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.