Chemical and Electrochemical Differences in Nonaqueous Li-O2 and Na-O2 Batteries

We present a comparative study of nonaqueous Li-O-2 and Na-O-2 batteries employing an ether-based electrolyte. The most intriguing difference between the two batteries is their respective galvanostatic charging over-potentials: a Na-O-2 battery exhibits a low overpotential throughout most of its charge, whereas a Li-O-2 battery has a low initial overpotential that continuously increases to very high voltages by the end of charge. However, we find that the inherent kinetic Li and Na-O-2 overpotentials, as measured on a flat glassy carbon electrode in a bulk electrolysis cell, are similar. Measurement of each batteries' desired product yield, Y-NaO2 and Y-Li2O2, during discharge and rechargeability by differential electrochemical mass spectrometry (DEMS) indicates that less chemical and electrochemical decomposition occurs in a Na-O-2 battery during the first Galvanostatic discharge-charge cycle. We therefore postulate that reactivity differences (Li2O2 being more reactive than NaO2) between the major discharge products lead to the observed charge overpotential difference between each battery.