Stabilizing oxygen redox reaction in phase-transition-free P2-type Co/Ni-free cathode via Cu doping for sodium-ion batteries

Abstract

Due to their high capacity, the P2-type layered oxide cathodes containing oxygen redox reaction processes have attracted wide attention for sodium-ion batteries. However, these materials usually exhibit poor electro- chemical properties, resulting from irreversible oxygen redox reactions and phase transition processes at high voltages, and thus hinder their large-scale application. This work reveals the mechanism for the significantly improved cycle stability and rate performance of Co/Ni-free Na_0.75Li_0.25−2/3xCu_xMn_0.75−1/3x O₂ via Cu doping. Ex-situ XPS demonstrates that Cu doping reduces the amount of Mn³⁺ that triggers the Jahn-Teller effect during the cycling. In addition, the electron enrichment of oxygen around Cu can alleviate the irreversible oxidation of oxygen, and thus suppressing the phase transition originates from the rapid weakening of the electrostatic repulsion between O-O. Meanwhile, in-situ XRD results verify that the Na_0.75Li_0.19Cu_0.09Mn_0.72O₂ maintains the P2 phase structure during charging and discharging, resulting in a near-zero strain characteristic of 1.9%. Therefore, the optimized cathode delivers a high reversible capacity of 194.9 mAh g⁻¹ at 0.1 C and excellent capacity retention of 88.6% after 100 cycles at 5 C. The full cell paired with commercial hard carbon anode delivers energy density of 240 Wh kg⁻¹. Our research provides an idea for designing a new type of intercalated cathode for sodium-ion batteries with low cost and high energy density.