Parasitic Products Formed during Discharge Limit Capacity and Rechargeability in Li–O2 Cells

Aprotic metal–oxygen batteries, especially Li–O2 and Na–O2 batteries, are considered high energy density alternatives to conventional Li-ion batteries. However, the rechargeability and, consequently, the cycle life of the metal–oxygen batteries are poor. In general, the poor rechargeability of these batteries is attributed to the oxidative instabilities of the carbon cathode and aprotic electrolytes at high oxidative potentials during charge. In this work, we employ complementary measurements, including electrochemical impedance spectroscopy, distribution of relaxation times analysis, chemical titrations of discharge products, and differential electrochemical mass spectrometry measurements to investigate electrochemical processes that limit rechargeability in these chemistries. Contrary to the extant understanding, our analysis shows that the origin of recharge inefficiencies in Li–O2 cells is the formation of parasitic side products during discharge. Significantly, our results suggest that cathode passivation by Li2O2 is not capacity-limiting during discharge, suggesting that increased capacities and rechargeability should be simultaneously possible in Li–O2 batteries.