The evolution of local structure of Mo6S8 during Li+ electrochemical storage studied by in-situ tender X-ray absorption spectroscopy

Abstract

Chevrel phase Mo6S8 has attracted great interest because of its capability to accommodate different types of small cations through reversible topotactic redox reactions. However, the intercalation mechanism of Mo6S8 is still far from being fully understood owing to the complexity of the crystal structure and limitation of the probe tools. In our study, the reaction mechanism of Li ​+ ​intercalation into Mo6S8 was studied as a model by in-situ tender X-ray absorption spectroscopy (itXAS) in the viewpoint of local structure. According to the isobestic points of the 1st derivative, the discharge process is divided into five regions while the charge process is divided into four regions, where most of the regions are assigned to the two-phase transition reactions. The intensity change of the two spectroscopic features of the 1st derivative is found to be correlated, implying that the intercluster bond length directly affects the localization of the unoccupied states of the intracluster bond. The asymmetric evolution of the intensity of a’ and b’ as well as the asymmetric partition of reaction regions demonstrate the asymmetry of the reaction path in discharge and charge process. The results help to clarify the remaining debates on the phase transitions. Besides, itXAS is demonstrated to be a powerful tool to study the reaction mechanism of Mo6S8, which is complementary to NMR, XRD and so on.