Non-aqueous lithium-oxygen batteries depend critically on the reversible formation and on the decomposition of lithium oxides on cycling ^[1-3]. The discharge reaction in the cathode of nonaqueous Li-O₂ batteries involves the reduction of O₂ and the formation of solid Li₂O₂. The process is reverse on charge. True reversibility of the cathode reaction in the Li-O₂ battery requires a set of qualities to obey the stoichiometry and to match each other during discharge and subsequent charge. These are 1) e^-/O₂ = 2 both on discharge and charge, 2) to produce/consume exactly one mole of Li₂O₂ and one mole of O₂ per 2e^- and on discharge/charge, 3) absence of any other gas evolution, e.g. CO₂, or generation of solid products other than Li₂O₂ during discharge and charge, 4) O₂ consumed during discharge matches the amount released on subsequent charge.

Typically these measures deviate more or less significantly from the ideal due to parasitic reactions, whose source has not been entirely identified. Irreversible parasitic reactions have predominantly been ascribed to the reactivity of reduced oxygen species, e.g., superoxide with cell components ^[4-6]. These species, however, cannot fully explain these observed side reactions. Only better knowledge of parasitic reactions will allow inhibiting them and reaching towards fully reversible cell reactions.

We will discuss recent insights into irreversible parasitic reactions during cycling of a Li-O₂ battery, that have so far been overlooked, their detection via newly developed methods and strategies to suppress them effectively ^[7]. Awareness of these reactions in non-aqueous Li-O₂batteries gives a rationale for future research towards achieving highly reversible cell operation.

References

^[1] P.G. Bruce; S.A. Freunberger; L.J. Hardwick; J.-M. Tarascon; Nature Mater. 11 (2012) 19.

^[2] A.C. Luntz, B.D. McCloskey, Chem. Rev. 114 (2014) 11721.

^[3] Y.-C. Lu; B.M. Gallant; D.G. Kwabi et al.; Energy Environ. Sci. 6 (2013) 750.

^[4] S.A. Freunberger, Y. Chen, N.E. Drewett, L.J. Hardwick, F. Bardé, P.G. Bruce, Angew. Chem. Int. Ed., 50 (2011) 8609.

^[5] M.M. Ottakam Thotiyl, S.A. Freunberger, Z. Peng, Y. Chen, Z. Liu, P.G. Bruce, Nature Mater.12 (2013) 1050.

^[6] B.D. McCloskey, A. Valery, A.C. Luntz, et al., J. Phys. Chem. Lett.(2013) 2989.

^[7]N. Mahne et al., submitted.