Ultrathin Yet Robust Quasi‐Solid‐State Electrolyte with Ion‐Selective Channel for Superior Alkali‐Metal Batteries

Abstract Solid‐state alkali‐metal batteries experience dendrite growth require a robust solid‐electrolyte interphase (SEI) and high deformation resistance of electrolytes to stabilize the electrolyte/metal interface. However, the traditional dual‐ion polymer electrolyte with large thickness has a low ion transference number and insufficient mechanical stability, leading to high ion concentration gradients and a brittle SEI that fails to suppress the dendrite's growth. Herein, an ultrathin yet robust single‐ion conducting quasi‐solid‐state polymer electrolyte is presented by copolymerizing sodium (4‐vinylphenyl)sulfonate with poly(vinylidene fluoride‐co‐hexafluoropropylene) (abbreviated as S‐PVSA) through atom transfer radical polymerization. The developed S‐PVSA gel polymer electrolyte exhibits a high ion transference number (up to 0.95) and achieves a tensile strength of up to 45.3 MPa even at ultrathin thickness (11 µm). When assembled in alkali metal batteries, the Na||Na cell demonstrates exceptional stability with over 8000 h of cycling (>11 months), and the Na||Na₃V₂(PO₄)₃ cell retains 97% of its initial capacity after 1400 cycles. The Li||Li cell enables long‐term reversibility (over 1200 h) and stable lithium plating/stripping at a current density of 1 mA cm⁻² at 60 °C. This work suggests new potential in polymer electrolyte engineering to rejuvenate alkali‐metal batteries.