Liquid-Like Ionic Conduction in Solid Lithium and Sodium Monocarba-closo-Decaborates Near or at Room Temperature

The search for solid electrolytes with sufficiently high ionic conductivities and stabilities is underway to enable the commercial viability of all-solid-state rechargeable batteries. LiCB₉H₁₀ and NaCB₉H₁₀ compounds exhibit the most impressive superionic conductivities yet among complex-hydride-based materials, including this class of large-polyhedral-anion-based salts. The pseudoaromatic nature of the CB₉H₁₀ anions makes them relatively stable like their B₁₂H₁₂^2-, B₁₀H₁₀^2-, and CB₁₁H₁₂^2- cousins, rendering their salts prime candidates for incorporation into next-generation, all-solid-state devices. Preliminary cyclic voltammetry measurements indicate that only cathodic and anodic currents are observed near 0 v corresponding to Li/Na deposition on the Au electrode and Li/Na stripping, respectively, without signifi cant anodic currents, at least ≤ 5 v for both LiCB₉H₁₀ (363 K) and NaCB₉H₁₀ (303 K). The similar conductivity behaviors with temperature for LiCB₉H₁₀ and NaCB₉H₁₀ compared to those for LiCB₁₁H₁₂ and NaCB₁₁H₁₂ , and their order-of-magnitude enhancements over disordered NaCB₉H₁₀, irrespective of structural symmetries, further reinforces the notion that anion monovalency better facilitates high cation translational mobility in these large- polyhedral-anion-based systems.