(289g) The Role of Superoxide in the Non-Aqueous Oxygen Reduction Reaction in Li-O2 Batteries

The non-aqueous Li-air (O₂) battery has attracted much research interest because it can potentially far exceed the gravimetric energy storage density than the current rechargeable battery technologies. The oxygen reduction reaction by Li (Li-ORR) is a key electrochemical process at a Li-O₂ cathode. Presently Li-O₂ batteries suffer from poor round-trip efficiency, low current density, and capacity fading,^1-3 To overcome these challenges, it is crucial to have a fundamental understanding of the reactive O₂ species involved in the ORR on different electrodes.

Recent studies have shown that the inclusion of metals in the cathode of Li-O₂ batteries can alter the cathode efficiency and possibly alter the nature of the discharge product.^1,2,4-8 We have performed a series of theoretical calculations of Li-ORR on metals and other materials. Our results show that the discharge potential is controlled by the formation of intermediates on the electrode surface. For instance, the initial O₂ reduction on Au occurs associatively with the formation of molecular lithium superoxide (LiO₂) at an equilibrium potential of ca. 2.0 V.⁹ The intrinsic activity of Li-ORR on different metals (including Au, Ag, Pt, Pd, Ir, and Ru) forms a volcano-like trend with respect to the adsorption energy of oxygen.⁹ Similar to the hydrogen-ORR, Pt and Pd are the most active of these metals, which has been borne out experimentally.10 This can be traced to the competition between the electrode on one hand and Li on the other hand for bonding with the oxygen.

The aprotic nature of typical Li-O₂ cells, however, makes the one-electron reduction of O₂ to the superoxide anion (O₂^-) possible (i.e., without concurrent Li⁺ transfer), and allows O₂^- to have appreciable lifetime, particularly on a relatively inert metal such as Au. O₂^- formation would occur at appreciably higher potential than molecular LiO₂ on many of the metals. DFT calculations, in conjunction with spectro-electrochemical studies,^11,12 therefore suggest that O₂^- formation needs to be taken into account when predicting the observed activity of ORR in Li-O₂ cells. On Au, for instance, O₂^- formation is the key process that corresponds to the discharge of a Li-O₂ cell at low overpotentials and determines how and where the solid discharge products are formed.¹² Our findings contribute to an improved understanding of the ORR in non-aqueous cells and to the further development of Li-O₂ batteries.

References

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(12) Zhang, Y.; Zhang, X.; Wang, J.; McKee, W. C.; Xu, Y.; Peng, Z. J. Phys. Chem. C 2016, 120, 3690.