Highly Plasticized Composite Solid Electrolyte via In Situ Polymerization for High‐Performance Solid‐State Lithium Metal Batteries

Solid‐state lithium metal batteries hold great promise for high‐energy‐density storage, but their practical application is hindered by the low ionic conductivity, limited Li + transference number, and unstable electrode/electrolyte interfaces inherent to composite solid electrolytes. Here, we report a highly plasticized composite solid electrolyte (denoted as PVLS) fabricated via the in situ polymerization of vinylene carbonate (PVCA), integrating succinonitrile (SN)‐LiTFSI plasticizer, and Li 6.4 La 3 Zr 1.4 Ta 0.6 O 12 (LLZTO) nanoparticles. LLZTO surface Lewis acidity, together with cooperative SN‐PVCA solvation, promotes salt dissociation and diversified Li + coordination, while in situ curing ensures intimate interfacial contact. The optimized PVLS exhibits a room‐temperature ionic conductivity of 6.2 × 10 −4 S cm −1 , a high Li + transference number of 0.72, and an expanded oxidative stability window of 4.8 V vs Li + /Li. Spectroscopic analyses reveal enhanced populations of free TFSI ‐ anions and the formation of organic‐rich outer layers with LiF‐rich inner structures in both the CEI and SEI, which effectively regulate Li + flux. As a result, PVLS enables stable Li||Li cycling for over 1000 h and delivers superior full‐cell performance with LiFePO 4 , LiCoO 2 , and LiNi 0.9 Co 0.05 Mn 0.05 O 2 cathodes. This work presents a multiscale strategy for simultaneously balancing ionic conductivity, interfacial stability, and safety in solid‐state batteries.