Void layer optimization in constant-composition yolk–shell SiO@C anodes for improved lithium storage performance

Abstract

Yolk–shell structures are effective in mitigating the severe volume expansion of silicon monoxide (SiO)-based anodes during lithiation/delithiation, due to the presence of internal void layers. However, optimization of void layer thickness has often been confounded by changes in material composition, obscuring its intrinsic structural effects on electrochemical performance. In this work, SiO@C yolk–shell anodes with constant composition and varied void layer proportions (1.0, 1.5, 2.0, and 2.5) were prepared via a controllable sol–gel templating and etching route. Electrochemical tests show that a void-to-core volume ratio of 2.0 yields optimal balance between expansion buffering and conductivity, achieving an initial coulombic efficiency of 76.1%, a reversible capacity of 836.77 mAh g⁻¹ after 200 cycles at 1000 mA g⁻¹, and 84.9% capacity retention at 5000 mA g⁻¹. Mechanistic analysis reveals that excessive void thickness, though further reducing electrode expansion, suppresses pseudocapacitive contribution and lowers Li⁺ diffusion coefficient (from 7.75 × 10⁻¹¹ to 1.74 × 10⁻¹¹ cm² s⁻¹), thus impairing rate capability. These results clarify the dual effects of void layer thickness while avoiding composition-induced bias, providing guidance for the structural optimization of yolk–shell Si-based anodes in lithium-ion batteries.