Enabling High‐Voltage “Superconcentrated Ionogel‐in‐Ceramic” Hybrid Electrolyte with Ultrahigh Ionic Conductivity and Single Li + ‐Ion Transference Number

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 7 Li 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||LiNi 0.5 Co 0.2 Mn 0.3 O 2 and Li||LiFePO 4 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.