In Situ Polymerization on a 3D Ceramic Framework of Composite Solid Electrolytes for Room‐Temperature Solid‐State Batteries

Solid‐state batteries (SSBs) are ideal candidates for next‐generation high‐energy‐density batteries in the Battery of Things era. Unfortunately, SSB application is limited by their poor ionic conductivity and electrode‐electrolyte interfacial compatibility. Herein, in situ composite solid electrolytes (CSEs) are fabricated by infusing vinyl ethylene carbonate monomer into a 3D ceramic framework to address these challenges. The unique and integrated structure of CSEs generates inorganic, polymer, and continuous inorganic–polymer interphase pathways that accelerate ion transportation, as revealed by solid‐state nuclear magnetic resonance (SSNMR) analysis. In addition, the mechanism and activation energy of Li(+) transportation are studied and visualized by performing density functional theory calculations. Furthermore, the monomer solution can penetrate and polymerize in situ to form an excellent ionic conductor network inside the cathode structure. This concept is successfully applied to both solid‐state lithium and sodium batteries. The Li|CSE|LiNi(0.8)Co(0.1)Mn(0.1)O(2) cell fabricated herein delivers a specific discharge capacity of 118.8 mAh g(−1) after 230 cycles at 0.5 C and 30 °C. Meanwhile, the Na|CSE|Na(3)Mg(0.05)V(1.95)(PO(4))(3)@C cell fabricated herein maintains its cycling stability over 3000 cycles at 2 C and 30 °C with zero‐fading. The proposed integrated strategy provides a new perspective for designing fast ionic conductor electrolytes to boost high‐energy solid‐state batteries.