Tuning the porous graphene interlayer structure for compact energy storage towards high volumetric performance of Zn-ion capacitor

For most consumer electronics, the volumetric performance of energy storage devices is usually more significant than traditional gravimetric performance. Zinc-ion capacitors (ZICs) are regarded as one of the most promising energy storage devices with high energy and power density. However, the low volumetric performance of the cathode is a serious problem that hinders its practical application. Herein, in this work, a compact freestanding graphene film was designed by tuning the interlayer structure of flame reduced graphene oxide sheets (FRGO) using graphene oxides (GO) via a self-assembly strategy. When the mass ratio of FRGO and GO is 75%:25%, the optimum of graphene film between density and electrochemical performance was achieved. The graphene film owns a bulk density of 0.82 g cm−3 and precisely regulated abundant mesoporous around 3.8 nm, which provides more active sites for ion storage. Profiting from the pseudocapacitance reaction between C=O and Zn2+, the compact graphene film with a thickness of 35 μm as ZICs cathode can deliver a high volumetric energy density of 113.1 Wh L-1 and outstanding gravimetric performance of 118.7 Wh kg−1 with a bulk density of 0.95 g cm−3, achieving the balance of the gravimetric and volumetric characteristics. The ZIC also presents a maximum volumetric power density of 21.2 kW L-1 at 7.9 Wh L-1 and 96.4% capacity retention after 8000 cycles, revealing excellent compact energy storage and cyclability. Moreover, the soft-package ZIC devices present admirable scalability, further exposing the application potential of the compact graphene film in the ZIC field.