Freestanding graphene/VO2 composite films for highly stable aqueous Zn-ion batteries with superior rate performance

Aqueous Zn-ion batteries (ZIBs) are promising energy storage systems owing to their high safety and low cost. However, their unsatisfactory energy and power densities as well as the cycling performance have hindered their practical application. Herein, we demonstrate a highly reversible zinc/vanadium dioxide system, where freestanding reduced graphene oxide/vanadium dioxide (RGO/VO2) composite films are used as the cathodes. Owing to the synergistic effects from continuously porous network of RGO and the robust structure of VO2, RGO/VO2 composite films not only enhance the transport of electrons and ions, but also accommodate the considerable deformations caused by Zn2+ extraction/insertion. Therefore, the Zn/VO2 batteries exhibit an energy density of 65 Wh kg-1 even at a high power density of 7.8 kW kg-1. More impressively, they deliver excellent capacity retention of 99% after 1000 cycles. In addition, the RGO/VO2 composite films can serve as the electrodes of flexible ZIBs. Flexible soft-packaged Zn/VO2 batteries demonstrated stable electrochemical performance at various bending states. Therefore, the rechargeable Zn/VO2 battery can bridge the gap between conventional batteries and supercapacitors, opening new opportunities for powering portable electronic devices and hybrid electric vehicles.