Capacitive charge storage enables an ultrahigh cathode capacity in aluminum-graphene battery

Aluminum-graphene battery is promising for its abundant raw materials, high power density, ultralong cycle life and superior safety. However, the development of aluminum-graphene battery is currently restricted by its insufficient cathode capacity, calling for a newly developed working mechanism. In addition, an irregular constant increase of the cathode capacity was always observed during cycling, but cannot be explained based on the current understanding. Here, we observed an increase of specific capacity by 60% with stable Coulombic efficiency of 98% during 7000 cycles life of Al-graphene batteries employing AlCl3/ET3NHCl electrolyte. We demonstrated this growing cathode capacity is attributed to an increasing contribution of capacitive charge storage during cycling, because a gradually enlarged surface area as capacitive active sites is enabled by the exfoliation of graphitic cathode during the periodic intercalation process. Moreover, the graphene cathode was exfoliated more significantly in AlCl3/ET3NHCl than 1-ethyl-3-methylimidazolium chloride-based electrolyte, which results from the heavier stress on the graphene layers caused by the larger intercalants in AlCl3/ET3NHCl. The common intercalation of cations with AlCl4− clusters was therefore supposed to occur during charging. This new proposed mechanism can offer the new thought for future design on high-capacity cathode of Al-ion battery.