Multielectron,\nCation and Anion Redox in Lithium-Rich\nIron Sulfide Cathodes

Abstract

Conventional Li-ion cathodes store\ncharge by reversible intercalation\nof Li coupled to metal cation redox. There has been increasing interest\nin new materials capable of accommodating more than one Li per transition-metal\ncenter, thereby yielding higher charge storage capacities. We demonstrate\nhere that the lithium-rich layered iron sulfide Li₂FeS₂ as well as a new structural analogue, LiNaFeS₂, reversibly store ≥1.5 electrons per formula unit and support\nextended cycling. <i>Ex situ</i> and <i>operando</i> structural and spectroscopic data indicate that delithiation results\nin reversible oxidation of Fe²⁺ concurrent with an increase\nin the covalency of the Fe–S interactions, followed by reversible\nanion redox: 2 S^2–/(S₂)^2–. S K-edge spectroscopy unequivocally proves the contribution of\nthe anions to the redox processes. The structural response to the\noxidation processes is found to be different in Li₂FeS₂ in contrast to that in LiNaFeS₂, which we suggest\nis the cause for capacity fade in the early cycles of LiNaFeS₂. The materials presented here have the added benefit of avoiding\nresource-sensitive transition metals such as Co and Ni. In contrast\nto Li-rich oxide materials that have been the subject of so much recent\nstudy and that suffer capacity fade and electrolyte degradation issues,\nthe materials presented here operate within the stable potential window\nof the electrolyte, permitting a clearer understanding of the underlying\nprocesses.