Transition metal Si-chalcogenides: a new two-dimensional anode material for Na-ion batteries

Sodium (Na) ion batteries (SIB) hold great importance in energy storage due to their potential to offer a sustainable and cost-effective alternative to traditional lithium-ion batteries. Na is abundantly available and less expensive than lithium, making it an attractive option for large-scale energy storage applications. In the present work, we have predicted a series of 2D transition metal (TM) Si-chalcogenides (TMSiCs), TM₂X₂Si (TM = Ta, Nb and X = S, Se), which exhibit metallic characteristics. All these materials are dynamically stable, but only Ta₂S₂Si, Ta₂Se₂Si and Nb₂Se₂Si are thermally stable even at an elevated temperature of 400 K. Through first-principles calculations, we show that Ta₂S₂Si, Ta₂Se₂Si and Nb₂Se₂Si are promising anode materials for SIB. These materials have a low Na migration barrier in the range of 0.13 to 0.17 eV, which could enhance the cycling performance of the SIB. The calculated average open circuit voltage (OCV) is small, i.e. 0.48, 0.4 and 0.47 V for Ta₂S₂Si, Ta₂Se₂Si and Nb₂Se₂Si, respectively, which suggests the possibility of higher output voltage and larger energy density of the battery. The maximum Na ion capacities for Ta₂S₂Si, Ta₂Se₂Si and Nb₂Se₂Si are calculated to be 206.6, 171.3 and 252.4 mA h g^-1, respectively. Our results could provide fundamental insights into TM₂X₂Si for energy storage applications.