Reversible Electrochemical Anionic Redox in Rechargeable Multivalent-Ion Batteries

Rechargeablemultivalent-ion batteries are of significantinterestdue to the high specific capacities and earth abundance of their metalanodes, though few cathode materials permit multivalent ions to electrochemicallyintercalate within them. The crystalline chevrel phases are amongthe few cathode materials known to reversibly intercalate multivalentcations. However, to date, no multivalent-ion intercalation electrodescan match their reversibility and stability, in part due to the lackof design rules that guide how ion intercalation and electron chargetransfer are coupled up from the atomic scale. Here, we elucidatethe electronic charge storage mechanism that occurs in chevrel phase(Mo6Se8, Mo6S8) electrodesupon the electrochemical intercalation of multivalent cations (Al3+, Zn2+), using solid-state nuclear magnetic resonancespectroscopy, synchrotron X-ray absorption near edge structure measurements,operando synchrotron diffraction, and density functional theory calculations.Upon cation intercalation, electrons are transferred selectively tothe anionic chalcogen framework, while the transition metal octahedraare redox inactive. This reversible electrochemical anionic redox,which occurs without breaking or forming chemical bonds, is a fundamentallydifferent charge storage mechanism than that occurring in most transitionmetal-containing intercalation electrodes using anionic redox to enhanceenergy density. The results suggest material design principles aimedat realizing new intercalation electrodes that enable the facile electrochemicalintercalation of multivalent cations.