Impedance analysis of Na0.44MnO2 positive electrode for reversible sodium batteries in organic electrolyte

Na0.44MnO2 (NMO) is one of the most promising positive electrode materials for the development of sodium ion secondary batteries. In this work, a NMO orthorhombic compound was prepared by a soft chemistry modified Pechini method and characterized. The material exhibits a good discharge capacity (about 110 mAh/g) at low current rate (11 mA/g, C/25) which decreases to 65 mAh/g at high rate (275 mA/g, 1C). The electrochemical behavior was investigated by the extensive use of electrochemical impedance spectroscopy analysis. It was observed that the kinetic limitations are mainly due to the low diffusion coefficient of Na+ ions in the structure (in the range 10−13/10−14 cm2/s) and to the high values of the surface resistance which is the sum of two contributes attributed to the charge transfer process and the presence of a passivating layer. The diffusion coefficient as well as the charge transfer resistance depends on the sodium amount in the electrode.