Impact of Fluorine Content on Structure, Redox, and Electrochemical Performance of Disordered Rock-Salt Li1.25Mn0.5+ y /2Nb0.25– y /2O2– y F y Electrode Materials

Abstract Disordered rocksalt (DRS) oxides are highly promising positive electrode materials for next-generation Li-ion batteries, yet their practical application is severely bottlenecked by the irreversibility of oxygen-redox processes. Fluorination (O2– by F– substitution) represents a powerful strategy to mitigate these drawbacks, but achieving and characterizing high-content fluorine incorporation remains a significant challenge. In this work, we demonstrate the successful mechanochemical synthesis of an extended series of Mn- and Nb-based DRS oxyfluorides, Li1.25Mn0.5+y/2Nb0.25–y/2O2–yFy (0 ≤ y ≤ 0.5). While average structure analysis via powder XRD indicates the formation of a well-disordered solid solution across the entire series, local-structure probes reveal that the ideal cubic NaCl-type model with mixed occupation of the cation and anion sites fails to fully describe the materials. Pair distribution function (PDF) and XANES analyses unveil significant local deviations, highlighted by strongly distorted Mn3+ octahedra induced by the Jahn–Teller effect, while solid-state 7Li and 19F MAS NMR spectroscopies reveal a short-range clustering tendency. Bridging the local and macroscopic scales, dark-field TEM imaging demonstrates that these local arrangements assemble into coherent crystalline nanodomains (1–5 nm), in excellent agreement with the PDF analysis. Regarding the electrochemical properties, this progressive fluorination successfully shifts the redox mechanism from irreversible oxygen oxidation toward more reversible cationic Mn3+/Mn4+ redox, reducing the irreversible capacity and substantially enhancing the average discharge voltage. However, a persistent irreversible capacity loss remains regardless of the fluorine content. This study highlights the critical role of multiscale characterization in understanding the complex local structure–property relationships of highly fluorinated DRS cathodes.