Development of lithium-free P2-type high-sodium content cathode materials with enhanced cycle and air stability for sodium-ion batteries

The effect of partial substitution of Mg for Ni on a high-sodium and lithium-free layered P2-type Na 45/54 Mg 6/54 Ni 12/54 Mn 34/54 O 2 cathode with high initial Coulombic efficiency and excellent cyclic stability has been investigated in this study. Based on the crystal structural analysis, the Mg doping can retain the P2 structure up to 4.3 V, thus restraining the detrimental phase transformation of P2–O2 during the Na-ion intercalation/deintercalation process. Therefore, the obtained Mg-doped P2-type cathode exhibits a reversible specific capacity of 109 mAh·g −1 at 0.1C between 2.0 and 4.3 V and a retention rate of 81.5% after 200 cycles at 1C. In addition, the full cell consisting of Mg-doped P2-type cathode and hard carbon anode shows a capacity retention rate of 85.6% after 100 cycles. This study provides new insight into the development of durable cathode materials for sodium-ion batteries. Graphical abstract