Reversible Electrochemical Anionic Redox in Rechargeable Multivalent-Ion Batteries

Rechargeable multivalent-ion batteries are of significant interest due to the high specific capacities and earth abundance of their metal anodes, though few cathode materials permit multivalent ions to electrochemically intercalate within them. The crystalline chevrel phases are among the few cathode materials known to reversibly intercalate multivalent cations. However, to date, no multivalent-ion intercalation electrodes can match their reversibility and stability, in part due to the lack of design rules that guide how ion intercalation and electron charge transfer are coupled up from the atomic scale. Here, we elucidate the electronic charge storage mechanism that occurs in chevrel phase (Mo₆Se₈, Mo₆S₈) electrodes upon the electrochemical intercalation of multivalent cations (Al³⁺, Zn²⁺), using solid-state nuclear magnetic resonance spectroscopy, synchrotron X-ray absorption near edge structure measurements, operando synchrotron diffraction, and density functional theory calculations. Upon cation intercalation, electrons are transferred selectively to the anionic chalcogen framework, while the transition metal octahedra are redox inactive. This reversible electrochemical anionic redox, which occurs without breaking or forming chemical bonds, is a fundamentally different charge storage mechanism than that occurring in most transition metal-containing intercalation electrodes using anionic redox to enhance energy density. The results suggest material design principles aimed at realizing new intercalation electrodes that enable the facile electrochemical intercalation of multivalent cations.