Networked Solid Polymer Electrolyte Enabling 5C Fast Cycling and Enhanced Stability of Oriented LiCoO ₂ All‐Solid‐State Thin Film Batteries

ABSTRACT Polyethylene oxide (PEO)‐based solid polymer electrolytes (SPEs) have emerged as promising candidates for advancing the all‐solid‐state lithium metal batteries (ASSLMBs) market. However, conventional PEO‐based SPEs exhibit low room‐temperature (RT) ionic conductivity and limited oxidative stability (restricted to ∼4.2 V), which impedes their compatibility with high‐voltage cathodes and diminishes the achievable energy density of ASSLMBs. Here, we report the development of polymer‐in‐salt PEO‐based networked solid polymer electrolyte (NSPE) that enables stable fast cycling at 5 C and extended electrochemical stability up to 4.5 V, demonstrated using oriented LiCoO 2 (LCO) sputtered film cathodes. This system effectively overcomes the typical irreversibility of LCO cathodes above 4.3 V in conventional liquid electrolytes (LEs), which is often attributed to complex cathode–electrolyte interphase (CEI) formation, structural phase transformations, and cobalt dissolution. Furthermore, we systematically compare the electrochemical performance and interfacial evolution of cells employing conventional electrolytes with those utilizing the NSPE membrane. Our results reveal that a thin and uniform LiF‐rich CEI layer forms at the interface, which facilitates rapid Li + transport between the well‐oriented LCO films and the NSPE. Here, we provide innovative mechanistic insights into interfacial interactions between layered oxide cathodes and SPEs and offer substantial potential to accelerate the development of next‐generation energy storage systems.