When lithium-oxygen batteries discharge, O2 is reduced at the cathode to form solid Li2O2. Understanding the fundamental mechanism of O2 reduction in aprotic solvents is therefore essential to realizing their technological potential. Two different models have been proposed for Li2O2 formation, involving either solution or electrode surface routes. Here, we describe a single unified mechanism, which, unlike previous models, can explain O2 reduction across the whole range of solvents and for which the two previous models are limiting cases. We observe that the solvent influences O2 reduction through its effect on the solubility of LiO2, or, more precisely, the free energy of the reaction LiO2(*) ⇌ Li(sol)(+) + O2(-)(sol) + ion pairs + higher aggregates (clusters). The unified mechanism shows that low-donor-number solvents are likely to lead to premature cell death, and that the future direction of research for lithium-oxygen batteries should focus on the search for new, stable, high-donor-number electrolytes, because they can support higher capacities and can better sustain discharge.

On discharge, the Li-O2 battery can form a Li2O2 film on the cathode surface, leading to low capacities, low rates and early cell death, or it can form Li2O2 particles in solution, leading to high capacities at relatively high rates and avoiding early cell death. Achieving discharge in solution is important and may be encouraged by the use of high donor or acceptor number solvents or salts that dissolve the LiO2 intermediate involved in the formation of Li2O2. However, the characteristics that make high donor or acceptor number solvents good (for example, high polarity) result in them being unstable towards LiO2 or Li2O2. Here we demonstrate that introduction of the additive 2,5-di-tert-butyl-1,4-benzoquinone (DBBQ) promotes solution phase formation of Li2O2 in low-polarity and weakly solvating electrolyte solutions. Importantly, it does so while simultaneously suppressing direct reduction to Li2O2 on the cathode surface, which would otherwise lead to Li2O2 film growth and premature cell death. It also halves the overpotential during discharge, increases the capacity 80- to 100-fold and enables rates >1 mA cmareal(-2) for cathodes with capacities of >4 mAh cmareal(-2). The DBBQ additive operates by a new mechanism that avoids the reactive LiO2 intermediate in solution.

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.

At the cathode of a Li-O2 battery, O2 is reduced to Li2O2 on discharge, the process being reversed on charge. Li2O2 is an insulating and insoluble solid, leading ultimately to low rates, low capacities and early cell death if formed on the electrode surface, problems overcome by forming/decomposing Li2O2 from solution. A Li-O2 cell is described that decouples completely the electrochemistry at the cathode surface from Li2O2 formation/decomposition. Mediators on discharge (2,5-Di-tert-butyl-1,4-benzoquinone [DBBQ]) and charge (2,2,6,6-tetramethyl-1-piperidinyloxy [TEMPO]) transfer electrons between the cathode surface and Li2O2. The cell cycles with a capacity of 2 mAh cm -2 areal at 1 mA cm -2 areal with low polarisation on charge/discharge, indicating that dual mediators combined with a true gas diffusion electrode could deliver 40 mAh cm -2 areal at rates >> 1 mA cm -2 areal. Arguably, the most important advantage of dual mediators is they avoid instability at the carbon cathode. Carbon is the most attractive material for the porous cathode in Li-O2 cells, but is too reactive degrading to Li2CO3. By forming/decomposing Li2O2 in solution and not in intimate contact with the carbon, by avoiding high charge potentials and because only mediators transfer electrons at the carbon surface, carbon instability is avoided (< 0.008 % carbon decomposition per cycle compared with 0.12 % without mediators), addressing one of the biggest barriers to the progress of Li-O2 cells.