We use XPS and isotope labeling coupled with differential electrochemical mass spectrometry (DEMS) to show that small amounts of carbonates formed during discharge and charge of Li–O₂ cells in ether electrolytes originate from reaction of Li₂O₂ (or LiO₂) both with the electrolyte and with the C cathode. Reaction with the cathode forms approximately a monolayer of Li₂CO₃ at the C–Li₂O₂ interface, while reaction with the electrolyte forms approximately a monolayer of carbonate at the Li₂O₂–electrolyte interface during charge. A simple electrochemical model suggests that the carbonate at the electrolyte–Li₂O₂ interface is responsible for the large potential increase during charging (and hence indirectly for the poor rechargeability). A theoretical charge-transport model suggests that the carbonate layer at the C–Li₂O₂ interface causes a 10–100 fold decrease in the exchange current density. These twin “interfacial carbonate problems” are likely general and will ultimately have to be overcome to produce a highly rechargeable Li–air battery.

In this work, we extend the activity volcano for oxygen reduction from the face-centered cubic (fcc) metal (111) facet to the (100) facet. Using density functional theory calculations, we show that the recent findings of constant scaling between OOH* and OH* holds on the fcc metal (100) facet, as well. Using this fact, we show the existence of a universal activity volcano to describe oxygen reduction electrocatalysis with a minimum overpotential, η_min = 0.37 ± 0.1 V. Specifically, we find that the (100) facet of Pt is found to bind oxygen intermediates too strongly and is not active for oxygen reduction reaction (ORR). In contrast, Au(100) is predicted to be more active than Au(111) and comparable in activity to Pt alloys. Using this activity volcano, we further predict that Au alloys that bind OH more strongly could display improved ORR activity on the (100) facet. We carry out a computational search over candidate alloys and suggest that alloying Au with early transition metals could lead to materials that exhibit enhanced ORR activity. We also prescribe a simple principle in the choice of nanoparticle catalysts in which the chosen catalyst should have its dense facet on the weak binding leg of the volcano so that its more open facets and defects could also be active.

Non-aqueous Li-air or Li-O₂ cells show considerable promise as a very high energy density battery couple. Such cells, however, show sudden death at capacities far below their theoretical capacity and this, among other problems, limits their practicality. In this paper, we show that this sudden death arises from limited charge transport through the growing Li₂O₂ film to the Li₂O₂-electrolyte interface, and this limitation defines a critical film thickness, above which it is not possible to support electrochemistry at the Li₂O₂–electrolyte interface. We report both electrochemical experiments using a reversible internal redox couple and a first principles metal-insulator-metal charge transport model to probe the electrical conductivity through Li₂O₂ films produced during Li-O₂ discharge. Both experiment and theory show a “sudden death” in charge transport when film thickness is ∼5 to 10 nm. The theoretical model shows that this occurs when the tunneling current through the film can no longer support the electrochemical current. Thus, engineering charge transport through Li₂O₂ is a serious challenge if Li-O₂ batteries are ever to reach their potential.