their excellent physical and chemical properties. A large number of the layered materials are van der Waals heterostructures, which are composed from atomically thin layers in one-unit cells through van der Waals interactions. Compared to their bulk counterparts, atomically thin layered nanosheets tend to exhibit remarkable electronic and optical properties, outstanding mechanical flexibility, and exceptional catalytic performance. This is mainly attributed to the reduced dimensionality and quantum-confinement effect. [6] To date, the abovementioned 2D layered materials hold great promise for various applications, such as semiconductors, optoelectronics, photovoltaics, quantum dots, sensors, filtration, lightweight/strong composite materials, medicine, biological engineering, water purification, and energy conversion/ storage. As a representative of 2D layered inorganic materials, TMDCs have received a lot of attention because of their many unusual physical and chemical properties. Such kinds of compounds have a general chemical formula MX 2 (M = transition metal, X = chalcogen), and are bonded by strong intralayer bonding (covalent bonding) and weak interlayer bonding. [7] Each individual TMDC monolayer is composed of hexagonally packed M atoms sandwiched by two layers of X atoms. For MX 2 , the M is often from group IV of the periodic table and the X is commonly seen as S, Se, and Te. As expected, the various MX 2 have diverse electronic properties, including as insulators (e.g., HfS 2 ), semiconductors (e.g., MoS 2 and WS 2 ), semimetals (e.g., WTe 2 and TiSe 2 ), and true metals (e.g., NbS 2 and VSe 2 ). In addition to the preserved properties from their bulk counterparts, the reduced dimension of these materials into atomically thin layers (mono-or few-layers) gives them additional extraordinary characteristics. [7c,8] For example, the absence of interlayer interactions in monolayer MX 2 gives rise to a charge-carrier redistribution, as well as conversion from indirect bandgap to direct bandgap. [9] This is critically important for MX 2 to be applied in some areas, such as electronics and transistors. Moreover, atomically thin MX 2 are excellent host materials for ion insertion and ion transport, which are beneficial for developing high-performance electrodes for energy storage and conversion devices. [9a,10] Furthermore, the atoms at edges or grain boundaries with low coordination number in atomically thin Since the discovery of graphene in 2004, research on 2D materials has grown rapidly. Compared to their bulk counterparts, 2D atomically thin, layered transition-metal dichalcogenides (TMDCs or MX 2 ) nanosheets exhibit excellent electronic and optical properties, outstanding mechanical flexibility, and exceptional catalytic performance. As a representative of 2D materials, MX 2 holds great promise in many potential applications, especially in energyrelated applications. Here, a brief overview of atomically thin layered MX 2 nanosheets applied in energy storage and conversion systems is presented. Fi...