Charge Distribution on S and Intercluster Bond Evolution in Mo6S8 during the Electrochemical Insertion of Small Cations Studied by X-ray Absorption Spectroscopy.

Mo6S8 is regarded as a promising cathode material in rechargeable Mg batteries. Despite extensive studies, some fundamental questions are still unclarified, including the origination of the chemical stability, key factors inducing the structural evolution, and the factors determining the electrochemical reversibility. Herein Mo L2,3 and S K-edge X-ray absorption spectroscopy are utilized to uncover the underlying mechanism. Two kinds of S with different effective charge are found, indicating the nonuniform charge distribution. With one cation inserted, the charge distribution becomes homogeneous, relevant to the chemical stability and electrochemical reversibility. The structural evolution is attributed to the change of bond length induced by the delocalization of inserted cations. Moreover, the evolution of intercluster Mo-Mo bond length can be revealed by the drastic change of the S K pre-edge and is closely related to the electrochemical reversibility. This study can shed light on the aforementioned questions and guide the development of Mg cathode material.