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

Abstract

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.