Structure-property relationships and application of molecular crystal solid electrolytes for next-generation metal-ion batteries

The development of safe, high-performance batteries requires solid electrolytes that combine high ionic conductivity, thermal stability, and interfacial compatibility. Conventional liquid electrolytes are often flammable and pose safety risks, while existing solid electrolytes face limitations in ion transport, mechanical robustness, and electrode integration. This dissertation addresses these challenges through the molecular and structural engineering of crystal-based and composite solid electrolytes for Li- and Na-ion batteries.This research establishes fundamental structure–property relationships governing ion transport in molecular solid electrolytes. By leveraging hard–soft acid–base (HSAB) principles, unfavorable hard–soft interactions, such as Li⁺–nitrile coordination, were shown to enhance lithium mobility. Systematic studies of dinitrile–LiPF₆ systems revealed that shorter Li–Li hopping distances and restricted anion mobility increase Li⁺ transference numbers. A Li–(CN)₆ coordination complex was observed for the first time, demonstrating how ligand geometry can regulate Li⁺ transport. Grain boundary engineering was introduced as a strategy to create rapid ion-conducting pathways. In adiponitrile–LiI crystals, disordered grain boundaries enriched with solvated ions transformed the material from an anion conductor to a high-performance Li⁺ conductor with conductivities up to 10⁻³ S cm⁻¹ and stable lithium redox behavior. Molecular crystal–polymer composite electrolytes were developed to overcome interfacial resistance, a major limitation in flexible solid-state battery films. By designing seamless ionic contact between crystal and polymer phases, Li⁺ migration across interfaces was significantly enhanced, enabling scalable, mechanically robust, and processable solid electrolytes. Temperature-driven phase transitions in co-crystal electrolytes were employed to enhance ionic transport. Controlled lattice reorganization at elevated temperatures generates highly ordered Li⁺ channels, increasing transference numbers and enabling safe operation near 100 °C, where conventional liquid electrolytes fail. Silver-doped molecular crystal electrolytes were developed to promote uniform lithium plating and suppress dendrite growth. In situ Li–Ag alloy formation at the electrode–electrolyte interface, combined with dual-ion transport, improves ionic conductivity and interfacial stability, establishing a scalable strategy for dendrite-free lithium metal batteries. Collectively, these studies provide a comprehensive framework linking coordination chemistry, crystal structure, grain boundary engineering, and interfacial design to ionic transport, offering safe, high-conductivity, and thermally robust solid electrolytes for next-generation Li- and Na-ion batteries.