Engineered phase of nickel sulfides inside hairy hollow fibers towards high-performance anodes for flexible potassium ion hybrid capacitors

The development of potassium-ion hybrid capacitors (PIHCs) is restricted by the cathode/anode kinetics mismatch and the limited working environments. Herein, a freestanding fiber assembled from tiny carbon nanotubes capped with nickel sulfide nanocrystallites (NSCN hollow fiber) is designed as an anode for PIHCs. The highly porous and conductive framework promotes fast electron/ion transport. Meanwhile, the nanoscale crystallites inside carbon nanotubes facilitate the accommodation of large volume change. Moreover, the interaction between N-doped carbon and nickel sulfide further promotes potassium storage. For the first time, the phase of nickel sulfide inside the NSCN hollow fiber is engineered and their properties are carefully evaluated. Both theoretical and experimental results demonstrate that the α-NiS phase shows the best kinetics and durability among all tailor-made phases of α-NiS, β-NiS and NiS 2 . The superior performance of the α-NiS-NSCN hollow fiber is associated with the advantages of its well-designed architecture and preferable phase. Moreover, the full PIHC based on the α-NiS-NSCN anode achieves high energy/power densities and superior high-rate long-term performance. More impressively, the flexible PIHC incorporated with the polymer electrolyte achieves good pliability and low-temperature tolerance even at −20 °C. Therefore, this work not only provides a high-performance anode for potassium ion storage, but also sheds light on the fabrication of flexible power sources under diverse working conditions. Benefitting from the well-designed structure and preferable hexagonal phase, an α-NiS-NSCN anode achieves high pliability, superior rate capability and good cycling stability for PIHCs.