Structural evolution of Na-rich spinel oxides involving anionic redox reaction for Na-ion batteries

Two sodium-rich transition metal (TM) oxides with the same spinel structure, Na₂MoO₄ and Na₂WO₄ have been investigated as cathode materials for Na-ion batteries for the first time. Although the oxidation state of TMs in the compounds are already at its highest value of 6+, both of them can be activated by anionic redox reaction during initial charge, as revealed by X-ray photoelectron spectroscopy, to give considerable reversible capacity between 1.2 and 4.7 V. In addition, ex-situ X-ray diffractometry (XRD) shows that both cathode materials undergo insignificant structural evolution during Na extraction/insertion, suggesting that the Mo-O₄ and W-O₄ tetrahedral framework are stable even when more than 1 Na is removed from the materials. Overall, Na₂WO₄ shows larger amount of Na extraction/insertion and better cycle stability than Na₂MoO₄. This is likely due to better structural integrity and better stability of Na₂WO₄ against oxygen loss from ex-situ XRD and differential electrochemical mass spectrometry results.