An excellent full sodium-ion capacitor derived from a single Ti-based metal-organic framework

Hybrid ion capacitors, especially sodium ion capacitors (SICs), have recently attracted enormous attention due to their combined merits of high energy density from the battery-type anode and high power density from the capacitor-type cathode. However, achieving high-performance SICs to overcome the sluggish kinetic energy storage feature and inferior cycling stability of the battery-type anode remains a challenge. In this work, N-doped porous carbon embedded with ultrasmall titanium oxynitride nanoparticles (TiOxNy/C) was developed from metal-organic frameworks (MOFs). As the SIC anode, the as-designed TiOxNy/C exhibited a high reversible capacity (275 mA h g(-1) at 50 mA g(-1)), ultrahigh rate capability, and superior cycling performance, which is attributed to the effective synergy between the ultrasmall TiOxNy nanoparticles and N-doped porous carbon. Furthermore, using a two-for-one strategy, N-doped hierarchical porous carbon (NHPC) with high surface area was prepared from TiOxNy/C by HF etching and displayed high specific capacity and rate capability when used as a SIC cathode. Considering the excellent electrochemical performances of both the anode and cathode, the as-assembled TiOxNy/C//NHPC SIC delivered a high energy density (80 W h kg(-1)) and high power density (4000 W kg(-1)).