphate composite materials of each embodiment and comparative example were tested for gram capacity, as follows:

[0142] Preparation of button-type lithium-ion batteries:

[0143] A1. The lithium iron manganese phosphate composite material and the conductive agent (acetylene black) of each Example or Comparative Example were placed in an oven and baked at 120 ° C for 4h, then cooled in a drying container; then, the lithium iron manganese phosphate composite material, the binder (polyvinylidene fluoride), the conductive agent (acetylene black) mass ratio of 80:10:10 was weighed in the corresponding proportion of the lithium iron manganese acid composite material and the conductive agent (acetylene black) was added to a 50mL small beaker, and then the binder solution (5% by mass fraction of polyvinylidene fluoride in N-methylpyrrolidone solution) was added and stirred with a stirrer to form a paste;

[0144] A2. Apply the paste evenly on the aluminum foil to obtain a single-sided surface density of 60g / m 2 The positive electrode sheet was then placed in a blast drying oven and dried at 120°C for 2 hours and pressed (the pressing density of the positive electrode sheet was 2.0 g / cm 3 ), cut into a circular positive electrode sheet with a diameter of 12 mm;

[0145] A3. In an argon atmosphere glove box, a button-type lithium-ion battery was assembled using a circular cathode sheet 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 in ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (EC:EMC volume ratio of 3:7) as the electrolyte.

[0146] Gram Capacity Test:

[0147] Under the condition of 25°C ± 2°C, the assembled button-type 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: the charge and discharge rate was 0.1C, the voltage range was 2.5V-4.5V, and the 0.1C discharge capacity of each battery was recorded; the number of button-type lithium-ion batteries in each embodiment or comparative example was 5, that is, the experiment was repeated 5 times, the 0.1C discharge capacity data was averaged, and the gram capacity of the lithium iron manganese phosphate composite material was calculated according to the following formula:

[0148] Gram capacity of the lithium iron manganese phosphate composite material (mAh / g) = average value of 0.1C discharge specific capacity (unit: mAh) / mass of the lithium iron manganese phosphate composite material (unit: g).

[0149] (3) Scanning electron microscopy (SEM) was used to perform microscopic surface testing on the porous iron phosphate particles obtained in Example 1 and the final manganese iron lithium phosphate composite material. The test results are as follows: Figure 1 and Figure 3 shown.

[0150] Example 1 The gram capacity test results of the lithium manganese iron phosphate composite material are as follows Figure 2 and Figure 3 The specific test results of compacted density and gram capacity are shown in Table 1.

[0151] Table 1

[0152]

[0153]

[0154] Depend on Figure 1 and 3 It can be seen that conventional iron phosphate has less pore structure and is prone to melting between particles, resulting in larger particles, which is generally called molten iron phosphate; the porous iron phosphate prepared by the present invention has no obvious melting phenomenon, the number of pore structures is significantly increased, the porous morphology is smaller, and the particles are smaller. And from Examples 1-12, it can be seen that the manganese iron phosphate lithium composite material prepared by the present invention has a high compacted density and gram capacity, wherein the compacted density reaches 2.25-2.5g / cm 3 , with a gram capacity of up to 150-160 mAh / g. Furthermore, in conjunction with Comparative Examples 1-5, it can be seen that, compared with conventional molten iron phosphate, the use of the porous iron phosphate of the present invention as the iron source for preparing the lithium manganese iron phosphate composite material can significantly increase the gram capacity and compacted density of the composite material.

[0155] As can be seen from Examples 1-12 above, the conditions of the first sintering, the type of pore-forming agent, and the time (reaction rate) of adding the raw materials have a certain influence on the particle size of the obtained iron phosphate, and thus on the compacted density and gram capacity of the prepared ferromanganese phosphate. The particle size of the iron phosphate after the first sintering, the particle size of the mixed slurry after ball milling, the conditions of the second sintering, the solid electrolyte, the type and content of the carbon source have a certain influence on the particle size of the obtained manganese iron phosphate lithium composite material, and thus affect its compacted density and gram capacity.

[0156] 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 the technical solutions of the present invention may be modified or replaced by equivalents 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: The steps include: (1) adding a solution containing a ferrous source, a solution containing a phosphorus source, and an oxidant dropwise into a reaction vessel simultaneously to carry out a first-stage reaction to obtain a mixed system; (2) adding a pore-forming agent to the mixed system, carrying out a second-stage reaction, and then washing, drying, and sintering for the first time to obtain porous iron phosphate; (3) The porous iron phosphate, lithium source, manganese source, phosphorus source, doped metal element M source, carbon source, optional solid electrolyte and solvent are subjected to ball milling, spray drying and second sintering in sequence to obtain a manganese iron phosphate lithium composite material.

2. The method for preparing the lithium iron manganese phosphate composite material according to claim 1, wherein: In step (1), the oxidant includes at least one of hydrogen peroxide, oxygen or chloric acid.

3. The method for preparing the lithium manganese iron phosphate composite material according to claim 1, wherein: In step (2), the pore-forming agent includes at least one of ammonium carbonate, ammonium bicarbonate, ammonium oxalate, polymethacrylate, methyl methacrylate, methyl cellulose, polyvinyl alcohol, carboxymethyl cellulose, ascorbic acid or polystyrene.

4. The method for preparing the lithium manganese iron phosphate composite material according to claim 1, wherein: In step (2), the temperature of the first sintering is 500-900° C., the time of the first sintering is 2-6 hours, and the rate of heating to the temperature of the first sintering is 3-15° C. / min.

5. The method for preparing the lithium manganese iron phosphate composite material according to claim 1, wherein: In step (3), the temperature of the second sintering is 550-850° C., the time of the second sintering is 4-26 hours, and the rate of heating to the temperature of the second sintering is 3-15° C. / min.

6. The method for preparing the lithium manganese iron phosphate composite material according to claim 1, wherein: Include at least one of the following: In step (1), the iron source in the solution containing a ferrous source includes at least one of ferrous chloride, ferrous sulfate or ferrous nitrate; In step (1), the pH value of the solution containing the ferrous source is 1.2-3; In step (1), the phosphorus source in the solution containing a phosphorus source and the phosphorus source in step (3) are each independently selected from at least one of ammonium phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate or phosphoric acid; In step (1), the molar ratio of the Fe element in the solution containing the ferrous source, the oxidant, and the P element in the solution containing the phosphorus source is 1:(1.3-1.8):(1-1.3); In step (1), the pH value of the solution containing the phosphorus source is 6-8; In step (1), the mass percentage of P element in the solution containing the phosphorus source is 3%-20%; In step (1), the mass percentage of Fe element in the solution containing ferrous source is 3%-20%; In step (1), in step (1), the temperature of the first stage reaction is 30-60° C., and the time of the first stage reaction is 0.5-8 h.

7. The method for preparing the lithium manganese iron phosphate composite material according to claim 1, wherein: Include at least one of the following: In step (2), the mass percentage of the pore-forming agent is 1% to 30% based on the mass of the iron source in the solution containing the ferrous source and the mass of the phosphorus source in the solution containing the phosphorus source; In step (2), the temperature of the second stage reaction is 55-110° C., and the time of the second stage reaction is 0.5-8 h; In step (2), the reaction is carried out under at least one of ultrasonic dispersion, microwave dispersion or stirring dispersion; In step (2), the drying temperature is 100-220°C; In step (2), the average particle size of the porous iron phosphate is less than 30 μm.

8. The method for preparing the lithium iron manganese phosphate composite material according to claim 1, wherein: Include at least one of the following: In step (3), the molar ratio of the Fe element in the porous iron phosphate, the Li element in the lithium source, the Mn element in the manganese source, the P element in the phosphorus source, and the M element in the doping metal element M source is (0.25-0.45):(1-1.06):(0.55-0.75):(0.55-0.9):(0.001-0.01); In step (3), the mass percentage of the carbon source is 1%-8% based on the total mass of the porous iron phosphate, lithium source, manganese source, phosphorus source and doping metal element M source; In step (3), based on the total mass of the porous iron phosphate, lithium source, manganese source, phosphorus source and doping metal element M source, the mass percentage of the solid electrolyte is 0.1%-10%; In step (3), based on the total mass of the porous iron phosphate, lithium source, manganese source, phosphorus source, doping metal element M source, carbon source, solid electrolyte and solvent, the mass percentage of the solvent is 40%-80%; In step (3), the average particle size of the particles in the slurry obtained after ball milling is 0.15-1 μm; In step (3), the spray drying temperature is 80-300°C; In step (3), the average particle size of the lithium manganese iron phosphate composite material is 0.2-12 μm; In step (3), the metal element M in the doping metal element M source includes at least one of Ti, Mg, Al, V, and Nb; In step (3), the carbon source includes at least one of starch, sucrose, glucose, citric acid, polyethylene glycol, polyvinyl pyrrolidone, polypropylene, conductive carbon black, acetylene black, carbon nanotubes or graphene; In step (3), the solid electrolyte includes at least one of lithium aluminum titanium phosphate, lithium aluminum germanium phosphate, lithium lanthanum titanate or lithium lanthanum zirconium oxide.

9. A lithium iron manganese phosphate composite material obtained by the method for preparing the lithium iron manganese phosphate composite material according to any one of claims 1 to 8.

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

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