Lithium manganese iron phosphate composite positive electrode 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 positive electrode material and a preparation method and application thereof. The lithium manganese iron phosphate composite positive electrode material is synthesized by a multi-step solid phase method, and the problems of non-uniform reaction, large particle size, difficulty in morphology control and poor batch consistency during preparation of the lithium manganese iron phosphate by the solid phase method are effectively solved. According to the method disclosed by the invention, the lithium manganese iron phosphate composite positive electrode material which is uniform in size, uniform and compact can be obtained, and the material is high in powder compaction density, high in gram volume and relatively high in energy density.

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 positive electrode material and a preparation method and application thereof. Background Art

[0002] Lithium manganese iron phosphate (LMFP), like LFP, has an olivine structure and excellent thermodynamic and kinetic stability. Its voltage platform is approximately 0.7V higher than LFP, and its energy density is approximately 15% higher than LFP. Its charge and discharge voltage platform is similar to that of ternary cathode materials, and its safety performance is significantly superior to that of ternary cathode materials. However, LMP's inherent olivine crystal structure results in poor conductivity and rate performance. Furthermore, during the charge and discharge process, the manganese element in the positive electrode undergoes a Jahn-Teller effect, dissolving manganese ions and increasing battery polarization. Furthermore, the manganese dissolved in the electrolyte deposits on the negative electrode surface, destroying the SEI layer structure within the negative electrode, resulting in capacity loss and poor cycle performance.

[0003] To address the defects of lithium manganese iron phosphate, single measures such as particle structure control, surface coating, and bulk doping, or multiple measures are used in concert to improve it. These improvement measures are mainly implemented through two methods: liquid phase method and solid phase method. The liquid phase method can be further divided into hydrothermal method / solvothermal method, sol-gel method, co-precipitation method, etc. The hydrothermal method is the most widely used, with low energy consumption and cost, and can control the particle size of the product. However, due to its production in a high temperature and high pressure environment, it has high requirements for equipment and operation control, cannot be promoted on a large scale, and the consistency of different batches of products is poor, making industrialization difficult. Although the solid phase synthesis method has low industrialization difficulty, the solid phase method for preparing lithium manganese iron phosphate has problems such as uneven reaction, large particle size, difficult morphology control, and poor batch consistency. Therefore, lithium manganese iron phosphate prepared by the existing solid phase process still has problems such as low compaction density, low electronic conductivity, slow lithium ion diffusion rate, and low specific capacity utilization. It cannot effectively improve the compaction density and specific capacity utilization of lithium manganese iron phosphate composite positive electrode materials, resulting in limited application scenarios of lithium manganese iron phosphate. 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 positive electrode 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 positive electrode material, comprising the following steps:

[0007] (1) A lithium source, an iron source, a manganese source, a phosphorus source, a doping metal element M source, a first carbon source, and a first solvent are ball-milled to obtain a first slurry; the first slurry is spray-dried to obtain a first dry powder; the first dry powder is sintered for a first time to obtain a first precursor material; the temperature of the first sintering is 350-550° C., and the time of the first sintering is 4.5-11 hours;

[0008] (2) a portion of the first precursor material, a second carbon source, and a second solvent are ball-milled to obtain a second slurry; the second slurry is spray-dried to obtain a second dry powder; the mass of the second carbon source is 4%-10% of the mass of the first precursor material in step (2); and the average particle size of the second slurry is 0.2-0.7 μm;

[0009] (3) Part of the first precursor material, a third carbon source, and a third solvent are ball-milled to obtain a third slurry; the third slurry and the second dry powder are sequentially mixed, spray-dried, and sintered for a second time to obtain a second precursor material; the mass of the third carbon source is 4%-10% of the mass of the first precursor material in step (3); the average particle size of the third slurry is 0.1-0.33 μm; the temperature of the second sintering is 650-800° C., and the time of the second sintering is 6-12 h;

[0010] (4) The second precursor material, solid electrolyte and forming aid are sequentially mixed, granulated, sintered for a third time and crushed to obtain a lithium manganese iron phosphate composite positive electrode material; the temperature of the third sintering is 600-750°C, and the time of the third sintering is 4-8h.

[0011] The compaction density of the material is affected by the crystallinity, morphology, surface smoothness, particle size distribution, etc. The lithium manganese iron phosphate composite positive electrode material prepared by the solid phase method is generally a secondary particle agglomerate composed of primary spherical particles. There will be gaps between the primary spherical particles. These gaps lead to unsatisfactory compaction density of the material. The present invention first prepares a first precursor material by low-temperature sintering, and then divides the first precursor material into two parts. Part of the precursor material is respectively mixed with a carbon source through ball milling to obtain two slurries with different specific particle sizes, and the particles of the two particle sizes are evenly mixed, so that the material can achieve large and small particle grading in the precursor, which can be used to form a material with high compaction density in the subsequent process; then high-temperature sintering is performed to obtain a second precursor material; finally, a solid electrolyte and a molding aid are added, and the mixture is granulated and then sintered, so that the material is surface-coated with a solid electrolyte and carbon, wherein the molding aid can not only increase the plasticity and fluidity of the second precursor material powder during the pressing and granulation molding process, but also increase the tap density of the second precursor material particles after granulation, so that the sintered material has high density, few lattice defects, low interface resistance, and improves the ion transfer rate of the material; and the molding aid is carbonized during the sintering process to achieve surface coating, which affects the sintering process to a certain extent, increases the shrinkage rate of the material during the sintering process, and further improves the compaction density of the material. In addition, surface coating of solid electrolyte and carbon can improve the lithium ion transmission rate of the material, greatly improving the specific capacity of the material.

[0012] Therefore, the present invention uses a multi-step solid-phase method to synthesize a lithium manganese iron phosphate composite cathode material, effectively addressing the poor material consistency caused by the large differences in the properties of the various elemental raw materials, large particle size, and uneven distribution during the solid-phase preparation of LMFP. The method of the present invention can produce a uniform, dense, and uniform material, thereby improving the material's compaction density and lithium ion transmission rate, significantly enhancing the specific capacity of LMFP.

[0013] Preferably, in step (1), the lithium source includes one or more of lithium hydroxide, lithium carbonate, lithium dihydrogen phosphate, lithium oxalate, lithium acetate, and lithium phosphate.

[0014] Preferably, in step (1), the iron source includes one or more of ferric phosphate, ferrous oxalate, ferric oxide, ferric oxide, and ferric acetate.

[0015] Preferably, in step (1), the manganese source includes one or more of manganese oxalate, manganese carbonate, manganese acetate, manganese dioxide, and trimanganese tetraoxide.

[0016] Preferably, in step (1), the phosphorus source includes one or more of ammonium phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and phosphoric acid.

[0017] Preferably, in step (1), the metal element M in the doping metal element M source includes at least one of Ti, Mg, Al, V, and Nb.

[0018] Preferably, in step (1), 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.

[0019] 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.

[0020] Preferably, in steps (1)-(3), the first carbon source, the second carbon source and the third carbon source are each independently selected from at least one of starch, sucrose, glucose, citric acid, PEG, PVP, polypropylene, conductive carbon black, acetylene black, carbon nanotubes and graphene.

[0021] Preferably, in steps (1) to (3), the first solvent, the second solvent and the third solvent are each independently selected from at least one of water and ethanol.

[0022] Preferably, in step (1), based on the molar amount n of the Li element in the lithium source Li , the molar amount of Fe element in the iron source n Fe , the molar amount of Mn element in the manganese source n Mn , the molar amount of P element in the phosphorus source n P and the molar amount n of the metal element M in the doping metal element M M , n Li :n Fe :n Mn :n P :n M =(1-1.06):(0.15-0.45):(0.55-0.8):(1-1.05):(0.01-0.03), more preferably (1-1.06):(0.35-0.40):(0.7-0.8):(1-1.05):(0.01-0.03).

[0023] Preferably, based on the total mass of the lithium source, the iron source, the manganese source, the phosphorus source and the doping metal element M source, the mass percentage of the first carbon source is 1%-3.5%.

[0024] Preferably, in step (1), the solid content of the first slurry is 30%-50%.

[0025] Preferably, in step (1), the average particle size of the first slurry is 0.35-0.64 μm, more preferably 0.41-0.58 μm.

[0026] The "slurry" referred to herein is a mixture containing solid particles and liquid, wherein the average particle size is obtained by testing with a laser particle size analyzer.

[0027] The inventors of the present invention have found that the average particle size of the first slurry affects the compacted density and gram capacity of the final LMFP particles. When the average particle size of the first slurry is within the above range, the LMFP has a better compacted density and gram capacity.

[0028] Preferably, in step (1), the temperature of the first sintering is 450-550° C., and the time of the first sintering is 5-10 h, more preferably 6.5-8 h.

[0029] The inventors of the present invention have found that the temperature and time of the first sintering affect the compacted density and gram capacity of the final LMFP particles. Within the above-mentioned first sintering temperature and time ranges, LMFP has better compacted density and gram capacity.

[0030] Preferably, in step (2), the solid content of the second slurry is 40%-60%, and the average particle size of the second slurry is 0.33-0.56 μm.

[0031] The inventors of the present invention have found that the average particle size of the second slurry affects the final compacted density and gram capacity of the LMFP particles. When the average particle size of the second slurry is within the above range, the LMFP has a better compacted density and gram capacity.

[0032] Preferably, in step (2), the mass of the second carbon source is 5.5%-8% of the mass of the first precursor material in step (2).

[0033] Preferably, in steps (2) and (3), based on the total mass of the first precursor material in step (2) and the first precursor material in step (3), the mass percentage of the first precursor material in step (2) is 50%-70%.

[0034] Preferably, in step (3), the mass of the third carbon source is 5.5%-8% of the mass of the first precursor material in step (3).

[0035] Preferably, in step (3), the mass ratio of the first precursor material to the second dry powder is (20-60): (40-80), more preferably (30-50): (50-70).

[0036] The inventors of the present invention have discovered that the mass ratio of the first precursor material to the second dry powder affects the combination effect of the two types of precursor materials of different sizes, and thus affects the final compacted density and gram capacity of the LMFP. Within the above-mentioned mass ratio range of the first precursor material to the second dry powder, the LMFP has a better compacted density and gram capacity.

[0037] Preferably, in step (3), the solid content of the third slurry is 40%-60%, and preferably the average particle size of the third slurry is 0.23-0.29 μm.

[0038] Preferably, in step (3), the temperature of the second sintering is 730-750° C., and the time of the second sintering is 6-8 hours.

[0039] The inventors of the present invention have found that the temperature and time of the second sintering affect the final compacted density and gram capacity of the LMFP. Within the above-mentioned second sintering temperature and time range, the LMFP has a better compacted density and gram capacity.

[0040] Preferably, in step (4), the mass ratio of the second precursor material, the solid electrolyte and the forming aid is 89:(8-10):(1-3).

[0041] The inventors of the present invention found that the mass ratio of the second precursor material, the solid electrolyte and the molding aid affects the sintering effect and the surface coating effect of the particles. Within the above-mentioned mass ratio range of the second precursor material, the solid electrolyte and the molding aid, LMFP has a better compaction density and gram capacity.

[0042] Preferably, in step (4), the average width of the material obtained after granulation is 2-4 mm and the average length is 6-10 mm.

[0043] Preferably, in step (4), the solid electrolyte comprises at least one of lithium aluminum titanium phosphate (LATP), lithium aluminum germanium phosphate (LAGP), lithium lanthanum titanate (LLTO), and lithium lanthanum zirconium oxide (LLZO). LATP, LAGP, and LLTO are NASICON structure ion conductors, and LLZO is a Garnet structure ion conductor.

[0044] More preferably, the chemical formula of the LATP is: Li 1+a Al a Ti 2-a-b M1 b(PO4)3, where 0 ≤ a ≤ 0.5 and 0 ≤ b ≤ 0.5; wherein, M1 is a doping element, and M1 includes one or more of Zn, Mg, Mn, Co, Ni, Cr, Al, Sc, In, Fe, Y, V, Nb, Ta, Sb, W, Mo, Ru, Ga, Ba, As, Hf, Ge, Sn.

[0045] Further preferably, the chemical formula of the LAGP is: Li 1+c Al c Ge 2-c-d M2 d (PO4)3, where 0 ≤ c ≤ 0.5 and 0 ≤ d ≤ 0.5; wherein, M2 is a doping element, and M2 includes one or more of Zn, Mg, Mn, Co, Ni, Cr, Al, Sc, In, Fe, Y, V, Nb, Ta, Sb, W, Mo, Ru, Ga, Ba, As, Hf, Ge, Sn.

[0046] Further preferably, the chemical formula of the LLTO is: Li 3e La 2 / 3-e Ti f M3 1-f O3, where 0 < e < 2 / 3 and 0 < f ≤ 1; wherein, M3 is a doping element, and M3 includes one or more of Nb, W, Ti, Hf, Ru, Mo, Nd, Ba, Ga, In, Ge, Sn, Sb, Se.

[0047] Further preferably, the chemical formula of the LLZO is: Li 7-x La3Zr 2-x M4 x O 12 , where 0 ≤ x ≤ 1; wherein, M4 is a doping element, and M4 includes one or more of Ga, Nb, Ta, Ti, Ge, Y, Gd, W, Mo, Sn, Sb, Se, Ru.

[0048] Preferably, in step (4), the molding aid includes at least one of phenolic resin, epoxy resin, polyvinyl alcohol (PVA), polyvinyl butyral (PVB), polyacrylic acid (PAA), polyvinylpyrrolidone (PVP), polyethylene (PE), polyethylene oxide (PEO), polypropylene (PP), dextrin, carboxymethyl cellulose (CMC), carboxymethyl cellulose (CMC-Li), hydroxypropyl methyl cellulose (HPMC), magnesium stearate, ascorbic acid (vitamin C), dibutylhydroxytoluene (BHT), styrene-butadiene rubber.

[0049] The above-mentioned molding aids have the thermoplastic or thermosetting properties of polymers, and also have good physical characteristics such as solubility, dispersibility, interfacial compatibility and lubricity. These properties enable them to form a flexible network, soften flow, disperse evenly, reduce interfacial tension and reduce friction between particles during processing, thereby effectively improving the plasticity and fluidity of the material. Molding aids can increase the plasticity and fluidity of the second material precursor powder during the compaction and granulation molding process, increase the tap density of the second precursor material particles after granulation, and to a certain extent affect the sintering process, increase the shrinkage rate of the material during sintering, and further improve the compaction density of the material.

[0050] Preferably, in step (4), the temperature of the third sintering is 680-700° C., and the time of the third sintering is 4-6 hours.

[0051] The inventors of the present invention have found that the temperature and time of the third sintering affect the sintering effect and surface coating effect of the particles. Within the above-mentioned temperature and time range of the third sintering, LMFP has a better compaction density and gram capacity.

[0052] In a second aspect, the present invention provides a lithium iron manganese phosphate composite positive electrode material prepared by the method for preparing the lithium iron manganese phosphate composite positive electrode material.

[0053] The lithium manganese iron phosphate composite positive electrode material of the present invention has a high compaction density and gram capacity, wherein the compaction density reaches 2.20-2.45g / cm 3 , the gram capacity is as high as 140-160mAh / g.

[0054] In a third aspect, the present invention provides a secondary battery comprising a positive electrode sheet and a negative electrode sheet, wherein the positive electrode sheet comprises the lithium manganese iron phosphate composite positive electrode material.

[0055] Compared to existing technologies, the present invention offers the following advantages: The multi-step solid-phase synthesis of lithium manganese iron phosphate (LMFP) composite cathode materials effectively addresses the problem of poor material consistency caused by large variations in the properties of the various elemental raw materials, large particle size, and uneven distribution during solid-phase preparation of LMFP. The method achieves uniform, uniform, and dense material, improving the material's compaction density and lithium ion transfer rate, significantly enhancing the specific capacity of LMFP. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] Figure 1 The gram-capacity curve of the lithium manganese iron phosphate positive electrode material in Example 1. DETAILED DESCRIPTION

[0057] 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.

[0058] The particle size of the solid electrolyte powder involved below is D50=0.1μm-0.3μm.

[0059] Example 1

[0060] A method for preparing ferromanganese phosphate material comprises the following steps:

[0061] S1. According to the molar ratio of Li, Fe, Mn, P and doping metal element vanadium of 1.05:0.38:0.6:1.04:0.02, weigh the raw materials lithium carbonate, ferric oxide, manganese dioxide, ammonium dihydrogen phosphate and doping source ammonium metavanadate. Based on the total mass of the above raw materials, weigh 1.5% of the total mass of glucose as the first carbon source, and add deionized water at a slurry solid content of 40%. The raw materials, doping source, carbon source and deionized water are mixed by ball milling in a sand mill to obtain a first slurry with a particle size of D50 = 0.53 μm. The first slurry is centrifugally spray-dried to obtain a first dry powder. The first dry powder is sintered for the first time in a nitrogen atmosphere sintering furnace at a sintering temperature of 450°C and a sintering time of 6.5h to obtain a first precursor material;

[0062] S2. Weigh 6 parts by weight of the first precursor material obtained in step S1, and then weigh sucrose as a second carbon source at 5.5% of the total mass of the 6 parts by weight of the precursor material, add deionized water at a slurry solid content of 50%, and mix by ball milling with a sand mill to obtain a second slurry with a particle size of D50 = 0.44 μm. The slurry is centrifugally spray-dried to obtain a second dry powder for later use;

[0063] S3, weighing 4 parts by weight of the first precursor material obtained in step S1, and then weighing sucrose as a third carbon source at 5.5% of the total mass of the 4 parts by weight of the precursor material, adding deionized water at a slurry solid content of 48%, and mixing by ball milling with a sand mill to obtain a third slurry, wherein the particle size of the slurry is D50 = 0.23 μm, and then adding the second dry powder obtained in step S2 thereto, stirring uniformly in a stirring tank, and then centrifugally spray drying to obtain a third dry powder, and sintering the third dry powder in a nitrogen atmosphere sintering furnace at a sintering temperature of 730° C. and a sintering time of 8 h to obtain a second precursor material;

[0064] S4, the second precursor material obtained in step S3, solid electrolyte powder LATP (Li 1.1 Al 0.1 Ti 1.9(PO4)3) and the molding aid polyvinyl alcohol (PVA) are uniformly mixed in a mechanical mixer at a mass ratio of 89:9:2, and then the mixed powder is sent to a granulator for granulation. After granulation, a sheet material with a thickness of 3 mm and a diameter of 8 mm is obtained; then it is sintered for the third time in a nitrogen atmosphere sintering furnace at a sintering temperature of 680°C and a sintering time of 5 hours. After the material temperature is lowered, it is crushed by roller air flow to obtain a finished product lithium manganese iron phosphate composite positive electrode material.

[0065] Example 2

[0066] This embodiment provides a lithium manganese iron phosphate, the preparation method of which is basically the same as that of Example 1, except that:

[0067] The raw materials weighed in step S1 are lithium hydroxide, ferric oxide, manganese tetraoxide, phosphoric acid, and magnesium chloride as a doping source; based on the total mass of the above raw materials, 3.0% of the total mass of citric acid is added as a first carbon source;

[0068] In step S1, the first sintering temperature is 550°C and the sintering time is 8 hours;

[0069] In step S3, the sintering temperature is 750° C. and the sintering time is 6 h;

[0070] In step S4, the sintering temperature is 700°C and the sintering time is 4 hours;

[0071] The molding aid added in step S4 is polyvinyl butyral (PVB).

[0072] Example 3

[0073] This embodiment provides a lithium manganese iron phosphate, the preparation method of which is basically the same as that of Example 1, except that:

[0074] The particle size of the first slurry in step S1 is D50 = 0.41 μm;

[0075] The particle size of the second slurry in step S2 is D50 = 0.33 μm;

[0076] The particle size of the third slurry in step S3 is D50=0.10 μm.

[0077] Example 4

[0078] This embodiment provides a lithium manganese iron phosphate, the preparation method of which is basically the same as that of Example 1, except that:

[0079] The particle size of the first slurry in step S1 is D50 = 0.58 μm;

[0080] The particle size of the second slurry in step S2 is D50 = 0.56 μm;

[0081] The particle size of the third slurry in step S3 is D50=0.29 μm.

[0082] Example 5

[0083] This embodiment provides a lithium manganese iron phosphate, the preparation method of which is basically the same as that of Example 1, except that:

[0084] The first sintering temperature in step S1 is 550° C. and the sintering time is 8 hours;

[0085] The second sintering temperature in step S3 is 800° C. and the sintering time is 10 h;

[0086] The third sintering temperature in step S4 is 750° C., and the sintering time is 6 hours.

[0087] Example 6

[0088] This embodiment provides a lithium manganese iron phosphate, the preparation method of which is basically the same as that of Example 1, except that:

[0089] The first sintering temperature in step S1 is 350° C. and the sintering time is 5 h;

[0090] The second sintering temperature in step S3 is 650° C. and the sintering time is 6 hours;

[0091] The third sintering temperature in step S4 is 600° C., and the sintering time is 4 hours.

[0092] Example 7

[0093] This embodiment provides a lithium manganese iron phosphate, the preparation method of which is basically the same as that of Example 1, except that:

[0094] The mass ratio of the first precursor material added in step S3 to the second dry powder added is 30:70.

[0095] Example 8

[0096] This embodiment provides a lithium manganese iron phosphate, the preparation method of which is basically the same as that of Example 1, except that:

[0097] The mass ratio of the first precursor material added in step S3 to the second dry powder added is 50:50.

[0098] Example 9

[0099] This embodiment provides a lithium manganese iron phosphate, the preparation method of which is basically the same as that of Example 1, except that:

[0100] In step S4, the mass ratio of the second lithium manganese iron phosphate precursor material, the solid electrolyte powder and the molding aid is 89:8:3.

[0101] Example 10

[0102] This embodiment provides a lithium manganese iron phosphate, the preparation method of which is basically the same as that of Example 1, except that:

[0103] In step S4, the mass ratio of the second lithium manganese iron phosphate precursor material, the solid electrolyte powder and the molding aid is 89:10:1.

[0104] Example 11

[0105] This embodiment provides a lithium manganese iron phosphate, the preparation method of which is basically the same as that of Example 1, except that:

[0106] The solid electrolyte powder added in step S4 is LLZO (Li7La3Zr2O 12 );

[0107] Example 12

[0108] This embodiment provides a lithium manganese iron phosphate, the preparation method of which is basically the same as that of Example 1, except that:

[0109] In step S2, the amount of the second carbon source added is 4% of the mass of the first precursor material added; and in step S3, the amount of the third carbon source added is 4% of the mass of the first precursor material added.

[0110] Example 13

[0111] This embodiment provides a lithium manganese iron phosphate, the preparation method of which is basically the same as that of Example 1, except that:

[0112] In step S2, the amount of the second carbon source added is 8% of the mass of the first precursor material added; and in step S3, the amount of the third carbon source added is 8% of the mass of the first precursor material added.

[0113] Example 14

[0114] This embodiment provides a lithium manganese iron phosphate positive electrode material, and its preparation method is basically the same as that of Example 1, except that:

[0115] In step S1, raw materials are weighed according to a molar ratio of Li, Fe, Mn, P and doping metal elements of 1.05:0.18:0.8:1.04:0.02.

[0116] Example 15

[0117] This embodiment provides a lithium manganese iron phosphate positive electrode material, and its preparation method is basically the same as that of Example 1, except that:

[0118] The particle size of the first slurry in step S1 is D50=0.35 μm.

[0119] Example 16

[0120] This embodiment provides a lithium manganese iron phosphate positive electrode material, and its preparation method is basically the same as that of Example 1, except that:

[0121] The particle size of the first slurry in step S1 is D50=0.64 μm.

[0122] Example 17

[0123] This embodiment provides a lithium manganese iron phosphate positive electrode material, and its preparation method is basically the same as that of Example 1, except that:

[0124] The particle size of the second slurry in step S2 is D50=0.28 μm.

[0125] Example 18

[0126] This embodiment provides a lithium manganese iron phosphate positive electrode material, and its preparation method is basically the same as that of Example 1, except that:

[0127] The particle size of the second slurry in step S2 is D50=0.62 μm.

[0128] Example 19

[0129] This embodiment provides a lithium manganese iron phosphate positive electrode material, and its preparation method is basically the same as that of Example 1, except that:

[0130] The particle size of the third slurry in step S3 is D50=0.33 μm.

[0131] Example 20

[0132] This embodiment provides a lithium manganese iron phosphate positive electrode material, and its preparation method is basically the same as that of Example 1, except that:

[0133] The first sintering time in step S1 is 10 hours;

[0134] The second sintering time in step S3 is 12 hours;

[0135] The third sintering time in step S4 is 8 hours.

[0136] Example 21

[0137] This embodiment provides a lithium manganese iron phosphate positive electrode material, and its preparation method is basically the same as that of Example 1, except that:

[0138] The mass ratio of the first precursor material added in step S3 to the second dry powder added is 20:80.

[0139] Example 22

[0140] This embodiment provides a lithium manganese iron phosphate positive electrode material, and its preparation method is basically the same as that of Example 1, except that:

[0141] The mass ratio of the first precursor material added in step S3 to the second dry powder added is 60:40.

[0142] Example 23

[0143] This embodiment provides a lithium manganese iron phosphate positive electrode material, and its preparation method is basically the same as that of Example 1, except that:

[0144] In step S2, the amount of the second carbon source added is 10% of the mass of the first precursor material added; and in step S3, the amount of the third carbon source added is 10% of the mass of the first precursor material added.

[0145] Comparative Example 1

[0146] This embodiment provides a lithium manganese iron phosphate positive electrode material, and its preparation method is basically the same as that of Example 1, except that:

[0147] The particle size of the third slurry in step S3 is D50=0.09 μm.

[0148] Comparative Example 2

[0149] This embodiment provides a lithium manganese iron phosphate positive electrode material, and its preparation method is basically the same as that of Example 1, except that:

[0150] The first sintering temperature in step S1 is 300°C;

[0151] The second sintering temperature in step S3 is 600°C;

[0152] The third sintering temperature in step S4 is 550°C.

[0153] Comparative Example 3

[0154] This embodiment provides a lithium manganese iron phosphate positive electrode material, and its preparation method is basically the same as that of Example 1, except that:

[0155] The first sintering temperature in step S1 is 600°C;

[0156] The second sintering temperature in step S3 is 850°C;

[0157] The third sintering temperature in step S4 is 800°C.

[0158] Comparative Example 4

[0159] This embodiment provides a lithium manganese iron phosphate positive electrode material, and its preparation method is basically the same as that of Example 1, except that:

[0160] The first sintering time in step S1 is 4 hours;

[0161] The second sintering time in step S3 is 5 hours;

[0162] The third sintering time in step S4 is 3 hours.

[0163] Comparative Example 5

[0164] This embodiment provides a lithium manganese iron phosphate positive electrode material, and its preparation method is basically the same as that of Example 1, except that:

[0165] In step S2, the amount of the second carbon source added is 3% of the mass of the first precursor material added; in step S3, the amount of the third carbon source added is 3% of the mass of the first precursor material added.

[0166] Comparative Example 6

[0167] This comparative example provides a lithium manganese iron phosphate positive electrode material, and its preparation method is basically the same as that of Example 1, except that:

[0168] In step S4, the mass ratio of the second lithium manganese iron phosphate precursor material, the solid electrolyte powder and the molding aid is 89:11:0.

[0169] Table 1 (Step S1)

[0170]

[0171]

[0172]

[0173] Table 2 (steps S2 and S3)

[0174]

[0175]

[0176]

[0177] Table 3 (Step S4)

[0178]

[0179] The compaction density test of the lithium iron manganese phosphate composite positive electrode material of each embodiment and comparative example was performed, and the test method referred to the method in GB / T 24533-2019;

[0180] The lithium manganese iron phosphate composite materials of each embodiment and comparative example were tested for gram capacity, which can refer to the method in T / CIAPS0029-2023, as follows:

[0181] (1) Preparation of button-type lithium-ion batteries

[0182] A1. The lithium iron manganese phosphate composite cathode material and the conductive agent (acetylene black) of each embodiment or comparative example were placed in an oven and baked at 120 ° C for 4h, then transferred to a drying container for cooling; 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 phosphate composite material and the conductive agent (acetylene black) was added to a 50mL small beaker, and then the binder solution (5% mass fraction of polyvinylidene fluoride in N-methylpyrrolidone solution) was added and stirred with a stirrer to form a paste;

[0183] 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;

[0184] 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 1 mol / L LiPF6 solution in ethylene carbonate and ethyl methyl carbonate (EMC) in a volume ratio of 3:7 as the electrolyte.

[0185] (2) Gram capacity test

[0186] 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:

[0187] Gram capacity of lithium iron manganese phosphate composite positive electrode material (mAh / g) = average value of 0.1C discharge specific capacity (unit: mAh) / mass of lithium iron manganese phosphate composite positive electrode material (unit: g).

[0188] Example 1 The gram capacity test results of lithium manganese iron phosphate composite positive electrode material are as follows Figure 1 The specific test results are shown in Table 4.

[0189] Table 4

[0190]

[0191]

[0192] As can be seen from Examples 1-23, the lithium manganese iron phosphate composite positive electrode material prepared by the present invention has a high compaction density and gram capacity, wherein the compaction density reaches 2.20-2.45 g / cm 3 , the gram capacity is as high as 140-160mAh / g.

[0193] It can be seen from Example 1, Examples 3-4, Examples 15-16, Examples 17-18, Example 19 and Comparative Example 1 that the average particle size of the first slurry, the second slurry and the third slurry affects the high compaction density and gram capacity of the lithium manganese iron phosphate composite positive electrode material. Among them, the average particle size of the first slurry can be 0.35-0.64 μm, and more preferably the average particle size of the first slurry is 0.41-0.58 μm; the average particle size of the second slurry can be 0.2-0.7 μm, and more preferably the average particle size of the second slurry is 0.33-0.56 μm; the average particle size of the third slurry can be 0.1-0.33 μm, and more preferably the average particle size of the third slurry is 0.23-0.29 μm.

[0194] It can be seen from Example 1, Examples 5-6, Example 20, and Comparative Examples 2-4 that the temperature and time of the three sinterings affect the high compaction density and gram capacity of the lithium manganese iron phosphate composite positive electrode material, wherein the temperature of the first sintering is 350-550°C, and the time of the first sintering is 4.5-11h; the temperature of the second sintering is 650-800°C, and the time of the second sintering is 6-12h; the temperature of the third sintering is 600-750°C, and the time of the third sintering is 4-8h.

[0195] It can be seen from Examples 1 and 14 that the molar ratios of Li, Fe, Mn, P, and the doping metal element M in the raw materials affect the high compaction density and gram capacity of the lithium manganese iron phosphate composite positive electrode material, wherein the molar amount can be n M , n Li :n Fe :n Mn :n P :n M =(1-1.06):(0.15-0.45):(0.55-0.8):(1-1.05):(0.01-0.03), more preferably (1-1.06):(0.35-0.40):(0.7-0.8):(1-1.05):(0.01-0.03).

[0196] It can be seen from Examples 1, 7-8, and 21-22 that the mass ratio of the first precursor material and the second dry powder in step S3 affects the compaction density and gram capacity of the lithium manganese iron phosphate composite positive electrode material, wherein the mass ratio of the two is (20-60): (40-80), preferably (30-50): (50-70).

[0197] It can be seen from Example 1, Examples 12-13, Example 23 and Comparative Example 5 that the amount of the second carbon source and the third carbon source added affects the compaction density and gram capacity of the lithium manganese iron phosphate composite positive electrode material, wherein the amount of the second carbon source and the third carbon source added can be 4%-10% of the mass of the first precursor material in each step, preferably 5.5%-8%.

[0198] It can be seen from Examples 1 and 9-11 that the mass ratio of the second precursor material, the solid electrolyte solid and the molding aid, as well as the type of electrolyte, have an impact on the compacted density and gram capacity of the lithium manganese iron phosphate composite positive electrode material.

[0199] It can be seen from Example 1 and Comparative Example 6 that adding a molding aid to the system can significantly improve the compaction density and gram capacity of the lithium manganese iron phosphate composite positive electrode material.

[0200] 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 positive electrode material, characterized in that: The steps include: (1) A lithium source, an iron source, a manganese source, a phosphorus source, a doping metal element M source, a first carbon source, and a first solvent are ball-milled to obtain a first slurry; the first slurry is spray-dried to obtain a first dry powder; the first dry powder is sintered for a first time to obtain a first precursor material; the temperature of the first sintering is 350-550° C., and the time of the first sintering is 4.5-11 hours; (2) a portion of the first precursor material, a second carbon source, and a second solvent are ball-milled to obtain a second slurry; the second slurry is spray-dried to obtain a second dry powder; the mass of the second carbon source is 4%-10% of the mass of the first precursor material in step (2); and the average particle size of the second slurry is 0.2-0.7 μm; (3) Part of the first precursor material, a third carbon source, and a third solvent are ball-milled to obtain a third slurry; the third slurry and the second dry powder are sequentially mixed, spray-dried, and sintered for a second time to obtain a second precursor material; the mass of the third carbon source is 4%-10% of the mass of the first precursor material in step (3); the average particle size of the third slurry is 0.1-0.33 μm; the temperature of the second sintering is 650-800° C., and the time of the second sintering is 6-12 h; (4) The second precursor material, solid electrolyte and forming aid are sequentially mixed, granulated, sintered for a third time and crushed to obtain a lithium manganese iron phosphate composite positive electrode material; the temperature of the third sintering is 600-750°C, and the time of the third sintering is 4-8h.

2. The method for preparing the lithium manganese iron phosphate composite positive electrode material according to claim 1, wherein: Include at least one of the following: In step (2), the mass of the second carbon source is 5.5%-8% of the mass of the first precursor material in step (2); In step (3), the mass of the third carbon source is 5.5%-8% of the mass of the first precursor material in step (3).

3. The method for preparing the lithium manganese iron phosphate composite positive electrode material according to claim 1, wherein: Include at least one of the following: In step (1), the temperature of the first sintering is 450-550° C., and the time of the first sintering is 5-10 hours; In step (3), the temperature of the second sintering is 730-750° C., and the time of the second sintering is 6-8 hours; In step (4), the temperature of the third sintering is 680-700° C., and the time of the third sintering is 4-6 hours.

4. The method for preparing the lithium manganese iron phosphate composite positive electrode material according to claim 1, wherein: Include at least one of the following: In step (1), based on the molar amount n of the Li element in the lithium source Li , the molar amount of Fe element in the iron source n Fe , the molar amount of Mn element in the manganese source n Mn , the molar amount of P element in the phosphorus source n P and the molar amount n of the metal element M in the doping metal element M M , n Li :n Fe :n Mn :n P :n M =(1~1.06):(0.15~0.45):(0.55~0.8):(1~1.05):(0.01~0.03); In step (1), the metal element M in the doping metal element M source includes at least one of Ti, Mg, Al, V, and Nb.

5. The method for preparing the lithium manganese iron phosphate composite positive electrode material according to claim 1, wherein: In step (4), the molding aid includes at least one of phenolic resin, epoxy resin, polyvinyl alcohol, polyvinyl butyral, polyacrylic acid, polyvinyl pyrrolidone, polyethylene, polyethylene oxide, polypropylene, dextrin, carboxymethyl cellulose, carboxymethyl cellulose, hydroxypropyl methylcellulose, magnesium stearate, ascorbic acid, di-tert-butyl paracresol, and styrene-butadiene rubber.

6. The method for preparing the lithium manganese iron phosphate composite positive electrode material according to claim 1, wherein: In step (4), the solid electrolyte includes at least one of lithium aluminum titanium phosphate, lithium aluminum germanium phosphate, lithium lanthanum titanate, and lithium lanthanum zirconium oxide.

7. The method for preparing the lithium manganese iron phosphate composite positive electrode material according to claim 1, wherein: In step (1), the solid content of the first slurry is 30%-50%; In step (1), the average particle size of the first slurry is 0.35-0.64 μm; In step (2), the solid content of the second slurry is 40%-60%; In step (2), the average particle size of the second slurry is 0.33-0.56 μm; In steps (2) and (3), based on the total mass of the first precursor material in step (2) and the first precursor material in step (3), the mass percentage of the first precursor material in step (2) is 50%-70%; In step (3), the mass ratio of the first precursor material to the second dry powder is (20-60): (40-80); In step (3), the solid content of the third slurry is 40%-60%; In step (3), the average particle size of the third slurry is 0.23-0.29 μm; In step (4), the mass ratio of the second precursor material, the solid electrolyte and the forming aid is 89: (8-10): (1-3); In step (4), the material obtained after granulation has an average thickness of 2-4 mm and an average diameter of 6-10 mm.

8. The method for preparing the lithium manganese iron phosphate composite positive electrode material according to claim 1, wherein: Include at least one of the following: In step (1), the lithium source includes one or more of lithium hydroxide, lithium carbonate, lithium dihydrogen phosphate, lithium oxalate, lithium acetate, and lithium phosphate; In step (1), the iron source includes one or more of ferric phosphate, ferrous oxalate, ferric oxide, ferric oxide, and ferric acetate; In step (1), the manganese source includes one or more of manganese oxalate, manganese carbonate, manganese acetate, manganese dioxide, and manganese tetraoxide; In step (1), the phosphorus source includes one or more of ammonium phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and phosphoric acid; In step (1), the metal element M in the doping metal element M source includes at least one of Ti, Mg, Al, V, and Nb; In step (1), 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; In steps (1)-(3), the first carbon source, the second carbon source and the third carbon source are each independently selected from at least one of starch, sucrose, glucose, citric acid, PEG, PVP, polypropylene, conductive carbon black, acetylene black, carbon nanotubes and graphene; In steps (1)-(3), the first solvent, the second solvent and the third solvent are each independently selected from at least one of water and ethanol; Based on the total mass of the lithium source, the iron source, the manganese source, the phosphorus source and the doping metal element M source, the mass percentage of the first carbon source is 1%-3.5%.

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

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

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