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

• Published in: Electrochimica acta 2023-02, Vol.440, p.141746, Article 141746
• Main Authors: Su, Bizhe, Liang, Hanqin, Zhao, Xiaohui, Zhang, Tao, Zhou, Yu, Yu, Denis Y.W.
• Format: Article
• Language: English
• Subjects: Anionic redox reactions; Cathode materials; Na-ion battery; Spinel oxides; Structural stability
• ISSN: 0013-4686; 1873-3859
• DOI: 10.1016/j.electacta.2022.141746

•Spinel Na2MoO4 and Na2WO4 can be used as cathode materials for Na-ion batteries despite TMs in the highest oxidation state.•Large initial charge capacity is attributed to anionic redox reactions in the materials.•Negligible structural evolution is observed for both materials during charge and discharge.•Na2WO4 shows a better stability against oxygen loss and structural degradation than Na2MoO4. 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.