Interlayer spacing regulation achieved by Mn/Li/Al dopants for higher performance O3-type layered oxides cathodes

O3-type NaNi 2/9 Fe 1/3 Cu 1/9 Mn 1/3 O 2 is considered a promising layered oxide cathode for sodium-ion batteries, owing to its high operating voltage and cost-effective elemental composition. However, its practical application is hindered by irreversible phase transitions at high voltages, which severely compromise structural integrity and cycling stability during prolonged operation. To address these challenges, elemental doping has emerged as an effective strategy to modulate crystal lattice parameters and suppress detrimental phase evolution. In this study, we propose a multi-element (Li/Mn/Al) doping approach, synthesized via a hydroxide-precursor-assisted sodiation process, to enhance the structural robustness and electrochemical performance of O3-type NaNi 2/9 Fe 1/3 Cu 1/9 Mn 1/3 O 2 . Specifically, Aluminum doping serves as a structural pillar to stabilize the transition metal-oxygen (TM-O) framework, effectively mitigating layer gliding. Meanwhile, Li doping activates the Fe 3+ /Fe 4+ redox couple, enabling charge compensation and enhancing reversible capacity. Benefiting from the synergistic effects of Li/Mn/Al co-doping, the resulting Na(Ni 2/9 Fe 1/3 Cu 1/9 Mn 1/3 ) 0.96 Mn 0.02 Al 0.01 Li 0.01 O 2 cathode demonstrates suppressed Jahn-Teller distortion, improved air stability, and superior cycling performance. Notably, a reduced interlayer distance ratio ( d (O Na-O) / d (O-TM-O) ) correlates with delayed O3-to-P3 phase transformation and enhanced lattice stability, as confirmed by crystallographic analyses. The synergistic effects of Li/Mn/Al co-doping not only stabilize the host structure but also unlock cationic redox reversibility, offering a pathway to balance energy density and longevity in sodium-based cathodes.