An Ultrathin Nonporous Polymer Separator Regulates Na Transfer Toward Dendrite‐Free Sodium Storage Batteries

Sodium storage batteries are one of the ever-increasing next-generation large-scale energy storage systems owing to the abundant resources and low cost. However, their viability is severely hampered by dendrite-related hazards on anodes. Herein, a novel ultrathin (8 µm) exterior-nonporous separator composed of honeycomb-structured fibers is prepared for homogeneous Na deposition and suppressed dendrite penetration. The unhindered ion transmission greatly benefits from honeycomb-structured fibers with huge electrolyte uptake (376.7%) and the polymer's inherent transport ability. Additionally, polar polymer chains consisting of polyethersulfone and polyvinylidene customize the highly aggregated solvation structure of electrolytes via substantial solvent immobilization, facilitating ion-conductivity-enhanced inorganic-rich solid-electrolyte interphase with remarkable interface endurance. With the reliable mechanical strength of the separator, the assembled sodium-ion full cell delivers significantly improved energy density and high safety, enabling stable operation under cutting and rolling. The as-prepared separator can further be generalized to lithium-based batteries for which apparent dendrite inhibition and cyclability are accessible and demonstrates its potential for practical application.