Oxygen (O2) reduction is one of the most studied reactions in chemistry.1 Widely investigated in aqueous media, O2 reduction in non-aqueous solvents, such as CH3CN, has been studied for several decades.2–7 Today, O2 reduction in non-aqueous Li+ electrolytes is receiving considerable attention because it is the reaction on which operation of the Li–air (O2) battery depends.8–29 The Li–O2 battery is generating a great deal of interest because theoretically its high energy density could transform energy storage.8, 9 As a result, it is crucial to understand the O2 reaction mechanisms in non-aqueous Li+ electrolytes. Important progress has been made using electrochemical measurements including recently by Laoire et al.29 No less than five different mechanisms for O2 reduction in Li+ electrolytes have been proposed over the last 40 years based on electrochemical measurements alone.25–29 The value of using spectroelectrochemical methods is that they can identify directly the species involved in the reaction. Here we present in situ spectroscopic data that provide direct evidence that LiO2 is indeed an intermediate on O2 reduction, which then disproportionates to the final product Li2O2. Spectroscopic studies of Li2O2 oxidation demonstrate that LiO2 is not an intermediate on oxidation, that is, oxidation does not follow the reverse pathway to reduction

There is increasing evidence that cyclic and linear carbonates, commonly used solvents in Li ion battery electrolytes, are unstable in the presence of superoxide and thus are not suitable for use in rechargeable Li-air batteries employing aprotic electrolytes. A detailed understanding of related decomposition mechanisms provides an important basis for the selection and design of stable electrolyte materials. In this article, we use density functional theory calculations with a Poisson-Boltzmann continuum solvent model to investigate the reactivity of several classes of aprotic solvents in nucleophilic substitution reactions with superoxide. We find that nucleophilic attack by O(2)(•-) at the O-alkyl carbon is a common mechanism of decomposition of organic carbonates, sulfonates, aliphatic carboxylic esters, lactones, phosphinates, phosphonates, phosphates, and sulfones. In contrast, nucleophilic reactions of O(2)(•-) with phenol esters of carboxylic acids and O-alkyl fluorinated aliphatic lactones proceed via attack at the carbonyl carbon. Chemical functionalities stable against nucleophilic substitution by superoxide include N-alkyl substituted amides, lactams, nitriles, and ethers. The results establish that solvent reactivity is strongly related to the basicity of the organic anion displaced in the reaction with superoxide. Theoretical calculations are complemented by cyclic voltammetry to study the electrochemical reversibility of the O(2)/O(2)(•-) couple containing tetrabutylammonium salt and GCMS measurements to monitor solvent stability in the presence of KO(2)(•) and a Li salt. These experimental methods provide efficient means for qualitatively screening solvent stability in Li-air batteries. A clear correlation between the computational and experimental results is established. The combination of theoretical and experimental techniques provides a powerful means for identifying and designing stable solvents for rechargeable Li-air batteries.

A major challenge in the development of rechargeable Li-O(2) batteries is the identification of electrolyte materials that are stable in the operating environment of the O(2) electrode. Straight-chain alkyl amides are one of the few classes of polar, aprotic solvents that resist chemical degradation in the O(2) electrode, but these solvents do not form a stable solid-electrolyte interphase (SEI) on the Li anode. The lack of a persistent SEI leads to rapid and sustained solvent decomposition in the presence of Li metal. In this work, we demonstrate for the first time successful cycling of a Li anode in the presence of the solvent, N,N-dimethylacetamide (DMA), by employing a salt, lithium nitrate (LiNO(3)), that stabilizes the SEI. A Li-O(2) cell containing this electrolyte composition is shown to cycle for more than 2000 h (>80 cycles) at a current density of 0.1 mA/cm(2) with a consistent charging profile, good capacity retention, and O(2) detected as the primary gaseous product formed during charging. The discovery of an electrolyte system that is compatible with both electrodes in a Li-O(2) cell may eliminate the need for protecting the anode with a ceramic membrane.

Solvent plays a major role in determining the nature of discharge products and the extent of rechargeability of the nonaqueous lithium-air (oxygen) battery. Here we investigate chemical stability for a number of aprotic solvents against superoxide, including N,N-dialkyl amides, aliphatic and aromatic nitriles, oxygenated phosphorus (V) compounds, substituted 2-oxazolidinones, and fluorinated ethers. The free energy barriers for nucleophilic attack by superoxide and the C-H acidity constants in dimethyl sulfoxide are reported, which provide a theoretical framework for computational screening of stable solvents for Li-air batteries. Theoretical results are complemented by cyclic voltammetry to study the electrochemical reversibility of the O2/O2− couple containing tetrabutylammonium salt and GCMS measurements to monitor solvent stability in the presence of KO2 and a Li salt. Excellent agreement among all quantum chemical, electrochemical, and chemical methods has been obtained in evaluating solvent stability against superoxide. The combined theoretical and experimental methodology provides a comprehensive testing ground to identify electrolyte solvents stable in the air cathode. Based upon this knowledge we report on the use of an amide-based electrolyte for rechargeable oxygen electrodes in Li-O2 secondary cells.

Li-O 2 cells with slight variations in design and material were employed in three different laboratories for this work, but all show consistent results. Figures 2, 3 and 6 were performed in one lab, Figure 1, 5, 7 and 8 in another, and Figure 4 in a third, but key experiments were repeated in all labs to confirm consistency. In order to provide specific repeatability, we will discuss specific cell materials, cathode preparation, and cell assembly pertaining to the data from Figures 2, 3 and 6. Materials.Lithium iodide was purchased from Sigma Aldrich and was dried under vacuum in a heated glove box antechamber at 150 °C for 24 hours before use. Lithium bis(trifluoromethane) sulfonamide (LiTFSI) and 1,2-dimethoxyethane (DME) were purchased from BASF and used as received. PTFE (both 60 wt% dispersion in H 2 O and 1 µm particle-size powder) was purchased from Sigma Aldrich. Vulcan XC72 was purchased from Fuel Cell Store and was filtered through a 60-mesh screen. Ketjenblack ® (KB) was received from Toyota. T316 stainless steel 120 mesh, with wire diameter 0.0026", was purchased from TWP Inc. Research-grade oxygen and argon were purchased from Praxair. 99% 18 O 2 was purchased from Sigma Aldrich. 90% H 2 18 O was purchased from Cambridge Isotopes. Water used to contaminate the electrolytes was ultrapure (18.2 MΩ cm, Millipore). All electrolyte and cell preparation was carried out in an argon glove box with < 0.1 ppm O 2 and <0.1 ppm H 2 O. Water was quickly and carefully added to electrolytes in the glovebox via micropipette. Graphene oxide was prepared via the oxidation of graphite (Alfa Aesar), and reduced it to rGO according to the procedure previously reported by Grey et al; the material was centrifuged in the initial washing steps.Cathode Preparation. The XC72 cathodes used for Figure 6 were prepared via a similar method to that described previously. 1,2 A mixture of 3:1 w:w ratio of Vulcan XC72 to PTFE binder in isopropanol (IPA) and water (4:1 water:IPA; and 15 mL total for 400 mg C) was sonicated for 30 seconds and homogenized for 6 minutes. A Badger model 250 air-sprayer was used to spray the