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

New insights into the polymorphism and electrochemical properties of the solid electrolyte NaCB 9 H 10 , investigating the usability of NaCB 9 H 10 as a battery electrolyte at room temperature. 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 (NaCB 9 H 10 ) is a promising Na + conducting solid electrolyte with polymorphic transitions at moderate temperatures. We revised the crystal structure of ht -NaCB 9 H 10 and discovered disordered [CB 9 H 10 ] − 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-NaCB 9 H 10 , at 250 K before the first heating. Electrochemical impedance spectroscopy of ht -NaCB 9 H 10 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 NaCB 9 H 10 was found to be 0.7 mA cm −2 and a Na|NaCB 9 H 10 |NaTi 2 (PO 4 ) 3 cell was cycled with a discharge capacity of 51%.