Room Temperature Halide‐Eutectic Solid Electrolytes with Viscous Feature and Ultrahigh Ionic Conductivity

A viscous feature is beneficial for a solid electrolyte with respect to assembling solid‐state batteries, which can change the solid‐solid contacts from point to face. Here, novel halide‐based deep eutectic solid electrolytes (DESEs) prepared by a facile ball milling method is reported. The mixture of halides triggers the deep eutectic phenomena by intermolecular interactions, leading to diverse morphologies and viscous statuses in terms of composition. Chemical‐ and micro‐structure analyses via the cryogenic technique reveal that the LiCl and LiF nanoparticles are dispersed in an amorphous halide matrix, which endow freely mobile ions for fast ion transport. The optimized DESE thus achieves low activation energy and high ionic conductivity of 16 mS cm(−1) at room temperature, one of the highest values among various electrolytes so far. By integrating with the active materials to form a composite cathode, the viscous DESE yields a super‐dense composite pellet which possesses intensively enhanced ionic conductivity in contrast to those formed by the sulfide‐based electrolyte additives, demonstrating an attractive application prospect.