Regulating Na-Ion Occupations of P-Type Na_0.67Ti_0.10Fe_0.05Mn_0.85O₂ for Improving Electrochemical Reversibility and...

P'2-type manganese-based layered oxides (Na_xMnO₂, 0.5 < x < 0.8 usually) have emerged as promising cathode materials for sodium-ion batteries (SIBs), primarily due to their ability to deliver higher capacity compared to P2-type layered oxides. However, the underlying mechanism behind this high capacity still remains unclear, and the cycling stability has been a challenge. Given that distinct Na⁺ occupation environments (edge-shared Na_e and face-shared Na_f) in P-type cathodes have different electrochemical kinetics, this study establishes a direct correlation between the high capacity of the P'2 structure and the high Na_e/Na_f ratio. The theoretical simulation confirms that the P'2 structure can accommodate more Na⁺ at the Na_e site, which features a lower migration energy barrier and enhanced migration. Guided by this insight, a dual-approach rational design─combining quenching treatment and Ti/Fe codoping─is proposed to harvest the high-capacity and high-stability P'2-Na_0.67Ti_0.1Fe_0.05Mn_0.85O₂ cathode. Quenching enables the formation of P'2-structure with a high Na_e/Na_f ratio of 2.1 (compared to the typical ∼ 1.00), delivering a higher capacity of 190.3 mAh g^-1 at 0.1 C between 2.0 and 4.0 V (the naturally cooled cathode only exhibits 130.2 mAh g^-1 at 0.1 C); Furthermore, Ti⁴⁺ (3d⁰, unfilled) and Fe³⁺ (3d⁵, half-filled) are introduced into P'2-Na_0.67MnO₂ for suppressing Na⁺/vacancy ordering and stabilizing the structure, resulting in excellent cycle stability with 96.9% capacity retention after 350 cycles at 5 C. This strategy provides a pathway to improve the reversible capacity of Mn-based layered cathodes for sodium-ion batteries.