In Situ Sodiation of CoSe ₂ Modulating Charge Transfer in Sodium−Selenium Batteries

Selenized polyacrylonitrile (SePAN), characterized by the unique C−Se covalent bonds, can suppress the formation of polyselenides and their associated shuttle effect in sodium−selenium (Na−Se) batteries. However, its polymeric structure and distinct conversion mechanism lead to sluggish redox kinetics, severely limiting the rate capability. To overcome these challenges, we propose a catalyst−charge transfer mediator strategy to accelerate the conversion kinetics of SePAN by leveraging the intrinsic dynamic electrochemical behavior of CoSe 2 additives. We demonstrate that CoSe 2 weakens both C−Se and Se−Se bonds, thereby facilitating their cleavage and subsequent conversion. Moreover, the synchronized sodiation of CoSe 2 forms Na x CoSe 2 intermediates, which reorganize the local charge density at the Na 2 Se 2 /Na x CoSe 2 interface. This process induces an additional interfacial charge transfer of 0.36 e , significantly enhancing conductivity. By incorporating CoSe 2 into SePAN nanofibers, the bifunctional strategy enables Na−Se batteries to achieve outstanding rate performance, delivering a high capacity of 404.3 mAh g −1 at an ultra‐high rate of 10 C. Overall, this study provides comprehensive insights into the rational design and mechanistic role of CoSe 2 catalysts in high‐performance Na−Se batteries.