Li2O‐Enhanced Solid Electrolyte Interphase Surpassing LiF‐Only SEI for High‐Performance All‐Solid‐State Li Batteries

Solid‐state lithium batteries face critical challenges in achieving stable electrode‐electrolyte interfaces, where the formation characteristics and architectural properties of the solid electrolyte interphase (SEI) critically influence battery performance. While LiF‐rich SEI layers have been widely studied for their ability to enhance interfacial stability, the contribution of Li2O—a key component in improving ionic conductivity and mechanical robustness—has been largely overlooked. This work tackles this deficiency by developing a cellulose acetate (CA)‐modified electrolyte system, which facilitates the cooperative generation of LiF and Li2O within the SEI layer. Consequently, the CA‐modified poly(ethylene oxide) (PEO)‐based electrolyte enabled exceptional electrochemical stability, ensuring reliable performance under elevated voltages (reaching 4.3 V) and across a wide temperature range (−10 °C–60 °C). Such improvements are ascribed to the synergistic LiF‐Li2O composite SEI layer, which enhances interfacial ion transport and mechanical stability. Furthermore, the scalability of this approach was demonstrated in practical pouch cells, which maintained a discharge capacity of 132 mAh g−1 over 300 cycles at 0.1 C, exhibiting an average Coulombic efficiency of 99.79%. This work highlights the critical role of Li2O in complementing LiF‐dominated SEI layers, offering a promising pathway toward the advancement of high‐efficiency all‐solid‐state energy storage systems.