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

Abstract

P2-Na0.67Ni0.33Mn0.67O2 is a promising cathode in sodium-ion batteries for large-scale energy storage applications due to their advantages of low cost, high energy density, superior sodium ion conductivity, and good air stability, whereas the structural rearrangement at high voltage over 4.2 V and the appearance of Na+/vacancy ordering result in fast capacity decay and poor rate capability, which leads to unsatisfactory performance to meet practical demands. In this paper, we propose a dual-ion doping strategy coupling both cation and anion. Thanks to their collaborative effect, the optimized cathode material P2-Na0.67Ni0.33Mn0.57Ti0.1O1.95F0.05 exhibits greatly improved stability in the voltage range of 2.3-4.3 V at high rate. Specifically, it retains 79.59% of its initial capacity after 500 cycles at 5 C, 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, the P2-Na0.67Ni0.33Mn0.57Ti0.1O1.95F0.05//hard carbon full cell delivers a high reversible capacity of 122 mAh g-1 with a high energy density of 365 Wh/Kg based on the mass of cathode material. This study provides an efficient strategy to promote the application of layered oxide cathode towards practical sodium-ion batteries.