Synergetic stability enhancement with magnesium and calcium ion substitution for Ni/Mn-based P2-type sodium-ion battery cathodes.

The conventional P2-type cathode material Na 0.67 Ni 0.33 Mn 0.67 O 2 suffers from an irreversible P2-O2 phase transition and serious capacity fading during cycling. Here, we successfully carry out magnesium and calcium ion doping into the transition-metal layers (TM layers) and the alkali-metal layers (AM layers), respectively, of Na 0.67 Ni 0.33 Mn 0.67 O 2 . Both Mg and Ca doping can reduce O-type stacking in the high-voltage region, leading to enhanced cycling endurance, however, this is associated with a decrease in capacity. The results of density functional theory (DFT) studies reveal that the introduction of Mg 2+ and Ca 2+ make high-voltage reactions (oxygen redox and Ni 4+ /Ni 3+ redox reactions) less accessible. Thanks to the synergetic effect of co-doping with Mg 2+ and Ca 2+ ions, the adverse effects on high-voltage reactions involving Ni-O bonding are limited, and the structural stability is further enhanced. The finally obtained P2-type Na 0.62 Ca 0.025 Ni 0.28 Mg 0.05 Mn 0.67 O 2 exhibits a satisfactory initial energy density of 468.2 W h kg -1 and good capacity retention of 83% after 100 cycles at 50 mA g -1 within the voltage range of 2.2-4.35 V. This work deepens our understanding of the specific effects of Mg 2+ and Ca 2+ dopants and provides a stability-enhancing strategy utilizing abundant alkaline earth elements.