Ceramic–Salt Composite Electrolytes from Cold Sintering

The development of solid electrolytes with the combination of high ionic conductivity, electrochemical stability, and resistance to Li dendrites continues to be a challenge. A promising approach is to create inorganic–organic composites, where multiple components provide the needed properties, but the high sintering temperature of materials such as ceramics precludes close integration or co‐sintering. Here, new ceramic–salt composite electrolytes that are cold sintered at 130 °C are demonstrated. As a model system, composites of Li 1.5 Al 0.5 Ge 1.5 (PO 4 ) 3 (LAGP) or Li 1+ x + y Al x Ti 2− x Si y P 3− y O 12 (LATP) with bis(trifluoromethanesulfonyl)imide (LiTFSI) salts are cold sintered. The resulting LAGP–LiTFSI and LATP–LiTFSI composites exhibit high relative densities of about 90% and ionic conductivities in excess of 10 −4 S cm −1 at 20 °C, which are comparable with the values obtained from LAGP and LATP sintered above 800 °C. It is also demonstrated that cold sintered LAGP–LiTFSI is electrochemically stable in Li symmetric cells over 1800 h at 0.2 mAh cm −2 . Cold sintering provides a new approach for bridging the gap in processing temperatures of different materials, thereby enabling high‐performance composites for electrochemical systems.