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

Abstract

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/cm3. 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.

Emergent Pseudocapacitance of 2D Nanomaterials

“…[168] They also showed that the RGO electrodes can deliver a high capacity of ≈160 mA h g −1 based on the redox reactions of the oxygen functional groups with Liions with exceptional cycling stability up to 50 000 charge and discharge cycles. [168] They also showed that the RGO electrodes can deliver a high capacity of ≈160 mA h g −1 based on the redox reactions of the oxygen functional groups with Liions with exceptional cycling stability up to 50 000 charge and discharge cycles.…”

“…[165] They also showed that the capacity of pristine CNT (26 mA h g −1 ) can be significantly increased to 118 mA h g −1 as increasing the amount of the oxygen functional groups on CNT. [100,[167][168][169][170][171][172][173][174][175] Chen et al demonstrated that partially oxidized graphene can facilitate the penetration of aqueous electrolyte and induced the pseudocapacitive effects (Figure 18a). [100,[167][168][169][170][171][172][173][174][175] Chen et al demonstrated that partially oxidized graphene can facilitate the penetration of aqueous electrolyte and induced the pseudocapacitive effects (Figure 18a).…”

“…[168,169,174] The RGO electrodes can exhibit effective pseudocapacitance since the conjugated structure is partially recovered after the reduction process. [168,169,174] The RGO electrodes can exhibit effective pseudocapacitance since the conjugated structure is partially recovered after the reduction process.…”

“…[168,169,174] The RGO electrodes can exhibit effective pseudocapacitance since the conjugated structure is partially recovered after the reduction process. [168] Recent computation studies showed that the pseudocapacitance of the oxygen functional groups depends on their local oxygen chemistry, including the number and position of the functional groups. Accordingly, controlling the amount of the oxygen functional on the RGO is critical to maximize the pseudocapacitance of the RGO electrodes.…”

“…However, full reduced graphene loses its pseudocapacitance due to the lack of the redox-active oxygen functional groups. [168,177] In addition, control of oxygen chemistry should be attempted to improve the pseudocapacitance of the RGO. [167,168] However, the pseudocapacitance mechanisms of the various oxygen functional groups, including hydroxyl, epoxide, carbonyl, and carboxylic acid, have not been clearly understood on the surface of the RGO.…”