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 3.08 V 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.