Preparation Of Si/SiO_x/C Anode Material By Micro-silica Fume Purification And Magnesium Thermal Reduction And Its Lithium Storage Performance

With the depletion of fossil fuels and serious environmental pollution,the search for new clean alternative energy has become the focus of human attention.Photovoltaics and lithium-ion batteries as the latest energy storage devices,have been playing an important role in responding to the energy crisis.In particular,lithium-ion batteries have been widely used in electronic equipment and electric vehicles(EV).Their large-scale applications have put forward higher requirements for lithium-ion battery electrode materials,especially anode materials.At present,the theoretical specific capacity of graphite anodes on the market is low,and it is difficult to meet human needs for high energy storage equipment.Silicon,as the negative material with the largest lithium storage capacity at present,has abundant resources,good safety,and is expected to become a promising candidate anode replacement material for the next-generation.However,the large volume expansion(300%)of silicon electrodes during the lithiation process and poor electrical conductivity,which has become an obstacle to its commercial development.And traditional silicon materials are prepared by the Siemens method.The preparation process is complex,energy consumption is high,large pollution,and high cost.Although some researchers have also prepared crystalline silicon through the magnesiothermic reduction routes,but its conversion yield is still unsatisfactory.In order to solve the above problems,in this paper using the by-product of silicon metallurgical industry silicon fume(SF,containing SiO₂>85wt%)as raw material to the preparation of crystalline silicon.After roasting and acid etching pretreatment of silica fume,a ball-milling mixing of SF and magnesium powder,then via magnesiothermic reduction reaction(MRR)combined acid etching routes preparation of porous crystalline silicon.Encapsulation of SiO_x and C layer on porous Si surface by the sol-gel method to prepare a Si@SiO_x@C composite material,and used as a Li-ion battery anode.The electrochemical tests results show that the structure can effectively alleviate the volume expansion of silicon anode and construct an efficient conductive network to improve its conductivity,and significantly improves the electrochemical performance of the silicon anode.Successfully achieved the transformation of industrial by-product silica fume to Li-ion batteries electrode material.The main results of this thesis are as follows:(1)Pretreatment of silica fume.The SF,by-product of the silicon metallurgical industry,is decarbonization treatment by roasting at 700°C for 4 hours.The amorphous characteristics of the SF can be maintained,and the removal rate of impurity carbon can reach 97.1%.Then the calcined SF further was leached initially in 10wt%HCl at 60 and 90°C for 1 and 4 hours,to obtain a SiO₂ with a purity of98.05%.(2)Preparation of crystalline Si by MRR combined acid etching routes from silica fume.By investigating the heat treatment temperature,holding time,and Mg/SiO₂mass ratio of the MRR,it is found that:The reduction reaction processes are two independent steps in which Mg and SiO₂ generate Mg₂Si,and then the Mg₂Si reduces SiO₂to Si.The phase analysis of XRD and the results of XRF show that the best reaction effect can be achieved,under the conditions of a mass ratio of Mg/SiO₂=0.85,heat treatment temperature of 700°C,and a holding time of 5h.The reactants are formed into Mg@SiO₂ core-shell structure by ball milling,and reduction with a"wrap-microreactor"model,which can achieve a reduction degree of up to 96%.The final prepared product of crystalline silicon achieves a purity of 99.88wt%after two acid-etching.(3)Preparation of silicon-based electrode materials.Encapsulation of SiO_x and C layer on porous Si surface by the sol-gel method to prepare a Si@SiO_x@C composite material,the volume change of silicon during cycling was effectively buffered and the electronic conductivity was enhanced.Greatly improved the electrochemical cycle and the rate performance of the silicon anode and expand the application.This simple and easy preparation method has improved the industrial application potential of silicon-based anode materials.At the same time,it also achieved a high value-added utilization of SF.