Reversible conversion of MoS2 upon sodium extraction

Intercalation and conversion are two of the most popular forms of lithium (Li) or sodium (Na) storage. Conversion reactions usually involve transference of more than one electron. Therefore, the electrode materials that store Li- and Na-ions by way of such reactions theoretically deliver specific capacities a few times higher than those of the intercalation materials. Intercalation and conversion reactions can occur in sequence upon Na storage in MoS2, but the reversibility of the latter upon Na extraction is still controversial. In this article, we clarify that the conversion reaction of nano-MoS2 can be reversible in a carbon matrix and the extent of reversibility is highly dependent on the dynamic properties of the composite. In addition, hexagonal NaMoS2 is recognized as a new phase in the early stage of Na extraction from the reduction products of MoS2, Mo+Na2S, on the basis of high-resolution transmission electron microscopy (HRTEM) characterization and first-principles calculations. These findings enrich the understanding of reaction mechanism of MoS2 upon Na storage and removal and are helpful to the design and applications of the transition metal sulfides.