Ca/Mg dual-doping P2-type Na0.67Ni0.17Co0.17Mn0.66O2 cathode material for sodium ion batteries

Ternary layered oxide Na0.67Ni0.17Co0.17Mn0.66O2 with P2 structure has emerged as alternative to the lithium-ion batteries. The main cause of the induced circulation fluctuation is the P2-O2 change that exists during the charging and discharging process. In this study, a Ca/Mg co-doped strategy has been employed to solve this problem and P2-type Na0.64Ca0.03(Ni0.17Co0.17Mn0.66)0.9Mg0.1O2 with satisfactory electrochemical performance has been successfully prepared. XRD results confirm that no impure phases formed after Ca/Mg co-substitution. Particularly, the co-doped Na0.64Ca0.03(Ni0.17Co0.17Mn0.66)0.9Mg0.1O2 shows an impressive initial discharge capacity of 158 mAh g−1, and the capacity after 100 cycles reaches 140 mAh g−1, which has been greatly improved than the raw Na0.67Ni0.17Co0.17Mn0.66O2. Small amount of Ca expands the lattice spacing and improves the specific capacity. The presence of inert Mg2+ inhibits P2-O2 transition, thus enhancing the structure stability. The study shows that the cycling stability and the rate capability of Na0.67Ni0.17Co0.17Mn0.66O2 can be improved by Ca/Mg co-doping.