Dual-Pillar Effect in P2-Type Na0.67Ni0.33Mn0.67O2 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_0.67Ni_0.33Mn_0.67O₂ 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_0.76Ni_0.23Cu_0.07Zn_0.03Mn_0.62Mg_0.05O₂ (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.