Encapsulation of SeS2 into Nitrogen-Doped Free-Standing Carbon Nanofiber Film Enabling Long Cycle Life and High Energy Density K‑SeS2 Battery

K-SeS2 batteries could provide a low-cost and high energy density energy storage system, because the earth-abundant element potassium (K) shows a low reduction potential and a high gravimetric capacity. But the K-SeS2 battery has never been reported because of the lack of high-performance electrode materials. Herein, we design an advanced K-SeS2 battery by encapsulation of SeS2 in the nitrogen-doped free-standing porous carbon matrix (SeS2@NCNFs). The self-supported SeS2@NCNFs electrode achieves a high reversible capacity of 417 mAh g–1 after 1000 cycles with 85% capacity retention at 0.5 Ag1– with nearly 100% Coulombic efficiency. The nanosized SeS2 nanoparticles are encapsulated in the carbon matrix, which minimizes the volume expansion during cycling and shortens the ion transport pathways, thus enhancing the rate capability. The interconnected porous carbon nanofiber structure could improve the flexibility and offer a continuous pathway for rapid ionic/electronic transport. The DFT calculations confirm that high content N-doping (11.2 at. %) can enhance the chemical affinity between the discharge product and the N-doped carbon. The pyrrolic and pyridinic N-doping lead to stronger adsorption than that of the graphitic N-doping. This proposed design holds great promise for practical application of high energy density K-SeS2 batteries.