Molten Salt Electrolyte Enables Micro-Sized Silicon Anode in Lithium-Ion Batteries

Micro-sized silicon (mSi) anodes offer high capacity for next-generation lithium-ion batteries but suffer from severe volume changes, causing unstable interphases and poor cycling. Traditional electrolytes derive unstable electrolyte/electrolyte interphases, and flammable solvents pose safety risks. Here, we introduce a non-flammable molten salt electrolyte, which consists of lithium bis(fluorosulfonyl)imide, potassium bis(fluorosulfonyl)amide, and cesium bis(fluorosulfonyl)imide in a mole ratio of 0.3:0.35:0.35 (noted as Li_0.3K_0.35Cs_0.35FSA), that forms an inorganic interphase on mSi, stabilizing the electrode/electrolyte interface. Computational and experimental insights elucidate the FSA⁻ anion decomposition-derived SEI predominantly of LiF, Li₃N, Li₂O, and Li₂S, which exhibits mechanical resilience and low interfacial resistance, effectively accommodating the significant volume expansion of silicon during lithiation/delithiation. As a result, the Li‖mSi half-cell achieves 60.7% capacity retention after 100 cycles with 99.5% average Coulombic efficiency. Overall, the Li_0.3K_0.35Cs_0.35FSA electrolyte eliminates flammability concerns while enabling robust cycling performance. This work demonstrates a safe, high-energy battery system by coupling mSi anodes with stable molten salt electrolytes, addressing both interfacial instability and safety challenges in mSi-based lithium-ion batteries.