Fast sodium ionic conduction in Na₂B₁₀H₁₀-Na₂B₁₂H₁₂ pseudo-binary complex hydride and application to a bulk-type all-solid-state battery

In the present work, we developed highly sodium-ion conductive Na₂B₁₀H₁₀-Na₂B₁₂H₁₂ pseudo-binary complex hydride via mechanically ball-milling admixtures of the pure Na₂B₁₀H₁₀ and Na₂B₁₂H₁₂ components. Both of these components show a monoclinic phase at room temperature, but ball-milled mixtures partially stabilized highly ion-conductive, disordered cubic phases, whose fraction and favored structural symmetry (body-centered cubic or face-centered cubic) depended on the conditions of mechanical ball-milling and molar ratio of the component compounds. First-principles molecular-dynamics simulations demonstrated that the total energy of the closo-borane mixtures and pure materials is quite close, helping to explain the observed stabilization of the mixed compounds. The ionic conductivity of the closo-borane mixtures appeared to be correlated with the fraction of the body-centered-cubic phase, exhibiting a maximum at a molar ratio of Na₂B₁₀H₁₀:Na₂B₁₂H₁₂ = 1:3. A conductivity as high as log(σ/S cm⁻¹) = -3.5 was observed for the above ratio at 303 K, being approximately 2-3 orders of magnitude higher than that of either pure material. A bulk-type all-solid-state sodium-ion battery with a closo-borane-mixture electrolyte, sodium-metal negative-electrode, and TiS₂ positive-electrode demonstrated a high specific capacity, close to the theoretical value of NaTiS₂ formation and a stable discharge/charge cycling for at least eleven cycles, with a high discharge capacity retention ratio above 91% from the second cycle.