Rechargeable aluminum batteries have been a topic of great interest as aluminum metal has a high volumetric capacity and is the most abundant metal in the earth’s crust. However, few crystalline materials have been shown to reversibly intercalate aluminum ions into their structures. Recently, it has been shown that aluminum ions can reversibly intercalate/de-intercalate into the Chevrel Mo₆S₈ phase and the spinel Mn₂O₄ compound. However, the intercalation mechanisms are not well understood, particularly for the spinel Mn₂O₄, in part due to poor capacity retention and slow solid-state diffusion of the highly charged tri-valent aluminum ions within these crystalline structures. Better understanding of the intercalation mechanism of aluminum ions into these materials, including the dynamics and populations of aluminum ions, is important for the design and optimization of next-generation intercalation electrodes for aluminum batteries.

Our research has focused on these intercalation electrode materials for use in aluminum batteries with AlCl₃/[EMIm]Cl ionic liquid electrolyte to study the intercalation process of aluminum ions into crystalline host structures. Cyclic voltammetry was performed at low scan rates to identify oxidation and reduction peaks corresponding to different intercalation sites during the charge-discharge process. Galvanostatic cycling was performed to observe capacity retention at different current densities. Ex situ multi-dimensional solid-state ²⁷Al magic-angle-spinning (MAS) NMR spectroscopy was used to observe changes in the populations and local environments of the aluminum ions, as well as their dynamics. X-ray diffraction and electron microscopy were also used to analyze structural changes in the cathode materials as a function of state-of-charge. Our studies shed light on the intercalation mechanisms of aluminum ions in crystalline transition metal oxide and sulfide host structures.