Manipulating Molecular Structure to Trigger Ultrafast and Long‐Life Potassium Storage of Fe_0.4Ni_0.6S Solid Solution

Abstract Currently, the main obstacle to the widespread utilization of metal chalcogenides (MS_x) as anode for potassium‐ion batteries (PIBs) is their poor rate capability and inferior cycling stability as a result of the undesirable electrical conductivity and severe pulverization of the nanostructure during large K‐ions intercalation‐extraction processes. Herein, an ultrafast and long‐life potassium storage of metal chalcogenide is rationally demonstrated by employing Fe_0.4Ni_0.6S solid‐solution (FNS/C) through molecular structure engineering. Benefiting from improved electroactivity and intense interactions within the unique solid solution phase, the electrical conductivity and structure durability of Fe_0.4Ni_0.6S are vastly improved. As anticipated, the FNS/C electrode delivers superior rate properties (538.7 and 210.5 mAh g⁻¹ at 0.1 and 10 A g⁻¹, respectively) and long‐term cycle stability (180.8 mAh g⁻¹ at 5 A g⁻¹ after 2000 cycles with a capacity decay of 0.011% per cycle). Moreover, the potassium storage mechanisms of Fe_0.4Ni_0.6S solid solution are comprehensively revealed by several in situ characterizations and theoretical calculations. This innovative molecular structure engineering strategy opens avenues to achieve high‐quality metal chalcogenides for future advanced PIBs.