Stabilizing High-Voltage Operation in O3-Type Na-Layered Cathodes by Suppressing Distortion-Driven Fe Migration

Abstract Fe-based O3-type Na-layered oxides such as Na[Ni1/3Fe1/3Mn1/3]O2 are attractive cathodes based on earth-abundant elements, yet their high-voltage operation is limited by voltage decay and capacity fading. Here, we identify a distortion−migration cascade initiated by Fe3+ oxidation to Fe4+. The resulting Jahn−Teller distortion of FeO6 octahedra destabilizes Fe within the transition-metal (TM) framework. This destabilization promotes Fe migration into the Na layer and drives irreversible layered to rocksalt-like reconstruction. A stepwise strategy that reduces the Fe content, reinforces the TM−O framework, and blocks Fe migration into the Na layer interrupts this cascade. The optimized cathode delivers 177.96 mAh g−1 with an initial Coulombic efficiency of 94.1% and retains 73.5% capacity after 100 cycles. Experimental and computational analyses reveal continuous structural evolution and suppressed Fe migration. These results establish suppression of distortion-driven Fe migration as a key design principle for stabilizing high-voltage operation in Fe-based Na-layered cathodes.