Enhancing stability and high voltage tolerance of P2-type layered Na0.70Ni0.25Mn0.75O2 cathodes via Cu/Mg dual-doping for Na-ion batteries

Abstract

P2-type Na0.70Ni0.25Mn0.75O2 layered cathode materials exhibit high theoretical capacity and operating voltage but suffer from capacity decay at high voltages, Na+/vacancy ordering, charge compensation issues, and limited cycling life. In this study, we report enhanced electrochemical performance, suppression of phase transition at high voltage, and stable performance at high C-rates achieved through the incorporation of dual dopants Cu and Mg into the P2-type Na0.70Ni0.25Mn0.75O2 layered cathode. Structural and chemical analyses using x-ray diffraction and x-ray photoelectron spectroscopy confirm the formation of the P2-phase after the incorporation of Mg and Cu dopants. Scanning electron microscopy and transmission electron microscopy reveal the material morphology and its microstructural integrity. The dual-doped cathode delivers a high initial discharge capacity of 136.38 mAh/g at a 0.1 C rate, demonstrating a capacity retention of 74.08% after 300 cycles, even at high current rates (1 C), and exhibits rapid reaction kinetics within the 2.0–4.3 V window. It also showed enhanced reaction kinetics, with a Na+ diffusion coefficient on the order of 10−9–10−12 cm2/s. The Cu and Mg co-doped sample exhibits a nominal voltage of 3.51 V and delivers a high energy density of 478.69 Wh/kg, underscoring its promising potential for advanced sodium-ion battery applications. Thus, the incorporation of Cu and Mg dopants significantly enhances the cyclic stability, diffusion coefficient, and electrochemical performance of the P2-type Na0.70Ni0.19Cu0.03Mg0.03Mn0.75O2 cathode, making it a strategic approach and a favorable candidate for sodium-ion battery applications.