Charge Transport Properties of Lithium Superoxide in Li-O2 Batteries

Abstract

The theoretical energy density of lithium–oxygen (Li–O2) batteries is extremely high, although there are many challenges that must be overcome to achieve high energy density in a manufactured cell. For example, little is known about the properties of one of the key intermediates, lithium superoxide (LiO2), which until recently had not been stabilized in bulk form. In this work, lithium superoxide was deposited onto iridium–reduced graphene oxide (Ir–rGO) cathodes in a Li–O2 system under a flow of O2. Lithium peroxide (Li2O2) was subsequently produced on the cathode surface in an inert Ar atmosphere. Based on a detailed analysis of electrochemical impedance spectroscopy data, it was demonstrated experimentally for the first time that the charge transport resistance through LiO2 was much lower than for Li2O2 and correlated with lower LiO2 charge overpotentials. This result indicates that LiO2 has good electronic conductivity and confirms previous theoretical predictions that bulk LiO2 has better charge transport properties than Li2O2. In addition, impedance and other characterization of Li2O2 formation from LiO2 in an Ar atmosphere revealed that when surface-mediated Li2O2 formation occurs, it has a significantly lower discharge potential than when it forms through a solution-phase-mediated process. These significant findings will contribute to the development of Li–O2 batteries through better understanding of LiO2 properties and formation mechanisms.