Fluorine-Mediated Molten-Salt Crystallization Regulates Phase Evolution and Structural Stability in Sodium Layered Oxides

Ni- and Co-free P2-type Fe-Mn layered oxides with high Na concentrations are promising cathodes for sodium-ion batteries, but their practical use is limited by phase instability during Na-excess synthesis and structural degradation during cycling and storage. Although large single-crystal-like architectures could improve stability, controlled phase-selective crystallization in these cathode systems remains challenging. Here, we show that fluorine-assisted molten-salt synthesis enables controlled formation of large-particle Fe-Mn-Ti P2 cathodes with enhanced phase selectivity and structural coherence. Compared with Na-excess processing alone, NaF-assisted flux growth expands the P2 phase stability window and promotes large particles with uniform morphologies while preserving local layered ordering. Multiscale characterization further reveals more-reversible structural evolution and more homogeneous Mn redox behavior in the NaF-assisted material, together with a gradient-like fluorine distribution involving surface-enriched NaF-like species and lower level interior fluorine environments. These coupled structural, morphological, and chemical features collectively mitigate crack formation and deliver good cycling stability. In summary, NaF-assisted molten-salt synthesis shows promise in addressing key limitations of Ni- and Co-free P2-type Fe-Mn layered cathodes.