Highly Ordered Graphene Solid: An Efficient Platform for Capacitive Sodium-Ion Storage with Ultrahigh Volumetric Capacity and Superior Rate Capability

As an emerging type of electrochemical energy storage devices, sodium-ion capacitors (SICs) are potentially capable of high energy density and high power density, as well as low cost and long lifespan. Unfortunately, the lack of high-performance capacitive cathodes that can fully couple with the well-developed battery-type anodes severely restricts the further development of SICs. Here, we develop a compact yet highly ordered graphene solid (HOGS), which combines the merits of high density and high porosity and, more attractively, possesses a highly ordered lamellar texture with low pore tortuosity. As the capacitive cathode of SICs, HOGS delivers a record-high volumetric capacity (303 F cm^-3 or 219 mA h cm^-3 at 0.05 A g^-1), a superior rate capability (185 F cm^-3 or 139 mA h cm^-3 even at 10 A g^-1), and an outstanding cycling stability (over 80% after 10000 cycles). The material design and construction strategies reported here can be easily extended to other metal-ion-based energy storage technologies, exhibiting universal potentials in compact electrochemical energy storage systems.