Patent Description:
As a new type of energy storage and conversion device, lithium-ion batteries are widely used in portable consumer electronics devices, new energy vehicles, energy storage grids and other fields. Lithium-ion batteries have many advantages, such as high working voltage, high energy density, high coulombic efficiency, no memory effect, long cycle life and environmental friendliness. The positive electrode materials for lithium-ion batteries determine the comprehensive performance of the batteries. Among the current mainstream positive electrode materials, the ternary positive electrode materials LiNixCoyM<NUM>-x-yO<NUM> ( M = Mn or Al ) have become a research focus, due to their high energy/power density and low-temperature performance.

With the increase of Ni content in the ternary positive electrode material, the capacity of the material increases, but its structural stability decreases. The reason is that the Ni-O bond energy is weak. In the charging and discharging process, oxidation-reduction reactions occur between the highly active interface and the electrolyte, and thus form an inactive rock-salt phase structure. In the process, the electrolyte is decomposed and a large amount of heat is released, resulting in a series of problems, such as cell inflation, safety performance degradation, discharge capacity decay, and cycle stability deterioration. At present, coating is one of the main ways to solve the problem. The coating agent can effectively avoid the direct contact between the highly active positive electrode interface and the electrolyte, and thus alleviate the occurrence of side reactions. The coating agent can also act as a fast ion conductor to provide a good channel for lithium ion diffusion and transmission. Therefore, the rate performance is improved. At present, the coating agent mainly includes oxides, such as Al<NUM>O<NUM>, TiO<NUM>, ZrO<NUM>, B<NUM>O<NUM>, SiO<NUM>, etc. The oxides as coating agent can improve the above interface stability problems. However, most of the oxides belong to semiconductors, which have low electronic conductivity and cannot meet the requirements of high current charging and discharging. Compared with oxides, nitrides have better chemical corrosion resistance, more excellent electronic conductivity and thermal stability. With nitrides as coating agent, the electrical properties of ternary materials can be improved to a greater extent.

Currently, among the coating technologies of nitrides, the Chinese patent <CIT> disclosed a gas phase deposition coating method. However, the gas phase reaction process is complex and not easy to produce on a large scale. The Chinese patent <CIT> disclosed that the ternary positive electrode material coated with titanium nitride was prepared by mixing and sintering the ternary positive electrode material, titanium source and nitrogenous compounds in one step. The method is simple and convenient for industrial production. However, the reducing gas of the nitrogenous compounds may directly react with the ternary material in the preparation process, thereby destroying the lattice structure of the ternary positive electrode main material and affecting the electrical performance of the material. <CIT> discloses a nitride and carbon composite coated lithium iron phosphate positive electrode material, wherein the carbon coating is formed in-situ during the nitride coating with TiN.

Technical problems to be solved by the disclosure are that the process of the gas phase deposition coating method is complex and the one-step mixed sintering method is easy to destroy the lattice structure of the ternary positive electrode main material, which will affect the performance of the material.

The scope of protection of the present invention is defined in the appended set of claims.

In the first aspect, the present invention provides a nitride/graphitized carbon nanosheet-coated ternary positive electrode material, which includes a ternary positive electrode material matrix and a coating layer; the coating layer is composed of nitride and graphitized carbon, and the graphitized carbon is formed in situ in the coating process of the nitride proposed by the invention.

The present invention provides a preparation method of a nitride/graphitized carbon nanosheet-coated ternary positive electrode material, including the following steps:.

The advantages of the technical scheme proposed in the disclosure are:
in the present invention, the graphitized carbon layer structure is generated in situ in the coating process of the nitride on the surface of the ternary positive electrode material. Compared with a physical mixing method, the in-situ generated carbon layer is connected to the material matrix more tightly, and the formed conductive network is denser. So that the rate performance of the material is improved to the maximum extent. The preparation method of a nitride/graphitized carbon nanosheet-coated ternary positive electrode material is simple and easy to realize industrial production. And the obtained nitride/graphitized carbon nanosheet-coated ternary positive electrode material has excellent rate performance and cycle stability.

In the first aspect, the present invention provides a nitride/graphitized carbon nanosheet-coated ternary positive electrode material, which includes a ternary positive electrode material matrix and a coating layer. The coating layer is composed of nitride and graphitized carbon, and the graphitized carbon is formed in situ in the coating process of the nitride. Compared with amorphous carbon, the graphitized carbon material has better conductivity and is more conducive to improving the rate performance.

In the present invention, the ternary positive electrode material matrix is one or more of nickel-cobalt-manganese ternary positive electrode materials or nickel-cobalt-aluminum ternary positive electrode materials, of which the structural formula is LiNixCoyM<NUM>-x-yO<NUM> ( M = Mn or Al ).

In the present invention, the nitride is one or more of aluminum nitride, silicon nitride, titanium nitride, zirconium nitride, tantalum nitride, and niobium nitride.

In the present invention, the thickness of the coating layer is <NUM> ~ <NUM>.

In the present invention, the transition metal nitride is produced by the reaction of the nitrogen element in the carbonaceous and nitrogenous compounds with the transition metal oxide coated on the surface of the intermediate product in a high-temperature environment. At the same time, carbon element forms graphitized carbon nanosheets in situ under the catalytic action of graphitization of transition metals. Thereby the nitride/graphitized carbon nanosheet-coated ternary positive electrode material was formed.

In the present invention, the specific steps of providing ternary positive electrode material matrix are as follows:
the precursor of ternary positive electrode material and the lithium source are mixed evenly, and then the ternary positive electrode material matrix is obtained after sintering, pulverizing, and sieving.

Further, the precursor of ternary positive electrode material is one or more of oxides, hydroxides, and oxyhydroxides of nickel-cobalt-manganese or nickel-cobalt-aluminum. The lithium source is one or more of lithium carbonate, lithium oxide, and lithium hydroxide. The molar ratio of the precursor of ternary positive electrode material to the lithium source is <NUM> : ( <NUM> ~ <NUM> ). The precursor of ternary positive electrode material and the lithium source are mixed evenly by a high mixer, and the speed of the high mixer is <NUM> ~ <NUM> rpm, further <NUM> rpm, the mixing time is <NUM> ~ <NUM>, further <NUM>. The sintering is carried out under the oxygen atmosphere, and the volume concentration of the oxygen atmosphere is≥ <NUM> %; the sintering temperature is <NUM> ~ <NUM>, further <NUM> ~ <NUM>, even further <NUM>; the sintering time is <NUM> ~ <NUM>, further <NUM> ~ <NUM>, even further <NUM>.

Further, dopant is also added to improve the structural stability of the ternary positive electrode material in the preparation process of ternary positive electrode material matrix. For example, the dopant can be zirconium dioxide, aluminum trioxide, titanium dioxide, magnesium oxide, etc., which is not limited in the present invention. Furthermore, the molar ratio of the dopant to the precursor of ternary positive electrode material is ( <NUM> ~ <NUM> ) : <NUM>.

In the present invention, the mass ratio of the coating element to the ternary positive electrode material matrix is ( <NUM> ~ <NUM> ) : <NUM>, further ( <NUM> ~ <NUM> ) : <NUM>, even further ( <NUM> ~ <NUM> ) : <NUM>.

In the present invention, the specific steps of wet coating are as follows:.

Further, the compounds containing coating element are the alkoxides containing coating element. In an exemplary embodiment, the compounds containing coating element is one or more of aluminum triethoxide, tetratetraethyl orthosilicate, tetrabutyl titanate, titanium isopropoxide, zirconium isopropoxide, tantalum pentaethoxide, and niobium pentaethoxide; the first solvent and the second solvent are one or more of water, methanol, ethanol, isopropanol, and ethylene glycol, respectively.

Further, in solution I, the dosage ratio of the ternary positive electrode material matrix to the first solvent is ( <NUM> ~ <NUM> ) g : <NUM>, further ( <NUM> ~ <NUM> ) g : <NUM>; in solution II, the dosage ratio of the compounds containing coating element to the second solvent is ( <NUM> ~ <NUM> ) g : <NUM>, further ( <NUM> ~ <NUM> ) g : <NUM>, even further ( <NUM> ~ <NUM> ) g : <NUM>, furthermore <NUM> : <NUM>.

Further, the heat preservation reaction temperature is <NUM> ~ <NUM>, further <NUM>, and the heat preservation reaction time is <NUM> ~ <NUM>, further <NUM> ~ <NUM>; the drying temperature is <NUM> ~ <NUM>, and the drying time is <NUM> ~ <NUM>.

In the present invention, the carbonaceous and nitrogenous compound is one or more of amino acids, melamine, and urea. Further, the mass ratio of the carbonaceous and nitrogenous compound to the intermediate product is ( <NUM> ~ <NUM> ) : <NUM>. If the mass ratio is too high, it will lead to the surface coating (nitride and graphitized carbon) excessive. This will affect the capacity of the material. If the mass ratio is too low, it will lead to the surface coating less. This cannot reach the effect of the coating. The further ratio is ( <NUM> ~ <NUM> ) : <NUM>, and even further <NUM> : <NUM>. The carbonaceous and nitrogenous compound and the intermediate product are mixed evenly by a high mixer. The speed of the high mixer is <NUM> ~ <NUM> rpm, further <NUM> rpm. The mixing time is <NUM> ~<NUM>, further <NUM>. Sintering is carried out under the protection of inert gas, such as argon or nitrogen, etc. The sintering temperature is <NUM> ~ <NUM>, further <NUM>. The sintering time is <NUM> ~ <NUM>, further <NUM>.

Button battery test: The obtained nitride/graphitized carbon nanosheet-coated ternary positive electrode material (LiNi<NUM>Co<NUM>Mn<NUM>) was mixed with acetylene black and PVDF in a ratio of <NUM> : <NUM> : <NUM>. Then, NMP as a solvent was mixed with the mixture evenly and coated on aluminum foils to make <NUM> button batteries for electrochemical performance test. The test voltage was <NUM> ~ <NUM> V, the charge-discharge current was <NUM> C / <NUM> C in the first week and <NUM> C / <NUM> C in the next <NUM>-week for cycle test. Final test results: the <NUM> C discharge capacity was <NUM> mAh/g, the <NUM> C discharge capacity was <NUM> mAh/g, the <NUM>-week cycle capacity retention rate was <NUM> %.

Button battery test: The obtained nitride/graphitized carbon nanosheet-coated ternary positive electrode material (LiNi<NUM>Co<NUM>Mn<NUM>) was mixed with acetylene black and PVDF in a ratio of <NUM>:<NUM>:<NUM>. Then, NMP as a solvent was mixed with the mixture evenly and coated on aluminum foils to make <NUM> button batteries for electrochemical performance test. The test voltage was <NUM> ~ <NUM> V, the charge-discharge current was <NUM> C / <NUM> C in the first week and <NUM> C / <NUM> C in the next <NUM>-week for cycle test. Final test results: the <NUM> C discharge capacity was <NUM> mAh/g, the <NUM> C discharge capacity was <NUM> mAh/g, the <NUM>-week cycle capacity retention rate was <NUM> %.

Button battery test: The obtained ternary positive electrode material coated with nitride (LiNi<NUM>Co<NUM>Mn<NUM>) was mixed with acetylene black and PVDF in a ratio of <NUM>:<NUM>:<NUM>. Then, NMP as a solvent was mixed with the mixture evenly and coated on aluminum foils to make <NUM> button batteries for electrochemical performance test. The test voltage was <NUM> ~ <NUM> V, the charge-discharge current was <NUM> C / <NUM> C in the first week and <NUM> C / <NUM> C in the next <NUM>-week for cycle test. Final test results: the <NUM> C discharge capacity was <NUM> mAh/g, the <NUM> C discharge capacity was <NUM> mAh/g, the <NUM>-week cycle capacity retention rate was <NUM>%.

Claim 1:
A preparation method of a nitride/graphitized carbon nanosheet-coated ternary positive electrode material, wherein graphitized carbon is formed in situ in the coating process of the nitride including the following steps:
providing ternary positive electrode material matrix;
the coating element is coated on the surface of the ternary positive electrode material matrix by wet coating to obtain an intermediate product; wherein the coating element is one or more of Al, Si, Ti, Zr, Ta, Nb;
the intermediate product is mixed with a carbonaceous and nitrogenous compound evenly, and the nitride/graphitized carbon nanosheet-coated ternary positive electrode material is obtained after sintering, pulvering, sieving and iron removal; wherein the sintering is carried out under the protection of inert gas.