Chemical Challenges that the Peroxide Dianion Presents to Rechargeable Lithium–Air Batteries

Understanding the fundamental redox reactions and processes that occur inlithium-air and, more generally, metal-air batteries is important to the progress of this promisingenergy-storage technology. Knowledge of the chemistry of the peroxide dianion, O22-, is especiallycrucial, as the dianion is at the nexus of the charge/discharge cycle of lithium-air batteries. Theintrinsic electron transfer properties and redox chemistry of peroxide dianion are poorly definedbecause it is difficult to isolate the dianion free of protons and metal ions. We review the results of(i) the electron transfer kinetics and (ii) the redox reaction chemistry of isolated peroxide dianionencapsulated within the cavity of a hexacarboxamide cryptand. With regard to the former, electrontransfer kinetics measurements provide fundamental Marcus parameters that will be useful formodels that seek to disentangle the precise contributions of Li+ion-coupled electron transfer,electron transfer across the Li2O2solid particle interface, and charge hopping among Li2O2particles. With regard to the latter, an underappreciated chemistry of peroxide dianion with CO2produces peroxymonocarbonate(OOCO22-) and peroxydicarbonate (O2COOCO22-). An autocatalytic cycle will lead to oxidative degradation of traditional organicelectrolytes and other vulnerable cell components employed in lithium-air batteries. This peroxycarbonate-derived chemistry, inaddition to more commonly recognized solution-based oxidation chemistry, will need to be mitigated to realize the long-termcyclability of rechargeable lithium-air batteries