The roles of graphene in advanced Li-ion hybrid supercapacitors

Lithium-ion hybrid supercapacitors (LIHSs), also called Li-ion capacitors, are electrochemical energy storage devices that combining the advantages of high power density of supercapacitor and high energy density of Li-ion battery. However, high power density and long cycle life are still challenges for the current LIHSs due to the imbalance of charge-storage capacity and electrode kinetics between capacitor-type cathode and battery-type anode. Therefore, great efforts have been made on designing novel cathode materials with high storage capacity and anode material with enhanced kinetic behavior for LIHSs. With unique two-dimensional form and numerous appealing properties, for the past several years, the rational designed graphene and its composites materials exhibit greatly improved electrochemical performance as cathode or anode for LIHSs. Here, we summarized and discussed the latest advances of the state-of-art graphene-based materials for LIHSs applications. The major roles of graphene are highlighted as (1) a superior active material, (2) ultrathin 2D flexible support to remedy the sluggish reaction of the metal compound anode, and (3) good 2D building blocks for constructing macroscopic 3D porous carbon/graphene hybrids. In addition, some high performance aqueous LIHSs using graphene as electrode were also summarized. Finally, the perspectives and challenges are also proposed for further development of more advanced graphene-based LIHSs.