Ultrafast high-volumetric sodium storage of folded-graphene electrodes through surface-induced redox reactions

Abstract Post-Li-ion high-volumetric electrochemical energy storage devices have been believed as the next generation power sources for portable electronics and electrified vehicles. Sodium-ion capacitors (SICs) are competent for the sake of low cost and high energy-power performance. The most promising positive electrodes for SICs are functionalized carbon electrodes that enable surface-induced redox reactions of sodium cations and exclude the performance degradation caused by sodium insertion/extraction. However, the surface charge storage cannot realize high-volumetric energy storage. In this work, we demonstrate for the first time that folded-graphene electrodes via three-dimensional densification are promising candidates for high-density sodium storage via the surface-induced process. The folded-graphene electrodes delivered the record high volumetric capacity of 132 mA h/cm 3 at 0.05 A/g. Even at a 100-times higher current density (5 A/g), the volumetric capacity still preserved 72 mA h/cm 3 indicating the great potential for pulse energy output. Moreover, the folded-graphene electrodes demonstrated long-term stability for over 1600 cycles with only 0.01% decay in capacity per cycle. The concept of using 3D folded-graphene electrodes for high volumetric sodium storage can be readily extended to Mg-/Li-ion capacitors and indicates a new avenue towards compact electrochemical energy storage.