Dual‐Pillar Effect in P2‐Type Na 0.67 Ni 0.33 Mn 0.67 O 2 Through Na Site Substitution Achieve Superior Electrochemical and Air/Water Dual‐Stability as Cathode for Sodium‐Ion Batteries

Abstract High‐voltage phase changes limit the capacity and cycle stability of P2‐type sodium‐layered transition metal oxides. In this study, Cu, Zn, and Mg ions are successfully co‐doped into Na 0.67 Ni 0.33 Mn 0.67 O 2 to restrain the phase transition and increase Na + diffusion with enhanced structural stability. In situ and ex situ evaluations elucidate the structural and charge compensation during high‐voltage operation. Remarkably, the resultant Na 0.76 Ni 0.23 Cu 0.07 Zn 0.03 Mn 0.62 Mg 0.05 O 2 (NNCZMMO) cathode exhibits superior rate capability (135 and 94.0 mA h g −1 at 0.1C and 5C), prolonged‐cycling stability (85.4% capacity retention over 1000 cycles at 5C) and excellent air/water stability over 40 days. According to density functional theory (DFT), scanning transmisson electron microscopy (STEM), and other assessments, Cu at 2a sites strengthens the metal layer, whereas Zn/Mg on Na sites creates a “dual‐pillar” effect to avoid cracks and O2 phase formation during desodiation. Impressively, NNCZMMO//hard carbon (HC) full‐cell achieved an exceptional average voltage of 3.58 V and an outstanding energy density of 284.7 Wh kg −1 at 60.8 W kg −1 and 104.23 Wh kg −1 at 2439.5 W kg −1 . This innovative approach enables the rapid fabrication of high‐energy‐density cathodes for grid‐scale energy‐storage usages.