Composite lithium manganese iron phosphate positive electrode material as well as preparation method and application thereof

Abstract

The invention provides a composite lithium manganese iron phosphate positive electrode material and a preparation method and application thereof, the preparation method comprises the following steps: (1) mixing a lithium source, an iron source, a manganese source, a phosphorus source, a fluxing agent, a dispersing agent and a solvent to obtain mixed slurry, spray-drying the mixed slurry, and calcining to obtain a lithium manganese iron phosphate intermediate; and (2) mixing the lithium iron manganese phosphate intermediate, a lithium source, a dispersing agent and a solvent to obtain mixed slurry, performing spray drying on the mixed slurry, and performing calcination treatment to obtain the composite lithium iron manganese phosphate positive electrode material. According to the method, the lithium iron manganese phosphate positive electrode material is prepared in a sectional manner, and the lithium iron manganese phosphate positive electrode material which is compact in structure, good in structural stability, high in grain strength, uniform in particle size distribution and high in compaction density can be prepared by adjusting the adding types of the materials and the adding amount of each section.

Description

Technical Field

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

[0002] Due to its high safety, low cost, and environmental friendliness, lithium iron phosphate cathode materials have been increasingly growing in market share in recent years and have exceeded ternary lithium batteries. However, the energy density of lithium iron phosphate cathode materials has approached its theoretical energy density, and it is very difficult to further improve the energy density of the battery cells. As an upgraded product of lithium iron phosphate materials, lithium iron manganese phosphate materials not only inherit their advantages but also greatly improve the energy density and low-temperature performance, and will surely be very competitive in the market in the future.

[0003] However, although the introduction of manganese element in lithium iron manganese phosphate materials increases the working voltage of the materials, it also leads to a decrease in the conductivity and tap density of the materials, resulting in insufficient volumetric energy density of the battery cells. In the application of battery cells, the tap density of the cathode material is often closely related to the specific capacity, internal resistance, charge and discharge efficiency, and cycle performance of the battery electrode sheet. Generally speaking, the greater the tap density, the higher the capacity of the battery usually is, and the corresponding volumetric energy density is also higher.

[0004] CN119943946A discloses a modified lithium iron manganese phosphate cathode material, a preparation method thereof, and a lithium ion battery. The cathode material is prepared by mixing a phosphorus source, an iron source, a manganese source, a lithium source, and a magnesium source and a titanium source in a solution, followed by hydrothermal reaction, centrifugal washing, and drying to synthesize lithium iron manganese phosphate, and coating with polypyrrole and glucose carbon layer to improve the electronic conductivity of lithium iron manganese phosphate.

[0005] CN119786544A discloses a preparation method of a high-tap-density lithium iron manganese phosphate cathode material, comprising the following steps: placing a manganese source, an iron source, and a phosphorus source in deionized water, stirring, performing primary high-speed ball milling, and performing a constant-temperature water bath, then adding an oxidant and stirring for oxidation, and then drying the slurry to obtain a lithium iron manganese phosphate precursor powder; performing primary sintering on the lithium iron manganese phosphate precursor powder to obtain a lithium iron manganese phosphate first-fired material; placing the lithium iron manganese phosphate first-fired material in deionized water, adding a lithium source and a carbon source at the same time, stirring, performing secondary high-speed ball milling, and drying to obtain a lithium iron manganese phosphate lithium precursor powder; and performing secondary sintering on the lithium iron manganese phosphate lithium precursor powder to obtain the high-tap-density lithium iron manganese phosphate cathode material.

[0006] The above solutions prepare a high-tap-density lithium iron manganese phosphate cathode material by doping or first preparing a lithium iron manganese phosphate precursor and then sintering with a lithium source. Although the tap density is increased, the strength of the crystal grains is reduced and the electronic conductivity is poor, resulting in a decline in the rate performance. Summary of the Invention

[0007] The object of the present invention is to provide a composite lithium manganese iron phosphate cathode material, a preparation method and an application thereof. The method of the present invention adopts a segmented preparation of the lithium manganese iron phosphate cathode material. By adjusting the types of materials added and the addition amounts in each segment, a lithium manganese iron phosphate cathode material with a dense structure, good structural stability, high grain strength, uniform particle size distribution and high tap density can be prepared.

[0008] To achieve the object of this invention, the following technical solutions are adopted in the present invention:

[0009] In the first aspect, the present invention provides a preparation method of a composite lithium manganese iron phosphate cathode material. The preparation method includes the following steps:

[0010] (1) A first lithium source, an iron source, a manganese source, a phosphorus source, a flux, a first dispersant and a first solvent are mixed for the first time to obtain a first mixed slurry. After the first mixed slurry is subjected to first spray drying, it is subjected to first calcination treatment to obtain a lithium manganese iron phosphate intermediate;

[0011] (2) The lithium manganese iron phosphate intermediate, a second lithium source, a second dispersant and a second solvent are mixed for the second time to obtain a second mixed slurry. After the second mixed slurry is subjected to second spray drying, it is subjected to second calcination treatment to obtain the composite lithium manganese iron phosphate cathode material;

[0012] Wherein, the molar amount of lithium element in the first mixed slurry is amol, the molar amount of iron element is bmol, the molar amount of manganese element is cmol, and the molar amount of lithium element in the second lithium source is a’mol, a / (b + c) = (0.7 - 0.85):1, (a + a’) / (b + c) = (1.01 - 1.03):1.

[0013] The present invention adopts a segmented method to prepare the lithium iron manganese phosphate cathode material. In the first mixed slurry, the particle dispersibility is regulated by the first dispersant to avoid hard agglomeration. In the first mixing, the atomic-level uniform mixing of iron, manganese, and phosphorus sources is ensured, and the local looseness caused by composition segregation is reduced. By adding a flux, the grain boundary energy can be reduced, the growth of particle densification can be promoted, large-grain and high-crystallinity precursors can be formed, and internal pores can be reduced. A porous precursor microsphere is formed during the first spray drying process, and then through the first calcination treatment, a preliminary olivine structure is formed, but a certain amount of lithium vacancies or non-stoichiometry is retained, providing space for lithium supplementation in the second step. The dense and low-porosity particle arrangement reduces the voids during electrode coating and directly increases the tap density. Then, a second lithium source is added for lithium supplementation and a second dispersant is added. Through the second spray drying and the second calcination treatment, the lithium vacancies of the intermediate can be repaired, while avoiding the coarsening of particles caused by over-sintering, improving the crystallization integrity and structural strength, reducing the hindrance of defects to ion diffusion, and maintaining the uniformity of particle size. The second dispersant described in the present invention serves both as a carbon source and modifies the surface of the intermediate through chelation, promoting the uniform lithium diffusion during the second calcination.

[0014] Preferably, the first lithium source in step (1) includes any one or at least two combinations of lithium hydroxide, lithium carbonate, lithium phosphate, lithium dihydrogen phosphate, lithium acetate, or lithium oxalate. Typical but non-limiting combinations include the combination of lithium hydroxide and lithium carbonate, the combination of lithium acetate and lithium hydroxide, or the combination of lithium phosphate and lithium oxalate, etc.

[0015] Preferably, the iron source in step (1) includes any one or at least two combinations of iron oxide, magnetite, ferrous oxalate, iron phosphate, iron sulfate, or iron chloride. Typical but non-limiting combinations include the combination of iron oxide and magnetite, the combination of iron phosphate and ferrous oxalate, or the combination of iron chloride and iron sulfate, etc.

[0016] Preferably, the manganese source in step (1) includes any one or at least two combinations of manganese dioxide, manganese pentoxide, manganese tetroxide, manganese hydrogen phosphate, manganese carbonate, or manganese oxalate. Typical but non-limiting combinations include the combination of manganese dioxide and manganese pentoxide, the combination of manganese pentoxide and manganese tetroxide, or the combination of manganese carbonate and manganese oxalate, etc.

[0017] Preferably, the phosphorus source in step (1) includes any one or at least two combinations of lithium dihydrogen phosphate, ammonium dihydrogen phosphate, or phosphoric acid.

[0018] Preferably, the flux in step (1) includes lithium fluoride and / or lithium bismuthate, preferably lithium bismuthate.

[0019] Preferably, based on the total mass of the first lithium source, iron source, manganese source and phosphorus source being 100%, the addition amount of the flux is 0.2% to 0.8%, such as 0.2%, 0.3%, 0.5%, 0.6% or 0.8%, etc.

[0020] Preferably, the first dispersant in step (1) includes any one or a combination of at least two of polyvinyl alcohol, polyethylene glycol or oxalic acid. Typical but non-limiting combinations include the combination of polyvinyl alcohol and polyethylene glycol, the combination of polyvinyl alcohol and oxalic acid, or the combination of polyethylene glycol and oxalic acid, etc.

[0021] Preferably, based on the total mass of the first lithium source, iron source, manganese source and phosphorus source being 100%, the addition amount of the first dispersant is 1% to 3%, such as 1%, 1.5%, 2%, 2.5% or 3%, etc. It is not limited to the listed values, and other unlisted values within this numerical range are equally applicable.

[0022] Preferably, the first solvent in step (1) includes water.

[0023] Preferably, the solid content of the first mixed slurry in step (1) is 35% to 50%, such as 35%, 38%, 40%, 45% or 50%, etc. It is not limited to the listed values, and other unlisted values within this numerical range are equally applicable.

[0024] Preferably, the median particle size D50 of the solid materials in the first mixed slurry in step (1) is 0.8 μm to 1.5 μm, such as 0.8 μm, 0.9 μm, 1 μm, 1.2 μm or 1.5 μm, etc. It is not limited to the listed values, and other unlisted values within this numerical range are equally applicable.

[0025] Preferably, the molar amount of phosphorus element in the first mixed slurry in step (1) is d mol, and (b + c) / d = (0.95 to 0.99):1.

[0026] Preferably, the molar amount of iron element in the first mixed slurry in step (1) is b mol, and the molar amount of manganese element is c mol, and c / b = (1 to 4):1.

[0027] Preferably, ball milling and / or sand milling are carried out during the first mixing in step (1).

[0028] Preferably, a first carbon source and an additive are also added during the first mixing in step (1).

[0029] Preferably, the first carbon source includes any one or a combination of at least two of citric acid, starch, sucrose, glucose, carbon black, or carbon nanotubes. Typical but non-limiting combinations include the combination of citric acid and starch, the combination of sucrose and glucose, or the combination of carbon black and carbon nanotubes, etc.

[0030] The first carbon source in the present invention forms a conductive network skeleton at high temperature to avoid excessive sintering and adhesion of particles.

[0031] Preferably, based on the mass of the lithium iron manganese phosphate intermediate being 100%, the addition amount of the first carbon source is 5% - 7%, for example: 5%, 5.5%, 6%, 6.5%, or 7%, etc. It is not limited to the listed values, and other unlisted values within this range are also applicable.

[0032] Preferably, the additive includes any one or a combination of at least two of niobium oxide, zirconium oxide, titanium oxide, magnesium oxide, aluminum oxide, tungsten oxide, or molybdenum oxide. Typical but non-limiting combinations include the combination of niobium oxide and zirconium oxide, the combination of zirconium oxide and titanium oxide, or the combination of aluminum oxide and tungsten oxide, etc.

[0033] Preferably, based on the mass of the lithium iron manganese phosphate intermediate being 100%, the addition amount of the additive is 0.5% - 5%, for example: 0.5%, 1%, 2%, 3%, 4%, or 5%, etc.

[0034] Preferably, the temperature of the first spray drying in step (1) is 200°C - 300°C, for example: 200°C, 220°C, 250°C, 280°C, or 300°C, etc.

[0035] Preferably, the first calcination treatment in step (1) includes first sintering and second sintering in sequence.

[0036] Preferably, the temperature of the first sintering is 300°C - 400°C, for example: 300°C, 320°C, 350°C, 380°C, or 400°C, etc. It is not limited to the listed values, and other unlisted values within this range are also applicable.

[0037] Preferably, the time of the first sintering is 2h - 4h, for example: 2h, 2.5h, 3h, 3.5h, or 4h, etc. It is not limited to the listed values, and other unlisted values within this range are also applicable.

[0038] Preferably, the temperature of the second sintering is 800°C - 950°C, for example: 800°C, 820°C, 850°C, 900°C, or 950°C, etc. It is not limited to the listed values, and other unlisted values within this range are also applicable.

[0039] Preferably, the time of the second sintering is 4 h to 8 h, for example: 4 h, 5 h, 6 h, 7 h or 8 h, etc. It is not limited to the listed values, and other unlisted values within this value range are equally applicable.

[0040] Preferably, after the first calcination treatment in step (1), crushing, iron removal and sieving treatments are carried out.

[0041] Preferably, the second lithium source in step (2) includes any one or a combination of at least two of lithium hydroxide, lithium carbonate, lithium phosphate, lithium dihydrogen phosphate, lithium acetate or lithium oxalate. Typical but non-limiting combinations include the combination of lithium hydroxide and lithium carbonate, the combination of lithium acetate and lithium hydroxide, or the combination of lithium phosphate and lithium oxalate, etc.

[0042] Preferably, the second dispersant in step (2) includes any one or a combination of at least two of polyvinyl alcohol, polyethylene glycol or oxalic acid. Typical but non-limiting combinations include the combination of polyvinyl alcohol and polyethylene glycol, the combination of polyvinyl alcohol and oxalic acid, or the combination of polyethylene glycol and oxalic acid, etc.

[0043] Preferably, based on the mass of the lithium iron manganese phosphate intermediate being 100%, the addition amount of the second dispersant is 1% to 3%, for example: 1%, 1.5%, 2%, 2.5% or 3%, etc. It is not limited to the listed values, and other unlisted values within this value range are equally applicable.

[0044] Preferably, the second solvent in step (2) includes water.

[0045] Preferably, ball milling and / or sand milling are carried out during the second mixing in step (2).

[0046] Preferably, a second carbon source is further added during the second mixing in step (2).

[0047] Preferably, the second carbon source includes any one or a combination of at least two of citric acid, starch, sucrose, glucose, carbon black or carbon nanotubes. Typical but non-limiting combinations include the combination of citric acid and starch, the combination of sucrose and glucose, or the combination of carbon black and carbon nanotubes, etc.

[0048] The second carbon source in the present invention forms a uniform coating layer on the particle surface to fill the gaps between primary particles.

[0049] Preferably, based on the mass of the lithium iron manganese phosphate intermediate being 100%, the addition amount of the second carbon source is 4% to 5%, for example: 4%, 4.2%, 4.5%, 4.8% or 5%, etc. It is not limited to the listed values, and other unlisted values within this value range are equally applicable.

[0050] Preferably, the solid content of the second mixed slurry in step (2) is 30% to 40%, for example: 30%, 32%, 35%, 38% or 40%, etc. It is not limited to the listed values, and other unlisted values within this value range are equally applicable.

[0051] Preferably, the median particle size D50 of the solid materials in the second mixed slurry in step (2) is 100 nm to 400 nm, for example: 100 nm, 150 nm, 200 nm, 300 nm or 400 nm, etc. It is not limited to the listed values, and other unlisted values within this value range are equally applicable.

[0052] Preferably, the temperature of the second spray drying in step (2) is 200 °C to 300 °C, for example: 200 °C, 220 °C, 250 °C, 280 °C or 300 °C, etc.

[0053] Preferably, the second calcination treatment in step (2) includes third sintering and fourth sintering in sequence.

[0054] Preferably, the temperature of the third sintering is 300 °C to 400 °C, for example: 300 °C, 320 °C, 350 °C, 380 °C or 400 °C, etc. It is not limited to the listed values, and other unlisted values within this value range are equally applicable.

[0055] Preferably, the time of the third sintering is 2 h to 4 h, for example: 2 h, 2.5 h, 3 h, 3.5 h or 4 h, etc.

[0056] Preferably, the temperature of the fourth sintering is 600 °C to 700 °C, for example: 600 °C, 620 °C, 650 °C, 680 °C or 700 °C, etc. It is not limited to the listed values, and other unlisted values within this value range are equally applicable.

[0057] Preferably, the time of the fourth sintering is 6 h to 8 h, for example: 6 h, 6.5 h, 7 h, 7.5 h or 8 h, etc. It is not limited to the listed values, and other unlisted values within this value range are equally applicable.

[0058] Preferably, after the second calcination treatment in step (2), crushing, iron removal and sieving treatments are carried out.

[0059] In a second aspect, the present invention provides a composite lithium manganese iron phosphate cathode material, and the composite lithium manganese iron phosphate cathode material is prepared by the preparation method as described in the first aspect.

[0060] The composite lithium manganese iron phosphate cathode material prepared by the method of the present invention has high uniformity, consistent mechanical strength of particles and relatively high tap density. It is not easy to break when used for pressing electrodes and can maintain a high density. The dense structure can reduce the erosion of the electrolyte penetration on the material and inhibit the dissolution of Mn.

[0061] In a third aspect, the present invention provides a positive electrode plate, which comprises the composite lithium iron manganese phosphate positive electrode material as described in the second aspect.

[0062] In a fourth aspect, the present invention provides a lithium ion battery, which comprises the positive electrode plate as described in the third aspect.

[0063] Compared with the prior art, the present invention has the following beneficial effects:

[0064] (1) The method of the present invention adopts a segmented preparation of the lithium iron manganese phosphate positive electrode material. By adjusting the types of materials added and the addition amounts in each segment, a lithium iron manganese phosphate positive electrode material with a dense structure, uniform particle size distribution and high tap density can be prepared.

[0065] (2) The lithium iron manganese phosphate positive electrode material prepared by the method of the present invention has a powder tap density of up to 2.3733 g / cm 3 at 221.6 MPa or more. The 0.1C discharge specific capacity of the battery made therefrom can reach 150.4 mAh / g or more, and the cycle capacity retention rate after 500 cycles can reach 95.1% or more. Specific Embodiments

[0066] The technical solutions of the present invention will be further described below through specific embodiments. Those skilled in the art should understand that the embodiments are only for helping to understand the present invention and should not be regarded as specific limitations on the present invention.

[0067] Example 1

[0068] This example provides a composite lithium iron manganese phosphate positive electrode material, which is prepared by the following method:

[0069] (1) Lithium hydroxide, iron oxide, manganese dioxide, phosphoric acid, lithium bismuthate, niobium oxide, carbon nanotubes, polyvinyl alcohol and water are mixed to obtain a mixed slurry with a solid content of 42%. The slurry is ball-milled until the median particle size of the solids in the slurry is 1 μm. After spray drying at 280 °C, it is sintered at 350 °C for 3 h and at 880 °C for 6 h. After crushing, iron removal and sieving, a lithium iron manganese phosphate intermediate is obtained;

[0070] Among them, the mass ratio of the total mass of lithium hydroxide, iron oxide, manganese dioxide, and phosphoric acid to the mass of lithium bismuthate is 100:0.5, and the mass ratio of the total mass of lithium hydroxide, iron oxide, manganese dioxide, and phosphoric acid to the mass of polyvinyl alcohol is 100:2. Based on the mass of the lithium iron manganese phosphate intermediate being 100%, the mass of carbon nanotubes is 6%, and the mass of niobium oxide is 3%. In the mixed slurry, Li:(Mn + Fe) = 0.8, Mn:Fe = 2:1, and (Mn + Fe) / P = 0.96:1;

[0071] (2) Mix lithium iron manganese phosphate intermediate, lithium hydroxide, glucose, polyvinyl alcohol and water to obtain a mixed slurry with a solid content of 35%, ball mill it until the median particle size of the solid in the slurry is 300 nm, spray dry it at 280 °C, sinter it at 350 °C for 3 h, sinter it at 650 °C for 7 h, and after air flow pulverization, iron removal and sieving treatment, the composite lithium iron manganese phosphate cathode material is obtained. In the composite lithium iron manganese phosphate cathode material, Li:(Mn + Fe) = 1.02:1, the mass ratio of glucose to lithium iron manganese phosphate intermediate is 4.5:100, and the mass ratio of polyvinyl alcohol to lithium iron manganese phosphate intermediate is 2:100.

[0072] Example 2

[0073] This example provides a composite lithium iron manganese phosphate cathode material, which is prepared by the following method:

[0074] (1) Mix lithium carbonate, ferric sulfate, manganese oxalate, phosphoric acid, lithium bismuthate, titanium oxide, sucrose, polyethylene glycol and water to obtain a mixed slurry with a solid content of 50%, ball mill it until the median particle size of the solid in the slurry is 1.5 μm, spray dry it at 280 °C, sinter it at 400 °C for 2 h, sinter it at 950 °C for 4 h, and after crushing, iron removal and sieving treatment, the lithium iron manganese phosphate intermediate is obtained.

[0075] Among them, the mass ratio of the total mass of lithium carbonate, ferric sulfate, manganese oxalate, and phosphoric acid to the mass of lithium bismuthate is 100:0.5, the mass ratio of the total mass of lithium carbonate, ferric sulfate, manganese oxalate, and phosphoric acid to polyethylene glycol is 100:3. Based on the mass of the lithium iron manganese phosphate intermediate being 100%, the mass of carbon nanotubes is 5%, the mass of niobium oxide is 3%, in the mixed slurry, Li:(Mn + Fe) = 0.85, Mn:Fe = 1.5:1, (Mn + Fe) / P = 0.95:1;

[0076] (2) Mix lithium iron manganese phosphate intermediate, lithium hydroxide, starch, polyethylene glycol and water to obtain a mixed slurry with a solid content of 30%, ball mill it until the median particle size of the solid in the slurry is 400 nm, spray dry it at 280 °C, sinter it at 300 °C for 4 h, sinter it at 600 °C for 8 h, and after air flow pulverization, iron removal and sieving treatment, the composite lithium iron manganese phosphate cathode material is obtained. In the composite lithium iron manganese phosphate cathode material, Li:(Mn + Fe) = 1.02:1, the mass ratio of starch to lithium iron manganese phosphate intermediate is 4:100, and the mass ratio of polyethylene glycol to lithium iron manganese phosphate intermediate is 3:100.

[0077] Example 3

[0078] This example provides a composite lithium iron manganese phosphate cathode material, which is prepared by the following method:

[0079] (1) Mix lithium phosphate, iron oxalate, manganese oxalate, ammonium dihydrogen phosphate, lithium bismuthate, titanium oxide, sucrose, polyethylene glycol with water to obtain a mixed slurry with a solid content of 35%. Ball mill it until the median particle size of the solids in the slurry is 0.8 μm. After spray drying at 280 °C, sinter it at 300 °C for 4 h and at 800 °C for 8 h. After crushing, iron removal and sieving, obtain a lithium iron manganese phosphate intermediate;

[0080] Among them, the mass ratio of the total mass of lithium phosphate, iron oxalate, manganese oxalate, ammonium dihydrogen phosphate to the mass of lithium bismuthate is 100:0.5, and the mass ratio of the total mass of lithium phosphate, iron oxalate, manganese oxalate, ammonium dihydrogen phosphate to the mass of polyvinyl alcohol is 100:1. Based on the mass of the lithium iron manganese phosphate intermediate being 100%, the mass of carbon nanotubes is 7% and the mass of niobium oxide is 3%. In the mixed slurry, Li:(Mn + Fe)=0.7, Mn:Fe = 3:1, (Mn + Fe) / P = 0.99:1;

[0081] (2) Mix the lithium iron manganese phosphate intermediate, lithium hydroxide, starch, polyethylene glycol and water to obtain a mixed slurry with a solid content of 40%. Ball mill it until the median particle size of the solids in the slurry is 100 nm. After spray drying at 280 °C, sinter it at 400 °C for 2 h and at 700 °C for 6 h. After air-flow pulverization, iron removal and sieving, obtain the composite lithium iron manganese phosphate cathode material. In the composite lithium iron manganese phosphate cathode material, Li:(Mn + Fe)=1.03:1, the mass ratio of starch to the lithium iron manganese phosphate intermediate is 5:100, and the mass ratio of polyethylene glycol to the lithium iron manganese phosphate intermediate is 1:100.

[0082] Example 4

[0083] The difference between this example and Example 1 is only that lithium bismuthate is replaced with lithium fluoride of equal mass, and other conditions and parameters are exactly the same as those in Example 1.

[0084] Example 5

[0085] The difference between this example and Example 1 is only that the mass ratio of the total mass of lithium hydroxide, iron oxide, manganese dioxide, phosphoric acid to the mass of lithium bismuthate is 100:1, and other conditions and parameters are exactly the same as those in Example 1.

[0086] Example 6

[0087] The difference between this example and Example 1 is only that the mass ratio of the total mass of lithium hydroxide, iron oxide, manganese dioxide, phosphoric acid to the mass of lithium bismuthate is 100:0.1, and other conditions and parameters are exactly the same as those in Example 1.

[0088] Example 7

[0089] The difference between this embodiment and Embodiment 1 is only that the mass ratio of the total mass of lithium hydroxide, iron oxide, manganese dioxide, and phosphoric acid to the mass of polyvinyl alcohol in step (1) is 100:0.5, and other conditions and parameters are exactly the same as those in Embodiment 1.

[0090] Example 8

[0091] The difference between this embodiment and Embodiment 1 is only that the mass ratio of the total mass of lithium hydroxide, iron oxide, manganese dioxide, and phosphoric acid to the mass of polyvinyl alcohol in step (1) is 100:4, and other conditions and parameters are exactly the same as those in Embodiment 1.

[0092] Example 9

[0093] The difference between this embodiment and Embodiment 1 is only that the mass ratio of polyethylene glycol to the lithium iron manganese phosphate intermediate in step (2) is 0.5:100, and other conditions and parameters are exactly the same as those in Embodiment 1.

[0094] Example 10

[0095] The difference between this embodiment and Embodiment 1 is only that the mass ratio of polyethylene glycol to the lithium iron manganese phosphate intermediate in step (2) is 4:100, and other conditions and parameters are exactly the same as those in Embodiment 1.

[0096] Comparative Example 1

[0097] The difference between this comparative example and Embodiment 1 is only that lithium bismuthate is not added, and other conditions and parameters are exactly the same as those in Embodiment 1.

[0098] Comparative Example 2

[0099] The difference between this comparative example and Embodiment 1 is only that lithium hydroxide is added only in step (1), and in the prepared composite lithium iron manganese phosphate cathode material, Li:(Mn + Fe) = 1.02:1, and other conditions and parameters are exactly the same as those in Embodiment 1.

[0100] Comparative Example 3

[0101] The difference between this comparative example and Embodiment 1 is only that in the mixed slurry in step (1), Li:(Mn + Fe) = 0.6, and other conditions and parameters are exactly the same as those in Embodiment 1.

[0102] Comparative Example 4

[0103] The difference between this comparative example and Embodiment 1 is only that in the mixed slurry in step (1), Li:(Mn + Fe) = 0.9, and other conditions and parameters are exactly the same as those in Embodiment 1.

[0104] Performance test:

[0105] The lithium iron manganese phosphate cathode materials prepared in the examples and comparative examples were tested. The lithium iron manganese phosphate cathode materials prepared in the examples and comparative examples were made into coin cells respectively, and the test results are shown in Table 1:

[0106] Table 1

[0107]

[0108] As can be seen from Table 1, from Examples 1-10, the lithium iron manganese phosphate cathode material prepared by the method of the present invention has a powder tap density of up to 2.3733 g / cm at 221.6 MPa 3 or more, the 0.1C discharge specific capacity of the made battery can reach 150.4 mAh / g or more, and the cycle capacity retention rate after 500 cycles can reach 95.1% or more.

[0109] Comparing Example 1 and Example 4, it can be obtained that during the preparation process of the composite lithium iron manganese phosphate cathode material of the present invention, the type of flux will affect its performance. In a high-performance lithium iron manganese phosphate system sensitive to manganese dissolution, using lithium bismuthate as a flux can form a liquid phase at a lower temperature, promote Li+ diffusion and crystal growth, reduce the mixing of manganese / iron ions caused by high temperature, improve the electrochemical performance of the material, and Bi may be partially doped into the lattice to stabilize the structure and reduce Mn3 + Jahn-Teller distortion and dissolution during charge and discharge, enhance the electronic conductivity, and extend the cycle life.

[0110] Comparing Example 1 and Examples 5-6, it can be obtained that during the preparation process of the composite lithium iron manganese phosphate cathode material of the present invention, the addition amount of the flux will affect its performance. Controlling the addition amount of the flux at 0.2% - 0.8% of the total mass of the lithium source, iron source, manganese source and phosphorus source, the prepared composite lithium iron manganese phosphate cathode material has better performance. If the addition amount of the flux is too low, the sintering driving force is insufficient and the material densification is incomplete. If the addition amount of the flux is too high, it will not be able to volatilize completely or participate in the reaction, and too much bismuth will enter the material lattice, reducing the conductivity of the material.

[0111] Comparing Example 1 with Examples 7-8, it can be seen that during the preparation process of the composite lithium manganese iron phosphate cathode material of the present invention, in the mixing process of step (1), the addition amount of the dispersant will affect its performance. When the addition amount of the dispersant in the mixing process of step (1) is controlled at 1% to 3% of the total mass of the lithium source, iron source, manganese source and phosphorus source, the performance of the prepared composite lithium manganese iron phosphate cathode material is better. If the addition amount of the dispersant in the mixing process of step (1) is too low, during the grinding process, when the material reaches the nanoscale, agglomeration occurs under the electrostatic force, making it difficult to further grind to reduce the size and resulting in poor slurry homogeneity. If the addition amount of the dispersant in the mixing process of step (1) is too high, the viscosity of the slurry increases during the grinding process, and the fluidity becomes poor. Excessive dispersant during the sintering process will hinder the effective combination between particles, resulting in a loose internal structure of the material.

[0112] Comparing Example 1 with Examples 9-10, it can be seen that during the preparation process of the composite lithium manganese iron phosphate cathode material of the present invention, in the mixing process of step (2), the addition amount of the dispersant will affect its performance. When the addition amount of the dispersant in the mixing process of step (2) is controlled at 1% to 3% of the mass of the lithium manganese iron phosphate intermediate, the performance of the prepared composite lithium manganese iron phosphate cathode material is better. If the addition amount of the dispersant in the mixing process of step (2) is too low, during the grinding process, when the material reaches the nanoscale, agglomeration occurs under the electrostatic force, making it difficult to further grind to reduce the size and resulting in poor slurry homogeneity. If the addition amount of the dispersant in the mixing process of step (2) is too high, the viscosity of the slurry increases during the grinding process, and the fluidity becomes poor. Excessive dispersant during the sintering process will hinder the effective combination between particles, resulting in a loose internal structure of the material.

[0113] Comparing Example 1 with Comparative Example 1, it can be seen that during the preparation process of the composite lithium manganese iron phosphate cathode material of the present invention, the flux can reduce the grain boundary energy, promote the densification growth of particles, form a large-grain and high-crystallinity precursor, reduce internal pores, and thus prepare a lithium manganese iron phosphate cathode material with excellent performance and high tap density.

[0114] Comparing Example 1 with Comparative Example 2, it can be seen that in the method of the present invention, the lithium source is added step by step. In step (1), the lithium source reacts with the manganese source, iron source and phosphorus source to generate preliminary lithium manganese iron phosphate crystal nuclei, but the lithium content is slightly lower than the stoichiometric ratio, which can avoid excessive volatilization of lithium at high temperatures, reduce lattice defects caused by lithium loss, and form denser primary particles. The supplementary lithium source replenishes the lattice Li vacancies of the primary particles during the secondary sintering and promotes the fusion between particles through a liquid phase (such as a eutectic of Li2O or Li3PO4), forming denser secondary spherical particles and reducing the porosity, thereby improving the tap density.

[0115] Comparing Example 1 with Comparative Example 3, in the method of the present invention, the lithium source is added step by step. If the amount of lithium source added at the beginning is too small, FePO4 or MnPO4 will remain in the precursor, and these inactive phases will reduce the content of effective active substances. When lithium is insufficient, the growth of particles is limited, forming nanoscale but loosely packed primary particles, and the strength of the secondary spheres is low and the tap density is low.

[0116] Comparing Example 1 with Comparative Example 4, in the method of the present invention, the lithium source is added step by step. If the amount of lithium source added at the beginning is too large, due to the large size of the primary particles during the first sintering process, excessive Li is difficult to enter the interior of the material bulk phase, and the excessive lithium will volatilize in the form of Li2O, and the excessive lithium will accelerate the particle fusion, forming primary particles with too large and uneven sizes, resulting in an increase in the porosity of the secondary spherical particles and a decrease in the tap density.

[0117] The applicant declares that the above description is only the specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention fall within the protection scope and the disclosure scope of the present invention.

Claims

1. A preparation method of a composite lithium iron manganese phosphate cathode material, characterized in that, The preparation method comprises the following steps: (1) Mix a first lithium source, an iron source, a manganese source, a phosphorus source, a flux, a first dispersant and a first solvent for the first time to obtain a first mixed slurry. After the first spray drying of the first mixed slurry, a lithium iron manganese phosphate intermediate is obtained through a first calcination treatment; (2) Mix the lithium iron manganese phosphate intermediate, a second lithium source, a second dispersant and a second solvent for the second time to obtain a second mixed slurry. After the second spray drying of the second mixed slurry, the composite lithium iron manganese phosphate cathode material is obtained through a second calcination treatment; Wherein, the molar amount of lithium element in the first mixed slurry is amol, the molar amount of iron element is bmol, the molar amount of manganese element is cmol, the molar amount of lithium element in the second lithium source is a'mol, a / (b + c) = (0.7 - 0.85):1, (a + a') / (b + c) = (1.01 - 1.03):

1.

2. The preparation method according to claim 1, characterized in that, The flux in step (1) includes lithium fluoride and / or lithium bismuthate, preferably lithium bismuthate; Preferably, based on the total mass of the first lithium source, iron source, manganese source and phosphorus source being 100%, the addition amount of the flux is 0.2% - 0.8%; Preferably, the first dispersant in step (1) includes any one or a combination of at least two of polyvinyl alcohol, polyethylene glycol or oxalic acid; Preferably, based on the total mass of the first lithium source, iron source, manganese source and phosphorus source being 100%, the addition amount of the first dispersant is 1% - 3%; Preferably, the first solvent in step (1) includes water.

3. The preparation method according to claim 1 or 2, characterized in that, The solid content of the first mixed slurry in step (1) is 35% - 50%; Preferably, the median particle size D50 of the solid materials in the first mixed slurry in step (1) is 0.8μm - 1.5μm; Preferably, the molar amount of phosphorus element in the first mixed slurry in step (1) is dmol, (b + c) / d = (0.95 - 0.99):1; Preferably, the molar amount of iron element in the first mixed slurry in step (1) is b, the molar amount of manganese element is cmol, c / b = (1 - 4):1; Preferably, ball milling and / or sand milling are carried out during the first mixing in step (1); Preferably, a first carbon source and an additive are further added during the first mixing in step (1); Preferably, based on the mass of the lithium iron manganese phosphate intermediate being 100%, the addition amount of the first carbon source is 5% - 7%; Preferably, the additive includes any one or a combination of at least two of niobium oxide, zirconium oxide, titanium oxide, magnesium oxide, aluminum oxide, tungsten oxide or molybdenum oxide; Preferably, based on the mass of the lithium iron manganese phosphate intermediate being 100%, the addition amount of the additive is 0.5% - 5%.

4. The preparation method according to any one of claims 1-3, characterized in that, The temperature of the first spray drying in step (1) is 200°C - 300°C; Preferably, the first calcination treatment in step (1) includes first sintering and second sintering in sequence; Preferably, the temperature of the first sintering is 300°C - 400°C; Preferably, the time of the first sintering is 2h - 4h; Preferably, the temperature of the second sintering is 800°C - 950°C; Preferably, the time of the second sintering is 4 h to 8 h; Preferably, after the first calcination treatment in step (1), crushing, iron removal and sieving treatments are carried out.

5. The preparation method according to any one of claims 1 to 4, characterized in that, The second dispersant in step (2) includes any one or a combination of at least two of polyvinyl alcohol, polyethylene glycol or oxalic acid; Preferably, based on the mass of the lithium iron manganese phosphate intermediate being 100%, the addition amount of the second dispersant is 1% to 3%; Preferably, the second solvent in step (2) includes water.

6. The preparation method according to any one of claims 1-5, characterized in that, During the second mixing in step (2), ball milling and / or sand milling are carried out; Preferably, a second carbon source is further added during the second mixing in step (2); Preferably, based on the mass of the lithium iron manganese phosphate intermediate being 100%, the addition amount of the second carbon source is 4% to 5%; Preferably, the solid content of the second mixed slurry in step (2) is 30% to 40%; Preferably, the median particle size D50 of the solid materials in the second mixed slurry in step (2) is 100 nm to 400 nm.

7. The preparation method according to any one of claims 1 to 6, characterized in that, The temperature of the second spray drying in step (2) is 200 °C to 300 °C; Preferably, the second calcination treatment in step (2) includes third sintering and fourth sintering in sequence; Preferably, the temperature of the third sintering is 300 °C to 400 °C; Preferably, the time of the third sintering is 2 h to 4 h; Preferably, the temperature of the fourth sintering is 600 °C to 700 °C; Preferably, the time of the fourth sintering is 6 h to 8 h; Preferably, after the second calcination treatment in step (2), crushing, iron removal and sieving treatments are carried out.

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

9. A positive electrode plate, characterized in that, The positive electrode sheet includes the composite lithium iron manganese phosphate cathode material according to claim 8.

10. A lithium-ion battery, characterized in that, The lithium ion battery includes the positive electrode sheet according to claim 9.

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