Functional carbon materials have recently attracted attention as positive electrode active materials for Al rechargeable batteries in conjunction with an Al negative electrode and a chloroaluminate ionic liquid electrolyte. This study explores a new carbon material, a graphene-decorated activated carbon fiber cloth (ACFC), for the positive electrode. Graphene-decorated ACFCs can be used in a binder-free manner and will directly contribute to increased gravimetric capacity of the whole electrode [mAh (g electrode ) -1 , not per the active material]. Specifically, the positive electrode behaviors of three types of ACFC-based electrodes are examined in a Lewis acidic AlCl 3 −1-ethyl-3-methylimidazolioum chloride ([C 2 mim]Cl) room-temperature ionic liquid. Uncoated and low-loading graphenecoated ACFC electrodes show an electric double layer capacitor (EDLC)-like behavior, whereas the intercalation/deintercalation reaction of chloroaluminate anion (e.g., [AlCl 4 ] -) is observed for the high-loading graphene-coated one. These electrodes exhibit a good cyclability and satisfactory coulombic efficiency up to 800 cycles at a current density of 500 mA g -1 . © The Author Our modern life is supported by many kinds of energy storage devices. Lithium-ion secondary batteries (LIBs) currently outperform other competitive batteries in terms of energy and power densities as well as cyclability. However, extensive improvements are still necessary to meet the requirements of next-generation applications such as all-electric vehicles and humanoids. In addition to the foreseen shortage of some of the chemical elements currently used in LIBs, cost effectiveness and safety risks have driven researchers to explore alternative energy storage solutions.1 In this context, Li-sulfur, 2,3 Liair, 3 sodium ion, 4 potassium ion, 5 and multivalent metal secondary batteries 6-10 have been proposed. Multivalent systems (Mg, Zn, Ca, Al, etc.) provide opportunities to build devices with higher capacities beyond monovalent LIBs because they involve more electron transfers per single metal redox center. 8,9 The rich abundance of these metals also holds promise to reduce the cost of a battery.8 However, one of the significant challenges for multivalent batteries is the lack of reliable electrolytes, which enable an efficient metal deposition/stripping reaction. Even if the metal ion is reduced, in most cases, it does not achieve a sufficient coulombic efficiency since the deposited metal on the negative electrode is highly reactive and readily reacts with the electrolyte. Furthermore, active materials for the positive electrode that allow facile ion transport are far from fully exploited.Room-temperature ionic liquids (RTILs) have been the center of attention as advanced reaction media due to their unique features, including flame retardation, negligible vapor pressure, relatively-high ionic conductivity, and high electrochemical stability.11-14 RTIL electrolytes will realize noticeable performance improvements in future batteries.15-17 Recently, w...
