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

Charging P2-type Na 0.66 Ni 0.33 Mn 0.67 O 2 (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 Zn 2+ 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 + /Zn 2+ dual-site-doped Na 0.63 K 0.03 Ni 0.26 Zn 0.07 Mn 0.67 O 2 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 .