First-principles study of the electrochemical properties of NaFeCl₄ for cathode applications in sodium-ion batteries

All-solid-state sodium-ion batteries (ASSIBs) have attracted significant attention in the energy storage technology field, but the selection of an appropriate cathode material is indispensable. In this work, we perform first-principles calculations on the structural, electrochemical, electronic and lattice vibrational properties of NaFeCl₄ for cathode applications in ASSIBs. Considering the crystalline symmetry, we perform bond valence sum analysis to predict additional positions for inserting Na atoms into a doubled supercell and determine the optimized structure with the lowest energy among different configurations for each Na composition. Our calculations demonstrate that the lattice constants of the stable intermediate phases Na_1+x FeCl₄ (x = 0-1) vary smoothly with increasing content of inserted Na x, confirming relative volume changes of less than 5%. We calculate the formation enthalpies of intermediate phases Na_1+x FeCl₄, plot the convex hull, and determine the step-like electrode potential, finding an average voltage of 3.45 V and a maximum specific capacity of 121 mAh g^-1. Furthermore, we compute activation energies for Na-ion migrations along three different pathways and demonstrate fast Na-ion diffusion with low activation energy. Finally, the electronic band structures and phonon dispersions are investigated to estimate the electron conduction properties and thermodynamic stability. Our work paves the way for designing advanced cathode materials for ASSIBs using iron-based chloride materials.