Enabling the Li-ion conductivity of Li-metal fluorosulphates by ionic liquid grafting

Recently unveiled ‘alkali metal fluorosulphate (AMSO₄F)’ class of compounds offers promising electrochemical and transport properties. Registering conductivity value as high as 10⁻⁷ S cm⁻¹ in NaMSO₄F phases, we explored the fluorosulphate group to design novel compounds with high Li-ion conductivity suitable for solid electrolyte applications. In the process, we produced sillimanite-structured LiZnSO₄F by low temperature synthesis (T ≤ 300 °C). Examining this phase, we accidentally discovered the possibility of improving the ionic conductivity of poor conductors by forming a monolayer of ionic liquid at their particle surface. This phenomenon was studied by solid-state NMR, XPS and AC impedance spectroscopy techniques. Further, similar trends were noticed in other fluorosulphate materials like tavorite LiCoSO₄F and triplite LiMnSO₄F. With this study, we propose ‘ionic liquid grafting’ as an interfacial route to enable good Li-ion conductivity in otherwise poor conducting ceramics.