Zwitterionic Covalent Organic Framework Solid Electrolyte with Ordered Ionic Channels for All-Solid-State Lithium-Metal Batteries

Organic solid electrolytes are considered a promising way to enhance the energy density of Li rechargeable batteries. However, practical applications of organic solid electrolytes have suffered from extremely low ionic conductivity at room-temperature due to strong ion pairs of Li salts and lack of ordered ionic channels for Li+ diffusion. Covalent organic frameworks (COFs) with well-defined chemical and pore structures can be considered excellent candidates for solid electrolytes due to their uniform channels for the migration of ionic species. Herein, the zwitterionic covalent organic framework (Zwitt-COF) was developed as a solid electrolyte with not only the superior ability to dissociate strong Li-ion pairs but also well-organized ionic channels for fast Li+ diffusion. Theoretical simulations revealed that zwitterion can effectively dissociate the strong Li-ion pairs, and the linear hexagonal ion channels can be reopened in the Zwitt-COF solid electrolyte by reconstructing AA stacking structures due to the dissociative adsorption of Li+ on Zwitt-COF. The Zwitt-COF electrolyte exhibited high ionic conductivity at room-temperature and stable Li plating/stripping performance without formation of Li dendrite and dead Li. Furthermore, the Zwitt-COF electrolyte displayed a wide electrochemical stability window and outstanding thermal stability. All-solid-state Li metal full cells prepared with a Zwitt-COF electrolyte exhibited excellent cyclic performance for a long duration with high retention of discharge capacity. The strategy for incorporating zwitterions into a COF structure can provide an effective way to develop various all-solid-state batteries.