A Stretchable and Safe Polymer Electrolyte with a Protecting‐Layer Strategy for Solid‐State Lithium Metal Batteries

An elastic and safe electrolyte is demanded for flexible batteries. Herein, a stretchable solid electrolyte comprised of crosslinked elastic polymer matrix, poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP), and flameproof triethyl phosphate (TEP) is fabricated, which exhibits ultrahigh elongation of 450%, nonflammability and ionic conductivity above 1 mS cm-1. In addition, in order to improve the interface compatibility between the electrolyte and Li anode and stabilize the solid-electrolyte interphase (SEI), a protecting layer containing poly(ethylene oxide) (PEO) is designed to effectively prevent the anode from reacting with TEP and optimize the chemical composition in SEI, leading to a tougher and more stable SEI on the anode. The LiFePO4/Li cells employing this double-layer electrolyte exhibit an 85.0% capacity retention after 300 cycles at 1 C. Moreover, a flexible battery based on this solid electrolyte is fabricated, which can work in stretched, folded, and twisted conditions. This design of a stretchable double-layer solid electrolyte provides a new concept for safe and flexible solid-state batteries.