Suppressing Metastable Phase in Si Alloy Anodes via Controlled Grain Design

Due to the enormous volume variation and rapid structure degradation of high-capacity microscale silicon (Si) anode particles upon cycling, it is preferable to utilize the class of particles with nano-sized Si grains in the robust matrixes (e.g., carbon, alloy). However, it is still crucial to control nano-Si grains to prevent the detrimental effect from the reaction metastable c-Li_3.75Si phase generated during the full lithiation of larger Si nanograins. Herein, we present a scalable grain-controlled (∼60 nm) Si alloy anode synthesized by equilibrium path, namely, recrystallization of the amorphous alloy at a specific temperature synthesized via melt-spinning technique. The optimized nanograins played a vital role in controlling the reaction phases by mitigating the metastable c-Li_3.75Si phase. Furthermore, fine-grained porous structure is obtained by acidic etching. The porous nature not only acts as a criterion to improve the kinetic parameters but acidic etching also increases capacity by removing inactive intermetallic phases, thereby enriching the active Si content. Consequently, the fine-grained porous Si alloy anode provides a stable cycling performance (reaching ∼99.9% Coulombic efficiency over 100 cycles). In addition, differential capacity (dQ/dV) vs. voltage profiles were served as a quantitative indicator of capacity fading upon cycling for the c-Li_3.75Si reaction phase. This research provides important insights into the rational design of Si alloy anodes based on grain regulation and broadens research strategies for other Si-based anode materials.