Are sodiation/de-sodiation reactions reversible in two-dimensional metallic NbSe₂?

Two-dimensional (2D) metallic transition metal dichalcogenides (TMDs) are attracting increasing attention as promising electrode materials with fast ion/electron transport due to their ultrahigh electrical conductivities and layered structures. However, their further development is hindered by the inadequate understanding of energy storage mechanisms and electrochemistry upon charging/discharging processes. Herein, 2D metallic niobium diselenide (NbSe₂) flakes are targeted to understand the underlying electrochemical reaction mechanism during sodiation/de-sodiation. The complementary characterization techniques, including operando synchrotron X-ray diffraction, Raman spectroscopy, X-ray absorption spectroscopy and electron microscopy, are performed to investigate the structural evolution of 2D metallic NbSe₂ under electrochemical conditions. Different from conventional wisdom that believes reversible conversion reactions in TMDs, our results correct the sodiation/de-sodiation mechanism of the NbSe₂ electrode, which is an initial irreversible conversion from NbSe₂ to Na₂Se/Nb composites and the subsequent reversible selenide conversion reaction. This work brings an end to the debate over the sodiation/de-sodiation mechanism of metallic NbSe₂, enabling a deeper understanding of conversion reactions of TMD materials for alkali metal ion batteries.