A lithium manganese iron phosphate composite material and its preparation method and application

Abstract

The present invention discloses a lithium manganese iron phosphate composite material and its preparation method and application, the preparation method comprises the following steps: S1. mixing a first lithium source, a first iron source, a first manganese source, a first phosphorus source, a first doped metal source, a first carbon source and a first solvent to obtain a first mixed slurry, spray drying, and performing a first sintering to obtain a seed crystal; S2. mixing a seed crystal, a second lithium source, a second iron source, a second manganese source, a second phosphorus source, a second doped metal source, a second carbon source and a second solvent to obtain a second mixed slurry, spray drying, and performing a second sintering to obtain a lithium manganese iron phosphate composite material; the iron source is a +3 valent iron source, and the manganese source is a non-+2 valent manganese source; the ratio of the sum of the amount of substance of the iron element in the first iron source and the manganese element in the first manganese source to the mass of the first carbon source is 1 mol: (3.0 7.5) g. The use of the first carbon source of the present invention is conducive to improving the gram capacity of the lithium manganese iron phosphate composite material.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium battery positive electrode materials, and more specifically, to a lithium manganese iron phosphate composite material and a preparation method and application thereof. Background Art

[0002] Lithium manganese iron phosphate (LiMn x Fe 1-x Lithium iron phosphate (LIFP) is a novel phosphate-based cathode material for lithium-ion batteries, prepared from lithium iron phosphate (LiFePO4, LFP). It is also an olivine-type cathode material. Lithium manganese iron phosphate (LMFP) has a similar crystal structure to LFP and boasts stable chemical properties and excellent safety performance. The addition of manganese increases the material's charging voltage from 3.4V (of LFP) to 4.1V, boosting the battery's energy density by 15-20%, further extending its battery life. Therefore, LMFP offers superior safety performance to ternary materials and a higher energy density than LFP, making it a promising candidate for future cathode materials. However, the specific capacity of LFP is currently relatively low and needs to be improved.

[0003] When preparing and synthesizing lithium manganese iron phosphate, its raw materials are commonly +2-valent iron source and +2-valent manganese source. However, the sintering atmosphere must be strictly controlled during the sintering process. Otherwise, the +2-valent iron source is easily oxidized, generating other impurities, seriously affecting the performance of the finished product. Using a +3-valent iron source instead of a +2-valent iron source makes it easier to control the sintering atmosphere. Therefore, it is necessary to use a +3-valent iron source instead of a +2-valent iron source to synthesize lithium manganese iron phosphate. The valence of manganese source is +2, +3, +4, +5, +6 and +7. Using only a +2-valent manganese source is not conducive to broadening the raw material selection of lithium manganese iron phosphate.

[0004] Therefore, it is of great economic value to develop a preparation method of a lithium manganese iron phosphate composite material that utilizes a +3 valent iron source and a non-+2 valent manganese source and can increase the specific capacity of lithium manganese iron phosphate. Summary of the Invention

[0005] The purpose of the present invention is to solve the shortcomings of the prior art and to provide a lithium manganese iron phosphate composite material and a preparation method and application thereof.

[0006] To achieve the above object, the technical solution adopted by the present invention is:

[0007] In a first aspect, the present invention provides a method for preparing a lithium manganese iron phosphate composite material, the preparation method comprising the following steps:

[0008] S1. mixing a first lithium source, a first iron source, a first manganese source, a first phosphorus source, a first doping metal source, a first carbon source and a first solvent to obtain a first mixed slurry, spray drying, and performing a first sintering to obtain a seed crystal;

[0009] S2. Mixing the seed crystals obtained in step S1, a second lithium source, a second iron source, a second manganese source, a second phosphorus source, a second doping metal source, a second carbon source, and a second solvent to obtain a second mixed slurry, spray drying, and performing a second sintering to obtain a lithium manganese iron phosphate composite material;

[0010] Wherein, the first iron source and the second iron source are both +3 valent iron sources, and the first manganese source and the second manganese source are both non-+2 valent manganese sources;

[0011] In step S1, the mass ratio of the sum of the amount of the iron element in the first iron source and the manganese element in the first manganese source to the mass of the first carbon source is 1 mol: (3.0-7.5) g.

[0012] In the present invention, the first carbon source can not only reduce the +3 valent iron source and the non-+2 valent manganese source to divalent iron (Fe 2+ ) and divalent manganese (Mn 2+ ), promoting Fe 2+ and Mn 2+ It enters the crystal lattice to form the crystal structure of lithium manganese iron phosphate, obtaining a seed crystal with high uniformity, consistency and crystallinity, which is beneficial to improving the gram capacity of the lithium manganese iron phosphate composite material prepared using the seed crystal; the first carbon source can also decompose to form carbon during the sintering process, and evenly wrap around the seed crystal, thereby enhancing the uniformity and density of the lithium manganese iron phosphate composite material prepared using the seed crystal, thereby improving the gram capacity of the lithium manganese iron phosphate composite material.

[0013] In the present invention, when the mass ratio of the sum of the amount of iron in the first iron source and the amount of manganese in the first manganese source to the mass of the first carbon source is too high (i.e., the amount of the first carbon source is too small), the +3 valent iron source and the non-+2 valent manganese source cannot be fully reduced to divalent iron (Fe 2+ ) and divalent manganese (Mn 2+ ), can not better promote Fe 2+ and Mn 2+ The carbon cannot be uniformly wrapped around the crystal seed to obtain a seed with high uniformity, consistency and crystallinity, and the carbon cannot be uniformly wrapped around the seed, which ultimately leads to a significant decrease in the gram capacity of the manganese iron phosphate lithium composite material. When the ratio of the sum of the amount of iron in the first iron source and the amount of manganese in the first manganese source to the mass of the first carbon source is too small (that is, the amount of the first carbon source is too much), the +3 valent iron source and the non-+2 valent manganese source will be excessively reduced to iron and manganese, hindering the Fe 2+ and Mn 2+It enters the crystal lattice to form the crystal structure of lithium manganese iron phosphate, and at the same time interferes with the formation of crystal seeds, which has an adverse effect on the uniformity, consistency and crystallinity of the crystal seeds, thereby destroying the stability, uniformity and density of the lithium manganese iron phosphate composite material prepared using the crystal seeds, and ultimately leads to a significant decrease in the gram capacity of the lithium manganese iron phosphate composite material.

[0014] Preferably, the mass ratio of the sum of the amounts of the iron element in the first iron source and the manganese element in the first manganese source to the mass of the first carbon source is 1 mol:(3.0-7.1) g.

[0015] More preferably, the mass ratio of the sum of the amount of the iron element in the first iron source and the manganese element in the first manganese source to the mass of the first carbon source is 1 mol:(3.09-7.01) g.

[0016] Preferably, in step S1, the first carbon source is starch, polypropylene and acetylene black in a mass ratio of 1:(0.4-1.0):(0.6-1.0).

[0017] The reason why the present invention selects starch, polypropylene and acetylene black as the first carbon source and controls the mass ratio of starch, polypropylene and acetylene black to be 1: (0.4-1.0): (0.6-1.0) is that the carbon formed by the decomposition of starch, polypropylene and acetylene black in a suitable mass ratio is not only conducive to the reduction of the +3 valent iron source and the non-+2 valent manganese source, but also to the reduction of Fe 2+ and Mn 2+ It can enter the crystal lattice to form the crystal structure of lithium manganese iron phosphate, and can also form a uniform, continuous and highly graphitized carbon layer on the surface of the seed crystal, thereby enhancing the uniformity and density of the lithium manganese iron phosphate composite material prepared using the seed crystal, thereby increasing the gram capacity of the lithium manganese iron phosphate composite material.

[0018] Starch, polypropylene and acetylene black commonly used in the art can be used in the present invention.

[0019] Preferably, in step S1, the temperature of the first sintering is 300-750°C.

[0020] More preferably, in step S1, the temperature of the first sintering is 350-650°C.

[0021] In the present invention, when the temperature of the first sintering is too high, after the second sintering is completed, the grains of the lithium iron manganese phosphate composite material obtained are likely to be too large, resulting in a longer transmission path for lithium ions, and the gram capacity of the lithium iron manganese phosphate composite material cannot be better improved; at the same time, when the temperature of the first sintering is too high, the crystal structure of the seed crystal is easily damaged, such as lattice distortion and unnecessary phase change, and it is impossible to obtain a seed crystal with higher uniformity, consistency and crystallinity, and the gram capacity of the lithium iron manganese phosphate composite material cannot be better improved. When the temperature of the first sintering is too low, the first carbon source cannot be better decomposed to form carbon uniformly wrapped around the seed crystal, and the uniformity and density of the lithium iron manganese phosphate composite material prepared using the seed crystal cannot be better enhanced, thereby failing to better improve the gram capacity of the lithium iron manganese phosphate composite material.

[0022] More preferably, in step S1, the temperature of the first sintering is 550-650°C.

[0023] Preferably, in step S1, the first sintering time is 3-15 hours.

[0024] Preferably, in step S1, the first sintering is performed under the protection of an inert gas.

[0025] More preferably, the inert gas is at least one of nitrogen, helium, neon and argon.

[0026] Preferably, in step S1, the molar ratio of the lithium element in the first lithium source, the iron element in the first iron source, the manganese element in the first manganese source, the phosphorus element in the first phosphorus source and the doping metal element in the first doping metal source is (1.00-1.06):(0.35-0.45):(0.55-0.65):(1.00-1.05):(0.01-0.04), and the molar ratio of the total molar amount of the iron element in the first iron source, the manganese element in the first manganese source and the doping metal element in the first doping metal source to the lithium element in the first lithium source is 1 mol:(1.00-1.06) mol.

[0027] Preferably, in step S1, the ratio of the total mass of the first lithium source, the first iron source, the first manganese source, the first phosphorus source, and the first doping metal source to the mass of the first solvent is (1-6):(5-9).

[0028] Preferably, in step S1, the D50 particle size of the first mixed slurry is ≤0.6 μm.

[0029] Because the present invention uses a solid-phase method to prepare seed crystals, the consistency and stability of the first mixed slurry will affect the performance of the lithium manganese iron phosphate composite material. The first lithium source, first iron source, first manganese source, first phosphorus source, first doping metal source, and first carbon source used in the present invention have high material density and specific gravity. If the D50 particle size of the first mixed slurry is too large, the consistency and stability of the first mixed slurry will be reduced, resulting in sedimentation problems.

[0030] Preferably, in step S2, the molar ratio of the lithium element in the second lithium source, the iron element in the second iron source, the manganese element in the second manganese source, the phosphorus element in the second phosphorus source and the doping metal element in the second doping metal source is (1.00-1.06):(0.35-0.45):(0.55-0.65):(1.00-1.05):(0.01-0.04), and the molar ratio of the total molar amount of the iron element in the first iron source, the manganese element in the first manganese source and the doping metal element in the first doping metal source to the lithium element in the first lithium source is 1 mol:(1.00-1.06) mol.

[0031] Preferably, the molar ratio of the lithium element in the first lithium source to the lithium element in the second lithium source is 1:(0.1-9).

[0032] More preferably, the molar ratio of the lithium element in the first lithium source to the lithium element in the second lithium source is 1:(0.11-9).

[0033] Preferably, in step S2, the mass ratio of the sum of the amounts of iron in the second iron source and manganese in the second manganese source to the mass of the second carbon source is 1 mol:(3.0-14.0) g.

[0034] More preferably, in step S2, the mass ratio of the sum of the amount of iron in the second iron source and the amount of manganese in the second manganese source to the mass of the second carbon source is 1 mol:(3.2-13.9) g.

[0035] Preferably, in step S2, the ratio of the total mass of the second lithium source, the second iron source, the second manganese source, the second phosphorus source, and the second doping metal source to the mass of the second solvent is (1-6):(5-9).

[0036] Preferably, in step S2, the temperature of the second sintering is 600-850°C.

[0037] Preferably, in step S2, the second sintering time is 3-15 hours.

[0038] Preferably, in step S2, the second sintering is performed under the protection of an inert gas.

[0039] More preferably, the inert gas is at least one of nitrogen, helium, neon and argon.

[0040] Preferably, in step S2, the D50 particle size of the second mixed slurry is ≤0.9 μm.

[0041] In the present invention, the D50 particle size of the first mixed slurry and the second mixed slurry is measured by a Malvern Panalytical laser particle size analyzer Mastersizer 3000.

[0042] Preferably, the first lithium source or the second lithium source is at least one of lithium carbonate, lithium hydroxide, lithium dihydrogen phosphate, lithium nitrate, lithium oxalate, lithium acetate, and lithium phosphate.

[0043] Preferably, the first phosphorus source or the second phosphorus source is at least one of ammonium phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, phosphoric acid, and lithium dihydrogen phosphate.

[0044] Preferably, in step S2, the second carbon source is starch, polypropylene and acetylene black in a mass ratio of 1:(0.2-0.4):(1.0-1.8).

[0045] Preferably, the first solvent or the second solvent is water.

[0046] Preferably, the +3 valent iron source is at least one of ferric phosphate, ferric oxide, ferric hydroxide, and ferric acetate.

[0047] Preferably, the non-+2 valent manganese source is at least one of manganese dioxide, dimanganese trioxide, and trimanganese tetraoxide.

[0048] In the present invention, the doping metal element in the first doping metal source includes but is not limited to at least one of Ti, Mg, Al, Ni, and Nb.

[0049] Preferably, the first doping metal source or the second doping metal source is a magnesium source.

[0050] More preferably, the magnesium source is at least one of magnesium oxide, magnesium hydrogen phosphate, magnesium hydroxide, and magnesium carbonate.

[0051] In a second aspect, the present invention provides a lithium manganese iron phosphate composite material prepared by the preparation method described in the first aspect.

[0052] In a third aspect, the present invention provides an application of a lithium iron manganese phosphate composite material in a lithium ion battery.

[0053] Compared with the prior art, the present invention has the following beneficial effects:

[0054] In the present invention, the first carbon source can not only reduce the +3 valent iron source and the non-+2 valent manganese source to divalent iron (Fe 2+ ) and divalent manganese (Mn 2+ ), promoting Fe 2+ and Mn 2+ It enters the crystal lattice to form the crystal structure of lithium manganese iron phosphate, obtaining a seed crystal with high uniformity, consistency and crystallinity, which is beneficial to improving the gram capacity of the lithium manganese iron phosphate composite material prepared using the seed crystal; the first carbon source can also decompose to form carbon during the sintering process, and evenly wrap around the seed crystal, thereby enhancing the uniformity and density of the lithium manganese iron phosphate composite material prepared using the seed crystal, thereby improving the gram capacity of the lithium manganese iron phosphate composite material.

[0055] The reason why the present invention selects starch, polypropylene and acetylene black as the first carbon source and controls the mass ratio of starch, polypropylene and acetylene black to be 1: (0.4-1.0): (0.6-1.0) is that the carbon formed by the decomposition of starch, polypropylene and acetylene black in a suitable mass ratio is not only conducive to the reduction of the +3 valent iron source and the non-+2 valent manganese source, but also to the reduction of Fe 2+ and Mn 2+ It can enter the crystal lattice to form the crystal structure of lithium manganese iron phosphate, and can also form a uniform, continuous and highly graphitized carbon layer on the surface of the seed crystal, thereby enhancing the uniformity and density of the lithium manganese iron phosphate composite material prepared using the seed crystal, thereby increasing the gram capacity of the lithium manganese iron phosphate composite material. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] Figure 1 This is a charge and discharge curve diagram of a button-type lithium-ion battery prepared using the lithium manganese iron phosphate composite material in Example 1. DETAILED DESCRIPTION

[0057] In order to better illustrate the purpose, technical solutions and advantages of the present invention, the present invention will be further described below in conjunction with specific embodiments.

[0058] The experimental methods in the following examples and comparative examples where specific conditions are not specified are generally based on conventional conditions in the art or conditions recommended by the manufacturers; the raw materials and reagents used, unless otherwise specified, are all raw materials and reagents that can be obtained from commercial channels such as conventional markets.

[0059] In the various embodiments and comparative examples of the present invention, the use of reagents is as follows:

[0060] Starch, S818265-5kg, Maclean;

[0061] Polypropylene, P816225-5kg, McLean;

[0062] Acetylene black, C742510-5g, Aladdin;

[0063] Sucrose, S818046-5kg, McLean;

[0064] carbon nanotubes, C805971-5g, MacLean;

[0065] Graphene, G762015-25g, MacLean.

[0066] In the present invention, the D50 particle size of the first mixed slurry and the second mixed slurry is measured by a Malvern Panalytical laser particle size analyzer Mastersizer 3000.

[0067] Example 1

[0068] This embodiment provides a lithium manganese iron phosphate composite material, the preparation method of which includes the following steps:

[0069] S1. 4.50 mol of a first lithium source Li2CO3 (332.5 g), 1.665 mol of a first iron source Fe2O3 (265.9 g), 5.4 mol of a first manganese source MnO2 (469.5 g), 9.00 mol of a first phosphorus source NH4H2PO4 (1035.3 g), 0.27 mol of a first doping metal source MgO (10.9 g), 43.9 g of a first carbon source and 2100 g of a first solvent of pure water were added to a sand mill containing 2 kg of 0.6-0.8 mm zirconium beads and ground for 5 h to obtain a first mixed slurry, at which point the D50 particle size of the first mixed slurry was 0.5-0.6 μm, spray dried (the inlet air temperature of the sprayer was set to 220 ° C, and the outlet air temperature was set to 100 ± 5 ° C), sintered in a box furnace at 500 ° C for 8 h under nitrogen protection, and cooled to obtain a seed crystal;

[0070] S2. The seed crystals obtained in step S1, 0.50 mol of the second lithium source Li2CO3 (36.95 g), 0.185 mol of the second iron source Fe2O3 (29.55 g), 0.60 mol of the second manganese source MnO2 (52.16 g), 1.00 mol of the second phosphorus source NH4H2PO4 (115.03 g), 0.03 mol of the second doping metal source MgO (1.21 g), 7.6 g of the second carbon source and 1700 g of the second solvent pure water were added to a sand mill containing 2 kg of 0.4-0.6 mm zirconium beads and ground for 2 h to obtain a second mixed slurry, at which time the second mixed slurry had a D50 particle size of 0.8-0.9 μm, spray dried (the inlet air temperature of the sprayer was set to 220 ° C, and the outlet air temperature was set to 90 ± 5 ° C), sintered in a box furnace at 730 ° C under nitrogen protection for 8 h for a second sintering, and cooled to obtain a lithium manganese iron phosphate composite material;

[0071] in:

[0072] In step S1, the mass ratio of the sum of the amount of iron in the first iron source and the amount of manganese in the first manganese source to the mass of the first carbon source is 1 mol:5.03 g;

[0073] In step S1, the first carbon source is starch, polypropylene and acetylene black in a mass ratio of 1:0.7:0.8;

[0074] In step S1, the molar ratio of the lithium element in the first lithium source, the iron element in the first iron source, the manganese element in the first manganese source, the phosphorus element in the first phosphorus source, and the doping metal element in the first doping metal source is 1:0.37:0.6:1:0.03, and the molar ratio of the total molar amount of the iron element in the first iron source, the manganese element in the first manganese source, and the doping metal element in the first doping metal source to the lithium element in the first lithium source is 1 mol:1 mol;

[0075] In step S1, the ratio of the total mass of the first lithium source, the first iron source, the first manganese source, the first phosphorus source, and the first doping metal source to the mass of the first solvent is 5.03:5;

[0076] In step S2, the mass ratio of the sum of the amount of the iron element in the second iron source and the manganese element in the second manganese source to the mass of the second carbon source is 1 mol:7.835 g;

[0077] In step S2, the second carbon source is starch, polypropylene and acetylene black in a mass ratio of 1:0.3:1.4;

[0078] In step S2, the molar ratio of the lithium element in the second lithium source, the iron element in the second iron source, the manganese element in the second manganese source, the phosphorus element in the second phosphorus source, and the doping metal element in the second doping metal source is 1:0.37:0.6:1:0.03, and the molar ratio of the total molar amount of the iron element in the second iron source, the manganese element in the second manganese source, and the doping metal element in the second doping metal source to the lithium element in the second lithium source is 1 mol:(1.00-1.06) mol;

[0079] In step S2, the ratio of the total mass of the second lithium source, the second iron source, the second manganese source, the second phosphorus source, and the second doping metal source to the mass of the second solvent is 1:7.24;

[0080] The molar ratio of the lithium element in the first lithium source to the lithium element in the second lithium source is 1:0.11.

[0081] Examples 2-3 and Comparative Examples 1-2

[0082] Examples 2-3 and Comparative Examples 1-2 provide different lithium manganese iron phosphate composite materials, which differ from Example 1 in that the amount of the first carbon source is different, and the rest are consistent with Example 1, as shown in the following table:

[0083] Table 1 Amount of the first carbon source in Examples 1-3 and Comparative Examples 1-2

[0084]

[0085] Examples 4-7 and Comparative Examples 3-6

[0086] Examples 4-7 and Comparative Examples 3-6 provide different lithium manganese iron phosphate composite materials, which differ from Example 1 in that the mass ratios of starch, polypropylene, and acetylene black in the first carbon source are different. The rest are consistent with Example 1, as shown in the following table:

[0087] Table 2 Mass ratio of starch, polypropylene and acetylene black in the first carbon source in Examples 1, 4-7 and Comparative Examples 3-6

[0088]

[0089] Examples 8-9 and Comparative Examples 7-8

[0090] Examples 8-9 and Comparative Examples 7-8 provide different lithium manganese iron phosphate composite materials, which differ from Example 1 in that the temperature of the first sintering in step S1 is different, and the rest are consistent with Example 1, as shown in the following table:

[0091] Table 3 The first sintering temperature in step S1 of Examples 1, 8-9 and Comparative Examples 7-8

[0092] The temperature of the first sintering in step S1 / °C Example 1 550 Example 8 350 Example 9 650 Comparative Example 7 750 Comparative Example 8 300

[0093] Comparative Example 9

[0094] This comparative example provides a lithium manganese iron phosphate composite material, which differs from Example 1 in that sucrose is used instead of starch, and the rest is consistent with Example 1.

[0095] Comparative Example 10

[0096] This comparative example provides a lithium manganese iron phosphate composite material, which differs from Example 1 in that carbon nanotubes are used instead of acetylene black, and the rest is consistent with Example 1.

[0097] Comparative Example 11

[0098] This comparative example provides a lithium manganese iron phosphate composite material, which differs from Example 1 in that graphene is used instead of acetylene black, and the rest is consistent with Example 1.

[0099] Performance Testing

[0100] The performance tests of the lithium manganese iron phosphate composite materials of each embodiment and comparative example are as follows:

[0101] (1) Preparation of button-type lithium-ion batteries

[0102] 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;

[0103] 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;

[0104] 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.

[0105] (2) Gram capacity test

[0106] 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:

[0107] Gram capacity of lithium iron phosphate composite material (mAh / g) = average value of 0.1C discharge specific capacity (unit: mAh) / mass of lithium iron phosphate composite material (unit: g);

[0108] The experimental results are shown in the following table:

[0109] Table 4 Gram capacity of lithium manganese iron phosphate composite materials of various embodiments and comparative examples

[0110]

[0111]

[0112] Figure 1 This is a charge and discharge curve diagram of a button-type lithium-ion battery prepared using the lithium manganese iron phosphate composite material in Example 1.

[0113] From Table 4 and Figure 1 It can be seen that in the present invention, the first carbon source can not only reduce the +3 valent iron source and the non-+2 valent manganese source to divalent iron (Fe 2+ ) and divalent manganese (Mn 2+ ) and promote Fe 2+ and Mn 2+ It enters the crystal lattice to form the crystal structure of lithium manganese iron phosphate, obtaining a seed crystal with high uniformity, consistency and crystallinity, which is beneficial to improving the gram capacity of the lithium manganese iron phosphate composite material prepared using the seed crystal; the first carbon source can also decompose to form carbon during the sintering process, and evenly wrap around the seed crystal, thereby enhancing the uniformity and density of the lithium manganese iron phosphate composite material prepared using the seed crystal, thereby improving the gram capacity of the lithium manganese iron phosphate composite material.

[0114] Specifically, by comparing Examples 1-3 and Comparative Examples 1-3, it can be seen that in the present invention, when the ratio of the sum of the amount of the iron element in the first iron source and the manganese element in the first manganese source to the mass of the first carbon source is too high (i.e., the amount of the first carbon source is too small), the +3 valent iron source and the non-+2 valent manganese source cannot be fully reduced to divalent iron (Fe 2+ ) and divalent manganese (Mn 2+ ), can not better promote Fe 2+ and Mn 2+ The carbon cannot be uniformly wrapped around the crystal seed to obtain a seed with high uniformity, consistency and crystallinity, and the carbon cannot be uniformly wrapped around the seed, which ultimately leads to a significant decrease in the gram capacity of the manganese iron phosphate lithium composite material. When the ratio of the sum of the amount of iron in the first iron source and the amount of manganese in the first manganese source to the mass of the first carbon source is too small (that is, the amount of the first carbon source is too much), the +3 valent iron source and the non-+2 valent manganese source will be excessively reduced to iron and manganese, hindering the Fe 2+ and Mn 2+ It enters the crystal lattice to form the crystal structure of lithium manganese iron phosphate, and at the same time interferes with the formation of crystal seeds, which has an adverse effect on the uniformity, consistency and crystallinity of the crystal seeds, thereby destroying the stability, uniformity and density of the lithium manganese iron phosphate composite material prepared using the crystal seeds, and ultimately leads to a significant decrease in the gram capacity of the lithium manganese iron phosphate composite material.

[0115] By comparing Examples 1, 4-7 and Comparative Examples 3-9, it can be seen that the reason why the present invention selects starch, polypropylene and acetylene black as the first carbon source and controls the mass ratio of starch, polypropylene and acetylene black to be 1: (0.4-1.0): (0.6-1.0) is that the carbon formed by the decomposition of starch, polypropylene and acetylene black in an appropriate mass ratio is not only conducive to the reduction of the +3 valent iron source and the non-+2 valent manganese source, but also to the reduction of Fe 2+ and Mn 2+ It can enter the crystal lattice to form the crystal structure of lithium manganese iron phosphate, and can also form a uniform, continuous and highly graphitized carbon layer on the surface of the seed crystal, thereby enhancing the uniformity and density of the lithium manganese iron phosphate composite material prepared using the seed crystal, thereby increasing the gram capacity of the lithium manganese iron phosphate composite material.

[0116] By comparing Examples 1 and 8-11, it can be seen that in the present invention, when the temperature of the first sintering is too high, after the second sintering is completed, the grains of the lithium iron manganese phosphate composite material obtained are likely to be too large, resulting in a longer transmission path for lithium ions, and the gram capacity of the lithium iron manganese phosphate composite material cannot be better improved; at the same time, when the temperature of the first sintering is too high, the crystal structure of the seed crystal is easily damaged, such as lattice distortion and unnecessary phase change, and it is impossible to obtain a seed crystal with higher uniformity, consistency and crystallinity, and the gram capacity of the lithium iron manganese phosphate composite material cannot be better improved. When the temperature of the first sintering is too low, the first carbon source cannot be better decomposed to form carbon uniformly wrapped around the seed crystal, and the uniformity and density of the lithium iron manganese phosphate composite material prepared using the seed crystal cannot be better enhanced, thereby failing to better improve the gram capacity of the lithium iron manganese phosphate composite material.

[0117] 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 preparation method comprises the following steps: S1. mixing a first lithium source, a first iron source, a first manganese source, a first phosphorus source, a first doping metal source, a first carbon source and a first solvent to obtain a first mixed slurry, spray drying, and performing a first sintering to obtain a seed crystal; S2. Mix the seed crystals obtained in step S1, the second lithium source, the second iron source, the second manganese source, the second phosphorus source, the second doping metal source, the second carbon source, and the second solvent to obtain a second mixed slurry, spray dry, and perform a second sintering to obtain a lithium manganese iron phosphate composite material; Wherein, the first iron source and the second iron source are both +3 valent iron sources, and the first manganese source and the second manganese source are both non-+2 valent manganese sources; In step S1, the mass ratio of the total amount of iron in the first iron source and the manganese in the first manganese source to the first carbon source is 1 mol:(3.0-7.5) g, and the first carbon source is starch, polypropylene and acetylene black in a mass ratio of 1:(0.4-1.0):(0.6-1.0).

2. The method for preparing the lithium manganese iron phosphate composite material according to claim 1, wherein: In step S1, the temperature of the first sintering is 350-650°C.

3. The method for preparing the lithium manganese iron phosphate composite material according to claim 1, wherein: Include at least one of the following (1)-(4): (1) In step S1, the first sintering time is 3-15 hours; (2) In step S1, the molar ratio of the lithium element in the first lithium source, the iron element in the first iron source, the manganese element in the first manganese source, the phosphorus element in the first phosphorus source, and the doping metal element in the first doping metal source is (1.00-1.06):(0.35-0.45):(0.55-0.65):(1.00-1.05):(0.01-0.04), and the molar ratio of the total molar amount of the iron element in the first iron source, the manganese element in the first manganese source, and the doping metal element in the first doping metal source to the lithium element in the first lithium source is 1 mol:(1.00-1.06) mol; (3) In step S1, the ratio of the total mass of the first lithium source, the first iron source, the first manganese source, the first phosphorus source, and the first doping metal source to the mass of the first solvent is (1-6):(5-9); (4) In step S1, the D50 particle size of the first mixed slurry is ≤0.6 μm.

4. The method for preparing the lithium manganese iron phosphate composite material according to claim 1, wherein: Include at least one of the following (1)-(7): (1) In step S2, the mass ratio of the sum of the amount of iron in the second iron source and the amount of manganese in the second manganese source to the mass of the second carbon source is 1 mol: (3.0-14.0) g; (2) In step S2, the second carbon source is starch, polypropylene and acetylene black in a mass ratio of 1:(0.2-0.4):(1.0-1.8); (3) In step S2, the temperature of the second sintering is 600-850°C; (4) In step S2, the second sintering time is 3-15 hours; (5) In step S2, the molar ratio of the lithium element in the second lithium source, the iron element in the second iron source, the manganese element in the second manganese source, the phosphorus element in the second phosphorus source, and the doping metal element in the second doping metal source is (1.00-1.06):(0.35-0.45):(0.55-0.65):(1.00-1.05):(0.01-0.04), and the molar ratio of the total molar amount of the iron element in the first iron source, the manganese element in the first manganese source, and the doping metal element in the first doping metal source to the lithium element in the first lithium source is 1 mol:(1.00-1.06) mol; (6) In step S2, the ratio of the total mass of the second lithium source, the second iron source, the second manganese source, the second phosphorus source, and the second doping metal source to the mass of the second solvent is (1-6):(5-9); (7) In step S2, the D50 particle size of the second mixed slurry is ≤0.9 μm.

5. The method for preparing the lithium manganese iron phosphate composite material according to claim 1, wherein: The molar ratio of the lithium element in the first lithium source to the lithium element in the second lithium source is 1:(0.1-9).

6. The method for preparing the lithium manganese iron phosphate composite material according to claim 1, wherein: Include at least one of the following (1)-(7): (1) The first lithium source or the second lithium source is at least one of lithium carbonate, lithium hydroxide, lithium dihydrogen phosphate, lithium nitrate, lithium oxalate, lithium acetate, and lithium phosphate; (2) The first phosphorus source or the second phosphorus source is at least one of ammonium phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, phosphoric acid, and lithium dihydrogen phosphate; (3) The first solvent or the second solvent is water; (4) The +3 valent iron source is at least one of ferric phosphate, ferric oxide, ferric hydroxide, and ferric acetate; (5) The non-+2 manganese source is at least one of manganese dioxide, manganese trioxide, and manganese tetraoxide; (6) The first doping metal source or the second doping metal source is a magnesium source.

7. The method for preparing the lithium iron manganese phosphate composite material according to claim 6, wherein: The magnesium source is at least one of magnesium oxide, magnesium hydrogen phosphate, magnesium hydroxide and magnesium carbonate.

8. A lithium manganese iron phosphate composite material, characterized in that: The method is prepared according to any one of claims 1 to 7.

9. Use of the lithium manganese iron phosphate composite material according to claim 8 in lithium ion batteries.

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