The novel compound Na2-x (CB11H12) x (B12H12)1-x shows a fast ionic Na conductivity of 2 mS cm−1 at room temperature, reaching the superionic regime above 333 K. A low activation energy of 314 meV, in a thermodynamically and electrochemically stable structure, suggest its usage as a potential solid electrolyte in future all-solid Na-ion batteries. Density functional theory simulations indicate the heterovalent anionic mixing to be the responsible in stabilizing a bcc structure, which allows a facile Na+ diffusion already at room temperature. Cyclic voltammetry measurement with a tailored setup for solid-state electrolytes reveals a wide operating window (up to 4.1 V vs. Na+/Na). The symmetrical cell featuring NaSn alloy-based electrodes shows reversible Na+ shuttling with limited polarization after more than 700 operating hours.