Discharge reaction mechanisms in Na/FeS₂ batteries: First-principles calculations

We have studied microscopic discharge reaction mechanisms in Na/FeS₂ batteries by first-principles calculations. The calculated Na-Fe-S phase diagram shows that the discharge reactions can proceed by converting 4Na and FeS₂ into 2Na₂S and Fe as a fully discharged state. As an intermediate discharge reaction, we find that Na_xFeS₂ (x ∼ 1.5) intermediate products can be generated in the cathode, giving two major plateaus in voltage-capacity curves. The calculated voltage-capacity characteristics and X-ray absorption spectra at S and Fe K-edges of Na-discharged FeS₂ cathode materials are compared with experimental results, showing that theoretically determined reaction formulas can account for the experimental discharge reactions.