The effect of H₂O on O₂ reduction in Li-O₂ batteries

Thesis

There is significant interest in aprotic lithium-air batteries due to their high theoretical specific energy. During discharge, O₂ is reduced at the positive electrode and forms Li2O2. Cells are typically discharged in O₂, not air, because CO₂ and H₂O can interfere with the discharge reaction. However, lithium-air batteries will need to use atmospheric O₂ if they are to supplant state-of-the-art lithium-ion cells. Therefore, it is important to establish how detrimental CO₂ and H₂O are to the battery. The former is well-known to induce the formation of Li₂CO₃ in Li-O₂ batteries, but the recent work has suggested that H₂O appears to be beneficial at low concentrations in some solvents (e.g. glyme ethers), increasing discharge capacities and still forming Li₂O₂, while in other solvents, such as acetonitrile (CH₃CN), 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 H₂O on O₂ reduction.

The purpose of this work was to understand how H₂O affects O₂ 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 H₂O concentrations. Electrochemical and spectroscopic analysis found that, in CH₃CN, 4e- reduction occurred at lower H2O concentrations than in DMSO and TEGDME. A mechanism based on the ability of the H₂O/solvent mixture to stabilise OH- was proposed, with mixtures that stabilise OH- promoting 4e- O₂ 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- O₂ reduction, even with 1 M H2O concentrations. Finally, TEGDME cells were discharged in a 13% RH at 25 °C O₂ atmosphere, corresponding to 1 M H₂O in solution, and Li₂O₂ was confirmed as the discharge product, demonstrating that it is possible for electrolytes to withstand a near-atmospheric humidity.

Authors: Holc, C

• Alternative title: The effect of H2O on O2 reduction in Li-O2 batteries
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: lithium-air battery; water; oxygen reduction
• Subjects: Electrochemistry; Batteries