Electronic structure perspective on cation and anion redox in Li2FeS2

Multielectron redox in Li2FeS2 involves both cation and anion contributions. While prior studies have investigated this process experimentally, questions remain about the Fe-to-S redox transition, persulfide formation, and incomplete delithiation. Here, we revisit the electrochemistry of Li2FeS2 using density functional theory (DFT) calculations. Initial Fe oxidation is shown to proceed via increased Fe–S covalency driven by ligand-to-metal rehybridization. This accounts for only ~60% of the Fe capacity, after which phase separation into Li1.5FeS2 and Li0.5FeS2 induces a shift to S oxidation. We identify nonbonding S 3p orbitals as redox-active and track their evolution using a newly developed electronic structure descriptor. These orbitals undergo S-to-Fe rehybridization, progressive oxidation, and eventual reorganization into persulfide bonds. At high states of charge, Li vacancies enable FeS4 tilting and facilitate S–S bond formation. Further delithiation beyond Li0.5FeS2 would require unstable S hole formation or conversion to pyrite FeS2, explaining why it is challenging to fully delithiate this material. These findings offer insights into the interplay of cation and anion redox, guiding the design of high-capacity sulfide cathodes.