Nanosheet-stacked flake graphite for high-performance Al storage in inorganic molten AlCl₃–NaCl salt

Aluminum storage systems with graphite cathode have been greatly promoting the development of state-of-the-art rechargeable aluminum batteries over the last five years; this is due to the ultra-stable cycling, high capacity, and good safety of the systems. This study discussed the change of electrochemical behaviors caused by the structural difference between flake graphite and expandable graphite, the effects of temperature on the electrochemical performance of graphite in low-cost AlCl₃–NaCl inorganic molten salt, and the reaction mechanisms of aluminum complex ions in both graphite materials by scanning electron microscopy, X-ray diffraction, Raman spectroscopy, cyclic voltammetry, and galvanostatic charge–discharge measurements. It was found that flake graphite stacked with noticeably small and thin graphene nanosheets exhibited high capacity and fairly good rate capability. The battery could achieve a high capacity of ∼219 mA hg⁻¹ over 1200 cycles at a high current density of 5 Ag⁻¹, with Coulombic efficiency of 94.1%. Moreover, the reaction mechanisms are clarified: For the flake graphite with small and thin graphene nanosheets and high mesopore structures, the reaction mechanism consisted of not only the intercalation of AlCl4− anions between graphene layers but also the adsorption of AlCl4− anions within mesopores; however, for the well-stacked and highly parallel layered large-size expandable graphite, the reaction mechanism mainly involved the intercalation of AlCl4− anions.