Designing Water/Air-Stable P2-Layered Cathodes with Delayed P2–O2 Phase Transition by Composition and Structure Engineering for Sodium-Ion Batteries at High Voltage

Abstract P2-NaxMnO2 are promising cathodes for sodium-ion batteries (SIBs) owing to their high specific capacities and fast 2D Na+ diffusion path. However, the poor storage stability, irreversible phase transition, and ultrahigh initial Coulombic efficiency (ICE) hinder their full-cells application. Herein, composition designing and structure engineering is combined to tackle these handicaps. A novel Ni2+/Cu2+/Mg2+ co-doped P2-Na0.8Mn0.6Ni0.2Cu0.1Mg0.1O2 (P2-NaMNCuMg) with 3D hierarchical structure is designed and synthesized. The divalent Ni2+/Cu2+/Mg2+ co-doping could postpone P2-O2 phase transition up to 4.2 V and restrain the Jahn-Teller effect of Mn3+, and the Ni2+ and Cu2+ can participate in the redox reaction process for charge compensation. The 3D hierarchical structure is conducive to enhancing structural stability and Na+ transfer rate. Benefiting from the co-doping elements and well-designed 3D hierarchical structure, the P2-NaMNCuMg exhibits good water/air stability and delivers a reversible capacity of 160.8 mAh g−1 with ICE of 113.8% at 20 mA g−1 and prolonged cycling stability of 82.9% capacity retention over 500 cycles at 500 mA g−1. Moreover, the full-cells based on P2-NaMNCuMg cathode achieve a power density of 1221.5 W kg−1 at 500 mA g−1 in 1.5-4.2 V, demonstrating the potential of P2-NaMNCuMg as high-performance cathode for high-voltage SIBs.