TECHNICAL FIELD

The present invention relates to the technical field of manufacture of a potassium ion battery, and particularly relates to a carbon quantum dot/carbon coated VSe2composite material (VSe2@CQD) and a preparation method thereof.

BACKGROUND OF THE PRESENT INVENTION

Lithium-ion batteries have the advantages of high open-circuit voltage, long cycle life, high energy density, no memory effect, etc., so that the lithium-ion batteries are widely used in the fields of portable digital products, electric vehicles and energy storage. However, due to the shortcomings of low lithium storage in nature, high price and the like at present, the further development of the lithium-ion batteries in the fields of the electric vehicles and large-scale energy storage is limited. Metal potassium has abundant reserves and low price in nature, thereby becoming an ideal material for substituting the lithium. In order to meet the sustainable demand of people for the high energy density potassium ion batteries, improving the specific capacity and cycle stability of negative materials of the potassium ion batteries has become an important research direction of the potassium ion batteries.

As a typical graphene-like interlayer transition metal selenide, vanadium diselenide has attracted wide attention in tribology, energy, electronic devices, optoelectronics and other fields because of the unique and excellent electrical, thermal and mechanical properties. In recent years, as the negative material of the lithium ion battery, the vanadium diselenide has already aroused the interest of researchers. As the negative material of the potassium ion battery, the vanadium diselenide has the advantages of moderate potassium insertion voltage (about 1.3 V), good safety, high specific capacity, etc. However, because the vanadium diselenide has poor conductivity and is easy to be re-stacked, the good electrical connection and lithium ion path in the cycle process may be lost, which eventually leads to the rapid decline of the capacity in the cycle process. In order to solve this problem well, it is a very effective method to prepare vanadium diselenide composite materials. Therefore, the composite materials of vanadium diselenide and amorphous carbon as well as carbon nanotubes and graphene have been synthesized and applied to the negative materials of potassium ion batteries, and their electrochemical properties have been greatly improved. In addition, because vanadium diselenide has great mechanical strength, some researchers believe that the vanadium diselenide can inhibit the volume expansion of other negative materials during charging and discharging. Therefore, the composite materials of vanadium diselenide, metal negative electrodes and transition metal oxides have also attracted the attention of the researchers.

SUMMARY OF THE PRESENT INVENTION

The technical problem to be solved by the present invention is to provide a carbon quantum dot/carbon coated VSe2composite material (VSe2@CQD) and a preparation method thereof. The method is simple and easy in operation, effectively improves the electronic conductivity of VSe2, improves the rate performance of the material, simultaneously inhibits the volume expansion and agglomeration of VSe2, and improves the cycle stability of the material.

In the carbon quantum dot/carbon coated VSe2composite material and the preparation method thereof disclosed by the present invention, the carbon quantum dot/carbon coated VSe2composite material is prepared by combining a hydrothermal method and a high-temperature pyrolysis method. The mass fraction of VSe2in carbon quantum dot/carbon coated VSe2composite material is 60%-70%, and the mass fraction of carbon quantum dots/carbon is 30%-40%. Steps are as follows:

1). Weighing vanadium oxide and selenium oxide, dissolving in water or organic solvent to prepare a solution with a concentration of 0.5-2 mol/L, and stirring for 0.5 h to obtain a brownish gray solution;

2). Adding organic acid into the salt solution obtained in the step 1), and continuously stirring for 0.5 h to obtain a mixed solution;

3). Transferring the mixed solution obtained in the step 2) into a polytetrafluoroethylene lined high-pressure hydrothermal reaction kettle, and keeping the temperature at 150-220° C. for 15-30 h;

4). After the solution obtained in the step 3) is cooled, repeatedly centrifuging the solution with deionized water and absolute ethyl alcohol at a rate of 5,000-10,000 r/m, and removing the solution to obtain black precipitates;

5). Drying the black precipitates obtained in the step 4) at 50-120° C. for 12-24 h to obtain black powder;

6). Washing an organic compound cellulose acetate butyrate in an alkaline mixed solution and filtering for 3-5 times at a low temperature of −20 to 0° C., and washing the filtered white precipitates with water and absolute ethyl alcohol for 3-5 times until the solution is neutral;

7). Drying the white precipitates obtained in the step 6) at 50-100° C. for 12-24 h to obtain white powder;

8). Weighing the black powder obtained in the step 5) and the white powder obtained in the step 6) in a mass ratio of 1:(3-10), dissolving the powder in organic solvent, and stirring for 12-24 h;

9). Drying the solution obtained in the step 8) at 50-120° C. for 12-24 h to obtain brown colloid;

10). Heating the brown colloid obtained in the step 9) from 25° C. to 180-250° C. at 1-5° C./min in an inert atmosphere, keeping the temperature for 1-5 h, then heating to 600-950° C. at 1-5° C./min, keeping the temperature for 2-5 h, and naturally cooling to the room temperature to obtain the carbon quantum dot/carbon coated VSe2composite material (VSe2@CQD).

In the above method,

In the step 1), the vanadium oxide is vanadium dioxide; the selenium oxide is selenium dioxide; and the solvent is one of deionized water or N-methyl pyrrolidone;

In the step 2), the organic acid is formic acid;

In the step 3), the keeping temperature is preferably controlled at 180-220° C., and the temperature keeping time is preferably controlled at 20-28 h;

In the step 4), the centrifuging rate is preferably controlled at 8,000-10,000 r/m;

In the step 5), the drying temperature is preferably controlled at 80-100° C., and the temperature keeping time is preferably controlled at 18-24 h;

In the step 6), the alkaline mixed solution is a 5-10% sodium hydroxide/8-15% urea mixed aqueous solution, and the temperature is preferably controlled at −20 to −15° C.;

In the step 7), the drying temperature is preferably controlled at 80-100° C., and the temperature keeping time is preferably controlled at 18-24 h;

In the step 8), a mass ratio of the black powder obtained in the step 5) to the white powder obtained in the step 6) is preferably controlled at 1:(4-8), the organic solvent is preferably N-methyl pyrrolidone, and the stirring time is preferably 20-24 h;

In the step 9), the drying temperature is preferably controlled at 80-100° C., and the temperature keeping time is preferably controlled at 18-24 h;

In the step 10), the inert atmosphere is one or more of nitrogen or argon and preferably argon; the heating rate is preferably 5° C./min, the first keeping temperature is preferably 200-220° C., and the temperature keeping time is preferably 2-3 h; the second keeping temperature is preferably 700-800° C., and the temperature keeping time is preferably 2-3 h;

The carbon quantum dot/carbon coated VSe2composite material is prepared through the above method and used as a negative material of a potassium ion battery and abbreviated as carbon quantum dot/carbon coated VSe2(VSe2@CQD).

The carbon quantum dot/carbon coated VSe2(VSe2@CQD) of the present invention has excellent rate performance and cycle stability. The three components, i.e. the carbon quantum dots, the vanadium diselenide and the carbon, generate a synergistic effect to effectively inhibit the agglomeration of the vanadium diselenide, and to simultaneously improve the electronic conductivity and lithium ion diffusion rate, thereby effectively improving the rate performance and cycle stability of the material.

