Rechargeable aluminum batteries are a promising alternative to lithium-ion batteries due to the high volumetric energy density (8040 mAh/cm³), high abundance in the earth’s crust, and low cost of aluminum metal. However, aluminum ion intercalation into crystalline electrode materials is challenging, as the high coulombic charge of trivalent aluminum ions significantly increases the activation energy of diffusion compared to monovalent ions. Chevrel phase Mo₆S₈ is one of the most well-studied and successful cathode materials for magnesium-ion batteries, partly due to the facile transport of divalent magnesium ions within its crystal structure. Reversible intercalation of aluminum ions within chevrel phase has been reported, though the intercalation mechanism is not well understood at a molecular level.

Here, we report enhanced understanding of aluminum-ion intercalation in chevrel using a combination of spectroscopic, diffraction, and imaging techniques. Ex situ solid-state ²⁷Al MAS NMR measurements were performed as a function of state-of-charge to observe changes in the environments and populations of aluminum ions within the host crystal structure, revealing both intercalated ions and additional aluminum species associated with surface layers and/or decomposition products. In situ diffraction methods were performed to observe changes in the crystal structure during the cycling process, while transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) were conducted to characterize surface structures and compositions. Variable-temperature electrochemical measurements including cyclic voltammetry (CV), galvanostatic cycling, and electrochemical impedance spectroscopy (EIS) were performed to study the effects of temperature on the diffusion of Al³⁺ ions, electrochemical performance, and capacity retention. In combination, the results yield molecular-level insights into the aluminum-ion intercalation mechanism in the chevrel phase as well as changes of surface compositions and structures upon cycling in the ionic liquid electrolyte.