High-energy flexible quasi-solid-state lithium-ion capacitors enabled by a freestanding rGO-encapsulated Fe3O4 nanocube anode and a holey rGO film cathode.

Flexible energy storage devices have become critical components for next-generation portable electronics. In the present work, a flexible quasi-solid-state lithium-ion capacitor (LIC) is developed based on graphene-based bendable freestanding films in a gel polymer electrolyte. A graphene encapsulated Fe3O4 nanocube hybrid film (rGO@Fe3O4) has been fabricated as the anode of LICs through a filtration assisted self-assembly and the subsequent thermal annealing process. In this hybrid architecture, flexible and ultrathin graphene shells uniformly enwrap the Fe3O4 within the whole film, which can effectively suppress the aggregation of Fe3O4 and also accommodate the volume change of Fe3O4 during the cycling process. As a consequence, the electrochemical performance of the rGO@Fe3O4 half-cell versus Li/Li+ shows high specific capacity (731 mA h g-1 at 0.1 A g-1), excellent rate capability (210 mA h g-1 at 10 A g-1) and superior cycling stability (98% retention after 600 cycles). After chemically etching rGO@Fe3O4 with hydrochloric acid, a holey rGO film is successfully obtained as a high-rate cathode of LICs. On the basis of such a flexible anode and cathode, the as-fabricated quasi-solid-state LIC device delivers a high energy density of 148 W h kg-1, a high power density of 25 kW kg-1 (achieved at 70 W h kg-1) and an excellent capacity retention of 82% after 2000 cycles. More importantly, the rGO@Fe3O4//holey rGO LIC shows good mechanical flexibility with stable Li-storage capacities under harsh bending.