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.