The goal of limiting global warming to 1.5 °C requires a drastic reduction in CO2 emissions across many sectors of the world economy. Batteries are vital to this endeavor, whether used in electric vehicles, to store renewable electricity, or in aviation. Present lithium-ion technologies are preparing the public for this inevitable change, but their maximum theoretical specific capacity presents a limitation. Their high cost is another concern for commercial viability. Metal−air batteries have the highest theoretical energy density of all possible secondary battery technologies and could yield step changes in energy storage, if their practical difficulties could be overcome. The scope of this review is to provide an objective, comprehensive, and authoritative assessment of the intensive work invested in nonaqueous rechargeable metal−air batteries over the past few years, which identified the key problems and guides directions to solve them. We focus primarily on the challenges and outlook for Li−O2 cells but include Na−O2, K−O2, and Mg−O2 cells for comparison. Our review highlights the interdisciplinary nature of this field that involves a combination of materials chemistry, electrochemistry, computation, microscopy, spectroscopy, and surface science. The mechanisms of O2 reduction and evolution are considered in the light of recent findings, along with developments in positive and negative electrodes, electrolytes, electrocatalysis on surfaces and in solution, and the degradative effect of singlet oxygen, which is typically formed in Li−O2 cells. CONTENTS 3.4.1. Electrolytes 3.4.2. Development of New Solvents for Li−O2 N 3.7. Novel Electrolytes and Electrodes AH 3.7.1. The Possibilities and Development of Active Metal (Li, Na) Protection AH 3.7.2. Solid-State Li−Air and Na−Air Batteries AJ 3.7.3. On the Use of Ionic Liquids and Molten Salts AL 3.7.4. On the Possible Use of Solid Li-Oxide Cathodes and the Connection to Lithiated Transition Metals AN 3.8. Studies with Consideration of Practical Metal−Air Batteries AN 3.8.1. Li Batteries with Lithium Oxygen Compound Cathodes (and Closed Systems) AN 3.8.2. Challenges of Capacity and Kinetics AO 3.8.3. On the Validity of E nergy Density Calculation of Li (Na)−Oxygen Batteries AO 3.8.4. From Oxygen to Air AP 3.8.5. Configuration of Li−Air Cells and the Balance of Plant AQ 4. Future Perspective AR 5. Conclusion AS Author Information AT Corresponding Authors AT Authors AT Author Contributions AT Notes AT Biographies AT Acknowledgments AV Abbreviations Used AV References AV G Figure 28. Representative methods for protecting Li metal in Li−O2 batteries. (A) Gel or solid electrolyte. Reproduced with permission from ref 256.

Oxygen reduction in nonaqueous electrolyte solutions containing Li salts is a complex field of research involving solution reactions with oxygen radicals and lithium oxides. The aprotic polar solvent dimethyl sulfoxide presents itself as a most promising candidate for a durable electrolyte for use in lithium−oxygen batteries. In the present study, we detail our in-depth study on dimethyl sulfoxide (DMSO) stability in the presence of electroactive lithium oxygen species on carbon electrodes. The question of the stability of DMSO is magnified by our use of carbon-fiber electrodes, which have relatively high specific surface-area and utilize low volumes of electrolyte solutions. This configuration has enabled us to identify even minor side-products such as LiOH, dimethyl sulfone, Li 2 SO 3 and Li 2 SO 4 . The proposed mechanism of DMSO decomposition is supported by analytical measurements. These analyses confirm that during the reduction of oxygen on carbon electrodes, the solvent undergoes oxidation by reactive oxygen species and lithium oxides. SECTION: Energy Conversion and Storage; Energy and Charge Transport

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Improved efficiency and cyclability of cells containing LiBr demonstrate that the appropriate choice of electrolyte solution is the key to a successful Li–O2 battery.

This work deals with core issues of Li–oxygen battery systems; intrinsic stability of polyether electrolyte solutions and the role of important redox mediators such as LiI/I2.

The development of a successful Li-O2 battery depends to a large extent on the discovery of electrolyte solutions that remain chemically stable through the reduction and oxidation reactions that occur during cell operations. The influence of the electrolyte anions on the behavior of Li-O2 cells was thought to be negligible. However, it has recently been suggested that specific anions can have a dramatic effect on the chemistry of a Li-O2 cell. In the present paper, we describe how LiNO3 in polyether solvents can improve both oxygen reduction (ORR) and oxygen evolution (OER) reactions. In particular, the nitrate anion can enhance the ORR by enabling a mechanism that involves solubilized species like superoxide radicals, which allows for the formation of submicronic Li2O2 particles. Such phenomena were also observed in Li-O2 cells with high donor number solvents, such as dimethyl sulfoxide dimethylformamide (DMF) and dimethylacetamide (DMA). Nevertheless, their instability toward oxygen reduction, lithium metals, and high oxidation potentials renders them less suitable than polyether solvents. In turn, using catalysts like LiI to reduce the OER overpotential might enhance parasitic reactions. We show herein that LiNO3 can serve as an electrolyte and useful redox mediator. NO2(-) ions are formed by the reduction of nitrate ions on the anode. Their oxidation forms NO2, which readily oxidizes to Li2O2. The latter process moves the OER overpotentials down into a potential window suitable for polyether solvent-based cells. Advanced analytical tools, including in situ electrochemical quartz microbalance (EQCM) and ESR plus XPS, HR-SEM, and impedance spectroscopy, were used for the studies reported herein.

Polyether solvents are considered interesting and important candidates for Li-O2 battery systems. Discharge of Li-O2 battery systems forms Li oxides. Their mechanism of formation is complex. The stability of most relevant polar aprotic solvents toward these Li oxides is questionable. Specially high surface area carbon electrodes were developed for the present work. In this study, several spectroscopic tools and in situ measurements using electrochemical quartz crystal microbalance (EQCM) were employed to explore the discharge-charge processes and related side reactions in Li-O2 battery systems containing electrolyte solutions based on triglyme/lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) electrolyte solutions. The systematic mechanism of lithium oxides formation was monitored. A combination of Fourier transform infrared (FTIR), NMR, and matrix-assisted laser desorption/ionization (MALDI) measurements in conjunction with electrochemical studies demonstrated the intrinsic instability and incompatibility of polyether solvents for Li-air batteries.

The kinetics and thermodynamics of oxygen reduction reactions (ORR) in aprotic Li electrolyte were shown to be highly dependent on the surrounding chemical environment and electrochemical conditions. Numerous reports have demonstrated the importance of high donor number (DN) solvents for enhanced ORR, and attributed this phenomenon to the stabilizing interactions between the reduced oxygen species and the solvent molecules. We focus herein on the often overlooked effect of the Li salt used in the electrolyte solution. We show that the level of dissociation of the salt used plays a significant role in the ORR, even as important as the effect of the solvent DN. We clearly show that the salt used dictates the kinetics and thermodynamic of the ORR, and also enables control of the reduced Li2O2 morphology. By optimizing the salt composition, we have managed to demonstrate a superior ORR behavior in diglyme solutions, even when compared to the high DN DMSO solutions. Our work paves the way for optimization of various solvents with reasonable anodic and cathodic stabilities, which have so far been overlooked due to their relatively low DN.