Harvesting Si Nanostructures and C–Si Composites by Paired Electrolysis in Molten Salt: Implications for Lithium Storage

Electrolysis of SiO2 to produce Si in MgCl2-based molten salt faces the challenge of generating chlorine gas at a graphite anode. Herein, we replace the graphite anode with a Mg2Si alloy anode that is paired with a C–SiO2 cathode in molten NaCl–KCl–MgCl2. During the paired electrolysis, the Mg2Si anode releases two Mg2+ ions into the electrolyte, resulting in Si nanostructures, while the C–SiO2 cathode is reduced to C–Si composites without generating SiC. Thus, both electrode reactions obtain desired products, which eliminates chlorine gas evolution and tailors the structure and composition of Si. In addition, the electrolytic Si nanostructures and C–Si composites show enhanced lithium-storage performances. The anodic Si nanostructures display a high initial Coulombic efficiency of 82.7% and achieve a cycling capacity of 1490 mAh g–1 300 cycles later under 2 A g–1. The cathodic C–Si composite that remains inside the carbon matrix derived from rice husks maintains a stable capacity of 780 mAh g–1 and 92.1% capacity retention after 200 cycles at 0.5 A g–1. In addition to Si-based anode materials, this study offers an economical and green paired electrolysis approach to preparing various functional materials.