Diamondoid-templated lithium single-ion conduction in solid state organic electrolytes

Abstract

Rapid growth in battery use increases the focus on battery safety and sustainability. While ionic liquids and organic ionic plastic crystals show promise as safe battery electrolytes, they often exhibit poor lithium transport due to excessive lithium-anion coordination and preferential organic ion conduction. We show that adamantane can be leveraged to hinder organic cation motion while templating lithium transport pathways to enable solid state lithium single-ion conduction. By using solid state NMR spectroscopy to study anion conformational changes and electrochemical impedance spectroscopy to evaluate lithium mobility in adamantane and diamantane salts, we reveal that adamantane-templated assembly of anion networks is key for enabling lithium-ion mobility. Further, we find that solid state conductivity is a strong function of lithium concentration, rising two orders of magnitude at room temperature upon increasing lithium from 5 to 15 mol percent. Our experimental findings are substantiated by density functional theory calculations, where we determine the lowest-energy transport pathway guiding lithium mobility within the adamantane-templated salt. Overall, temperature-dependent conductivity measurements, solid state NMR spectroscopy, and electronic structure calculations establish that lithium-single ion conduction occurs via hopping between anion coordination sites, rising to exceed 0.1 mS cm–1 at 70°C. This demonstrates that diamondoid-derived electrolytes show promise as organic lithium single-ion conductors that can address longstanding battery performance challenges associated with low lithium transference in conventional ionic liquid and polymeric electrolytes.