Suppressing Organic Cation Reactivity in Locally Concentrated Ionic Liquid Electrolytes for Lithium Metal Batteries

Abstract

The quest for highly stable ionic liquid electrolytes is vital for longer, safer cycling of Li-metal batteries (LMBs), given their nonflammable nature and broad electrochemical window. Locally concentrated ionic liquid electrolytes (LCILEs) have emerged by incorporating anti-solvating co-solvents to address the high viscosity and poor conductivity of Li⁺-concentrated ionic liquids. Although solvation and interface chemistry are crucial in determining cell performance, the impacts of organic cations in LCILEs remain overlooked. This work unravels the co-solvent-guided mediation of organic cation reactivity toward Li metal anodes. The donor number (DN) of co-solvents is found to significantly influence their local distribution within LCILEs, modulating Coulombic interactions between Li⁺–anion complexes and organic cations. Low DN co-solvents, such as hydrofluoroethers, hardly interact with Li⁺–anion complexes but dissociate and destabilize organic cations, adversely promoting organic cation decomposition at Li metal anodes. Conversely, high DN co-solvents prefer to occupy the Li⁺ solvation sheath, promoting organic cation–anion association and mitigating the cathodic decomposition. Suppressing the reactivity of organic cations in LCILEs is essential for proper anion-derived solid-electrolyte interphase formation and stable cycling of LMBs. The controlled reactivity of organic cations in concentrated ionic liquid electrolytes incorporating high DN co-solvent enables stable cycling of LMBs under stringent conditions, achieving 95% capacity retention over 200 cycles.