Carbon/two-dimensional MoTe₂ core/shell-structured microspheres as an anode material for Na-ion batteries

Unique-structured composite microspheres of carbon and MoTe₂ were prepared by a two-step process. Precursor C-MoO_x composite microspheres were prepared by spray pyrolysis, and then the precursor was transformed into C-MoTe₂ composite microspheres by a tellurization process. C-MoTe₂ composites with a uniform distribution of MoTe₂ nanocrystals (C/MoTe₂) and core-shell-structured C-MoTe₂ composites (C@MoTe₂) were synthesized at tellurization temperatures of 450 and 600 °C, respectively. At a higher tellurization temperature of 600 °C, all of the MoTe₂ nanocrystals moved to the surface of the microsphere because of the Ostwald ripening process. The initial discharge capacities of the C/MoTe₂, C@MoTe₂, and bare MoTe₂ (i.e., containing no carbonaceous materials) powders for Na-ion storage at a current density of 1.0 A g^-1 were 328, 388, and 341 mA h g^-1, respectively. The discharge capacities of the C/MoTe₂, C@MoTe₂, and bare MoTe₂ powders for the 200^th cycle were 241, 286, and 104 mA h g^-1, respectively, and the corresponding capacity retentions, which were measured from the second cycle were 100%, 99%, and 37%, respectively. The high structural stability and well-developed two-dimensional layer of MoTe₂ of the C@MoTe₂ microspheres provide superior Na-ion storage properties compared to those of the C/MoTe₂ microspheres and bare MoTe₂ powder.