Precise Na⁺ Stoichiometry Engineering of O3-Type Na_xNi_0.4Fe_0.2Mn_0.4O₂ in Sodium-Ion Batteries: Balancing Redox Activity and Structural Stability

Layered transition metal oxides represent promising cathode materials for sodium-ion batteries, yet their electrochemical performance is critically hindered by the intrinsic trade-off between redox activity and structural stability. Herein, we systematically investigate the Na_xNi_0.4Fe_0.2Mn_0.4O₂ (x = 0.7, 0.83, 1) system to elucidate how Na⁺ stoichiometry tuning modulates charge storage mechanisms and degradation pathways. Contrary to conventional charge-balance predictions, Na-deficiency (x = 0.7) induces a P2/O3 hetero-phase and activates detrimental Mn^3+/4+ redox, triggering severe Jahn-Teller distortions and phase boundary instability. Na-excess (x = 1) results in lattice contraction and reduced transition metal valence, leading to sluggish Na+ diffusion kinetics and irreversible oxygen redox. Remarkably, the intermediate composition (x = 0.83) is identified as the optimal stoichiometry, achieving a delicate synergy between ionic mobility and lattice rigidity. This composition maximizes Ni/Fe redox utilization while effectively suppressing structural degradation. Consequently, the Na_0.83NFM cathode delivers a high specific capacity (146.2 mAh g^-1), superior rate capability (97.0 mAh g^-1 at 5 C), and outstanding long-term voltage stability. These findings establish precise Na⁺ stoichiometric engineering as a pivotal strategy for high-performance sodium-ion cathode design.