Cation-synergy stabilizing anion redox of Chevrel phase Mo6S8 in aluminum ion battery

Anion redox chemistry is an essential component of many high energy density electrode materials, while accompanying with voltage hysteresis and fade, which currently hinders its widespread use. Here we demonstrate cation-synergy stabilizing anion redox of Chevrel phase Mo6S8 in aluminum ion battery. EELS and XAS reveal that S is fully reduced, and Mo6 cluster is firstly oxidized and then reduced with Al3+ ions insertion, which originates from the contraction and elongation of Mo-Mo bond in Mo6 cluster verified by atomic-resolved imaging. DFT calculations uncover that the energy level of [Mo-Mo]* antibonding orbitals is lifted and fell by Mo-Mo bond evolution, resulting in the synergetic cationic and anionic redox, which contributes to the stable structure upon cycling. Our work figures out the electrochemical redox mechanism of Mo6S8 in aluminum ion batteries and provides implications generally for the design of materials employing anion redox chemistry.