The Dual Fine Fiber Separator Simultaneously Optimizes Ion Transport Kinetics and Interfacial Stability in Sodium Metal Batteries

ABSTRACT Sodium metal batteries hold great promise for energy storage applications, but are constrained by limited charge transfer kinetics and unstable solid electrolyte interphases (SEI). Here, we design a dual fine fiber separator to simultaneously enhance the ion transport kinetics and construct a stable SEI. Through synergistic regulation of solvent molecules and anions by dual fine fibers with distinct potential distributions, solvent‐separated ion pairs with small solvation shells become the dominant solvation structure, which significantly enhances ion transport kinetics. Meanwhile, through the strong dipole interaction of anions originating from the inner Helmholtz layer, the designed separator promotes the formation of a stable, low‐resistance SEI. Finally, the Na||Na cell with the designed separator exhibits excellent reversibility, operating efficiently for over 1800 h, while the Na||Na 3 V 2 (PO 4 ) 3 pouch cell (228.6 Wh kg −1 ) with the designed separator exhibits a Coulombic efficiency exceeding 99.7% under conditions of limited sodium excess and lean electrolyte.