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

We present a comparative study of nonaqueous Li-O2 and Na-O2 batteries employing an ether-based electrolyte. The most intriguing difference between the two batteries is their respective galvanostatic charging overpotentials: a Na-O2 battery exhibits a low overpotential throughout most of its charge, whereas a Li-O2 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-O2 overpotentials, as measured on a flat glassy carbon electrode in a bulk electrolysis cell, are similar. Measurement of each batteries' desired product yield, YNaO2 and YLi2O2, during discharge and rechargeability by differential electrochemical mass spectrometry (DEMS) indicates that less chemical and electrochemical decomposition occurs in a Na-O2 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.