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

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, Rb_0.5Cs_0.5NH₂, 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.