Swelling-Controlled Double-Layered SiO_x/Mg₂SiO₄/SiO_x Composite with Enhanced Initial Coulombic Efficiency for Lithium-Ion Battery

Si-based anode materials are considered as potential materials for high-energy lithium-ion batteries (LIBs) with the advantages of high specific capacities and low operating voltages. However, significant initial capacity loss and large volume variations during cycles are the primary restrictions for the practical application of Si-based anodes. Herein, we propose an affordable and scalable synthesis of double-layered SiO_x/Mg₂SiO₄/SiO_x composites through the magnesiothermic reduction of micro-sized SiO with Mg metal powder at 750 °C for 2 h. The distinctive morphology and microstructure of the double-layered SiO_x/Mg₂SiO₄/SiO_x composite are beneficial as they remarkably improve the reversibility in the first cycle and completely suppress the volume variations during cycling. In our material design, the outermost layer with a highly porous SiO_x structure provides abundant active sites by securing a pathway for efficient access to electrons and electrolytes. The inner layer of Mg₂SiO₄ can constrain the large volume expansion to increase the initial Coulombic efficiency (ICE). Owing to these promising structural features, the composite prepared with a 2:1 molar ratio of SiO to Mg exhibited initial charge and discharge capacities of 1826 and 1381 mA h g^-1, respectively, with an ICE of 75.6%. Moreover, it showed a stable cycle performance, maintaining high capacity retention of up to >86.0% even after 300 cycles. The proposed approach provides practical insight into the mass production of advanced anode materials for high-energy LIBs.