Surface-functionalized graphene-based quasi-solid-state Na-ion hybrid capacitors with excellent performance

Simultaneous integration of high power density, high energy density, long cycle life and superior safety in a single energy storage system is still a huge challenge. Addressing this issue requires the design of new energy storage systems with novel electrodes. Herein, we propose a novel electrochemical energy storage device called a quasi-solid-state Na-ion hybrid capacitor (QSS–NIC) based on surface oxygen-functionalized crumpled graphene (OCG) as both the negative and positive electrodes and a Na-ion conducting gel polymer as the electrolyte. The as-prepared OCG with dense and porous structure ensures abundant ion-accessible active sites and short ion diffusion path. The surface oxygen functional groups within OCG are favorable for high energy storage when applied as both battery-type negative electrodes and capacitor-type positive electrodes. Benefiting from the elaborate design of electrode materials and device configuration, the QSS–NIC achieves a high energy density of 121.3 W h kg−1, high power density of 8000 W kg−1 and a long cycling life of over 2500 cycles with a capacitance retention of ~ 86.7%. This work successfully demonstrates a proof of concept of quasi-solid-state Na-ion hybrid capacitors as a high performance energy storage device based on two graphene electrodes, which narrows the performance gap between conventional electrochemical capacitors and batteries.