Exceptional Superionic Conductivity in Disordered Sodium Decahydro‐closo‐decaborate

Graphical Abstract Na2B10H10 exhibits exceptional superionic conductivity above ca. 360 K (e.g., ca. 0.01 S cm−1 at 383 K) concomitant with its transition from an ordered monoclinic structure to a face-centered-cubic arrangement of orientationally disordered B10H102− anions harboring a vacancy-rich Na+ cation sublattice. This discovery represents a major advancement for solid-state Na+ fast-ion conduction at technologically relevant device temperatures.