Influence of Dissolved Oxygen on the Composition and Stability of the Solid-Electrolyte Interphase on Lithium Electrodes

Abstract

Aprotic Li-O2 batteries are considered promising energy storage devices for mobile applications, as they have a high theoretical energy density that significantly exceeds that of state of the art lithium-ion batteries. However, they are still far from commercialization as they suffer from low lifetime and high charge overpotential. A further problem are numerous degradation reactions, many of which are related to dissolved O2. This work investigates the influence of O2 dissolved in the liquid electrolyte on the lithium anode, which could not be clarified in the literature so far. First, a systematic study of the solubility and diffusion of O2 in different electrolytes is performed. Both an experimental approach and molecular dynamics simulations are used to determine the diffusion coefficient. Good agreement is found between theory and experiment, demonstrating that both methods are suitable for the determination of the diffusion coefficient. Moreover, a correlation between the experimentally determined O2 solubility and the surface tension of the solvent is established, which will allow the prediction of O2 solubility in other solvents in the future. Overall, these findings significantly reduce the amount of work required to determine O2 solubility and diffusivity, thus facilitating the transfer of the results to other electrolytes. Since it was previously unclear, due to conflicting literature, whether dissolved O2 has a positive or negative effect on the stability of the lithium metal anode, this issue is further investigated. Lithium deposition and dissolution experiments demonstrate that the Coulomb efficiency can be significantly increased by dissolved O2. This can be attributed to reduced degradation of the conducting salt using X-ray photoelectron spectroscopy. In addition, freshly deposited lithium is compared to a commercial lithium foil. The reactivity of the native passivation layer on the lithium foil differs significantly from freshly deposited lithium with respect to the influence of dissolved O2, which is a possible reason for the apparently contradictory statements in the literature. Overall, the results obtained in this work significantly improve the understanding of the interaction between dissolved O2 and the lithium metal anode. Furthermore, the results are not only relevant for research on Li-O2 batteries, but also of interest for other metal-O2 batteries or lithium metal batteries.