Exploring the Origin of Anionic Redox Activity in Super Li-Rich Iron Oxide-Based High-Energy-Density Cathode Materials

Super alkali-rich materials (alkali:transition metal ratio of ≥2:1), such as Li5FeO4, exhibit the potential to realize anionic redox upon deep delithiation. Li5FeO4 was shown to undergo reversible cycling between Li4FeO3.5 and Li3FeO3.5 with combined cation/anion redox, a remarkable capacity of 189 mAh/g, and no O2 release. However, the impact of phase transformations on the reaction thermodynamics and the correlation between the structural changes and reaction reversibility remain unclear. In this study, we use first-principles calculations to examine the delithiation and (re)lithiation reactions of Li5FeO4. We show that the experimentally observed charge and discharge processes go through non-equilibrium pathways. Upon delithiation, the compound undergoes a phase transformation from Li5FeO4, with tetrahedrally coordinated (Td) Fe ions, to a delithiated disordered rocksalt structure, with octahedral (Oh) Fe ions. Fe-ion migration has an asymmetric kinetic barrier that makes Td → Oh migration facile, whereas the reverse process has a much larger barrier, explaining the difficulties in reaction reversibility. We further elucidate the transition metal and O redox sequences during the charge cycle and identify the complex electrochemistry associated with the dual participation of cationic redox (Fe3+/Fe4+) and anionic redox (O2–/O–/O0). Armed with this knowledge, we conduct high-throughput screening of known alkali-rich transition metal oxides by evaluating their potential to enable reversible anionic redox, with multiple candidates proposed for further experimental trials. Our work provides a useful guide for the further development of super alkali-rich anionic-redox-active electrodes for high-energy-density batteries.