<p>Layered transition metal oxide cathode materials have garnered increasing attention for sodium-ion batteries (SIBs). However, they are plagued by the Jahn–Teller distortion of MnO₆, Na⁺/vacancy ordering, and irreversible lattice oxygen loss, which collectively lead to capacity fading and voltage decay. Herein, we report a P2-type material, Na_0.67Ni_0.3Mn_0.6Li_0.09Sn_0.01O₂ (NNMO-Li0.09Sn0.01), modified with two closed-shell dopants (i.e., Li⁺ and Sn⁴⁺). Benefiting from the unique electronic configurations of closed-shell ions, NNMO-Li0.09Sn0.01 exhibits enhanced structural and electrochemical stability. Specifically, the incorporation of Li⁺ increases the Mn⁴⁺/Mn³⁺ ratio, thereby mitigating Jahn–Teller distortion during (de)sodiation process. In addition, Li⁺ disrupts the Ni/Mn ordering in the transition metal layer, suppressing Na⁺/vacancy ordering. Meanwhile, the introduction of Sn⁴⁺ forms stronger Sn–O bonds (548 kJ mol⁻¹), thereby enhancing the bonding strength between neighboring transition metal ions and surrounding oxygen atoms, effectively reducing oxygen loss during cycling. NNMO-Li0.09Sn0.01 exhibits significantly improved cycling stability, delivering a specific capacity of 90.3 mAh g⁻¹ with 62.9% capacity retention after 50 cycles at 0.1 C (1 C = 200 mA g⁻¹), along with 90.3% voltage retention. This substitution strategy based on closed-shell ions offers a viable approach for enhancing the structural stability of wide-voltage layered oxide cathodes.</p>