Li-O₂ secondary batteries are attracting much researcher’s attentions as a candidate for the post lithium-ion secondary batteries due to the superior theoretical energy capacity. Oxygen reduction reaction forming Li₂O₂ deposition takes place at the cathode of Li-O₂ secondary batteries during the discharge process. The theoretical capacity of the Li₂O₂ electrode (8 kWh L^-1) is comparable to the energy density of diesel fuel (9.7 kWh L^-1). However, the mechanisms of the cathode reactions are complicated and are not well understood. The trace amount of water in electrolyte solutions is known to affect the reaction mechanisms^1,2 but how water affect the reactions are still under discussions.

In this study, the effects of trace amount of water on the discharge product at gold electrode in the DMSO-based electrolyte solution are studied by using electrochemical quartz crystal microbalance (EQCM), rotating ring-disk electrode (RRDE) method, in situ surface enhanced Raman scattering (SERS) spectroscopy and other instrumental analysis. In situ SERS showed that ca.1000 ppm of water results in the decrease of crystal Li₂O₂ signal intensity and new peaks of LiHO₂ and H₂O₂ at the initial stage of the discharge process. Application of negative potential, which causes surface mediated electrochemical reduction of LiO₂ in the anhydrous electrolyte solution,³ results in the decomposition of DMSO forming Li₂SO₄ while amorphous phase of Li₂O₂ was formed in the anhydrous electrolyte solution. The series of decomposition reactions of DMSO induced by water clearly indicated by EQCM measurement as M/z = 55 g mol^-1 corresponding to the formation of 1 mol of Li₂SO₄ by consumption of 2 mol of electron. These results show that effects of water should be considered when developing the better battery system utilizing water to change morphology of Li₂O₂ or converting Li₂O₂ to LiOH.^1,2

References

 (1) D. G. Kwabi, T. P. Batcho, S. Feng, L. Giordano, C. V. Thompson and Y. Shao-Horn, Physical Chemistry Chemical Physics, 18, 24944 (2016).

 (2) F. Li, S. Wu, D. Li, T. Zhang, P. He, A. Yamada and H. Zhou, Nature Communications, 6, 7843 (2015).

(3) D. G. Kwabi, M. Tułodziecki, N. Pour, D. M. Itkis, C. V. Thompson and Y. Shao-Horn, Journal of Physical Chemistry Letters, 7, 1204 (2016).