Precisely modulating the structural stability and redox potential of sodium layered cathodes through the synergetic effect of co-doping strategy

Abstract

Although P2-type Na0.67Ni0.33Mn0.67O2 (NNMO) is a promising cathode material for sodium-ion batteries (SIBs), it usually suffers from complex phase transition, multiple ordering rearrangements, and irreversible oxygen loss upon electrochemical cycling, incurring rapid voltage decay and capacity degradation. Herein, a novel optimization strategy with simultaneous modulation of the crystal and electronic structure of NNMO is realized through Zn/Cu synergistic doping. It is revealed that partial substitution of Ni with inert Zn possessing similar ionic radii but different Fermi levels suppresses the unfavorable phase transition and disrupts the Na+/vacancy ordering, while active Cu doping not only elevates the redox potential but also alleviates the irreversible oxygen redox through reinforced orbital hybridization. Moreover, the cooperation of Zn and Cu inhibits the Ni/Mn ordering distribution, thereby promoting the Na+ transportation kinetics. Benefiting from the cooperative advantages, the resultant Na0.67Ni0.21Mn0.67Cu0.05Zn0.07O2 exhibits a complete solid-solution reaction upon cycling, a high operating voltage of 3.65 V with a low decay rate of 0.48 mV per cycle, and a superior rate capability of 84.1 mAh/g at 20 C. This work demonstrates the promise of co-doping strategy for optimizing the structural stability and redox potential from the atomic and orbital levels to construct high-energy cathode materials for SIBs.