| Summary: | <p>There is significant interest in aprotic lithium-air batteries due to their high theoretical specific energy. During discharge, O<sub>2</sub> is reduced at the positive electrode and forms Li2O2. Cells are typically discharged in O<sub>2</sub>, not air, because CO<sub>2</sub> and H<sub>2</sub>O can interfere with the discharge reaction. However, lithium-air batteries will need to use atmospheric O<sub>2</sub> if they are to supplant state-of-the-art lithium-ion cells. Therefore, it is important to establish how detrimental CO<sub>2</sub> and H<sub>2</sub>O are to the battery. The former is well-known to induce the formation of Li<sub>2</sub>CO<sub>3</sub> in Li-O<sub>2</sub> batteries, but the recent work has suggested that H<sub>2</sub>O appears to be beneficial at low concentrations in some solvents (e.g. glyme ethers), increasing discharge capacities and still forming Li<sub>2</sub>O<sub>2</sub>, while in other solvents, such as acetonitrile (CH<sub>3</sub>CN), LiOH is the discharge product. Several mechanisms have been proposed to rationalise these findings, but as yet, there is no consensus on the role of H<sub>2</sub>O on O<sub>2</sub> reduction. </p>
<p>The purpose of this work was to understand how H<sub>2</sub>O affects O<sub>2</sub> reduction in electrolytes using acetonitrile (CH3CN), dimethyl sulfoxide (DMSO) and tetraethylene glycol dimethyl ether (TEGDME) solvents, and, why the 4e- reduction appears to be unfavourable at low H<sub>2</sub>O concentrations. Electrochemical and spectroscopic analysis found that, in CH<sub>3</sub>CN, 4e- reduction occurred at lower H2O concentrations than in DMSO and TEGDME. A mechanism based on the ability of the H<sub>2</sub>O/solvent mixture to stabilise OH- was proposed, with mixtures that stabilise OH- promoting 4e- O<sub>2</sub> reduction. The mechanism was confirmed by using pressure cells to identify the electrochemical reaction occurring during discharge. Cells using DMSO and TEGDME solvents underwent 2e- O<sub>2</sub> reduction, even with 1 M H2O concentrations. Finally, TEGDME cells were discharged in a 13% RH at 25 °C O<sub>2</sub> atmosphere, corresponding to 1 M H<sub>2</sub>O in solution, and Li<sub>2</sub>O<sub>2</sub> was confirmed as the discharge product, demonstrating that it is possible for electrolytes to withstand a near-atmospheric humidity.</p>
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