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.

Heterogeneous electrocatalysis has become a focal point in rechargeable Li-air battery research to reduce overpotentials in both the oxygen reduction (discharge) and especially oxygen evolution (charge) reactions. In this study, we show that past reports of traditional cathode electrocatalysis in nonaqueous Li-O(2) batteries were indeed true, but that gas evolution related to electrolyte solvent decomposition was the dominant process being catalyzed. In dimethoxyethane, where Li(2)O(2) formation is the dominant product of the electrochemistry, no catalytic activity (compared to pure carbon) is observed using the same (Au, Pt, MnO(2)) nanoparticles. Nevertheless, the onset potential of oxygen evolution is only slightly higher than the open circuit potential of the cell, indicating conventional oxygen evolution electrocatalysis may be unnecessary.

Quantitative differential electrochemical mass spectrometry (DEMS) is used to measure the Coulombic efficiency of discharge and charge [(e(-)/O2)dis and (e(-)/O2)chg] and chemical rechargeability (characterized by the O2 recovery efficiency, OER/ORR) for Li-O2 electrochemistry in a variety of nonaqueous electrolytes. We find that none of the electrolytes studied are truly rechargeable, with OER/ORR <90% for all. Our findings emphasize that neither the overpotential for recharge nor capacity fade during cycling are adequate to assess rechargeability. Coulometry has to be coupled to quantitative measurements of the chemistry to measure the rechargeability truly. We show that rechargeability in the various electrolytes is limited both by chemical reaction of Li2O2 with the solvent and by electrochemical oxidation reactions during charging at potentials below the onset of electrolyte oxidation on an inert electrode. Possible mechanisms are suggested for electrolyte decomposition, which taken together, impose stringent conditions on the liquid electrolyte in Li-O2 batteries.

Quantitative differential electrochemical mass spectrometry and cyclic voltammetry have been combined to probe possible mechanisms and the kinetic overpotentials, responsible for discharge and charge in a Li−O 2 battery, using C as the cathode and an electrolyte based on dimethoxyethane as the solvent. Previous spectroscopy experiments (X-ray diffraction, μRaman, IR, XPS) have shown that Li 2 O 2 is the principle product formed during Li−O 2 discharge using this electrolyte/cathode combination. At all discharge potentials and charge potentials <4.0 V, the observed electrochemistry is ∼2e − /O 2 consumed or produced, also implying that Li 2 O 2 is the dominant thermodynamically stable species formed and consumed in the electrochemistry. No evidence exists at any potential for formation of stable LiO 2 (1e − /O 2 ) or Li 2 O (4e − /O 2 ) during discharge. At charging potentials >4.0 V, the electrochemistry requires significantly more than 2e − /O 2 , and we take this as evidence for electrolyte decomposition. We find that sequential concerted (Li + + e − ) ion transfers to/from adsorbed O 2 * and LiO 2 * to produce/consume Li 2 O 2 is the mechanism that is most compatible with these experiments. The kinetic overpotentials are extremely low relative to aqueous O 2 reduction and evolution, and this implies that in principle a discharge−charge Li−O 2 cycle is possible with high voltaic efficiency (∼85%) if electrolyte and cathode stability issues are resolved.

We describe Li−O 2 discharge experiments in a bulk electrolysis cell as a function of current density and temperature. In combination with a simple model, these imply that charge transport through Li 2 O 2 in Li−O 2 batteries at practical current densities is based principally on hole tunneling, with hole polaron conductivity playing a significant role near the end of very low current discharges and at temperatures greater than 30 °C. We also show that charge-transport limitations are much less significant during charging than those in discharge. A key element of the model that qualitatively explains all results is the alignment of the Li 2 O 2 valence band maximum close to the electrochemical Fermi energy and how this alignment varies with overpotentials during discharge and charge. In fact, comparison of the model with the experiments allows determination of the alignment of the bands relative to the electrochemical Fermi level.