Modified positive electrode material and preparation method and application thereof

Abstract

The invention relates to the technical field of lithium ion battery positive electrode materials, in particular to a modified positive electrode material and a preparation method and application thereof. The modified positive electrode material comprises a positive electrode material core, and a boron-containing coating layer and a metal oxide coating layer which sequentially coat the surface of the positive electrode material core from inside to outside; the boron-containing coating layer and the positive electrode material core are combined through a B-O-Li bond, the boron-containing coating layer and the metal oxide coating layer are combined through a B-O-M bond, and M is a metal element in the metal oxide coating layer. Organic boron is used as a molecular bridge, and a chemical bonding interface is constructed between the positive electrode material and the coating layer through chemical reaction, so that the uniformity and compactness of the coating layer are remarkably improved, the problem that the coating layer of the traditional inorganic coating positive electrode material is easy to fall off is solved, and the electrochemical performance and thermal safety of the positive electrode material are remarkably improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium ion battery cathode material, in particular to a modified cathode material and a preparation method and application thereof. BACKGROUND

[0002] As the current mainstream electrochemical energy storage device, lithium ion batteries have been widely used in electric vehicles, portable electronic devices and large-scale energy storage systems. The ternary cathode material (NCM) has become one of the key core materials of high energy density lithium ion batteries due to its high specific capacity, high energy density and good cycle performance.

[0003] One of the ways to improve the energy density of the ternary cathode material is to charge to a high voltage (>4.3V vs. Li / Li + However, at high voltage, the oxidation decomposition of the electrolyte and the degradation of the surface structure of the cathode material will accelerate sharply, which seriously affects the interface stability. Inorganic coating of the cathode material can effectively isolate the cathode material from the electrolyte, protect the core cathode material, and significantly improve the cycle life and thermal safety of the material; however, the traditional inorganic coating layer (such as Al2O3, ZrO2) is mostly physically attached to the substrate, with weak adhesion, and is prone to cracking and peeling under the action of long-term electrochemical cycling or volume expansion stress, resulting in failure of the protection effect; secondly, the coating process often fails to achieve uniform and dense coverage, and is prone to island-like agglomeration or local exposure, affecting the overall performance improvement.

[0004] Therefore, how to build a firm, uniform and functionally cooperative chemical bonding interface between the cathode material and the coating layer, realize stable anchoring and efficient protection of the coating layer, and effectively consume the surface residual alkali and inhibit the interface side reaction, is of great significance to improve the electrochemical performance and thermal safety of the cathode material.

[0005] Therefore, the present application is proposed. SUMMARY

[0006] The first object of the present application is to provide a modified cathode material, which establishes a firm chemical bonding between the inorganic coating layer and the cathode material through a boron-containing interface layer, replacing the traditional physical adsorption or simple inorganic coating, so that the uniformity and density of the coating layer are significantly improved, solving the problem of easy peeling of the coating layer of the traditional inorganic coated cathode material.

[0007] The second object of the present application is to provide a preparation method of the modified cathode material as described above, which uses an organic boron compound as a "molecular bridge" to react with the cathode material and the inorganic coating layer respectively to build a firm chemical bonding interface between the core of the cathode material and the inorganic coating layer, realizing stable and dense coating.

[0008] A third object of the present application is to provide a positive electrode sheet comprising the modified positive electrode material as described above.

[0009] A fourth object of the present application is to provide a lithium ion battery comprising the positive electrode sheet as described above.

[0010] In order to achieve the above object of the present application, the following technical solutions are adopted: A modified positive electrode material comprising a positive electrode material core and a boron-containing coating layer and a metal oxide coating layer successively coated on the surface of the positive electrode material core from inside to outside; the boron-containing coating layer and the positive electrode material core are combined by B-O-Li bonds, and the boron-containing coating layer and the metal oxide coating layer are combined by B-O-M bonds, wherein M is a metal element in the metal oxide coating layer.

[0011] Preferably, the chemical formula of the positive electrode material core comprises LiNi x Co y Mn z A a O2, x+y+z+a =1, x≥0.5, 0.05≤y≤0.25, 0≤a≤0.015, A comprises at least one of Zr, Ti, W, Nb, Sr, Al, Mg, Ca, Ba, La, Ce, Nd, Mo, F, P, B or Y.

[0012] Preferably, the metal oxide coating layer comprises at least one of cobalt-manganese oxide, nickel-cobalt-manganese oxide, cobalt-aluminum oxide, nickel-cobalt-aluminum oxide.

[0013] A preparation method of the modified positive electrode material according to any one of the preceding embodiments, comprising the following steps: S1. Dissolve an organic boron compound in a solvent to obtain a bridging agent solution; S2. Take a powder positive electrode material as a core and add it to the bridging agent solution, and react at a constant temperature to make the organic boron compound react with residual alkali on the surface of the positive electrode material to form B-O-Li bonds. After the reaction, remove the solvent and free organic matter to obtain an organic boron-modified positive electrode material; S3. Disperse the organic boron-modified positive electrode material in water, and simultaneously add a metal salt solution and a precipitating agent to form a metal hydroxide coating layer on the surface of the organic boron-modified positive electrode material through a co-precipitation reaction. The metal hydroxide reacts with the organic boron compound to form B-O-M bonds, and then the product is washed and dried; S4. Sinter and solidify the dried material and crush it to obtain the modified positive electrode material.

[0014] Preferably, in step S1, the organic boron compound comprises at least one of phenylboronic acid, 4-vinylphenylboronic acid, polyborate, tributyl borate, trimethyl borate, polyborosiloxane, and triphenyl boron.

[0015] Preferably, in step S1, the solvent comprises at least one of alcohol-water mixture, tetrahydrofuran, dichloromethane, toluene, and n-hexane.

[0016] Preferably, the alcohol in the alcohol-water mixture comprises ethanol and / or isopropanol.

[0017] Preferably, in step S2, the mass ratio of the powder positive electrode material to the organic boron compound is 80-120:1.

[0018] Preferably, in step S2, the temperature of the constant-temperature reaction is 40-50℃, and the reaction time is 2-6h.

[0019] Preferably, in step S2, after the reaction, the physically adsorbed free organic matter is removed by alcohol washing.

[0020] Preferably, in step S3, the metal salt comprises at least one of cobalt salt, manganese salt, and aluminum salt; the cobalt salt comprises at least one of cobalt nitrate, cobalt acetate, cobalt sulfate, and cobalt chloride; the manganese salt comprises at least one of manganese nitrate, manganese sulfate, manganese chloride, and manganese acetate; and the aluminum salt comprises at least one of aluminum nitrate, aluminum isopropoxide, aluminum chloride, and aluminum sulfate.

[0021] Preferably, in step S3, the total concentration of metal ions in the metal salt solution is 0.1-1mol / L.

[0022] Preferably, in step S3, the precipitant comprises at least one of ammonia water, sodium hydroxide solution, and potassium hydroxide solution.

[0023] Preferably, in step S3, the concentration of the precipitant is 0.5-4mol / L.

[0024] Preferably, in step S3, the metal salt is added in a molar ratio of 1:5-1:50 of the organic boron compound to metal ions.

[0025] Preferably, in step S3, the pH of the co-precipitation reaction is 8-12, the reaction temperature is 50-90℃, and the reaction time is 2-6h.

[0026] A positive electrode sheet comprising the modified positive electrode material of any one of the preceding embodiments or the modified positive electrode material prepared by the preparation method of any one of the preceding embodiments.

[0027] A lithium ion battery comprising the positive electrode sheet of the preceding embodiments.

[0028] Compared with the prior art, the present application has the following beneficial effects: When coating the positive electrode material, the present application uses organic boron compounds as "molecular bridges", utilizes the specific functional groups to occur directional chemical reaction with residual lithium or hydroxyl on the surface of the matrix, and simultaneously coordinates or combines with the coating agent precursor, builds firm chemical bonding between the core of the positive electrode material and the inorganic coating layer, guides the subsequent inorganic coating agent to grow more uniformly and densely on the basis of the organic boron molecules "bridging", and thus obtains a coating layer with more excellent performance. The present application replaces the traditional physical adsorption or simple inorganic coating with chemical bonding, thereby solving the problems of non-uniform coating layer, weak bonding force, easy peeling of the coating layer, and large interface impedance, realizing uniform, stable and dense coating, and more effectively isolating the corrosion of the electrolyte to the core positive electrode material; and significantly improving the cycle life and thermal safety of the material. BRIEF DESCRIPTION OF DRAWINGS

[0029] In order to more clearly illustrate the technical solutions in the specific embodiments or the prior art, the drawings needed to be used in the specific embodiments or the prior art description will be briefly introduced. Obviously, the drawings in the following description are some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor.

[0030] Figure 1 SEM image of the modified positive electrode material prepared in Example 1 of the present application; Figure 2 SEM image of the modified positive electrode material prepared in Comparative Example 1 of the present application. DETAILED DESCRIPTION

[0031] The technical solutions of the present application will be described clearly and completely in combination with the drawings and specific embodiments, but those skilled in the art will understand that the following described embodiments are part of the embodiments of the present application, not all the embodiments, and are only used to illustrate the present application, and should not be regarded as limiting the scope of the present application. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application. The specific conditions not mentioned in the embodiments are carried out according to the conventional conditions or the conditions recommended by the manufacturer. The reagents or instruments not mentioned by the manufacturer are all conventional products that can be purchased on the market.

[0032] The first aspect of the present application provides a modified positive electrode material, comprising a positive electrode material core and a boron-containing coating layer and a metal oxide coating layer successively coated on the surface of the positive electrode material core from inside to outside; the boron-containing coating layer and the positive electrode material core are combined through B-O-Li bonds, and the boron-containing coating layer and the metal oxide coating layer are combined through B-O-M bonds, wherein M is a metal element in the metal oxide coating layer.

[0033] The present application introduces a boron-containing coating layer as an interface layer between the positive electrode material core and the inorganic coating layer, and makes the boron-containing coating layer respectively connected with the positive electrode material core and the inorganic coating layer through chemical bonds, thereby constructing a strong chemical bonding interface between the positive electrode material core and the inorganic coating layer, realizing uniform, stable and dense coating, solving the problems of non-uniform coating layer, weak interface bonding force, easy peeling of coating layer and large interface impedance of traditional inorganic coating products, so as to make the coating layer more effectively isolate the electrolyte, protect the positive electrode material core, and significantly improve the electrochemical performance and thermal safety of the material.

[0034] In some embodiments of the present application, the chemical formula of the positive electrode material core comprises LiNi x Co y Mn z A a O2, wherein x+y+z+a=1, x≥0.5, 0.05≤y≤0.25, 0≤a≤0.015, and A comprises at least one of Zr, Ti, W, Nb, Sr, Al, Mg, Ca, Ba, La, Ce, Nd, Mo, F, P, B or Y; for example, it can be LiNi 0.8 Co 0.1 Mn 0.1 O2, LiNi 0.68 Co 0.1 Mn 0.22 O2, LiNi 0.72 Co 0.12 Mn 0.16 O2, etc.

[0035] In some embodiments of the present application, the metal oxide coating layer comprises at least one of cobalt-manganese oxide, nickel-cobalt-manganese oxide, cobalt-aluminum oxide, and nickel-cobalt-aluminum oxide; the use of Co-Mn or Co-Al or Ni-Co-Mn oxide coating can significantly improve the surface stability, especially the cycle performance at high voltage; the coating layer can form a barrier more resistant to high voltage on the surface of the particles; the Mn (especially +4) in the Mn-rich shell is very stable at high voltage, which can effectively inhibit oxygen precipitation and phase transition; the Co in the Co-rich shell can improve the electronic conductivity, stabilize the layered structure, and reduce Li / Ni mixing, thus the improvement can make the positive electrode material more suitable for working at high voltage, thereby tapping its higher capacity potential, achieving the improvement of energy density, and ensuring the cycle life.

[0036] The second aspect of the present application provides a preparation method of the modified positive electrode material according to any one of the preceding embodiments, comprising the following steps: S1. Dissolving the organic boron compound in a solvent to obtain a bridging agent solution; S2. Taking the powder positive electrode material as the core and adding it to the bridging agent solution, and performing constant temperature reaction to make the organic boron compound react with the residual alkali on the surface of the positive electrode material to form B-O-Li bond; after the reaction, the solvent and free organic matter are removed to obtain the organic boron modified positive electrode material; S3. Dispersing the organic boron modified positive electrode material in water, synchronously adding a metal salt solution and a precipitating agent, and forming a metal hydroxide coating layer on the surface of the organic boron modified positive electrode material through a co-precipitation reaction; the metal hydroxide reacts with the organic boron compound to form B-O-M bond, and then washing and drying are performed; S4. Sintering and solidifying the dried material, and crushing to obtain the modified positive electrode material.

[0037] The present application uses organic boron compounds as molecular bridge linkers to realize the close coating modification of metal oxides and ternary positive electrode materials; the introduction of organic boron compounds aims to utilize the specific functional groups (such as -B(OH)2) to occur directional chemical reaction with the residual lithium or hydroxyl on the surface of the positive electrode material, and at the same time utilize the organic functional groups or boron atoms to coordinate or combine with the coating agent precursor; this interface layer is a unique organic-inorganic hybrid structure, the organic boron molecules act as "reinforcing bars", and the oxides act as "concrete", forming a synergistic protective layer with high strength (inorganic) and flexibility (organic). Through the "bridging" of the organic boron molecules, the subsequent inorganic coating agent is guided to grow more uniformly and densely on the basis, so as to obtain a coating layer with more excellent performance. The organic boron compounds used can effectively reduce the surface residual alkali, inhibit the interface side reaction, and better adapt to the volume change in the charging and discharging process, reduce the generation of cracks, and provide a more optimal ion migration channel; and the outer inorganic coating layer can more effectively isolate the electrolyte and protect the core positive electrode material, significantly improving the cycle life and thermal safety of the material.

[0038] The present application first pretreats the surface of the positive electrode material by using organic boron compounds, utilizes the organic functional groups (such as boronic acid groups -B(OH)2) in the organic boron molecules to react with LiOH, Li2CO3 and other residual alkali on the surface of the positive electrode material, forms B-O-Li bond, so that the organic boron molecules are firmly anchored on the surface of the positive electrode material, and the surface is rich in reactive boronic acid groups or hydroxyl groups; then directional reaction is carried out with metal hydroxide to realize precise coating. Specifically, the positive electrode material modified by organic boron is dispersed in water to form a suspension, and then metal hydroxide is generated through coprecipitation reaction. The -OH on the surface of the newly generated metal hydroxide nanoparticles will undergo dehydration condensation reaction with the organic groups of the organic boron molecules pre-modified on the surface of the positive electrode material, forming a firm B-O-M covalent bond, and obtaining a core-shell structure of positive electrode material (core)-organic boron molecule bridge-metal hydroxide (shell); then sintering solidification is carried out to make the organic boron pyrolysis form a boron-containing interface layer, and at the same time promote the fusion of the hydroxide coating layer and the substrate and the boride layer, so as to obtain an oxide-coated modified positive electrode material.

[0039] In some embodiments of the present application, in step S1, the organic boron compound includes at least one of phenylboronic acid, 4-vinylphenylboronic acid, polyborate, tributyl borate, trimethyl borate, polyborosiloxane, and triphenyl boron; as an example, the purity of the organic boron compound is ≥98%.

[0040] In some embodiments of the present application, in step S1, the solvent used includes at least one of alcohol water mixture, tetrahydrofuran, dichloromethane, toluene, and n-hexane; the organic boron compound is placed in the solvent, stirred at 40-60°C until completely dissolved, to obtain a clear and transparent organic boron bridging agent solution.

[0041] In some embodiments of the present application, the alcohol used in the alcohol-water mixture includes ethanol and / or isopropanol.

[0042] In some embodiments of the present application, in step S2, the mass ratio of the added powder positive electrode material to the organic boron compound is 80-120:1, for example, it can be any one value or a range value composed of any two point values in 80:1, 90:1, 100:1, 110:1, 120:1.

[0043] In some embodiments of the present application, in step S2, the temperature of the constant temperature reaction is 40-50℃, for example, it can be any one value or a range value composed of any two point values in 40℃, 42℃, 45℃, 48℃, 50℃; the reaction time is 2-6h, for example, it can be any one value or a range value composed of any two point values in 2h, 3h, 4h, 5h, 6h.

[0044] In some embodiments of the present application, in step S2, after the reaction, the physically adsorbed free organic matter is removed by alcohol washing.

[0045] In some embodiments of the present application, in step S2, the D50 particle size of the powder positive electrode material used is 3-20μm, for example, it can be any one value or a range value composed of any two point values in 3μm, 5μm, 10μm, 15μm, 20μm.

[0046] In some embodiments of the present application, in step S2, the powder positive electrode material is dried at 100-120℃ for 8-12h before use, for example, the drying temperature can be any one value or a range value composed of any two point values in 100℃, 105℃, 110℃, 115℃, 120℃; the drying time can be any one value or a range value composed of any two point values in 8h, 9h, 10h, 11h, 12h.

[0047] In some embodiments of the present application, in step S3, the added metal salt includes at least one of a cobalt salt, a manganese salt, and an aluminum salt; the cobalt salt includes at least one of cobalt nitrate, cobalt acetate, cobalt sulfate, and cobalt chloride; the manganese salt includes at least one of manganese nitrate, manganese sulfate, manganese chloride, and manganese acetate; the aluminum salt includes at least one of aluminum nitrate, aluminum isopropanolate, aluminum chloride, and aluminum sulfate.

[0048] In some embodiments of the present application, in step S3, the metal salt added includes Co salt and Mn salt or Co salt and Al salt, so that the positive electrode material surface is coated with a high-stability manganese-rich layer or a high-voltage-stability cobalt-rich layer. This shell layer is a physical barrier that can more effectively isolate the electrolyte from the core positive electrode material corrosion, and it is also an electrochemically active substance that can provide part of the capacity. In some embodiments, the metal salt includes Co salt and Mn salt, wherein the molar ratio of Co to Mn is 1:1-5:1, for example, it can be any one value or a range value composed of any two point values in 1:1, 2:1, 3:1, 4:1, 5:1; in other embodiments, the metal salt includes Co salt and Al salt, wherein the molar ratio of Co to Al is 1:1-4:1, for example, it can be any one value or a range value composed of any two point values in 1:1, 2:1, 3:1, 4:1.

[0049] In some embodiments of the present application, in step S3, the total concentration of metal ions in the metal salt solution is 0.1-1 mol / L, for example, it can be any one value or a range value composed of any two point values in 0.1 mol / L, 0.2 mol / L, 0.3 mol / L, 0.4 mol / L, 0.5 mol / L, 0.8 mol / L, 1 mol / L.

[0050] In some embodiments of the present application, in step S3, the precipitant includes at least one of ammonia, sodium hydroxide solution, potassium hydroxide solution.

[0051] In some embodiments of the present application, in step S3, the concentration of the precipitant is 0.5-4 mol / L, for example, it can be any one value or a range value composed of any two point values in 0.5 mol / L, 1 mol / L, 1.5 mol / L, 2 mol / L, 3 mol / L, 4 mol / L.

[0052] In some embodiments of the present application, in step S3, the amount of metal salt added is calculated according to the molar ratio of organic boron compound to metal ion of 1:5-1:50, for example, the molar ratio of organic boron compound to metal ion can be any one value or a range value composed of any two point values in 1:5, 1:10, 1:15, 1:20, 1:30, 1:40, 1:50.

[0053] In some embodiments of the present application, in step S3, the pH of the co-precipitation reaction is 8-12, for example, it can be any one of 8, 9, 10, 11, 12 or a range value composed of any two of them; the reaction temperature is 50-90℃, for example, it can be any one of 50℃, 60℃, 70℃, 80℃, 90℃ or a range value composed of any two of them; the reaction time is 2-6h, for example, it can be any one of 2h, 3h, 4h, 5h, 6h or a range value composed of any two of them.

[0054] In some embodiments of the present application, in step S3, after the reaction is completed, suction filtration is performed, and the filter is washed with deionized water until the filtrate is neutral to remove excess salt ions.

[0055] In some embodiments of the present application, in step S4, the sintering and solidification step includes: placing the dried material in a box furnace or a tube furnace, and under the protection of an inert atmosphere (such as argon or nitrogen), heating to 400-600℃ at a heating rate of 1-5℃ / min, and holding for 2-5h; typically but not limitedly, for example, the heating rate can be any one of 1℃ / min, 2℃ / min, 3℃ / min, 4℃ / min, 5℃ / min or a range value composed of any two of them; the holding temperature can be any one of 400℃, 450℃, 500℃, 550℃, 600℃ or a range value composed of any two of them; the holding time can be any one of 2h, 3h, 4h, 5h or a range value composed of any two of them.

[0056] In some embodiments of the present application, in step S4, after crushing, it is passed through a 200-400 mesh sieve.

[0057] The third aspect of the present application provides a positive electrode sheet comprising the modified positive electrode material of any one of the preceding embodiments or the modified positive electrode material prepared by the preparation method of any one of the preceding embodiments.

[0058] The fourth aspect of the present application provides a lithium ion battery comprising the positive electrode sheet of the preceding embodiments.

[0059] Embodiments of the present application will be described in detail below with reference to specific examples, but those skilled in the art will understand that the following examples are only for illustration of the present application and should not be regarded as limiting the scope of the present application. If the specific conditions are not specified in the examples, they are carried out under conventional conditions or according to the manufacturer's recommendations. If the reagents or instruments used are not specified by the manufacturer, they are all conventional products that can be obtained commercially.

[0060] Example 1 S1. Weigh 1.0 g of 4-vinylphenylboronic acid (4-VPBA) into 200 mL of a mixed solvent of anhydrous ethanol and deionized water (volume ratio 1:1), stir in a 45°C water bath until completely dissolved, to obtain a clear and transparent organic boron bridging agent solution; S2. Weigh 100 g of LiNi 0.68 Co 0.1 Mn 0.22 O2 positive electrode material with a D50 of 3.5 μm into the bridging agent solution prepared in step one, and stir the mixed system at a constant temperature of 45°C for 4 hours. After the reaction is completed, perform suction filtration and wash with anhydrous ethanol 3 times to remove physically adsorbed free organic matter. Dry the filter cake in a vacuum drying oven at 100°C for 12 hours to obtain an organic boron-modified positive electrode material; S3. Prepare a mixed salt solution of Co(NO3)2 and Mn(NO3)2 with a total metal ion concentration of 0.5 mol / L and a molar ratio of Co to Mn of 1:1, and a total volume of 200 mL. Prepare a 2.0 mol / L NaOH solution; Re-disperse the organic boron-modified positive electrode material obtained in step S2 in 400 mL of deionized water to form a suspension, and stir in a 60°C water bath. Use two constant-pressure dropping funnels to simultaneously and slowly add the mixed salt solution and the NaOH solution to the above suspension. Control the reaction pH value to be 10.5-11.0 and the temperature to be 60°C, and the dropping time is about 4 hours. After the dropping is completed, continue to mature for 2 hours to allow the reaction to proceed fully. After the reaction is completed, perform suction filtration and wash with deionized water until the filtrate is neutral to remove excess salt ions. Dry the obtained filter cake in a vacuum drying oven at 120°C for 24 hours; S4. Dry the material after drying under nitrogen gas protection, and heat to 400°C at a rate of 2.5°C / min, and keep the temperature for 5h to obtain a modified positive electrode material.

[0061] Example 2 S1. Weigh 5.0 g of phenylboronic acid (C6H7BO2) into 200 mL of a mixed solvent of anhydrous ethanol and deionized water (volume ratio 1:1), stir in a 45°C water bath until completely dissolved, to obtain a clear and transparent organic boron bridging agent solution; S2. Weigh 400 g of LiNi 0.72 Co 0.12 Mn 0.16O2 cathode material, add to the bridging agent solution prepared in step one, the mixed system is constant temperature stirring under 45℃ for 5 hours, after the reaction is finished, carries on the suction filtration, and washes 3 times with anhydrous ethanol to remove the free organic matter of physical adsorption, the filter cake is placed in the vacuum drying oven of 100℃ and dried for 12 hours, the organic boron modified cathode material is obtained; S3. Prepare a mixed salt solution of Co(NO3)2 and Al(NO3)2 with a total metal ion concentration of 1.0 mol / L and a molar ratio of Co to Mn of 5:1, with a total volume of 200 mL. Prepare a 4.0 mol / L NaOH solution. Disperse the organic boron-modified cathode material obtained in step S2 in 400 mL of deionized water to form a suspension, and stir in a 60℃ water bath. Use two constant-pressure dropping funnels to simultaneously and slowly add the mixed salt solution and the NaOH solution to the suspension. Control the pH value of the reaction to be 10.5-11.0, and maintain the temperature at 60℃. The dropping time is about 4 hours. After the dropping is completed, continue to mature for 2 hours to ensure that the reaction is fully carried out. After the reaction is completed, perform suction filtration and wash with deionized water until the filtrate is neutral to remove excess salt ions. Place the obtained filter cake in a vacuum drying oven at 120℃ and dry for 24 hours. S4. Dry the material under nitrogen gas protection, heat it to 450℃ at a rate of 4℃ / min, and keep it at this temperature for 5 hours to obtain the modified cathode material.

[0062] Example 3 S1. Weigh 5.0g of triphenylboron (BPh3) and dissolve it in 200mL of a mixture of anhydrous ethanol and deionized water (volume ratio 2:1). Stir in a 60℃ water bath until completely dissolved to obtain a clear and transparent organic boron bridging agent solution. S2. Weigh 450g of LiNi 0.8 Co 0.1 Mn 0.1 O2 cathode material, add to the bridging agent solution prepared in step one, the mixed system is constant temperature stirring under 45℃ for 5 hours, after the reaction is finished, carries on the suction filtration, and washes 3 times with anhydrous ethanol to remove the free organic matter of physical adsorption; the filter cake is placed in the vacuum drying oven of 80℃ and dried for 12 hours, the organic boron modified cathode material is obtained; S3. Prepare a mixed salt solution of Ni(NO3)2, Co(NO3)2 and Mn(NO3)2 with a total metal ion concentration of 1.0 mol / L and a molar ratio of Ni to Co to Mn of 5:2:3, with a total volume of 200 mL. Prepare a 4.0 mol / L NaOH solution. The organic boron modified cathode material obtained in step S2 is redispersed in 400 mL of deionized water to form a suspension, which is stirred in a 60°C water bath; a mixed salt solution and a NaOH solution are simultaneously and slowly added to the above suspension using two constant pressure dropping funnels; the pH value of the reaction is controlled at 11.0-12.0, the temperature is kept at 80°C, and the dropping time is about 4 hours; after the dropping is completed, the reaction is allowed to continue for 2 hours for maturation; after the reaction is completed, the filter cake is washed with deionized water until the filtrate is neutral to remove excess salt ions; and the obtained filter cake is dried in a vacuum drying oven at 120°C for 24 hours. S4. The dried material is heated to 350°C at a rate of 1.5°C / min under the protection of nitrogen gas, and kept at 350°C for 5 hours to obtain the modified cathode material.

[0063] Comparative Example 1 Comparative Example 1 is similar to Example 1, except that steps S1 and S2 are not performed, and the other conditions are the same as in Example 1; that is, the cathode material is not modified with 4-VPBA, and is directly inorganic coated.

[0064] Comparative Example 2 Comparative Example 2 is similar to Example 1, except that steps S3 and S4 are not performed, and the other conditions are the same as in Example 1; that is, only organic boron coating modification is performed, and no inorganic coating is performed.

[0065] Comparative Example 3 Comparative Example 3 is similar to Example 1, except that the order of steps S2 and S3 is exchanged, and the other conditions are the same as in Example 1; that is, co-precipitation coating is performed first, then organic boron modification is performed, and finally sintering and solidification are performed.

[0066] Test Example 1. The modified cathode materials prepared in Example 1 and Comparative Example 1 are respectively subjected to SEM morphology characterization, and the results are shown in Figure 1 and Figure 2 As shown in the figures, in Example 1, there is a uniform, dense and extremely thin coating layer (about 20-30 nm) on the surface of the cathode material, and the boundary with the matrix is clear; while in Comparative Example 1, the coating layer is unevenly distributed in the form of agglomerated islands.

[0067] 2. Surface residual alkali test Test Method: A T5 potentiometric titrator, multi-point magnetic stirrer, 0.01% electronic balance, and vacuum pump were used. The sample was weighed into a clean 150mL beaker, and the batch number, bottle number, and sample weight (5.0000g ± 0.0100g) were recorded. The sample bag should be sealed promptly to prevent contamination or moisture absorption. 100mL of distilled water was added to the weighed beaker, a magnetic stir bar was placed inside, and the beaker mouth was sealed with sealing film. The beaker was stirred on a magnetic stirrer at 350 rpm for 5 minutes. The vacuum filtration device and 0.45μm filter membrane were installed, ensuring they were clean and free of contamination. The vacuum pump was started, and the suspension was poured in for filtration. The filtrate should be clear and transparent; otherwise, the filtration process was repeated. All the filtrate was transferred to a titration beaker and awaited testing. The electrodes and titration head were rinsed with distilled water, and the titration beaker containing the test solution was installed. The pre-set test method was selected, and all parameters in the program were confirmed. The residual alkali was determined using the equivalence point automatic endpoint identification method. Record the test results and directly obtain the calculated result value according to the set measurement method.

[0068] The test results are shown in Table 1.

[0069] 3. Electrochemical performance testing Battery Assembly: Button Cell Manufacturing Acetylene black was used as a conductive agent and PVDF as a binder. The test sample, conductive agent, binder, and N-methyl-2-pyrrolidone were weighed according to the specified mass ratios and stirred evenly to prepare a paste-like positive electrode mixture. This positive electrode mixture was coated onto the surface of aluminum foil and dried at high temperature. Test electrodes of appropriate size were cut out and pressed to the required thickness using a calendering method, then dried for later use. CR2032 coin cells were selected for the test, with lithium metal sheets used as the negative electrode. A separator was sandwiched between the positive and negative electrodes, and the positive electrode, separator, and negative electrode were assembled using a stacking process. The assembled coin cells were left to stand for 12 hours to obtain the test coin cells.

[0070] Test: Cyclic test requirements: 45℃, constant current and constant voltage charging to 4.45V, cutoff current 0.05C, constant current discharging to 2.5V, 0.5CC / 1.0CD, 50 cycles; Rate test requirements: test temperature 25℃, first static 10h, constant current constant voltage charging to 4.45V, cut-off current 0.05C, constant current discharge to 2.5V, 0.1CC / 0.1CD; constant current constant voltage charging to 4.45V, cut-off current 0.05C, constant current discharge to 2.5V, 0.33CC / 0.33CD; constant current constant voltage charging to 4.45V, cut-off current 0.05C, constant current discharge to 2.5V, 0.5CC / 0.1CD; constant current constant voltage charging to 4.45V, cut-off current 0.05C, constant current discharge to 2.5V, 0.5CC / 0.5CD; constant current constant voltage charging to 4.45V, cut-off current 0.05C, constant current discharge to 2.5V, 0.5CC / 1.0CD; constant current constant voltage charging to 4.45V, cut-off current 0.05C, constant current discharge to 2.5V, 0.5CC / 2.0CD; DSC test requirements: 2.8-4.5V, 400℃ cut-off.

[0071] "C" represents "Capacity", "1C" is a reference current value, which indicates the current size required by a battery to completely discharge its rated capacity from 100% to 0% in 1 hour; 2C indicates twice the value of 1C current, which is a standard unit for indicating the size of charge and discharge current. It means that when using this current for constant current discharge, the battery will theoretically discharge the battery in 0.5 hours (i.e. 30 minutes). Rate performance (capacity retention rate) = (discharge capacity @ high rate / discharge capacity @ low rate) x 100%, the higher the percentage, the better the rate performance of the battery.

[0072] The test results are shown in Table 1.

[0073] Table 1

[0074] From the data in Table 1, it can be seen that using organic boron compounds as "molecular bridges" and using their specific functional groups to react with residual lithium or hydroxyl groups on the surface of the matrix, and at the same time coordinating or combining with the coating agent precursor, the cycle performance and thermal safety performance of the modified positive electrode material are better, which is specifically reflected in the 50 th The cycle retention rate increases by about 4%, the DSC peak temperature increases by about 10℃, the 2C rate increases by 1%~2%, and the rate performance is obviously improved.

[0075] Although the present application has been described and illustrated with a certain degree of particularity, it is understood that the present application has been made by way of examples only and that numerous changes in the details of execution can be made by those skilled in the art without departing from the spirit and scope of the application. It is therefore intended to cover in the appended claims all such changes and modifications that come within the scope of the application.

Claims

1. A modified cathode material, characterized in that, The modified positive electrode material comprises a positive electrode material core and a boron-containing coating layer and a metal oxide coating layer successively coated on the surface of the positive electrode material core from inside to outside; the boron-containing coating layer and the positive electrode material core are combined through B-O-Li bonds, and the boron-containing coating layer and the metal oxide coating layer are combined through B-O-M bonds, wherein M is a metal element in the metal oxide coating layer.

2. The modified cathode material of claim 1, wherein, The chemical general formula of the positive material core includes LiNi x Co y Mn z A a O2, wherein x+y+z+a=1, x≥0.5, 0.05≤y≤0.25, 0≤a≤0.015, A includes at least one of Zr, Ti, W, Nb, Sr, Al, Mg, Ca, Ba, La, Ce, Nd, Mo, F, P, B or Y.

3. The modified cathode material of claim 1, wherein, The metal oxide coating layer comprises at least one of cobalt-manganese oxide, nickel-cobalt-manganese oxide, cobalt-aluminum oxide, and nickel-cobalt-aluminum oxide.

4. The method for producing the modified cathode material according to any one of claims 1 to 3, characterized in that, The method comprises the following steps: S1. Dissolve an organic boron compound in a solvent to obtain a bridging agent solution; S2. Take a powder positive electrode material as the core, add it to the bridging agent solution, and react at a constant temperature to make the organic boron compound react with residual alkali on the surface of the positive electrode material to form B-O-Li bonds; after the reaction, remove the solvent and free organic matter to obtain an organic boron-modified positive electrode material; S3. Disperse the organic boron-modified positive electrode material in water, synchronously add a metal salt solution and a precipitant, form a metal hydroxide coating layer on the surface of the organic boron-modified positive electrode material through a coprecipitation reaction, make the metal hydroxide react with the organic boron compound to form B-O-M bonds, and perform washing and drying; S4. Sinter and solidify the dried material and crush it to obtain the modified positive electrode material.

5. The method for preparing the modified cathode material according to claim 4, characterized in that, The organic boron compound comprises at least one of phenylboronic acid, 4-vinylphenylboronic acid, polyborate, tributyl borate, trimethyl borate, polyborosiloxane, and triphenyl boron.

6. The method of claim 4, wherein the modified cathode material is prepared by the steps of: preparing a precursor material by mixing a lithium source, a transition metal source, and a carbon source; and calcining the precursor material in a non-oxidizing atmosphere. In step S1, the solvent comprises at least one of an alcohol-water mixture, tetrahydrofuran, dichloromethane, toluene, and n-hexane; Preferably, the alcohol in the alcohol-water mixture comprises ethanol and / or isopropanol.

7. The method for preparing the modified cathode material according to claim 4, characterized in that, In step S2, at least one of the following characteristics is met: (1) The mass ratio of the powder positive electrode material to the organic boron compound is 80-120:1; (2) The temperature of the constant-temperature reaction is 40-50°C, and the reaction time is 2-6h; (3) After the reaction, free organic matter physically adsorbed is removed through alcohol washing.

8. The method of claim 4, wherein the modified cathode material is prepared by the steps of: preparing a precursor material by mixing a lithium source, a transition metal source, and a carbon source; and calcining the precursor material in a non-oxidizing atmosphere. In step S3, at least one of the following characteristics is met: (1) The metal salt comprises at least one of a cobalt salt, a manganese salt, and an aluminum salt; the cobalt salt comprises at least one of cobalt nitrate, cobalt acetate, cobalt sulfate, and cobalt chloride; the manganese salt comprises at least one of manganese nitrate, manganese sulfate, manganese chloride, and manganese acetate; and the aluminum salt comprises at least one of aluminum nitrate, aluminum isopropanolate, aluminum chloride, and aluminum sulfate; (2) The total concentration of metal ions in the metal salt solution is 0.1-1mol / L; (3) The precipitant comprises at least one of ammonia water, a sodium hydroxide solution, and a potassium hydroxide solution; (4) The concentration of the precipitant is 0.5-4mol / L; (5) The amount of the metal salt is added according to a molar ratio of the organic boron compound to metal ions of 1:5-1:50; (6) The pH of the coprecipitation reaction is 8-12, the reaction temperature is 50-90°C, and the reaction time is 2-6h.

9. A positive electrode sheet characterized by comprising: The modified positive electrode material according to any one of claims 1 to 3 or the modified positive electrode material prepared by the preparation method according to any one of claims 4 to 8.

10. A lithium-ion battery, characterized by, The positive electrode sheet according to claim 9.

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