Two-dimensional materials as anodes for sodium-ion batteries

Sodium-ion batteries (NIBs) are considered as promising alternatives to lithium-ion batteries (LIBs) especially in large-scale energy storage systems of renewable energy owing to their potentially low production cost. In view of the larger ionic size of Na ions than Li ions, the commercial graphite anode in LIBs is not suitable for NIBs. To achieve NIBs with a high energy density, various anode materials have been studied in recent years. Among these, two-dimensional (2D) materials have attracted considerable attention on account of their unique 2D-layered structure with infinite planar lengths; these materials provide short paths for sodium-ion transportation and large surface areas for sodium ion adsorption. Furthermore, some 2D materials exhibit a high electronic conductivity (e.g. graphene and metal selenide), which also aids in increasing the capacity and enhancing the rate performance. This review provides an insight into the recent progress in 2D anode materials in NIBs, including graphene and its derivatives, transition metal sulfides/selenides, phosphorene/metal phosphides, transition metal carbides/nitrides (MXene), and other graphene-like elemental analogs (silicene, germanene, stanene, and borophene). Moreover, a series of in situ characterization techniques, which have been utilized to investigate the fundamental sodium storage mechanism of the aforementioned 2D anode materials, are explained in-depth in this paper. This review is focused on providing a pathway for comprehending the electrochemical properties and methods to study the sodium storage mechanism of 2D anode materials for further research.