Unraveling the Nature of Excellent Potassium Storage in Small‐Molecule Se@Peapod‐Like N‐Doped Carbon Nanofibers

The potassium–selenium (K–Se) battery is considered as an alternative solution for stationary energy storage because of abundant resource of K. However, the detailed mechanism of the energy storage process is yet to be unraveled. Herein, the findings in probing the working mechanism of the K‐ion storage in Se cathode are reported using both experimental and computational approaches. A flexible K–Se battery is prepared by employing the small‐molecule Se embedded in freestanding N ‐doped porous carbon nanofibers thin film (Se@NPCFs) as cathode. The reaction mechanisms are elucidated by identifying the existence of short‐chain molecular Se encapsulated inside the microporous host, which transforms to K2Se by a two‐step conversion reaction via an “all‐solid‐state” electrochemical process in the carbonate electrolyte system. Through the whole reaction, the generation of polyselenides (K2Sen, 3 ≤ n ≤ 8) is effectively suppressed by electrochemical reaction dominated by Se2 molecules, thus significantly enhancing the utilization of Se and effecting the voltage platform of the K–Se battery. This work offers a practical pathway to optimize the K–Se battery performance through structure engineering and manipulation of selenium chemistry for the formation of selective species and reveal its internal reaction mechanism in the carbonate electrolyte.