Mixed anion closo-carbaborates as Na-superionic conductors

Abstract

Sodium closo-borates have recently received significant attention as solid electrolytes for all-solid-state sodium batteries. The closo-borate compounds exhibit high thermal and electrochemical stability, and the reorientational motion and weak coordination of the anion facilitate fast Na-ion conductivity. Here, we report a simple synthesis method to directly form a mixed anion sodium closo-(carba)borate, Na_1.24[(CB₈H₉)_0.26(CB₉H₁₀)_0.50(B₁₂H₁₂)_0.24]. The [B₁₂H₁₂]²⁻ can readily be removed from the sample, but complete separation of [CB₈H₉]⁻ and [CB₉H₁₀]⁻ is challenging owing to their chemical similarities. Na[(CB₈H₉)_1−x(CB₉H₁₀)_x] and Na_x+y+2z[(CB₈H₉)_x(CB₉H₁₀)_y(B₁₂H₁₂)_z] form solid solutions that are isostructural with the high temperature polymorph of NaCB₉H₁₀ (P31c), which is also maintained upon cooling to low temperatures (T < −54 °C). A second high temperature polymorph (Imm) is also observed in minor amounts in [CB₈H₉]⁻-rich samples. The highest sodium conductivity was observed for the composition Na[(CB₈H₉)_0.38(CB₉H₁₀)_0.62], demonstrating a conductivity of σ(Na⁺) = 5.5 × 10⁻³ S cm⁻¹ at 20 °C. The initial synthesis product, Na_1.24[(CB₈H₉)_0.26(CB₉H₁₀)_0.50(B₁₂H₁₂)_0.24], has a high ionic transport number of t_ion > 0.9999 and a negligible electronic conductivity of σ_e = 2 × 10⁻¹¹ S cm⁻¹ at room temperature. The solid electrolyte shows excellent stability against Na-metal and demonstrates a stable stripping/plating behavior. An all-solid-state battery is demonstrated using Na-metal as the anode, Na[(CB₈H₉)_0.04(CB₉H₁₀)_0.96] as the electrolyte and NaTi₂(PO₄)₃ as the cathode.