Understanding Cation and Anion Redox in Li ₂ FeS ₂

Multielectron redox in Li 2 FeS 2 is achieved through both cation and anion redox. 1,2 While the redox mechanism has been investigated experimentally, questions remain regarding the transition from Fe to S redox, persulfide formation, and the inability to reach the large theoretical capacity. Here, we revisit the charging process by analyzing atomic and electronic structures using density functional theory (DFT) calculations. The initial redox stage is found to proceed by Fe oxidation with increased Fe–S covalency driven by a ligand-to-metal charge transfer. The observed Fe redox limit (60–75%) 2 arises from phase separation as further oxidation is calculated to result in decomposition into Li-rich and S-rich phases. This phase separation induces a shift from Fe to S redox with charge compensation transitioning from oxidation of nonbonding Fe states to oxidation of nonbonding S states. Li vacancies at high states of charge create space for FeS 4 tetrahedra to tilt, facilitating S–S bond formation. Subsequent delithiation beyond ~1.5 e – would require unstable S hole formation in the layered structure or a conversion to pyrite FeS 2 , hence the inability to completely delithiate Li 2 FeS 2 . This work provides fundamental insights into the nature of cation and anion redox in Li 2 FeS 2 , informing the development of other high-capacity sulfide cathodes for Li-ion batteries. [1] Hansen, C. J.; Zak, J. J.; Martinolich, A. J.; Ko, J. S.; Bashian, N. H.; Kaboudvand, F.; Van der Ven, A.; Melot, B. C.; Nelson Weker, J.; See, K. A. Multielectron, J. Am. Chem. Soc. 2020 , 142 (14), 6737–6749. [2] Patheria, E. S.; Guzman, P.; Soldner, L. S.; Qian, M. D.; Morrell, C. T.; Kim, S. S.; Hunady, K.; Priesen Reis, E. R.; Dulock, N. V.; Neilson, J. R.; Nelson Weker, J.; Fultz, B.; See, K. A., J. Am. Chem. Soc. 2025 . https://doi.org/10.1021/jacs.4c18440. Figure 1