Zinc-substituted P2-type Na0.67Ni0.23Zn0.1Mn0.67O2 cathode with improved rate capability and cyclic stability for sodium-ion storage at high voltage

Abstract

P2-type layered transition metal oxides are considered one of the ideal cathode materials for sodium ion batteries because of its high working voltage and excellent theoretical specific capacity. P2-Na0.67Ni0.33Mn0.67O2 electrodes deliver a theoretical specific capacity of 173 mAh g-1, which is dependent on Ni2+/Ni3+/Ni4+ charge compensation. High operating voltage leads to higher specific capacity and energy density, which are better adapted to practical development needs, but at the same time it causes the material to undergo a destructive P2-O2 phase transition, which greatly affect cycle stability and Na+ diffusion kinetics. In this study, we propose Zn2+ doping and spherical morphology modification strategy. The spherical morphology not only reduces the distance of Na+ diffusion and improves the rate performance, but also the spherical structure can alleviate the volume change during Na+ intercalation/extraction. Furthermore, the doping of Zn2+ changes the valence distribution of Mn, suppressing the phase transition at high voltage and further improving the cycling stability. At a high cut-off voltage of 4.4 V, the cathode material with Zn2+ doping and spherical shaped possesses an average operating voltage of up to 3.6 V and exhibits 90% capacity retention after 30 cycles (compared to only 56% for the pristine material). Above that, the rate performance is also greatly improved, with the modified material contributing an excellent specific capacity of 95 mAh g-1, even at a high current density of 5 C (1 C = 170 mA g-1).