In Situ Polymerized Composite Electrolytes for High‐Performance Solid‐State Lithium Batteries: A Review

ABSTRACT Solid‐state electrolytes offer improved safety compared to liquid counterparts but suffer from inherent limitations when used as single‐component systems. Composite electrolytes, combining complementary materials, provide a promising route to overcome these issues. Among fabrication techniques, in situ polymerization stands out for its ability to construct intimately integrated composite polymer electrolytes with enhanced interfacial stability and tunable architectures. However, a systematic and comprehensive review of recent advances in in situ polymerized composite polymer electrolytes (CPEs) is still lacking. In this work, recent developments in in situ polymerized CPEs are systematically reviewed and categorized based on different polymer matrices and their specific interactions with various inorganic fillers. The fundamental reaction mechanisms governing in situ polymerization are discussed, with particular attention to the coupling between polymerization kinetics, interfacial structure evolution, and ion‐transport pathway reconstruction. Building on this mechanistic perspective, representative polymer–filler systems are analyzed to illustrate how organic–inorganic interactions regulate ionic conductivity, mechanical integrity, and electrochemical stability. Key challenges in current systems are critically assessed, and finally, future perspectives, guidelines, and strategies toward the resolution of these challenges are analyzed and discussed.