Microscopic Mechanism of the Intercalation Behavior of AlCl₄ ^- in Graphite Cathode Materials of Aluminum-ion Batteries

Rechargeable aluminum-ion batteries with graphite as cathode material are highly attractive for energy storage due to their low cost, high abundance, and high capacity. Although some techniques have been employed to investigate intercalation/deintercalation behavior of AlCl₄ ^- in graphite materials, the microscopic mechanism remains ambiguous and controversial, leading to a question that needs to be answered. Therefore, direct and in-situ characterization of nanoscale variations in the z direction is a reliable avenue to finding the answer to the intercalation stage mechanism. In this work, in-situ AFM was used to directly monitor the intercalation/deintercalation process of AlCl₄ ^- in highly oriented pyrolytic graphite, pyrolytic graphite and natural graphite electrode materials. We demonstrate that during the initial stage of intercalation, AlCl₄ ^- ions tend to intercalate intensively into the same graphite layer, and that as potential increases the intercalation of AlCl₄ ^- ions displays a mixed-staged process. Moreover, during the intercalation of AlCl₄ ^- into the pyrolytic graphite electrode, noticeable exfoliation of the graphite layers was observed. This phenomenon accounts for the energy loss that occurs in the initial cycles of the graphite/Al battery system. Finally, we propose an optimization strategy to prevent exfoliation.