Scandium Doping and Coating for Improving O3-NaNi_1/2Mn_1/2O₂ Electrode in Sodium Battery

Scandium ion, Sc³⁺, was introduced into O3-Na[Ni_1/2Mn_1/2]O₂ either by bulk doping or by surface coating. We prepared O3-type Na[(Ni^II _1/2Mn^IV _1/2)_{1- x}Sc^III _x]O₂ and Sc₂O₃-coated Na[Ni_1/2Mn_1/2]O_2, and the resulting materials were systematically compared for application to a rechargeable battery. Electrochemical evaluation revealed that both strategies remarkably improved cycling performance in Na half-cells, increasing the capacity retention after 100 cycles from 18.6% to 67.8% for Sc-doped and to 75.4% for Sc-coated samples, and enabled sustained long-term operation for over 300 cycles in sodium-ion cells. We found the two strategies enhanced cycling performance through distinct mechanisms. The bulk doping, inducing a structural Na-pillar between two MeO₂ slabs (Me = Ni, Mn, Sc), stabilizes the lattice by attracting local Na⁺ with redox-inactive Sc³⁺ and suppressing rapid lattice shrinkage during sodium extraction. Furthermore, the discharge potential can be smoothened by suppressing the unwanted O'3 phase, which is validated by both ex situ XRD and DFT calculations. The coating appears to improve interfacial stability, possibly by suppressing transition-metal dissolution. These findings clarify the distinct contributions of bulk Sc doping and Sc-based surface modification to the cycling stability of O3-Na[Ni_1/2Mn_1/2]O₂ and suggest that separate optimization of bulk and interfacial modifications is a promising strategy for developing more durable Na-layered oxide cathodes.