Sodium decahydrido-closo-1-carbadecaborate as a solid electrolyte: new insight into polymorphism and electrochemical performance

Abstract

Solid-state batteries receive increasing attention due to their potentially increased safety and energy density in the pursuit of next-generation energy storage technologies. Sodium decahydrido-closo-1-carbadecaborate (NaCB₉H₁₀) is a promising Na⁺ conducting solid electrolyte with polymorphic transitions at moderate temperatures. We revised the crystal structure of ht-NaCB₉H₁₀ and discovered disordered [CB₉H₁₀]⁻ anions and sodium distributed at four crystallographic positions by Rietveld refinement of synchrotron radiation powder X-ray diffraction data. The large number of available Na⁺ positions are connected into a three-dimensional conduction pathway, consistent with the extremely high Na⁺ ionic conductivity. The polymorphic transition is investigated using in situ synchrotron radiation powder X-ray diffraction revealing the presence of a new polymorph, δ-NaCB₉H₁₀, at 250 K before the first heating. Electrochemical impedance spectroscopy of ht-NaCB₉H₁₀ reveals a high ionic conductivity of σ(Na⁺) = 0.013 S cm⁻¹ at 296 K, an activation energy of E_a = 0.18 eV and an ionic conductivity of σ(Na⁺) = 0.072 S cm⁻¹ at 332 K. Slow scan rate cyclic voltammetry combined with a carbon additive revealed an oxidative stability of 3.1 V vs. Na⁺/Na. The highest sustainable current density of NaCB₉H₁₀ was found to be 0.7 mA cm⁻² and a Na|NaCB₉H₁₀|NaTi₂(PO₄)₃ cell was cycled with a discharge capacity of 51%.