Transformative Effect of Li Salt for Proactively Mitigating Interfacial Side Reactions in Sodium-Ion Batteries

• LiPF6 integration into sodium-ion battery electrolytes strengthens solid electrolyte interphase (SEI) film and stabilizes O3 electrode surfaces, enhancing cycleability with 92.7% at 400 cycles. • Li-based SEI exhibits reduced solubility, effectively suppressing sodium-ion and electron leakage, and mitigating electrolyte decomposition on hard carbon electrode. • The formation of Li-ion pillars on O3-type electrode surfaces significantly reduces oxygen release and electrolyte degradation, resulting in improved capacity retention. • Compared with conventional fluoroethylene carbonate additive, the dual-interface strengthening approach yields superior capacity retention and long-term stability.