Mitigating Structural Degradation in O3-Layered Sodium-Ion Cathodes: Insights from Mg Doping in NaNi_0.2Fe_0.4Mn_0.4O₂

The long-term viability of sodium-ion batteries (SIBs) hinges on improving cathode stability while maximizing energy density and efficiency. O3-layered cathodes, with high sodium content, are ideal for achieving high capacity, but their long-term structural stability is challenged by the electrochemical conditions in SIBs. Here, the impact of Mg doping on the transition metal site in O3-type NaNi_0.2Fe_0.4Mn_0.4O₂ (NFM244) cathodes is investigated, revealing its influence on phase stability, anion redox ability, and Na-ion diffusion kinetics. Experimental and theoretical studies confirmed that Mg²⁺ preferentially substitutes the Fe³⁺ sites, mitigating transition metal ions migration and lattice strain while suppressing irreversible oxygen release. Optimal 5% Mg doping maintains a stable O3-layered structure, enhanced capacity, and Na-ion kinetics, improving capacity retention during extended cycling over 200 cycles. In contrast, excessive Mg substitution causes structural distortion, impeding Na-ion mobility, and results in severe capacity fading during prolonged cycling. In situ online electrochemical mass spectroscopy (OEMS) combined with X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy revealed that Mg doping also suppresses detrimental electrolyte decomposition. These findings provide a strategic pathway for tailoring next-generation sodium-ion cathodes for scalable high-performance energy storage solutions, underscoring the potential of Mg doping in O3-layered cathodes.