Ionic Conduction Mechanism in the Na-2(B12H12)(0.5)(B10H10)(0.5) closo-Borate Solid-State Electrolyte: Interplay of Disorder and Ion-Ion Interactions

摘要

The conduction mechanism of Na-2(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 degrees 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 degrees 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., Na-23 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-elastic neutron scattering experiment to fast librations of the anions. In the third regime, at temperatures above 70 degrees C, the thermal energy increases above the background energy fluctuations and the activation energy decreases to 0.34 eV, reflecting the local energy barrier for noncorrelated ion diffusion. We discuss the link between this behavior and the different frustrations responsible for the high conductivity of closo-borate electrolytes and show that our interpretation can also explain the complex conductivity behavior observed in related compounds.