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, Al4/3Mo6S8 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. (C) 2016 The Electrochemical Society. All rights reserved.