Conversion of 1T-MoSe₂ to 2H-MoS₂ : XSe_{2-2 x} mesoporous nanospheres for superior sodium storage performance

S-Doped 2H-MoSe₂ (i.e., 2H-MoS_2xSe_2-2x) mesoporous nanospheres assembled from several-layered nanosheets are synthesized by sulfurizing freshly-prepared 1T-MoSe₂ nanospheres, and they serve as a robust host material for sodium storage. The sulfuration treatment is found to be beneficial for removing surface/interface insulating organic contaminants and converting the 1T phase to the 2H phase with improved crystallinity and electrical conductivity. These result in significantly enhanced sodium storage performance, including charge/discharge capacity, first Coulombic efficiency, cycling stability, and rate capability. Coupled with benefits from in situ carbon modification and its mesoporous morphology, the 2H-MoS_2xSe_2-2x (x = 0.22) nanosphere anode can maintain a reversible capacity of 407 mA h g^-1 after 100 cycles with no observable capacity fading at a high current density of 2.0 A g^-1. This value is much higher than those of the anode fabricated with the freshly-prepared 1T-MoSe₂ (95 mA h g^-1) and the annealed 2H-MoSe₂ (144 mA h g^-1) samples. As the current density rises from 0.05 to 5.0 A g^-1 (100-fold increase), the discharge capacity retention is significantly increased from 39% before sulfuration to 65% after sulfuration. This superior electrochemical performance of the 2H-MoS_2xSe_2-2x electrode suggests a promising way to design advanced sodium host materials by surface/interface engineering.