Breaking the Trade‐Off between Ionic Conductivity and Mechanical Strength in Solid Polymer Electrolytes for High‐Performance Solid Lithium Batteries

Abstract Solid polymer electrolytes (SPEs) are among the most promising candidates for solid‐state batteries due to their easy processibility, interface compatibility, and cost efficiency. However, the trade‐off between the ionic conductivity and mechanical strength of SPEs, which has persisted for decades, hinders their application in solid‐state lithium (Li) metal batteries. In this study, the aim is to break this trade‐off by utilizing poly(p‐phenylene benzobisoxazole) (PBO) nanofibers as a mechanically strong backbone and polyethylene oxide (PEO) as an ionically conductive network. The PBO/PEO composite electrolyte reduces the crystallinity of PEO while increasing the mechanical strength (74.4 MPa, ≈14 times that of PEO). Thus, PBO/PEO simultaneously improves ionic conductivity and mechanical strength both at room temperature and elevated temperatures, enabling uniform and smooth Li deposition. Thus, a long cycle life of solid‐state Li symmetric cells for 1000 h at 60 °C is achieved, and stable cycling of solid‐state Li metal full batteries at 60 °C and even 100 °C. Furthermore, the solid‐state pouch cell using this SPE exhibits excellent performance reliably after bending. The study clearly indicates that simultaneously improving mechanical properties and conductivity is the indispensable path to the practical application of solid‐state electrolytes.