Nanostructured Bi₂S₃ encapsulated within three-dimensional N-doped graphene as active and flexible anodes for sodium-ion batteries

Sodium-ion batteries (SIBs) have been increasingly attracting attention as a sustainable alternative to lithium-ion batteries for scalable energy storage. The key to advanced SIBs relies heavily upon the development of reliable anodes. In this respect, Bi₂S₃ has been extensively investigated because of its high capacity, tailorable morphology, and low cost. However, the common practices of incorporating carbon species to enhance the electrical conductivity and accommodate the volume change of Bi₂S₃ anodes so as to boost their durability for Na storage have met with limited success. Herein, we report a simple method to realize the encapsulation of Bi₂S₃ nanorods within three-dimensional, nitrogen-doped graphene (3DNG) frameworks, targeting flexible and active composite anodes for SIBs. The Bi₂S₃/3DNG composites displayed outstanding Na storage behavior with a high reversible capacity (649 mAh·g^–1 at 62.5 mA·g^–1) and favorable durability (307 and 200 mAh·g^–1 after 100 cycles at 125 and 312.5 mA·g^–1, respectively). In-depth characterization by in situ X-ray diffraction revealed that the intriguing Na storage process of Bi₂S₃ was based upon a reversible reaction. Furthermore, a full, flexible SIB cell with Na_0.4MnO₂ cathode and as-prepared composite anode was successfully assembled, and holds a great promise for next-generation, wearable energy storage applications.