Elucidating hindered reactivity of high-manganese content mixed-olivine cathodes for Na-ion batteries: an operando X-ray absorption study

Sodium-based olivine cathodes appear to be a viable, sustainable and low-cost alternative in commercial batteries, especially for the stationary electrochemical energy storage. However, high-manganese-content mixed olivine cathodes have not reached desired performance, particularly due to the material's structural instability and limited reactivity. Hence, to better understand the structural dynamics during battery operation, an in-depth X-ray absorption spectroscopy (XAS) analysis under operando conditions of the electrochemically converted NaFe0.6Mn0.4PO4 (NFMP) cathode is performed. The X-ray absorption near edge structure (XANES) study conducted at both Fe and Mn K-edges reveals uncompleted Mn oxidation in both ex-situ and operando measurements, accompanied by a progressive distortion of the FeO6 octahedra during de-sodiation. The local structure modifications are confirmed via extended X-ray absorption fine structure (EXAFS), which also reveals the occurrence of Jahn-Teller distortion of Mn3+, similarly to lithium-based materials. The obtained results elucidate the hindered reactivity of the material observed in Na-cell, particularly related to Mn3+/Mn2+ redox couple, and clarify the structural dynamics of the sodium (de-)insertion process. Furthermore, the presented results shed the light on the reasons of the limited Mn3+/Mn2+ redox activity, and might pave the way to an optimization of the Mn content in mixed olivine cathodes to achieve low-cost, efficient and sustainable materials with improved performances.