SILICON/CARBON COMPOSITE AND PREPARATION METHOD AND USE THEREOF TECHNICAL FIELD The present invention belongs to the field of silicon/carbon anode materials for lithium ion batteries, and specifically, relates to a silicon/carbon composite and a preparation method and use thereof. BACKGROUND In the past five years, electric vehicles have been developed rapidly, and lithium-ion batteries have also received worldwide attention as a main power source of electric vehicles. However, there are still problems such as low energy density in the lithium ion batteries. It is an effective method to find an anode material with a higher specific capacity. Silicon is not only rich in reserves, but also has a specific capacity up to 4200 mAh/g and an appropriate discharge voltage (Li/Li>0.5 V), and therefore it is an extremely potential anode material. However, a silicon anode is subject to a volume expansion/shrinkage change of more than 300% during lithium intercalation/delithiation. Consequently, not only active particles are crushed or even fall off from a current collector, but also an SEI film on the surface of silicon particles is repeatedly destructed and grows. This increases an irreversible capacity and intensifies a cycle failure. Studies have shown that the influence of a volume effect on the battery performance can be effectively reduced by reducing a size of a silicon anode material, for example, making the silicon anode material into nanoparticles, nanowires, nanotubes, etc. However, because the above material has high manufacturing costs, it is difficult to apply the material to industrial production. Diatomite is a natural porous mineral with silica as a main component. It has advantages of a low price and abundant reserves, and is suitable for large-scale production. An internal structure of diatomite can be well preserved through diatomite reduction by a magnesium thermal reaction, and therefore there are a large number of pores inside an obtained silicon-based material. During charging and discharging, not only silicon particles can be effectively prevented from being crushed, but also an effective diffusion distance of lithium ions can be shortened, thereby improving the electrochemical performance of a battery. However, a reversible specific capacity of the silicon-based material prepared only through the magnesium thermal reaction of diatomite still needs to be improved. SUMMARY An objective of the present invention is to provide a silicon/carbon composite obtained by coating carbon and a silicon-based material that is prepared from diatomite, and a preparation method and use thereof, to overcome a defect in the prior art that a reversible specific capacity of a silicon-based material prepared only through a magnesium thermal reaction of diatomite is relatively low. The silicon/carbon composite can significantly increase a reversible specific capacity of a lithium ion battery. Specifically, the present invention provides a silicon/carbon composite, where the silicon/carbon composite includes a carbon shell and a core inside the carbon shell; the core is a silicon-based material; and there is a hollow gap between the carbon shell and the core. Preferably, the silicon/carbon composite has 50-95wt.% of a silicon element and 5-50wt.% of a carbon element. Preferably, a pore diameter of the silicon-based material is 10-200 nm, and a particle size thereof is 100 nm-10 [m. Preferably, a particle size of the silicon/carbon composite is 100 nm-15 m. Preferably, the hollow gap between the carbon shell and the core is 5-100 nm. The present invention further provides a preparation method of a silicon/carbon composite, where the method includes the following steps: (1) uniformly mixing metal magnesium powder, diatomite powder, and a moderator, subjecting an obtained mixture to a redox reaction, then cooling an obtained product of the redox reaction to room temperature, and conducting washing and drying to obtain a silicon-based material; (2) dispersing the silicon-based material in an alcohol-water mixed solvent, adding an alkaline catalyst and tetraethyl orthosilicate to an obtained dispersion and stirring for reaction to coat a silica layer on the surface of the silicon-based material, and conducting washing and drying to obtain a silica-coated silicon-based material; (3) dispersing the silica-coated silicon-based material in water, adding cetyltrimethylammonium bromide and an alkaline catalyst and conducting uniform stirring, adding resorcinol and formaldehyde and stirring for reaction to coat a phenolic resin layer on the surface of the silica layer, conducting washing and drying, and conducting high-temperature calcination on a dried product obtained to carbonize the phenolic resin layer contained in the dried product, to obtain a carbon and silica coated silicon-based material; and (4) etching the carbon and silica coated silicon-based material to remove the silica layer to obtain the silicon/carbon composite. Preferably, in step (1), a mass ratio of the metal magnesium powder, the diatomite powder, and the moderator is 1:(1-2):(10-20). Preferably, in step (1), the moderator is sodium chloride powder. During the redox reaction, silica contained in the diatomite powder is reduced to monatomic silicon by the metal magnesium powder. A function of the moderator to make the redox reaction proceed smoothly and inhibit the occurrence of side reactions, so as to obtain the silicon-based material with more uniform pore diameter distribution.
Preferably, in step (1), the redox reaction is conducted by heating the mixture to 600-800°C under the protection of an inert gas at a rate of 2-10°C/min, and keeping the temperature for 2-8 h. Preferably, in step (2), a dosage ratio of the silicon-based material and the alcohol-water mixed solvent is 100 mg:(100-400) mL; a dosage of the alkaline catalyst is 0.5-2% of a volume of the dispersion; and a dosage of the tetraethyl orthosilicate is 0 . 1 -1% of the volume of the dispersion. In step (2), the tetraethyl orthosilicate is subject to hydrolytic polycondensation in the presence of the alkaline catalyst to form silica, to form a silica-coated layer on the surface of the silicon-based material. Preferably, in step (3), the cetyltrimethylammonium bromide is used in a form of an aqueous solution and the concentration of the cetyltrimethylammonium bromide aqueous solution is 0.5-2 mmol/L and a dosage thereof is 1/15-1/80 of a volume of water; and a dosage of the alkaline catalyst is 1/200-1/800 of the volume of the water. A function of the alkaline catalyst is to allow the subsequently added resorcinol and formaldehyde to react with each other to form phenolic resin. The cetyltrimethylammonium bromide can be attached to the surface of the silica-coated silicon-based material to provide positive charges. This is more conducive to the subsequent coating of the phenolic resin layer (negatively charged). In this case, a structure of each layer of a coated product obtained is more compact, which is extremely conducive to the improvement of a reversible specific capacity of a lithium ion battery. Preferably, in step (3), relative to 100 mg of the silica-coated silicon-based material, a dosage of the resorcinol is 5-200 mg, and a dosage of the formaldehyde is 10-100 L. Preferably, in step (3), the high-temperature calcination is conducted by heating the dried product to 800-1200°C under the protection of an inert gas at a rate of 2-10°C/min, and keeping the temperature for 2-8 h. Preferably, in step (2) and step (3), the alkaline catalysts are both concentrated ammonia water. In the present invention, the concentration of the concentrated ammonia water may be 20-40wt.%. Preferably, in step (2) and step (3), the stirring for reaction is conducted for independently 12-48 h. Preferably, in step (4), the etching is conducted by soaking the carbon and silica coated silicon-based material in a 5-15wt.% hydrofluoric acid solution, stirring for reaction for 20-300 min, conducting solid-liquid separation, and conducting centrifugal cleaning and drying on obtained solid. According to a preferred implementation of the present invention, the preparation method of the silicon/carbon composite includes the following steps: (1) placing the metal magnesium powder, the diatomite powder, and sodium chloride powder in a ball mill at a mass ratio of 1:(1-2):(10-20), and conducting grinding at a rotational speed of
120-200 r/min for 0.5-10 h; placing an obtained mixture in a tubular furnace, heating the mixture to 600-800°C under the protection of argon gas at a rate of 2-10°C/min, and keeping the temperature for 2-8 h; and naturally cooling an obtained product to room temperature, washing and centrifuging the product with a hydrochloric acid solution with a concentration of 0.1-3 mol/L for 1-5 times, and conducting drying to obtain a silicon-based material; (2) dispersing the silicon-based material in the alcohol-water mixed solvent, conducting ultrasonic treatment, adding the concentrated ammonia water to the obtained dispersion, adding the tetraethyl orthosilicate under vigorous stirring, conducting the stirring reaction for 12-48 h, washing and centrifuging an obtained product with deionized water for 1-5 times, and conducting freeze drying to obtain a silica-coated silicon-based material; (3) mixing 100 mg of the silica-coated silicon-based material and water at a mass ratio of 1:(100-1000), adding a cetyltrimethylammonium bromide aqueous solution with a concentration of 0.5-2 mmol/L and a dosage of 1/15-1/80 of a volume of the water and the concentrated ammonia water with a dosage of 1/200-1/800 of the volume of the water, vigorously stirring for 10-60 min, adding 5-200 mg of the resorcinol and 10-100 L of a formaldehyde solution, conducting the stirring reaction for 12-48 h, washing and centrifuging an obtained product with anhydrous ethanol for 1-5 times, conducting vacuum drying, placing a dried product in the tubular furnace, heating the dried product to 800-1200°C under the protection of the argon gas at a rate of 2-10°C/min, and keeping the temperature for 2-8 h to obtain a carbon and silica coated silicon-based material; and (4) soaking the carbon and silica coated silicon-based material in the 5-15wt.% hydrofluoric acid solution, conducting the stirring reaction for 20-300 min, conducting solid-liquid separation, and conducting centrifugal cleaning and drying on obtained solid to obtain the silicon/carbon composite. The present invention further provides a silicon/carbon composite prepared by the above method. In addition, the present invention further provides use of the silicon/carbon composite as an anode material for a lithium ion battery. Beneficial effects of the present invention are as follows: On one hand, a silicon source used in the present invention is diatomite; not only the diatomite has advantages of a low price and abundant reserves, but also it is porous material; and a silicon-based material prepared from the diatomite through a magnesium thermal reduction reaction can well preserve an internal structure of the diatomite, and therefore there are a large number of pores inside the obtained silicon-based material, thereby greatly improving the cycle performance and stability of a battery. On the other hand, in the present invention, a carbon layer is coated on the outer surface of the silicon-based material, and there is a gap between the silicon-based material and the carbon layer, thereby forming a carbon-coated porous silicon material with an egg yolk structure. Due to such a unique structure, not only a stable SEI film can be formed to reduce the consumption of Li * ions and increase a reversible specific capacity, but also the gap between the silicon-based material and the carbon layer can accommodate the volume expansion of the silicon-based material and avoid the collapse of an anode material structure. To sum up, the silicon/carbon composite provided in the present invention has excellent electrochemical performance such as cycling stability, and has a relatively high reversible specific capacity. BRIEF DESCRIPTION OF DRAWINGS In FIG. 1, a is a scanning electron microscope image of a silicon-based material obtained in Example 1, and b is a scanning electron microscope image of a raw material diatomite in Example 1; FIG. 2 is an energy spectrum diagram of a silicon/carbon composite obtained in Example 1; FIG. 3 is an X-ray diffraction pattern of the silicon-based material obtained in Example 1; FIG. 4 is a cycle performance diagram of the silicon/carbon composite obtained in Example 1; FIG. 5 is a cycle performance diagram of a silicon/carbon composite obtained in Example 2; and FIG. 6 is a rate performance diagram of a silicon/carbon composite obtained in Example 3. DETAILED DESCRIPTION The following describes in detail examples of the present invention, and examples in the examples of the present invention are intended to explain the present invention, and cannot construed as a limitation of the present invention. If a specific technology or condition is not specified in the examples, it shall be carried out according to a technology or condition described in the literature in the art or according to a product specification. All used reagents or instruments for which manufacturers are not specified are conventional commercially-available products. Example 1 (1) 0.1625 g of metal magnesium powder, 0.1993 g of diatomite powder, and 2.2025 g of sodium chloride powder were placed in a ball mill, and ground at a rotational speed of 180 r/min for 2.5 h; an obtained mixture was placed in a tubular furnace, heated to 650°C under the protection of argon gas at a rate of 2°C/min, and kept at the temperature for 3 h; an obtained product was naturally cooled to room temperature, placed in 65 mL of a 1mol/L hydrochloric acid solution for stirring and washing for 5.5 h and then centrifuged, where the product was repeatedly washed and centrifuged for 3 times according to the method; and finally the product was placed in a vacuum drying oven for drying for 12 h to obtain a silicon-based material. (2) 120 mg of the silicon-based material obtained in step (1) was dispersed in a mixed solution composed of 320 mL of anhydrous ethanol and 80 mL of deionized water, and was subject to ultrasonic treatment; 4 mL of concentrated ammonia water (with a mass fraction of 28%, the same below) was added to an obtained dispersion, 1.6 mL of tetraethyl orthosilicate was added under vigorous stirring (a stirring speed is 2000 rpm, the same below), and a stirring reaction was conducted for 12 h; and an obtained product was washed and centrifuged with deionized water for 3 times, and then was subject to freeze drying for 8 h to obtain a silica-coated silicon-based material. (3) 100 mg of the silica-coated silicon-based material obtained in step (2) was dispersed in 80 mL of deionized water, then mixed with 1 mL of a cetyltrimethylammonium bromide aqueous solution (with a concentration of 1 mmol/L) and 0.1 mL of concentrated ammonia water (with a mass fraction of 28%), and vigorously stirred for 20 min; 60 mg of resorcinol and 56 L of a formaldehyde solution (with a mass fraction of 37%, where a solvent is water, the same below) were added, and subject to a stirring reaction for 16 h; an obtained product was washed and centrifuged with anhydrous ethanol for 3 times, and was subject to vacuum drying for 8 h; and a dried product was placed in the tubular furnace, heated to 800°C under the protection of the argon gas at a rate of 5°C/min, and kept at the temperature for 2 h to obtain a carbon and silica coated silicon-based material. (4) The carbon and silica coated silicon-based composite obtained in step (3) was soaked in a 5wt.% HF aqueous solution, and was subject to a stirring reaction for 40 min; and an obtained product was washed and centrifuged with deionized water for 3 times, and finally dried in a vacuum freezing drying oven for 6 h to obtain a silicon/carbon composite. a and b in FIG. 1 are a scanning electron microscope image of the silicon-based material obtained in step (1) and a scanning electron microscope image of a raw material diatomite. It can be seen from a and b in FIG. 1 that, the raw material diatomite itself is a type of material with a porous structure and has high porosity, and after the diatomite is subject to a magnesium thermal reaction, its original porous structure is completely preserved. FIG. 2 is an energy spectrum diagram of the silicon/carbon composite. It can be seen from FIG. 2 that, the silicon/carbon composite is composed of silicon and carbon elements. FIG. 3 is an X-ray diffraction pattern of the silicon-based material. It can be seen from FIG. 3 that, monatomic silicon is produced through a magnesium thermal reaction, and a wide peak at approximately 230should be unreacted silica. The silicon/carbon composite includes a carbon shell and a core inside the carbon shell; the core is the silicon-based material; and there is a hollow gap with a size of 5-100 nm between the carbon shell and the core. A pore diameter of the silicon-based material is 10-200 nm, and a particle size thereof is 100 nm-10 [m; and a particle size of the silicon/carbon composite is 100 nm-15 m. The silicon/carbon composite was mixed with carbon black and PVDF according to a mass ratio of 7:2:1, and then NMP was added as a solvent and stirred overnight; stirred slurry is coated on a copper foil, and placed in a vacuum drying oven and dried at 50°C for 2 h and then at 100°C for 12 h to obtain a battery pole piece; then the battery pole piece was punched into a circular electrode sheet by using a steel punching die; and the electrode sheet was weighed, then placed in a glovebox fully filled with argon gas, and assembled in a CR2025 battery case with lithium metal as a counter electrode and an electrolyte that is prepared by dissolving IM LiPF in ethylene carbonate (EC) and dimethyl carbonate (DEC) (1:1vol%) and that contains 5vol% FEC, to obtain a lithium ion battery. A cycle performance test was conducted on the lithium ion battery after the lithium ion battery stood for 24 h, and an obtained result is shown in FIG. 4. It can be seen from the result in FIG. 4 that, when current density is 100 mA/g, after 20 cycles, a reversible capacity is 428 mAh/g, and a capacity retention rate reaches 80%; and starting from the sixth cycle, the coulomb efficiency reaches higher than 95%. Example 2 (1) 0.2400 g of metal magnesium powder, 0.3041 g of diatomite powder, and 3.0451 g of sodium chloride powder were placed in a ball mill, and ground at a rotational speed of 120 r/min for 10 h; an obtained mixture was placed in a tubular furnace, heated to 700°C under the protection of argon gas at a rate of 5°C/min, and kept at the temperature for 3 h; an obtained product was naturally cooled to room temperature, placed in 65 mL of a 1mol/L hydrochloric acid solution for stirring and washing for 6 h and then centrifuged, where the product was repeatedly washed and centrifuged for 3 times according to the method; and finally the product was placed in a vacuum drying oven for drying for 12 h to obtain a silicon-based material. (2) 150 mg of the silicon-based material obtained in step (1) was dispersed in a mixed solution composed of 320 mL of anhydrous ethanol and 80 mL of deionized water, and was subject to ultrasonic treatment; 4 mL of concentrated ammonia water was added to an obtained dispersion, 1.6 mL of tetraethyl orthosilicate was added under vigorous stirring, and a stirring reaction was conducted for 12 h; and an obtained product was washed and centrifuged with deionized water for 3 times, and then was subject to freeze drying for 8 h to obtain a silica-coated silicon-based material. (3) A step same as step (3) in Example 1 was conducted to obtain a carbon and silica coated silicon-based composite. (4) A step same as step (4) in Example 1 was conducted to obtain a silicon/carbon composite. The silicon/carbon composite includes a carbon shell and a core inside the carbon shell; the core is the silicon-based material; and there is a hollow gap with a size of 5-100 nm between the carbon shell and the core. A pore diameter of the silicon-based material is 10-200 nm, and a particle size thereof is 100 nm-10 [m; and a particle size of the silicon/carbon composite is 100 nm-15 m. A lithium ion battery was fabricated from the silicon/carbon composite according to the method in Example 1. A cycle performance test was conducted on the lithium ion battery after the lithium ion battery stood for 24 h, and an obtained result is shown in FIG. 5. It can be seen from in FIG. 5 that, when extremely high current density is 1000 mA/g, after 35 cycles, a reversible capacity is 300 mAh/g, and a capacity retention rate reaches up to 60%; and starting from the seventh cycle, the coulomb efficiency reaches higher than 97%. Example 3 (1) 0.3120 g of metal magnesium powder, 0.4010 g of diatomite powder, and 4.1430 g of sodium chloride powder were placed in a ball mill, and ground at a rotational speed of 150 r/min for 6 h; an obtained mixture was placed in a tubular furnace, heated to 650°C under the protection of argon gas at a rate of 5°C/min, and kept at the temperature for 6 h; an obtained product was naturally cooled to room temperature, placed in 65 mL of a 3mol/L hydrochloric acid solution for stirring and washing for 8 h and then centrifuged, where the product was repeatedly washed and centrifuged for 3 times according to the method; and finally the product was placed in a vacuum drying oven for drying for 12 h to obtain a silicon-based material. (2) 150 mg of the silicon-based material obtained in step (1) was dispersed in a mixed solution composed of 200 mL of anhydrous ethanol and 50 mL of deionized water, and was subject to ultrasonic treatment; 2.5 mL of concentrated ammonia water was added to an obtained dispersion, 1 mL of tetraethyl orthosilicate was added under vigorous stirring, and a stirring reaction was conducted for 15 h; and an obtained product was washed and centrifuged with deionized water for 3 times, and then was subject to freeze drying for 8 h to obtain a silica-coated silicon-based material. (3) 100 mg of the silica-coated silicon-based material obtained in step (2) was dispersed in 40 mL of deionized water, then mixed with 0.8 mL of a cetyltrimethylammonium bromide aqueous solution (with a concentration of 1 mmol/L) and 0.08 mL of concentrated ammonia water (with a mass fraction of 28%), and vigorously stirred for 20 min; 60 mg of resorcinol and 40 L of a formaldehyde solution were added, and subject to a stirring reaction for 14 h; an obtained product was washed and centrifuged with anhydrous ethanol for 3 times, and was subject to vacuum drying for 10 h; and a dried product was placed in the tubular furnace, heated to 900°C under the protection of the argon gas at a rate of 8°C/min, and kept at the temperature for 2 h to obtain a carbon and silica coated silicon-based material. (4) The carbon and silica coated silicon-based composite obtained in step (3) was soaked in a 2wt.% HF aqueous solution, and was subject to a stirring reaction for 120 min; and an obtained product was washed and centrifuged with deionized water for 3 times, and finally dried in a vacuum freezing drying oven for 6 h to obtain a silicon/carbon composite. The silicon/carbon composite includes a carbon shell and a core inside the carbon shell; the core is the silicon-based material; and there is a hollow gap with a size of 5-100 nm between the carbon shell and the core. A pore diameter of the silicon-based material is 10-200 nm, and a particle size thereof is 100 nm-10 [m; and a particle size of the silicon/carbon composite is 100 nm-15 m.
A lithium ion battery was fabricated from the silicon/carbon composite according to the method in Example 1. A cycle performance test and a rate performance test were conducted on the lithium ion battery after the lithium ion battery stood for 24 h. The rate performance test was separately conducted when current density is 100 mA/g, 200 mA/g, 500 mA/g, 1000 mA/g, and 5000 mA/g. Results show that an initial specific capacity is 837 mAh/g when the current density is 100 mA/g, and after 25 cycles, a remaining capacity rate is 86%. The rate performance of the lithium ion battery is shown in FIG. 6. It can be seen from FIG. 6 that, when the current density returns to 100 mA/g, a specific capacity returns to a higher level, showing excellent rate performance. Comparative Example 1 A silicon-based material was prepared according to the method in Example 1. A lithium ion battery was assembled according to the method in Example 1 by replacing a silicon/carbon composite with the silicon-based material. A cycle performance test was conducted on the lithium ion battery after the lithium ion battery stood for 24 h. Results show that when current density is 100 mA/g, a reversible capacity is 213 mAh/g and a capacity retention rate is 17.6% after 20 cycles. Comparative Example 2 A carbon and silica coated silicon-based composite was prepared according to the method in Example 1. A lithium ion battery was assembled according to the method in Example 1 by replacing a silicon/carbon composite with the carbon and silica coated silicon-based composite. A cycle performance test was conducted on the lithium ion battery after the lithium ion battery stood for 24 h. Results show that when current density is 100 mA/g, a reversible capacity is 236 mAh/g and a capacity retention rate is 23.6% after 20 cycles. Although the examples of the present invention have been illustrated and described, it can be understood that the above examples are exemplary and cannot be construed as a limitation to the present invention. A person of ordinary skill in the art may make various changes, modifications, replacements and variations to the above examples without departing from the principle and spirit of the present invention.