Active Interphase Engineering via In Situ Polymerization of Solid‐State Polymer Electrolytes

ABSTRACT The rapid growth of electric vehicles and portable electronics has intensified the demand for high‐energy‐density batteries. Accordingly, the development of advanced battery systems with high performance and broad compatibility has emerged as a dominant research direction in energy storage. The combination of solid‐state polymer electrolytes (SPEs) and lithium (Li) metal anodes is considered a core and effective strategy to realize high‐energy‐density and high‐safety batteries. However, issues such as poor solid–solid interface contact between the solid‐state electrolyte and electrode and uncontrollable interfacial side reactions have long been the critical bottlenecks restricting their performance and practical application. This review systematically revisits the evolution of electrolyte fabrication strategies from conventional ex situ preparation to in situ polymerization and reveals the technological progression from simple electrolyte fabrication toward integrated interphase engineering for high‐performance solid‐state batteries. In particular, we focus on the engineering of the interphase formed during the polymerization, revealing the conceptual shift and scientific connotations from suppressing side reactions to rationally utilizing them. Finally, we present perspectives on the essential challenges and development opportunities of in situ polymerization toward practical solid‐state batteries.