Two-dimensional Na_{: X}SiS as a promising anode material for rechargeable sodium-based batteries: Ab initio material design

The rapidly rising demand for energy storage systems presents an imperative need to develop sodium-ion batteries with high energy density, high conductivity, and low barrier energy. In this work, we present a Density Functional study on the properties of two-dimensional Na_xSiS as a promising anode material for rechargeable sodium-ion batteries. Energetically stable structures of Na-adsorbed silicene sulfide Na_xSiS with various Na contents were explored. It is found that the adsorption energy of a Na atom is higher than -0.4 eV and it decreases with increasing Na content. The electronic structure of pristine silicene sulfide shows semiconductor behaviour with a bandgap of 0.99 eV, while the Na-adsorbed SiS exhibits metallic characteristics. The highest theoretical capacity of 187.2 mA h g^-1, which is higher than that of well-known two dimensional materials, is found in the fully intercalated phase of SiS Na_0.5SiS which corresponds to per side layer. Furthermore, Na ions can diffuse along two typical pathways on the surface of SiS with a small barrier of 183 meV which is much smaller than that of the two dimensional Li_xSiS, Na_xTiS₂, and Na_xMoS₂. All these characteristics suggest that silicene sulfide SiS can be expected to be a promising anode material for sodium ion batteries.