Na2FeS2 Cathode for Sodium-Ion Batteries: A Theoretical Study

Sodium-ion batteries (SIBs) with high energy density, improved safety, and low cost are exciting candidates for next-generation energy storage and electrical vehicles. Cathode materials are the core component for SIBs. Recently, an experimental study reported a promising Na2FeS2 cathode with a specific structure consisting of edge-shared and chained FeS4 tetrahedra as the host structure and a high capacity of 320 mA h g–1 for sodium storage. However, the underlying reaction mechanisms and Na migration pathways have not been fully understood. In this study, density functional theory (DFT) and DFT + U calculations are performed to study the structural stability, phase stability, electronic properties (spin polarization density of states), average voltage using total energy based on fully charged and discharged states, and Na-ion transport and diffusion channel using ab initio molecular dynamic simulations of the NaXFeS2 (X = 2, 1.5, and 1) cathode materials. It is revealed that Na2FeS2 is unstable at 0 K and possesses a theoretical capacity of 323 mA h g–1 with a low diffusion barrier of 0.40 eV in NaxFeS2 series. Moreover, some transition metals are substituted at Fe sites to evaluate the structural effect of Na2FeS2, in which Na2MnS2 exhibits excellent structural stability, low hull energy, and high theoretical capacity of 325 mA h g–1, which could be appealing for researchers in the future.