PNNL

Abstract

We report a new sodium fast-ion conductor, Na3B5S9, that exhibits a high Na ion conductivity in the bulk grains of 1.39 mS·cm-1 and a low bulk activation energy barrier for Na-ion migration of 0.35 eV. It consists of corner-sharing B10S20 supertetrahedral clusters, which create a framework that supports 3D Na ion diffusion channels. The Na ions are well-distributed in the channels and form a disordered sublattice spanning five Na sites. The combination of structural elucidation via single crystal X-ray diffraction and powder synchrotron X-ray diffraction at variable temperature, solid-state nuclear magnetic resonance spectra and ab initio molecular dynamics simulations reveal high Na-ion mobility and the nature of the 3D diffusion pathways. Notably, the Na ion sublattice orders at low temperatures, resulting in much lower ionic conductivity. This highlights the importance of a disordered Na ion sublattice in dictating Na ion diffusion.