Role of Dual-Redox Couples in Antiperovskite Li₂FeSeO Cathodes

In recent years, cathode materials for Li-ion batteries with antiperovskite (AP) structure have attracted growing interest due to their high capacity, chemical flexibility, low cost, and relatively simple synthesis. Among AP cathodes, Li₂FeSeO has emerged as a promising candidate, demonstrating high capacity and good cycling stability. However, the lack of fundamental knowledge of the operating mechanism of AP cathodes significantly impedes efforts for further improvement. In this work, the redox mechanism and structural evolution of Li₂FeSeO cathode material during electrochemical cycling are revealed through a combination of operando and ex situ X-ray diffraction, X-ray absorption spectroscopy, and Raman spectroscopy coupled with density functional theory calculations. Within the voltage range of 1-3 V vs Li/Li⁺, two stages in the (de)-lithiation process during the initial cycle are determined, involving simultaneous redox activity of Se^2- and Fe²⁺ with strong local structural distortion, leading to the formation of Se₂ ^2- and Fe³⁺ species in the delithiated state and possible formation of Fe⁰ in the lithiated state. While the dual-redox activity of Se^2- and Fe²⁺ ions is the key to high capacity, the -O-Fe-O- framework serves as a crucial element for reversibility during cycling. The study highlights the potential for optimizing the structural frameworks of AP cathodes.