Modified Lithium Iron Phosphate Cathode Material, Its Preparation Method and Application

Abstract

The present invention provides a modified lithium iron phosphate cathode material, a preparation method thereof and an application. The modified lithium iron phosphate cathode material comprises a lithium iron phosphate doping material and aluminum dihydrogen phosphate coated on the surface thereof; wherein, the lithium iron phosphate doping material is a lithium iron phosphate material doped with lanthanum and fluorine, and its chemical general formula is Li<subgt;1‑a+b< / subgt;Fe<subgt;1‑a< / subgt;PO<subgt;4< / subgt;La<subgt;a< / subgt>F<subgt;b< / subgt>, where 0.0004 ≤ a ≤ 0.03 and 0.0004 ≤ b ≤ 0.03. Through the synergistic effect of double modification by doping and coating, the modified lithium iron phosphate cathode active material of the present invention has good electrochemical performance consistency, reduced residual lithium amount on the surface, suppressed surface side reactions, significantly improved electronic conductivity and lithium ion diffusion rate, and at the same time has significantly improved tap density and low temperature resistance.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium ion batteries, and in particular, to a modified lithium iron phosphate cathode material, a preparation method thereof, and an application thereof. Background Art

[0002] With the rapid development of social economy, modern science and technology, and information industrialization, more and more new electronic products are widely used in different fields, and environmentally friendly batteries and their related industries play an increasingly important role in the high-tech industrial chain. Lithium iron phosphate materials have the characteristics of good safety performance, light weight, large energy storage, long life, excellent cycle performance, environmental friendliness, and wide raw material sources. They are recognized as one of the most important cathode materials for the new generation of lithium ion batteries and are widely used in power sources, power tools, lamps, meters, communications, automotive electronics, and some defense industrial fields. Driven by the market of power tools, electric scooters, and hybrid vehicles, the high-energy lithium iron phosphate battery industry is growing rapidly.

[0003] Lithium iron phosphate (LiFePO4) with an olivine structure has always accounted for a large proportion in the cathode materials of lithium ion batteries due to its advantages of stable working voltage (3.4V), relatively high specific capacity (170 mAh / g), large discharge power, fast chargeability, long cycle life, no memory effect, and good stability in high temperature and high heat environments. Although the advantages of LiFePO4 are very obvious, it also has serious disadvantages, such as low electronic conductivity and low lithium ion diffusion rate, low tap density, and poor low-temperature resistance, which to a certain extent affect the market application of LiFePO4. Summary of the Invention

[0004] The main object of the present invention is to provide a modified lithium iron phosphate cathode material, a preparation method thereof, and an application thereof, so as to solve at least one of the problems of low electronic conductivity, low lithium ion diffusion rate, low tap density, and poor low-temperature resistance of lithium iron phosphate materials in the prior art.

[0005] To achieve the above object, according to one aspect of the present invention, there is provided a modified lithium iron phosphate cathode material, including a lithium iron phosphate doping material and aluminum dihydrogen phosphate coated on its surface; wherein, the lithium iron phosphate doping material is a lithium iron phosphate material doped with lanthanum and fluorine, and the chemical general formula is Li 1-a+b Fe 1-a PO4La a F b , where 0.0004 ≤ a ≤ 0.03 and 0.0004 ≤ b ≤ 0.03.

[0006] Further, 0.0004 ≤ a ≤ 0.0025, 0.0004 ≤ b ≤ 0.0025; and / or the tap density of the modified lithium iron phosphate cathode material ≥ 1.25 g / cm 3 .

[0007] According to another aspect of the present invention, there is provided a method for preparing the above-mentioned modified lithium iron phosphate cathode material of the present invention, comprising the following steps: Step S1, mixing a lithium source, an iron source, a phosphorus source, a lanthanum source, a fluorine source, a carbon source and water, and successively performing first grinding and drying to obtain a mixed material A; Step S2, performing first sintering on the mixed material A to obtain a lithium iron phosphate doped material; Step S3, mixing the lithium iron phosphate doped material with an aluminum dihydrogen phosphate precursor, and then performing second grinding to obtain a mixed material B; Step S4, performing second sintering on the mixed material B to obtain a modified lithium iron phosphate cathode material.

[0008] Further, the weight ratio of the lithium iron phosphate doped material to the aluminum dihydrogen phosphate precursor is 100:(0.35 - 6.5); preferably, the aluminum dihydrogen phosphate precursor comprises aluminum phosphate and diammonium hydrogen phosphate; more preferably, the weight ratio of aluminum phosphate to diammonium hydrogen phosphate is (0.0005 - 0.005):(0.003 - 0.06).

[0009] Further, in Step S1, the lithium source is one or more of lithium hydroxide, lithium carbonate, lithium nitrate and lithium acetate; and / or the iron source is iron phosphate; and / or the phosphorus source is iron phosphate; and / or the lanthanum source is one or more of lanthanum oxide, lanthanum acetate and lanthanum carbonate; and / or the fluorine source is one or more of lithium fluoride, sodium fluoride and potassium fluoride; and / or the carbon source is one or more of glucose, sucrose, starch, cyclodextrin and citric acid.

[0010] Further, in Step S1, the first grinding method is sand grinding, the rotation speed is 100 - 800 r / min, and the time is 1 - 6 h; and / or the drying method is spray drying, the inlet air temperature is 280 - 380 °C, and the drying time is 1 - 6 h.

[0011] Further, in Step S2, the first sintering is carried out in an inert gas environment, and the inert gas is nitrogen and / or argon; and / or the heating rate of the first sintering is 1 - 10 °C / min, the sintering temperature is 600 - 800 °C, and the sintering time is 6 - 15 h.

[0012] Further, in Step S3, the second grinding method is ball grinding, the rotation speed is 100 - 800 r / min, and the time is 1 - 6 h.

[0013] Further, in step S4, the second sintering is carried out in an inert gas atmosphere, and the inert gas is nitrogen and / or argon; and / or the heating rate of the second sintering is 1-10 °C / min, the sintering temperature is 500-780 °C, and the sintering time is 6-10 h.

[0014] According to another aspect of the present invention, there is provided a battery, including a positive electrode material, and the positive electrode material includes the modified lithium iron phosphate positive electrode material of the present invention as described above, or the modified lithium iron phosphate positive electrode material obtained by using the preparation method of the present invention as described above.

[0015] Applying the technical solution of the present invention, doping La and F enables the surface of the lithium iron phosphate material to have good lithium ion conduction characteristics and electronic conductivity characteristics, which can increase the crystal plane spacing, thereby improving the tap density. At the same time, there is aluminum dihydrogen phosphate coating on the surface of the lithium iron phosphate dopant, which can effectively resist the erosion of the electrolyte on the surface of the positive electrode material, avoid side reactions between the positive electrode material and the electrolyte, and improve the low-temperature performance and cycle performance of the lithium iron phosphate material. In summary, the modified lithium iron phosphate positive electrode active material of the present invention has La and F doping modification and aluminum dihydrogen phosphate coating modification. Through the synergistic effect of double modification of doping and coating, the electrochemical performance of the modified lithium iron phosphate positive electrode active material can be made consistent, the residual lithium amount on the surface can be reduced, the side reactions on the surface can be inhibited, and the electronic conductivity and lithium ion diffusion rate are significantly improved. At the same time, the tap density and low-temperature performance are significantly improved. Description of the Drawings

[0016] The specification drawings forming a part of this application are used to provide a further understanding of the present invention. The schematic embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation to the present invention. In the drawings:

[0017] Figure 1 Shows a scanning electron micrograph of the modified lithium iron phosphate positive electrode material according to Embodiment 1 of the present invention;

[0018] Figure 2 Shows a scanning electron micrograph of the lithium iron phosphate positive electrode material according to Comparative Example 1; and

[0019] Figure 3 Shows a scanning electron micrograph of the lithium iron phosphate positive electrode material according to Comparative Example 2. Detailed Embodiments

[0020] It should be noted that, without conflict, the embodiments in this application and the features in the embodiments can be combined with each other. The present invention will be described in detail below with reference to the drawings and in combination with the embodiments.

[0021] As described in the background art of the present invention, there are problems in the prior art such as low electronic conductivity, low lithium ion diffusion rate, low tap density, or poor low-temperature resistance of lithium iron phosphate materials. To solve the above problems, in a typical embodiment of the present invention, a modified lithium iron phosphate cathode material is provided, which includes a lithium iron phosphate doping material and aluminum dihydrogen phosphate coated on its surface; wherein, the lithium iron phosphate doping material is a lithium iron phosphate material doped with lanthanum and fluorine, and its chemical general formula is Li 1-a+b Fe 1-a PO4La a F b , where 0.0004 ≤ a ≤ 0.03 and 0.0004 ≤ b ≤ 0.03.

[0022] In the lithium iron phosphate doping material of the present invention, lanthanum and fluorine elements are doped in a specific proportion, which can increase the crystal plane spacing, thereby promoting the migration of lithium ions, making the chemical bonds formed with metal ions in the lithium iron phosphate material more firm, thereby improving the stability of the material structure, increasing the tap density and low-temperature resistance of the modified lithium iron phosphate cathode material, and improving the electronic conductivity and lithium ion diffusion rate. It should be noted that the inventor unexpectedly found during the research process that only by doping lanthanum elements can the unit cell parameters of the modified material be reduced and the unit cell shrink, which is beneficial to the stability of the material, and other rare earth elements cannot achieve the above properties of the present invention.

[0023] At the same time, the surface of the lithium iron phosphate doping material of the present invention is coated with a specific phosphate, and the material surface has good lithium ion conduction characteristics and certain electronic conductivity characteristics, making it have higher delithiated state stability under high voltage. Moreover, the coating of aluminum dihydrogen phosphate can effectively resist the erosion of the electrolyte on the surface of the cathode material, avoid side reactions between the cathode material and the electrolyte, further increase the tap density, electronic conductivity and lithium ion diffusion rate of the modified lithium iron phosphate cathode material, and improve its low-temperature resistance and cycle performance. When the modified lithium iron phosphate cathode material of the present invention is used for the positive electrode sheet, the compaction density of the sheet can reach 2.65 g / cm 3 , and the discharge capacity retention rate of the soft-pack battery made therefrom can reach 90% at -20 °C, and the low-temperature resistance of the battery is significantly improved.

[0024] In a preferred embodiment, 0.0004 ≤ a ≤ 0.0025 and 0.0004 ≤ b ≤ 0.0025. Most preferably, the modified lithium iron phosphate cathode material is Li 1.0021 Fe 0.9996 PO4La 0.0004 F 0.0025 ; and / or the tap density of the modified lithium iron phosphate cathode material ≥ 1.25 g / cm 3 , such as 1.25 - 3 g / cm 3, the low-temperature resistance and cycling performance of the modified lithium iron phosphate cathode material are better under the above conditions.

[0025] In another typical embodiment of the present invention, a preparation method of the above-mentioned modified lithium iron phosphate cathode material is further provided, including the following steps: Step S1, mixing a lithium source, an iron source, a phosphorus source, a lanthanum source, a fluorine source, a carbon source and water, and sequentially performing first grinding and drying to obtain a mixed material A; Step S2, performing first sintering on the mixed material A to obtain a lithium iron phosphate doped material; Step S3, mixing the lithium iron phosphate doped material with an aluminum dihydrogen phosphate precursor, and then performing second grinding to obtain a mixed material B; Step S4, performing second sintering on the mixed material B to obtain a modified lithium iron phosphate cathode material.

[0026] In the present invention, first, a lithium source, an iron source, a phosphorus source, a lanthanum source, a fluorine source, a carbon source and water are mixed and preliminarily dispersed in water, and then first grinding and drying are sequentially performed. After mixing evenly, a mixed material A is obtained; secondly, the mixed material A is subjected to first sintering, so that lanthanum and fluorine further enter the lithium iron phosphate material to achieve doping. After obtaining the lithium iron phosphate doped material, it is mixed with an aluminum dihydrogen phosphate precursor, and then second grinding is performed. After mixing evenly, a mixed material B is obtained and subjected to second sintering. Among them, the carbon source is used as a fuel, so that aluminum dihydrogen phosphate is coated on the surface of the lithium iron phosphate doped material to obtain a modified lithium iron phosphate cathode material.

[0027] In the process of preparing the lithium iron phosphate doped material in the present invention, lanthanum and fluorine elements are doped, so that the lanthanum and fluorine elements enter the interior of the cathode material, ensuring the uniformity of the distribution of the doped ions in the cathode material, which can increase the crystal plane spacing and thus promote the migration of lithium ions, making the chemical bonds formed with metal ions in the cathode material more firm. Doping lanthanum and fluorine can not only promote sintering and make the structure more uniform, but also improve the stability of the material structure, increase the tap density of lithium iron phosphate, improve the low-temperature resistance, and increase the electronic conductivity and lithium ion diffusion rate. Then, a modified lithium iron phosphate cathode material is prepared by surface coating with aluminum dihydrogen phosphate, so that the material surface has good lithium ion conduction characteristics and certain electronic conductivity characteristics, making it have higher delithiated state stability under high voltage. And the preparation process of the present invention is simple, easy to operate, and easy to realize industrial production, and has broad application prospects.

[0028] For the purpose of enabling aluminum dihydrogen phosphate to complete coating on the surface of the lithium iron phosphate doping material while avoiding excessive coating on the surface of the doping material, which reduces the capacity of the cathode material, thereby further improving the electronic conductivity and lithium ion diffusion rate of the material. In a preferred embodiment, in step S3, the weight ratio of the lithium iron phosphate doping material to the aluminum dihydrogen phosphate precursor is 100:(0.35 - 6.5); preferably, the aluminum dihydrogen phosphate precursor includes aluminum phosphate and diammonium hydrogen phosphate. When sintered, diammonium hydrogen phosphate decomposes at high temperature to generate phosphoric acid, which reacts with aluminum phosphate to generate aluminum dihydrogen phosphate to achieve coating. More preferably, the weight ratio of aluminum phosphate to diammonium hydrogen phosphate is (0.0005 - 0.005):(0.003 - 0.06), the coating reaction is more complete, and the coating modification effect of the material is better. In the specific preparation process, aluminum phosphate and diammonium hydrogen phosphate can be added in the form of an aqueous solution. Preferably, the diammonium hydrogen phosphate aqueous solution is an aqueous solution with a mass percentage content of diammonium hydrogen phosphate of 20 - 40%, and the coating is more uniform.

[0029] For the purpose of further improving the stability of the lithium iron phosphate doping material, thereby further improving the tap density, low-temperature resistance of lithium iron phosphate, as well as improving the electronic conductivity and lithium ion diffusion rate. In a preferred embodiment, in step S1, the lithium source is one or more of lithium hydroxide, lithium carbonate, lithium nitrate, and lithium acetate; and / or the iron source is iron phosphate; and / or the phosphorus source is iron phosphate; and / or the lanthanum source is one or more of lanthanum oxide, lanthanum acetate, and lanthanum carbonate; and / or the fluorine source is one or more of lithium fluoride, sodium fluoride, and potassium fluoride; and / or the carbon source is one or more of glucose, sucrose, starch, cyclodextrin, and citric acid. The above raw materials can further reduce the preparation cost. Among them, each raw material can be added according to the elemental stoichiometric ratio of the modified lithium iron phosphate cathode material, which can be understood by those skilled in the art and will not be elaborated here. The lithium source can be added in an amount of 1.0 - 1.15 times the stoichiometric ratio, so as to further compensate for the loss caused by the sublimation of lithium during high-temperature sintering.

[0030] The specific grinding and dispersion process adopted by the present invention can make the particle size distribution of the lithium iron phosphate doping material more uniform. In a preferred embodiment, in step S1, the first grinding method is sand grinding, the rotation speed is 100 - 800 r / min, and the time is 1 - 6 h, and the dispersion effect is better; and / or the drying method is spray drying, the inlet air temperature is 100 - 150 °C, and the drying time is 1 - 6 h. The drying rate is fast, and the material can be dried more uniformly.

[0031] Correspondingly, in a preferred embodiment, in step S3, the second grinding method is ball grinding, the rotation speed is 100 - 800 r / min, and the time is 1 - 6 h, so as to make the materials mix more fully.

[0032] In a preferred embodiment, in step S2, the first sintering is carried out in an inert gas environment, and the inert gas is nitrogen and / or argon, which can avoid the oxidation side effects of oxygen and the like on the materials or raw materials; and / or the heating rate of the first sintering is 1-10 °C / min, the sintering temperature is 600-800 °C, and the sintering time is 6-15 h, so that the raw materials become liquid, which is more conducive to the formation of lithium iron phosphate. At the same time, as described above, lanthanum and fluorine elements are also doped in the materials of the present invention, which can promote sintering. Therefore, a relatively low sintering temperature and a shorter sintering time can be used to achieve relatively sufficient sintering and obtain materials with uniform structure.

[0033] For similar reasons, in a preferred embodiment, in step S4, the second sintering is carried out in an inert gas environment, and the inert gas is nitrogen and / or argon; and / or the heating rate of the second sintering is 1-10 °C / min, the sintering temperature is 500-780 °C, and the sintering time is 6-10 h, which is more conducive to the sufficient coating of aluminum dihydrogen phosphate on the surface of the lithium iron phosphate doping material.

[0034] Typical but non-limiting, in the above lithium iron phosphate doping material, a is 0.0005, 0.001, 0.002, 0.003, 0.004, 0.005, 0.006, 0.007, 0.008, 0.009, 0.01, 0.011, 0.012, 0.013, 0.014, 0.015, 0.016, 0.017, 0.018, 0.019, 0.02, 0.021, 0.022, 0.023, 0.024, 0.025, 0.026, 0.027, 0.028, 0.029, 0.03 or a range value composed of any two of these values. b is 0.0005, 0.001, 0.002, 0.003, 0.004, 0.005, 0.006, 0.007, 0.008, 0.009, 0.01, 0.011, 0.012, 0.013, 0.014, 0.015, 0.016, 0.017, 0.018, 0.019, 0.02, 0.021, 0.022, 0.023, 0.024, 0.025, 0.026, 0.027, 0.028, 0.029, 0.03 or a range value composed of any two of these values.

[0035] Typical but non-limiting, in the preparation method of the modified lithium iron phosphate cathode material, the weight ratio of the lithium iron phosphate doping material to the aluminum dihydrogen phosphate precursor is 100:0.35, 100:0.5, 100:1, 100:1.5, 100:2, 100:2.5, 100:3, 100:3.5, 100:4, 100:4.5, 100:5, 100:5.5, 100:6, 100:6.5 or a range value composed of any two of these ratios.

[0036] Typically but not limitedly, the mass ratio of the above-mentioned aluminum phosphate, diammonium hydrogen phosphate, and lithium iron phosphate dopant is the following values or a range value composed of any two of these values: taking the mass of the lithium iron phosphate dopant as 1, the aluminum phosphate is 0.0005, 0.001, 0.0015, 0.002, 0.0025, 0.003, 0.0035, 0.004, 0.0045, 0.005; the diammonium hydrogen phosphate is 0.003, 0.004, 0.005, 0.006, 0.007, 0.008, 0.009, 0.01, 0.015, 0.02, 0.025, 0.03, 0.035, 0.04, 0.045, 0.05, 0.055, 0.06.

[0037] Typically but not limitedly, the inlet air temperature of the above spray drying is 280 °C, 290 °C, 300 °C, 310 °C, 320 °C, 330 °C, 340 °C, 350 °C, 360 °C, 370 °C, 380 °C or a range value composed of any two of these values, and the drying time is 1 h, 2 h, 3 h, 4 h, 5 h, 6 h or a range value composed of any two of these values.

[0038] Typically but not limitedly, the sintering temperature of the above first sintering is 600 °C, 620 °C, 640 °C, 660 °C, 680 °C, 700 °C, 720 °C, 740 °C, 760 °C, 780 °C, 800 °C or a range value composed of any two of these values, and the sintering time is 6 h, 7 h, 8 h, 9 h, 10 h, 11 h, 12 h, 13 h, 14 h, 15 h or a range value composed of any two of these values.

[0039] Typically but not limitedly, the sintering temperature of the above second sintering is 500 °C, 520 °C, 540 °C, 560 °C, 580 °C, 600 °C, 620 °C, 640 °C, 660 °C, 680 °C, 700 °C, 720 °C, 740 °C, 760 °C, 780 °C or a range value composed of any two of these values, and the sintering time is 6 h, 7 h, 8 h, 9 h, 10 h or a range value composed of any two of these values.

[0040] In another typical embodiment of the present invention, a battery is further provided, including a positive electrode material, and the positive electrode material includes the modified lithium iron phosphate positive electrode material of the present invention, or the modified lithium iron phosphate positive electrode material obtained by using the above preparation method of the present invention. The electronic conductivity and lithium ion diffusion rate of the above battery can be significantly improved, and at the same time, the tap density and low temperature resistance of the material can be improved.

[0041] The following further describes the present application in detail with specific embodiments, and these embodiments should not be construed as limiting the scope claimed by the present application.

[0042] Example 1

[0043] Step S1: Anhydrous ferric phosphate, La2O3, LiF, glucose, and Li2CO3 were weighed in a molar ratio of 1:0.0008:0.025:0.001:1.05, mixed evenly, and added to a dispersion (water) for sand milling at a speed of 300 r / min for 3 hours to obtain a sand-milled slurry. The sand-milled slurry was spray dried at an inlet air temperature of 300°C for 3 hours to obtain a mixed material A.

[0044] Step S2: In a nitrogen environment, the mixture A is placed in a muffle furnace and heated to 770°C at a heating rate of 5°C / min, calcined for 10 hours, and then naturally cooled to room temperature, ground, crushed and sieved to prepare a lithium iron phosphate doping material. The obtained lithium iron phosphate doping material has the chemical formula Li 1.0021 Fe 0.9996 PO4La 0.0004 F 0.0025 ;

[0045] Step S3: Aluminum phosphate, diammonium hydrogen phosphate, and lithium iron phosphate doping material are mixed in a mass ratio of 0.003:0.015:1 and ball milled at a speed of 300 r / min for 5 hours to obtain a mixed material B, wherein the diammonium hydrogen phosphate is added in the form of an aqueous solution, wherein the mass fraction of the diammonium hydrogen phosphate is 30%;

[0046] Step S4: Under an inert gas environment, the mixed material B is placed in a tube furnace and heated from room temperature to 730°C at a heating rate of 5°C / min, calcined for 8 hours, and naturally cooled to room temperature. The mixed material is then ground, crushed, and sieved to prepare a modified lithium iron phosphate positive electrode material.

[0047] Example 2

[0048] Step S1: Anhydrous ferric phosphate, La2O3, LiF, glucose, and Li2CO3 were weighed in a molar ratio of 1:0.0008:0.025:0.001:1.05, mixed evenly, and added to a dispersion (water) for sand milling at a speed of 300 r / min for 3 hours to obtain a sand-milled slurry. The sand-milled slurry was spray dried at an inlet air temperature of 300°C for 3 hours to obtain a mixed material A.

[0049] Step S2: In an inert gas environment, the mixture A is placed in a muffle furnace and heated to 770°C at a heating rate of 5°C / min, calcined for 10 hours, and then naturally cooled to room temperature, ground, crushed and sieved to prepare a lithium iron phosphate doping material. The obtained lithium iron phosphate doping material has the chemical formula Li 1.0021 Fe 0.9996 PO4La0.0004 F 0.0025 ;

[0050] Step S3: Mix aluminum phosphate, aqueous solution of diammonium hydrogen phosphate, and lithium iron phosphate doping material in a mass ratio of 0.003:0.015:1, and then ball mill them. The rotation speed of the ball mill is 300 r / min and the time is 5 h to obtain a mixed material B. The diammonium hydrogen phosphate is added in the form of an aqueous solution, and the mass fraction of the diammonium hydrogen phosphate is 30%;

[0051] Step S4: Under an inert gas environment, place the mixed material B in a tube furnace and heat it from room temperature to 730 °C at a heating rate of 5 °C / min, calcine for 8 h, naturally cool to room temperature, then grind, crush, and screen to prepare a modified lithium iron phosphate cathode material.

[0052] Example 3

[0053] Step S1: Weigh anhydrous iron phosphate, La2O3, LiF, glucose, and Li2CO3 according to a molar ratio of 1:0.0008:0.025:0.001:1.12, mix them evenly, then add them to a dispersion liquid (water) for sand milling. The rotation speed of the sand milling is 300 r / min and the time is 3 h to obtain a sand-milled slurry. Spray dry the sand-milled slurry. The inlet air temperature for spray drying is 300 °C and the time is 3 h to obtain a mixed material A;

[0054] Step S2: Under an inert gas environment, place the mixed material A in a muffle furnace and heat it to 770 °C at a heating rate of 5 °C / min, calcine for 10 h, then naturally cool to room temperature, grind, crush, and screen to prepare a lithium iron phosphate doping material. The chemical formula of the obtained lithium iron phosphate doping material is Li 1.0021 Fe 0.9996 PO4La 0.0004 F 0.0025 ;

[0055] Step S3: Mix aluminum phosphate, aqueous solution of diammonium hydrogen phosphate, and lithium iron phosphate doping material in a mass ratio of 0.003:0.015:1, and then ball mill them. The rotation speed of the ball mill is 300 r / min and the time is 5 h to obtain a mixed material B. The diammonium hydrogen phosphate is added in the form of an aqueous solution, and the mass fraction of the diammonium hydrogen phosphate is 30%;

[0056] Step S4: Under an inert gas environment, place the mixed material B in a tube furnace and heat it from room temperature to 730 °C at a heating rate of 5 °C / min, calcine for 8 h, naturally cool to room temperature, then grind, crush, and screen to prepare a modified lithium iron phosphate cathode material.

[0057] Example 4

[0058] Example 4 is different from Example 1 in that the lithium source is lithium hydroxide, the iron source is iron phosphate, the phosphorus source is iron phosphate, the lanthanum source is lanthanum carbonate, the fluorine source is sodium fluoride, the carbon source is sucrose, and the chemical formula of the lithium iron phosphate doped material obtained in step S2 is LiFe 0.9996 PO4La 0.0004 F 0.03 .

[0059] Example 5

[0060] Example 5 is different from Example 1 in that the lithium source is lithium nitrate, the iron source is iron phosphate, the phosphorus source is iron phosphate, the lanthanum source is lanthanum acetate, the fluorine source is potassium fluoride, the carbon source is starch, and the chemical formula of the lithium iron phosphate doped material obtained in step S2 is Li 1.0296 Fe 0.9996 PO4La 0.0004 F 0.03 .

[0061] Example 6

[0062] Example 6 is different from Example 1 in that the lithium source is lithium acetate, the iron source is iron phosphate, the phosphorus source is iron phosphate, the lanthanum source is lanthanum carbonate, the fluorine source is sodium fluoride, the carbon source is cyclodextrin, and the chemical formula of the lithium iron phosphate doped material obtained in step S2 is Li 0.9704 Fe 0.97 PO4La 0.03 F 0.0004 .

[0063] Example 7

[0064] Example 7 is different from Example 1 in that the lithium source is lithium hydroxide, the iron source is iron phosphate, the phosphorus source is iron phosphate, the lanthanum source is lanthanum carbonate, the fluorine source is sodium fluoride, the carbon source is citric acid, and the chemical formula of the lithium iron phosphate doped material obtained in step S2 is LiFe 0.97 PO4La 0.03 F 0.0004 .

[0065] Example 8

[0066] Example 8 is different from Example 1 in that in step S1, after the raw materials are mixed evenly, they are added to a dispersion liquid (water) for sanding. The rotation speed of the sanding is 100 r / min and the time is 6 h to obtain a sanded slurry; the sanded slurry is spray-dried. The inlet air temperature of the spray drying is 280 °C and the time is 6 h to obtain a mixed material A.

[0067] Example 9

[0068] Example 9 is different from Example 1. In step S1, after uniformly mixing the raw materials, they are added to a dispersion liquid (water) for sand grinding. The rotation speed of the sand grinding is 800 r / min, and the time is 1 h to obtain a sand-ground slurry. The sand-ground slurry is subjected to spray drying. The inlet air temperature of the spray drying is 380 °C, and the time is 1 h to obtain a mixed material A.

[0069] Example 10

[0070] Example 10 is different from Example 1. In step S2, in a nitrogen environment, the mixed material A is placed in a muffle furnace and heated to 600 °C at a heating rate of 1 °C / min, calcined for 15 h, then naturally cooled to room temperature, ground, crushed, and sieved to prepare a lithium iron phosphate doping material.

[0071] Example 11

[0072] Example 11 is different from Example 1. In step S2, in a nitrogen environment, the mixed material A is placed in a muffle furnace and heated to 800 °C at a heating rate of 10 °C / min, calcined for 6 h, then naturally cooled to room temperature, ground, crushed, and sieved to prepare a lithium iron phosphate doping material.

[0073] Example 12

[0074] Example 12 is different from Example 1. In step S3, aluminum phosphate, diammonium hydrogen phosphate, and the lithium iron phosphate doping material are mixed in a mass ratio of 0.0005:0.003:1 and then ball milled. The rotation speed of the ball milling is 100 r / min, and the time is 6 h to obtain a mixed material B.

[0075] Example 13

[0076] Example 13 is different from Example 1. In step S3, aluminum phosphate, diammonium hydrogen phosphate, and the lithium iron phosphate doping material are mixed in a mass ratio of 0.005:0.06:1 and then ball milled. The rotation speed of the ball milling is 800 r / min, and the time is 1 h to obtain a mixed material B.

[0077] Example 14

[0078] Example 14 is different from Example 1. In step S4, in an inert gas environment, the mixed material B is placed in a tube furnace and heated from room temperature to 500 °C at a heating rate of 1 °C / min, calcined for 10 h, naturally cooled to room temperature, then ground, crushed, and sieved to prepare a modified lithium iron phosphate cathode material.

[0079] Example 15

[0080] Example 15 is different from Example 1 in that in step S4, in an inert gas environment, the mixed material B is placed in a tube furnace and heated from room temperature to 780 °C at a heating rate of 10 °C / min, calcined for 6 h, naturally cooled to room temperature, ground, crushed and sieved to prepare a modified lithium iron phosphate cathode material.

[0081] Comparative Example 1

[0082] Step S1: Weigh anhydrous iron phosphate, Li2CO3, and glucose at a molar ratio of 1:0.53:0.001, mix them evenly, add them to a dispersion liquid (water) for sand milling. The rotation speed of sand milling is 300 r / min and the time is 3 h to obtain a sand-milled slurry. Spray dry the sand-milled slurry. The inlet air temperature of spray drying is 300 °C and the time is 3 h to obtain a mixture.

[0083] Step S2: In an inert gas environment, place the mixture in a muffle furnace and heat it to 770 °C at a heating rate of 5 °C / min, calcine for 10 h, then naturally cool to room temperature, grind, crush and sieve to prepare a lithium iron phosphate material.

[0084] Step S3: In an inert gas environment, place the lithium iron phosphate material in a tube furnace and heat it from room temperature to 730 °C at a heating rate of 5 °C / min, calcine for 8 h, naturally cool to room temperature, grind, crush and sieve to prepare a lithium iron phosphate cathode material.

[0085] Comparative Example 2

[0086] Step S1: Weigh anhydrous iron phosphate, Li2CO3, and glucose at a molar ratio of 1:0.53:0.001, mix them evenly, add them to a dispersion liquid (water) for sand milling. The rotation speed of sand milling is 300 r / min and the time is 3 h to obtain a sand-milled slurry. Spray dry the sand-milled slurry. The inlet air temperature of spray drying is 300 °C and the time is 3 h to obtain a mixture.

[0087] Step S2: In an inert gas environment, place the mixture in a muffle furnace and heat it to 760 °C at a heating rate of 5 °C / min, calcine for 10 h, then naturally cool to room temperature, grind, crush and sieve to prepare a lithium iron phosphate material.

[0088] Step S3: In an inert gas environment, place the lithium iron phosphate material in a tube furnace and heat it from room temperature to 730 °C at a heating rate of 5 °C / min, calcine for 8 h, naturally cool to room temperature, grind, crush and sieve to prepare a lithium iron phosphate cathode material.

[0089] Comparative Example 3

[0090] Step S1: Weigh anhydrous iron phosphate, Li2CO3, and glucose at a molar ratio of 1:0.53:0.001. After mixing them evenly, add them to the dispersion liquid (water) and perform sand grinding. The rotation speed of the sand grinding is 300 r / min, and the time is 3 h to obtain a sand grinding slurry. Spray-dry the sand-ground slurry. The inlet air temperature for spray drying is 300 °C, and the time is 3 h to obtain a mixture;

[0091] Step S2: In an inert gas environment, place the mixture in a muffle furnace and heat it to 770 °C at a heating rate of 5 °C / min, calcine for 10 h, then naturally cool to room temperature, grind, crush, and sieve to prepare lithium iron phosphate material;

[0092] Step S3: Mix aluminum phosphate, diammonium hydrogen phosphate, and lithium iron phosphate material at a mass ratio of 0.003:0.015:1 and perform ball milling. The rotation speed of the ball milling is 300 r / min, and the time is 5 h to obtain a coated material. Among them, diammonium hydrogen phosphate is added in the form of an aqueous solution, and the mass fraction of diammonium hydrogen phosphate is 30%;

[0093] Step S4: In an inert gas environment, place the coated material in a tube furnace and heat it from room temperature to 730 °C at a heating rate of 5 °C / min, calcine for 8 h, naturally cool to room temperature, then grind, crush, and sieve to prepare a single-coated modified lithium iron phosphate cathode material.

[0094] Comparative Example 4

[0095] Step S1: Weigh anhydrous iron phosphate, La2O3, LiF, glucose, and Li2CO3 at a molar ratio of 1:0.0008:0.025:0.001:1.05. After mixing them evenly, add them to the dispersion liquid (water) and perform sand grinding. The rotation speed of the sand grinding is 300 r / min, and the time is 3 h to obtain a sand grinding slurry. Spray-dry the sand-ground slurry. The inlet air temperature for spray drying is 300 °C, and the time is 3 h to obtain a mixed material A;

[0096] Step S2: In a nitrogen environment, place the mixed material A in a muffle furnace and heat it to 770 °C at a heating rate of 5 °C / min, calcine for 10 h, then naturally cool to room temperature, grind, crush, and sieve to prepare a lithium iron phosphate doped material. The chemical formula of the obtained lithium iron phosphate doped material is Li 1.0021 Fe 0.9996 PO4La 0.0004 F 0.0025 ;

[0097] Step S3: In an inert gas environment, place the lithium iron phosphate doped material in a tube furnace and heat it from room temperature to 730 °C at a heating rate of 5 °C / min, calcine for 8 h, naturally cool to room temperature, then grind, crush, and sieve to prepare a single-doped modified lithium iron phosphate cathode material.

[0098] The scanning electron microscope image of the modified lithium iron phosphate cathode material obtained in Example 1 is shown in Figure 1 , and the scanning electron microscope image of the lithium iron phosphate cathode material obtained in Comparative Example 1 is shown in Figure 2 , and the scanning electron microscope image of the lithium iron phosphate cathode material obtained in Comparative Example 2 is shown in Figure 3 .

[0099] The modified lithium iron phosphate cathode materials prepared in the above examples and comparative examples were made into 10 Ah soft-pack full electric lithium batteries, and the performance results are shown in Table 1.

[0100] Performance test:

[0101] Tap density: Tap density tester.

[0102] Electrochemical performance test: Electrochemical workstation, charge and discharge voltage range: 2.0 - 3.65 V.

[0103] Table 1

[0104]

[0105]

[0106] As can be seen from the above, in the process of synthesizing the lithium iron phosphate doping material in each embodiment of the present invention, lanthanum and fluorine elements are doped, and then the modified lithium iron phosphate cathode material is prepared by surface coating with phosphate, which has a significant improvement effect on improving the tap density of the lithium iron phosphate powder and the discharge capacity retention rate at low temperature of the battery. The soft-pack full electric lithium batteries made of this material in each embodiment of the present invention have better performance than the materials without doping and / or without coating in the comparative examples. Therefore, the lithium iron phosphate cathode material prepared by the present invention, when applied to lithium ion batteries, can not only improve the tap density of lithium iron phosphate, which is beneficial to the improvement of the battery energy density, but also greatly improve the low temperature resistance performance of the battery.

[0107] As can be seen from the above, in each embodiment of the present invention, doping La and F endows the lithium iron phosphate material with good lithium ion conduction characteristics and electronic conductivity characteristics on the surface, which can increase the interplanar spacing and thus improve the tap density. At the same time, the surface of the lithium iron phosphate doping material is coated with aluminum dihydrogen phosphate, which can effectively resist the erosion of the electrolyte on the surface of the positive electrode material, avoid side reactions between the positive electrode material and the electrolyte, and improve the low-temperature performance and cycling performance of the lithium iron phosphate material. In summary, the modified lithium iron phosphate positive electrode active material of the present invention has La and F doping modification and aluminum dihydrogen phosphate coating modification. Through the synergistic effect of doping and coating double modification, the electrochemical performance of the modified lithium iron phosphate positive electrode active material is good, the residual lithium amount on the surface is reduced, the side reactions on the surface are inhibited, the electronic conductivity and lithium ion diffusion rate are significantly increased, and at the same time, the tap density and low-temperature performance are significantly improved. In addition, it can be seen that when each process parameter is within the preferred range of the present invention, the comprehensive performance of the modified lithium iron phosphate positive electrode active material is better.

[0108] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A modified lithium iron phosphate cathode material, characterized in that, It includes lithium iron phosphate doping material and aluminum dihydrogen phosphate coated on its surface; Among them, the lithium iron phosphate doping material is a lithium iron phosphate material doped with lanthanum and fluorine, and its chemical general formula is Li 1-a+b Fe 1- a PO4La a F b , where 0.0004 ≤ a ≤ 0.03 and 0.0004 ≤ b ≤ 0.

03.

2. The modified lithium iron phosphate cathode material according to claim 1, wherein 0.0004 ≤ a ≤ 0.0025, 0.0004 ≤ b ≤ 0.0025; and / or The tap density of the modified lithium iron phosphate cathode material ≥ 1.25 g / cm 3 .

3. The preparation method of the modified lithium iron phosphate cathode material according to claim 1 or 2, characterized in that, It includes the following steps: Step S1: Mix a lithium source, an iron source, a phosphorus source, a lanthanum source, a fluorine source, a carbon source and water, and successively perform first grinding and drying to obtain a mixed material A; Step S2: Perform first sintering on the mixed material A to obtain a lithium iron phosphate doping material; Step S3: Mix the lithium iron phosphate doping material with an aluminum dihydrogen phosphate precursor, and then perform second grinding to obtain a mixed material B; Step S4: Perform second sintering on the mixed material B to obtain the modified lithium iron phosphate cathode material.

4. The preparation method according to claim 3, characterized in that, The weight ratio of the lithium iron phosphate doping material to the aluminum dihydrogen phosphate precursor is 100:(0.35 - 6.5).

5. The preparation method according to claim 4, characterized in that, The aluminum dihydrogen phosphate precursor includes aluminum phosphate and diammonium hydrogen phosphate.

6. The preparation method according to claim 5, characterized in that, The weight ratio of the aluminum phosphate to the diammonium hydrogen phosphate is (0.0005 - 0.005):(0.003 - 0.06).

7. The preparation method according to claim 3, characterized in that, In the step S1, The lithium source is one or more of lithium hydroxide, lithium carbonate, lithium nitrate and lithium acetate; and / or The iron source is iron phosphate; and / or The phosphorus source is iron phosphate; and / or The lanthanum source is one or more of lanthanum oxide, lanthanum acetate and lanthanum carbonate; and / or The fluorine source is one or more of lithium fluoride, sodium fluoride and potassium fluoride; and / or The carbon source is one or more of glucose, sucrose, starch, cyclodextrin and citric acid.

8. The preparation method according to claim 3, characterized in that, In the step S1, The first grinding method is sand grinding, the rotation speed is 100 - 800 r / min, and the time is 1 - 6 h; and / or The drying method is spray drying, the inlet air temperature is 280 - 380 °C, and the drying time is 1 - 6 h.

9. The preparation method according to claim 3, wherein In the step S2, The first sintering is carried out in an inert gas environment, and the inert gas is nitrogen and / or argon; and / or The heating rate of the first sintering is 1 - 10 °C / min, the sintering temperature is 600 - 800 °C, and the sintering time is 6 - 15 h.

10. The preparation method according to claim 3, characterized in that, In the step S3, The second grinding method is ball grinding, the rotation speed is 100 - 800 r / min, and the time is 1 - 6 h.

11. The preparation method according to claim 3, characterized in that, In the step S4, The second sintering is carried out in an inert gas environment, and the inert gas is nitrogen and / or argon; and / or The heating rate of the second sintering is 1 - 10 °C / min, the sintering temperature is 500 - 780 °C, and the sintering time is 6 - 10 h.

12. A battery, comprising a positive electrode material, characterized in that, The cathode material includes the modified lithium iron phosphate cathode material according to claim 1 or 2, or the modified lithium iron phosphate cathode material obtained by using the preparation method described in any one of claims 3 to 11.

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