Parasitic\nProducts Formed during Discharge Limit Capacity\nand Rechargeability in Li–O<sub>2</sub> Cells

Aprotic metal–oxygen batteries, especially Li–O<sub>2</sub> and Na–O<sub>2</sub> batteries, are considered high\nenergy density alternatives to conventional Li-ion batteries. However,\nthe rechargeability and, consequently, the cycle life of the metal–oxygen\nbatteries are poor. In general, the poor rechargeability of these\nbatteries is attributed to the oxidative instabilities of the carbon\ncathode and aprotic electrolytes at high oxidative potentials during\ncharge. In this work, we employ complementary measurements, including\nelectrochemical impedance spectroscopy, distribution of relaxation\ntimes analysis, chemical titrations of discharge products, and differential\nelectrochemical mass spectrometry measurements to investigate electrochemical\nprocesses that limit rechargeability in these chemistries. Contrary\nto the extant understanding, our analysis shows that the origin of\nrecharge inefficiencies in Li–O<sub>2</sub> cells is the formation\nof parasitic side products during discharge. Significantly, our results\nsuggest that cathode passivation by Li<sub>2</sub>O<sub>2</sub> is\nnot capacity-limiting during discharge, suggesting that increased\ncapacities and rechargeability should be simultaneously possible in\nLi–O<sub>2</sub> batteries.