Preparation method of lithium manganese iron phosphate positive electrode material with low specific surface area and lithium manganese iron phosphate positive electrode material

Abstract

The invention discloses a preparation method of a low-specific-surface-area lithium manganese iron phosphate positive electrode material and the low-specific-surface-area lithium manganese iron phosphate positive electrode material. The preparation method specifically comprises the following steps: a) raw material screening: ball-milling and screening a precursor prepared by taking a lithium source, an iron source, a manganese source and a phosphorus source as raw materials to obtain powder with D50 of 1-3 microns; the powder obtained in the step a and the nucleating agent containing the V element are subjected to dry mixing according to the proportion of 0.1 wt%-1 wt%, then heating sintering is conducted in two stages under the inert gas atmosphere, a sintered material is obtained, heat preservation is conducted for 1-3 h at the first stage at the temperature of 400-500 DEG C, and heat preservation is conducted for 8-12 h at the second stage at the temperature of 750-850 DEG C; and c) surface coating: sequentially coating the surface of the sintered material with a carbon layer and a metal oxide layer. The lithium manganese iron phosphate positive electrode material prepared by the invention has a relatively low specific surface area, and also has good processability and high electrochemical performance.

Description

Technical Field

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

[0002] With the rapid development of new energy vehicles and energy storage industries, the improvement of the performance of lithium-ion battery cathode materials has become the 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 relatively low working voltage platform (about 3.4V vs. Li / Li + ) limits the further improvement of energy density. In recent years, lithium iron manganese phosphate (LiMn x Fe 1-x PO4, LMFP) has attracted much attention as an upgraded material of LFP. By introducing manganese element, the working voltage of LMFP can be increased to 4.1V (corresponding to the Mn 3+ / Mn 2+ redox pair), and the theoretical energy density is increased by about 20% compared with LFP. At the same time, it has the advantage of controllable cost and is regarded as an important candidate for the next generation of high-performance and cost-effective cathode materials.

[0003] However, the practical application of LMFP materials still faces severe challenges. First, during the material synthesis process, it is easy to form nanoscale particles with a high specific surface area. In the prior art, the primary particle size of LMFP synthesized by the solid-phase method or the liquid-phase method (such as the hydrothermal method) is generally less than 1μm, resulting in a specific surface area as high as 20 - 40m² / g. Although a high specific surface area is beneficial 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), leading to gas generation, capacity attenuation, and a decrease in cycle life of the battery. Especially under high-temperature (>45°C) or high-voltage (>4.2V) conditions, the side reactions intensify, and the battery performance deteriorates more significantly. Second, the valence state stability problem of manganese element. In LMFP, Mn 3+ is prone to disproportionation reaction (2Mn 3+ → Mn 2+ + Mn 4+ ) during high-temperature sintering or long cycling, resulting in crystal structure collapse and manganese ion dissolution, further accelerating capacity attenuation. Third, it is difficult for the prior art to balance a low specific surface area and high conductivity. For example, although high-temperature sintering (≥800°C) can promote grain growth to reduce the specific surface area, excessive sintering will cause excessive particle agglomeration and a decrease in porosity, resulting in blocked lithium-ion transport paths; in addition, the high-temperature environment will intensify the disproportionation reaction of manganese and damage the structural stability of the material.

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

[0005] Therefore, there is an urgent need for a multi-dimensional collaborative regulation strategy to optimize the whole process from raw material screening, crystal growth control to surface modification, so as 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 lithium iron manganese phosphate cathode material with a low specific surface area and the lithium iron manganese phosphate cathode material.

[0007] In the first aspect, the present application provides a preparation method of a lithium iron manganese phosphate cathode material with a low specific surface area, adopting the following technical scheme: A preparation method of a lithium iron manganese phosphate cathode material with a low specific surface area includes the following steps: a) Raw material screening: Ball-mill the precursor made of lithium source, iron source, manganese source and phosphorus source, and screen to obtain a powder with a D50 of 0.1 - 0.3 μm; b) Nucleating agent premixing and high-temperature sintering: Mix the powder obtained in step a with a nucleating agent containing V element, and then sinter in two stages under an inert gas atmosphere to obtain a sintered material. The first stage is to keep the temperature at 400 - 500 °C for 1 - 3 h, and the second stage is to keep the temperature at 750 - 850 °C for 8 - 12 h; c) Surface coating: Coat a carbon layer and an alumina layer on the surface of the sintered material in sequence.

[0008] Further preferably, 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.

[0009] V2O5 is used as a nucleating agent and mixed with the precursor after ball-milling. 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 nuclei. The nucleating agent promotes crystal growth mainly through the following mechanisms: Liquid-phase assisted sintering: The melting point of V2O5 is 690 °C, and a small amount of liquid phase is formed at 750 - 850 °C, thus wetting the surface of the precursor particles, reducing the grain boundary migration energy barrier, and promoting grain fusion; Lattice doping regulation: V 5+ partially replaces P in PO4 3- or Li 5+ or Li +Sites induce the preferential growth of crystals along the crystal plane of the lithium ion diffusion channel, forming a quasi-single crystal structure with uniform size, reducing the number of grain boundaries, and thus decreasing the specific surface area.

[0010] 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.

[0011] Further preferably, the iron source is selected from at least one of ferrous sulfate, ferric nitrate, ferric acetate, and ferric chloride.

[0012] Further preferably, the manganese source is selected from at least one of manganese sulfate, manganese nitrate, manganese acetate, and manganese chloride.

[0013] Further preferably, the phosphorus source includes phosphoric acid and / or phosphate, and the phosphate is selected from at least one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, triammonium phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, and trisodium phosphate.

[0014] Further preferably, the preparation step of the precursor is as follows: dissolve the lithium source, iron source, manganese source, and phosphorus source in deionized water, add citric acid and ethylene glycol, heat and stir to form a sol, and obtain the precursor after drying.

[0015] Further preferably, the inert gas is a mixed gas of argon and hydrogen. Preferably, the volume ratio of argon to hydrogen is 90 - 95%:5 - 10%.

[0016] Hydrogen can be used as a reducing agent to maintain the balance of oxygen partial pressure in the reaction system and prevent Mn 3+ from being oxidized to Mn 4+ , and 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.

[0017] Further preferably, the heating rate of the first-stage sintering is 5 - 10 °C / min.

[0018] Slowly raise the temperature and keep it warm to promote the decomposition of organic matter and the preliminary melting of some components, and avoid the collapse of internal pores of the precursor caused by rapid heating.

[0019] Further preferably, the heating rate of the second-stage sintering is 3 - 5 °C / min.

[0020] Under the combined action of the nucleating agent and the hydrogen reduction atmosphere, the directional growth and densification of LiMn x Fe 1-x PO4 crystals are completed, and finally quasi-single crystal particles are obtained.

[0021] Further preferably, the specific preparation step of the carbon layer is as follows: ball-mill and mix the sintered material with the carbon source, and form a carbon layer after carbonization.

[0022] Preferably, the carbon source is selected from at least one of glucose, sucrose, starch, and citric acid.

[0023] Preferably, the carbonization temperature is 600 - 610 °C and the carbonization time is 3 - 3.5 h.

[0024] Preferably, the thickness of the carbon layer is 5 - 10 nm. For example, it can 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 value between any two of them.

[0025] More preferably, the alumina layer is specifically deposited on the surface of the carbon layer by atomic layer deposition, and the thickness of the alumina layer is 2.5 - 3.5 nm. For example, it can be any one of 2.5 nm, 3 nm, and 3.5 nm or a range value between any two of them.

[0026] In a second aspect, the present application provides a lithium iron manganese phosphate cathode material with a low specific surface area, and the lithium iron manganese phosphate cathode material with a low specific surface area is obtained by the above preparation method.

[0027] More preferably, the specific surface area of the lithium iron manganese phosphate cathode material is 5 m 2 / g to 15 m 2 / g. For example, it can be any one of 5 m 2 / g, 6 m 2 / g, 7 m 2 / g, 8 m 2 / g, 9 m 2 / g, 10 m 2 / g, 11 m 2 / g, 12 m 2 / g, 13 m 2 / g, 14 m 2 / g, and 15 m 2 / g or a range value between any two of them.

[0028] More preferably, the D50 of the lithium iron manganese 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 value between any two of them.

[0029] In summary, the present application has the following beneficial effects: This application uses a V2O5 nucleating agent and a two-stage gradient sintering process, which can induce liquid-phase-assisted grain orientation growth in the high-temperature stage, enabling the prepared lithium iron manganese phosphate particles to have both a low specific surface area and structural stability. In addition, combined with a double-layer coating structure of a carbon layer and an alumina layer and sintering in an argon / hydrogen mixed reducing atmosphere, the structural stability of the particles is further improved, and the dissolution of manganese ions can be efficiently inhibited, significantly improving the electrochemical performance of the cathode material. Description of the Drawings

[0030] Figure 1 SEM image of the lithium iron manganese phosphate cathode material prepared in Example 1; Figure 2 SEM image of the lithium iron manganese phosphate cathode material prepared in Example 1 after preparing the cathode slurry and compressing it. Detailed Description of the Embodiments

[0031] The following specific examples illustrate the embodiments of the present application. Those skilled in the art can easily understand other advantages and effects of the present application from the content disclosed in this specification. The present application can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present application.

[0032] In addition, it should be understood that one or more method steps mentioned in the present application do not exclude the existence of other method steps before and after the combined steps or the insertion of other method steps between these clearly mentioned steps, unless otherwise stated. Moreover, unless otherwise stated, the numbers of the method steps are only convenient tools for identifying the method steps, rather than limiting the arrangement order of the method steps or the scope in which the present application can be implemented. The change or adjustment of their relative relationship, without substantial change in the technical content, should also be regarded as the scope in which the present application can be implemented.

[0033] If the specific experimental conditions are not specified in the examples, they are usually in accordance with the conventional conditions in the art or the conditions recommended by the reagent company; the materials, reagents, etc. used in the examples, unless otherwise specified, can be obtained through commercial channels.

[0034] Examples Example 1 Preparation of lithium iron manganese phosphate cathode material: a) Take 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 and dissolve them 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 h to form a viscous sol. Vacuum dry the sol at 120 °C for 12 h to obtain a precursor. Ball mill the precursor for 4 h and screen to obtain a powder with D50 = 0.1 μm.

[0035] b) Take the powder obtained in step a), add V2O5 accounting for 0.1% of the total mass of the powder, mechanically stir evenly at 150 rpm, and then carry out two-stage sintering in an atmosphere of a mixed gas of argon and hydrogen (volume ratio Ar:H2 = 90:10). The first-stage sintering: heat up to 450 °C at a rate of 5 °C / min and hold for 2 h. Then carry out the second-stage sintering: heat up to 800 °C at a rate of 4 °C / min and hold for 10 h to obtain the sintered material.

[0036] c) First, carry out carbon layer coating on the surface of the sintered material: Take the sintered material obtained in step c), add sucrose accounting for 3% of the total mass of the sintered material, heat to 600 °C, and carbonize for 3 h to form a carbon layer with a thickness of 5 nm. Then carry out alumina layer coating: Using atomic layer deposition (ALD), the deposition temperature is 150 °C, trimethylaluminum and water are used as precursors, the single deposition thickness is 0.1 nm, and the total number of cycles is 25 times, and an alumina layer with a total thickness of 2.5 nm is deposited on the surface of the carbon layer, thus obtaining the lithium iron manganese phosphate cathode material.

[0037] Figure 1 SEM image of the lithium iron manganese phosphate cathode material prepared in this example. Figure 2 SEM image of the lithium iron manganese phosphate cathode material prepared in this example after preparing the cathode slurry and compressing it. It can be seen that its surface is flat, the connection between particles is tight but there are still some voids, and the infiltration of the electrolyte is not affected due to too high compaction density.

[0038] Example 2 Preparation of lithium iron manganese phosphate cathode material: The preparation steps are the same as those in Example 1, except for the different raw material dosages, specifically: 0.525 mol of lithium carbonate, 0.6 mol of manganese acetate, 0.4 mol of ferrous oxalate, and 1 mol of ammonium dihydrogen phosphate.

[0039] Example 3 Preparation of lithium iron manganese phosphate cathode material: The preparation steps are the same as those in Example 1, except for the different raw material dosages, specifically: 0.525 mol of lithium carbonate, 0.7 mol of manganese acetate, 0.3 mol of ferrous oxalate, and 1 mol of ammonium dihydrogen phosphate.

[0040] Example 4 Preparation of lithium iron manganese phosphate cathode material: The preparation steps are the same as those in Example 1, except for the different raw material dosages, specifically: 0.525 mol of lithium carbonate, 0.8 mol of manganese acetate, 0.2 mol of ferrous oxalate, and 1 mol of ammonium dihydrogen phosphate.

[0041] Example 5 Preparation of lithium iron manganese phosphate cathode material: The preparation steps are the same as those in Example 1, except for the different addition amount of V2O5, specifically: 0.5% V2O5.

[0042] Example 6 Preparation of lithium iron manganese phosphate cathode material: The preparation steps are the same as those in Example 1, except that the addition amount of V2O5 is different, specifically: 1% V2O5.

[0043] Example 7 Preparation of lithium iron manganese phosphate cathode 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.

[0044] Example 8 Preparation of lithium iron manganese phosphate cathode material: a) Take 0.525 mol of lithium carbonate, 0.5 mol of manganese acetate, 0.5 mol of iron oxalate, and 1 mol of ammonium dihydrogen phosphate and dissolve them 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 h to form a viscous sol. Vacuum dry the sol at 120 °C for 12 h to obtain a precursor. Ball mill the precursor for 4 h and screen to obtain a powder with D50 = 0.1 μm.

[0045] b) Take the powder obtained in step a), add 0.1% of V2O5 based on the total mass of the powder, stir evenly at 150 rpm, and then carry out two-stage sintering under an argon gas atmosphere. First-stage sintering: Raise the temperature to 450 °C at a rate of 5 °C / min and hold for 2 h. Then carry out the second-stage sintering: Raise the temperature to 800 °C at a rate of 4 °C / min and hold for 10 h to obtain a sintered material.

[0046] c) First, carry out carbon layer coating on the surface of the sintered material: Take the sintered material obtained in step c), add 3% of sucrose based on the total mass of the sintered material, heat to 600 °C, and carbonize for 3 h to form a 5-nm-thick carbon layer; then carry out alumina layer coating: Use atomic layer deposition (ALD), deposition temperature 150 °C, trimethylaluminum and water as precursors, deposit 0.1 nm thick per cycle, and a total of 25 cycles, deposit a 2.5-nm-thick alumina layer on the surface of the carbon layer to obtain the lithium iron manganese phosphate cathode material.

[0047] The difference between this example and Example 1 is that in step b), 100% argon is used to replace the mixed gas composed of argon and hydrogen.

[0048] Comparative Example Comparative Example 1 Preparation of lithium iron manganese phosphate cathode material: a) Take 0.525 mol of lithium carbonate, 0.5 mol of manganese acetate, 0.5 mol of iron oxalate, and 1 mol of ammonium dihydrogen phosphate and dissolve them 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 h to form a viscous sol. Vacuum dry the sol at 120 °C for 12 h to obtain a precursor. Add V2O5 accounting for 0.1% of the total mass of the precursor, then ball mill for 4 h and sieve to obtain a powder with D50 = 0.1 μm.

[0049] b) Take the powder obtained in step a) and carry out two-stage sintering in an atmosphere of a mixed gas of argon and hydrogen (volume ratio Ar:H2 = 90:10). First-stage sintering: Raise the temperature to 450 °C at a rate of 5 °C / min and hold for 2 h. Then carry out the second-stage sintering: Raise the temperature to 800 °C at a rate of 4 °C / min and hold for 10 h to obtain a sintered material.

[0050] c) First, carry out carbon layer coating on the surface of the sintered material: Take the sintered material obtained in step c), add sucrose accounting for 3% of the total mass of the sintered material, heat to 600 °C, and carbonize for 3 h to form a carbon layer with a thickness of 5 nm. Then carry out alumina layer coating: Use atomic layer deposition (ALD), with a deposition temperature of 150 °C, trimethylaluminum and water as precursors, deposit 0.1 nm thick per cycle, and a total of 25 cycles, to deposit an alumina layer with a total thickness of 2.5 nm on the surface of the carbon layer, thus obtaining the lithium iron manganese phosphate cathode material.

[0051] The difference between this comparative example and Example 1 is that V2O5 is added before the ball milling of the precursor.

[0052] Comparative Example 2 Preparation of lithium iron manganese phosphate cathode material: a) Take 0.525 mol of lithium carbonate, 0.5 mol of manganese acetate, 0.5 mol of iron oxalate, and 1 mol of ammonium dihydrogen phosphate and dissolve them 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 h to form a viscous sol. Vacuum dry the sol at 120 °C for 12 h to obtain a precursor. Add V2O3 accounting for 0.1% of the total mass of the precursor, then ball mill for 4 h and sieve to obtain a powder with D50 = 0.1 μm.

[0053] b) Take the powder obtained in step a) and carry out two-stage sintering in an atmosphere of a mixed gas of argon and hydrogen (volume ratio Ar:H2 = 90:10). First-stage sintering: Raise the temperature to 450 °C at a rate of 5 °C / min and hold for 2 h. Then carry out the second-stage sintering: Raise the temperature to 800 °C at a rate of 4 °C / min and hold for 10 h to obtain a sintered material.

[0054] c) First, perform carbon layer coating on the surface of the sintered material: Take the sintered material obtained in step c), add sucrose accounting for 3% of the total mass of the sintered material, heat to 600 °C, and carbonize for 3 h to form a carbon layer with a thickness of 5 nm; then perform alumina layer coating: Use atomic layer deposition (ALD), with a deposition temperature of 150 °C, trimethylaluminum and water as precursors, deposit 0.1 nm thick per cycle, and perform a total of 25 cycles to deposit an alumina layer with a total thickness of 2.5 nm on the surface of the carbon layer, thus obtaining the lithium iron manganese phosphate cathode material.

[0055] The difference between this comparative example and Example 1 is that: V2O3 is used instead of V2O5 as the nucleating agent.

[0056] Comparative Example 3 Preparation of lithium iron manganese phosphate cathode material: a) Take 0.525 mol of lithium carbonate, 0.5 mol of manganese acetate, 0.5 mol of iron oxalate, and 1 mol of ammonium dihydrogen phosphate and dissolve them 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 h to form a viscous sol. Vacuum dry the sol at 120 °C for 12 h to obtain a precursor. Ball mill the precursor for 4 h and sieve to obtain a powder with D50 = 0.1 μm.

[0057] b) Take the powder obtained in step a), add V2O5 accounting for 0.1% of the total mass of the powder, mechanically stir evenly at 150 rpm, and then perform two-stage sintering in an atmosphere of a mixed gas of argon and hydrogen (volume ratio Ar:H2 = 90:10). First-stage sintering: Raise the temperature to 450 °C at a rate of 5 °C / min and hold for 2 h. Then perform the second-stage sintering: Raise the temperature to 800 °C at a rate of 4 °C / min and hold for 10 h to obtain the sintered material.

[0058] c) Perform carbon layer coating on the surface of the sintered material: Take the sintered material obtained in step c), add sucrose accounting for 3% of the total mass of the sintered material, heat to 600 °C, and carbonize for 3 h to form a carbon layer with a thickness of 5 nm, thus obtaining the lithium iron manganese phosphate cathode material.

[0059] The difference between this comparative example and Example 1 is that: In step c), alumina layer coating is not performed.

[0060] Performance detection test 1. Physical and chemical property test Particle size D50: Use a Malvern Mastersizer 3000 laser particle size analyzer and test according to GB / T 19077-2016 (Particle size distribution - Laser diffraction method).

[0061] Specific surface area (BET): The specific surface area of the lithium iron manganese phosphate cathode material was tested using a Micromeritics TriStar II 3020 device in the United States. Test method: Weigh the mass of the empty tube, then load 10 g of the lithium iron manganese phosphate cathode material. After degassing at 150 °C for 1.5 h, place the tube containing the sample in the test station for testing for 90 min, and record the test results.

[0062] Tap density: The tap density of the lithium iron manganese phosphate cathode material was tested using a powder tap density meter. The tablet thickness was 0.185 mm and the pressure was 325 Mpa.

[0063] 2. Electrochemical performance test The cathode materials prepared in each example and preparation example, conductive carbon black, and PVDF were added to N-methyl-2-pyrrolidone in a mass ratio of 8:1:1. After mixing evenly, a slurry was made. The slurry was coated on an aluminum foil and dried in an 80 °C oven to make a cathode electrode sheet. The cathode electrode sheet was assembled into a button battery. The electrolyte was 1 mol / L LiPF6 (EC:DMC = 1:1), the negative electrode used a lithium sheet, and the separator used a Celgard 2400 polypropylene porous membrane.

[0064] Initial coulombic efficiency: At 25 °C, the battery was charged and discharged at a current of 0.1C, and the voltage was 2.5 - 4.2V. The initial coulombic efficiency was calculated as discharge capacity / charge capacity × 100%.

[0065] Long cycle stability: At 25 °C, the battery was charged and discharged at a current of 3C. After 1000 cycles, the battery capacity retention rate was calculated.

[0066] Table 1 Test results Comparing the results of Example 1 and Comparative Example 1, it can be seen that in Comparative Example 1, each raw material was directly mixed with a nucleating agent and then ball-milled. For the obtained cathode material, its particle size showed a decreasing trend, the specific surface area showed an increasing trend, while the tap density decreased instead. In addition, the electrochemical performance all decreased. This may be because when V2O5 was added to the raw materials before ball-milling, it participated in the reaction of the precursor and could not play the role of wetting the precursor particles and promoting grain fusion during the sintering process.

[0067] Comparing the results of Example 1 and Comparative Example 2, it can be seen that the physical properties of the cathode material prepared in Comparative Example 2 are quite different from those of Example 1. Especially, the tap density of the cathode material prepared in Comparative Example 2 decreased significantly. This indicates that the breakage rate of lithium iron manganese phosphate particles during compaction is relatively high. This is because the melting point of V2O3 is as high as 1970 °C, and no liquid phase can be formed during the sintering stage, resulting in the inability to promote the preferred growth of crystals and the inability to reduce the specific surface area of particles while maintaining good electrochemical performance.

[0068] Comparing the results of Example 1 and Comparative Example 3, it can be seen that for the positive electrode material prepared in Comparative Example 3, its tap density and electrochemical performance also decreased to some extent. This is because on the one hand, the alumina layer can improve the structural stability of the lithium iron phosphate manganese particles, and on the other hand, it can effectively prevent the dissolution of manganese ions, thereby improving its electrochemical performance.

[0069] From the results of Example 1 and Comparative Example 4, it can be seen that by further coating the alumina layer on the surface of the carbon layer, the cycling ability can be improved. After the surface of the alumina layer is coated, the dissolution of Mn 2+ can be effectively inhibited, the polarization phenomenon is significantly weakened, and the positive electrode material of the battery shows good long-term cycling stability.

[0070] The above are only the preferred embodiments of the present invention, and do not limit the present invention in any form or substance. It should be pointed out that for those of ordinary skill in the art in this technical field, without departing from the method of the present invention, several improvements and supplements can still be made, and these improvements and supplements should also be regarded as the protection scope of the present invention. Any equivalent changes in the form of slight changes, modifications and evolutions that can be made by those who are familiar with this professional technology 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 in the form of slight changes, modifications and evolutions made to the above embodiments according to the essential technology of the present invention still fall within the scope of the technical solution of the present invention.

Claims

1. A preparation method of a lithium iron manganese phosphate cathode material with a low specific surface area, characterized in that, It includes the following steps: a) Raw material screening: The precursor made of lithium source, iron source, manganese source and phosphorus source as raw materials is ball-milled and screened to obtain a powder with 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 V element, and then sintered by heating in two stages under an inert gas atmosphere to obtain a sintered material. The first stage is to keep the temperature at 400 - 500 °C for 1 - 3 h, and the second stage is to keep the temperature at 750 - 850 °C for 8 - 12 h; c) Surface coating: A carbon layer and an alumina layer are sequentially coated on the surface of the sintered material; 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, wherein 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, lithium citrate; The iron source is selected from at least one of ferrous sulfate, iron nitrate, iron acetate, iron chloride; The manganese source is selected from at least one of manganese sulfate, manganese nitrate, manganese acetate, manganese chloride; The phosphorus source includes phosphoric acid and / or phosphate, and the phosphate is selected from at least one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, triammonium phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, trisodium phosphate; 3. The preparation method according to claim 1, characterized in that, The preparation steps of the precursor are: Dissolve the lithium source, iron source, manganese source and phosphorus source in deionized water, add citric acid and ethylene glycol, heat and stir to form a sol, and then obtain the precursor after drying.

4. The preparation method according to claim 1, wherein 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: Ball-mill and mix the sintered material with a carbon source, and form a carbon layer after carbonization.

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

8. The preparation method according to claim 1, wherein The thickness of the carbon layer is 5 - 10 nm, and the thickness of the alumina layer is 2.5 - 3.5 nm.

9. A lithium iron manganese phosphate cathode material with a low specific surface area, characterized in that, It is prepared by using the preparation method of the lithium iron manganese phosphate cathode material with a low specific surface area as described in any one of claims 1-8, and the specific surface area of the lithium iron manganese phosphate cathode material with a low specific surface area is 5 m 2 / g to 15 m 2 / g, and the D50 of the lithium iron manganese phosphate cathode material with a low specific surface area is 0.65 - 0.75 μm.

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