**5.3. Redox mediators and recharge**

The use of redox mediators has opened an important strategy to recharge lithium‐oxygen batteries [30]. These are redox‐active soluble molecules, which are able to readily oxidize electrochemically during charge, and then reach the peroxide deposits giving place to a chemical redox reaction that evolves oxygen and switches back the molecule to its reduced state. By providing alternative electron paths from the insulating peroxide to the electrode collector, they act as catalysts, with considerable decrease of overpotentials increased charging rates and efficiencies. One of the requirements for the mediator is its stability to the electrochemical conditions, in effect we found UV‐visible spectroscopy evidence that oxygen actually reacts with the mediator iodide [31]. Other authors instead have shown that the product may also be significantly affected by iodide [32]. By comparing **Figure 12** with **Figure 11** we could not notice any remarkable difference in the morphology or composition of fully discharged electrodes with or without iodide. Although in this case it is difficult to unambiguously detect subtle spectral changes that may prove small deviations in the irreversible processes, the information we obtain is more representative of the relative amounts of oxygen in different chemical states. This allows affirming. That overall the mediator iodine has practically no effect on the proportions of main and side Li/O2 discharge products. When we eventually recharge the battery, using these small TEM grid‐based electrodes, the voltage usually increases fast above 4 V vs. Li. With the iodide mediator we instead obtain a large charge profile (**Figure 12a**). During this profile, we can observe that toroids disappear fast (sample B). Then, we can see that the sample just shows circular imprints inside a carbonate‐like material at the place of the toroids (sample C). The spectrum still shows some peroxide, in agreement with the tiny yellow fragments that can be noted embedded in the carbonate matrix, without being accessible even to the mediators, making the corresponding capacity highly irreversible.

**Figure 11.** (a and b) TXM images of a carbon‐coated Au TEM grid after being fully discharged in a Li‐O2 cell. The images are the result of overlapping three color maps with intensities proportional to the amounts of Li superoxide (cyan), Li peroxide (green), and carbonate (red). (c) Map of the LiO2 /Li2 O2 ratio. The respective LiO2 and Li2 O2 intensities have been obtained by the same method. The gray noisy area results from regions with low LiO2 and Li2 O2 values. (d) Corresponding O K‐edge XANES spectra at the selected points indicated by arrows in figures a and b, together with reference Li2 O2 and Li2 CO3 spectra. Reprinted with permission from Olivares‐Marín et al. [11]. Copyright 2015 American Chemical Society.

**Figure 12.** Galvanostatic discharge‐charge profile of a TEM mesh‐based cathode in a 1 M Li triflate electrolyte in TEGDME with a 0.02 M LiI additive (a), showing the position of three different samples. TXM images of the three samples (b) and corresponding overall O K spectra compared with references (c).
