Suppressing Fe Migration for Highly Reversible Oxygen Redox of Sodium-Ion Layered Oxide Cathode

Sodium-ion batteries are promising energy storage device candidates, yet are limited by low energy density. NaNi_0.33Fe_0.33Mn_0.33O₂ (NFM) activates oxygen redox under high voltage and delivers a high capacity, but suffers from irreversible oxygen loss and rapid capacity fading. Moreover, the underlying failure mechanism is still debatable. Herein, we highlight the critical role of Fe migration in NFM electrochemical degradation and develop approaches to suppress Fe migration and facilitate reversible oxygen redox. Incorporating Ca and Sn into NFM mitigates the OP2 phase, alleviates NiO₆/FeO₆ Jahn-Teller distortions, and deters incoming Fe³⁺ ions. Consequently, the out-of-plane Fe migration is effectively suppressed, thus minimizing undercoordinated oxygen species formation and achieving highly reversible oxygen redox. As-optimized NFM cathode exhibits a high capacity of 158.3 mAh g^-1 and excellent electrochemical stability even under high voltage (4.5 V) and high temperature (50 °C). The pouch cells using this NFM cathode achieve a high capacity retention of 80.7% after 2000 cycles. This interlayer-migration-free cathode offers guidelines for developing long-life sodium-ion layered oxides.