P2-Type Moisture-Stable and High-Voltage-Tolerable Cathodes for High-Energy and Long-Life Sodium-Ion Batteries

P2-Na0.67Ni0.33Mn0.67O2 represents a promising cathode for Na-ion batteries, but it suffers from severe structural degradation upon storing in a humid atmosphere and cycling at a high cutoff voltage. Here we propose an in situ construction to achieve simultaneous material synthesis and Mg/Sn cosubstitution of Na0.67Ni0.33Mn0.67O2 via one-pot solid-state sintering. The materials exhibit superior structural reversibility and moisture insensitivity. In-operando XRD reveals an essential correlation between cycling stability and phase reversibility, whereas Mg substitution suppressed the P2-O2 phase transition by forming a new Z phase, and Mg/Sn cosubstitution enhanced the P2-Z transition reversibility benefiting from strong Sn-O bonds. DFT calculations disclosed high chemical tolerance to moisture, as the adsorption energy to H2O was lower than that of the pure Na0.67Ni0.33Mn0.67O2. A representative Na0.67Ni0.23Mg0.1Mn0.65Sn0.02O2 cathode exhibits high reversible capacities of 123 mAh g-1 (10 mA g-1), 110 mAh g-1 (200 mA g-1), and 100 mAh g-1 (500 mA g-1) and a high capacity retention of 80% (500 mA g-1, 500 cycles).