A method for preparing a low specific surface area lithium iron manganese phosphate positive electrode material and a lithium iron manganese phosphate positive electrode material

Abstract

The present application discloses a preparation method of a low specific surface area lithium iron manganese phosphate positive electrode material and a low specific surface area lithium iron manganese phosphate positive electrode material. The preparation method specifically comprises the following steps: a) raw material screening: ball milling a precursor made of a lithium source, an iron source, a manganese source and a phosphorus source as raw materials, and sieving to obtain a powder with a D50 of 1-3 μm; b) nucleating agent premixing and high-temperature sintering: dry-mixing the powder obtained in step a with a nucleating agent containing a V element at a ratio of 0.1-1wt%, and then sintering in two stages under an inert gas atmosphere to obtain a sintered material, the first stage being 400-500°C for 1-3h, and the second stage being 750-850°C for 8-12h; c) surface coating: sequentially coating a carbon layer and a metal oxide layer on the surface of the sintered material. The lithium iron manganese phosphate positive electrode material prepared in the present application has a low specific surface area and has both good processing performance and high electrochemical properties.

Description

Technical Field

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

[0002] With the rapid development of new energy vehicles and energy storage industries, the performance improvement of lithium-ion battery cathode materials has become a core demand of the industry. Lithium iron phosphate (LiFePO4, LFP) has been widely used in the field of power batteries due to its advantages such as high safety, long cycle life and low cost. However, its low operating voltage platform (about 3.4V vs. Li / Li + ) limits the further improvement of energy density. In recent years, lithium manganese iron phosphate (LiMn x Fe 1-x PO4, LMFP) has attracted much attention as an upgraded material for LFP. By introducing manganese, the operating voltage of LMFP can be increased to 4.1V (corresponding to Mn 3+ / Mn 2+ Redox couple), the theoretical energy density is about 20% higher than that of LFP, while also having the advantage of controllable cost, and is regarded as an important candidate for the next generation of cost-effective positive electrode materials.

[0003] However, the practical application of LMFP materials still faces severe challenges. First, nano-scale particles with high specific surface area are easily formed during the material synthesis process. In the existing technology, the particle size of LMFP primary particles synthesized by solid-phase method and liquid-phase method (such as hydrothermal method) is generally less than 1μm, resulting in a specific surface area as high as 20-40m² / g. Although an excessively high specific surface area is conducive to the diffusion of lithium ions, it will significantly increase the contact area between the material and the electrolyte, triggering continuous interfacial side reactions (such as electrolyte decomposition and transition metal dissolution), resulting in battery gas production, capacity attenuation and decreased cycle life. Especially under high temperature (>45°C) or high voltage (>4.2V) conditions, the side reactions are intensified and the battery performance deteriorates more significantly. Second, the valence stability of the manganese element. Mn in LMFP 3+ Disproportionation reaction is easy to occur during high temperature sintering or long cycle (2Mn 3+ →Mn 2+ +Mn 4+ ), leading to crystal structure collapse and manganese ion dissolution, further accelerating capacity decay. Third, existing technologies struggle to achieve both low specific surface area and high conductivity. For example, while high-temperature sintering (≥800°C) can promote grain growth and reduce specific surface area, over-sintering can lead to excessive particle agglomeration and decreased porosity, hindering lithium-ion transport pathways. Furthermore, high temperatures exacerbate manganese disproportionation reactions, undermining the stability of the material structure.

[0004] In response to the above problems, existing public technologies have proposed some improvement plans, such as optimizing the sintering process and surface coating, but they all have limitations: for example, simply relying on the sintering process to reduce the specific surface area will sacrifice structural stability, and it is difficult to reduce the number of active sites from the root through surface coating alone.

[0005] Therefore, a multi-dimensional coordinated regulation strategy is urgently needed to optimize the entire process from raw material screening, crystal growth control to surface modification to break through the performance bottleneck of LMFP materials. Summary of the Invention

[0006] In order to solve the above technical problems, the present application provides a preparation method of a low specific surface area lithium iron manganese phosphate positive electrode material and a lithium iron manganese phosphate positive electrode material.

[0007] In a first aspect, the present application provides a method for preparing a low specific surface area lithium manganese iron phosphate positive electrode material, using the following technical solution:

[0008] A method for preparing a low specific surface area lithium manganese iron phosphate positive electrode material comprises the following steps:

[0009] a) Raw material screening: ball milling a precursor prepared from a lithium source, an iron source, a manganese source, and a phosphorus source, and sieving to obtain a powder with a D50 of 0.1-0.3 μm;

[0010] b) Nucleating agent premixing and high-temperature sintering: The powder obtained in step a is mixed with a nucleating agent containing the V element, and then sintered in two stages under an inert gas atmosphere to obtain a sintered material. The first stage is 400-500°C for 1-3 hours, and the second stage is 750-850°C for 8-12 hours.

[0011] c) Surface coating: A carbon layer and an aluminum oxide layer are sequentially coated on the surface of the sintered material.

[0012] Further preferably, the nucleating agent containing V element is V2O5, and the added amount of the nucleating agent is 0.1-1% of the total mass of the precursor.

[0013] V2O5 is mixed with the precursor after ball milling as a nucleating agent. The nucleating agent mainly takes effect in the second stage of sintering. At this time, the precursor decomposes to form LiMn x Fe 1-x PO4 crystal nucleus, nucleating agent mainly promotes crystal growth through the following mechanisms:

[0014] Liquid-phase assisted sintering: The melting point of V2O5 is 690°C, and a trace liquid phase is formed at 750-850°C, thereby wetting the surface of the precursor particles, reducing the energy barrier of grain boundary migration, and promoting grain fusion;

[0015] Lattice doping control: V 5+ Partially replace PO43- P in 5+ or Li + sites, inducing the preferential growth of crystals along the crystal plane of lithium ion diffusion channels, forming a single-crystal structure with uniform size, reducing the number of grain boundaries, and thus reducing the specific surface area.

[0016] Further preferably, the lithium source is selected from at least one of lithium oxide, lithium hydroxide, lithium chloride, lithium nitrate, lithium nitrite, lithium acetate, lithium oxalate, lithium carbonate, and lithium citrate.

[0017] Further preferably, the iron source is selected from at least one of ferrous sulfate, ferric nitrate, ferric acetate and ferric chloride.

[0018] More preferably, the manganese source is selected from at least one of manganese sulfate, manganese nitrate, manganese acetate and manganese chloride.

[0019] Further preferably, the phosphorus source includes phosphoric acid and / or phosphate, and the phosphate is selected from at least one of monoammonium phosphate, diammonium phosphate, triammonium phosphate, monosodium phosphate, disodium phosphate, and trisodium phosphate.

[0020] Further preferably, the precursor is prepared by dissolving a lithium source, an iron source, a manganese source and a phosphorus source in deionized water, adding citric acid and ethylene glycol, heating and stirring to form a sol, and drying to obtain the precursor.

[0021] Further preferably, the inert gas is a mixture of argon and hydrogen. Preferably, the volume ratio of argon to hydrogen is 90-95%:5-10%.

[0022] Hydrogen can be used as a reducing agent to maintain the balance of oxygen partial pressure in the reaction system and prevent Mn 3+ Oxidized to Mn 4+ In addition, hydrogen can reduce the oxygen vacancies generated during the sintering process, reduce the occurrence of lattice defects, and improve the structural stability of the material.

[0023] Further preferably, the heating rate of the first stage sintering is 5-10°C / min.

[0024] Slowly increase the temperature and keep it warm to promote the decomposition of organic matter and the initial melting of some components, and avoid the collapse of pores in the precursor caused by rapid heating.

[0025] Further preferably, the heating rate of the second stage sintering is 3-5°C / min.

[0026] Under the combined action of nucleating agent and hydrogen reducing atmosphere, the LiMn x Fe 1-x The directional growth and densification of PO4 crystals eventually lead to the acquisition of single-crystal-like particles.

[0027] Further preferably, the carbon layer is prepared by the following specific steps: mixing the sintered material with the carbon source by ball milling, and forming the carbon layer after carbonization.

[0028] Preferably, the carbon source is selected from at least one of glucose, sucrose, starch and citric acid.

[0029] Preferably, the carbonization temperature is 600-610° C. and the carbonization time is 3-3.5 h.

[0030] Preferably, the carbon layer has a thickness of 5-10 nm, for example, it may be any one of 5 nm, 5.5 nm, 6 nm, 6.5 nm, 7 nm, 7.5 nm, 8 nm, 8.5 nm, 9 nm, 9.5 nm and 10 nm, or a range between any two of them.

[0031] More preferably, the aluminum oxide layer is formed by depositing the aluminum oxide layer on the surface of the carbon layer by atomic layer deposition, and the thickness of the aluminum oxide layer is 2.5-3.5 nm, for example, any one of 2.5 nm, 3 nm and 3.5 nm, or a range between any two of them.

[0032] In a second aspect, the present application provides a low specific surface area lithium manganese iron phosphate positive electrode material, which is prepared by the above-mentioned preparation method.

[0033] Further preferably, the specific surface area of ​​the lithium manganese iron phosphate positive electrode material is 5m 2 / g~15m 2 / g. For example: it can be 5m 2 / g、6m 2 / g、7m 2 / g、8m 2 / g、9m 2 / g、10m 2 / g、11m 2 / g、12m 2 / g、13m 2 / g、14m 2 / g and 15m 2 / g or the range between any two.

[0034] More preferably, the D50 of the lithium manganese iron phosphate cathode material is 0.65-0.75 μm, for example, it can be any one of 0.65 μm, 0.66 μm, 0.67 μm, 0.68 μm, 0.69 μm, 0.70 μm, 0.71 μm, 0.72 μm, 0.73 μm, 0.74 μm and 0.75 μm, or a range between any two of them.

[0035] In summary, this application has the following beneficial effects:

[0036] This application utilizes a V2O5 nucleating agent and a two-stage gradient sintering process to induce liquid-phase-assisted directional grain growth at high temperatures, resulting in the prepared lithium manganese iron phosphate particles with both low specific surface area and structural stability. Furthermore, the combination of a double-layer coating structure of carbon and aluminum oxide layers and sintering in an argon / hydrogen mixed reducing atmosphere further enhances the particles' structural stability, effectively suppresses manganese ion dissolution, and significantly improves the electrochemical performance of the cathode material. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 This is an SEM image of the lithium manganese iron phosphate positive electrode material prepared in Example 1;

[0038] Figure 2 This is an SEM image of the lithium manganese iron phosphate positive electrode material prepared in Example 1 after the positive electrode slurry is prepared and compacted. DETAILED DESCRIPTION

[0039] The following describes the embodiments of the present application through specific examples. Those skilled in the art can easily understand the other advantages and effects of the present application from the contents disclosed in this specification. The present application can also be implemented or applied through different specific embodiments, and the details in this specification can be modified or changed based on different viewpoints and applications without departing from the spirit of the present application.

[0040] Furthermore, it should be understood that the mention of one or more method steps in this application does not preclude the presence of other method steps before or after the combination of steps, or the insertion of other method steps between these explicitly mentioned steps, unless otherwise stated. Furthermore, unless otherwise stated, the numbering of each method step is merely a convenient tool for identifying each method step, and is not intended to limit the order of the method steps or the scope of the application. Changes or adjustments to their relative relationships, without substantially changing the technical content, should also be considered within the scope of the application.

[0041] If the specific experimental conditions are not specified in the examples, they are usually based on conventional conditions in the field or conditions recommended by the reagent company; the materials, reagents, etc. used in the examples can be purchased through commercial channels unless otherwise specified.

[0042] Example

[0043] Example 1

[0044] Preparation of lithium manganese iron phosphate positive electrode material:

[0045] a) Dissolve 0.525 mol of lithium carbonate, 0.5 mol of manganese acetate, 0.5 mol of ferrous oxalate, and 1 mol of ammonium dihydrogen phosphate in 200 mL of deionized water. Add 1.5 mol of citric acid and 1.8 mol of ethylene glycol. Heat to 80°C and stir for 6 hours to form a viscous sol. Dry the sol at 120°C in a vacuum for 12 hours to obtain a precursor. Ball mill the precursor for 4 hours and sieve to obtain a powder with a D50 of 0.1 μm.

[0046] b) To the powder obtained in step a), 0.1% by weight of V2O5 was added to the powder, and the mixture was mechanically stirred at 150 rpm for uniformity. The mixture was then sintered in two stages in a mixed gas atmosphere of argon and hydrogen (volume ratio Ar:H2 = 90:10). The first stage sintering temperature was increased to 450°C at 5°C / min and held for 2 h. The second stage sintering temperature was increased to 800°C at 4°C / min and held for 10 h to obtain a sintered material.

[0047] c) first performing a carbon layer coating on the surface of the sintered material: taking the sintered material obtained in step c), adding sucrose accounting for 3% of the total mass of the sintered material, heating to 600°C, and carbonizing for 3 hours to form a 5nm thick carbon layer; then performing an aluminum oxide layer coating: using atomic deposition (ALD) at a deposition temperature of 150°C, trimethylaluminum and water as precursors, a single deposition of 0.1nm thickness, a total of 25 cycles, and depositing an aluminum oxide layer with a total thickness of 2.5nm on the surface of the carbon layer to obtain the lithium manganese iron phosphate positive electrode material.

[0048] Figure 1 This is the SEM image of the lithium manganese iron phosphate positive electrode material prepared in this example. Figure 2 The SEM image of the lithium manganese iron phosphate positive electrode material prepared in this embodiment after the positive electrode slurry is prepared and compacted shows that its surface is smooth, the particles are closely connected but some gaps are retained, and the infiltration of the electrolyte is not affected by the high compaction density.

[0049] Example 2

[0050] Preparation of lithium manganese iron phosphate positive electrode material: The preparation steps are the same as those in Example 1, except that the amounts of raw materials used are different, specifically: 0.525 mol lithium carbonate, 0.6 mol manganese acetate, 0.4 mol ferrous oxalate, and 1 mol ammonium dihydrogen phosphate.

[0051] Example 3

[0052] Preparation of lithium manganese iron phosphate positive electrode material: The preparation steps are the same as those in Example 1, except that the amounts of raw materials used are different, specifically: 0.525 mol lithium carbonate, 0.7 mol manganese acetate, 0.3 mol ferrous oxalate, and 1 mol ammonium dihydrogen phosphate.

[0053] Example 4

[0054] Preparation of lithium manganese iron phosphate positive electrode material: The preparation steps are the same as those in Example 1, except that the amounts of raw materials used are different, specifically: 0.525 mol lithium carbonate, 0.8 mol manganese acetate, 0.2 mol ferrous oxalate, and 1 mol ammonium dihydrogen phosphate.

[0055] Example 5

[0056] Preparation of lithium manganese iron phosphate positive electrode material: The preparation steps are the same as those in Example 1, except that the amount of V2O5 added is different, specifically: 0.5% V2O5.

[0057] Example 6

[0058] Preparation of lithium manganese iron phosphate positive electrode material: The preparation steps are the same as those in Example 1, except that the amount of V2O5 added is different, specifically: 1% V2O5.

[0059] Example 7

[0060] Preparation of lithium manganese iron phosphate positive electrode material: The preparation steps are the same as those in Example 1, except that the volume ratio of argon to hydrogen is different, specifically Ar:H2=95:5.

[0061] Example 8

[0062] Preparation of lithium manganese iron phosphate positive electrode material:

[0063] a) Dissolve 0.525 mol of lithium carbonate, 0.5 mol of manganese acetate, 0.5 mol of ferrous oxalate, and 1 mol of ammonium dihydrogen phosphate in 200 mL of deionized water. Add 1.5 mol of citric acid and 1.8 mol of ethylene glycol. Heat to 80°C and stir for 6 hours to form a viscous sol. Dry the sol at 120°C in a vacuum for 12 hours to obtain a precursor. Ball mill the precursor for 4 hours and sieve to obtain a powder with a D50 of 0.1 μm.

[0064] b) To the powder obtained in step a), 0.1% by weight of V2O5 was added, and the mixture was mechanically stirred at 150 rpm for uniformity. The mixture was then sintered in two stages under an argon atmosphere. The first stage was sintering at a temperature of 5°C / min to 450°C and holding for 2 h. The second stage was sintering at a temperature of 4°C / min to 800°C and holding for 10 h to obtain a sintered material.

[0065] c) first performing a carbon layer coating on the surface of the sintered material: taking the sintered material obtained in step c), adding sucrose accounting for 3% of the total mass of the sintered material, heating to 600°C, and carbonizing for 3 hours to form a 5nm thick carbon layer; then performing an aluminum oxide layer coating: using atomic deposition (ALD) at a deposition temperature of 150°C, trimethylaluminum and water as precursors, a single deposition of 0.1nm thickness, a total of 25 cycles, and depositing an aluminum oxide layer with a total thickness of 2.5nm on the surface of the carbon layer to obtain the lithium manganese iron phosphate positive electrode material.

[0066] The difference between this embodiment and embodiment 1 is that in step b), 100% argon is used instead of the mixed gas consisting of argon and hydrogen.

[0067] Comparative Example

[0068] Comparative Example 1

[0069] Preparation of lithium manganese iron phosphate positive electrode material:

[0070] a) Dissolve 0.525 mol of lithium carbonate, 0.5 mol of manganese acetate, 0.5 mol of ferrous oxalate, and 1 mol of ammonium dihydrogen phosphate in 200 mL of deionized water. Add 1.5 mol of citric acid and 1.8 mol of ethylene glycol. Heat to 80°C and stir for 6 hours to form a viscous sol. Dry the sol in a vacuum at 120°C for 12 hours to obtain a precursor. Add 0.1% of the total weight of the precursor to the sol. Ball mill the mixture for 4 hours and sieve to obtain a powder with a D50 of 0.1 μm.

[0071] b) The powder obtained in step a) was sintered in a two-stage atmosphere of a mixed gas of argon and hydrogen (volume ratio Ar:H2=90:10). The first stage sintering temperature was raised to 450°C at 5°C / min and held for 2 h. The second stage sintering temperature was raised to 800°C at 4°C / min and held for 10 h to obtain a sintered material.

[0072] c) first performing a carbon layer coating on the surface of the sintered material: taking the sintered material obtained in step c), adding sucrose accounting for 3% of the total mass of the sintered material, heating to 600°C, and carbonizing for 3 hours to form a 5nm thick carbon layer; then performing an aluminum oxide layer coating: using atomic deposition (ALD) at a deposition temperature of 150°C, trimethylaluminum and water as precursors, a single deposition of 0.1nm thickness, a total of 25 cycles, and depositing an aluminum oxide layer with a total thickness of 2.5nm on the surface of the carbon layer to obtain the lithium manganese iron phosphate positive electrode material.

[0073] The difference between this comparative example and Example 1 is that V2O5 is added before the precursor is ball-milled.

[0074] Comparative Example 2

[0075] Preparation of lithium manganese iron phosphate positive electrode material:

[0076] a) Dissolve 0.525 mol of lithium carbonate, 0.5 mol of manganese acetate, 0.5 mol of ferrous oxalate, and 1 mol of ammonium dihydrogen phosphate in 200 mL of deionized water. Add 1.5 mol of citric acid and 1.8 mol of ethylene glycol. Heat to 80°C and stir for 6 hours to form a viscous sol. Dry the sol in a vacuum at 120°C for 12 hours to obtain a precursor. Add 0.1% of the total weight of the precursor to the sol. Ball mill the mixture for 4 hours and sieve to obtain a powder with a D50 of 0.1 μm.

[0077] b) The powder obtained in step a) was sintered in a two-stage atmosphere of a mixed gas of argon and hydrogen (volume ratio Ar:H2=90:10). The first stage sintering temperature was raised to 450°C at 5°C / min and held for 2 h. The second stage sintering temperature was raised to 800°C at 4°C / min and held for 10 h to obtain a sintered material.

[0078] c) first performing a carbon layer coating on the surface of the sintered material: taking the sintered material obtained in step c), adding sucrose accounting for 3% of the total mass of the sintered material, heating to 600°C, and carbonizing for 3 hours to form a 5nm thick carbon layer; then performing an aluminum oxide layer coating: using atomic deposition (ALD) at a deposition temperature of 150°C, trimethylaluminum and water as precursors, a single deposition of 0.1nm thickness, a total of 25 cycles, and depositing an aluminum oxide layer with a total thickness of 2.5nm on the surface of the carbon layer to obtain the lithium manganese iron phosphate positive electrode material.

[0079] The difference between this comparative example and Example 1 is that V2O3 is used instead of V2O5 as the nucleating agent.

[0080] Comparative Example 3

[0081] Preparation of lithium manganese iron phosphate positive electrode material:

[0082] a) Dissolve 0.525 mol of lithium carbonate, 0.5 mol of manganese acetate, 0.5 mol of ferrous oxalate, and 1 mol of ammonium dihydrogen phosphate in 200 mL of deionized water. Add 1.5 mol of citric acid and 1.8 mol of ethylene glycol. Heat to 80°C and stir for 6 hours to form a viscous sol. Dry the sol at 120°C in a vacuum for 12 hours to obtain a precursor. Ball mill the precursor for 4 hours and sieve to obtain a powder with a D50 of 0.1 μm.

[0083] b) To the powder obtained in step a), 0.1% by weight of V2O5 was added to the powder, and the mixture was mechanically stirred at 150 rpm for uniformity. The mixture was then sintered in two stages in a mixed gas atmosphere of argon and hydrogen (volume ratio Ar:H2 = 90:10). The first stage sintering temperature was increased to 450°C at 5°C / min and held for 2 h. The second stage sintering temperature was increased to 800°C at 4°C / min and held for 10 h to obtain a sintered material.

[0084] c) coating the surface of the sintered material with a carbon layer: taking the sintered material obtained in step c), adding sucrose (3% by weight of the total mass of the sintered material), heating to 600° C., and carbonizing for 3 hours to form a 5 nm thick carbon layer, thereby obtaining the lithium manganese iron phosphate positive electrode material.

[0085] The difference between this comparative example and Example 1 is that in step c), no aluminum oxide coating is performed.

[0086] Performance testing

[0087] 1. Physical and chemical properties test

[0088] Particle size D50: Malvern Mastersizer 3000 laser particle size analyzer was used and tested according to GB / T 19077-2016 (particle size distribution-laser diffraction method).

[0089] BET surface area: The BET surface area of ​​lithium iron phosphate cathode material was measured using a Micromeritics TriStar II 3020 instrument. Test method: Weigh the empty tube, then load 10g of lithium iron phosphate cathode material. After degassing at 150°C for 1.5 hours, place the tube containing the sample in the test station and test for 90 minutes. Record the test results.

[0090] Compaction density: The compaction density of the lithium manganese iron phosphate positive electrode material was tested using a powder compaction density meter with a tablet thickness of 0.185 mm and a pressure of 325 MPa.

[0091] 2. Electrochemical performance test

[0092] The positive electrode materials prepared in each Example and Preparation Example were added to N-methyl-2-pyrrolidone with conductive carbon black and PVDF at a mass ratio of 8:1:1. After mixing, the mixture was formed into a slurry. The slurry was coated on aluminum foil and dried in an 80°C oven to form a positive electrode sheet. The positive electrode sheets were assembled into button cells. The electrolyte was 1 mol / L LiPF6 (EC:DMC = 1:1), the negative electrode was a lithium sheet, and the separator was a Celgard 2400 polypropylene porous membrane.

[0093] Initial Coulombic Efficiency: At 25°C, charge and discharge the battery with a current of 0.1C and a voltage of 2.5-4.2V. The initial Coulombic Efficiency is calculated as discharge capacity / charge capacity × 100%.

[0094] Long-cycle stability: At 25°C, charge and discharge the battery with a 3C current. After 1,000 cycles, calculate the battery capacity retention rate.

[0095] Table 1 Test results

[0096]

[0097] Comparing the results of Example 1 and Comparative Example 1 reveals that in Comparative Example 1, where the raw materials are directly mixed with the nucleating agent and then ball-milled, the resulting cathode material exhibits a decreasing particle size, an increasing specific surface area, and a decreasing compaction density. Furthermore, electrochemical performance declines. This may be because V2O5, added to the raw materials before ball milling, participates in the precursor reaction, preventing it from wetting the precursor particles and promoting grain fusion during sintering.

[0098] Comparing the results of Example 1 and Comparative Example 2, it can be seen that the physical properties of the positive electrode material prepared in Comparative Example 2 are quite different from those in Example 1. In particular, the compaction density of the positive electrode material prepared in Comparative Example 2 is significantly reduced, which indicates that the breakage rate of the lithium manganese iron phosphate particles is high during compaction. This is because the melting point of V2O3 is as high as 1970°C, and a liquid phase cannot be formed during the sintering stage, resulting in the inability to promote preferential crystal growth and the inability to reduce the specific surface area of ​​the particles while maintaining good electrochemical properties.

[0099] By comparing the results of Example 1 and Comparative Example 3, it can be seen that the compaction density and electrochemical performance of the positive electrode material prepared in Comparative Example 3 are also reduced. This is because the aluminum oxide layer can, on the one hand, improve the structural stability of the lithium manganese iron phosphate particles, and on the other hand, effectively prevent the dissolution of manganese ions, thereby improving its electrochemical performance.

[0100] From the results of Example 1 and Comparative Example 4, it can be seen that the cycle capacity can be improved by coating the carbon layer with an aluminum oxide layer. The aluminum oxide layer can effectively inhibit the Mn 2+ dissolution, significantly weakening the polarization phenomenon and enabling the battery positive electrode material to exhibit good long-term cycle stability.

[0101] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any form or substance. It should be pointed out that ordinary technicians in this technical field can make several improvements and supplements without departing from the method of the present invention. These improvements and supplements should also be regarded as the scope of protection of the present invention. Any equivalent changes, modifications and evolutions made by technicians familiar with this profession without departing from the spirit and scope of the present invention by using the technical content disclosed above are all equivalent embodiments of the present invention; at the same time, any equivalent changes, modifications and evolutions made to the above embodiments based on the essential technology of the present invention are still within the scope of the technical solution of the present invention.

Claims

1. A method for preparing a low specific surface area lithium manganese iron phosphate positive electrode material, characterized in that: The steps include: a) Raw material screening: ball milling a precursor prepared from a lithium source, an iron source, a manganese source, and a phosphorus source, and sieving to obtain a powder with a D50 of 0.1-0.3 μm; b) Nucleating agent premixing and high-temperature sintering: The powder obtained in step a is mixed with a nucleating agent containing the V element, and then sintered in two stages under an inert gas atmosphere to obtain a sintered material. The first stage is 400-500°C for 1-3 hours, and the second stage is 750-850°C for 8-12 hours. c) Surface coating: The surface of the sintered material is first coated with a carbon layer, and then coated with an aluminum oxide layer by atomic layer deposition; The nucleating agent containing V element is V2O5, and the addition amount of the nucleating agent is 0.1-1% of the total mass of the precursor.

2. The preparation method according to claim 1, characterized in that The lithium source is selected from at least one of lithium oxide, lithium hydroxide, lithium chloride, lithium nitrate, lithium nitrite, lithium acetate, lithium oxalate, lithium carbonate, and lithium citrate; The iron source is selected from at least one of ferrous sulfate, ferric nitrate, ferric acetate, and ferric chloride; The manganese source is selected from at least one of manganese sulfate, manganese nitrate, manganese acetate, and manganese chloride; The phosphorus source includes phosphoric acid and / or phosphate, and the phosphate is selected from at least one of diammonium phosphate, triammonium phosphate, monosodium phosphate, disodium phosphate, and trisodium phosphate.

3. The preparation method according to claim 1, characterized in that The steps of preparing the precursor are as follows: dissolving a lithium source, an iron source, a manganese source and a phosphorus source in deionized water, adding citric acid and ethylene glycol, heating and stirring to form a sol, and drying to obtain the precursor.

4. The preparation method according to claim 1, characterized in that The inert gas is a mixed gas of argon and hydrogen, and the volume ratio of argon to hydrogen is 90-95%:5-10%.

5. The preparation method according to claim 1, characterized in that The heating rate of the first stage sintering is 5-10°C / min, and the heating rate of the second stage sintering is 3-5°C / min.

6. The preparation method according to claim 1, characterized in that The specific preparation steps of the carbon layer are: mixing the sintered material and the carbon source by ball milling, and forming the carbon layer after carbonization.

7. The preparation method according to claim 1, characterized in that The aluminum oxide layer is specifically obtained by depositing the aluminum oxide layer on the surface of the carbon layer by atomic layer deposition.

8. The preparation method according to claim 1, characterized in that The thickness of the carbon layer is 5-10 nm, and the thickness of the aluminum oxide layer is 2.5-3.5 nm.

9. A low specific surface area lithium manganese iron phosphate positive electrode material, characterized in that: The method for preparing a low specific surface area lithium manganese iron phosphate positive electrode material according to any one of claims 1 to 8 is used, wherein the specific surface area of ​​the low specific surface area lithium manganese iron phosphate positive electrode material is 5 m 2 / g~15m 2 / g, and the D50 of the low specific surface area lithium manganese iron phosphate positive electrode material is 0.65-0.75 μm.

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