Multirole Integrated Filler Design for Polymer‐in‐Salt Electrolytes Enables Long‐Life, Safe Solid‐State Lithium Batteries

ABSTRACT Although polymer‐in‐salt (PIS) electrolyte strategy is widely employed in solid‐state lithium batteries for interface stabilization through ion transport modulation and interphase chemistry tailoring, its application remains plagued by interfacial degradation induced by residual solvent and inherent ionic conductivity‐mechanics trade‐off. Herein, we propose a multirole integrated filler design to simultaneously address these limitations through defect chemistry and interface engineering. CaF 2 filler, as a representative case, synergistically promotes salt dissociation and ion‐conducting sites generation through fluorine vacancy defects while reinforcing the polymer matrix via hydrogen bonding interactions and high bulk modulus. This enables roll‐to‐roll scalable fabrication of ultra‐thin (ca. 8 µm) PIS membrane with exceptional ionic conductivity of 3.32 × 10 −4 S cm −1 at 25°C. Crucially, the in situ generated LiF/Li‐Ca interphase suppresses both residual solvent decomposition and dendrite formation, achieving stable cycling in lithium symmetric cells for nearly 1600 h and high‐capacity retention in full cells with LiFePO 4 or NCM811 cathodes over 1000 cycles. The outstanding thermal stability (melting point: 1418°C) of CaF 2 further boosts the inherent safety of pouch cells under mechanical and thermal abuse conditions. This single‐component filler strategy effectively addresses multiple performance bottlenecks in polymer electrolytes, offering a scalable and cost‐effective pathway for practical solid‐state batteries.