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

P2-Na_0.67Ni_0.33Mn_0.67O₂ 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 Na_0.67Ni_0.33Mn_0.67O₂ 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 Na_0.67Ni_0.33Mn_0.67O₂. A representative Na_0.67Ni_0.23Mg_0.1Mn_0.65Sn_0.02O₂ 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).