N-doped carbon coated Ni-Mo sulfide tubular structure decorated with nanobubbles for enhanced sodium storage performance

The electrochemical performances of transition metal sulfides for sodium-ion batteries (SIBs) depend on their scrupulous designed nanostructures. In particular, numerous efforts have been devoted to synthesize bubble-like structures that consisted of nano-sized hollow spheres for their engaging features that enhanced electrochemical properties; increased surface areas, shortened diffusion length between active material and electrolyte and reduced stress during the Na-ion insertion and extraction. However, fabricating multicomponent transition metal sulfides with bubble-like structure has scarcely succeeded until now. In this study, we demonstrate the rational design and fabrication of hierarchical tubular structures decorated with nanobubbles, which consist of multicomponent (Mo, Ni) metal sulfides (NB-NiMoS). To achieve this structure, the sulfidation process in an autoclave and subsequent heat-treatment were employed to hydrated NiMoO₄ nanorod precursors. By comparing between samples obtained in each step, the transformation mechanism of hydrated NiMoO₄ nanorods was examined. In order to further enhance the electrochemical properties, NB-NiMoS was coated with polydopamine followed by carbonization in an inert condition, forming the N-doped carbon layers on the tubes. Benefiting from the rational structure and conductive N-doped carbon layers, carbon-coated NB-NiMoS (NB-NiMoS@C) exhibited excellent electrochemical performances as an anode for SIB. It delivered reversible discharge capacity of 420 mA h g⁻¹ for the 200th cycle at the current density of 0.5 A g⁻¹, and its capacity retention calculated from the second cycle was 84%. Even at a high current density of 10 A g⁻¹, the high and stable discharge capacity of 309 mA h g⁻¹ could be achieved.