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

Understanding the fundamental redox reactions and processes that occur in lithium–air and, more generally, metal–air batteries is important to the progress of this promising energy-storage technology. Knowledge of the chemistry of the peroxide dianion, O22–, is especially crucial, as the dianion is at the nexus of the charge/discharge cycle of lithium–air batteries. The intrinsic electron transfer properties and redox chemistry of peroxide dianion are poorly defined because 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 dianion encapsulated within the cavity of a hexacarboxamide cryptand. With regard to the former, electron transfer kinetics measurements provide fundamental Marcus parameters that will be useful for models that seek to disentangle the precise contributions of Li+ ion-coupled electron transfer, electron transfer across the Li2O2 solid particle interface, and charge hopping among Li2O2 particles. With regard to the latter, an underappreciated chemistry of peroxide dianion with CO2 produces peroxymonocarbonate (OOCO22–) and peroxydicarbonate (O2COOCO22–). An autocatalytic cycle will lead to oxidative degradation of traditional organic electrolytes and other vulnerable cell components employed in lithium–air batteries. This peroxycarbonate-derived chemistry, in addition to more commonly recognized solution-based oxidation chemistry, will need to be mitigated to realize the long-term cyclability of rechargeable lithium–air batteries.