Avoiding the Irreversible Four-electron Reduction of Oxygen in Water-containing Lithium-air Batteries

The specific energy of the aprotic lithium-air (Li-O ₂ ) battery has the potential to significantly exceed those of other rechargeable batteries at the system level. However, it is assumed to be intolerant to H ₂ O, in part because its presence can induce the irreversible formation of LiOH, rather than the desired product Li ₂ O ₂ , at the positive electrode. As a result, system designs incorporate gas handling to remove H ₂ O, adding weight and volume to the battery. Here, we describe how the presence of H ₂ O impacts the chemistry of the O ₂ reduction reaction in the organic solvents used in the Li-O ₂ battery and identify the factors that determine when LiOH is formed on discharge. Electrochemical and chemical analysis of the cell reactions were performed in a range of solvents and our analysis shows that LiOH formation in the Li-O ₂ battery occurs by a 2 + 2e ^– mechanism where O ₂ is first reduced by 2e ^– to Li ₂ O ₂ at carbon which is followed by a further 2e ^– reduction to LiOH at steel. We show that the impact of H ₂ O is greatest when electrolyte solutions with high acceptor numbers are used, due to stabilisation of the conjugate base of H ₂ O ( ^– OH), which favours deprotonation and new reaction routes. Using this new understanding, we demonstrate a H ₂ O tolerant Li-O ₂ electrode at 100% relative humidity at 20 °C (ca. 9 M H2O) which forms Li ₂ O ₂ , the desired discharge product. These results show that removal of H ₂ O by a gas handling system may not be required, which would drastically improve the system specific energy of a real-world lithium-air battery.