Operando UV-visible spectroscopy evidence of the reactions of iodide as redox mediator in Li–O2 batteries

• A simple homemade Li/O2 cell has been built up for operando UV-visible spectroscopy.
• Iodide redox mediator has been evaluated using this cell.
• Triiodide formation during charge has been observed.
• Side reactions associated to the use of iodine have also been identified.

We monitored by means of UV-visible spectroscopy the formation and consumption of triiodide ion (I3−), generated when iodide ion (I−) is used as redox mediator in lithium–oxygen batteries. Results evidence the initial formation of I3− during oxidation, and its decomposition during reduction. Correlation of absorbance with capacity is consistent with the mediation of peroxide oxidation above 3 V. Decrease of I3− absorbance and the appearance of a peak at a wavelength of 410 nm after 14 hours at open circuit voltage revealed side reactions associated to electrolyte degradation leading to the formation of HOI. This work shows that UV-visible spectroscopy is a valuable tool for following reactions involved in the operation of lithium–oxygen batteries based in the absorbance of the species formed or consumed.

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