Charles J. Hansen, Joshua J. Zak, Andrew J. Martinolich, Jesse S. Ko, Nicholas H. Bashian, Farnaz Kaboudvand, Anton Van der Ven, Brent C. Melot, Johanna Nelson Weker, and Kimberly A. See,
J. Am. Chem. Soc. 2020, 142, 6737-3749.
tl;dr Two isostructural alkali-rich metal sulfides - Li2FeS2 and previously unreported LiNaFeS2 - demonstrate reversible multielectron redox (>= 1.5 electrons). We probe the charge storage mechanism and find that both cationic and anionic redox contribute. In the beginning of the charge profile, the materials undergo a deintercalation-like mechanism in which Fe2+ is oxidized to Fe2+/3+. Oxidation of Fe2+ causes the Fe and S bands to rehybridize, increasing the covalency of the Fe-S correlations and pushing the S 2p states closer to the Fermi level. Subsequent oxidation occurs on the anions, (S)2-, to form (S2)2- moieties causing loss of long-range order. The anion oxidation is clearly observed in S K-edge X-ray absorption spectroscopy.