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.

A major unsolved problem with rechargeable Li/O2 batteries is the identification of electrolyte compositions that allow efficient and stable cycling of both Li metal and O2 electrodes simultaneously. Previously, lithium nitrate (LiNO3) was employed in a rechargeable Li/O2 battery to stabilize the solid–electrolyte interphase on Li metal in an electrolyte based on N,N-dimethylacetamide (DMA), a solvent with favorable properties vis-à-vis the O2 electrode. We show that LiNO3 is regenerated following reaction with Li metal in the presence of dissolved O2, which may account for the surprising long-term cycling previously demonstrated in DMA. According to this new concept, nitrate anions incorporated into the electrolyte react with Li metal to form soluble nitrite anions and a passivating layer of Li2O on the Li electrode surface. The soluble nitrite anions subsequently react with dissolved O2 through a combined electrochemical and chemical process that results in regeneration of nitrate. Discovery of this regenerative principle provides a strategy for using other solvents that have favorable characteristics in the O2 electrode but are highly unstable toward Li metal without the use of a ceramic Li-ion-conducting membrane.

Despite the promise of extremely high theoretical capacity (2Li + O2 ↔ Li2O2, 1675 mAh per gram of oxygen), many challenges currently impede development of Li/O2 battery technology. Finding suitable electrode and electrolyte materials remains the most elusive challenge to date. A radical new approach is to replace volatile, unstable and air-intolerant organic electrolytes common to prior research in the field with alkali metal nitrate molten salt electrolytes and operate the battery above the liquidus temperature (>80 °C). Here we demonstrate an intermediate temperature Li/O2 battery using a lithium anode, a molten nitrate-based electrolyte (e.g., LiNO3–KNO3 eutectic) and a porous carbon O2 cathode with high energy efficiency (∼95%) and improved rate capability because the discharge product, lithium peroxide, is stable and moderately soluble in the molten salt electrolyte. The results, supported by essential state-of-the-art electrochemical and analytical techniques such as in situ pressure and gas analyses, scanning electron microscopy, rotating disk electrode voltammetry, demonstrate that Li2O2 electrochemically forms and decomposes upon cycling with discharge/charge overpotentials as low as 50 mV. We show that the cycle life of such batteries is limited only by carbon reactivity and by the uncontrolled precipitation of Li2O2, which eventually becomes electrically disconnected from the O2 electrode.