Unparalleled lithium and sodium superionic conduction in solid electrolytes with large monovalent cage-like anions

Solid electrolytes with sufficiently high conductivities and stabilities are the elusive answer to the inherent shortcomings of organic liquid electrolytes prevalent in today's rechargeable batteries. We recently revealed a novel fast-ion-conducting sodium salt, Na₂B₁₂H₁₂, which contains large, icosahedral, divalent B₁₂H₁₂^2- anions that enable impressive superionic conductivity, albeit only above its 529 K phase transition. Its lithium congener, Li₂B₁₂H₁₂, possesses an even more technologically prohibitive transition temperature above 600 K. Here we show that the chemically related LiCB₁₁H₁₂ and NaCB₁₁H₁₂ salts, which contain icosahedral, monovalent CB₁₁H₁₂^- anions, both exhibit much lower transition temperatures near 400 K and 380 K, respectively, and truly stellar ionic conductivities (> 0.1 S cm^-1) unmatched by any other known polycrystalline materials at these temperatures. With proper modifications, we are confident that room-temperature-stabilized superionic salts incorporating such large polyhedral anion building blocks are attainable, thus enhancing their future prospects as practical electrolyte materials in next-generation, all-solid-state batteries.