K-doped P2-Na0.67−xKxNi0.33−yMn0.67CuyO2 cathode material with enhanced cycle performance for sodium-ion batteries

As one of the typical compositions for Ni/Mn-based layered oxides, P2-Na0.67Ni0.33Mn0.67O2 is widely studied due to its high capacity and wide voltage. However, its electrochemical performance requires further improvement for practical application as a cathode material of sodium-ion batteries. In this study, we successfully synthesized a series of K-doped P2-Na0.67−xKxNi0.33−yMn0.67CuyO2 (x = 0, 0.01, 0.06, 0.10, 0 < y < 0.16) cathode materials using a convenient sol-gel method. During the initial discharge, the NKNMC0.06 electrode delivers a high capacity of 120.3 mAh g−1 at 100 mA g−1 when charged and discharged from 1.5 to 4.2 V. And the electrode retains a specific capacity of 101.6 mAh g−1, with a good capacity retention rate of 84.5 % after 100 cycles. The enhanced cycle performance can be attributed to the appropriate amount of K+ ions occupying the Na+ sites and providing support between the transition metal layers, which stabilizes the crystal structure and improves cycle stability of the electrode. Moreover, larger-sized K+ ions expand the Na+ diffusion channel, facilitating Na+ diffusion. Furthermore, the pouch cell of NKNMC0.06 delivers a capacity of 0.28 Ah during the first discharge and maintains a capacity retention rate of 78.2 % after 100 cycles.