Investigation on the structural evolutions during the insertion of aluminum ions into Mo₆S₈ Chevrel phase

Herein, we evaluate the detailed insertion mechanism of aluminum ions into the Mo₆S₈ Chevrel phase in the 1-ethyl-3-methylimidazolium tetrachloroaluminate ionic liquid through combined structural and electrochemical characterization. This involves two-step two-phase insertion reactions, where the first and the second step account for 3/4 and 1/4 of the total reversible capacity, respectively. During the first step, aluminum ions occupy the inner ring sites in the Chevrel phase, forming AlMo₆S₈ with a higher activation energy for cation diffusion. Subsequently, Al_4/3Mo₆S₈ emerges as a discharge product during the second step, where one aluminum ion fills one outer ring site in every three AlMo₆S₈ units. The diffusion coefficient for aluminum ions through the Chevrel phase was found much lower than that of lithium or magnesium ions, reflecting the difficulties in redistributing trivalent charge carried by each aluminum ion.