Metal−Air Batteries: Will They Be the Future Electrochemical Energy Storage Device of Choice? Yanguang Li*,† and Jun Lu*,‡ †
Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices, Soochow University, Suzhou 215123, China ‡ Chemical Sciences and Engineering Division, Argonne National Laboratory, Lemont, Illinois 60439, United States ABSTRACT: Metal−air batteries have a theoretical energy density that is much higher than that of lithium-ion batteries and are frequently advocated as a solution toward next-generation electrochemical energy storage for applications including electric vehicles or grid energy storage. However, they have not fulfilled their full potential because of challenges associated with the metal anode, air cathode, and electrolyte. These challenges will have to be properly resolved before metal−air batteries can become a practical reality and be deployed on a large scale. Here we survey the current status and latest advances in metal−air battery research for both aqueous (e.g., Zn−air) and nonaqueous (e.g., Li−air) systems. An overview of the general technical issues confronting their development is presented, and our perspective on possible solutions is offered.
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depending on the nature of the anode employed; the airbreathing cathode often has an open porous architecture that permits continuous oxygen supply from surrounding air. Metal−air batteries combine the design features of both conventional batteries and fuel cells. They have large theoretical energy density that is about 3−30 times greater than those of lithium-ion batteries (Figure 1). Research on metal−air batteries commenced much earlier than lithium-ion batteries. The first primary zinc−air battery was designed by Maiche in 1878,4 and its commercial products started to enter the market in 1932.5 Following that, aqueous iron−air, aluminum−air, and magnesium−air batteries were developed in 1960s.6−8 Nonaqueous metal−air batteries first emerged about two decades ago, initially for Li−air, and more recently for Na−air and K−air.9−11 Despite their early beginning, the development of metal−air batteries has been hampered by problems associated with metal anodes, air catalysts, and electrolytes. None of them at present are at a stage for large-scale industrial deployment. Their viability to replace lithium-ion batteries for future EV applications also remains unclear. In this Perspective, we do not intend to provide a comprehensive overview of metal−air batteries (excellent review articles on these topics can be found in the recent literature12−21); instead, we aim to highlight major technical
lectrochemical energy storage devices are essential components in the future energy network to buffer the unpredictable energy generation and supply derived from renewable sources. Of the many available options, lithiumion batteries play the most important role and have deeply penetrated every corner of our life since their advent. In spite of their great success, there has been a call to continuously improve their energy and power density. This need has become increasingly urgent in recent years and is driven by the demand to electrify transportation and promote grid-scale stationary energy storage.1,2 Unfortunately, conventional lithium-ion technology based on intercalation chemistry is approaching its performance limit. It is now generally believed that further improvements in Li-ion battery technology can bring in at most an additional 30% increase in energy density.3 Such an upper limit means that it would be difficult to achieve long driving ranges (e.g., 500 km) when conventional lithium-ion batteries are used to power electrical vehicles (EVs). New chemistry and systems are therefore being actively sought with a targeted energy density of 500 Wh/kg and a cost of 1000 cycles), low-cost (1000 cycles) of any nonaqueous metal−air batteries. Second, improving battery discharge performance would require the integral optimization of the electrolyte−cathode couple. It is suggested that the solvent donor number (DN) greatly impacts its solubility toward the reaction intermediate (LiO2) and hence the discharge characteristic of Li−air batteries.34,42 In high-DN solvents, LiO2 has greater solubility 1373
DOI: 10.1021/acsenergylett.7b00119 ACS Energy Lett. 2017, 2, 1370−1377
ACS Energy Letters
Perspective
Figure 4. Li−air batteries using the Ir−rGO cathode. (a) Schematic showing lattice match between LiO2 and Ir3Li that may be responsible for the LiO2 discharge product found on the Ir−rGO cathode. (b) Battery voltage profiles of the Ir−rGO cathode at 100 mA/g. Inset shows capacity as a function of cycle number. (c) SEM images of discharge product on Ir−rGO at different magnifications. Reproduced with permission from ref 49. Copyright 2016 Nature Publishing Group.
obviously is not suited for any practical application. Moreover, efforts are also devoted to developing catalysts to facilitate the reversible Li2O2 decomposition. A number of materials including Pt, Co3O4, MnO2, and RuO2 have been investigated, and some of them are proven to be effective.18,19,46 The use of redox mediators may also be beneficial. Soluble mediators can better access Li2O2 and mediate the electron transfer to accelerate its dissociation at lower potentials.47,48 The disadvantage, however, is the parasitic shuttle effect between anode and cathode. At last, the rechargeability of Li−air batteries might also be dramatically improved if its reaction pathway could be altered. Even though Li2O2 is the thermodynamically stable discharge product, we recently demonstrated that crystalline LiO2 was selectively formed as the discharge product by using an iridium−graphene hybrid catalyst (Figure 4).49 LiO2 could be more facilely decomposed during recharge because of its half metallic nature and the catalytic effect of Ir. As a result, the battery was repeatedly charged and discharged with a very low charge potential (3 times greater than those using low-DN solvents.34 One can envisage that the optimized cathode structure for the solution growth of Li2O2 should have large pore volume to accommodate as many Li2O2 deposits as possible and that for the surface growth of Li2O2 should have high surface area commensurate with pores of sufficient size to permit mass transport within the pores. When the pore size is too small, the pore entrance would be easily blocked, rendering the electrode inner surface area inaccessible and causing premature discharge termination. Third, reversible Li2O2 decomposition would need to be achieved at a lower overpotential (1 V) is commonly required to reverse the reaction and electrochemically decompose Li2O2 at the cathode.18,19 This not only lowers the energy efficiency of Li−air batteries to
