Sodium halide solid state electrolyte of NaYBr with low activation energy.

Halide solid-state electrolytes (SSEs) are considered promising candidates for practical applications in all-solid-state batteries (ASSBs), due to their outstanding high voltage stability and compatibility with electrode materials. However, Na halide SSEs suffer from low ionic conductivity and high activation energy, which limit their applications in sodium all-solid-state batteries. Here, sodium yttrium bromide solid-state electrolytes (NaYBr) with a low activation energy of 0.15 eV is prepared solid state reaction. Structure characterization using X-ray diffraction reveals a monoclinic structure (2/) of NaYBr. First principle calculations reveal that the low migration activation energy comes from the larger size and vibration of Br anions, both of which expand the Na ion migration channel and reduce its activation energy. The electrochemical window of NaYBr is determined to be 1.43 to 3.35 V Na/Na, which is slightly narrower than chlorides. This work indicates bromides are a good catholyte candidate for sodium all solid-state batteries, due to their low ion migration activation energy and relatively high oxidation stability.