Two-dimensional (2D) layered transition-metal dichalcogenides has been regarded as highly useful electrode materials for rechargeable lithium-ion and sodium-ion batteries. However, their metal ion storage mechanism (conversion reaction or intercalation) and cyclic stability depends on the potential region as well as nanostructure design. In this talk, we will present our detailed investigation on ultrathin MoS2-xSex nanoflakes vertically aligned on the 3D lightweight graphene foam (MoS2-xSex/GF) in Na-ion storage. Sample is fabricated by a hydrothermal reaction followed by a selenization process. As a freestanding electrode, the MoS2-xSex/GF demonstrates high-rate reversible Na-ion storage, where both the capacity and rate-performance are enhanced by the selenium substitution. We show that by adjusting the potential range it is possible to maintain the 2D layered structure and improve the capacitance retention by the intercalation mechanism. The irreversible conversion reaction has been verified with in-situ Raman spectroscopy and ex-situ X-ray diffraction measurements. This study shed new light on better understanding the electrochemical performance of 2D transition metal chalcogenides in batteries.