Decoupling Electronic and Ionic Conductivities of Electrochemically Active LiCoO₂ as Solid‐State Electrolyte for Li Metal Batteries

Solid‐state electrolytes (SSEs) are pivotal to advancing lithium metal batteries (LMBs). Electrochemically active LiCoO 2 (LCO) has high ionic conductivity (1.45 × 10 −3 S cm −1 at 35 °C), but its inherent electronic conductivity limits its application as an SSE. To address this limitation, a sandwich‐structured composite solid electrolyte (SSCSE) is presented by integrating an LCO layer between non‐electron‐conducting porous polypropylene (PP). This design decouples the electronic and ionic conductivities of LCO, enabling LCO to be an SSE for LMBs for the first time. Meanwhile, the intermediate fast‐ion conductor layer homogenizes Li⁺ flux and enhances mechanical strength, suppressing lithium dendrites growth and improving safety. Li||Li symmetric cell and Li||LFP full cell using LCO as the SSE achieve excellent performance (800 h at 0.5 mA cm −2 and 0.5 mAh cm −2 ; 93.6% capacity retention after 800 cycles with 99.8% coulombic efficiency). This breakthrough in utilizing LCO with high ionic conductivity as an SSE promotes the development of safer and higher energy‐density batteries.