Fullerene-like MoSe₂ nanoparticles-embedded CNT balls with excellent structural stability for highly reversible sodium-ion storage

Three-dimensional (3D) porous-structured carbon nanotube (CNT) balls embedded with fullerene-like MoSe₂ nanocrystals were successfully prepared by the spray pyrolysis process and subsequent selenization process. The MoO₂-CNT composite balls prepared by spray pyrolysis transformed into the fullerene-like MoSe₂/CNT (F-MoSe₂/CNT) composite balls by the selenization process. The F-MoSe₂/CNT composite balls exhibited superior sodium-ion storage properties to bare MoSe₂ and MoSe₂/CNT with a filled structure (N-MoSe₂/CNT), both of which were prepared as comparison samples. The 250^th discharge capacities of bare MoSe₂, N-MoSe₂/CNT composite balls, and F-MoSe₂/CNT composite balls were 144, 200, and 296 mA h g^-1, respectively, at a high current density of 1.0 A g^-1, and their capacity retentions measured from the second cycle were 37%, 66%, and 83%, respectively. The 10^th discharge capacities of the F-MoSe₂/CNT composite balls were 382, 346, 310, 280, and 255 mA h g^-1 at current densities of 0.2, 0.5, 1.5, 3.0, and 5.0 A g^-1, respectively. The synergetic effect of the fullerene-like MoSe₂ nanocrystals with ultrafine sizes and the CNT balls with a tangled and 3D porous structure and high electrical conductivity resulted in excellent sodium-ion storage properties of the F-MoSe₂/CNT composite balls.