Na2FeS2 Cathode for Sodium-Ion Batteries: A Theoretical Study

Sodium-ion batteries (SIBs) with high energy density,improvedsafety, and low cost are exciting candidates for next-generation energystorage and electrical vehicles. Cathode materials are the core componentfor SIBs. Recently, an experimental study reported a promising Na2FeS2 cathode with a specific structure consistingof edge-shared and chained FeS4 tetrahedra as the hoststructure and a high capacity of 320 mA h g(-1) forsodium storage. However, the underlying reaction mechanisms and Namigration pathways have not been fully understood. In this study,density functional theory (DFT) and DFT + U calculationsare performed to study the structural stability, phase stability,electronic properties (spin polarization density of states), averagevoltage using total energy based on fully charged and discharged states,and Na-ion transport and diffusion channel using ab initio moleculardynamic simulations of the Na X FeS2 (X = 2, 1.5, and 1) cathode materials. Itis revealed that Na2FeS2 is unstable at 0 Kand possesses a theoretical capacity of 323 mA h g(-1) with a low diffusion barrier of 0.40 eV in Na x FeS2 series. Moreover, some transition metals aresubstituted at Fe sites to evaluate the structural effect of Na2FeS2, in which Na2MnS2 exhibitsexcellent structural stability, low hull energy, and high theoreticalcapacity of 325 mA h g(-1), which could be appealingfor researchers in the future.