Magnesium thermal reduction of phytolith for high-performance three-dimensional hierarchical C/Si@Si nanoparticle@reduced graphene oxide composite anodes

Abstract

Constructing three-dimensional (3D) porous silicon/C is an effective strategy to improve the electrochemical cyclability of silicon (Si) anodes. In this study, a 3D micro-sized Si-/C-containing anode, namely, a C/Si@SiNPs composite with C/Si as the inner core and porous SiNPs as the outer layer, is fabricated via magnesium thermal reduction with phytolith and Mg₂Si as feedstocks. The plant-derived carbon in phytolith can not only significantly improve the electrical conductivity of the inner C/Si, but also provide a substantial core for depositing the porous SiNP outer layer. The porous structure of the SiNP outer layer can effectively accommodate the volume expansion of the Si species. By adopting electrostatic assembly, reduced graphene oxide (rGO) is further wrapped around the surface of C/Si@SiNPs, and C/Si@SiNPs@rGO with better mechanical integrity is successfully fabricated. The resulting C/Si@SiNPs@rGO exhibits a specific capacity of 3064.9 mA h g⁻¹ with a high initial coulombic efficiency of 82.4% at 0.2 A g⁻¹. Even at a high current density of 4 A g⁻¹, a high capacity of 994.5 mA h g⁻¹ can be achieved. The C/Si@SiNPs@rGO composite also has excellent cyclability and can deliver a high capacity of 1526.36 mA h g⁻¹ after 200 cycles at a current density of 1 A g⁻¹ with a high capacity retention rate of 80%, significantly superior to that of C/Si@SiNPs and nanometer-sized silicon (nom-Si) anodes. This study not only offers a novel method for the efficient preparation of silicon-based materials but also paves a promising pathway for the high-value utilization of inexpensive phytolith.