Elucidating the stabilizing effect of copper substitution in high voltage P2-type layered oxides for sodium-ion batteries

Abstract

P2-type layered sodium transition-metal oxides are promising high-energy cathodes for sodium-ion batteries but suffer from structural degradation and irreversible redox reactions particularly in the high-voltage region, resulting in rapid capacity fade. This study investigates how Cu substitution affects the structural evolution, redox mechanisms, and electrochemical performance of P2-type Na_2/3Mn_2/3Ni_1/3−yCu_yO₂ (y = 0, 1/6, 1/3) cathodes for sodium-ion batteries. Cu substitution not only raises the average voltage of the practically delivered capacity upon extended cycling, thereby increasing energy density, but also shifts the undesired high-voltage plateau, associated with irreversible Ni⁴⁺/Ni³⁺ and/or oxygen redox, to potentials beyond the practical operating window. This enables the partially substituted Na_2/3Mn_2/3Ni_1/6Cu_1/6O₂ to deliver 94.8 mAh g⁻¹ at an average voltage of 3.45 V (320 Wh kg⁻¹) with 92% capacity retention after 100 cycles in half cells. Operando synchrotron X-ray diffraction reveals that this enhanced stability arises from a distinct phase evolution: while Na_2/3Mn_2/3Ni_1/3O₂ develops a P2 to O2 transition together with a loss of crystallinity in the form of stacking faults, the partially Cu-substituted sample forms a more reversible OP4 structure above 4.0 V with less pronounced interlayer-spacing changes. Operando X-ray absorption reveals sequential Ni²⁺ and Cu²⁺ oxidation to Ni³⁺ and Cu³⁺, respectively. Extended X-ray absorption fine structure (EXAFS) analysis, corroborated by density functional theory (DFT) calculations, shows that in Na_2/3Mn_2/3Ni_1/6Cu_1/6O₂ NiO₆ and CuO₆ octahedra undergo almost-simultaneous opposite Jahn–Teller-distortion trends. This phenomenon reduces the effective overall Jahn–Teller-related lattice strain during (de)sodiation, compared to Na_2/3Mn_2/3Ni_1/3O₂ and Na_2/3Mn_2/3Cu_1/3O₂. Full-cell tests vs. hard carbon demonstrate the practical relevance, with Na_2/3Mn_2/3Ni_1/6Cu_1/6O₂ retaining 80% of its initial energy after 310 cycles and 50% after 1130 cycles at 100 mA g⁻¹, establishing partial Cu substitution as an effective route to stabilize high-voltage P2-type layered oxides for durable sodium-ion batteries.