Tailoring High Na Content in P2-Type Layered Oxide Cathodes Via Cu-Li Dual Doping for Sodium-Ion Batteries

P2 and O3-type sodium layered oxides (NaxTMO2) have been considered as most promising cathodes for sodium-ion batteries. The P2-type oxides are superior to O3 in terms of fast Na diffusion and high-rate kinetics. However, the deficient Na (x < 0.7) in the P2-structure has low initial charge capacity and it limits practical application. In addition to Na deficiency, P2-O2 phase transition and Na+/vacancy ordering degrades the performance during cycling. To address this issue, dual doping strategy can be employed, synergistic effect of the dopants can mitigate both the Na+/vacancy ordering and the P2-O2 phase transitions. Herein, effect of Li-Cu dual doping in P2-type Na0.67Ni0.33Mn0.67O2 is systematically investigated. Dual doped high-Na P2-type cathode achieved high capacity ~110 mAh g-1 with a cycling stability of ~85% even following 200 cycles. Remarkably, the voltage profile is completely changed from multiple plateaus to a slow gradient which indicates a solid-solution pathway. Li present in TM layer and the strong Cu-O binding can suppress the irreversible P2-O2 phase transition. Structural analysis has ensured that there is no O2/OP4 phase formation during the charge up to 4.2 V. The present result reflects that the sufficient sodium and structural stabilization can improve the performance of P2-type cathodes.