Suppressed P3–O3′\nPhase Transition and\nEnhanced Na<sup>+</sup> Diffusion Kinetics of O3-Type Layered Oxide\nCathode via Multivariate Doping

O3-NaNi<sub>1/3</sub>Fe<sub>1/3</sub>Mn<sub>1/3</sub>O<sub>2</sub> has attracted much attention as a cathode for sodium-ion\nbatteries,\nbecause of its low cost and high sodium-ion storage capacity. However,\nits slow Na<sup>+</sup> diffusion kinetics and harmful P3–O3′\nphase transition with severe bulk strain at high voltage leads to\npoor rate capability and fast capacity fading. Herein, we propose\na multivariate doping strategy with Cu, Mg, and Ti ions to solve the\nabove problems of the O3-NaNi<sub>1/3</sub>Fe<sub>1/3</sub>Mn<sub>1/3</sub>O<sub>2</sub> cathode. The O3–Na(Ni<sub>1/3</sub>Fe<sub>1/3</sub>Mn<sub>1/3</sub>)<sub>0.9</sub>­Cu<sub>0.03</sub>Mg<sub>0.02</sub>Ti<sub>0.05</sub>O<sub>2</sub> (NFMCMT) cathode\nexhibits enlarged Na<sup>+</sup> diffusion channels and a strengthened\nlayered structure, which improves the Na<sup>+</sup> diffusion kinetics,\nsuppresses the harmful P3–O3′ phase transition at high\nvoltages, and inhibits the intragranular fatigue cracks, leading to\nenhanced rate capability and cycling performance. As a result, the\nNFMCMT exhibits outstanding performance in the 2–4.1 V voltage\nwindow, delivers a discharge capacity of 151.2 mAh g<sup>–1</sup> with the 81.5% capacity retention for 100 cycles at 0.1 C, and 83.1%\ncapacity retention for 300 cycles at 5 C. Especially in the rate performance,\nthe NFMCMT delivers a 115.6 mAh g<sup>–1</sup> and 100.1 mAh\ng<sup>–1</sup> discharge capacity even at 5 and 10 C. This\nwork provides an effective multivariate doping strategy for development\nof high-performance layered oxide cathodes for sodium-ion batteries.