Revealing Redox Pathways and Structural Evolution in Cation-Disordered Li2MnO2F via Operando X-ray Absorption Spectroscopy

Cation-disordered rocksalt lithium-rich compounds are promising materials for lithium-ion batteries as positive electrodes due to their potential low cost and high energy density. In this paper, we studied in detail redox processes of the composition Li₂MnO₂F using a combination of operando and ex situ X-ray absorption spectroscopy. Oxygen redox and manganese redox were deconvoluted during the first cycle by using a chemometric approach. We show a stepwise redox during charge with most of the manganese oxidized before 4.3 V, followed mainly by oxygen redox. In discharge, most of the reduction of Mn⁴⁺ occurs at high voltage above 4.3 V, but a non-negligible amount of Mn³⁺ is reduced to Mn²⁺ below 2 V, while oxygen redox occurs near 3 V with a strong hysteresis. The local order of the material is characterized by a strong Jahn–Teller deformation that disappears with the oxidation of Mn³⁺. The analysis of aged materials highlights that the capacity fade in the cathode material occurs in two stages: during the first few cycles, due to a decrease in oxygen redox activity and subsequently, due to a reduced manganese redox activity, linked with manganese reduction, O-loss, and probable cation-densification.