The Interplay of Oxygen Functional Groups and Folded Texture in Densified Graphene Electrodes for Compact Sodium-Ion Capacitors

Sodium-ion capacitors (SICs) can effectively combine high energy density with high power density, and are especially appropriate for high-power demanding applications of large-scale stationary energy storage. Surface-induced pseudocapacitive charge storage based on porous or nano carbon materials is regarded as the most promising candidate for SICs. Unfortunately, their ultralow packing densities severely restrict their practical applications. A novel approach toward ultrafast high-volumetric SICs based on folded-graphene electrodes has already been demonstrated and showed quite competitive performance. In this work, it is further proved that oxygen functional groups and folded texture are two key elements for high-volumetric sodium storage of folded-graphene electrodes. Through a simple and controllable method, of thermal treatment in inert atmosphere, both the oxygen functional groups and folded texture can be quantitatively manipulated to better investigate the individual contribution and mutual interplay. It is illustrated that oxygen functional groups are crucial to superior capacitive sodium storage while folded texture is not only the origin for high-volumetric sodium storage but also beneficial for both capacitive and additional diffusion-controlled sodium storage. Inspired by above-mentioned conclusion, more rational designs and effective preparation of advanced structure and novel materials can be realized to better promote the development of high-volumetric SICs.