Aluminum metal has long been envisioned as an electrode for high-performance rechargeable batteries due to its high global abundance, low cost, non-flammability, and high volumetric capacity. Graphite has recently been shown to pair as a cathode with aluminum anodes when using chloroaluminate-containing ionic liquid electrolytes, exhibiting capacities that are stable at high charge/discharge rates and over long cycle lifetimes. However, there are presently unanswered questions related to the intercalation of sterically bulky chloroaluminate anions (e.g. AlCl₄^-), structural changes of the cathode after prolonged cycling, and more generally, the relationship between the local structures of different graphitic materials and their electrochemical properties.

Here, we report on the investigation of charge storage and ion transport mechanisms in graphitic cathodes and their implications on bulk electrochemical behavior. Three types of graphites (pyrolytic, natural and synthetic) were studied. Solid-state ²⁷Al magic-angle-spinning (MAS) NMR measurements on intercalated graphitic cathodes reveal the presence of chloroaluminate anions in different local environments and with different mobilities that evolve as a function of state-of-charge. Cyclic voltammetry experiments at varying scan rates were conducted to yield insights into ion transport phenomena and to disentangle the Faradaic and capacitive contributions to the total discharge capacity. Additionally, the surface area and porosity of the cathode materials, which affect the number of accessible sites for ion electrosorption, were quantified and correlated to the observed capacitance and overall energy density. Our findings reveal molecular-level insights into the intercalation processes and redox mechanisms of graphitic cathodes that can be collectively used to better understand and optimize aluminum-graphite battery performance.