New insights into designing high-rate performance cathode materials for sodium ion batteries by enlarging the slab-spacing of the Na-ion diffusion layer
Recently, the design and synthesis of high performance cathode materials for sodium ion batteries have attracted great interest. In this study, we propose a novel strategy to design high-rate performance cathode materials for sodium ion batteries through enlarging the d-spacing of the Na-ion diffusion layer. More importantly, some new insights into the expansion mechanism of the interplanar spacing for Na0.67Mn0.8Ni0.1Mg0.1O2 induced by Ni and Mg co-doping and the resulting high-rate capability have been presented for the first time. We find that Mg and Ni co-doping leads to the shortening of the TM–O (TM = transition metal) bond lengths and the shrinkage of the TMO6 octahedrons, which might be largely responsible for the expansion of the interplanar spacing of the Na-ion diffusion layer. In comparison with Na0.67Mn0.8Ni0.2O2 and Na0.67Mn0.8Mg0.2O2, Mg and Ni co-doped Na0.67Mn0.8Ni0.1Mg0.1O2 has a higher Na-ion diffusion coefficient and can deliver around 160, 145, 133 and 124 mA h g−1 at 24, 48, 120 and 240 mA g−1, respectively. In particular, at the high current densities of 480 (2C), 1200 (5C) and 1920 mA g−1 (8C), MMN can still offer reversible capacities of 110, 66 and 37 mA h g−1, respectively. In addition, the cycling stability has also been enhanced via Mg and Ni co-doping at the same time, which means that Mg and Ni co-doping also has a positive effect on the stability of the layered structure.