Suppressed P3–O3′ Phase Transition and Enhanced Na⁺ Diffusion Kinetics of O3-Type Layered Oxide Cathode via Multivariate Doping

O3-NaNi 1/3 Fe 1/3 Mn 1/3 O 2 has attracted much attention as a cathode for sodium-ion batteries, because of its low cost and high sodium-ion storage capacity. However, its slow Na + diffusion kinetics and harmful P3–O3′ phase transition with severe bulk strain at high voltage leads to poor rate capability and fast capacity fading. Herein, we propose a multivariate doping strategy with Cu, Mg, and Ti ions to solve the above problems of the O3-NaNi 1/3 Fe 1/3 Mn 1/3 O 2 cathode. The O3–Na(Ni 1/3 Fe 1/3 Mn 1/3 ) 0.9 Cu 0.03 Mg 0.02 Ti 0.05 O 2 (NFMCMT) cathode exhibits enlarged Na + diffusion channels and a strengthened layered structure, which improves the Na + diffusion kinetics, suppresses the harmful P3–O3′ phase transition at high voltages, and inhibits the intragranular fatigue cracks, leading to enhanced rate capability and cycling performance. As a result, the NFMCMT exhibits outstanding performance in the 2–4.1 V voltage window, delivers a discharge capacity of 151.2 mAh g –1 with the 81.5% capacity retention for 100 cycles at 0.1 C, and 83.1% capacity retention for 300 cycles at 5 C. Especially in the rate performance, the NFMCMT delivers a 115.6 mAh g –1 and 100.1 mAh g –1 discharge capacity even at 5 and 10 C. This work provides an effective multivariate doping strategy for development of high-performance layered oxide cathodes for sodium-ion batteries.