Preparation method of multi-element co-coated lithium ion ternary positive electrode material

Abstract

The invention discloses a multi-element co-coated lithium ion ternary positive electrode material and a preparation method thereof, the multi-element co-coated lithium ion ternary positive electrode material comprises an inner core and a shell, the inner core is a high-nickel ternary positive electrode material NCM, the nickel content is greater than or equal to 80%, and the median particle size D50 of the powder is 6-12 [mu] m; the shell is a composite coating layer and coats the surface of the inner core, the composite coating layer comprises LATP, TiO2 and AlPO4, the total mass of the composite coating layer accounts for 0.5%-3% of the mass of the inner core, and the mass ratio of the LATP to the TiO2 to the AlPO4 is (80%-90%): (12%-6%): (8%-4%). The surface of the ternary material is simultaneously coated with a layer of LATP, TiO2 and AlPO4 spontaneously at one time through a hydrothermal method, so that the contact between the ternary material and an electrolyte is reduced, the surface lithium ion mobility is improved, the polarization is reduced, and the cycling stability and safety of the material are further improved.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of lithium ion batteries, in particular to a preparation method of a ternary co-coated lithium ion ternary positive electrode material. BACKGROUND

[0002] Lithium ion batteries have been widely studied and successfully applied in many fields such as 3C electronic products, new energy vehicles, low-altitude unmanned aerial vehicles and electrochemical energy storage systems due to the advantages of high energy density, long cycle life, low self-discharge, wide temperature range and environmental friendliness. With the rapid development of new energy electric vehicles, the energy density of lithium ion power batteries is increasingly required by the society, and improving the energy density and safety of lithium ion batteries has become the focus of the academic and industrial circles.

[0003] There are various material systems in lithium ion battery positive electrode materials, including polyanion type LiMPO4 materials, spinel type lithium manganate and layered structure LiMO2 (M=Ni, Co, Mn and other transition metals) materials. Among them, the theoretical specific capacity of lithium iron phosphate is 170 mAh / g, the theoretical specific capacity of lithium manganate material is 148 mAh / g, and the specific capacity of ternary layered transition metal oxide is higher, and the theoretical specific capacity is about 274 mAh / g. The high gram capacity of ternary material can effectively improve the energy density of lithium ion battery, alleviate the range anxiety of electric vehicles, and has become a research hotspot at home and abroad.

[0004] The ternary material increases the nickel content ratio to improve the specific capacity of the powder material, but this brings a series of problems, such as Ni 2+ is easy to react with the electrolyte, resulting in the loss of active lithium, and the ternary material undergoes irreversible phase transition from layered structure to spinel structure to rock salt structure during charging and discharging, and the anisotropy in the phase transition process causes cracks in the powder material, and the side reaction increases. Ni 2+ in the ternary material is easy to mix with Li + to form inversion defects, which will lead to slow Li + diffusion dynamics and poor electrochemical performance; after the high-nickel ternary material is delithiated, Ni 4+ ion is easy to form rock salt phase NiO in the reduction process and release O2, resulting in capacity attenuation of the battery and safety hazards.

[0005] A large number of studies have shown that the recognized effective method to improve the electrochemical performance of ternary positive electrode materials is to use specific metal oxides and metal phosphates, such as Al2O3, Fe2O3, TiO2, MgO, CoPO4, AlPO4 and Mg3(PO4)2, to coat the surface thereof. However, these materials can only reduce the direct contact area between the electrode surface and the electrolyte to prevent the side reaction of the material and the electrolyte, and these coatings have a certain hindering effect on the migration of lithium ions on the surface of the material. In recent years, the Nasicon type solid-state electrolyte material LATP has been widely studied due to its high ionic conductivity. When it is applied to the coating modification of high-nickel ternary materials, it can prevent the electrolyte from eroding the surface of the positive electrode while improving the ionic conductivity of the interface, thereby improving the electrochemical performance. However, the LATP coated positive electrode is usually prepared by a mechanical mixing sintering method, which results in an uneven coating layer due to the large particle size of the LATP powder, and the effect of improving the electrochemical performance is not obvious. In a few methods of coating LATP in ternary materials by sol-gel, titanium ester compounds are often used, which leads to a complex process and increased cost. SUMMARY

[0006] In view of the deficiencies of the existing coating technology in ternary materials, such as poor coating uniformity, complex coating process, high cost, and limited coating effect, the purpose of the present application is to provide a preparation method of a lithium ion ternary positive electrode material coated with multiple elements. By using a hydrothermal method, a layer of LATP, TiO2 and AlPO4 products is simultaneously coated on the surface of the ternary material at one time, the contact between the ternary material and the electrolyte is reduced, the surface lithium ion migration rate is improved, the polarization is reduced, and the cycle stability and safety of the material are improved.

[0007] To achieve the above purpose, the technical scheme adopted by the present application is:

[0008] A lithium ion ternary positive electrode material coated with multiple elements, comprising a core and a shell:

[0009] The core is a high-nickel ternary positive electrode material NCM, the content of Ni is ≥80%, and the median particle size D50 of the powder is 6-12 μm;

[0010] The shell is a composite coating layer, which is continuously and uniformly coated on the surface of the core, and the composite coating layer comprises LATP, TiO2 and AlPO4;

[0011] The chemical formula of the LATP is Li 1.3 Al 0.3 Ti 1.7 (PO4)3;

[0012] The total mass of the composite coating layer accounts for 0.5%-3% of the mass of the core;

[0013] The mass ratio of the LATP, TiO2 and AlPO4 is 80%-90%: 12%-6%: 8%-4%.

[0014] A preparation method of a multi-element co-coated lithium ion ternary positive electrode material, comprising the following steps:

[0015] S1, proportionally weighing NCM powder, lithium source, aluminum source, titanium dioxide powder and phosphorus source;

[0016] S2, the lithium source, the aluminum source and the phosphorus source are dissolved in deionized water respectively, and stirred until completely dissolved; the amount of deionized water should be combined with the amount of NCM powder, titanium dioxide powder and organic solvent in step S3, so that the total solid content of the final system is controlled between 20% and 60%.

[0017] S3, the NCM powder, the titanium dioxide powder and the organic solvent are added to the hydrothermal reaction kettle, and continuously stirred for 1h;

[0018] S4, the three solutions in step S2 are sequentially added to the reaction kettle of step S3, the pH is adjusted to 5-8 with ammonia water, and the stirring is continued for 0.5h to 2h;

[0019] S5, seal the reaction kettle, and perform hydrothermal reaction at 190℃-230℃ for 16-48 hours;

[0020] S6, the reaction product is filtered and washed with water for 6 times, and dried at 80℃-120℃ to obtain an intermediate product powder;

[0021] S7, the intermediate product is calcined at 500℃-700℃ for 2 to 6 hours to obtain the final multi-element co-coated ternary positive electrode material.

[0022] As a preferred technical solution, the atomic molar ratio of the lithium source, the aluminum source, the titanium dioxide powder and the phosphorus source in step S1 is Li: Al: Ti: P = 1.3-1.8: 0.3: 1.7: 3-3.1.

[0023] As a preferred technical solution, the organic solvent in step S3 includes one of ethanol, ethylene glycol, polyethylene glycol (200-1000) and diethylene glycol.

[0024] As a preferred technical solution, the lithium source compound in step S1 includes one or more combinations of lithium hydroxide, lithium nitrate, lithium chloride, lithium sulfate and lithium acetate.

[0025] As a preferred technical solution, the aluminum source compound in step S1 includes one or more combinations of aluminum nitrate, aluminum sulfate and aluminum chloride.

[0026] Preferably, the titanium dioxide powder in step S1 is in one of the following forms: rutile, anatase, or a mixture of rutile and anatase, and the primary particle size of the titanium dioxide powder is less than or equal to 60 nm.

[0027] Preferably, the phosphorus source compound in step S1 comprises one or more of the following: phosphoric acid, diammonium hydrogen phosphate, and ammonium dihydrogen phosphate.

[0028] Preferably, the total solid content of the powder in step S3 is controlled to be between 20% and 60%.

[0029] In the present application, in order to achieve the specific mass ratio of LATP, TiO2, and AlPO4 in the composite coating layer, the molar ratio of raw materials needs to be accurately controlled. The basic conversion principle is as follows: first, according to the mass of LATP (Li 1.3 Al 0.3 Ti 1.7 (PO4)3), TiO2, and AlPO4 in the target product, the corresponding molar amount of each component is calculated, and then the total molar amount of the four elements Li, Al, Ti, and P and the solid TiO2 is calculated. For example, to prepare a coating layer with a mass ratio of LATP:TiO2:AlPO4 of a:b:c, the theoretical molar ratio of Li:Al:Ti:P can be converted according to the stoichiometric relationship. Considering factors such as reaction yield, the actual feeding ratio can be adjusted slightly based on the theoretical value. The Li:Al:Ti:P atomic molar ratio range given in the present application is based on this and is the result of experimental optimization.

[0030] Advantages of the present application:

[0031] 1. The present application realizes the primary coating of various inorganic salts on the surface of NCM ternary material by hydrothermal method. The coating of fast ion conductor LATP can improve lithium ion migration, reduce polarization, and alleviate the accumulation of anisotropic stress during the material delithiation process, thereby stabilizing the structure of the material. The coating of TiO2 and AlPO4 can inhibit the side reaction between electrolyte and material and reduce the dissolution of transition metals, thereby improving the cycle stability and safety of high-nickel ternary material.

[0032] 2. The raw materials used in the present application are widely available and low in cost. The raw materials are easily available on the market and are well known to those skilled in the art. The cost is slightly increased based on NCM production, but the performance is significantly improved, and the cost performance is higher.

[0033] 3. The method of the present application realizes high uniformity, high bonding force, and functional optimization of the coating layer through the multi-element co-coating process of hydrothermal reaction combined with high-temperature calcination. The specific effects and principles are as follows:

[0034] In steps S2-S4, the precursor solutions of LATP, TiO2 and AlPO4 are in full contact with the NCM particles in the hydrothermal reactor, and hydrolysis / precipitation reactions occur at a suitable pH (5-8). This process can generate coated precursor particles in situ on the particle surface, avoiding the defects of uneven coating caused by mechanical mixing, thereby significantly improving the uniformity of the coating layer.

[0035] Hydrothermal conditions promote the directional combination of crystal nuclei: In a hydrothermal environment of 190-230℃, the precursor particles are directionally attached to the inner core crystal surface on the NCM surface through hydrogen bonding, electrostatic adsorption, etc. This process helps to form a continuous and dense shell structure during subsequent calcination, reduces the porosity of the coating layer, and improves the protection effect.

[0036] Precise control of the mass ratio of LATP:TiO2:AlPO4 in the solution phase allows the coating layer to have high lithium ion conductivity (LATP), chemical passivation (TiO2), and high thermal stability (AlPO4), with significant functional synergy. The total mass of the coating layer is controlled in the range of 0.5%-3%, which not only retains the high capacity advantage of high-nickel NCM but also realizes effective surface protection.

[0037] High-temperature calcination promotes solid-phase reaction and densification: During calcination at 500-700℃, LATP crystallizes and partially reacts with TiO2 and AlPO4 to form a stable composite phase; at the same time, sintering occurs between coated particles, significantly improving the bonding strength between the shell and the core. This step can effectively reduce the risk of peeling of the coating layer during long cycling, thereby maintaining the integrity of the interface.

[0038] The multi-element co-coated high-nickel NCM prepared by this method exhibits low interfacial impedance, high rate performance, and excellent cycle stability; when cycled at high voltage of 4.3-4.5 V, the capacity retention rate is significantly higher than that of uncoated or single-coated control samples; under high temperature (≥55℃) conditions, the surface structure of the material remains stable, reducing metal ion dissolution and side reactions. BRIEF DESCRIPTION OF DRAWINGS

[0039] In order to more clearly illustrate the specific embodiments of the present application or the technical solutions in the prior art, the following will briefly introduce the drawings needed to be used in the specific embodiments or prior art description. Obviously, the drawings described below are some embodiments of the present application, and those skilled in the art can also obtain other drawings according to these drawings without creative labor.

[0040] FIG. 1 The charge-discharge test curve of the powder material in implementation 1;

[0041] FIG. 20.5C charge-discharge cycle test of the powder material in implementation 1

[0042] FIG. 3 DSC test curve of the powder material in implementation 1. DETAILED DESCRIPTION

[0043] The following description is provided to enable any person skilled in the art to practice the present application. The preferred embodiments described herein are only examples of the present application and various modifications can be made by those skilled in the art.

[0044] Example 1, the present embodiment provides a multi-element co-coated lithium ion ternary positive electrode material and a preparation method thereof, the Ni content in the ternary material NCM is 92%, the powder particle size D50 is 11 um; the total mass ratio of the multi-element coating LATP and TiO2 and AlPO4 on the surface of NCM is 0.5%, and the mass ratio of LATP:TiO2:AlPO4 is 90%:6%:4%.

[0045] The following steps are used to complete the preparation:

[0046] S1, according to the mass ratio, 1000g of NCM powder material (Ni content 92%, D50=11um) is weighed. According to the total mass of the coating layer of the final product accounting for the core 0.5% (i.e. 5g), and the mass ratio of LATP:TiO2:AlPO4 is 90%:6%:4%, i.e. LATP 4.5g, TiO2 0.3g, AlPO4 0.2g, and lithium hydroxide (LiOH·H2O) 0.85g, aluminum sulfate heptahydrate (Al2(SO4)3·7H2O) 0.41g, anatase titanium dioxide (TiO2) 0.3g, phosphoric acid (H3PO4, 85% concentration) 1.21g are weighed.

[0047] S2, respectively, lithium hydroxide, aluminum sulfate and phosphoric acid are completely dissolved in deionized water;

[0048] S3, add NCM powder, TiO2 powder and organic solvent diethylene glycol to the hydrothermal reaction kettle, and continue to stir for 1h;

[0049] S4, add the three solutions in step S2 to the reaction kettle in step S3, adjust the pH value to 5 with ammonia water, and continue to stir for 1h;

[0050] S5, the reaction kettle of step S4 is subjected to hydrothermal reaction at a temperature range of 230℃ for 16h;

[0051] S6, the slurry after reaction in step S5 is subjected to suction filtration and water washing for 6 times, and then dried at 80℃ environment;

[0052] S7, calcining the powder in step S6 at 500℃ for 4h in air atmosphere to obtain the final multi-element coated ternary cathode material.

[0053] The prepared material was subjected to charge-discharge test by using a button half-cell to test its electrochemical performance, and the material in full charged state was subjected to DSC test, and the specific test results are shown in FIG. 1 and FIG. 2 The specific capacity of the material at 0.2C reached 226mAh / g, and the DSC decomposition temperature reached 240℃.

[0054] The forming mechanism of the application is specifically that: in the hydrothermal reaction stage (step S5), the lithium source, aluminum source and phosphorus source precursors added on the surface of the NCM particles undergo in-situ co-precipitation reaction to form amorphous Li-Al-P-O mixed precursors, and the titanium dioxide particles added as solid phase raw materials are physically attached to the surface of the NCM through electrostatic adsorption or van der Waals force to form a mixed precursor coating layer together with the co-precipitation product. In the subsequent high-temperature calcination stage (step S7), the amorphous Li-Al-P-O precursor undergoes solid phase reaction to crystallize and form LATP phase with high ionic conductivity and AlPO4 phase with stable chemical properties, and interface sintering occurs with the TiO2 particles, and finally a core-shell structure is formed, which is composed of LATP, AlPO4 and TiO2 three phases, has a dense structure and is firmly combined with the core. By accurately controlling the molar ratio of the raw materials in step S1, it can be ensured that the final product obtained after calcination meets the defined mass ratio range.

[0055] The application forms a core-shell structure by constructing a LATP / TiO2 / AlPO4 multi-element composite coating layer on the surface of high-nickel ternary cathode material (Ni ≥ 80%), which brings significant improvement in material structure, electrochemical reaction kinetics and interface stability, and the principle beneficial effects are as follows:

[0056] LATP (Li 1.3 Al 0.3 Ti 1.7 (PO4)3) is of NASICON type structure, has a three-dimensional lithium ion conduction network, and its room temperature ionic conductivity can reach 10 -3 S / cm level. When LATP is uniformly coated on the surface of NCM particles, a low-impedance Li + migration path can be formed at the solid-solid interface, which reduces the impedance film formed when the traditional high-nickel cathode directly contacts with the electrolyte, and improves the rate performance and low-temperature performance.

[0057] Can inhibit the corrosion of electrolyte and the side reaction induced by high-valence Ni 4+ The high-nickel cathode is prone to Ni 2+ / Ni 4 +The redox reaction of the surface layer, accompanied by the release of lattice oxygen, promotes the decomposition of the electrolyte and generates corrosive substances such as HF. The outer TiO2 layer has chemical inertness and high electronic insulation, which can act as a physical barrier to prevent HF from directly reacting with the positive electrode surface, thereby delaying the dissolution of Ni and the collapse of the lattice structure.

[0058] The stability of the coating layer is improved by chemical bonding. During the sintering process, AlPO4 chemically combines with the metal ions (Ni, Co, Mn) on the surface of the positive electrode or TiO2 to form a stable phosphate phase (such as LiMPO4, M is a transition metal). The layer structure is dense and has high thermal stability, which can effectively prevent the coating layer from peeling off under cyclic stress.

[0059] Multi-element synergy reduces interface stress and phase change tendency. The cell volume of high-nickel NCM changes greatly during charging and discharging, which easily leads to H2-H3 phase change and particle cracking. The ion conductivity of LATP ensures uniform lithium distribution during charging and discharging, and the mechanical rigidity of TiO2 and AlPO4 can share part of the stress, thereby inhibiting the crack propagation induced by phase change and improving the cycle life.

[0060] Controlling the total amount of the coating layer to 0.5% to 3% can achieve interface protection without significantly sacrificing specific capacity; when the ratio of LATP:TiO2:AlPO4 is 80% to 90%:12% to 6%:8% to 4%, both sufficient ion conductivity and interface chemical stability and mechanical strength are ensured, and the balance between capacity retention rate and rate performance is optimized from the mechanism.

[0061] Hydrothermal method can promote the in-situ deposition of LATP precursor on the particle surface, and cooperate with subsequent calcination at 500-700°C to crystallize LATP and sinter it with TiO2 and AlPO4, forming a continuous, dense and firmly combined shell layer with the core, reducing the adverse effects of coating defect areas on performance.

[0062] Example 2, compared with Example 1, the difference lies in that the total mass ratio of LATP and TiO2 and AlPO4 is 1%, among which the mass ratio of LATP:TiO2:AlPO4 is 80%:12%:8%, and the raw materials used in step S1 are lithium sulfate, aluminum sulfate, rutile titanium dioxide and ammonium dihydrogen phosphate, respectively; the solvent used in step S3 is ethylene glycol; the pH value in step S4 is 8. The electrochemical performance and DSC test data of the powder material prepared in Example 3: the specific capacity of the material at 0.2C reaches 225mAh / g, and the capacity retention rate after 0.5C charging and discharging for 50 times is 98.5%, and the DSC decomposition temperature reaches 242°C.

[0063] Example 3, compared with Example 1, the difference is that the total mass ratio of LATP and TiO2 and AlPO4 is 3%, wherein the mass ratio of LATP:TiO2:AlPO4 is 90%:8%:4%, the raw materials used in step S1 are lithium acetate, aluminum sulfate, rutile titanium dioxide and ammonium dihydrogen phosphate respectively; the solvent used in step S3 is ethylene glycol; the pH value in step S4 is 6. The electrochemical performance and DSC test data of the powder material prepared in Example 2: the gram capacity of the material at 0.2C reached 223mAh / g, the capacity retention rate after 50 times of 0.5C charge-discharge was 98.8%, and the DSC decomposition temperature reached 238℃.

[0064] Example 4, compared with Example 1, the difference is that the total mass ratio of LATP and TiO2 and AlPO4 is 3%, wherein the mass ratio of LATP:TiO2:AlPO4 is 80%:12%:8%, the raw materials used in step S1 are lithium sulfate, aluminum sulfate, rutile titanium dioxide and ammonium dihydrogen phosphate respectively; the solvent used in step S3 is ethylene glycol; the pH value in step S4 is 8. The electrochemical performance and DSC test data of the powder material prepared in Example 3: the gram capacity of the material at 0.2C reached 222mAh / g, the capacity retention rate after 50 times of 0.5C charge-discharge was 98.4%, and the DSC decomposition temperature reached 245℃.

[0065] Comparative Example 1

[0066] According to the mass percentage of TiO2 being 1.5%, NCM powder and nano-TiO2 powder are weighed, and TiO2 is uniformly attached to the surface of the ternary material by mechanical ball milling, and then calcined at a temperature of 500℃ for 6h. The electrochemical performance and DSC test data of the powder material prepared in Comparative Example 1: the gram capacity of the material at 0.2C reached 218mAh / g, the capacity retention rate after 50 times of 0.5C charge-discharge was 97%, and the DSC decomposition peak temperature reached 225℃.

[0067] Comparative Example 2

[0068] According to the mass percentage of LATP being 1%, NCM powder and LATP powder are weighed, and LATP is uniformly attached to the surface of the ternary material by mechanical ball milling, and then calcined at a temperature of 550℃ for 6h. The electrochemical performance and DSC test data of the powder material prepared in Comparative Example 2: the gram capacity of the material at 0.2C reached 220mAh / g, the capacity retention rate after 50 times of 0.5C charge-discharge was 97.2%, and the DSC decomposition peak temperature reached 230℃.

[0069] Comparative Example 3

[0070] According to the mass percentage of AlPO4 2%, NCM powder, LATP powder is weighed, and LATP is uniformly attached to the surface of the ternary material by mechanical ball milling, and then baked at 500°C for 6h. The electrochemical performance and DSC test data of the powder material prepared in Comparative Example 3: the specific capacity of the material at 0.2C reached 215mAh / g, the capacity retention rate after 50 times of 0.5C charge-discharge was 97%, and the DSC decomposition peak temperature reached 228°C.

[0071] The above shows and describes the basic principles, main features and advantages of the present application. Those skilled in the art should understand that the present application is not limited to the above examples, and the above examples and descriptions in the specification are only the principles of the present application. Without departing from the spirit and scope of the present application, various changes and improvements can be made to the present application, and these changes and improvements all fall within the scope of the claimed present application.

Claims

1. A multi-element co-coated lithium ion ternary cathode material comprising an inner core and an outer shell, characterized in that, The inner core is a high-nickel ternary positive electrode material NCM, the nickel content is greater than or equal to 80%, and the powder median particle size D50 is 6-12 microns; the shell is a composite coating layer coated on the surface of the inner core, the composite coating layer comprises LATP, TiO2 and AlPO4, the total mass of the composite coating layer accounts for 0.5-3% of the mass of the inner core, and the mass ratio of LATP:TiO2:AlPO4 is 80-90:12-6:8-4%. The shell is formed in situ and integrally by placing the inner core, a lithium source, an aluminum source, titanium dioxide powder and a phosphorus source in a reaction kettle for hydrothermal reaction, and then high-temperature calcining at 500-700 DEG C.

2. A method for preparing a multi-element co-coated lithium ion ternary cathode material, characterized in that, The method comprises the following steps: S1, proportionally weighing NCM powder, a lithium source, an aluminum source, titanium dioxide powder and a phosphorus source; S2, dissolving the lithium source, the aluminum source and the phosphorus source in deionized water respectively, and fully stirring until completely dissolved; S3, adding NCM powder, titanium dioxide powder and an organic solvent into a reaction kettle, and continuously stirring to fully disperse the powder; S4, sequentially adding the three solutions prepared in step S2 into the reaction kettle in step S3, adjusting the pH value of the system to 5-8 with ammonia water, and continuing to stir for a period of time; S5, after sealing the reaction kettle, placing it in an oven for hydrothermal reaction at 190-230 DEG C for 16-48 hours; S6, filtering and washing the reaction product, and drying at 80-120 DEG C to obtain an intermediate product powder; S7, calcining the intermediate product at 500-700 DEG C to obtain the final multi-element co-coated ternary positive electrode material.

3. The method of claim 2, wherein: The atomic molar ratio of the lithium source, the aluminum source, the titanium dioxide powder and the phosphorus source in step S1 is Li:Al:Ti:P = 1.3-1.8:0.3:1.7:3-3.

1.

4. The method of claim 2, wherein: The organic solvent in step S3 comprises one of ethanol, ethylene glycol, polyethylene glycol (200-1000) and diethylene glycol.

5. The method of claim 2, wherein, The lithium source compound in step S1 comprises one or more combinations of lithium hydroxide, lithium nitrate, lithium chloride, lithium sulfate and lithium acetate.

6. The method of claim 2, wherein, The aluminum source compound in step S1 comprises one or more combinations of aluminum nitrate, aluminum sulfate and aluminum chloride.

7. The method of claim 2, wherein, The titanium dioxide powder in step S1 is one of rutile type, anatase type or a mixture of rutile and anatase, and the primary particle size of the titanium dioxide powder is less than or equal to 60 nm.

8. The method of claim 2, wherein, The phosphorus source compound in step S1 comprises one or more combinations of phosphoric acid, diammonium hydrogen phosphate and ammonium dihydrogen phosphate.

9. The method of claim 2, wherein, The total solid content of the powder in step S3 is controlled between 20% and 60%.

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