CN112436131B - A method for preparing silicon-carbon composite materials by molten salt-assisted magnesia thermal reduction - Google Patents

Abstract

The invention provides a method for preparing a silicon-carbon composite material by using molten salt to assist magnesiothermic reduction, which comprises the steps of firstly carbonizing alginate and attapulgite serving as raw materials at high temperature to obtain an amorphous carbon coating-coated attapulgite composite material, then adding a reducing agent and molten salt to perform heat-assisted reduction reaction, and performing acid pickling treatment to obtain the silicon-carbon composite material. The invention realizes the method for preparing the silicon-carbon composite material by combining high-temperature carbonization and low-temperature molten salt heat assistance, effectively reduces the generation of silicon carbide through the molten salt heat assistance reduction reaction, and the carbon coating in the prepared silicon-carbon composite material coats silicon nano particles reduced by attapulgite and forms a gap structure through acid etching. The composite material is used for a lithium ion battery cathode material, and the carbon layer, the gaps and the porous structure can effectively relieve the volume expansion effect caused in the lithium intercalation and deintercalation process, and simultaneously improve the electronic conductivity, so that the composite material has excellent electrochemical lithium storage performance.

Description

A method for preparing silicon-carbon composite materials by molten salt-assisted magnesia thermal reduction

technical field

The invention relates to a method for preparing a silicon-carbon composite material through magnesia thermal reduction assisted by molten salt. The silicon-carbon composite material is mainly used for negative electrode materials of lithium-ion batteries, and belongs to the technical field of composite materials and the technical field of new energy.

Background technique

The endurance of new energy vehicles and electronic products depends on the energy density of batteries. With the continuous improvement of consumer demand, automobiles and electronic products with long endurance are very popular among consumers. Therefore, the search for batteries with high energy density will become the direction and driving force for future development. Among all the materials that can meet this requirement, silicon-based materials are one of the most candidate materials, because of their advantages such as high theoretical capacity (Si, 4200 mAh g ^-1 ), low operating potential, and wide sources, so that It becomes a high-spot material in applied research. However, the huge volume expansion (~300%) and serious interfacial side reactions of silicon anode materials in the process of lithium intercalation and delithiation are the key problems hindering its further realization of high energy density and stable applications. Reasonable design of the structure of silicon anode materials Coating with carbon layer can solve these problems to the greatest extent. At the same time, the selection of silicon source and carbon source is also very important, and it is very important to explore their preparation and application.

Attapulgite, also known as attapulgite, is a natural inorganic material with a wide range of sources and low cost. The ideal chemical composition is Mg ₅ Si ₈ O ₂₀ (OH) ₂ (H ₂ O) ₄ .nH ₂ O, Among them, the content of SiO ₂ is about 57%, and the content of SiO ₂ in attapulgite after pretreatment can be increased to more than 65%. Using it as a silicon source to obtain silicon nanomaterials through reduction has broad application prospects and practical significance. Alginate is a pure natural polysaccharide and also a highly viscous polymer compound. Because of its high-strength viscosity, choosing it as a carbon source is easier to achieve the coating effect of the carbon coating, making the silicon negative electrode material have excellent specific capacity and long cycle life. Therefore, the application of attapulgite in silicon-based negative electrode materials has far-reaching influence and great application prospects and economic and commercial value.

The invention patent (publication number: CN109873150A) discloses a method of directly reducing palygorskite to silicon nanoparticles at high temperature, and then compounding it with carbon source graphene to obtain a silicon-carbon composite material. The reduction temperature used in this patent is high, and the process is relatively complicated. Its preparation process is the process of obtaining simple silicon by high-temperature reduction, and then carbonizing and coating it to prepare silicon-carbon composite materials.

Contents of the invention

The object of the present invention is to provide a method for preparing silicon-carbon composite materials through molten salt-assisted magnesia thermal reduction.

1. Preparation of silicon-carbon composite materials

The method for preparing a silicon-carbon composite material by molten salt-assisted magnesia thermal reduction of the present invention comprises the following steps:

(1) After pretreatment, the attapulgite was uniformly dispersed in the ethanol-water mixed solution to obtain the attapulgite dispersion, and the alginate was dissolved in deionized water to obtain the alginate solution, and the attapulgite dispersion and seaweed The acid salt solution is mixed evenly, then dried, and then carbonized at high temperature under the protection of inert gas to obtain an amorphous carbon coating-coated attapulgite composite material. Wherein, the pretreatment of the attapulgite is to first dry the attapulgite at 100° C. to 300° C. to remove water, and then wash the attapulgite with HCl at a concentration of 2 to 10 mol/L. In the ethanol-water mixed solution, the volume ratio of ethanol to water is 1:0.25~1:4. The alginate is potassium alginate or sodium alginate. The mass ratio of the alginate to the attapulgite is 1:1-1:5. The high temperature carbonization temperature is 700° C. to 1000° C., and the high temperature carbonization time is 1 to 5 hours.

(2) Mix amorphous carbon coating-coated attapulgite composite material, reducing agent magnesium powder and molten salt, and react at 650°C~850°C for 2~8h under the protection of inert gas, cool to room temperature, and wash with hydrochloric acid Wash with hydrofluoric acid, then wash with deionized water and absolute ethanol, and dry to obtain the silicon-carbon composite material. Among them, the mass ratio of the amorphous carbon coating-coated attapulgite composite material to the reducing agent magnesium powder is 1:1-1:5. The molten salt is sodium chloride or potassium chloride, and the mass ratio of the amorphous carbon-coated attapulgite composite material to the molten salt is 1:10-1:30.

2. Structural characterization of silicon-carbon composites

The structure of the silicon-carbon composite material prepared in the present invention is characterized below by means of X-ray diffraction (XRD) and scanning electron microscope (SEM).

Fig. 1 is an X-ray diffraction pattern (XRD) of the silicon-carbon composite material prepared in the present invention. It can be seen from Figure 1 that the diffraction peaks of the silicon-carbon composites prepared in Examples 1 and 2 are completely consistent with the strong diffraction peaks of the simple Si phase (JCPDS card number 27-1402), indicating that the silicon-carbon composites were successfully synthesized , it can be seen from the figure that the material prepared in Example 2 contains more impurity silicon carbide, indicating that the increase of the reduction temperature will increase the generation content of impurity silicon carbide.

Fig. 2 is a scanning electron microscope image (SEM) of the silicon-carbon composite material prepared in the present invention. From Fig. 2, the morphology of the silicon-carbon composite material prepared in Example 1 and Example 2 can be seen. From the two figures, it can be seen that the reduced silicon nanoparticles are coated in the carbon layer and agglomerate into large blocks. The void groove structure formed by acid etching can be seen. Through comparison, it is found that the particles in Example 1 are smaller than those in Example 2, indicating that the increase of the reduction temperature will lead to the increase of the sample particles.

3. Performance testing of silicon-carbon composite materials as anode materials for lithium-ion batteries

Test method: After a certain amount of silicon-carbon composite material, sodium alginate and acetylene black are ground to prepare a slurry, they are assembled into a button battery, and the cycle performance is tested by a blue electric test system.

Fig. 3 is a diagram of the cycle performance when the silicon-carbon composite material prepared by the present invention is used as the negative electrode of a lithium-ion battery. It can be seen from Fig. 3 that the silicon-carbon composite materials prepared in Example 1 and Example 2 have 1221.1 mAh g ^-1 and 1352.8 mAh g -1 respectively at a current density of 200 mA g ^-1 when they are used as negative electrode materials for lithium ^- ion batteries. ¹ high initial reversible specific capacity, the first Coulombic efficiencies are 68.66%, 59.38%, respectively. After 100 cycles, the reversible specific capacities remained at 731.3 mAh g ^-1 and 514.7 mAh g ^-1 , respectively, indicating that the silicon-carbon composites prepared in the present invention have good cycle stability.

In summary, the present invention uses alginate and attapulgite as raw materials to obtain amorphous carbon coating-coated attapulgite composite material through high-temperature carbonization firstly, and then adds reducing agent and molten salt for heat-assisted reduction reaction, acid Washing treatment to obtain a silicon-carbon composite material. The invention realizes the method of preparing silicon-carbon composite materials by combining high-temperature carbonization and low-temperature molten salt heat-assisted reduction reaction, effectively reduces the generation of silicon carbide, and the carbon coating in the prepared silicon-carbon composite materials The silicon nano-particles reduced by the stone are coated, and the void structure is formed by acid etching. The composite material is used as a negative electrode material for lithium-ion batteries. The carbon layer, voids and porous structure can effectively alleviate the volume expansion effect caused by the lithium intercalation and desorption process, and at the same time improve the electronic conductivity, so that the composite material has excellent electrochemical storage properties. Lithium properties.

Description of drawings

Figure 1 is the X-ray diffraction pattern (XRD) of silicon-carbon composite materials prepared under different embodiments of the present invention;

Fig. 2 is a scanning electron microscope image (SEM) of a silicon-carbon composite material prepared under different embodiments of the present invention;

Fig. 3 is a cycle performance diagram of silicon-carbon composite materials prepared under different embodiments of the present invention when used as a negative electrode of a lithium-ion battery.

detailed description

The preparation and performance of the silicon-carbon composite material of the present invention will be described in detail below through specific examples.

Example 1

(1) First, dry the attapulgite at 200°C to remove water, wash it with 4mol/L HCl solution, and then take 2g of the pretreated attapulgite and evenly disperse it in 300mL of ethanol-water mixed solution (V _ethanol : V _water = 1:4) to obtain the attapulgite dispersion; dissolve 2g of sodium alginate in 100ml deionized water to form a sodium alginate solution; mix the attapulgite dispersion and sodium alginate solution to dry, Under gas protection, it was carbonized at 800°C for 5 hours to obtain an amorphous carbon coating-coated attapulgite composite material.

(2) Mix the amorphous carbon-coated attapulgite composite material with magnesium powder at a mass ratio of 1:1, mix and grind potassium chloride at a mass ratio of 1:15, and react at 650°C under the protection of an inert gas 5h, cooled to room temperature, then washed with hydrochloric acid and hydrofluoric acid, finally washed with deionized water and absolute ethanol, and dried to obtain a silicon-carbon composite material.

When the composite material is used as the anode material of lithium-ion batteries, it has a high initial reversible specific capacity of 1221.1mAh g ^-1 at a current density of 200 mA g ^-1 , and the first Coulombic efficiency is 68.66%. After 100 cycles, a reversible capacity up to ^731.3 mAh g can be maintained with excellent cycle stability.

Example 2

(1) First, dry the attapulgite at 300°C to remove water, wash it with 8mol/L HCl solution, and then take 4g of the pretreated attapulgite and evenly disperse it in 300mL ethanol-water mixed solution (V _ethanol : V _water = 1:4) to obtain the attapulgite dispersion; dissolve 2g of potassium alginate in 100ml deionized water to form a potassium alginate solution; mix the attapulgite dispersion and potassium alginate solution to dry, Under gas protection, it was carbonized at 900°C for 2 hours to obtain an amorphous carbon coating-coated attapulgite composite material.

(2) Mix the amorphous carbon-coated attapulgite composite material with magnesium powder at a mass ratio of 1:3, mix and grind potassium chloride at a mass ratio of 1:25, and react at 750°C under the protection of an inert gas 6h, cooled to room temperature, then washed with hydrochloric acid and hydrofluoric acid, finally washed with deionized water and absolute ethanol, and dried to obtain a silicon-carbon composite material.

When the composite material is used as the negative electrode material of lithium-ion batteries, it has a reversible specific capacity of 1352.8mAh g ^-1 at a current density of 200 mA g ^-1 , and the first Coulombic efficiency is 59.38%. After 100 cycles, a reversible capacity of ^514.7 mAh g can be maintained, showing good cycle stability.

Claims (8)

1. A method for preparing silicon-carbon composite material through magnesia thermal reduction assisted by molten salt, comprising the following steps: (1) After pretreatment, the attapulgite was uniformly dispersed in the ethanol-water mixed solution to obtain the attapulgite dispersion, and the alginate was dissolved in deionized water to obtain the alginate solution, and the attapulgite dispersion and seaweed The acid salt solution is mixed and dried, and then carbonized at a high temperature under the protection of an inert gas to obtain an amorphous carbon coating-coated attapulgite composite material; the high-temperature carbonization temperature is 700°C~1000°C, and the high-temperature carbonization time is 1~5h; (2) Mix amorphous carbon coating-coated attapulgite composite material, reducing agent magnesium powder and molten salt, and react at 650°C~850°C for 2~8h under the protection of inert gas, cool to room temperature, and wash with hydrochloric acid Wash with hydrofluoric acid, then wash with deionized water and absolute ethanol, and dry to obtain the silicon-carbon composite material. 2. A method for preparing silicon-carbon composite materials by molten salt-assisted magnesia thermal reduction as claimed in claim 1, characterized in that: in step (1), the pretreatment of the attapulgite is to firstly treat the attapulgite at 100 Dry at ℃~300℃ to remove water, and then wash with HCl with a concentration of 2~10 mol/L. 3. A kind of molten salt assisted magnesia thermal reduction method for preparing silicon-carbon composite material as claimed in claim 1, is characterized in that: in step (1), in ethanol-water mixed solution, the volume ratio of ethanol and water is 1: 0.25~1:4. 4 . A method for preparing silicon-carbon composite materials through molten salt-assisted magnesia thermal reduction according to claim 1 , wherein in step (1), the alginate is potassium alginate or sodium alginate. 5. A method for preparing silicon-carbon composite materials by molten salt-assisted magnesia thermal reduction as claimed in claim 1, characterized in that: in step (1), the mass ratio of alginate to attapulgite is 1:1 ~1:5. 6. A method for preparing silicon-carbon composite materials by molten salt-assisted magnesia thermal reduction as claimed in claim 1, characterized in that: in step (2), the amorphous carbon coating coated attapulgite composite material and reduction The mass ratio of magnesium powder is 1:1~1:5. 7. A method for preparing silicon-carbon composite materials assisted by molten salt magnesia thermal reduction as claimed in claim 1, characterized in that: in step (2), the molten salt is sodium chloride or potassium chloride, amorphous carbon The mass ratio of coating-coated attapulgite composite material to molten salt is 1:10~1:30. 8. the application of the silicon-carbon composite material prepared by the method as claimed in claim 1 in lithium ion battery negative electrode material.

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