The influence ofwater and protons on Li₂O₂ crystal growth in aprotic Li-O₂ cells

Aprotic Li-O₂ cells have attracted considerable research interest due to its outstandingly high theoretical specific capacity. However, published discharge capacities vary considerably among different researchers despite only minor differences in the tested cell components. Some research groups observe lowdischarge capacities and formation of passivating layers of Li₂O₂ on the electronically conducting cathode support, while other groups report large capacities and toroidal Li₂O₂ crystals as discharge product. In this study we show that these differences may be due to water and protons, both possible impurities in Li-O₂ cells, having a large effect on discharge capacity and Li₂O₂ morphology. As evidenced by XRD, FTIR and UV-visible analysis, Li₂O₂ is still the main discharge product in Li-O₂ cells containing water, and moreover the Li₂O₂ yield increases with the concentration of water in the electrolyte. On-line electrochemical mass spectrometry was employed to understand the differences in the discharge-charge behavior due to the addition of water and protons. While water seems to get oxidized at high potentials during charge, protons are consumed at the beginning of the discharge leading to a variety of reactive oxygen species and thus to degradation of cell components.