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 reaction 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 average voltage 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 1/6Cu1/6O2 to deliver 94.8 mAh g^-1 at an average voltage of 3.45 V (327 Wh kg^-1 ) with a 97% 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 partial Cu-substituted sample forms a more reversible OP4 structure above 4.0 V with less pronounced interlayerspacing changes. Operando X-ray absorption reveals sequential Ni²⁺ and Cu²⁺ oxidation to Ni³⁺ and Cu³⁺, respectively. Extend X-ray absorption fine structure (EXAFS) analysis, corroborated by density functional theory calculations (DFT), shows that in Na2/3Mn2/3Ni1/6Cu1/6O₂ 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_1/3O 2 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.