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.

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.

A novel lithium-oxygen battery exploiting PYR14TFSI-LiTFSI as ionic liquid-based electrolyte medium is reported. The Li/PYR14TFSI-LiTFSI/O2 battery was fully characterized by electrochemical impedance spectroscopy, capacity-limited cycling, field emission scanning electron microscopy, high-resolution transmission electron microscopy, and X-ray photoelectron spectroscopy. The results of this extensive study demonstrate that this new Li/O2 cell is characterized by a stable electrode-electrolyte interface and a highly reversible charge-discharge cycling behavior. Most remarkably, the charge process (oxygen oxidation reaction) is characterized by a very low overvoltage, enhancing the energy efficiency to 82%, thus, addressing one of the most critical issues preventing the practical application of lithium-oxygen 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.

Although lithium-oxygen batteries are attracting considerable attention because of the potential for an extremely high energy density, their practical use has been restricted owing to a low energy efficiency and poor cycle life compared to lithium-ion batteries. Here we present a nanostructured cathode based on molybdenum carbide nanoparticles (Mo2C) dispersed on carbon nanotubes, which dramatically increase the electrical efficiency up to 88% with a cycle life of more than 100 cycles. We found that the Mo2C nanoparticle catalysts contribute to the formation of well-dispersed lithium peroxide nanolayers (Li2O2) on the Mo2C/carbon nanotubes with a large contact area during the oxygen reduction reaction (ORR). This Li2O2 structure can be decomposed at low potential upon the oxygen evolution reaction (OER) by avoiding the energy loss associated with the decomposition of the typical Li2O2 discharge products.

Non-aqueous lithium-oxygen batteries cycle by forming lithium peroxide during discharge and oxidizing it during recharge. The significant problem of oxidizing the solid insulating lithium peroxide can greatly be facilitated by incorporating redox mediators that shuttle electron-holes between the porous substrate and lithium peroxide. Redox mediator stability is thus key for energy efficiency, reversibility, and cycle life. However, the gradual deactivation of redox mediators during repeated cycling has not conclusively been explained. Here, we show that organic redox mediators are predominantly decomposed by singlet oxygen that forms during cycling. Their reaction with superoxide, previously assumed to mainly trigger their degradation, peroxide, and dioxygen, is orders of magnitude slower in comparison. The reduced form of the mediator is markedly more reactive towards singlet oxygen than the oxidized form, from which we derive reaction mechanisms supported by density functional theory calculations. Redox mediators must thus be designed for stability against singlet oxygen.

this problem, solid [4][5][6][7][8][9] and liquid [10][11][12][13][14][15][16] catalysts have been introduced in Li-O 2 batteries. However, solid catalysts such as noble metals have been reported to promote not only the reversible decomposition of Li 2 O 2 but also electrolyte decomposition. [17][18][19] In addition, when direct contact between the solid catalyst and Li 2 O 2 is prevented or the surface of the catalyst is covered with Li 2 O 2 , no catalytic activation effect is expected. [10,11] Alternatively, liquid catalysts, called redox mediators (RMs), [10][11][12][13][14][15][16] have been added to the electrolyte as electron-hole transfer agents to promote reversible Li 2 O 2 formation/ decomposition. RMs constitute a more efficient solution for charge-discharge reversibility, resulting in a reduction of the charge overpotential without severe electrolyte decomposition. However, it has recently been reported that RMs can be deactivated during cycling through chemical reduction at the Li metal electrode (self-discharge of the electrochemically oxidized RM) and attack by oxygen species at the cathode. [20][21][22] Thus, more stable RMs are still required for practical application of Li-O 2 batteries with longer cycle life and higher efficiency.To alleviate the aforementioned problems associated with RMs in Li-O 2 batteries, Li metal, which is an essential anode component for obtaining a high energy density, should be protected through surface treatment and the employment of functional separators. [20,[22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40] However, reported Li protection materials and Li-protective layers mainly consist of polymers, which have insufficient stiffness to suppress the growth of Li dendrites. These polymer materials can also be decomposed by reactive oxygen radicals that are produced as intermediates during the formation and decomposition of Li 2 O 2 . [29,41] Herein, we attempted to solve these problems by controlling the concentration of the RM (LiBr) in the electrolyte and using a stable and stiff Li metal protective layer (a graphenepolydopamine composite layer, GPDL), which can help maintain high efficiency over prolonged cycling in Li-O 2 batteries. An appropriate concentration of LiBr in diethylene glycol dimethyl ether (DEGDME) was used as the RM based on previous reports. [15,42] A graphene-polydopamine composite was developed as a protective layer and was uniformly coated onto the Li metal electrode to suppress undesired reactions with LiBr in the electrolyte, as well as with oxygen and moisture. Using these approaches, we successfully demonstrated high-efficiency Recently, various approaches for adding redox mediators to electrolytes and introducing protective layers onto Li metal have been suggested to overcome the low energy efficiency and poor cycle life of Li-O 2 batteries. However, the catalytic effect of the redox mediator for oxygen evolution gradually deteriorates during repeated cycling owing to its decomposition at the surfaces of both t...