Fast-ion conductor-coated lithium iron phosphate cathode material, and preparation method and application thereof

Abstract

The application belongs to the technical field of lithium ion battery cathode materials, and particularly relates to a fast ion conductor coated lithium iron phosphate cathode material and a preparation method and application thereof. The cathode material comprises a lithium iron phosphate base and a coating layer coated on the outer surface of the base. The molecular formula of the lithium iron phosphate base is LiFe 1‑x M x PO4, wherein 0<=x<=0.02, and M is at least one of manganese, titanium, magnesium, aluminum, zirconium and rare earth elements. From the base outward, the coating layer comprises a lithium selenate inner layer and a carbon outer layer in sequence. According to the application, a carbon selenate coated inner layer is first formed on the surface of a lithium iron phosphate precursor through in-situ reaction, and then a carbon source layer is coated on the outer surface of the carbon selenate. Through high-temperature calcination, a lithium selenate coated inner layer and a carbon outer layer are obtained. Due to the coating of lithium selenate, the material agglomeration phenomenon is obviously reduced, the primary particles of the material are more uniform, and the primary particles and secondary particles are smaller, which is beneficial to guarantee the super-long cycle performance of the lithium iron phosphate.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of lithium ion battery cathode materials, and particularly relates to a fast ion conductor coated lithium iron phosphate cathode material and a preparation method and application thereof. BACKGROUND

[0002] Lithium iron phosphate batteries have become a research hotspot in the new energy field due to their low price and long cycle life. In recent years, with the development of battery technologies such as blade battery, CTC and CTP, lithium iron phosphate batteries have also experienced rapid development. At present, lithium iron phosphate is divided into power and energy storage according to different application fields. The power has high requirements for energy density, while the energy storage has more stringent requirements for cycle. Conventional energy storage can generally cycle 4000 times, and high-end energy storage generally requires 8000 or even 10000 cycles. In view of the requirement of the battery for long cycle, the lithium iron phosphate should have good conductivity and fast ion conduction capacity to ensure low impedance and polarization of the lithium iron phosphate material in the cycle process. The specific requirements for the lithium iron phosphate cathode material are that the conductive material is uniformly and densely coated, the particle size distribution is concentrated, and the particle size is small. The uniform and dense coating layer ensures the good electronic conductivity of the material in the cycle process and the integrity of the material structure; the particle size distribution is concentrated and the particle size is small, which ensures the high ion conduction and consistent conduction rate, and reduces the polarization of the material.

[0003] At present, the industry and academia mainly improve the conductivity of the material by carbon coating, including organic and inorganic carbon, by optimizing the type, addition amount and coating method of the coating raw material to improve the coating effect. For example, Chinese patent CN103441269B discloses a machine preparation method of lithium pyrophosphate / carbon coated lithium iron phosphate composite material, wherein the lithium source, iron source and phosphorus source are mixed and ground, then dried and sintered, and then mixed with the phosphorus source and organic carbon source in an organic solvent system, sintered and sieved. In the prior art, the particle size of the primary particles is controlled by controlling the calcination temperature and time, and the particle size of the secondary particles is controlled by airflow crushing, so as to finally realize the purpose of concentrated size distribution and small particle size of the primary particles and the secondary particles. The carbon coating is generally formed by coating organic matter and then carbonizing at high temperature. However, due to the low carbonization rate and the brittleness of the carbon coating, the coating layer is broken during the crushing process, resulting in uneven coating, and finally leading to high internal resistance of the material, serious polarization phenomenon in the charging and discharging process, and serious influence on the cycle performance.

[0004] However, reducing the particle size of the primary particles by reducing the calcination temperature or shortening the calcination time may reduce the crystallinity of the material, thereby affecting the cycle performance of the material. If the particle size of the secondary particles is controlled by airflow crushing, the coating layer will be damaged. In addition, airflow crushing cannot control the particle size of the primary particles. SUMMARY

[0005] In view of the above technical defects, one of the purposes of the present application is to provide a fast ion conductor coated lithium iron phosphate positive electrode material, and the second purpose of the present application is to provide a preparation method of the fast ion conductor coated lithium iron phosphate positive electrode material. The third purpose of the present application is to provide the application of the fast ion conductor coated lithium iron phosphate positive electrode material.

[0006] In order to achieve the above-mentioned purposes, the present application adopts the following technical solutions:

[0007] In the first aspect, the present application provides a fast ion conductor coated lithium iron phosphate positive electrode material, which comprises a lithium iron phosphate base and a coating layer coated on the outer surface of the base. The molecular formula of the lithium iron phosphate base is LiFe 1-x M x PO4, wherein 0≤x≤0.02, M is at least one of manganese, titanium, magnesium, aluminum, zirconium and rare earth elements; from the base outward, the coating layer comprises a lithium selenate inner layer and a carbon outer layer in sequence.

[0008] In the fast ion conductor coated lithium iron phosphate positive electrode material, as a preferred embodiment, the outer surface layer of the lithium iron phosphate base is doped with a second additive, and the second additive is a nanomaterial for improving the ion conductivity of the positive electrode material; more preferably, the nanomaterial comprises at least one of nano-aluminum oxide, nano-zirconium oxide, nano-yttrium oxide, nano-rare earth oxide, nano-niobium oxide and boric acid.

[0009] Further, the mass of the second additive accounts for 0.01-1% (for example: 0.1%, 0.3%, 0.5%, 0.7%, 0.9%) of the mass of the fast ion conductor coated lithium iron phosphate positive electrode material.

[0010] In the fast ion conductor coated lithium iron phosphate positive electrode material, as a preferred embodiment, the mass of the lithium selenate inner layer accounts for 0.01-2% (for example: 0.05%, 0.2%, 0.5%, 0.8%, 1.5%, 1.8%) of the mass of the fast ion conductor coated lithium iron phosphate positive electrode material.

[0011] And / or, the mass of the carbon outer layer accounts for 0.5-3% (for example: 0.5%, 2%, 2.5%) of the mass of the fast ion conductor coated lithium iron phosphate positive electrode material.

[0012] In the above-mentioned fast-ion conductor coated lithium iron phosphate positive electrode material, as a preferred embodiment, the secondary particle size D50 of the fast-ion conductor coated lithium iron phosphate positive electrode material is 0.5-2.5 μm (for example: 0.7 μm, 0.9 μm, 1.1 μm, 1.3 μm, 1.5 μm, 1.7 μm, 1.9 μm, 2.1 μm, 2.3 μm), and the primary particle size is 200-450 nm (for example: 210 nm, 250 nm, 300 nm, 350 nm, 400 nm, 440 nm), wherein the primary particle size or primary particle size of the present application is the average value.

[0013] In a second aspect, the present application provides a preparation method of the above-mentioned fast-ion conductor coated lithium iron phosphate positive electrode material, comprising the following steps:

[0014] Preparation of a precursor: after dissolving a soluble ferrous salt in water, a soluble salt additive containing an M element is added or not added, and a first solution is obtained after one-time heating and stirring. Then, a phosphorus source is added to the first solution while maintaining the pH at 1-4. After the phosphorus source is completely added, secondary heating and stirring are performed, followed by filtration and washing to obtain an M element bulk phase doped iron phosphate precursor.

[0015] Preparation of a positive electrode material: lithium carbonate, a carbon source and a selenium source are added to the precursor, and ball milling, sand milling and spraying are performed, followed by calcination and jet milling to obtain the fast-ion conductor coated lithium iron phosphate positive electrode material.

[0016] In the preparation of the positive electrode material, the carbon source, lithium carbonate and selenium source are added at one time. The lithium carbonate and selenium source react to generate precipitated selenium carbonate, which is first deposited on the surface of the precursor material. Then, the soluble carbon source is coated on the surface of the selenium carbonate. The coating of the selenium carbonate and the carbon source on the surface of the material is completed at one time, the process is simple, and the coating effect is better. Then, the selenium carbonate is converted into the inner layer of lithium selenate and the carbon source layer is converted into the outer layer of carbon through calcination. The pH is maintained at 1-4 when the phosphorus source is added in the preparation of the precursor in order to make the precipitation complete.

[0017] In the above-mentioned preparation method, the selenium source can be an inorganic substance containing selenium elements, such as selenium oxide, selenic acid or selenate, etc. As a preferred embodiment, the selenium source includes one or more of selenium trioxide, selenic acid, ammonium selenate and lithium selenate. In the above-mentioned preparation method, the carbon source includes an organic carbon source and an inorganic carbon source. As a preferred embodiment, the organic carbon source includes one or more of glucose, white sugar and polyethylene glycol; and the inorganic carbon source includes one or more of natural graphite, artificial graphite, acetylene black, carbon nanotubes, carbon fibers and graphene.

[0018] In the above preparation method, as a preferred embodiment, the soluble ferrous salt includes one or more of ferrous sulfate, ferrous nitrate, ferrous chloride;

[0019] And / or, the phosphorus source includes one or more of phosphoric acid, monohydrogen phosphate, dihydrogen phosphate.

[0020] Preferably, the monohydrogen phosphate includes at least one of diammonium hydrogen phosphate, disodium hydrogen phosphate, dipotassium hydrogen phosphate, and the dihydrogen phosphate includes at least one of ammonium dihydrogen phosphate, sodium dihydrogen phosphate, potassium dihydrogen phosphate.

[0021] In the above preparation method, as a preferred embodiment, the soluble salt additive includes one or more of soluble sulfate, nitrate of titanium, magnesium, aluminum, zirconium, rare earth elements; the soluble salt additive realizes bulk doping in the precursor, and the addition of the soluble salt additive is to modify the material by bulk doping to improve the ion conductivity of the material.

[0022] In the above preparation method, as a preferred embodiment, in the step of preparing the positive electrode material, the second additive is added to the precursor at the same time as lithium carbonate, a carbon source, and a selenium source are added, the second additive is a nano material for improving the ion conductivity of the positive electrode material, and more preferably, the nano material includes at least one of nano aluminum oxide, nano zirconium oxide, nano yttrium oxide, nano rare earth oxide, nano niobium oxide, and boric acid. The second additive is doped on the surface of the precursor, and the addition of the second additive is to modify the material by surface doping to improve the ion conductivity of the material.

[0023] In the above preparation method, as a preferred embodiment, the concentration of the solution formed after the soluble ferrous salt is dissolved in water is 1 mol / L;

[0024] In the above preparation method, as a preferred embodiment, the addition amount of the soluble salt additive is 0.01%-1% (for example: 0.1%, 0.3%, 0.7%, 0.9%) of the mass of the lithium iron phosphate positive electrode material coated with the fast ion conductor;

[0025] In the above preparation method, as a preferred embodiment, the molar ratio of lithium, iron, and phosphorus in lithium carbonate, the soluble ferrous salt, and the phosphorus source is (1-1.1):(0.9-1):1;

[0026] In the above preparation method, as a preferred embodiment, the addition amount of the carbon source is 0.5-3% (for example: 0.5%, 2%, 2.5%) of the mass content of carbon in the lithium iron phosphate positive electrode material coated with the fast ion conductor;

[0027] In the present application, when the carbon content of the final product is too high, the free carbon is too much, the side reaction is more, and in addition, the specific surface area is also larger, which affects the processing performance and the capacity is low, when the carbon content is too low, the material coating effect is not good and the conductivity is poor.

[0028] In the above preparation method, as a preferred embodiment, the adding amount of the selenium source is 0.01-2% (for example: 0.1%, 0.5%, 1%, 1.4%, 1.6%, 1.8%) of the mass of the fast ion conductor coated lithium iron phosphate positive electrode material;

[0029] In the present application, when the carbon content of the final product is too high, the free carbon is too much, the side reaction is more, and in addition, the specific surface area is also larger, which affects the processing performance and the capacity is low, when the carbon content is too low, the material coating effect is not good and the conductivity is poor.

[0030] In the above preparation method, as a preferred embodiment, the adding amount of the second additive is 0.01-1% (for example: 0.1%, 0.3%, 0.5%, 0.7%, 0.9%) of the mass of the fast ion conductor coated lithium iron phosphate positive electrode material.

[0031] In the above preparation method, as a preferred embodiment, in the first heating and stirring, the heating temperature is 40-80℃ (for example: 50℃, 70℃), and the heating and stirring time is 2-8h (for example: 3h, 5h, 6h, 7h);

[0032] In the above preparation method, as a preferred embodiment, in the second heating and stirring, the heating temperature is 80-95℃ (for example: 85℃); the heating and stirring time is 2-8h (for example: 3h, 4h, 5h, 7h). The washing is conventional water washing.

[0033] In the above preparation method, as a preferred embodiment, the ball milling adopts wet ball milling;

[0034] In the wet ball milling, the solid content is 30-50%; the liquid medium used in the wet ball milling is water or alcohol;

[0035] In the present application, the alcohol used in the liquid medium of the wet ball milling is the commonly used liquid medium in the art, and therefore is not described here. By using ball milling and spraying, the organic carbon source can be coated on the surface of the material.

[0036] In the above preparation method, as a preferred embodiment, the particle size of the product after sand milling is 100-500nm (for example: 300nm, 400nm), and more preferably 200-500nm. In the above preparation method, the spraying not only achieves the purpose of granulation but also achieves the purpose of rapid drying, so that the soluble carbon source is uniformly coated on the surface of the insoluble substance.

[0037] In the above preparation method, as a preferred embodiment, the temperature of the calcination is 650-750℃ (for example: 670℃, 690℃, 710℃); and / or, the time of the calcination is 4-16h (for example: 6h, 8h, 12h, 14h);

[0038] and / or, the calcination is carried out under a protective gas atmosphere, preferably, the protective gas comprises one of nitrogen, helium, neon;

[0039] In a third aspect, the application also provides the application of the above-mentioned fast-ion-conductor-coated lithium iron phosphate cathode material in lithium ion batteries.

[0040] Compared with the prior art, the application has the following beneficial effects:

[0041] The application uses lithium carbonate as a lithium source, and the lithium carbonate and selenium source can generate a precipitated selenium carbonate, so that through in-situ reaction, a layer of selenium carbonate coating inner layer is first formed on the surface of the iron phosphate precursor, and then a layer of carbon source layer is coated outside the selenium carbonate, and through high-temperature calcination, a fast-ion-conductor-coated lithium iron phosphate cathode material with a lithium selenate coating inner layer and a carbon outer layer is obtained. Due to the coating of lithium selenate, the material agglomeration phenomenon is obviously reduced, the primary particles of the material are more uniform, and the primary particles and secondary particles are smaller, which is beneficial to guarantee the super-long cycle performance of lithium iron phosphate. In addition, the inner layer of lithium selenate can play a role in stabilizing the structure and reducing the crushing phenomenon of lithium iron phosphate material in the super-long cycle process. Furthermore, the inner layer of lithium selenate belongs to a fast-ion-conductor, which can effectively improve the ionic conductivity of lithium iron phosphate and effectively reduce the internal resistance of the material, thereby further improving the cycle performance of the material.

[0042] In the calcination process, due to the limitation of the inorganic coating layer (i.e. selenium-containing substance), the growth of lithium iron phosphate crystals is inhibited, so that a material with uniform particle size can be obtained. In the crushing process, the crushing of lithium iron phosphate particles can also be reduced. In addition, by coating the selenium source, lithium selenate is formed after high-temperature calcination. Lithium selenate belongs to a fast-ion-conductor, which can effectively improve the lithium ion conductivity of the material and reduce the ionic resistance. By limiting the growth of lithium iron phosphate crystals, stabilizing the structure of lithium iron phosphate crystals, and improving the ionic conductivity of lithium iron phosphate, the cycle performance of lithium iron phosphate is improved. BRIEF DESCRIPTION OF DRAWINGS

[0043] Figure 1 FIG. 1 is an SEM image of the fast-ion-conductor-coated lithium iron phosphate cathode material of Example 1 of the application. DETAILED DESCRIPTION

[0044] The preparation method of the fast ion conductor coated lithium iron phosphate positive electrode material is further described in detail below through examples. The examples are given only to illustrate the present application and are not intended to limit the scope of the present application. Based on the examples in the present application, all other examples obtained by those of ordinary skill in the art belong to the scope of protection of the present application. The examples provided below can serve as a basis for further improvement or application by those of ordinary skill in the art, and do not constitute any specific limitation on the present application in any way.

[0045] Example 1

[0046] The present embodiment provides a preparation method of a fast ion conductor coated lithium iron phosphate positive electrode material, comprising the following steps:

[0047] S1: Dissolve ferrous sulfate salt in water (the concentration of ferrous sulfate in the solution is 1 mol / L), add 0.5wt% of titanium sulfate based on the mass of the fast ion conductor coated lithium iron phosphate positive electrode material, then heat it to 60℃, maintain the temperature and stir for 4h, after stirring, add ammonium dihydrogen phosphate and phosphoric acid, and maintain the pH value at 2, after the material is completely added, stir at 90℃ for 6h, filter and wash to obtain a precursor, wherein the molar ratio of iron to phosphorus is 1:1.

[0048] S2: Add lithium carbonate, glucose, selenium trioxide, and nano-niobium oxide to the precursor, ball mill, sand mill, and spray to obtain a mixture, wherein the molar ratio of lithium to phosphorus is 1.05:1; the addition amount of glucose is 1.0wt% based on the carbon content in the final fast ion conductor coated lithium iron phosphate positive electrode material; the addition amount of selenium trioxide is 1.2wt% based on the mass of the fast ion conductor coated lithium iron phosphate positive electrode material; the addition amount of nano-niobium oxide is 0.05wt% based on the mass of the fast ion conductor coated lithium iron phosphate positive electrode material; and the particle size (D50) of the product after sand milling is 200nm.

[0049] S3: Put the mixture into a bowl and perform calcination in a roller kiln under nitrogen atmosphere, the calcination temperature is 730℃, the calcination time is 10h, and finally the fast ion conductor coated lithium iron phosphate positive electrode material with a particle size (D50) of 1.2μm of secondary particles is obtained through jet milling, wherein the primary particle size is 321nm.

[0050] Figure 1 The SEM image of the fast ion conductor coated lithium iron phosphate positive electrode material of the present embodiment is shown in Figure 1. Figure 1 It can be seen that the particle size uniformity of the positive electrode material prepared by the method of the present application is good, and the coating effect is good.

[0051] Example 2

[0052] The present embodiment provides a preparation method of a fast ion conductor coated lithium iron phosphate positive electrode material, comprising the following steps:

[0053] The difference between this embodiment and Example 1 is that the amount of added selenium trioxide is 0.5 wt%.

[0054] Example 3

[0055] This embodiment provides a method for preparing a fast-ion conductor-coated lithium iron phosphate positive electrode material, comprising the following steps:

[0056] The difference between this embodiment and Example 1 is that the amount of added selenium trioxide is 2 wt%.

[0057] Example 4

[0058] This embodiment provides a method for preparing a fast-ion conductor-coated lithium iron phosphate positive electrode material, comprising the following steps:

[0059] The difference between this embodiment and Example 1 is that the amount of added glucose is 0.5 wt%.

[0060] Example 5

[0061] This embodiment provides a method for preparing a fast-ion conductor-coated lithium iron phosphate positive electrode material, comprising the following steps:

[0062] The difference between this embodiment and Example 1 is that the amount of added glucose is 3 wt%.

[0063] Example 6

[0064] This embodiment provides a method for preparing a fast-ion conductor-coated lithium iron phosphate positive electrode material, comprising the following steps:

[0065] The difference between this embodiment and Example 1 is that the phosphorus-to-lithium ratio is 1:1.

[0066] Example 7

[0067] This embodiment provides a method for preparing a fast-ion conductor-coated lithium iron phosphate positive electrode material, comprising the following steps:

[0068] The difference between this embodiment and Example 1 is that the phosphorus-to-lithium ratio is 1:1.1.

[0069] Example 8

[0070] This embodiment provides a method for preparing a fast-ion conductor-coated lithium iron phosphate positive electrode material, comprising the following steps:

[0071] The difference between this embodiment and Example 1 is that the temperature of calcination is 700°C.

[0072] Example 9

[0073] The embodiment provides a preparation method of a fast-ion-conductor-coated lithium iron phosphate positive electrode material, and comprises the following steps:

[0074] Compared with the embodiment 1, the embodiment 1 is different in that the calcination temperature is 750 DEG C.

[0075] Embodiment 10

[0076] The embodiment provides a preparation method of a fast-ion-conductor-coated lithium iron phosphate positive electrode material, and comprises the following steps:

[0077] Compared with the embodiment 1, the embodiment 1 is different in that the calcination time is 6h.

[0078] Embodiment 11

[0079] The embodiment provides a preparation method of a fast-ion-conductor-coated lithium iron phosphate positive electrode material, and comprises the following steps:

[0080] Compared with the embodiment 1, the embodiment 1 is different in that the calcination time is 16h.

[0081] Comparative example 1

[0082] The embodiment provides a preparation method of a fast-ion-conductor-coated lithium iron phosphate positive electrode material, and comprises the following steps:

[0083] Compared with the embodiment 1, the embodiment 1 is different in that the addition amount of the selenium trioxide is 0wt%.

[0084] Test example

[0085] The primary particle size of the positive electrode materials prepared in the embodiments 1-11 and the comparative example 1 is measured; the test method of the primary particle size is as follows: the primary particle size is counted through an SEM image.

[0086] The positive electrode materials prepared in the embodiments 1-11 and the comparative example 1 are respectively used as active materials, mixed with polyvinylidene fluoride (PVDF) and superconducting carbon black (SuperP) in a mass ratio of 93.5:4.2:2.3, ball milled for 60min with NMP as a solvent; then the slurry is uniformly coated on a metal aluminum foil and dried at 80 DEG C for 2h in a vacuum, finally, a punch is used to cut the circular electrode piece with a diameter of 14mm as a working electrode; in an Ar-filled purification glove box (O2 content is less than 0.1ppm, H2O content is less than 0.1ppm), a metal lithium sheet is used as a counter electrode, a Celgard 2400 porous polypropylene membrane (PP) is used as a separator, and a 1M LiPF6 / EC+DMC+EMC (volume ratio is 1:1:1) electrolyte is used as an electrolyte. -1The lithium hexafluorophosphate (LiPF6) solution with solvent of ethylene carbonate (EC) : dimethyl carbonate (DMC) = 1:1 volume ratio mixture is used to prepare R2032 type button cell according to certain assembly process, and after completion, the electrolyte is allowed to stand for 3h to fully infiltrate the electrode material. The constant current charge-discharge experiment of the battery is carried out at room temperature (25℃±3) in the voltage range of 2.5-3.65V, and the initial specific capacity and 200 cycle retention rate are tested.

[0087] The calculation method of 200 cycle retention rate is to calculate the cycle 200 cycle retention rate based on the initial cycle specific discharge capacity.

[0088] The specific test results are shown in the following table:

[0089] Primary particle size (nm) Initial specific capacity (mAh / g) 200 cycle retention (%) Example 1 321 161 99.7 Example 2 420 160 97.6 Example 3 380 161 99.1 Example 4 310 159 98.5 Example 5 323 157 97.6 Example 6 350 159 98.2 Example 7 380 159 98.5 Example 8 310 157 98.1 Example 9 450 157 96.5 Example 10 309 156 97.8 Example 11 430 158 97.7 Comparative Example 1 490 155 94.6

[0090] As can be seen from the above table, by double coating of carbon-coated outer layer and lithium selenate fast ion conductor material inner layer, controlling the particle size of primary particles, the specific capacity and cycle performance of the material can be effectively improved.

[0091] The above only describes the preferred embodiments of the present application and is not intended to limit the present application, and any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application is within the protection scope of the claims of the present application.

Claims

1. A fast-ion conductor-coated lithium iron phosphate cathode material, characterized in that, It includes a lithium iron phosphate matrix and a coating layer covering the outer surface of the matrix, wherein the molecular formula of the lithium iron phosphate matrix is ​​LiFe. 1-x M x PO4, wherein 0≤x≤0.02, and M is at least one of manganese, titanium, magnesium, aluminum, zirconium, and rare earth elements; from the substrate outward, the coating layer sequentially includes a lithium selenate inner layer and a carbon outer layer; The mass of the lithium selenate inner layer accounts for 0.01-2% of the mass of the fast ion conductor-coated lithium iron phosphate cathode material; The mass of the carbon outer layer accounts for 0.5-3% of the mass of the fast-ion conductor-coated lithium iron phosphate cathode material; The preparation method of the fast ion conductor coated lithium iron phosphate cathode material includes the following steps: Preparation of precursor: Dissolve soluble ferrous salt in water and add or not add soluble salt additive containing element M. After heating and stirring once, a first solution is obtained. Then add phosphorus source to the first solution and keep the pH between 1 and 4. After the phosphorus source is added, heat and stir a second time. After filtration and washing, ferric phosphate precursor is obtained. Preparation of cathode material: Lithium carbonate, carbon source and selenium source are added to the precursor in one step, followed by ball milling, sand milling and spraying, and then calcination and air jet pulverization to obtain the fast ion conductor coated lithium iron phosphate cathode material.

2. The fast-ion conductor-coated lithium iron phosphate cathode material according to claim 1, characterized in that, In the step of preparing the cathode material, a second additive is added to the precursor along with lithium carbonate, a carbon source, and a selenium source. The second additive is a nanomaterial that improves the ionic conductivity of the cathode material.

3. The fast-ion conductor-coated lithium iron phosphate cathode material according to claim 2, characterized in that, The nanomaterials include at least one of nano-alumina, nano-zirconia, nano-yttrium oxide, nano-rare earth oxides, nano-niobium oxide, and boric acid; And / or, the mass of the second additive accounts for 0.01-1% of the mass of the fast ion conductor-coated lithium iron phosphate cathode material.

4. The fast-ion conductor-coated lithium iron phosphate cathode material according to claim 2, characterized in that, The concentration of the solution formed after the soluble ferrous salt is dissolved in water is 1 mol / L; And / or, the amount of the soluble salt additive added is 0.01%-1% of the mass of the fast ion conductor-coated lithium iron phosphate cathode material; And / or, the molar ratio of lithium, iron, and phosphorus in lithium carbonate, soluble ferrous salt, and phosphorus source is (1-1.1):(0.9-1):1; And / or, the amount of carbon source added is based on the carbon mass content in the fast ion conductor-coated lithium iron phosphate cathode material being 0.5-3%; And / or, the amount of selenium source added is 0.01-2% of the mass of the fast ion conductor-coated lithium iron phosphate cathode material; And / or, the amount of the second additive added is 0.01-1% of the mass of the fast ion conductor-coated lithium iron phosphate cathode material.

5. The fast-ion conductor-coated lithium iron phosphate cathode material according to claim 1, characterized in that, The secondary particle size D50 of the fast ion conductor-coated lithium iron phosphate cathode material is 0.5-2.5 μm, and the primary particle size is 200-450 nm.

6. The fast-ion conductor-coated lithium iron phosphate cathode material according to claim 1, characterized in that, The selenium source includes one or more of selenium trioxide, selenic acid, ammonium selenate, and lithium selenate. And / or, the carbon source includes organic carbon sources and / or inorganic carbon sources; And / or, the phosphorus source includes one or more of phosphoric acid, monohydrogen phosphate, and dihydrogen phosphate; And / or, the soluble salt additive includes one or more of the soluble sulfates and nitrates of titanium, magnesium, aluminum, zirconium, and rare earth elements.

7. The fast-ion conductor-coated lithium iron phosphate cathode material according to claim 1, characterized in that, In the first heating and stirring process, the heating temperature is 40-80℃, and the heating and stirring time is 2-8 hours. And / or, in the secondary heating and stirring, the heating temperature is 80-95℃; the heating and stirring time is 2-8 hours; And / or, the ball milling is performed using a wet ball milling method; during the wet ball milling, the solid content is 30-50%; And / or, the particle size of the product after sand milling is 100-500 nm; And / or, the calcination temperature is 650-750℃, and the calcination time is 4-16h; And / or, the calcination is carried out under a protective gas atmosphere.

8. The application of the fast ion conductor-coated lithium iron phosphate cathode material as described in any one of claims 1-7 in lithium-ion batteries.

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