Premagnesiated Si-rich SiO _x composite delivers high ICE and reversible capacity. Si/Mg₂SiO₄ composite is synthesized via high-energy mechanical milling followed by MgH₂ treatment. Cycle performance is improved without compromising ICE by forming a Mg₂SiO₄ phase that suppresses irreversible a-SiO₂ reactions and buffers volume changes in the Si-rich structure.

Silicon monoxide (SiO)-based materials have significant potential as high-capacity anode materials for lithium-ion batteries (LIBs). However, the low initial Coulombic efficiency (ICE) associated with the irreversible electrochemical reaction of the amorphous SiO₂ phase (a-SiO₂) in SiO hinders its application in commercial LIBs. The preemptive phase transition of a-SiO₂ to an inactive silicate phase using a metal hydride is a promising strategy for improving the ICE. However, this process inevitably leads to reversible capacity loss. In this study, a high-capacity Si-rich SiO _x composite prepared by high-energy mechanical milling is premagnesiated using MgH₂, resulting in a significantly improved capacity and ICE compared to those of pristine SiO and Si-rich SiO _x composites. The resulting Si/Mg₂SiO₄ composite electrode exhibited a high initial discharge capacity of 1961 mAh g⁻¹ with a high ICE of 87.0% and maintained highly stable capacity retention after 200 cycles compared to the Si-rich SiO _x . These improved electrochemical properties are attributed to the preemptively synthesized Mg₂SiO₄, which not only prevents irreversible reactions between lithium and a-SiO₂ during the initial lithiation but also acts as a buffer phase that effectively reduces volume expansion during cycling.