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www.advmat.de www.advancedsciencenews.com Electrically Rechargeable Zinc–Air Batteries: Progress, Challenges, and Perspectives REVIEW Jing Fu, Zachary Paul Cano, Moon Gyu Park, Aiping Yu, Michael Fowler, and Zhongwei Chen* which are expected to replace internal Zinc–air batteries have attracted much attention and received revived combustion engine vehicles in the coming research efforts recently due to their high energy density, which makes them years, are another industry where batteries a promising candidate for emerging mobile and electronic applications. have the potential to be the dominant form of energy storage. However, many believe Besides their high energy density, they also demonstrate other desirable that widespread consumer adoption of EVs characteristics, such as abundant raw materials, environmental friendliness, could still be decades away due to the issues safety, and low cost. Here, the reaction mechanism of electrically recharge- of range anxiety and high upfront cost.[2] able zinc–air batteries is discussed, different battery configurations are Most EVs today use lithium-ion batteries, compared, and an in depth discussion is offered of the major issues that which have dominated the rechargeable battery market since their advent in the late affect individual cellular components, along with respective strategies to 1990s. Going forward, the main disadvan- alleviate these issues to enhance battery performance. Additionally, a section tages of lithium-ion batteries are their high dedicated to battery-testing techniques and corresponding recommendations cost and concerns regarding both their for best practices are included. Finally, a general perspective on the current safety and the supply of lithium and cobalt limitations, recent application-targeted developments, and recommended (the latter of which is most commonly used future research directions to prolong the lifespan of electrically rechargeable in the positive electrode). Their energy density is also limited by the capacity of the zinc–air batteries is provided. electrode materials. These factors have led to intense research activity involving alter- native rechargeable battery technologies. 1. Introduction Metal–air batteries display considerably high energy densities, because oxygen is used as the reactant at the positive Ever-increasing demands in energy and awareness of climate electrode and is stored outside of the battery until it is dis- change has propelled and accelerated the inevitable transition charged. Primary and secondary metal–air batteries, with from fossil fuels to clean renewable energy. Given the limited metals such as zinc, aluminum, iron, lithium, potassium, fossil-fuel resources on Earth, it is reasonable to expect a more sodium, and magnesium have attracted much attention.[3,4] The volatile energy economy in the near future. Even with the lev- theoretical specific energies (i.e., gravimetric energy densities), elized energy cost for renewable energy sources approaching volumetric energy densities, and nominal cell voltages of var- or headed below that of fossil fuels, their intermittent nature ious metal anodes in metal–air batteries are shown in Figure 1. remains a challenge to widespread adoption in the global For secondary metal–air batteries, lithium metal is considered energy mix.[1] With this in mind, the task of developing new to be a strong anode candidate since it has the highest theo- energy-storage systems is more urgent than ever. retical specific energy (5928 W h kg−1) and a high cell voltage For centuries, batteries have been known for their excellence (nominally 2.96 V). However, lithium in the metallic form is in converting and storing chemical energy. One of their biggest plagued by its inherent instability when exposed to air and advantages over traditional forms of energy storage is the ability aqueous electrolytes.[5] Magnesium and aluminum–air batteries to be scaled down to small sizes, which has made them indis- are both compatible with aqueous electrolytes and have energy pensable for portable electronic devices. Electric vehicles (EVs), densities comparable to lithium–air; however, their low reduc- tion potentials typically lead to rapid self-discharge and poor Coulombic charging efficiency.[3] Zinc and iron are more stable J. Fu, Z. P. Cano, M. G. Park, Prof. A. Yu, and can be charged more efficiently in aqueous electrolytes; Prof. M. Fowler, Prof. Z. Chen out of these two, zinc has received far more attention due to Department of Chemical Engineering Waterloo Institute for Nanotechnology its greater energy and cell voltage within an aqueous metal–air Waterloo Institute for Sustainable Energy battery. Compared with lithium, zinc is inexpensive and more University of Waterloo abundant in the earth’s crust. More importantly, zinc metal 200 University Avenue West, Waterloo, within a metal–air battery has a relatively high specific energy ON N2L 3G1, Canada −1 −1 E-mail: [email protected] (1218 W h kg ) and a volumetric energy density (6136 W h L ) comparable to that of lithium–air. A high volumetric energy DOI: 10.1002/adma.201604685 density is particularly desirable for mobile and portable devices 1604685 (1 of 34) wileyonlinelibrary.com © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2017, 29, 1604685 www.advancedsciencenews.com www.advmat.de REVIEW (e.g., EVs and personal electronics) because there is a limited volume for mounting the batteries in these applications.[6] Jing Fu received her Master’s Moreover, the inherent safety of zinc means that zinc–air bat- degree (2011) under the teries can be placed in the front hood of an automobile, where supervision of Prof. Jinli Qiao provision for air access is already well established in today’s and Prof. Jianxin Ma at East vehicles. China University of Science Zinc, due to its low cost and high capacity, is the most and Technology, Shanghai, common anode material in primary metal–air batteries. China, where she specialized Primary zinc–air batteries are most notable for being in alkaline fuel cells. After the predominant energy source for hearing aids, where graduation, she worked with they provide an impressive volumetric energy density of Volvo Cars Corporation as 1300–1400 W h L−1.[7] Rechargeable zinc–air batteries for EVs a chassis design engineer were heavily investigated between ca. 1975 and 2000.[8–10] (2011–2014). She is now pur- Mechanically rechargeable and electrically rechargeable forms suing her Ph.D. in chemical engineering under the supervi- of the battery were both proposed. In mechanically recharged sion of Prof. Zhongwei Chen at the University of Waterloo, zinc–air batteries (also referred to as zinc–air fuel cells), the Waterloo, Canada. Her current research interests include battery was charged by removing spent zinc and re-supplying a the development of advanced nanostructured electrode fresh zinc anode. This avoided the issues of poor zinc electrode materials and solid-state electrolytes for flexible recharge- reversibility and unstable bifunctional air electrodes. However, able metal–air batteries. this concept was never widely adopted due to the high costs of setting up a network of zinc recharging and supplying sta- Zachary Paul Cano received tions. The most successful electrically rechargeable zinc–air his bachelor’s degree (2012) batteries employed a flowing electrolyte design, which greatly and master’s degree (2015) improved the durability of the zinc electrode.[8,11] However, in materials science and their power performance has traditionally been a drawback engineering from McMaster due mainly to fundamental challenges in catalyzing reactions University, Hamilton, Canada, involving oxygen gas at the air electrode. Moreover, corrosion where he specialized in corro- of the air electrode during the charging reaction was another sion science. He is now pur- critical issue, which, along with the advent of lithium ion bat- suing his Ph.D. in chemical teries, slowed down zinc–air battery development at the end of engineering (nanotechnology) the 20th century. under the supervision of Notwithstanding these problems, advances in materials sci- Prof. Zhongwei Chen and ence and nanotechnology have brought about renewed interest Prof. Michael Fowler at the University of Waterloo, in electrically rechargeable zinc–air batteries in this decade. Waterloo, Canada. His research is currently focused on the Several companies have developed unique zinc–air systems; the development of novel cathode structures and cell designs most prominent ones are EOS Energy Storage, Fluidic Energy, for long–lasting rechargeable metal–air batteries. and ZincNyx Energy Solutions. EOS Energy Storage’s Aurora product is a 1 MW/4 MW h zinc–air battery for utility scale −1 [12] grid storage, offered for as low as 160 US$ (kW h) . Fluidic Zhongwei Chen is Canada Energy has partnered with Caterpillar and Indonesia’s public Research Chair Professor utility to provide over 250 MW h of its zinc–air batteries to in Advanced Materials for [13] store photovoltaic solar energy in 500 remote communities. Clean Energy at University of ZincNyx Energy Solutions has developed a 5 kW/40 kW h zinc– Waterloo, Waterloo, Canada. air backup system that is undergoing a field test at a subsidiary He is also the Director of [14] −1 of Teck Resources. The 160 US$ (kW h) price offered by Collaborative Graduate EOS Energy Storage is remarkable given that manufacturing Program in Nanotechnology of rechargeable zinc–air batteries is far from mature; for com- at University

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