Rapid and Phase–Selective Routes to Layered Na–Ion Cathodes

Abstract

Layered Na–Mn–Fe–O oxides are among the leading cathode candidates for Na-ion batteries owing to their low cost, earth-abundant constituents, and competitive electrochemical performance. Their rich structural chemistry, spanning O-type (octahedral Na) and P-type (prismatic Na) polymorphs with distinct stacking sequences, offers multiple avenues for property optimization. However, controllable access to specific polymorphs, particularly nonequilibrium ones, remains a synthetic challenge. Here we show that susceptor-assisted microwave heating can produce the P2, O3, and P3 phases of layered Na–Mn–Fe–O cathodes in minutes, with phase selectivity set by the nominal Na/M ratio of the precursor alone. The rapid reaction times suppress Na volatilization, preserving the target stoichiometry without the excess sacrificial Na precursors typically required by conventional solid-state routes. The high mobility of Na+ combined with the large size difference between Na and Mn/Fe, yields well-ordered frameworks with no evidence for antisite disorder between Na+ and the transition metals. Accessing all three structures from one rapid route enables a controlled comparison of how stacking sequence governs electrochemistry. On cycling, we observe Fe3+/Fe4+ redox and behavior consistent with Fe3+ migration into the Na layer at high voltage in all three polymorphs, with P2 showing the most stable high-voltage cycling. Operando diffraction shows that all three converge toward disordered O-type stacking on deep desodiation, and voltage-resolved distribution-of-relaxation-times analysis shows that Na+ diffusion kinetics are governed by stacking transitions and Na–vacancy ordering, with pronounced kinetic barriers at glide-type structural transitions. These results establish microwave synthesis as a versatile route to both equilibrium and nonequilibrium layered Na cathodes and clarify how stacking sequence and local disorder jointly control redox behavior and ion transport.