Two-dimensional layered metal diseleniums and its application in the electrochemical energy

Clean and sustainable energy supply is regarded as the most significant problems in the 21st century, which is ultimately related to our daily lives, global environment, economy, and human health. Although fossil fuels as the main energy sources will continue to play a crucial role in responding our energy needs in the future, they come at a tremendous price, including a rapid increase in greenhouse gas emissions and long-lasting environmental pollution. The imminent shortage of fossil fuels and growing ecological concerns is pushing scientists and engineers to exploit sustainable, clean, and highly efficient technologies to supply and store energy. With the permanently increasing demand in energy resources, massive efforts have been devoted to developing advanced energy storage and conversion systems. Novel materials hold the key to fundamental advances in energy conversion and storage, both of which are vital in order to meet the challenge of global warming and the finite nature of fossil fuels. Graphene as one of the most successful functionally nanomaterials, which have attracted great attention due to their unique properties of large surface area, superior electric and thermal conductivities, high mechanical flexibility, chemical stability, which render them great choices as alternative electrode materials for electrochemical energy storage systems. The ultrathin two-dimensional (2D) morphology of graphene with unique properties is triggering a great deal of attention toward the family of 2D structures. The types of 2D inorganic graphene analogues nanomaterials such as metal dichalcogenides have also been studied and applied in various applications including electronics, optoelectronics, energy storage devices, solar energy, electrocatalysts for hydrogen evolution reaction and so on. Layered transition metal dichalcogenides (MoS 2 , MoSe 2 , WS 2 , WSe 2 , etc.) as the typical graphene analogues, which are a chemically diverse class of compounds having band gaps from 0 to ∼2 eV and remarkable electrochemical properties. The band gaps and electrochemical properties of layered transition metal dichalcogenides can be tuned by exchanging the transition metal or chalcogenide elements. Among numerous transition metal dichalcogenides, layered metal seleniums exhibit many novel properties, especially in the electrochemical energy field, which may be beyond those existing in layered metal disulfide. The excellent electrochemical performances of the layered metal seleniums materials could be attributed to their unique intrinsic structure. Firstly, the layered metal seleniums have a higher electrical conductivity than layered metal disulfide owing to its narrower band-gap energies. In addition, the larger diameter of Se atom provides the layered metal seleniums with expanded interlayer spacing, which will afford more active reaction sites for electrolyte ion storage and reduce the energy barrier for electrolyte ion insertion. Benefiting from their remarkable electrochemical properties, these layered metal seleniums will play meaningful roles for low-cost and environmentally friendly energy storage and electrocatalysts for hydrogen evolution technologies. In this review, we summarize the physic structures, synthesis methods of 2D layered metal diseleniums, as well as its application in the field of electrochemical energy, including the Li ion battery, Na ion battery, supercapacitor, Mg ion battery and hydrogen evolution reaction. Finally, we make the prospects and the development trends on the layered metal diseleniums.