Sodium halide solid state electrolyte of Na₃YBr₆ 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 (Na₃YBr₆) with a low activation energy of 0.15 eV is prepared via solid state reaction. Structure characterization using X-ray diffraction reveals a monoclinic structure (P2₁/c) of Na₃YBr₆. 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 Na₃YBr₆ is determined to be 1.43 to 3.35 V vs. 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.