Metal–Air and Redox Flow Batteries

Battery powerered: Hot topics centering on energy conversion and storage are showcased. This special issue on ”metal–air and redox flow batteries“ assembled by guest editors Jaephil Cho, Linda Nazar and Peter Bruce contains reviews and research articles by top scientists outlining the most recent developments on these important topics. New materials and battery systems that exploit innovative concepts to improve the energy density of rechargeable batteries are being actively investigated for applications in fully electrical vehicles (EV) and large-scale energy storage. In this special issue, contributions focus on metal–air batteries, such as Li–O2 and Fe–Air, as well as on redox flow batteries (RFBs). Metal–air batteries are expected to be a new type of power source because many researchers predict that lithium-ion cells will never give electric cars the 800 km range of a petrol tank. Among many metal–air batteries, the Li–O2 battery, in particular, is one of the most promising candidates for EVs and hybrid electric vehicles (HEVS) owing to its extremely high energy density (theoretically 11 140 Wh kg−1). However, the specifics of how the Li–O2 battery will operate are still being determined. Winfried Wilcke, Principle Investigator of the Battery 500 Project at IBM, estimates that Li–O2 or Li–air batteries might be ready for production by 2020 at the earliest.1 But first, many problems have to be overcome. For example, for the anode, pure lithium metal can provide a significant amount of energy, but has several disadvantages, such as the formation of metallic dendrites, as well as corrosion of the metal electrode when exposed to water, carbon dioxide, or other contaminants (John T. Vaughey et al., page 363; Jinwoo Lee, Youngsik Kim et al., page 349; Hye Ryung Byon et al., page 344). For the cathode, carbon materials are cheap and have a large capacity, but they are not stable during cycling, resulting in carbon corrosion and evolution of carbon dioxide (Zhaoyin Wen et al., page 270; Yining Zhang, Huamin Zhang et al., page 390). Finally, as yet, no ideal electrolytes have been developed and further R&D efforts should be directed to addressing the problem (Khalil Amine, Larry A. Curtiss et al., page 336; Tatsumi Ishihara et al., page 359—featured on the front cover). Also presented in this special issue is the hot topic of iron–air batteries (Carlos Ponce de Leon, page 323) and other metal–air systems involving layered LiMO2 oxides in alkaline media (M=Mn, Co, Ni; Veronica Augustyn and Arumugam Manthiram, page 422). Recently, a prototype EV powered by a flow battery system with a driving range of approximately 400 to 600 km was demonstrated at the Geneva Motor Show. This EV presented a specific energy density of 600 Wh kg−1, which was comparable to that of conventional lithium-based batteries. For decades, RFBs have been studied as a next-generation large-scale EES system. The most prominent feature is the flexibility of the design, which means that the energy and power can be separately customised by controlling the external electrolyte tank size and stack numbers. Conventional aqueous RFBs based on all-V, V/Fe and Zn/Br have come into widespread use and been successfully deployed on a commercial scale (Frank C. Walsh et al., page 288; Murugesan Vijayakumar, Wei Wang et al., page 428; Cristina Flox et al., page 354). However, the low energy density and high cost of redox couples still cause problems. Another challenge is that the operating voltage of an aqueous RFB is relatively low compared with that of a lithium-ion battery. As for the cost of an active material, Aziz’s group found a potential organic redox couple with high performance as an inexpensive option (Nature­ 2014, 505, 195–198). This novel approach presented the possibility of using earth-abundant compounds as reversible redox couples. In a similar way, to increase the energy density and operation voltage range, non-aqueous RFBs have already received much attention from various research groups. Recently, researchers have reported a novel hybrid flow battery system having a high energy density and voltage that used organic redox couples and lithium metal; this showed a comparable performance to that of conventional lithium-ion batteries (Adv. Energy Mater.­ 2012, 2, 770–779). Many researchers in this field have paid careful attention to the development of next-generation RFBs (Qizhao Huang and Qing Wang, page 312).1 1 1 1 1 The outstanding properties of recent state-of-the-art RFB systems have resulted in the cost of redox couples becoming more competitive and a substantial decrease in the size of the flow system owing to a high energy density (Ao Tang and Maria Skyllas-Kazacos, page 368). If the reliability and long-term cyclability were guaranteed, advanced RFBs could further offer the possibility of not only large-scale EES systems, but also high-performance EVs (Adam Z. Weber et al., page 402). To date, major energy-storage devices in EVs are lithium-based secondary batteries. However, the relatively short cycle life and low stability of the lithium-based battery system are considered to be a big obstacle to their use for longer than 10 years. The inherent advantage of RFBs is long-term cycle life (Tuti M. Lim and Maria Skyllas–Kazacos et al., page 376; Vijay Ramani et al., page 412). The current research trend has focused on large-scale EES systems. However, RFB systems with high energy and power density could be the most ideal candidates for EVs in the long term (Bernard Lestriez et al., page 396; Qinzhi Lai, Huamin Zhang et al., page 382). Together with fellow guest editors, Linda Nazar and Peter Bruce, I believe you will enjoy the insightful reviews and cutting-edge research presented here, and which will lead the way in overcoming the technical hurdles mentioned above. 1 Prof. Jaephil Cho (UNIST, Korea) Guest Editor 1 Prof. Linda F. Nazar (University of Waterloo, Canada) Guest Editor 1 Prof. Peter G. Bruce (Oxford University, UK) Guest Editor