Influence of Hydrocarbon and CO2 on the Reversibility of Li-O2 Chemistry Using In Situ Ambient Pressure X-ray Photoelectron Spectroscopy

Identifying fundamental barriers that hinder reversible lithium-oxygen (Li-O-2) redox reaction is essential for developing efficient and long-lasting rechargeable Li-O-2 batteries. Addressing these challenges is being limited by parasitic reactions in the carbon-based O-2-electrode with aprotic electrolytes. Understanding the mechanisms of these parasitic reactions is hampered by the complexity that multiple and coupled parasitic reactions involving carbon, electrolytes, and Li-O-2 reaction intermediates/products can occur simultaneously. In this work, we employed solid-state cells free of carbon and aprotic electrolytes to probe the influence of surface adventitious hydrocarbons and carbon dioxide (CO2) on the reversibility of the Li-O-2 redox chemistry using in situ synchrotron-based ambient pressure X-ray photoelectron spectroscopy. Direct evidence was provided, for the first time, that surface hydrocarbons and CO2 irreversibly react with Li-O-2 reaction intermediates/products such as Li2O2 and Li2O, forming carboxylate and carbonate-based species, which cannot be removed fully upon recharge. The slower Li2O2 oxidation kinetics was correlated with increasing coverage of surface carbonate/carboxylate species. Our work critically points out that materials design that mitigates the reactivity between Li-O-2 reaction products and common impurities in the atmosphere is needed to achieve long cycle-life Li-O-2 batteries.