Preparation And Electrochemical Properties Of Silicon-based Anode Materials By Aluminothermic Reduction Of Sepiolite

With the characteristics of high energy density,excellent cycle performance and low self-discharge rate,lithium-ion battery has been widely used in consumer electronics,electric vehicles,and microgrid energy storage.The theoretical specific capacity of the commonly used graphite anode material is 372m Ah/g,which is difficult to meet people’s increasing capacity demand for the lithium-ion battery.Silicon is the anode material with the most development potential,which has the theoretical capacity as high as 4200m Ah/g.Meanwhile,it has the advantages of abundant reserves and environmental friendliness.However,due to the large volume expansion of silicon materials during the charging and discharging process,the electrochemical performance of the silicon anode rapidly decays,which seriously restricts its large-scale application.The preparation of silicon anode materials with special structures by means of natural minerals can significantly improve their electrochemical performance,which provides a new way for the research of silicon anode materials.In this paper,using natural sepiolite as raw material,silicon-based anode materials were prepared by aluminothermic reduction method,which is of great significance to the research of silicon-based anode materials for lithium-ion batteries and the efficient utilization of mineral resources.The main research contents and results of this paper are as follows:(1)Silicon anode material was prepared through low-temperature aluminum reduction method by using natural sepiolite as raw material and self-template.The analysis shows that the reduction product is silicon crystal,and the prepared silicon anode material has a nanotube structure with a specific surface area of 42.63m~2/g,which is higher than 38.35m~2/g of the sepiolite raw material,and the pores are mainly mesopores.This paper also studies the effect of acid washing on the structure of the reduction products,revealing that the existence of magnesia octahedra is the microscopic mechanism by which the reduction products can inherit the tubular structure of natural sepiolite.The anode material shows good cycle performance when used in the lithium-ion battery.At a current density of 0.5A/g,the initial discharge specific capacity of silicon nanotubes is 3150.2m Ah/g,and it still has a high discharge capacity of 1786.0m Ah/g after 50cycles.When the current density is increased to 2.0A/g,the specific capacity of silicon nanotubes can still be maintained at 997.6m Ah/g after 200 cycles.(2)Silicon carbon composites with core-shell structure were prepared by high-temperature pyrolysis with silicon material prepared by low-temperature aluminothermic reduction of natural sepiolite as raw material and asphalt as carbon precursor.The analysis shows that the surface of silicon-based material is successfully coated with uniform amorphous carbon material.With the increase of carbon content,the amorphous carbon coating becomes thicker and the surface tubular structure becomes blurred;At the same time,the specific surface area decreased from 37.3 m~2/g when the carbon content was 5%to 29.5 m~2/g when the carbon content was 20%.Silicon carbon composites with core-shell structure show excellent electrochemical properties.With the increase of carbon coating amount,the initial capacity of silicon carbon composites decreases,but the capacity retention rate increases gradually from 46.1%when the carbon content is 5%to 73.3%when the carbon content is 20%.With the increase of carbon content,the residual capacity of silicon carbon composites after 500 cycles first increases and then decreases.When the carbon content is 15%,the maximum residual capacity of silicon carbon composites is 1072.4m Ah/g.In addition,the rate performance of silicon carbon composites is also enhanced compared with the silicon anode material obtained by aluminothermic reduction of sepiolite.(3)Silicon carbon composites with three-dimensional skeleton support structure were prepared by high-temperature pyrolysis with nano tubular silicon material and carbon nanotubes as raw materials and asphalt as carbon precursor.The results show that most of the carbon nanotubes are dispersed on the surface of the nano tubular silicon material and coated inside the composite by the carbon layer,and only a small amount of carbon nanotubes appear on the surface through the carbon layer.Finally,the carbon nanotubes closely connect the silicon nanotubes and the carbon layer to form a three-dimensional skeleton support structure.In this paper,the silicon carbon composite with core-shell structure is directly mixed with carbon nanotubes as the anode material.The analysis shows that the carbon nanotubes in the composite are distributed outside the carbon coating and do not form a three-dimensional skeleton support structure.Finally,the silicon carbon composite with three-dimensional skeleton support structure shows the best electrochemical performance.At the current density of 2A/g,the initial capacity of the composite is 2162.8 m Ah/g.Finally,after 500 cycles,the composite shows a residual capacity of 1227.9m Ah/g,and the average capacity attenuation rate in the process of 500 cycles is only 0.05%.In addition,compared with the nano tubular silicon anode material and core-shell silicon carbon composite obtained by reducing sepiolite,silicon carbon composites with three-dimensional skeleton support structure show the smallest charge transfer impedance and the largest lithium ion diffusion coefficient,reflecting excellent electrochemical kinetic characteristics.The paper has 64 figures,8 tables,154 references.