Improving the high-voltage high-rate performance of a P2 layered oxide cathode by a dual-ion doping strategy for sodium-ion batteries

Abstract

P2-Na_0.67Ni_0.33Mn_0.67O₂ is a promising cathode for sodium-ion batteries in large-scale energy storage applications due to their advantages of low cost, high energy density, superior sodium ion conductivity, and good air stability, whereas their structural rearrangement at high voltage above 4.2 V and the appearance of Na⁺/vacancy ordering result in fast capacity decay and poor rate capability, which lead to unsatisfactory performance that fails to meet practical demands. In this paper, we propose a dual-ion doping strategy coupling both cations and anions. Thanks to their collaborative effect, the optimized cathode material P2-Na_0.67Ni_0.33Mn_0.57Ti_0.1O_1.95F_0.05 exhibits greatly improved stability in the voltage range of 2.3–4.3 V at a high rate. Specifically, it retains 79.59% of its initial capacity after 500 cycles at 5C, in contrast to 19.91% of the undoped material. Experimental and theoretical investigations on working mechanisms illustrate that Ti⁴⁺ and F⁻ dual-ion doping effectively suppresses the P2–O2 phase transition and facilitates sodium ion diffusion kinetics. Moreover, a P2-Na_0.67Ni_0.33Mn_0.57Ti_0.1O_1.95F_0.05//hard carbon full cell delivers a high reversible capacity of 122 mA h g⁻¹ with a high energy density of 365 W h kg⁻¹ based on the mass of the cathode material. This study provides an efficient strategy to promote the application of layered oxide cathodes in practical sodium-ion batteries.