High-Density Lithium-Ion Energy Storage Utilizing the Surface Redox Reactions in Folded Graphene Films

Reduced graphene oxides are active as positive electrodes for lithium-ion energy storage based on the surface redox reactions between oxygen functional groups and Li ions. For effective Li-ion energy storage within a confined mass and volume, free-standing, high-packing density, and redox-active graphene films were fabricated by a simple two-step compression and vacuum-drying process from a hydrothermally reduced graphene hydrogel. The assembled graphene films showed a folded microstructure with high packing densities up to ∼0.64 g/cm 3 . Redox-active oxygen functional groups on the graphene oxide were activated and controlled by the hydrothermal reduction temperature. Density functional theory (DFT) calculations revealed that the carbonyl and epoxide groups among various oxygen functional groups on graphene were the main contributors for the high potential redox reactions with Li ions. The folded graphene film electrodes delivered both a high gravimetric energy of ∼419 Wh/kg and a high volumetric energy of ∼239 Wh/L. In addition, the folded graphene film electrodes exhibited an exceptional cycling stability, retaining a gravimetric capacity of ∼160 mAh/g after 50 000 cycles. These results provide significant insights on the effective utilization of surface redox reactions to design graphene-based electrodes for high-performance Li-ion energy storage devices.