High Ionic Conduction in Rb- and Cs-Mixed Cation Amide for Energy Storage

Abstract

Ionic conductivity is one of the key parameters in designing advanced solid-state batteries and energy storage materials. This study presents the first observation of high ionic conductivity in the newly developed mixed cation amide solid solution, Rb0.5Cs0.5NH2, within the RbNH2-CsNH2 system. In particular, the solid solution formed shows an unexpectedly high ionic conductivity that is four orders of magnitude higher than that of the individual compounds, RbNH2 and CsNH2. This substantial improvement is ascribed to the Rb+/Cs+ cation exchange process. This exchange significantly stabilizes the cubic structure, thereby enhancing ionic conductivity in the solid solution compared to the parent compounds. A combined experimental and computational study using quasielastic neutron scattering (QENS) and density functional theory (DFT) elucidates the mechanism of Rb+/Cs+ ion migration in solid solution. The findings indicate intrinsically correlated with the reorientation dynamics of [NH₂]⁻ anions, which activates and facilitates Rb⁺/Cs⁺ ion transport within the lattice via the paddlewheel mechanism. A deep understanding of the crystal structure, anion reorientation dynamics, and cation migration mechanisms is crucial for advancing the ionic conductivity and hydrogen storage characteristics of these amide materials.