Superoxide (O₂^-) is the key discharge intermediary driving many non-aqueous metal-oxygen battery cathode chemistries^1–4. Fundamental investigations of the interphase at a roughened Au electrode in a variety of ionic liquid and solvent blended electrolytes have been performed to study electrolyte effects on the chemical nature of O₂^- using in situ surface enhanced Raman spectroscopy (SERS). The observed (1095-1165cm^-1) and (460-490cm^-1) bond vibrations for O₂^- at the electrode surface can vary greatly depending on the immediate coordinating environment^5–7. Electrolytes that interact weakly with O₂^- produce more radical oxide species that may be more likely to undergo parasitic side reactions and degrade alternatively, if stable, O₂^- can diffuse away from the surface into the bulk electrolyte enabling a solution reaction pathway that has been linked with higher discharge capacities. Analysis of Raman peak positions, intensities and shifts in oxide spectral bands in different electrolytes and potentials allows for key information on O₂^- radical character and its coordination environment to be elucidated. Overall four main parameters were shown to affect the radical character of O₂^- at the interface in ionic liquid based electrolytes; (1) surface potential, (2) electrolyte cation, (3) electrolyte anion and (4) solvent electron acceptor/ donor numbers.

Ionic liquids have gained significant attention due to a number of innate physical and electrochemical properties. The significantly large combinations of ions provide the opportunity for electrolytes to be tailored towards specific application, however a clear electrolyte tailoring methodology is remains under developed. Our study provides important information in the oxygen reduction reaction mechanism that can aid in the ionic liquid selection and tailoring process.

References

1. Lu, J. et al. Nature 529,1–7 (2016).

2. Johnson, L. et al., Nat. Chem. 6,1091–1099 (2014).

3. Hartmann, P. et al., Nat. Mater. 12,228–32 (2013).

4. Ren, X. & Wu, Y., J. Am. Chem. Soc. 135,2923–2926 (2013).

5. Galloway, T. A. & Hardwick, L. J., J. Phys. Chem. Lett. 7,2119–2124 (2016).

6. Aldous, I. M. & Hardwick, L. J., J. Phys. Chem. Lett. 5,3924–3930 (2014).

7. Aldous, I. M. & Hardwick, L. J., Angew. Chemie Int. Ed. 55, 8254–8257 (2016).