CN110137483B - Nickel cobalt lithium manganate composite material and preparation method and application thereof - Google Patents
Nickel cobalt lithium manganate composite material and preparation method and application thereof Download PDFInfo
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Abstract
The invention discloses a nickel cobalt lithium manganate composite material and a preparation method and application thereof. The nickel cobalt lithium manganate composite material prepared by the method can inhibit the precipitation of lithium and nickel cobalt manganese heavy metals, has good conductivity, specific capacity, cycle performance, rate capability and thermal stability, further improves the thermal stability of the material and meets the use requirement of the material on high specific capacity, meets the development trend of high-rate and high-specific-capacity lithium ion batteries, has simple process, controllable structure and easy popularization, and is suitable for the application and popularization of the nickel cobalt lithium manganate material in the lithium ion batteries.
Description
Technical Field
The invention belongs to the technical field of lithium ion battery materials, and particularly relates to a nickel cobalt lithium manganate composite material and a preparation method and application thereof.
Background
A lithium ion battery is a secondary battery (rechargeable battery) that mainly operates by movement of lithium ions between a positive electrode and a negative electrode. During charging and discharging, Li+Intercalation and deintercalation to and from two electrodes: upon charging, Li+The lithium ion battery is extracted from the positive electrode and is inserted into the negative electrode through the electrolyte, and the negative electrode is in a lithium-rich state; the opposite is true during discharge. As a clean and efficient energy source with the most development potential in the 21 st century, a lithium ion battery has been widely applied to portable mobile devices such as cameras, mobile phones, and notebook computers, which are commonly used in daily life, and is gradually applied to electric vehicles and has received much attention from people. The positive electrode material of lithium ion battery is in the lithium ion batteryThe most critical part is that many problems of the positive electrode material limit some applications of the lithium ion battery, and many problems still exist after years of development, and the problems seriously limit the further popularization and application of the lithium ion battery. In order to make the application range of the lithium ion battery wider and to make the lithium ion battery more easily develop to a large scale, it is very important to improve the performance of the positive electrode material of the lithium ion battery.
Common positive electrode materials of lithium ion batteries include lithium cobaltate, lithium iron phosphate, lithium manganate, lithium nickel manganese cobalt, and the like. The market share of lithium cobaltate is the largest, but the price of the material of lithium cobaltate is high due to the serious shortage of cobalt resources, and the application cost is high; the lithium iron phosphate material has low tap density, poor processing performance and low specific capacity, and limits the further application of the material; the structural stability, especially the high-temperature stability of the lithium manganate needs to be further improved; the nickel-manganese-cobalt lithium replaces a large amount of cobalt in the cobalt lithium by nickel and manganese, so that the material cost advantage is greatly improved, the specific capacity of the material is greatly improved by increasing the content of nickel, and meanwhile, the material has a stable structure and good safety performance, is a material which can most possibly replace the cobalt lithium, and has a great market application prospect.
Although the nickel cobalt lithium manganate has numerous advantages, the nickel cobalt lithium manganate has low specific discharge capacity and is easy to generate a lithium precipitation phenomenon, and at present, the technical problem is solved by improving the nickel content. However, the lithium precipitation phenomenon caused by the mixed exclusion of nickel and lithium in the material is serious just because of the substitution of high nickel, and particularly under high humidity, the lithium precipitation phenomenon is serious, and the overall conductivity of the material is also reduced. At present, there are some documents about improving the performance of the lithium nickel cobalt manganese oxide composite material, such as:
1. patent application CN201310739997.8 discloses a preparation method of high specific capacity graphene coated nickel cobalt lithium manganate material, which mainly solves the technical problems that in the prior art, the discharge specific capacity of nickel cobalt lithium manganate is low, the phenomenon of mixed discharge of nickel and lithium in the high nickel cobalt lithium manganate, nickel and lithium in the material is easy to occur, so that the lithium is separated out, and when the air humidity is high, the lithium separation phenomenon is serious, and the like. Dissolving a nickel cobalt lithium manganate material and a graphene sheet in a DMF (dimethyl formamide) solution at 180 ℃ to prepare a graphene DMF solution with the concentration of 100 plus 1000ppm, dropwise adding the DMF solution into the prepared nickel cobalt lithium manganate material under the stirring condition, and drying the obtained material in a vacuum drying oven at the temperature of 110 plus 140 ℃ to obtain a finished product. According to the method, a liquid phase doping mode is adopted, the contact area is increased compared with that of powder doping, however, the graphene is connected with the nickel cobalt lithium manganate material through Van der Waals force, the contact acting force of the graphene and the nickel cobalt lithium manganate material is small, and the effect of improving the ion mixed-arrangement phenomenon of the nickel cobalt lithium manganate material through graphene coating is limited.
2. Patent application CN201711154582.9 discloses a preparation method of a graphene-coated nickel cobalt lithium manganate composite material. The method comprises the following steps: combined aqueous phase "intercalation-expansion-oxidation" process and gamma-The ray irradiation reduction technology is used for preparing the graphene which is easy to separate, good in water dispersibility and high in conductivity in an environment-friendly and efficient manner. A high-temperature calcination method is adopted, spherical manganese dioxide is used as a template, a spherical ternary material which is of a hollow structure and is formed by nano nickel cobalt lithium manganate primary particles is ingeniously prepared, and surface modification treatment is carried out on the spherical ternary material. And further controlling appropriate conditions to enable the graphene to be uniformly and firmly coated on the surface of the nickel cobalt lithium manganate, so as to obtain the target composite material. The raw materials related by the invention have low price, the preparation process has strong controllability, and the used water or ethanol solvent is environment-friendly. The composite material has excellent electrochemical performance and specific capacity up to 265mAhg-1After 500 cycles, the capacity retention rate is 86%, so that the lithium ion battery can be widely applied to electrochemical energy storage devices such as lithium ion batteries. However, the method has relatively complex operation process and higher requirement on equipment.
However, the graphene-coated lithium nickel cobalt manganese oxide composite material prepared by the above documents or the preparation method is relatively complex in process and high in equipment requirement, and the stability or electrical property of the prepared material needs to be further improved. Therefore, a simple preparation method is urgently needed to further improve the material performance.
Disclosure of Invention
The invention provides a nickel cobalt lithium manganate composite material and a preparation method and application thereof for solving the technical problems. According to the method, in-situ deposition of graphene is realized on the surfaces of nickel cobalt lithium manganate particles, and the graphene coating layer and the nickel cobalt lithium manganate material are connected through a chemical bond, so that the protection of the nickel cobalt lithium manganate material is better realized while the process is simplified, the conductivity of the nickel cobalt lithium manganate composite material can be improved, lithium precipitation in the material and the dissolution of nickel cobalt manganese heavy metal ions in an electrolyte can be inhibited, the cycling stability of the material is further improved, and the use requirement of the material with high specific capacity is met; the prepared nickel cobalt lithium manganate composite material has good conductivity, the discharge platform is obviously increased compared with the nickel cobalt lithium manganate material which is not coated, the rate capability and the cycling stability are obviously improved, and the nickel cobalt lithium manganate composite material conforms to the development trend of high-power and high-specific energy lithium ion batteries.
In order to achieve the above purpose, the invention adopts the following technical scheme:
a graphene coating layer grows on the surface of a nickel cobalt lithium manganate composite material in situ, and the preparation method of the nickel cobalt lithium manganate composite material comprises the following steps: ultrasonically dispersing a nickel cobalt lithium manganate material in ethanol, adding reducing acid to carry out reduction reaction, carrying out in-situ nickel ion reduction on nickel ions on the surface of the nickel cobalt lithium manganate material, and then carrying out pre-calcination, calcination and sieving treatment under the protection of inert gas atmosphere to obtain the nickel cobalt lithium manganate composite material with a graphene coating layer grown in situ on the surface.
Further, the preparation method of the nickel cobalt lithium manganate composite material comprises the following steps:
step 1, preparation of pretreatment materials: dispersing a nickel cobalt lithium manganate material into an ethanol solution through ultrasonic treatment, then slowly dropwise adding a reducing acid under a stirring state to perform a reduction reaction, and then performing suction filtration, absolute ethanol washing and drying to obtain a nickel cobalt lithium manganate pretreatment material with reducing nickel on the surface;
and step 4, sieving: and after the calcination is finished, naturally cooling to room temperature under the protection of inert gas, taking out and sieving to obtain the nickel cobalt lithium manganate composite material with the surface in-situ grown with one graphene coating layer.
Further, in the step 1, the mass ratio of the nickel cobalt lithium manganate material to the ethanol solution is 1: 10-50.
Further, in the step 1, the mass ratio of the nickel cobalt lithium manganate material to the reducing acid is 1: 0.5-100.
Further, in step 1, the reducing acid is H2One or more of S, tartaric acid and citric acid, and the concentration is 1-5 wt.%.
Further, in the step 1, the stirring speed is 50-800 r/min, and the reduction reaction time is 0.5-2 h.
Further, in the step 2, the pre-calcining temperature is 200-400 ℃, and the time is 0.5-2 h.
Further, in the step 2, the inert gas is nitrogen or argon, and the inert gas introduction rate is 10-80 mL/min.
Further, in the step 3, the calcining temperature is 450-1500 ℃, and the time is 3-6 h.
Further, in step 3, the unsaturated hydrocarbon gas is one or more of acetylene, ethylene and propylene, and the introduction rate of the unsaturated hydrocarbon gas is 20-120 mL/min.
Further, the nickel cobalt lithium manganate composite material prepared by the method is provided.
Further, the nickel cobalt lithium manganate composite material contains a nickel cobalt lithium manganate base and a graphene coating layer grown on the surface of the nickel cobalt lithium manganate base in situ, and the thickness of the graphene coating layer is 10 nm-50 nm.
Further, the nickel cobalt lithium manganate composite material is applied as a lithium battery anode material.
The reaction principle of the invention is as follows: according to the method, nickel ions are easy to generate disproportionation reaction in an acidic solution to generate elemental nickel, and the generated elemental nickel can be used as a catalyst and can catalyze unsaturated hydrocarbon gas to be decomposed and reduced into graphene under a high-temperature condition. And (3) carrying out in-situ nickel ion reduction on nickel ions on the surface of the nickel cobalt lithium manganate material by adopting reducing acid to obtain the nickel cobalt lithium manganate material with the surface attached with a reducing nickel catalyst. Removing residual moisture of the pretreated nickel cobalt lithium manganate material through low-temperature calcination treatment, purifying an unsaturated hydrocarbon gas reduction environment, and simultaneously finely adjusting the crystal lattice of the pretreated nickel cobalt lithium manganate material; by further increasing the calcination temperature, the reductive nickel catalyst can catalyze unsaturated hydrocarbon gas to perform gas-phase reduction decomposition reaction on the surface of the nickel cobalt lithium manganate particles, so that a graphene protective layer is deposited in situ on the surface of the lithium cobalt manganese oxide material; the reductive nickel catalyst promotes the generation of a graphene protective layer, can improve the conductivity of the nickel cobalt lithium manganate composite material, improve the high rate performance of the composite material, simultaneously can inhibit the precipitation of lithium and nickel cobalt manganese heavy metals, further improves the thermal stability of the material and meets the use requirement of the material with high specific capacity.
Due to the adoption of the technical scheme, the invention has the following beneficial effects:
(1) according to the method, in-situ deposition of graphene is realized on the surfaces of the nickel cobalt lithium manganate particles, and the graphene coating layer and the nickel cobalt lithium manganate material are connected through a chemical bond, so that the protection of the nickel cobalt lithium manganate material is better realized while the process is simplified, the conductivity of the nickel cobalt lithium manganate composite material can be improved, lithium precipitation in the material and the dissolution of nickel cobalt manganese heavy metal ions in an electrolyte can be inhibited, the cycling stability of the material is further improved, and the use requirement of the material on high specific capacity is met.
(2) The nickel cobalt lithium manganate composite material prepared by the method has good conductivity, the discharge platform is obviously increased compared with the nickel cobalt lithium manganate material which is not coated, the rate capability and the cycle stability are obviously improved, and the nickel cobalt lithium manganate composite material conforms to the development trend of high-power and high-specific-energy lithium ion batteries.
(3) The method has the advantages of simple process and controllable structure, can obviously improve the cycle performance and rate capability of the lithium nickel cobalt manganese oxide material, and is suitable for application and popularization of the lithium nickel cobalt manganese oxide material in lithium ion batteries.
Drawings
In order to illustrate the embodiments of the present invention or the technical solutions in the prior art more clearly, the drawings needed in the description of the embodiments or the prior art will be briefly introduced below, it is obvious that the drawings in the following description are only some examples of the present invention, and for a person skilled in the art, without inventive step, other drawings can be obtained according to these drawings:
FIG. 1 is a flow chart of a preparation process of a nickel cobalt lithium manganate composite material of the invention;
FIG. 2 is a graph showing discharge curves at 0.2C and 5C rates of cells composed of the materials obtained in examples 1 and 2, comparative examples 1 and 2 according to the present invention.
In the drawings: 1-battery number one; 2-battery No. two; 3-battery No. three; 4-battery number four.
Detailed Description
The following is a detailed description of the embodiments of the present invention, but the present invention is not limited to these embodiments, and any modifications or substitutions in the basic spirit of the embodiments are included in the scope of the present invention as claimed in the claims.
Example 1
A graphene coating layer grows on the surface of the nickel cobalt lithium manganate composite material in situ.
A preparation method of a nickel cobalt lithium manganate composite material comprises the following steps:
step 1, preparation of pretreatment materials: dispersing a nickel cobalt lithium manganate material into an ethanol solution through ultrasonic treatment, then slowly dropwise adding a reducing acid under a stirring state to perform a reduction reaction, and then performing suction filtration, absolute ethanol washing and drying to obtain a nickel cobalt lithium manganate pretreatment material with reducing nickel on the surface;
the mass ratio of the nickel cobalt lithium manganate material to the ethanol solution is 1: 10; the mass ratio of the nickel cobalt lithium manganate material to the reducing acid is 1: 0.5; the reducing acid is citric acid and tartaric acid, and the concentration is 1 wt.%; the stirring speed is 50r/min, and the reduction reaction time is 0.5 h;
the calcining temperature is 450 ℃, and the calcining time is 3 hours; the unsaturated hydrocarbon gas is acetylene gas, and the gas introduction rate is 20 mL/min;
and step 4, sieving: and after the calcination is finished, naturally cooling to room temperature under the protection of inert gas, taking out and sieving to obtain the nickel cobalt lithium manganate composite material with the surface in-situ grown with one graphene coating layer.
The nickel cobalt lithium manganate composite material prepared by the preparation method comprises a nickel cobalt lithium manganate base and a graphene coating layer grown on the surface of the nickel cobalt lithium manganate base in situ, wherein the thickness of the graphene coating layer is 10 nm; the nickel cobalt lithium manganate composite material can be used as a positive electrode material to be applied to a lithium battery.
Example 2
A graphene coating layer grows on the surface of the nickel cobalt lithium manganate composite material in situ.
A preparation method of a nickel cobalt lithium manganate composite material comprises the following steps:
step 1, preparation of pretreatment materials: dispersing a nickel cobalt lithium manganate material into an ethanol solution through ultrasonic treatment, then slowly dropwise adding a reducing acid under a stirring state to perform a reduction reaction, and then performing suction filtration, absolute ethanol washing and drying to obtain a nickel cobalt lithium manganate pretreatment material with reducing nickel on the surface;
the mass ratio of the nickel cobalt lithium manganate material to the ethanol solution is 1: 50; the mass ratio of the nickel cobalt lithium manganate material to the reducing acid is 1: 100; the reducing acid is tartaric acid and the concentration is 5 wt.%; the stirring speed is 800r/min, and the reduction reaction time is 2 h;
the calcining temperature is 1500 ℃, and the time is 6 h; the unsaturated hydrocarbon gas is ethylene gas, and the gas introduction rate is 120 mL/min;
and step 4, sieving: and after the calcination is finished, naturally cooling to room temperature under the protection of inert gas, taking out and sieving to obtain the nickel cobalt lithium manganate composite material with the surface in-situ grown with one graphene coating layer.
The nickel cobalt lithium manganate composite material prepared by the preparation method comprises a nickel cobalt lithium manganate base and a graphene coating layer grown on the surface of the nickel cobalt lithium manganate base in situ, wherein the thickness of the graphene coating layer is 50 nm; the nickel cobalt lithium manganate composite material can be used as a positive electrode material to be applied to a lithium battery.
Example 3
A graphene coating layer grows on the surface of the nickel cobalt lithium manganate composite material in situ.
A preparation method of a nickel cobalt lithium manganate composite material comprises the following steps:
step 1, preparation of pretreatment materials: dispersing a nickel cobalt lithium manganate material into an ethanol solution through ultrasonic treatment, then slowly dropwise adding a reducing acid under a stirring state to perform a reduction reaction, and then performing suction filtration, absolute ethanol washing and drying to obtain a nickel cobalt lithium manganate pretreatment material with reducing nickel on the surface;
the mass ratio of the nickel cobalt lithium manganate material to the ethanol solution is 1: 20; the mass ratio of the nickel cobalt lithium manganate material to the reducing acid is 1: 10; the reducing acid is H2S, and concentration 2 wt.%; the stirring speed is 100r/min, and the reduction reaction time is 1 h;
the calcining temperature is 600 ℃, and the time is 4 h; the unsaturated hydrocarbon gas is propylene gas, and the feeding rate is 50 mL/min;
and step 4, sieving: and after the calcination is finished, naturally cooling to room temperature under the protection of inert gas, taking out and sieving to obtain the nickel cobalt lithium manganate composite material with the surface in-situ grown with one graphene coating layer.
The nickel cobalt lithium manganate composite material prepared by the preparation method comprises a nickel cobalt lithium manganate base and a graphene coating layer grown on the surface of the nickel cobalt lithium manganate base in situ, wherein the thickness of the graphene coating layer is 20 nm; the nickel cobalt lithium manganate composite material can be used as a positive electrode material to be applied to a lithium battery.
Example 4
A graphene coating layer grows on the surface of the nickel cobalt lithium manganate composite material in situ.
A preparation method of a nickel cobalt lithium manganate composite material comprises the following steps:
step 1, preparation of pretreatment materials: dispersing a nickel cobalt lithium manganate material into an ethanol solution through ultrasonic treatment, then slowly dropwise adding a reducing acid under a stirring state to perform a reduction reaction, and then performing suction filtration, absolute ethanol washing and drying to obtain a nickel cobalt lithium manganate pretreatment material with reducing nickel on the surface;
the mass ratio of the nickel cobalt lithium manganate material to the ethanol solution is 1: 40; the mass ratio of the nickel cobalt lithium manganate material to the reducing acid is 1: 90; the reducing acid is H2S, citric acid, and at a concentration of 4 wt.%; the stirring speed is 700r/min, and the reduction reaction time is 1.5 h;
the calcining temperature is 1300 ℃, and the time is 5 h; the unsaturated hydrocarbon gas is acetylene, ethylene and propylene gas, and the introduction rate is 100 mL/min;
and step 4, sieving: and after the calcination is finished, naturally cooling to room temperature under the protection of inert gas, taking out and sieving to obtain the nickel cobalt lithium manganate composite material with the surface in-situ grown with one graphene coating layer.
The nickel cobalt lithium manganate composite material prepared by the preparation method comprises a nickel cobalt lithium manganate base and a graphene coating layer grown on the surface of the nickel cobalt lithium manganate base in situ, wherein the thickness of the graphene coating layer is 40 nm; the nickel cobalt lithium manganate composite material can be used as a positive electrode material to be applied to a lithium battery.
Example 5
A graphene coating layer grows on the surface of the nickel cobalt lithium manganate composite material in situ.
A preparation method of a nickel cobalt lithium manganate composite material comprises the following steps:
step 1, preparation of pretreatment materials: dispersing a nickel cobalt lithium manganate material into an ethanol solution through ultrasonic treatment, then slowly dropwise adding a reducing acid under a stirring state to perform a reduction reaction, and then performing suction filtration, absolute ethanol washing and drying to obtain a nickel cobalt lithium manganate pretreatment material with reducing nickel on the surface;
the mass ratio of the nickel cobalt lithium manganate material to the ethanol solution is 1: 30; the mass ratio of the nickel cobalt lithium manganate material to the reducing acid is 1: 50; the reducing acid is tartaric acid and citric acid, and the concentration is 3 wt.%; the stirring speed is 400r/min, and the reduction reaction time is 1.2 h;
the calcining temperature is 1000 ℃, and the calcining time is 4.5 h; the unsaturated hydrocarbon gas is acetylene and ethylene gas (volume ratio is 1:1), and the gas introduction rate is 70 mL/min;
and step 4, sieving: and after the calcination is finished, naturally cooling to room temperature under the protection of inert gas, taking out and sieving to obtain the nickel cobalt lithium manganate composite material with the surface in-situ grown with one graphene coating layer.
The nickel cobalt lithium manganate composite material prepared by the preparation method comprises a nickel cobalt lithium manganate base and a graphene coating layer grown on the surface of the nickel cobalt lithium manganate base in situ, wherein the thickness of the graphene coating layer is 30 nm; the nickel cobalt lithium manganate composite material can be used as a positive electrode material to be applied to a lithium battery.
Comparative example 1
According to the examples in patent application CN 201310739997.8.
Comparative example 2
According to the examples in patent application CN 201711154582.9.
The nickel cobalt lithium manganate composite material prepared by the method of the embodiment 1 of the invention is used as a positive electrode material, SP and CNTS are conductive agents, PVDF is a binder, and the preparation method comprises the following steps: conductive agent: uniformly mixing a binder at a mass ratio of 80:10:10 to prepare positive electrode slurry, coating the positive electrode slurry on an aluminum foil, drying at 100 ℃, taking metal lithium as a negative electrode, assembling a lithium battery in a drying room to obtain a first battery, and taking the material prepared in the example 2 as a positive electrode by adopting the same method to prepare a second battery; meanwhile, the materials prepared by the methods of the comparative example 1 and the comparative example 2 are respectively used as anode materials, SP and CNTS are used as conductive agents, PVDF is used as a binder, and the lithium battery is assembled by adopting the method to obtain a third battery and a fourth battery. The discharge curves of the first battery and the third battery at 0.2C rate and the second battery and the fourth battery at 5C rate are shown in fig. 2 below.
As shown in figure 2, the nickel cobalt lithium manganate composite material prepared by the method has good high-rate discharge performance, discharges at a rate of 0.2C, and has a discharge capacity of 203.01mAhg-1(ii) a Discharge at 5C rate and discharge capacity of 170.16mAhg-1The capacity retention ratio was 85.08% with respect to 0.2C. In contrast, the material prepared by comparative example 1 was discharged at a 0.2C rate and had a discharge capacity of 185.36mAhg-1(ii) a The material prepared by comparative example 2 was discharged at 5C rate and the discharge capacity was only 148.49mAhg-1Therefore, the nickel cobalt lithium manganate composite material prepared by the invention has more excellent rate capability.
In conclusion, in the method, the graphene is deposited on the surface of the nickel cobalt lithium manganate particles in situ, and the graphene coating layer and the nickel cobalt lithium manganate material are connected through the chemical bond, so that the protection of the nickel cobalt lithium manganate material is better realized while the process is simplified, the conductivity of the nickel cobalt lithium manganate composite material can be improved, the precipitation of lithium in the material and the dissolution of nickel cobalt manganese heavy metal ions in an electrolyte can be inhibited, the cycling stability of the material is further improved, and the use requirement of the material with high specific capacity is met; the prepared nickel cobalt lithium manganate composite material has good conductivity, the discharge platform is obviously increased compared with the nickel cobalt lithium manganate material which is not coated, the rate capability and the cycle stability are obviously improved, and the nickel cobalt lithium manganate composite material conforms to the development trend of high-power and high-specific energy lithium ion batteries; the process is simple, the structure is controllable, the cycle performance and the rate capability of the lithium nickel cobalt manganese oxide material can be obviously improved, and the method is suitable for application and popularization of the lithium nickel cobalt manganese oxide material in a lithium ion battery.
It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrative embodiments, and that the present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.
Claims (5)
1. A preparation method of a nickel cobalt lithium manganate composite material is characterized by comprising the following steps: a graphene coating layer grows on the surface of the nickel cobalt lithium manganate composite material in situ, and the preparation method of the nickel cobalt lithium manganate composite material comprises the following steps:
step 1, preparation of pretreatment materials: dispersing a nickel cobalt lithium manganate material into an ethanol solution through ultrasonic treatment, then slowly dropwise adding a reducing acid under a stirring state to perform a reduction reaction, and then performing suction filtration, absolute ethanol washing and drying to obtain a nickel cobalt lithium manganate pretreatment material with reducing nickel on the surface;
step 2, precalcination: transferring the nickel cobalt lithium manganate pretreated material prepared in the step 1 into a calcining furnace, introducing inert gas for 10-30 min, evacuating the air in the furnace, and then heating and raising the temperature under the protection of the inert gas for precalcination;
step 3, calcining: after the precalcination is finished, continuing heating and raising the temperature under the protection of inert gas, and then introducing unsaturated hydrocarbon gas for calcination to form a graphene coating layer on the surface of the nickel cobalt lithium manganate material;
and step 4, sieving: after calcination is finished, under the protection of inert gas, naturally cooling to room temperature, taking out and sieving to obtain the nickel cobalt lithium manganate composite material with the surface in-situ grown with one graphene coating layer;
in the step 1, the mass ratio of the nickel cobalt lithium manganate material to the ethanol solution is 1: 10-50; the mass ratio of the nickel cobalt lithium manganate material to the reducing acid is 1: 0.5-100;
in step 1, the reducing acid is more than one of H2S, tartaric acid and citric acid, and the concentration is 1-5 wt%; the stirring speed is 50-800 r/min, and the reduction reaction time is 0.5-2 h;
in the step 3, the unsaturated hydrocarbon gas is more than one of acetylene, ethylene and propylene, and the introduction rate of the unsaturated hydrocarbon gas is 20-120 mL/min;
the nickel cobalt lithium manganate composite material contains a nickel cobalt lithium manganate base and a graphene coating layer grown on the surface of the nickel cobalt lithium manganate base in situ, and the thickness of the graphene coating layer is 10 nm-50 nm.
2. The method for preparing a lithium nickel cobalt manganese oxide composite material according to claim 1, characterized in that: in the step 2, the pre-calcining temperature is 200-400 ℃, and the time is 0.5-2 h; the inert gas is nitrogen or argon, and the introduction rate of the inert gas is 10-80 mL/min.
3. The method for preparing a lithium nickel cobalt manganese oxide composite material according to claim 1, characterized in that: in the step 3, the calcining temperature is 450-1500 ℃, and the calcining time is 3-6 h.
4. A lithium nickel cobalt manganese oxide composite material prepared by the method of any one of claims 1 to 3.
5. The use of the nickel cobalt lithium manganate composite material according to claim 4 as a lithium battery positive electrode material.
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