Discharge Reaction Mechanism of FeS2 Cathodes in Na Batteries: First-Principles Calculations

Several types of materials have been extensively studied to find suitable electrode materials for Na secondary batteries, and recently pyrite-FeS₂ has attracted much interest as a cathode material of Na batteries [1]. However charge and discharge reactions in the Na/FeS₂ batteries, which are governed by conversion mechanisms, are not fully understood yet especially concerning intermediate states involved in the reactions.

In this study, by using first-principles calculations for probable phases in Na/FeS₂ conversion reactions, we theoretically estimate electromotive forces (EMFs) characterizing battery performances. To study atomic structure changes during the reactions, we calculate x-ray absorption spectra (XAS) of Na/FeS₂ before and after Na discharges. Obtained EMF and XAS are compared with experimental results [1], and atomistic discharge reaction mechanisms in Na/FeS₂ battery systems are discussed.

We use stable crystal phases as model structures of Fe-S, Na-S and Na-Fe-S systems. Calculations are based on the density functional theory within the generalized gradient approximation adopting the all-electron full-potential linearized augmented plane wave method, and XAS spectra are computed on the basis of Fermi’s golden rule with the one-electron ground states [2]. All the computations are done using the HiLAPW code.

We first calculate EMFs assuming stable Na-S crystals as a final product of discharge reactions of Na/pyrite-FeS₂. Calculated results show that the most likely final product is Na₂S+Fe generating EFM of about 1.2V vs. Na, consistent with the experimental results. We further investigate multi-step reactions considering Fe-S, Na-S and Na_xFeS₂ (x = 1 and 2) models as intermediate products. All assumed multi-step reactions are energetically higher, indicating that the assumed intermediates can be minor products relative to the two-phase form as FeS₂-Na₂S. Among the assumed multi-step reactions, we find energetically acceptable two-step reactions that produce FeS and Na_xFeS₂ intermediate phases, and the two-step reactions can give EFMs close to the two-phase average 1.2V vs. Na. Thus FeS as well as Na_xFeS₂ may be plausible intermediate products. Comparison of the calculated XAS spectra at the S K-edge with the experimental ones [1] also shows that the intermediates of FeS+Na₂S likely exist during the reactions.

[1] A. Kitajou, J. Yamaguchi, S. Hara and S. Okada, J. Power Sources 247, 391 (2014).

[2] T. Oguchi and H. Momida, J. Phys. Soc. Jpn. 82, 065004 (2013).