Hollow Co₉S₈ cores encapsulated in hierarchical MXene@Bi₂O₃ multiple shells for constructing binder-free electrodes of foldable supercapacitors

Rational structure design and regulation are of paramount importance for obtaining electrode materials with desirable electrochemical performance. Here, a novel binder-free electrode with the hollow Co₉S₈ core@multi-shell structure (CS-x@MXene@Bi₂O₃) derived from metal-organic frameworks (MOFs) precursor is well designed by the electrospinning, sulfuration, carbonization, and hydrothermal processes. In this architecture, the concentration of Co₉S₈ (CS-x) is optimized for an ideal flexible substrate, which alleviates the dimensional variation for long cycle life. The unique cores and the MXene flakes engineered by Bi₂O₃ multiple shells can be responsible for the superior characteristics, including a fast electronic pathway, large specific surface area, enhanced electrical conductivity, and improved electrochemical performance. As expected, the obtained CS-2@MXene@Bi₂O₃ binder-free electrode exhibits a high discharge capacitance of 646.1 F g^–1 (1 A g^–1). Two binder-free electrodes can be assembled into a solid-state supercapacitor with desirable energy and power density, and long-term cyclic stability is demonstrated through 5000 cycles. Given these advantages, the CS-2@MXene@Bi₂O₃ is selected as the electrode in a foldable supercapacitor. More importantly, the specific capacitance is reserved after various deformations. Therefore, it is expected that binder-free electrode materials with the unique core@shell structure design could be applied in wearable and portable energy conversion devices.