First-Principles Study of X-Ray Absorption Spectra in NaFeSO4F for Exploring Na-Ion Battery Reactions

By means of first-principles calculations, we study electronic and spectroscopic properties of NaFeSO4F, which has been expected recently to be a candidate cathode material for Na-ion batteries. The NaFeSO4F cathodes have been often synthesized from NaF and FeSO4 in experiments, and present structural stability analysis shows that NaFeSO4F is slightly more stable in energy than the two-phase mixture of NaF and FeSO4. An average voltage of NaFeSO4F cathode is theoretically estimated to be 3.08V with respect to a metal Na anode, and that comes dominantly from electronic structure changes of Fe2+/Fe3+ in NaFeSO4F/FeSO4F. X-ray absorption spectra (XAS) near K-edges at Fe, Na, and F are calculated for NaFeSO4F and FeSO4F as the cathode materials in fully discharged and charged states, respectively, and also for NaF and FeSO4 as possible residual materials in actual cathodes. We show that XAS near K-edges, especially at F sites as well as Fe sites, provides us useful information to clarify battery reactions and structures in actual cathodes.