Ultra-small ZnS enhanced by Fe-N-C for advanced potassium-ion hybrid capacitors: Electronic transfer dynamics and ion adsorption capability

Metal sulfides are considered as potential anode candidates for potassium ion batteries (PIBs) and hybrid capacitors (PIHCs) due to their abundant energy reserves and decent theoretical capacity. Unfortunately, sulfide has severe volume expansion and slow kinetic reactions, resulting in unsatisfactory electrochemical properties. Herein, we have prepared both internal ultra-small ZnS nanoparticle cores and Fe-N-C shells (ZnS@FeNC), which exhibit excellent K+ storage properties, by a " kill two birds with one stone " approach. The synergistic effect between the ultra-small ZnS nanoparticles and the Fe-N-C network was verified as the origin of the enhanced K+ storage. The presence of the Fe-N-C bond can effectively enhance the potassium energy storage property of ZnS. More specifically, the Fe-N-C backbone alleviates the volume change of ZnS nanoparticles and facilitates the diffusion of KFSI electrolytes and the insertion/extraction of K+. When applied as PIBs, the ZnS@FeNC presents excellent specific capacity (471 mAh g−1 under 0.1 A g−1), stable cycling capability (0.011% capacity decay per cycle under 1 A g−1). More importantly, the PIHC devices exhibit the coexistence of excellent energy density (142.88 Wh kg−1 at 200.5 W kg−1) and excellent-power density (10025 W kg−1 with 36.1 Wh kg−1 retained) with ultra-long lifespan (88% capacity retention at 1 A g−1 after 3000 cycles). This unique structural design of this KFSI electrode provides indispensable guidance for energy storage materials.