The non-aqueous Li-air (O₂) battery has generated a great deal of attention because it potentially possesses far greater gravimetric energy storage density than today’s rechargeable battery technologies.^1-4  This has re-kindled the research interest in the O₂ reduction reaction (ORR) in non-aqueous electrolytes because the ORR is the central electrochemical process that occurs at a Li-air cathode.  Currently Li-air cells suffer from poor cycling efficiency and low current density.  Substantial improvements in these key aspects are critical to making rechargeable Li-air batteries a reality, for which it is necessary to understand the ORR and its reaction mechanism.

To that end, we have performed DFT-based theoretical studies of the ORR on metals.  Previously we have shown that the intrinsic activity of oxygen reduction by Li forms a volcano-like trend with respect to the adsorption energy of oxygen on different metals.⁵  Similar to the hydrogen version of the ORR, Pt and Pd are likewise found to be the most active metals and are located at the top of the volcano.  The aprotic nature of non-aqueous Li-O₂ cells, however, makes the one-electron reduction of O₂ to the superoxide anion (O₂^-) possible besides concerted Li⁺/e^- transfer, and indeed often at higher potential than molecular LiO₂.⁵  Our calculations, in conjunction with spectro-electrochemical studies, now identify O₂^- as a common intermediate in dimethyl sulfoxide (DMSO) electrolytes containing proton or lithium.⁶  When the DMSO contains a poor proton source, O₂^- appears as the first stable intermediate during the ORR, with H₂O₂ appearing at higher overpotentials.  When DMSO contains a Li⁺ source instead, O₂^- is again the first reaction intermediate, whereas surface LiO₂ does not appear until the potential is much more negative of bulk Li₂O₂ formation potential.  Therefore, the superoxide anion plays a key role in the ORR under a diverse range of electrode conditions, and needs to be taken into consideration when predicting the activity of ORR in non-aqueous Li-O₂ cells.

1. P.G. Bruce, S.A. Freunberger, L.J. Hardwick, J.-M. Tarascon, Nat. Mater. 2012, 11, 19.

2. J. Lu, L. Li, J.-B. Park, Y.-K. Sun, F. Wu, K. Amine, Chem. Rev. 2014, 114, 5611.

3. A.C. Luntz, B.D. McCloskey, Chem. Rev. 2014, 114, 11721.

4. R. Black, B. Adams, L.F. Nazar, Adv. Energy Mater. 2012, 2, 801.

5. G. K. P. Dathar, W.A. Shelton, Y. Xu, J. Phys. Chem. Lett. 2012, 3, 891.

6. Z. Peng, Y. Chen, P.G. Bruce, Y. Xu, Angew. Chem. Int. Ed. 2015, 54, 8165.