Lithium iron phosphate positive electrode material, preparation method therefor, and use thereof

Abstract

The present invention provides a lithium iron phosphate positive electrode material, a preparation method therefor, and a use thereof. The preparation method comprises: in a calcination and cooling phase for lithium iron phosphate synthesis, introducing hydrogen to react with lithium iron phosphate, so as to generate highly-conductive Fe2P on the surface of the lithium iron phosphate; then mixing the lithium iron phosphate with ammonium fluoride and carrying out secondary calcination to form a fluorine coating; and forming a stable and robust positive electrode-electrolyte layer on the surface of a positive electrode. By means of composite surface modification, the method of the present invention successfully improves the conductivity of the material and inhibits the decomposition of Fe2P in an electrolytic solution, thereby greatly improving the rate performance and the cycle performance of the material.

Description

A lithium iron phosphate positive electrode material and its preparation method and application Technical Field

[0001] The present invention belongs to the technical field of lithium-ion battery positive electrode materials, and relates to a lithium-ion battery positive electrode material and a preparation method and application thereof, and specifically relates to a lithium iron phosphate positive electrode material with good conductivity and a preparation method and application thereof. Background Art

[0002] Lithium-ion batteries offer advantages such as high energy density, high power density, and long cycle life, and are widely used in equipment such as electric vehicles and energy storage power stations. The four key materials of lithium-ion batteries are the positive electrode, negative electrode, electrolyte, and separator. The positive electrode material accounts for a large proportion of lithium-ion batteries, and its performance directly determines the performance of the lithium-ion battery. Among the various existing lithium-ion battery positive electrode materials, lithium iron phosphate (LIFP) has the advantages of high energy density, long cycle life, good safety performance, and low cost, making it one of the most promising lithium-ion battery positive electrode materials.

[0003] New energy vehicles have increasingly higher requirements for high-rate charge and discharge performance of power batteries. The conductivity of the positive electrode material is an important factor affecting the power performance and life of lithium-ion batteries; however, the poor conductivity of lithium iron phosphate positive electrode materials limits the performance of its electrochemical performance.

[0004] Currently, the best way to improve the conductivity of lithium iron phosphate is carbon coating, but excessive carbon coating leads to a large specific surface area of ​​the material, which reduces the processing performance and compaction density of the material; and the coated carbon is amorphous semiconductor carbon, and its conductivity (<10 -3 Scm -1 ) is much lower than the conductivity of metals, which limits its effect on improving the conductivity of lithium iron phosphate positive electrode materials. Studies have found that excessive reduction of lithium iron phosphate will produce Fe2P with excellent electronic conductivity, whose conductivity is close to that of metals and much higher than that of coated carbon; an appropriate amount of Fe2P can improve the conductivity of lithium iron phosphate, reduce polarization, and improve the rate performance and cycle performance of the material. However, if Fe2P is in direct contact with the electrolyte, it will decompose and produce Fe ions during the charge and discharge process. The Fe ions migrate to the negative electrode through the electrolyte, resulting in a significant reduction in battery capacity and a serious attenuation of cycle life, and even affecting battery safety; therefore, how to prevent Fe2P from decomposing due to direct contact with the electrolyte becomes the key to using Fe2P to improve the conductivity of lithium iron phosphate.

[0005] Summary of the Invention

[0006] In order to improve the conductivity of lithium iron phosphate positive electrode materials, the present invention proposes a method for generating Fe2P with metallic-grade electronic conductivity on the surface of lithium iron phosphate through H2 high-temperature reduction. At the same time, through fluorination treatment, F reacts with residual lithium on the surface of lithium iron phosphate to generate a LiF protective layer coated on the surface of Fe2P, forming a stable and strong positive electrode-electrolyte interphase protective layer on the surface, which can effectively block direct contact between Fe2P and the electrolyte and cause decomposition. The attachment of a certain amount of LiF-coated Fe2P on the surface of lithium iron phosphate can improve the rate performance and cycle performance of the material, and effectively inhibit performance degradation during the cycle process.

[0007] In order to achieve the above object, the present invention adopts the following technical solutions:

[0008] In one aspect, the present invention provides a method for preparing a lithium iron phosphate positive electrode material, the steps of which include:

[0009] 1) Mixing the precursor raw material and the solvent to obtain a slurry, and then grinding;

[0010] 2) drying the ground slurry to obtain a lithium iron phosphate precursor;

[0011] 3) calcining the lithium iron phosphate precursor under a protective gas atmosphere, and then entering a cooling stage after calcination. When the temperature drops to a certain temperature, hydrogen is introduced into the protective gas and kept warm for a period of time; after the insulation is completed, the introduction of H2 is stopped and the temperature is cooled to room temperature to obtain a lithium iron phosphate sintered body; the chemical reactions involved in this step are:

[0012] 6LiFePO4+16H2→2Fe2P+2Li3PO4+2FeP+16H2O↑; It should be noted that since Fe2P itself is not electrochemically active, excessive generation of Fe2P will reduce the specific capacity of the material, so its generation ratio must be strictly controlled;

[0013] 4) mixing the lithium iron phosphate sintered body with ammonium fluoride and calcining the mixture under a protective gas atmosphere to obtain a fluorine-coated lithium iron phosphate sintered body;

[0014] 5) The fluorine-coated lithium iron phosphate sintered body is subjected to air flow pulverization to obtain a finished lithium iron phosphate positive electrode material.

[0015] In some embodiments, in step 1), the slurry includes the following substances in parts by weight: 30-50 parts of precursor raw materials and 50-70 parts of solvent; the amount of precursor raw materials used can be, for example, 35 parts, 40 parts, or 45 parts; the amount of solvent used can be, for example, 55 parts, 60 parts, or 65 parts. By adjusting their relative amounts, the concentration of the precursor slurry can be adjusted.

[0016] In some embodiments, the precursor raw material includes a molar ratio of Li:Fe:P:doping element of (1.02 1.05):1:(1.01-1.05):(0.01-0.02) of lithium source, iron source, phosphorus source and doping element source; the element molar ratio of Li:Fe:P: doping element can be, for example, 1.02:1:1.02:0.01, 1.02:1:1.03:0.01, 1.02:1:1.04:0.01, 1.02:1:1.05:0.01, 1.03:1:1.02:0.01, 1.04:1:1.02:0.01, 1.05:1:1.02:0.01, 1.03:1:1.04:0.01, 1.04:1:1.04:0.01, 1.05:1:1.04:0.01, 1.02:1:1.02:0.02, 1.02:1:1.03:0.02、1.02:1:1.04:0.02、1.02:1:1.05:0.02、1.03:1:1.04:0.02、1.04:1:1.04:0.02、1.05:1:1.04:0.02。

[0017] In some embodiments, the precursor raw material further includes a carbon source, and the carbon source accounts for 8% to 15% of the total weight of the iron source and the phosphorus source, for example, 9%, 10%, 11%, 12%, or 14%.

[0018] In some embodiments, in step 1), the lithium source is selected from at least one of lithium carbonate, lithium hydroxide, lithium acetate, and lithium dihydrogen phosphate;

[0019] The iron source is selected from at least one of ferric phosphate, ferrous oxalate, ferric nitrate and ferric oxide;

[0020] The phosphorus source is selected from at least one of phosphoric acid, lithium dihydrogen phosphate and monoammonium phosphate; and the phosphorus source may also be iron phosphate;

[0021] The doping element source is selected from at least one of Mg compounds, Nb compounds, Ti compounds and Zr compounds;

[0022] The carbon source is selected from at least one of sucrose, glucose, polyethylene glycol and citric acid;

[0023] The solvent is selected from at least one of pure water, ethanol and methanol.

[0024] In some embodiments, in step 1), the solid content of the slurry is not particularly limited, and is preferably 30-50 wt%, for example, 35 wt%, 40 wt%, or 45 wt%; the grinding particle size can be adjusted by controlling the grinding time, the grinding equipment, etc., and the preferred grinding particle size D50 is 0.3-0.6 μm, for example, 0.4 μm, 0.5 μm, or 0.55 μm.

[0025] In some embodiments, in step 2), the drying method is not particularly limited as long as the drying effect can be achieved. Spray drying is preferred. There are no particular restrictions on the drying time and temperature, and those skilled in the art can make routine selections. From the perspective of optimizing material performance, it is preferred to remove moisture as much as possible.

[0026] In some embodiments, the protective gas in step 3) and step 4) can be any non-reactive inert gas, preferably nitrogen from a cost perspective.

[0027] In some embodiments, in step 3), the calcination temperature is 700-800°C (for example, 720°C, 740°C, 750°C, 780°C), the holding time is 8-12h (for example, 9h, 10h, 11h), and the heating rate is not particularly required, preferably 2-5°C / min (for example, 2.5°C / min, 3°C / min, 4°C / min); after the holding stage is completed, the cooling stage is then entered. During the cooling stage, the heat source can be removed and the cooling can be carried out naturally, or the cooling rate can be controlled by external cooling, etc., which is not required. Preferably, hydrogen is introduced when the temperature drops to 600-700°C, for example, 610°C, 620°C, 630°C, 640°C, 650°C, 660°C, 670°C, 680°C, 690°C;

[0028] In some embodiments, in step 3), during the process of introducing hydrogen into the protective gas and keeping warm, the flow ratio of H2:N2 is (1-10):(99-90), for example, 2:98, 3:97, 4:96, 5:95, 6:94, 7:93, 8:92, 9:91, and the insulation time is preferably 5-10h, such as 6h, 7h, 8h, 9h.

[0029] In some embodiments, in step 4), the weight ratio of ammonium fluoride to lithium iron phosphate sintered body is (1-10): (99-90), for example, 2:98, 3:97, 4:96, 5:95, 6:94, 7:93, 8:92, 9:91; the calcination temperature is 400-500°C (for example, 410°C, 420°C, 440°C, 450°C, 460°C, 480°C), the holding time is 5-10h (such as 6h, 7h, 8h, 9h), and the heating rate is 2-5°C / min (for example, 2.5°C / min, 3°C / min, 4°C / min, 4.5°C / min).

[0030] In some embodiments, in step 5), the particle size D50 of the material obtained by air flow milling is 0.5 to 2.0 μm, for example, 0.8 μm, 1.0 μm, 1.5 μm, or 1.8 μm.

[0031] On the other hand, the present invention also provides a lithium iron phosphate positive electrode material prepared by the above preparation method.

[0032] Finally, the present invention also provides the use of the above lithium iron phosphate positive electrode material in a battery.

[0033] The beneficial effects of the present invention are at least:

[0034] 1) Lithium iron phosphate reacts with H2 at high temperatures, partially reducing it to form Fe2P, a material with excellent electronic conductivity. By controlling the amount of Fe2P attached to the surface of lithium iron phosphate, the conductivity of the material is greatly improved, while the proportion of carbon coating can be reduced, improving the compaction performance of the material.

[0035] 2) The F coating reacts with excess lithium on the lithium iron phosphate surface to produce LiF, forming a stable and robust cathode-electrolyte layer on the Fe2P surface, effectively blocking direct contact between the Fe2P and the electrolyte and causing decomposition. Simultaneously, the nitrogen produced by the decomposition of ammonium fluoride can be doped into the carbon-coated skeleton, forming chemically active sites and further enhancing the conductive properties of the coated carbon. Ultimately, this improves the rate capability and cycling performance of the resulting cathode material, effectively suppressing performance degradation during cycling. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 shows the SEM characterization of the obtained products: F-Fe2P-LFP-01;

[0037] FIG2 shows the XRD test of the obtained products: A: LFP-D1, B: F-Fe2P-LFP-01. DETAILED DESCRIPTION

[0038] The embodiments of the present invention will be described in detail below with reference to specific examples, but those skilled in the art will appreciate that the following examples are intended only to illustrate the present invention and should not be construed as limiting the scope of the present invention. Where specific conditions are not specified in the examples, the methods were performed according to conventional conditions or the conditions recommended by the manufacturer. Where the manufacturers of the reagents or instruments are not specified, they are all conventional products that can be purchased commercially.

[0039] Raw materials and sources:

[0040] The lithium source, iron source, phosphorus source, doping element source, hydrogen source and ammonium fluoride source are all purchased from outside.

[0041] The materials were characterized by a Rigaku X-ray powder diffractometer (XRD) and a Zeiss Sigma 500 field emission scanning electron microscope (SEM).

[0042] Test method:

[0043] Full battery formula:

[0044] Cathode system: ratio LFP:SP:PVDF=95.8%:2%:2.2%; solid content 50%; compacted density: 2.40g / cm 3 ; Surface density: 36mg / cm 2 Designed gram capacity: 144 mAh / g (0.5C); aluminum foil 13 μm;

[0045] Anode system: C:SP:CMC:SBR ratio = 96.5%:1:0.9:1.6; designed gram capacity 340mAh / g; surface density 15.8mg / cm 2 ; Compacted density 1.55g / cm 3 ;Copper foil 8μm;

[0046] Diaphragm: 12μm; injection coefficient: 3.5g / Ah.

[0047] DC discharge internal resistance DCR (50% SOC) test method:

[0048] At a constant temperature of 25°C, the battery was allowed to rest for 30 minutes, then charged at a constant current rate of 1C to 3.65V. It was then charged at a constant voltage rate of 3.65V until the current was ≤0.05C. The battery was then allowed to rest for 15 minutes, and then discharged at a constant current rate of 1C to 2.5V. The resulting discharge capacity was taken as the battery's rated capacity, i.e., 100% SOC. The battery was then charged at a current of 1C for 30 minutes, adjusting the battery's state of charge to 50% SOC. The battery was then discharged at a constant current of 4C for 30 seconds, and the voltages before and after discharge were recorded. The DC discharge internal resistance (DCR) was calculated as: DCR = (U0 - U1) / I. Here, U0 is the voltage before 30 seconds of 4C discharge, U1 is the voltage after 30 seconds of discharge, and I is the current at 4C.

[0049] Full capacitance 1C test method:

[0050] At a constant temperature of 25°C, the battery was allowed to stand for 30 minutes, charged at a constant current rate of 1C to 3.65V, then charged at a constant voltage of 3.65V to a current ≤ 0.05C, allowed to stand for 15 minutes, and then discharged at a constant current rate of 1C to 2.5V, thus obtaining the full 1C discharge capacity.

[0051] 5C / 0.5C test method:

[0052] At a constant temperature of 25°C, the battery was left to stand for 30 minutes, charged to 3.65V at a constant current rate of 0.5C, then charged to a current of ≤0.05C at a constant voltage of 3.65V, and then left to stand for 15 minutes. Then, it was discharged to 2.5V at a constant current rate of 0.5C, and the full-electric 0.5C discharge capacity was obtained. Then, it was charged to 3.65V at a constant current rate of 0.5C, then charged to a current of ≤0.05C at a constant voltage of 3.65V, and then left to stand for 15 minutes. Then, it was discharged to 2.5V at a constant current rate of 5C, and the full-electric 5C discharge capacity was obtained. The 5C discharge capacity was divided by the 0.5C discharge capacity to obtain the 5C rate retention rate.

[0053] Cycle 200 cycles (1C, 40°C) test method:

[0054] The battery was placed in a 40°C constant temperature test chamber and allowed to rest for 30 minutes to reach a constant temperature. The battery was then charged to 3.65V at a constant current rate of 1C, then charged at a constant voltage rate of 3.65V until the current was ≤0.05C. The battery was then allowed to rest for 15 minutes, and then discharged to 2.5V at a constant current rate of 1C. This was recorded as the initial discharge capacity C1. This procedure was repeated for 200 cycles, and the discharge capacity after 200 cycles, C200, was recorded. The battery's 200 cycle capacity retention rate was calculated using the formula: 200-cycle capacity retention rate = (C200 / C1) * 100%.

[0055] Example 1

[0056] A method for preparing a lithium iron phosphate positive electrode material comprises the following steps:

[0057] (1) Li2CO3, FePO4, and TiO2 were weighed in a molar ratio of 0.525:1:0.01; the Fe / P (molar ratio) in the FePO4 used was 0.965; sucrose and PEG6000 were weighed in a weight ratio of 1:1, and the total weight of the two accounted for 14% of the weight of FePO4; the above materials were raw material precursors, and pure water was weighed at 1.2 times the weight of the raw material precursor (the solid content of the slurry was 45%);

[0058] Pure water and the raw material precursor were sequentially added into a stirring tank and stirred for 30 minutes, then ground with a sand mill to a particle size of D50 = 0.5 ± 0.05 μm and then spray-dried to obtain a lithium iron phosphate precursor;

[0059] The precursor was placed in a nitrogen atmosphere box furnace and calcined at a temperature of 5°C / min to 770°C and kept at that temperature for 12 h.

[0060] (2) After calcination, the temperature was lowered to 700°C, during which time H2 with a concentration of 2% (i.e., a flow ratio of H2:N2 of 2:98) was introduced into the furnace and the temperature was kept at this temperature for 5 h. After the end of the temperature holding period, the hydrogen introduction was stopped and the temperature was lowered to room temperature to obtain a Fe2P-LFP sintered body;

[0061] (3) The Fe2P-LFP sintered body was mixed with NH4F in a weight ratio of 98:2; then the temperature was raised to 500°C at 5°C / min in a nitrogen atmosphere and calcined for 5 hours to obtain a fluorine-coated lithium iron phosphate sintered body; the temperature was cooled to room temperature and air flow pulverized to obtain the lithium iron phosphate positive electrode material F-Fe2P-LFP-01.

[0062] Example 2

[0063] A method for preparing a lithium iron phosphate positive electrode material comprises the following steps:

[0064] (1) Weigh LiH2PO4, Fe2O3, and MgO in a molar ratio of 1.05:0.5:0.01; weigh sucrose in an amount of 8% of the total weight of LiH2PO4 and Fe2O3; the above materials are raw material precursors, and weigh pure water at 2.3 times the mass of the raw material precursor (the slurry solid content is 30%).

[0065] Pure water and the raw material precursor were sequentially added into a stirring tank and stirred for 30 minutes, then ground with a sand mill to a particle size of D50 = 0.3 ± 0.05 μm and then spray-dried to obtain a lithium iron phosphate precursor;

[0066] The precursor was placed in a nitrogen atmosphere box furnace and calcined at a temperature of 5°C / min to 780°C and kept at that temperature for 10 h.

[0067] (2) After calcination, the temperature was lowered to 650°C, during which time 3% H2 (i.e., a flow ratio of H2:N2 of 3:97) was introduced into the furnace and the temperature was maintained for 8 h. After the end of the temperature maintenance, the hydrogen introduction was stopped and the temperature was lowered to room temperature to obtain a Fe2P-LFP sintered body;

[0068] (3) The Fe2P-LFP sintered body was mixed with NH4F in a weight ratio of 96:4; then the temperature was raised to 450°C at 5°C / min in a nitrogen atmosphere and calcined for 8 hours to obtain a fluorine-coated lithium iron phosphate sintered body; the mixture was cooled to room temperature and subjected to air flow pulverization to obtain the lithium iron phosphate positive electrode material F-Fe2P-LFP-02.

[0069] Example 3

[0070] A method for preparing a lithium iron phosphate positive electrode material comprises the following steps:

[0071] (1) LiOH, FeC2O4·2H2O, NH4H2PO4, and NbO2 were weighed in a molar ratio of 1.02:1:1.01:0.02; glucose was weighed in an amount of 15% of the total weight of FeC2O4·2H2O and NH4H2PO4; the above materials were used as raw material precursors, and ethanol was weighed at 1 times the mass of the raw material precursors (the solid content of the slurry was 50%);

[0072] Ethanol and the raw material precursor were added to a stirring tank and stirred for 30 minutes, then ground with a sand mill to a particle size of D50 = 0.6 ± 0.05 μm, and then dried in a double-cone vacuum dryer to form a lithium iron phosphate precursor;

[0073] The precursor was placed in a nitrogen atmosphere box furnace and calcined at a temperature of 5°C / min to 800°C and kept at that temperature for 8 h.

[0074] (2) After calcination, the temperature was lowered to 600°C, during which time H2 with a concentration of 10% (i.e., a flow ratio of H2:N2 of 10:90) was introduced into the furnace and the temperature was kept at this temperature for 10 h. After the end of the temperature holding period, the hydrogen introduction was stopped and the temperature was lowered to room temperature to obtain a Fe2P-LFP sintered body;

[0075] (3) The Fe2P-LFP sintered body was mixed with NH4F in a weight ratio of 94:6; then the temperature was raised to 400°C at 5°C / min in a nitrogen atmosphere and calcined for 10 hours to obtain a fluorine-coated lithium iron phosphate sintered body; the temperature was cooled to room temperature and air flow pulverized to obtain the lithium iron phosphate positive electrode material F-Fe2P-LFP-03.

[0076] Example 4

[0077] A method for preparing a lithium iron phosphate positive electrode material comprises the following steps:

[0078] (1) Weigh CH3COOLi, Fe(NO3)3, H3PO4, and ZrO2 in a molar ratio of 1.02:1:1.01:0.02; weigh citric acid in an amount of 15% of the total weight of Fe(NO3)3 and H3PO4; the above materials are raw material precursors, and methanol (solid content 50%) is weighed at 1 times the mass of the raw material precursor;

[0079] Anhydrous ethanol and the raw material precursor were added to a stirring tank and stirred for 30 minutes, then ground with a sand mill to a particle size of D50 = 0.6 ± 0.05 μm, and then dried in a double-cone vacuum dryer to form a lithium iron phosphate precursor;

[0080] The precursor was placed in a nitrogen atmosphere box furnace and calcined at a temperature of 5°C / min to 700°C and kept at that temperature for 12 h.

[0081] (2) After calcination, the temperature was lowered to 650°C, during which time H2 with a concentration of 1% (i.e., a flow ratio of H2:N2 of 1:99) was introduced into the furnace and the temperature was maintained for 10 h. After the end of the temperature maintenance, the hydrogen introduction was stopped and the temperature was lowered to room temperature to obtain a Fe2P-LFP sintered body;

[0082] (3) The Fe2P-LFP sintered body was mixed with NH4F in a weight ratio of 94:6; then the temperature was raised to 400°C at 5°C / min in a nitrogen atmosphere and calcined for 10 hours to obtain a fluorine-coated lithium iron phosphate sintered body; the temperature was cooled to room temperature and air flow pulverized to obtain the lithium iron phosphate positive electrode material F-Fe2P-LFP-04.

[0083] Example 5

[0084] The lithium iron phosphate positive electrode material was prepared according to the method of Example 1, except that in step (2), the reaction with H2 was carried out at 700°C for 8 hours to obtain F-Fe2P-LFP-05.

[0085] Example 6

[0086] The lithium iron phosphate positive electrode material was prepared according to the method of Example 1, except that in step (2), the reaction with H2 was carried out at 700°C for 10 hours to obtain F-Fe2P-LFP-06.

[0087] Example 7

[0088] The lithium iron phosphate positive electrode material was prepared according to the method of Example 1, except that in step (3), the Fe2P-LFP sintered body was mixed with NH4F in a weight ratio of 95:5 to obtain F-Fe2P-LFP-07.

[0089] Example 8

[0090] The lithium iron phosphate positive electrode material was prepared according to the method of Example 1, except that in step (3), the Fe2P-LFP sintered body was mixed with NH4F in a weight ratio of 90:10 to obtain F-Fe2P-LFP-08.

[0091] Comparative Example 1

[0092] Lithium iron phosphate positive electrode material was prepared according to the method of Example 1, except that there was no step (2) of reacting with H2 and no step (3) of reacting with NH4F; after step (1) was completed and cooled to room temperature, the product was directly crushed to obtain LFP-D1.

[0093] Comparative Example 2

[0094] The lithium iron phosphate positive electrode material was prepared according to the method of Example 1, except that step (3) of reacting with NH4F was omitted; after step (2) was completed and cooled to room temperature, the product was directly crushed to obtain Fe2P-LFP-D2.

[0095] Comparative Example 3

[0096] A lithium iron phosphate positive electrode material was prepared according to the method of Example 1, except that step (2) of reacting with H2 was omitted, and step (3) was performed after step (1); thereby obtaining F-LFP-D3.

[0097] The performance of the positive electrode materials prepared in the above examples and comparative examples was tested, and the test results are shown in Table 1 below:

[0098] Table 1: Electrochemical performance

[0099] Figure 1 shows the particle morphology of the product F-Fe2P-LFP-01 obtained in Example 1 observed by SEM. As can be seen from Figure 1, F-Fe2P-LFP-01 is a spherical mixture of large and small particles, and the particle size is concentrated between 0.1 and 1 μm. Figure 2 shows the XRD patterns of LFP-D1 and F-Fe2P-LFP-01. As can be seen from Figure 2, the peaks of samples LFP-D1 and F-Fe2P-LFP-01 appear at the same position, with sharp peaks and complete crystal structures, both reflecting the typical olivine structure of LiFePO4 (JCPDS-83-2092) and belonging to the Pmnb space group of the orthorhombic crystal system. However, there are obvious Fe2P peaks (40.3°, 44.2°, 47.3°) in the XRD pattern of sample F-Fe2P-LFP-01, while there is no Fe2P in sample LFP-D1.

[0100] By comparing the data in Table 1, it can be found that the partial formation of Fe2P and its coating by F can reduce the DC discharge internal resistance of the material, which is beneficial to the performance of the capacity / rate / cycle capacity retention rate of the positive electrode material; however, excessive Fe2P will reduce the capacity because it has no electrochemical activity; at the same time, coating by F is the key to improving the rate / cycle capacity retention rate performance; although Fe2P not coated by F is beneficial to the initial capacity and rate, the cycle capacity retention rate performance will deteriorate because Fe2P is exposed in the electrolyte and decomposed.

[0101] In summary, controlling the appropriate Fe2P ratio and F coating are the key to Fe2P improving the electrochemical performance of LFP materials. The product batch with the best overall performance in the table is F-Fe2P-LFP-01, which has the most balanced and excellent electrochemical performance.

[0102] Unless otherwise defined, the terms used in the present invention have the same meanings as those commonly understood by those skilled in the art.

[0103] The embodiments described in the present invention are for illustrative purposes only and are not intended to limit the scope of protection of the present invention. Those skilled in the art may make various other substitutions, changes and improvements within the scope of the present invention. Therefore, the present invention is not limited to the above-mentioned embodiments, but is only limited by the claims.

Claims

1. A method for preparing a lithium iron phosphate positive electrode material, characterized in that: The steps of the preparation method include: 1) Mixing the precursor raw material and the solvent to obtain a slurry, and then grinding; 2) drying the ground slurry to obtain a lithium iron phosphate precursor; 3) calcining the lithium iron phosphate precursor under a protective gas atmosphere, and entering a cooling stage after calcination. When the temperature drops to a certain temperature, hydrogen is introduced into the protective gas and the temperature is kept warm. After the insulation is completed, the introduction of H2 is stopped and the temperature is cooled to room temperature to obtain a lithium iron phosphate sintered body; 4) mixing the lithium iron phosphate sintered body with ammonium fluoride and calcining the mixture under a protective gas atmosphere to obtain a fluorine-coated lithium iron phosphate sintered body; 5) The fluorine-coated lithium iron phosphate sintered body is subjected to air flow pulverization to obtain a finished lithium iron phosphate positive electrode material.

2. The preparation method according to claim 1, characterized in that In step 1), the slurry includes the following substances in parts by weight: 30-50 parts of precursor raw materials and 70-50 parts of solvent; and / or, The precursor raw materials include a lithium source, an iron source, a phosphorus source and a doping element source in a molar ratio of Li:Fe:P:doping element of (1.02 1.05):1:(1.01-1.05):(0.01-0.02).

3. The preparation method according to claim 2, characterized in that: In step 1), the precursor raw material also includes a carbon source, and the carbon source accounts for 8% to 15% of the total weight of the iron source and the phosphorus source.

4. The preparation method according to any one of claims 2 to 3, characterized in that: In step 1), the lithium source is selected from at least one of lithium carbonate, lithium hydroxide, lithium acetate and lithium dihydrogen phosphate; and / or, The iron source is selected from at least one of ferric phosphate, ferrous oxalate, ferric nitrate and ferric oxide; and / or, The phosphorus source is selected from at least one of phosphoric acid, lithium dihydrogen phosphate and monoammonium phosphate; and / or, The doping element source is at least one selected from Mg compounds, Nb compounds, Ti compounds and Zr compounds; and / or, The carbon source is selected from at least one of sucrose, glucose, polyethylene glycol and citric acid; and / or, The solvent is selected from at least one of pure water, ethanol and methanol.

5. The preparation method according to any one of claims 1 to 4, characterized in that: In step 1), the solid content of the slurry is 30-50 wt %, and the grinding particle size D50 is 0.3-0.6 μm; and / or, In step 2), the drying method is spray drying.

6. The preparation method according to any one of claims 1 to 5, characterized in that: In step 3), the calcination temperature is 700-800°C, the holding time is 8-12h, and the heating rate is 2-5°C / min; and / or, In step 3), when the temperature drops to 600-700°C, hydrogen is introduced, the flow ratio of H2:N2 is (1-10):(99-90), and the insulation time is 5-10h.

7. The preparation method according to any one of claims 1 to 6, characterized in that: In step 4), the weight ratio of ammonium fluoride to lithium iron phosphate sintered body is (1-10): (99-90); and / or, The calcination temperature is 400-500°C, the heat preservation time is 5-10h, and the heating rate is 2-5°C / min.

8. The preparation method according to any one of claims 1 to 7, characterized in that: In step 5), the particle size D50 of the material obtained by air flow milling is 0.5-2.0 μm.

9. The lithium iron phosphate positive electrode material prepared by the preparation method according to any one of claims 1 to 8.

10. Use of the lithium iron phosphate positive electrode material prepared by the preparation method according to any one of claims 1 to 9 in a battery.

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