Charge Transport Properties of Lithium Superoxide in Li–O₂ Batteries

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.