Mechanically flexible reduced graphene oxide/carbon composite films for high-performance quasi-solid-state lithium-ion capacitors

Practical applications of diverse flexible wearable electronics require electrochemical energy storage (EES) devices with multiple configurations. Moreover, to fabricate flexible EES devices with high energy density and stability, organic integration from electrode design to device assembly is required. To address these challenges, a free-standing reduced graphene oxide (rGO)/carbon film with a unique sandwich structure has been designed via the assistance of vacuum-assistant filtration for lithium-ion capacitors (LICs). The graphene acts as not only a binder to construct a three-dimensional conductive network but also an active material to provide additional capacitive lithium storage sites, thus enabling fast ion/electron transport and improving the capacity. The designed rGO/hard carbon (rGO/HC) and rGO/activated carbon (rGO/AC) free-standing films exhibit enhanced specific capacities (513.7 mA h g−1 for rGO/HC and 102.8 mA h g−1 for rGO/AC) and excellent stability. Moreover, the integrated flexible quasi-solid-state rGO/AC//rGO/HC LIC devices possess a maximum energy density of 138.3 Wh kg−1, a high power density of 11 kW kg−1, and improved cycling performance (84.4% capacitance maintained after 10,000 cycles), superior to the AC//HC LIC (43.5% retention). Such a strategy enlightens the development of portable flexible LICs.