High Sulfur-doped hollow carbon sphere with multicavity for high-performance Potassium-ion hybrid capacitors.

S doping is an effective strategy to improve the potassium-ion storage performance of carbon-based materials. However, due to the large atomic radius of S and poor thermal stability, it is challenging to synthesize carbon materials with high sulfur content by solid-phase transformation. In this work, we designed a multi-cavity structure that can confine the molten S during heat treatment and make it fully react, then achieving high S doping (7.6 at. %). As we known, S doping can also effectively increase the active sites of carbon materials to obtain higher capacity. In addition, through different ex/in-situ characterizations and DFT calculations, we confirmed that the S atoms can effectively expand the interlayer spacing of carbon, which facilitates the intercalation/deintercalation reaction of K, thereby significantly improving the rate performance. Therefore, benefiting from the effect of S-doping, the sample exhibits high reversible specific capacity (401.0 mAh g at 0.1 A/g) and rate performance (167.2 mAh g at 5 A/g). The as-assembled K hybrid capacitor delivers both high energy density and power density (138.5 W h kg and 7692.5 W kg, respectively). This work provides a new approach to design S content carbon-based materials for high performance K storage.