Confining Pyrrhotite Fe7S8 in Carbon Nanotubes Covalently Bonded onto 3D Few‐Layer Graphene Boosts Potassium‐Ion Storage and Full‐Cell Applications

Abstract

The pyrrhotite Fe₇S₈ with mixed Fe‐valence possesses high theoretical capacity, high conductivity, low discharge/charge voltage plateaus, and superior redox reversibility but suffers from structural degradation upon (de)potassiation process due to severe volume variations. Herein, to conquer this issue, a novel hierarchical architecture of confining nano‐Fe₇S₈ in carbon nanotubes covalently bonded onto 3D few‐layer graphene (Fe₇S₈@CNT@3DFG) is designed for potassium storage. Notably, CNTs could successfully grow on the surface of 3DFG via a tip‐growth model under the catalytic effect of Fe₃C. Such structure enables the hierarchical confinement of 0D nano‐Fe₇S₈ to 1D CNTs and further 1D CNTs to 3DFG, effectively buffering the volume variations, prohibiting the agglomeration of Fe₇S₈ nanograins, and boosting the ionic/electronic transportation through the stable and conductive CNTs‐grafted 3DFG framework. The as‐prepared Fe₇S₈@CNT@3DFG electrode delivers an exceptional rate capability (502 mAh g⁻¹ at 50 mA g⁻¹ with 277 mAh g⁻¹ at 1000 mA g⁻¹) and an excellent long‐term cyclic stability up to 1300 cycles. Besides, the in‐situ XRD and ex‐situ XPS/HRTEM results first elucidate the highly reversible potassium‐storage mechanism of Fe₇S₈. Furthermore, the designed potassium full‐cell employing Fe₇S₈@CNT@3DFG anode and potassium Prussian blue (KPB) cathode delivers a promising energy density of ≈120 Wh kg⁻¹, demonstrating great application prospects.