Diffusion behavior of sodium ions in Na0.44MnO2 in aqueous and non-aqueous electrolytes

Abstract

The slow kinetics of bigger-sized sodium ions in intercalation compounds restricts the practical applications of sodium batteries. In this work, sodium ion intercalation/de-intercalation behavior of Na0.44MnO2 (NMO), which is one of the promising cathode materials for sodium batteries, is presented in both aqueous and non-aqueous electrolyte systems. The NMO samples synthesized using modified Pechini method shows better rate capability in 0.5 M sodium sulfate aqueous electrolyte system than the 1 M sodium perchlorate non-aqueous system. The difference in the rate performance is extensively investigated using electrochemical impedance spectroscopy (EIS) measurements and the apparent diffusion coefficients of sodium in NMO are determined to be in the range of 1.08 × 10-13 to 9.15 × 10 -12 cm2 s-1 in aqueous system and in the range of 5.75 × 10-16 to 2.14 × 10-14 cm2 s-1 in non-aqueous systems. The differences in the evaluated rate capability are mainly attributed to nearly two to three orders of magnitude difference in the apparent diffusion coefficient along with the charge transfer resistance and the resistance from the formed SEI layer.