K/Zn dual-site doping toward ultralow-strain P2-type Ni/Mn-based cathode materials for sodium-ion batteries

Charging P2-type Na0.66Ni0.33Mn0.67O2 (NNM) to a high upper cut-off voltage of 4.3 V (vs. Na+/Na) can yield a competitive discharge voltage (3.6 V-class) and capacity. However, achieving stable cycling for NNM under this condition remains challenging owing to an unfavorable phase transition and a large lattice-volume change. Herein, a dual-site doping strategy is used to simultaneously realize a pinning effect at the Na and transition metal sites. Incorporating pinned K+ and Zn2+ ions considerably enhances the structural stability of NNM, affording a complete single-phase reaction with ultralow strain (only 0.54 % variation in volume) during Na+ insertion/extraction. The synergistic contributions of dual-site doping to the enhanced electronic conductivity, lowered Na+ migration barrier, and increased Na+ diffusion coefficients are confirmed via theoretical calculations. Consequently, the K+/Zn2+ dual-site-doped Na0.63K0.03Ni0.26Zn0.07Mn0.67O2 delivers satisfactory discharge capacities of 118.6 and 63.2 mAh g−1 at 10 and 2000 mA g−1, respectively, and achieves an excellent capacity retention of 81.2 % after 200 cycles at 500 mA g−1. This study provides a promising method to strengthen the stability of the Ni − Mn-based layered framework and optimize other high-voltage and high-capacity layered oxide cathode materials.