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

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 (NaCBH) is a promising Na conducting solid electrolyte with polymorphic transitions at moderate temperatures. We revised the crystal structure of ht-NaCBH and discovered disordered [CBH] 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-NaCBH, at 250 K before the first heating. Electrochemical impedance spectroscopy of ht-NaCBH reveals a high ionic conductivity of σ(Na) = 0.013 S cm at 296 K, an activation energy of E = 0.18 eV and an ionic conductivity of σ(Na) = 0.072 S cm 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 NaCBH was found to be 0.7 mA cm and a Na|NaCBH|NaTi(PO) cell was cycled with a discharge capacity of 51%.