Hydrothermal Synthesis And Electrochemical Performances Of Silicon-based Composites

Silicon is one of attractive and promising anode materials for lithium-ion batteries due to its remarkable capacity(4200 mAh/g),more than ten times that of graphite.Besides,silicon,the second most abundant element in the earth’s crust,is environmental-friendly.However,the practical application of silicon-based material in LIBs is restricted because of the large volume change(300-400%)during the charge-discharge process.It may lead to the pulverization of silicon,contact failure between the active material and the current collector as well as the unstable solid electrolyte interphase(SEI)film.Furthermore,the poor intrinsic conductivity of silicon combined with the reduced electrochemical activity results in unsatisfactory charging rate capability.The core-shell structure is considered as an effective strategy to solve the expansion problem of silicon-based anodes.In this paper,the core-shell structured Si@SnO2 and Si@SnO2@C nanocomposite are synthesized by a facile hydrothermal method.The influencing factors in the preparation process of composite materials and the mechanism of capacity fading were systematically investigated using transient absorption spectroscopy,electrochemical and photoelectrochemical measurements.The core-shell structured Si@SnO2 nanocomposite was prepared by hydrothermal method.The effects of reaction time,temperature and concentration of Na2SnO3 on the products of Si@SnO2 composite powder were investigated.The composition and structure of the composite were characterized by XRD,SEM and TEM.The optimal conditions are 165℃ reaction temperature,18 h reaction time and 2.10 g/L Na2SnO3 concentration.The electrochemical properties of the Si@SnO2 composite were examined using halfcell.The results show that SnO2 as the shell layer of Si can effectively improve the rate performance of silicon.When the current density returnes to 100 mA/g after 50 cycles,the reversible capacity recoveres to 584 mAh/g,and the capacity recovery rate reaches 78.6%.Thus,demonstrating its excellent rate performance.This is mainly because SnO2 is converted into Sn after the first discharge,and Sn has excellent conductivity.However,the cycle performance of Si@SnO2 composite powder is poor,and its capacity retention rate is only 6.9%after 300 cycles at a current density of 500 mA/g.This is because SnO2 has a large volume change during charge and discharge,up to 300%.When its directly contact with the electrolyte as a shell can lead to the formation of unstable SEI film,resulting in rapid cell capacity decay.In order to further improve the cycle performance of Si@SnO2 composite,the surface of Si@SnO2 was coated with amorphous carbon to form Si@SnO2@C composite with double shell structure.The effects of carbon inclusion temperature,glucose concentration and heat treatment temperature on Si@SnO2@C composite powders were studied.The optimal conditions are 190℃ reaction temperature,30.0 g/L glucose concentration and 500℃ temperature for heat treatment.The Si@SnO2@C nanocomposite is composed of crystalline Si,crystalline SnO2 and amorphous C,and the contents of them are 42.1 wt%,37.8 wt%and 20.1 wt%,respectively.The electrochemical properties of the Si@SnO2@C composite were examined using half-cell.The Si@SnO2@C electrode exhibits an extremely high initial discharge capacity of 2777 mAh/g at 100 mA/g and an excellent rate capability of 340 mAh/g at 1500 mA/g.The good capacity retention is 50.2%after 300 cycles over a potential of 0.01 to 2.00 V(vs.Li/Li+)at 500 mA/g.This is mainly because in the double-layer core-shell structure with silicon as the core,SnO2 as the second outer layer,and carbon as the outermost layer,the nano-core silicon helps to increase its specific capacity while inhibiting the volume expansion of silicon.The sub-outer SnO2 will form a conductive network,which effectively improves the conductivity of the composite powder.The outermost carbon acts as the outermost shell,which effectively avoids the direct contact between silicon,SnO2 and electrolyte,and helps to stabilize the formation of SEI film.