Diamondoid-templated lithium single-cation 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-cation 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 computations, where we determine the lowest-energy transport pathway guiding lithium transport within the adamantane-templated salt. Together, temperature-dependent conductivity measurements, solid-state NMR spectroscopy, pulsed gradient spin echo (PGSE) NMR spectroscopy and electronic structure calculations establish that lithium is the only mobile cation in the solid electrolyte phase, as adamantyl groups completely inhibit organic cation motion. Hence, our findings show that lithium transport occurs within dynamic bis(trifluoromethanesulfonyl)imide (TFSI) channels within the diamondoid templated electrolyte, achieving conductivities exceeding 0.1 mS cm⁻¹ at 70 °C. This demonstrates that diamondoid-derived electrolytes show promise as organic lithium single-cation conductors that can address longstanding battery performance challenges associated with low lithium transference in conventional ionic liquid and polymeric electrolytes.