Study On The Preparation And Electrochemical Performances Of Porous Silicon And Silicon-based Composite Materials

With the development of electric vehicles in recent years,higher requirements are put forward for the energy density and power density of lithium-ion batteries.The cathode and anode materials are the key to determining the battery capacity.Due to the abundant silicon resources,mature preparation technology and high theoretical specific capacity(4200m Ah g^-1),silicon is most promising to replace graphite anode and become the next generation of commercial anode materials.However,the silicon-based anode produces large volume change（～300%）in the process of lithiation/delithiation,which makes the electrode material easily broken and pulverized,resulting in continuous capacity decay.Due to the low conductivity,these shortcomings greatly hinder the commercialization of silicon-based anodes.In response to the problems of silicon-based anodes,a series of porous Si-based composites were prepared by combining porous structure design and carbon-coated method to alleviate the volume expansion during lithiation/delithiation processes,enhance the ion/electron transportation rate of the material and improve the electrochemical properties of silicon-based anode materials.The main conclusions are as follows.（1）Porous silicon materials with abundant macropores were successfully synthesized by ball milling and magnesiothermic reduction using cheap and readily available silica gel powder as raw material.A specific charge capacity of 2108.0 m Ah g^-1（capacity retention of 72.1%）was maintained after 100 cycles at a current density of 0.2 A g^-1.Then the magnesiothermic reduction products were coated with carbon by an improved carbon-coated method to obtain a porous and unique flower-like structure silicon-carbon composite with a specific surface area of 339.9m²/g.A specific charge capacity of 943.2 m Ah g^-1（capacity retention of75.4%）was maintained after 100 cycles at a current density of 0.2 A g^-1.The porous structure can well relieve the volume expansion problem of the silicon-based anode,shorten the diffusion distance of Li⁺,and improve the electrochemical performance of the material greatly.（2）In order to solve the problem that the carbon layer is easily cracked and fell off when inhibiting the volume expansion of the silicon substrate,due to the loose connection between the silicon and carbon composite materials,a silicon carbide layer is introduced in situ between silicon and carbon.The porous Si/Si C/C composite materials with good electrochemical performance can be obtained by changing the experimental procedure,firstly directly introducing a carbon source in the ball milling process,and then adjusting the carbon source ratio,the reducing agent ratio,the magnesium thermal reduction reaction temperature and time.The specific surface area of the porous Si/Si C/C composite is 390.7 m²/g.A specific charge capacity of 601.0 m Ah g^-1（capacity retention of 87.9%）was maintained after 100 cycles at a current density of 0.2 A g^-1,and a specific charge capacity of 491.9 m Ah g^-1 was maintained after 500 cycles at a current density of 1 A g^-1.Due to the good connection of the silicon carbide layer and the porous structure,the composite material can keep the structure from being broken in the cycle process,and thus the transmission distance of Li⁺is shortened and the ion/electron diffusion rate of the material is increased.Finally,it is verified that the presence of amorphous carbon materials can catalyze the side reaction between silicon and electrolyte,and the silicon carbide layer can inhibit the side reaction to a certain extent because of its compact characteristics.