Synergistic Gradient Interphase Engineering for Ultra‐Stable and High‐Rate Quasi‐Solid‐State Sodium Metal Batteries

ABSTRACT Quasi‐solid‐state sodium metal batteries (QSSMBs) have emerged as a pivotal technology for safe, high‐energy‐density, and large‐scale energy storage. However, the practical application of QSSMBs is severely hindered by the formation of an unstable and high‐impedance solid electrolyte interphase (SEI) that fails to accommodate the dramatic volume fluctuations of sodium (Na) metal anodes, consequently leading to rapid capacity decay during high‐rate cycling. Herein, we propose a rational design strategy to construct a rigid‐inner/flexible‐outer gradient SEI architecture by modulating the stepwise reduction behavior of functional additives within a poly(1,3‐dioxolane) electrolyte. By introducing 2,2,2‐trifluoroethyl trifluoromethanesulfonate to undergo synergistic decomposition with fluoroethylene carbonate, this formulation enables the in situ construction of the dual‐layer gradient interphase. This unique SEI efficiently facilitates desolvation and accelerates Na + transport, while achieving even Na plating after cycling. Consequently, the designed electrolyte delivers outstanding electrochemical performance, enabling Na || sodium vanadium phosphate full cells to exhibit long cycle‐life of 5500 cycles at 20 C and 97.5% capacity retention after 1000 cycles at 25 C. This work highlights the critical role of gradient SEI engineering and offers an insightful design principle for developing next‐generation QSSMBs.