Elucidating the optimized P2 type Na_0.67Mn_1−xCu_xO₂ cathode active material via operando XAS

Here we report on the structural and electrochemical properties of P2-type Na_0.67Mn_1−xCu_xO₂ (where x = 0.20–0.50) via various techniques. X-ray diffraction (XRD) reveals a reduction of the unit cell volume upon substitution of Cu elucidated through detailed Rietveld analysis. The cyclic voltammetry (CV) behavior is also affected by the Cu substitution indicating new redox reactions stemming from Cu substitution. Galvanostatic cycling measurements at room temperature show that when x = 0.35 in a P2-type Na_0.67Mn_1−xCu_xO₂ cathode active material, the best electrochemical performance is obtained. The Na-ion diffusion rate is found to be strongly dependent upon the environmental temperature. Changes in the valence state and the local structures of P2-type Na_0.67Mn_1−xCu_xO₂ during the charge/discharge are investigated through the operando X-ray absorption spectroscopy (XAS) technique.