Enhanced air stability and cycling stability of P'2-type layered cathodes for sodium-ion batteries via Pb–Mg synergistic doping

Air stability issues in layered oxides present significant challenges for sodium-ion batteries due to the high activity in humid air. The fundamental instability arises from inherently weak Na–O bonding, exacerbated by their wider interlayer spacing. Here, we propose that incorporating ions with high electronegativity can effectively modulate the Na+ binding strength, thereby enhancing intrinsic air stability. As a case study, P'2-type Na0.67MnO2 (NM), one of the promising cathodes that benefits from its cost-effectiveness and high specific capacity, was selected to verify the above-mentioned concept by introducing strongly electronegative Pb4+ ions. To synergistically enhance structural stability, electrochemically inactive Mg2+ ions were also incorporated, resulting in the P'2-type Na0.67Pb0.04Mg0.08Mn0.88O2 (NMP4M8). As confirmed by x-ray diffraction, the reduced layer spacing in NMP4M8 effectively inhibits spontaneous Na+/H+ exchange, resulting in excellent air stability. After 4-day air-exposure (20 °C, 50% RH), no significant hydrated phases were detected, whereas the unmodified NM exhibits clear structural degradation. After 7-day air-exposure, NMP4M8 achieved a capacity retention rate of 80.6% after 200 cycles, significantly higher than the 20.8% observed in aged NM. Density functional theory calculation revealed that Pb4+ incorporation strengthened Na–O attraction, while Gibbs free energy analysis showed thermodynamically unfavorable degradation for NMP4M8. This co-doping strategy presents a promising pathway toward developing air-stable layered oxides.