LAUSR

High room‐temperature ionic conductivities, large Li+‐ion transference numbers, and good compatibility with both Li‐metal anodes and high‐voltage cathodes of the solid electrolytes are the essential requirements for practical solid‐state lithium‐metal batteries. Herein, a unique “superconcentrated ionogel‐in‐ceramic” (SIC) electrolyte prepared by an in situ thermally initiated radical polymerization is reported. Solid‐state static 7Li NMR and molecular dynamics simulation reveal the roles of ceramic in Li+ local environments and transport in the SIC electrolyte. The SIC electrolyte not only exhibits an ultrahigh ionic conductivity of 1.33 × 10−3 S cm−1 at 25 °C, but also a Li+‐ion transference number as high as 0.89, together with a low electronic conductivity of 3.14 × 10−10 S cm−1 and a wide electrochemical stability window of 5.5 V versus Li/Li+. Applications of the SIC electrolyte in Li||LiNi0.5Co0.2Mn0.3O2 and Li||LiFePO4 batteries further demonstrate the high rate and long cycle life. This study, therefore, provides a promising hybrid electrolyte for safe and high‐energy lithium‐metal batteries.