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-MoOx 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 degrees C, respectively. At a higher tellurization temperature of 600 degrees 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 200th 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.