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

The conventional P2-type cathode material Na_0.67Ni_0.33Mn_0.67O₂ 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_0.67Ni_0.33Mn_0.67O₂. 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 Mg²⁺ and Ca²⁺ make high-voltage reactions (oxygen redox and Ni⁴⁺/Ni³⁺ redox reactions) less accessible. Thanks to the synergetic effect of co-doping with Mg²⁺ and Ca²⁺ 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_0.62Ca_0.025Ni_0.28Mg_0.05Mn_0.67O₂ 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 Mg²⁺ and Ca²⁺ dopants and provides a stability-enhancing strategy utilizing abundant alkaline earth elements.