Na2 B10 H10 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 B10 H10 (2-) 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.

Both LiCB9H10 and NaCB9H10 exhibit liquid‐like cationic conductivities (≥0.03 S cm−1) in their disordered hexagonal phases near or at room temperature. These unprecedented conductivities and favorable stabilities enabled by the large pseudoaromatic polyhedral anions render these materials in their pristine or further modified forms as promising solid electrolytes in next‐generation, power devices.

Solid lithium and sodium closo-polyborate-based salts are capable of superionic conductivities surpassing even liquid electrolytes, but often only at above-ambient temperatures where their entropically driven disordered phases become stabilized. Here we show by X-ray diffraction, quasielastic neutron scattering, differential scanning calorimetry, NMR, and AC impedance measurements that by introducing “geometric frustration” via the mixing of two different closo-polyborate anions, namely, 1-CB9H10 – and CB11H12 –, to form solid-solution anion-alloy salts of lithium or sodium, we can successfully suppress the formation of possible ordered phases in favor of disordered, fast-ion-conducting alloy phases over a broad temperature range from subambient to high temperatures. This result exemplifies an important advancement for further improving on the remarkable conductive properties generally displayed by this class of materials and represents a practical strategy for creating tailored, ambient-temperature, solid, superionic conductors for a variety of upcoming all-solid-state energy devices of the future.

The disordered phases of the 1-carba-closo-decaborates LiCB 9 H 10 and NaCB 9 H 10 exhibit the best solid-state ionic conductivities to date among all known polycrystalline competitors, likely facilitated in part by the highly orientationally mobile CB 9 H 10 − anions. We have undertaken both NMR and quasielastic neutron scattering (QENS) measurements to help characterize the monovalent anion reorientational mobilities and mechanisms associated with these two compounds and to compare their anion reorientational behaviors with those for the divalent B 10 H 10 2− anions in the related Li 2 B 10 H 10 and Na 2 B 10 H 10 compounds. NMR data show that the transition from the low-T ordered to the high-T disordered phase for both LiCB 9 H 10 and NaCB 9 H 10 is accompanied by a nearly two-orders-of-magnitude increase in the reorientational jump rate of CB 9 H 10 − anions. QENS measurements of the various disordered compounds indicate anion jump correlation frequencies on the order of 10 10 −10 11 s −1 and confirm that NaCB 9 H 10 displays jump frequencies about 60% to 120% higher than those for LiCB 9 H 10 and Na 2 B 10 H 10 at comparable temperatures. The Q-dependent quasielastic scattering suggests similar small-angular-jump reorientational mechanisms for the different disordered anions, changing from more uniaxial in character at lower temperatures to more multidimensional at higher temperatures, although still falling short of full three-dimensional rotational diffusion below 500 K within the nanosecond neutron window.

To study the dynamical properties of the monocarba-closo-dodecaborates LiCB11H12 and NaCB11H12 showing the exceptionally high ionic conductivities in the high-temperature disordered phases, we have measured the temperature dependences of the 1H, 7Li, and 23Na NMR spectra and spin–lattice relaxation rates in these compounds below and above the phase transition points. It has been found that for both compounds the transition from the low-T ordered to the high-T disordered phase (near 384 and 376 K for LiCB11H12 and NaCB11H12, respectively) is accompanied by a nearly 3 orders of magnitude increase in the reorientational jump rate of [CB11H12]− anions. The results of our 7Li and 23Na NMR measurements indicate that the phase transitions from the low-T to the high-T phases of both LiCB11H12 and NaCB11H12 are also accompanied by a strong acceleration of translational diffusion of cations (Li+ or Na+). In the high-T phases of LiCB11H12 and NaCB11H12, the cation diffusion is characterized by low activation energies: 92 (7) and 152 (8) meV, respectively. These results are consistent with the high superionic conductivity in the disordered phases of LiCB11H12 and NaCB11H12; furthermore, they suggest that the enhanced reorientational mobility of large nearly spherical anions may facilitate the translational mobility of the cations.