A modified ternary material with a double-layer coating structure and its preparation method

Abstract

The present invention discloses a modified ternary material with a double-layer coating structure and a preparation method thereof, comprising the following steps: adding the ternary material into an alcohol-based solvent containing ammonium phosphate and dopamine, enabling the polymerization reaction of dopamine on the surface of the ternary material to form a polydopamine coating layer, i.e., the first coating structure; then coating a solid electrolyte coating layer doped with cobalt element on the surface of the ternary material coated with the polydopamine coating layer, i.e., the second coating structure, thus obtaining the modified ternary material with a double-layer coating structure. Without introducing water as a solvent, the present invention sequentially coats a polydopamine coating layer and a solid electrolyte coating layer on the outer surface of the ternary material, and reduces energy consumption by optimizing the synthesis process. At the same time, an additive capable of removing residual lithium is selected during the operation process to achieve the purpose of improving the stability and electrochemical performance of the ternary material.

Description

Technical Field

[0001] The present invention relates to the technical field of solid-state battery materials, and particularly to a modified ternary material with a double-layer coating structure and a preparation method thereof. Background Art

[0002] Traditional liquid electrolyte lithium-ion batteries have advantages such as high discharge voltage, high energy density, long cycle life, and no memory effect, and have been widely used in various consumer electronics and electric vehicles and other fields. However, the uneven deposition and dissolution of lithium metal in traditional electrolyte-based lithium-ion batteries, as well as the chemical reactions occurring with the electrolyte, have led to many performance and safety problems such as battery capacity attenuation, increased internal resistance, short circuit, and easy combustion. Due to the many technical challenges and performance limitations such as energy density in liquid electrolyte lithium-ion batteries, many researchers have gradually shifted their focus to the field of solid-state batteries with higher energy density and high safety potential. Solid-state batteries use all-solid electrolytes to replace liquid electrolytes, which can not only broaden the electrochemical window of the electrolyte, improve the energy density of the battery, but also avoid battery leakage and combustion phenomena, improve the safety and service life of the battery, and eliminate the use of battery separators and optimize the battery assembly process.

[0003] Garnet-type solid electrolyte (LLZTO) has good ionic conductivity at room temperature, a wide electrochemical stability window, and does not chemically react with lithium metal. Therefore, LLZTO with the above excellent properties has also become one of the main research directions for exploring solid electrolytes. By coating LLZTO material on the surface of high-nickel ternary materials, contact between the high-nickel materials and humid air during storage can be avoided, thereby achieving a protective effect on their electrochemical properties. Currently, the solid-phase synthesis method is mainly used for coating ternary materials with solid electrolytes. The solid-phase method for preparing solid electrolyte-coated ternary materials has problems with the poor contact area between solid particles of the electrolyte and the cathode material, resulting in a decrease in the lithium-ion conductivity inside the battery and an increase in the interfacial impedance. The liquid-phase coating process can greatly increase the coating contact area between materials and achieve efficient conduction of ions inside the battery.

[0004] Patent Application No. 202010641220.8 discloses a method for coating a solid electrolyte on the surface of ternary materials by liquid-phase coating. In order to ensure the uniform coating of the solid electrolyte on the ternary materials, this method proposes to use the method of injecting gas to make the ternary materials perform suspended reciprocating motion to achieve the uniform coating of the solid electrolyte on the ternary materials. This method can achieve the uniform coating of the solid electrolyte on the ternary materials. However, because the ternary materials themselves have small particles and are easy to agglomerate, the actual operation process of suspending and coating the ternary materials requires very high requirements for experimental equipment. At the same time, during the liquid-phase coating process of the LLZTO solution on the ternary materials, it is easy for the LLZTO ion source to enter the lattice of the ternary materials, doping its crystal form and affecting its electrochemical performance. This problem can be solved by first coating a grafting layer on the surface of the ternary materials to block the doping phenomenon of the LLZTO ion source on the ternary materials and ensure the expected coating of the LLZTO on the ternary materials.

[0005] Patent Application No. 201310309260.2 discloses a synthesis method for coating ternary materials by the coprecipitation method of triblock copolymer PS-b-PAA. In this method, by adding the ternary material ion source to the tetrahydrofuran solution of the triblock copolymer PS-b-PAA and using (NH4)2CO3 as a precipitating agent, the PS-b-PAA-coated ternary materials are obtained by precipitation method. The coating of the copolymer PS-b-PAA on the ternary materials can play a role in reducing the residual alkali on the surface of the ternary cathode product, achieving the effect of improving the stability and rate performance of the ternary materials. However, the solvent tetrahydrofuran used in this method has high toxicity, making the experimental requirements for the operating environment relatively high, and there is an urgent need for a new, more environmentally friendly and safe improvement plan.

[0006] Patent Application No. 202011215087.6 discloses a synthesis process for a high-nickel ternary aqueous cathode slurry. By using water as a solvent and coating the water-based binder PAA on the surface of the ternary materials, the effect of reducing the residual alkali on the surface of the ternary materials and improving the stability of the cathode slurry is achieved. This scheme has a simple process and is environmentally friendly. However, the introduced aqueous solvent has a great damage to the electrical properties of the ternary materials. Therefore, this scheme needs to find a new anhydrous solvent as an alternative.

[0007] Patent Application No. 201510543694.8 discloses a scheme for coating cobalt salt on the surface of ternary materials. By using the chemical reaction between cobalt salt and the residual lithium on the surface of high-nickel materials to generate lithium cobaltate, the effect of removing residual alkali, improving the material stability, increasing the battery capacity and cycle performance is achieved. This scheme has a simple process and high feasibility. The disadvantage is that it requires secondary mixing and sintering in industrial production, increasing the production cost and time. Summary of the Invention

[0008] The present invention provides a modified ternary material with a double-layer coating structure and a preparation method thereof. Under the condition of not introducing water as a solvent, the present invention first constructs a polydopamine coating layer on the surface of the ternary material by a liquid-phase method, and then forms a solid electrolyte coating layer, reducing energy consumption by optimizing the synthesis process. At the same time, an additive capable of removing residual lithium is selected during the operation to improve the stability and electrochemical performance of the ternary material.

[0009] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0010] A preparation method of a modified ternary material with a double-layer coating structure includes the following steps:

[0011] (1) Preparation of the polydopamine coating layer:

[0012] In an oxygen-containing atmosphere, the ternary material and dopamine are added to an alcohol-based solvent of ammonium phosphate salt, and stirred for 8 - 12 h under heating conditions. The purpose of heating is to accelerate the polymerization reaction of dopamine. Then, the solvent is evaporated until the mixed solution becomes a slurry state, that is, a polydopamine coating layer is formed on the surface of the ternary material, which is the first coating structure. Further, the molecular formula of the ternary material is Li(Ni 1-x-y Co x Mn y )O2, where 0.07 ≤ x ≤ 0.1, 0.05 ≤ y ≤ 0.15; the ammonium phosphate salt is at least one of ammonium dihydrogen phosphate and ammonium hydrogen phosphate. The alkaline condition required during the polymerization reaction of polydopamine is provided by the ammonium phosphate salt. At the same time, the ammonium phosphate salt can chemically react with the residual lithium carbonate on the surface of the ternary material to generate lithium phosphate, reducing the residual alkali amount on the material surface and improving the thermal stability of the material; and the introduced P=O bond strengthens the chemical interaction between the transition metal ions and PO4 3- , thereby improving the structural stability of the material; the coating amount of the polydopamine coating layer on the surface of the ternary material is 3000 - 8000 ppm; the alcohol-based solvent is ethanol or isopropanol.

[0013] (2) Preparation of the solid electrolyte coating layer:

[0014] The lithium source, lanthanum source, zirconium source, and tantalum source are added to an alcohol-based solvent and dissolved, and then an acidic complexing agent is added. After stirring at 40 - 60 °C for 1 - 4 h, a solid electrolyte precursor solution is obtained. Further, the lithium source is lithium acetate or lithium nitrate, and the lithium source is in an excess of 5% - 30%; the lanthanum source is lanthanum nitrate, lanthanum oxide, or lanthanum chloride; the zirconium source is zirconium nitrate, zirconyl chloride, or zirconium tetrachloride; the tantalum source is tantalum ethoxide; the acidic complexing agent is citric acid, ethylenediaminetetraacetic acid, or oxalic acid. The role of the acidic complexing agent is to neutralize the residual alkaline ammonium phosphate salt in the polydopamine adhesive film.

[0015] The ternary material coated with a polydopamine coating layer is added to the solid electrolyte precursor solution. After stirring and reacting, a cobalt salt is added thereto. Then, the alcohol-based solvent is heated and evaporated. After the obtained slurry is sealed and left standing for 1-3 h, it is successively dried, ground, and sintered at a high temperature in an oxygen-containing atmosphere. A solid electrolyte coating layer is formed on the outer surface of the ternary material coated with a polydopamine coating layer, that is, the second coating structure; and the solid electrolyte coating layer is doped with cobalt element. Further, the cobalt salt is cobalt carbonate, cobalt tetroxide or cobalt oxalate; the mass of the cobalt salt is 2000-5000 ppm of the ternary material; the drying temperature is 90±5 °C, and the drying time is 12-16 h. The specific process parameters for high-temperature sintering in the oxygen-containing atmosphere are as follows: First, keep the temperature at 650-1050 °C for 2-4 h; then cool down to a temperature of 350-500 °C and continue to keep the temperature for 4-8 h. The molecular formula of the solid electrolyte coating layer is Li 7-x La3Zr 2-x Ta x O 12 , where x ranges from 0.25 to 0.6; the coating amount of the solid electrolyte coating layer is 5000-20000 ppm; the molar ratio of the acidic complexing agent to the total amount of all metal ions in the solid electrolyte precursor solution is 1:1-2.0.

[0016] The second object of the present invention is to provide a modified ternary material having a double-layer coating structure, which is prepared by using the preparation method described above.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0018] The modified ternary material with a double-layer coating structure prepared by the present invention is obtained by sequentially coating a polydopamine coating layer and a solid electrolyte coating layer on the surface of the ternary material. Among them: the polydopamine coating layer is located between the ternary material and the solid electrolyte coating layer, and plays a strong inorganic-organic covalent-non-covalent interaction, strongly combining the modified material into an overall structure. The polydopamine coating layer simultaneously achieves three functions, including: (1) removing residual lithium on the surface of the ternary material; (2) avoiding the doping of ion sources in the solid electrolyte into the interior of the ternary material during subsequent liquid-phase coating; (3) increasing the contact area between the solid electrolyte and the ternary material, making the coating stability and consistency of the solid electrolyte on the ternary material better. The second coating structure, that is, the solid electrolyte coating layer, can reduce the sintering temperature required for material synthesis by doping with tantalum and improve the ionic conductivity of the ternary material.

[0019] The liquid-phase coating process using alcohol-based solvents in the present invention can obtain product particles with a more uniform particle size distribution, increase the interfacial contact area between the solid electrolyte coating layer and the polydopamine coating layer, improve the lithium-ion migration rate and diffusion coefficient, and thus improve the electrochemical performance of the cathode material; using an alcohol-based solution as the synthesis solvent to avoid contact between the ternary material and moisture and prevent damage to the material performance caused by moisture; during the preparation process of the present invention, there are two processes for removing residual alkali, namely: using ammonium phosphate salt to provide an alkaline environment for the dopamine polymerization reaction to remove the residual alkali on the surface of the ternary material and doping cobalt ions during the formation of the solid electrolyte coating layer to remove the residual alkali on the surface of the solid electrolyte coating layer. By removing residual alkali twice before and after, the stability and battery capacity of the synthesized material are improved. Description of the Drawings

[0020] Figure 1 XRD patterns of the materials prepared in Example 1, Comparative Example 1, Comparative Example 2, and the uncoated Li(Ni 0.75 Co 0.10 Mn 0.15 )O2 material;

[0021] Figure 2 SEM images of the material prepared in Example 1 and the uncoated Li(Ni 0.75 Co 0.10 Mn 0.15 )O2 material;

[0022] Figure 3 DSC curves of the batteries made of the materials prepared in Example 3, Comparative Example 3, and the uncoated Li(Ni 088 Co 0.07 Mn 0.05 )O2 material. Detailed Embodiments

[0023] In order to make the objectives, technical solutions, and advantages of the present invention clearer, the following is a detailed and clear description of the present invention. The described embodiments do not represent all embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without making significant creative efforts fall within the protection scope of the present invention. Unless otherwise specified, the specific conditions in the embodiments are carried out according to conventional conditions or the conditions recommended by the manufacturer. All reagents or instruments not indicating the manufacturer can be obtained as conventional commercial products through market purchase.

[0024] A preparation method of a modified ternary material with a double-layer coating structure includes the following steps:

[0025] (1) Preparation of the polydopamine coating layer:

[0026] In an oxygen-containing atmosphere, ternary material and dopamine are added to an alcohol-based solvent of ammonium phosphate salt, and stirred for 8 - 12 h under heating conditions. The purpose of heating is to accelerate the polymerization reaction of dopamine. Then, the solvent is evaporated until the mixed solution becomes a slurry state, that is, a polydopamine coating layer is formed on the surface of the ternary material. Further, the ammonium phosphate salt is at least one of ammonium dihydrogen phosphate and ammonium hydrogen phosphate. The alkaline environment required during the coating process of the ternary material by polydopamine is provided by the ammonium phosphate salt, and at the same time, the residual lithium on the surface of the ternary material is removed. The coating amount of the polydopamine coating layer on the ternary material is 3000 - 8000 ppm. The alcohol-based solvent is ethanol or isopropanol.

[0027] (2) Preparation of solid electrolyte coating layer:

[0028] A lithium source, a lanthanum source, a zirconium source, and a tantalum source are added to an alcohol-based solvent for dissolution, and then an acidic complexing agent is added. After stirring at 40 - 60 °C for 1 - 4 h, a solid electrolyte precursor solution is obtained. Further, the lithium source is lithium acetate or lithium nitrate; the lanthanum source is lanthanum nitrate, lanthanum oxide, or lanthanum chloride; the zirconium source is zirconium nitrate, zirconyl chloride, or zirconium tetrachloride; the tantalum source is tantalum ethoxide; the acidic complexing agent is citric acid, ethylenediaminetetraacetic acid, or oxalic acid.

[0029] The ternary material coated with a polydopamine coating layer is added to the solid electrolyte precursor solution. After stirring and reacting, a cobalt salt is added thereto, and then the alcohol-based solvent is evaporated by heating. After the obtained slurry is sealed and left standing for 1 - 3 h, it is successively dried, ground, and sintered at a high temperature in an oxygen-containing atmosphere, and a solid electrolyte coating layer is formed on the outer surface of the ternary material coated with a polydopamine coating layer; and cobalt element is doped in the solid electrolyte coating layer. Further, the cobalt salt is cobalt carbonate, cobalt tetroxide, or cobalt oxalate; the mass of the cobalt salt is 2000 - 5000 ppm of the ternary material; the drying temperature is 90 ± 5 °C, and the drying time is 12 - 16 h. The specific process parameters of the high-temperature sintering in the oxygen-containing atmosphere are as follows: First, keep the temperature at 650 - 1050 °C for 2 - 4 h; then cool down to a temperature of 350 - 500 °C and continue to keep the temperature for 4 - 8 h. The molecular formula of the solid electrolyte coating layer is Li 7-x La3Zr 2-x Ta x O 12 , where x ranges from 0.25 to 0.6; the coating amount of the solid electrolyte coating layer is 5000 - 20000 ppm; the lithium source is in an excess of 5% - 30%; the molar ratio of the acidic complexing agent to the total amount of all metal ions in the solid electrolyte precursor solution is 1:1 - 2.0.

[0030] Example 1

[0031] A preparation method of a modified ternary material with a double-layer coating structure. In this embodiment, the NCM ternary material used is Li(Ni 0.75 Co 0.10 Mn 0.15 )O2, simply referred to as Ni75; the coating amount of the polydopamine (PDA) coating layer is 5000 ppm of the mass of the ternary material; the molecular formula of the solid electrolyte coating layer is Li 6.4 La3Zr 1.6 Ta 0.4 O 12 , and the coating amount is 15000 ppm of the ternary material; the preparation method of this material includes the following steps:

[0032] (1) Preparation of the polydopamine coating layer:

[0033] First, sequentially weigh the corresponding amount of the ternary material and add it to the ethanol solution. After stirring evenly, add dopamine and ammonium dihydrogen phosphate, adjust the pH to 8.5, adjust the temperature to 35 °C, introduce oxygen, and the oxygen flow rate is 5 mL / min. Continue to stir the solution while evaporating the solvent for 12 h to form a polydopamine coating layer on the surface of the ternary material;

[0034] (2) Preparation of the solid electrolyte coating layer:

[0035] According to the molecular formula of the solid electrolyte Li 6.4 La3Zr 1.6 Ta 0.4 O 12 , weigh the corresponding amounts of lithium nitrate, lanthanum nitrate hexahydrate, and zirconium nitrate pentahydrate and add them to anhydrous ethanol. Carry out sealed magnetic stirring and dissolution at 50 °C to obtain a solution containing lithium, lanthanum, and zirconium; dissolve tantalum ethoxide in anhydrous ethanol to obtain a tantalum ethoxide solution; add the tantalum ethoxide solution to the above solution containing lithium, lanthanum, and zirconium, and continue to stir; add citric acid to the above mixture, heat and stir for 2 h to obtain a solid electrolyte precursor solution; the lithium in this solid electrolyte precursor solution is 5% in excess, and the addition amount of citric acid is n(Li + +La 3+ +Zr 4+ +Ta 5+ ): n(citric acid) = 1:1.3.

[0036] The ternary material with a polydopamine coating layer was added to the solid electrolyte precursor solution and stirred at 70 °C for 1 h. Based on the mass of the ternary material, 3000 ppm of cobalt carbonate was added. The mixture was heated and stirred to evaporate the solvent. After the obtained slurry was sealed and allowed to stand for 1 h, it was dried at 90 °C for 12 h. After grinding, it was subjected to a sintering process. The sintering process was as follows: heating at 5 °C / min to 750 °C and holding for 2 h, then cooling to 500 °C and holding for 4 h, so that a solid electrolyte coating layer was formed on the outer surface of the ternary material with a polydopamine coating layer on its surface, and a modified ternary material with a double-layer coating structure was obtained, denoted as Ni75@PDA@LLZTO.

[0037] Example 2

[0038] Compared with Example 1, the difference in Example 2 is that the NCM ternary material is Li(Ni 0.88 Co 0.07 Mn 0.05 )O2, simply referred to as Ni88, and other processes are the same as those in Example 1.

[0039] Example 3

[0040] Compared with Example 1, the difference in Example 3 is that the ternary material is Li(Ni 0.88 Co 0.07 Mn 0.05 )O2; the molecular formula of the solid electrolyte coating layer is Li 6.75 La3Zr 1.75 Ta 0.75 O 12 , and its coating amount is 20000 ppm of the mass of the ternary material. Other processes are the same as those in Example 1. The prepared material is denoted as Ni88@PDA@LLZTO.

[0041] Comparative Example 1

[0042] According to the molecular formula of the solid electrolyte Li 6.4 La3Zr 1.6 Ta 0.4 O 12 , the corresponding amounts of lithium nitrate, lanthanum nitrate hexahydrate, and zirconium nitrate pentahydrate were weighed and added to anhydrous ethanol, and magnetically stirred and dissolved at 50 °C to obtain a solution containing lithium, lanthanum, and zirconium; tantalum ethoxide was dissolved in anhydrous ethanol to obtain a tantalum ethoxide solution; the tantalum ethoxide solution was added to the above solution containing lithium, lanthanum, and zirconium, and stirring was continued; citric acid was added to the above mixture, and it was heated and stirred for 2 h to obtain a solid electrolyte precursor solution; the lithium in this solid electrolyte precursor solution was in excess by 5%, and the addition amount of citric acid was n(Li + +La 3+ +Zr 4+ +Ta 5+):n(citric acid) = 1:1.3.

[0043] After standing the solid electrolyte precursor solution for 1 h, it was dried at 90 °C for 12 h, ground, and then sintered. The sintering process was as follows: heating to 750 °C at a rate of 5 °C / min, holding for 2 h, then cooling to 500 °C and holding for 4 h to obtain the solid electrolyte Li 6.4 La3Zr 1.6 Ta 0.4 O 12 material, denoted as LLZTO.

[0044] Comparative Example 2

[0045] Comparative Example 2 is different from Example 1 in that it does not include the preparation process of the polydopamine coating layer in step (1), only performs the preparation of the solid electrolyte coating layer in step (2) and does not dope cobalt carbonate during the preparation of the solid electrolyte coating layer. The remaining processes are the same as those in Example 1, that is, a ternary material with only a solid electrolyte coating layer as the coating structure is prepared, denoted as LLZTO@Ni75. Other processes are the same as those in Example 1.

[0046] Comparative Example 3

[0047] Comparative Example 3 is different from Example 3 in that it does not include the preparation process of the polydopamine coating layer in step (1), only performs the preparation of the solid electrolyte coating layer in step (2) and does not dope cobalt carbonate during the preparation of the solid electrolyte coating layer. The remaining processes are the same as those in Example 3, that is, a ternary material with only a solid electrolyte coating layer as the coating structure is prepared, denoted as Ni88@LLZTO.

[0048] Performance testing:

[0049] The products prepared in Example 1, Comparative Example 1, and Comparative Example 2 were respectively subjected to XRD and SEM tests, and the results are as Figure 1 shown. It can be seen from Figure 1 that a cubic solid electrolyte material (LLZTO) was synthesized in Comparative Example 1. The XRD patterns of Example 1 and Comparative Example 2 showed that the coating of LLZTO on the NCM ternary material did not change the diffraction peak shape of the ternary material, but weak LLZTO diffraction peaks appeared. Moreover, the LLZTO diffraction peak in Example 1 was significantly stronger than the diffraction peak of Comparative Example 2 without polydopamine (PDA) coating, indicating that the coating of PDA is helpful for the synthesis and coating of the second-layer solid electrolyte coating layer; Figure 2 The uncoated Li(Ni 0.75 Co 0.10 Mn 0.15)The morphological differences between the O2 material and the ternary material coated with LLZTO synthesized in Example 1. Compared with the uncoated material, the surface of the NCM ternary material particles after LLZTO coating is smoother, and the average particle size increases slightly. Through the XRD pattern and SEM image of the material, it is shown that cubic-phase LLZTO was successfully synthesized by this liquid-phase method and successfully coated on the surface of the ternary material particles.

[0050] Charge the ternary material of Example 3, Comparative Example 3 and the uncoated Ni88 to 4.35V, disassemble the battery and test the thermal stability performance of the material. The test parameters are: from room temperature to 400°C; heating rate: 10°C / min, and the test results are as Figure 3 . The test curve of the uncoated Ni88 shows an obvious exothermic peak at about 210°C. This exothermic peak is mainly attributed to the transformation of the ternary material from a layered structure to a spinel-phase structure, as well as the detachment of a small amount of oxygen atoms in the material lattice and the release of oxygen. The exothermic peak values of Example 3 and Comparative Example 3 appear at 219°C and 216°C respectively, indicating that the coating of LLZTO improves the stability of the ternary material, and the first-layer coating with polydopamine improves the improvement effect of LLZTO on the stability of the ternary material. By calculating the integral area of the region enclosed by the DSC curve of each test sample and its baseline, the integral area of the sample prepared in Example 3 is 0.6 and 0.4 times that of Comparative Example 3 and the Ni88 material without LLZTO coating respectively, indicating that by coating polydopamine and LLZTO in this heating stage, the heat release of the high-nickel ternary material can be reduced and the thermal stability performance of the ternary material can be improved.

[0051] Make CR2016 button batteries from the NCM ternary materials used in Examples 1-3, Comparative Examples 2-3 and the experiment respectively to test the electrochemical performance of the materials. The current density of all batteries is 100 mA / g, and the voltage range is 2.8-4.3V. The test results are shown in Table 1 below:

[0052] Table 1 Detection results of electrical performance

[0053]

[0054] It can be seen from the performance data of each synthesized material in Table 1 that coating the ternary material with LLZTO can improve its initial charge and discharge capacity, but the addition of polydopamine coating and the introduction of cobalt element can further improve the improvement effect of LLZTO on the stability of the ternary material and the capacitance of the material.

[0055] The above-described are only some specific embodiments of the present invention. However, the protection scope of the present invention is not limited thereto. Any person familiar with the technical field of the present invention can easily think that changes or substitutions within the technical scope disclosed by the present invention should be covered by the protection scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims.

Claims

1. A preparation method of a modified ternary material with a double-layer coating structure, characterized in that: It includes the following steps: Adding the ternary material into an alcohol-based solvent containing ammonium phosphate salt and dopamine, and enabling dopamine to perform a polymerization reaction on the surface of the ternary material to form a polydopamine coating layer; Coating a solid electrolyte coating layer on the surface of the ternary material coated with the polydopamine coating layer, wherein cobalt element is doped in the solid electrolyte coating layer, thus obtaining a modified ternary material with a double-layer coating structure; The method for coating a solid electrolyte coating layer on the surface of the ternary material coated with the polydopamine coating layer is as follows: Adding a lithium source, a lanthanum source, a zirconium source, and a tantalum source into an alcohol-based solvent for dissolution, then adding an acidic complexing agent, and stirring for 1 to 4 h at 40 - 60 °C to obtain a solid electrolyte precursor solution; Adding the ternary material coated with the polydopamine coating layer into the solid electrolyte precursor solution, adding cobalt salt thereto after stirring and reacting, then heating and evaporating the alcohol-based solvent, and after the obtained slurry is subjected to sealed static treatment, successively through drying, grinding, and high-temperature sintering in an oxygen-containing atmosphere, a solid electrolyte coating layer is formed on the outer surface of the ternary material coated with the polydopamine coating layer, and cobalt element is doped in the solid electrolyte coating layer; the cobalt salt is cobalt carbonate, cobalt tetroxide, or cobalt oxalate.

2. The preparation method of the modified ternary material with a double-layer coating structure according to claim 1, characterized in that: The ammonium phosphate salt is at least one of ammonium dihydrogen phosphate and ammonium hydrogen phosphate.

3. The preparation method of the modified ternary material with a double-layer coating structure according to claim 1, characterized in that: The molecular formula of the ternary material is Li(Ni 1-x-y Co x Mn y )O2, where 0.07 ≤ x ≤ 0.1 and 0.05 ≤ y ≤ 0.15; the coating amount of the polydopamine coating layer on the surface of the ternary material is 3000 - 8000 ppm.

4. The preparation method of the modified ternary material with a double-layer coating structure according to claim 1, characterized in that: The lithium source is lithium acetate or lithium nitrate, and the lithium source is in an excess of 5% - 30%; the lanthanum source is lanthanum nitrate, lanthanum oxide, or lanthanum chloride; the zirconium source is zirconium nitrate, zirconyl chloride, or zirconium tetrachloride; the tantalum source is tantalum ethoxide; the acidic complexing agent is citric acid, ethylenediaminetetraacetic acid, or oxalic acid.

5. The preparation method of the modified ternary material with a double-layer coating structure according to claim 1, characterized in that: The molecular formula of the solid electrolyte coating layer is Li 7-x La3Zr 2-x Ta x O 12 , where the value of x ranges from 0.25 to 0.6; the coating amount of the solid electrolyte coating layer is 5000 - 20000 ppm; the molar ratio of the acidic complexing agent to the total amount of all metal ions in the solid electrolyte precursor solution is 1:1 - 2.

0.

6. The preparation method of the modified ternary material with a double-layer coating structure according to claim 1, characterized in that: The mass of the cobalt salt is 2000 - 5000 ppm of the ternary material; the drying temperature is 90 ± 5 °C, and the drying time is 12 - 16 h.

7. The preparation method of the modified ternary material with a double-layer coating structure according to claim 1, characterized in that: The specific process parameters for high-temperature sintering in the oxygen-containing atmosphere are as follows: First, keep the temperature at 650 - 1050 °C for 2 - 4 h; then cool down to a temperature of 350 - 500 °C and continue to keep the temperature for 4 - 8 h.

8. The preparation method of the modified ternary material with a double-layer coating structure according to any one of claims 1 to 7, characterized in that: The alcohol-based solvent is ethanol or isopropanol.

9. A modified ternary material with a double-layer coating structure, characterized in that: It is prepared by using the preparation method described in any one of claims 1 to 8.

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