MXene derived TiS₂ nanosheets for high-rate and long-life sodium-ion capacitors

Sodium-ion capacitors (SICs) have received increasing attention because of the abundance of sodium resources and the combined feature of high energy and power density derived from the battery-type anode and capacitor-type cathode. The lack of suitable anode has been one of the major challenges for SICs. In this study, carbon coated TiS₂ nanosheets are synthesized by in-situ conversion of polyvinyl pyrrolidone (PVP) modified Ti₃C₂T_x MXene (TiS₂@C_pvp) and present outstanding electrochemical performance for high energy and high power Na-ion storage. The ultrathin nanostructure with large interlayer spaces provides easy Na⁺ access, leading to reduced entropic barrier for the reversible conversion and thus ultrafast rate capability and high reversibility. The unique structure exhibits a high discharge capacity of 448 ​mA ​h ​g⁻¹ ​at 0.1 ​A ​g⁻¹, superior rate capability with 387 ​mA ​h ​g⁻¹ even at 10 ​A ​g⁻¹ and a good cycling stability with 92.5% capacity retention after 5000 cycles. Combined with an activated carbon (AC) cathode, the TiS₂@C_pvp//AC SICs delivers high energy density (101.7 ​Wh kg⁻¹) and power density (200 ​W ​kg⁻¹), which is among the best performances of TiS₂-based SICs reported so far. This study sheds light on the development of ultrathin metal sulfides materials for high performance SICs and also expands the application of 2D hybrid materials in energy storage field.