Feasibility of NaMNbO4 and NaMSbO4 (M = Ni, Co, Mn, Fe) for their use as high voltage cathode materials in sodium ion battery—a DFT study

Density functional theory study of NaMNbO₄ and NaMSbO₄ (M = Ni/Co/Mn/Fe), for using them as cathode materials in sodium ion battery, was computed in this work. The study showed that while transition metal atoms M act as the redox center, Nb/Sb acts as a matrix and holds the structural integrity. The electrochemical voltages were found to have values 4.71 V, 4.12 V, 2.82 V, and 3.52 V for NaMSbO₄ while having values 4.34 V, 4.04 V, 2.96 V, and 3.50 V for NaMNbO₄, respectively for M = Ni, Co, Fe, and Mn. The voltage trend is in the increasing order in accordance with Ni > Co > Mn > Fe. This trend can be explained by the Hund’s rule. The structural studies indicated that the volume strain during charging is < 5% for all the materials. The high redox potential with minimal volume strain of NaMNb/SbO₄ (with M = Ni/Co/Mn/Fe) presents them as excellent cathode material for Na ion battery applications. Simulation of the diffusion barrier (activation energy) and room temperature ionic conductivity indicates that a moderate diffusion barrier ~ 0.274–0.527 eV exists in these composites with average ionic conductivity ~ 10⁻⁶ to 10⁻⁸ S/cm.