Synthesis of hollow silicon nanospheres encapsulated with a carbon shell through sol–gel coating of polystyrene nanoparticles

Abstract

In this study a templating method to form hollow Si nanospheres encapsulated with a carbon shell (HSi@C) has been investigated. The key synthesis parameters and their optimization are studied. The positively charged surface of sacrificial polystyrene (PS) nanospheres is found to be essential in forming a uniform SiO2 shell on PS through sol–gel reactions of tetraethylorthosilicate in a basic medium. Both the sol–gel reaction time and hydrolysis ratio play an important role in controlling the thickness of the SiO2 shell. The PS core is burnt via a step-wise programmed heating to prevent the formation of cracks in hollow SiO2 nanospheres which are subsequently converted to hollow Si nanospheres through magnesiothermic reduction using MgH2. Removal of the byproducts from the reduction is critical for superior electrochemical performance. The hollow Si nanospheres are coated with a carbon shell to form HSi@C which are evaluated as the active material for Li-ion battery anodes. Electrochemical results indicate significant improvements in the specific capacity and cycling stability of half-cells in comparison to other designs. The improvement is attributed to the synergistic effects of the engineered void and conductive carbon shell in HSi@C. Specific capacity and Coulombic efficiency of HSi@C half cells vs. cycle number at different current densities.