Enhanced Structural Reversibility and Electrochemical Stability Via Fe 3+ /Fe 4+ Stabilization in a P3 ‐Type Layered Na 0.7 [Mn 0.5 Fe 0.4 Li 0.1 ] O 2 Cathode at High‐Voltages

This study investigates a lithium‐substituted layered sodium transition metal oxide, Na 0.7 [Mn 0.5 Fe 0.4 Li 0.1 ]O 2 (Li‐NMFO), which has a P3‐type structure instead of the commonly reported P2 or O3 phases. Structural characterization reveals that Li + promotes the formation of a P3 phase via stabilizing interlayer interactions and partially occupying transition metal sites. Electrochemical analysis shows that Li‐NMFO displays highly reversible charge–discharge behavior and superior cycling performance when charged up to 4.5 V, outperforming cells charged to 4.0 or 4.2 V. Unlike conventional Fe‐based cathodes that suffer from degraded high‐voltage performance due to limited Fe 4+ formation and irreversible phase transitions, Li‐NMFO charged to 4.5 V exhibits stabilized Fe redox activity and reversible phase evolution. XRD and GITT analyses indicate that sluggish Na + diffusion and structural rigidity at lower cutoff voltages originate from incomplete Fe oxidation and the absence of phase reorganization. In contrast, XAS confirms that Fe 4+ formation is maximized at 4.5 V, enabling favorable local structural relaxation. Mn exhibits much smaller redox changes than Fe throughout cycling and primarily contributes to stabilizing the layered framework. These findings demonstrate that strategic Fe 4+ activation within a Li‐stabilized P3 framework, combined with suppressed irreversible phase transitions is an effective approach to enhancing the reversibility and cycling performance of Fe‐based layered oxide cathodes.