The nonaqueous lithium-oxygen (also called lithium-air) electrochemistry has an extraordinarily high theoretical specific energy, but critical electrochemical challenges prevent the realization of a high-capacity, reversible battery. Consisting of a lithium-metal negative electrode, an anhydrous, organic solvent-based lithium electrolyte, and a gaseous oxygen-saturated porous carbon positive electrode, the battery operates via the formation and decomposition of lithium peroxide (Li₂O₂). Unfortunately, in the most stable, ether-based electrolytes studied to-date, Li₂O₂ forms as a thin film on the carbon surface. As Li₂O₂ is an insulator, the thin film quickly leads to a large increase in the surface overpotential, effectively shutting off the battery before a high capacity can be achieved. Additionally, both the most stable electrolytes and porous carbons studied to-date still exhibit electrochemical decomposition in the presence of Li₂O₂ in operating cells, leading to parasitic products and poor cyclability.

In this work, we present electrolyte salt and solvent considerations toward addressing these challenges. We will particularly focus on recent reports that the introduction of lithium iodide (LiI) salt to a water-contaminated ether-based electrolyte induces a 4e^- per O₂ reduction reaction forming lithium hydroxide (LiOH) rather than the typical 2e^- per O₂ reduction reaction forming Li₂O₂.^1,2 This could potentially mitigate both charge transport and instability issues related to Li₂O₂ by entirely changing the discharge electrochemistry. We present our recent quantitative analysis of the influence of LiI and H₂O on the electrochemistry in a typical ether-based Li-O₂ battery via differential electrochemical mass spectrometry and titrations of extracted cathodes.³ We confirm that LiI and H₂O promote the efficient production of LiOH on discharge, but we find that LiOH is not reversibly oxidized to O₂ on charge. Rather, the charge electrochemistry is a complicated mix of redox shuttling and side reactions. We hope to inspire further research toward understanding the complex chemistry involved upon the addition of redox mediators to lithium-oxygen batteries.

(1) Liu, T.; Leskes, M.; Yu, W.; Moore, A. J.; Zhou, L.; Bayley, P. M.; Kim, G.; Grey, C. P. Science 2015, 350, 530.

(2) Kwak, W.-J.; Hirshberg, D.; Sharon, D.; Shin, H.-J.; Afri, M.; Park, J.-B.; Garsuch, A.; Chesneau, F. F.; Frimer, A. A.; Aurbach, D.; Sun, Y.-K. J. Mater. Chem. A 2015, 3, 8855.

(3) Burke, C. M.; Black, R.; Kochetkov, I. R.; Giordani, V.; Addison, D.; Nazar, L. F.; McCloskey, B. D. ACS Energy Letters 2016, 1, 747.