Ionic Conduction Mechanism in the Na2(B12H12)0.5(B10H10)0.5 closo-Borate Solid-State Electrolyte: Interplay of Disorder and Ion–Ion Interactions

The conduction mechanism of Na2(B12H12)0.5(B10H10)0.5, a particularly promising solid-state electrolyte for sodium-ion batteries, is elucidated. We find from electrochemical impedance spectroscopy that the temperature-dependent conductivity is characterized by three distinct regimes of conductivity. In the first regime, at temperatures below −50 °C, conductivity remains low before a glasslike transition identified by X-ray diffraction and calorimetry causes a faster increase of sodium conductivity through site disordering. The second regime of faster diffusion above −50 °C is characterized by an apparent activation energy of 0.6 eV, higher than expected from the local microscopic barrier of 0.35 eV observed by, e.g., 23Na nuclear magnetic resonance spin-lattice relaxation. This mechanism of so-called correlated ion diffusion originates from the coupling of the cation and anion motion due to short-range ion–ion interactions combined with background energy fluctuations, which we can associate through quasi-...