Elsevier

EnergyChem

Volume 3, Issue 4, July 2021, 100055
EnergyChem

Rechargeable zinc-air batteries with neutral electrolytes: Recent advances, challenges, and prospects

https://doi.org/10.1016/j.enchem.2021.100055Get rights and content

Highlights

  • A comprehensive review on the latest development of rechargeable zinc-air batteries with neutral electrolytes.

  • Critical issues of alkaline electrolytes and challenges in neutral electrolytes.

  • Neutral electrolytes: aqueous inorganic and organic salt solutions, water-in-salt electrolytes, and quasi-solid gel polymer electrolytes.

  • Common strategies for creating long-lasting zinc anodes.

  • A summary of novel electrocatalysts for oxygen reduction and evolution reaction in neutral electrolytes.

  • Future research directions of rechargeable zinc-air batteries with neutral electrolytes.

Abstract

Rechargeable zinc-air batteries (R-ZABs) are attractive for many essential energy storage applications – from portable electronics, electric vehicles to incorporation of renewable energy due to their high energy storage density, abundant raw materials, and inherent safety. However, alkaline electrolytes cause critical obstacles in realizing a long battery life. Thus, neutral electrolytes are attracting growing interest. However, the current understandings of R-ZABs in neutral/near-neutral electrolytes are far behind those in alkaline electrolytes. This review summarizes the latest research progress of neutral electrolytes used in R-ZABs, including aqueous inorganic and organic salt solutions, water-in-salt electrolytes, and quasi-solid electrolytes based on polymer hydrogels. Research efforts in improving the stability of Zn anodes in neutral electrolytes are also reviewed. Reaction mechanisms of oxygen reduction and evolution reactions in alkaline and neutral electrolytes are compared in the context of R-ZABs, together with a summary of potential oxygen electrocatalysts applicable in neutral conditions. Different device configurations are introduced. We further provide our perspectives on future research directions of R-ZABs with neutral electrolytes.

Introduction

Shifting our energy consumption from fossil fuels to renewable energy sources is critical to building a sustainable society. Electrochemical storage systems with high energy density and low cost are needed to incorporate electricity generated from various renewable energy sources into our daily energy consumption efficiently. Metal-air batteries use pure metals as an anode and redox reactions of external oxygen gas (O2) as cathode reactions. During discharging, the metal anode is oxidized while O2 in the air is reduced. The specific capacity and energy density of metal-air batteries are often higher than existing batteries because air is not required to be stored inside batteries.1, 2, 3, 4 Among them, zinc-air batteries (ZABs) have attracted significant interest due to their inherent safety of using non-flammable aqueous electrolytes and nontoxic Zn, low cost and abundance of Zn metals, and high theoretical specific energy density (1218 Wh kg−1 or 6136 Wh L  1).2,3

The first primary (non-rechargeable) ZAB was patented by Maiche in 1878 using a neutral NH4Cl aqueous solution as the electrolyte. In 1932, Heise and Schumadcher commercialized neutral ZABs, which were used in many applications. Unlike alkaline electrolytes, neutral NH4Cl solution does not react with CO2 to form carbonates. Jindra et al.5 in 1973 demonstrated that 5 M NH4Cl aqueous solution possessed sufficient buffering capacity to serve as a quasi-neutral electrolyte for primary ZABs. However, to create ZABs with higher energy density, alkaline electrolytes (e.g., KOH, NaOH, and LiOH aqueous solutions) are preferred because of their higher ionic conductivity, which are more widely used in commercial primary ZABs. For example, 35 wt.% aqueous KOH solvent at 25 °C has high ionic conductivity (0.55 S cm−1) and low viscosity (2.2339 mPa s), providing excellent electrochemical kinetics and mass transfer for ZABs.3,6,7 More importantly, commonly used electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) have much better catalytic activities in alkaline electrolytes than in neutral electrolytes.1,8,9 From 1997 onwards, rechargeable ZABs (R-ZABs) with alkaline electrolytes entered the spotlight. For example, 3.2 M of KOH mixed with 1.5 M of KF solution has often been used as an electrolyte in R-ZABs. However, the use of alkaline electrolytes brings various problems. The timeline of significant events in the development of ZABs is shown in Scheme 1.

In addition to providing efficient ion transport and favorable solvent environments for ORR/OER electrocatalysts, electrolytes play a crucial role in influencing the cycling life of R-ZABs. During discharging, Zn electrode oxidation yields various discharging products (e.g., ZnO, Zn(OH)2, Zn(OH)3, and Zn(OH)42−). They have high solubility in KOH aqueous electrolytes, enabling fast Zn oxidation reaction and inhibiting Zn electrodes’ surface passivation.3 However, during repeated charging, supersaturated Zn(OH)42− in electrolytes leads to Zn dendrite formation.10 Concentration-controlled Zn electrodeposition results in dendritic morphologies. A positively sloped concentration gradient of Zn(OH)42− is formed as a function of distance from the Zn surface. Zn(OH)42− preferentially deposits on surface heterogeneous sites with high concentration gradients. Upon continued deposition, these deposits cross the boundary of the diffusion-limited region, leading to rapid dendrite growth under a near-pure activation controlled condition.11 Severe overgrowth of Zn dendrites causes a short-circuit between anodes and cathodes, resulting in battery failure. A large number of electrolyte additives have been explored to regulate the mass transfer of discharging products in electrolytes and inhibit dendrite formation, including both inorganic materials (ZnO, SnO, CdO, PbO, Pb3O4, BeO, Bi2O3, In2O3, MgO, KF, LiF, LiOH, Co(OH)2, K2CO3, K3PO3, K3BO3, Na2CO3, and Li3BO3)12, 13, 14, 15, 16, 17, 18, 19, 20, 21 and organic materials (cetyl trimethyl ammonium bromide (CTAB), sodium dodecyl benzenesulfonate (SDBS), polyethyleneglycol (PEG), 1-ethyl-3-methylimidazolium dicyanamide (EMI-DCA), polyoxyethylen alkyl phosphate ester acid (PAPEA), tartaric acid (TA), tetra-alkyl ammonium hydroxides).22, 23, 24, 25, 26, 27 Besides, high concentration alkaline electrolytes also cause corrosions of carbon materials used in air electrodes by Eq. (1),28 which degrades air electrodes’ performance.C+6OH=CO32+3H2O+2e

Further, ZABs is an open battery system, having material exchanges with the external environment. CO2 present in the atmosphere can enter ZABs, and dissolved CO2 reacts with alkaline electrolytes to form CO32−, lowering electrolytes’ ionic conductivity.1 Further, reaction products (e.g., K2CO3 or KHCO3) with low solubility in alkaline electrolytes may clog micropores in the gas diffusion layer of air electrodes, impeding ORR/OER. Although some CO2 adsorbents have been used to mitigate this issue,9,29,30 additional storage space requirements for adsorbents and routine adsorbent replacements prevent their practical implementation. These issues have been critical obstacles to create practical R-ZABs with a long cycling life. Thus, neutral electrolytes are again attracting interest.

The commonly used neutral or near-neutral electrolytes are based on aqueous solutions of inorganic salts, such as NH4Cl and ZnCl2. They are widely used in primary ZABs and Zn-carbon batteries.5 Since 2012, neutral electrolytes have also been used in rechargeable Zn-ion batteries, in which Zn metal is used as the anode, and Zn-intercalating materials are used as the cathode.31 Neutral electrolytes are a potential competitor to alkaline electrolytes for rechargeable Zn batteries because Zn dendrite formation can be minimized in neutral electrolytes. The most significant difference between neutral and alkaline electrolytes lies in their different reactions with Zn electrodes. Zn dissolution reactions during discharging follow Eqs. (2)–(3) in alkaline electrolytes and Eqs. (4)–(6) in neutral electrolytes, respectively.32 There are no supersaturated Zn(OH)42− in neutral electrolytes that contribute to dendrite formation. Besides, the problem of CO2 contamination is also avoided in neutral electrolytes. Further, some neutral electrolytes are also nontoxic and non-corrosive, safer for creating batteries for flexible and wearable electronics.

Zn dissolution in alkaline electrolytes:Zn+4OH=Zn(OH)42+2eZn(OH)42=ZnO+2OH+H2O

Zn dissolution in neutral solution:Zn+H2O=ZnOH+H++eZnOH=ZnO+H++eZnO+H2O=Zn2++2OH

Based on the above advantages, Thomas Goh et al. reported the first R-ZAB using a neutral electrolyte in 2014.33 The aqueous electrolyte was comprised of 2.34 M NH4Cl and 0.51 M ZnCl2, 1000 ppm PEG and 1000 ppm thiourea with a pH of 6. They found no carbonate formation and minimal Zn dendrite formation in the ZAB with 120 discharging/charging cycles (1440 h) under 1 mA with a capacity of 4 mAh. Subsequently, In 2016, Sumboja et al.28 reported an improved neutral aqueous electrolyte (pH = 7) with similar ingredients, containing 5 M NH4Cl, 35 g L  1 ZnCl2, and 1000 ppm thiourea. R-ZABs were assembled using this electrolyte, a Zn metal anode, and an air electrode loaded with bifunctional MnOx electrolytes. The ZAB achieved 540 cycles (about 90 days) under a discharging/charging current of 1 mA cm−2 (4 h per cycle). In comparison, similar ZABs based on an alkaline electrolyte (5 M KOH, 35 g L  1 ZnCl2, and 1000 ppm thiourea, pH=14) were only stable for 200 cycles (about 33 days). These studies showed that Cl-based neutral electrolytes, which are widely used in primary ZABs, can also work in R-ZABs. The first flexible R-ZAB using a Cl-based neutral polymer electrolyte was reported in 2018, indicating that neutral electrolytes can also be applied to quasi-solid stable flexible ZABs.34

Although the above pioneer studies have demonstrated the feasibility of R-ZABs using Cl-based neutral electrolytes, several challenges arise in developing practical ZABs. First, Cl-based neutral electrolytes (containing Cl) may result in chlorine evolution reaction (CER). As shown in Eqs. (7)–(8), OER and CER's overpotential are similar.35 During battery charging, CER competes with OER, reducing the Columbic efficiency of ZABs and generating Cl2. The resulting Cl2 dissolves in aqueous electrolytes to form HClO, reducing electrolytes’ pH. Simultaneously, a decrease in electrolytes’ pH makes it easier for CER to occur, especially at pH below 2, forming a vicious cycle. NH4OH has been used to adjust electrolytes’ pH and maintain electrolytes’ ionic conductivity, which can partially inhibit CER. Moreover, several inorganic salts, such as CoCl2, IrO2, or soluble Mn salts, have also been used for the same purpose.35 Alternatively, other neutral electrolytes, such as ZnSO4, Zn(Ac)2, and organic aqueous solvents, have also been explored.362H2O=4H++O2+4e,E0=1.23V2Cl=Cl2+2e,E0=1.36V

Second, common ORR and OER electrocatalysts’ catalytic activity is much lower in neutral electrolytes than in alkaline electrolytes, which significantly lower air electrodes’ performance, limiting ZABs’ power density. The first study on bifunctional O2 electrocatalysts tailored for neutral electrolytes appeared in 2018.37 However, their performance still requires substantial improvement. Third, the oxidation and redeposition of Zn electrodes are also much slower in neutral electrolytes than in alkaline electrolytes. The development of Zn metal electrodes optimized for neutral electrolytes is still at an early stage.

Overall, the current understandings of R-ZABs in neutral/near-neutral electrolytes are far behind those in alkaline electrolytes. There are growing interests in developing practical ZABs based on neutral/near-neutral electrolytes. Thus, it is timely to summarize the latest research progress of novel electrolytes, electrocatalysts, and Zn electrodes for R-ZABs. This review will also help to rationalize future research directions. We first introduce the most commonly used neutral electrolytes, including inorganic and organic salt solutions, water-in-salt electrolytes, and quasi-solid electrolytes based on polymer hydrogels. We discuss their compositional characteristics and reaction mechanisms in ZABs. Next, we summarize recent research efforts in inhibiting Zn dendrite formation, Zn corrosion, and side reactions for Zn anodes in neutral electrolytes. Afterward, we compare ORR and OER mechanisms in alkaline and neutral electrolytes in the context of R-ZABs. We then summarize the latest development of electrocatalysts for efficient ORR/OER in neutral conditions. Different device configurations for R-ZABs are also discussed. Last, we provide our perspectives on future research directions to realize neutral R-ZABs’ practical applications.

Section snippets

Aqueous electrolytes based on inorganic or organic salt solutions

Early developments of neutral R-ZABs suggested that aqueous Cl-based inorganic salt solutions are promising electrolyte candidates. ZnCl2-NH4Cl electrolytes have been used in many studies, in which ZnCl2 provides Zn2+ for Zn stripping/platting process on Zn electrodes. Simultaneously, NH4Cl serves as a buffer to maintain electrolytes’ neutral pH, avoiding Zn electrodes’ corrosion.28,33,38

Fig. 1 shows a mixed electrolyte's composition containing 0.51 M ZnCl2 and 2.34 M NH4Cl at different pH.39

Zn anodes in neutral electrolytes

Zn metal is an ideal anode material for ZABs due to its high theoretical gravimetric and volumetric capacity (820 mAh g  1 and 5855 mAh cm−3) and low redox potential (−0.762 V vs. standard hydrogen electrode (SHE)).69,70 Moreover, Zn metal electrodes can be directly used in aqueous electrolytes, much safer than flammable organic electrolytes.71,72 Besides, Zn metal is naturally abundant with low toxicity.73, 74, 75 Therefore, Zn metal has been used as anode materials for ZABs since their early

Air cathodes in neutral electrolytes

Air cathodes consume O2 from the air during discharge via ORR and release O2 during the battery charge via OER. The reaction formula in alkaline electrolytes is shown in Eq. (20).O2+2H2O+4e4OH(0.401Vvs.SHE)

ORR occurs at three-phase interfaces (air/solid catalysts/liquid electrolytes) in air cathodes, and OER takes place at two-phase interfaces (solid catalysts/liquid electrolytes). Typical air cathodes consist of three components: an O2 electrocatalyst layer facing electrolytes, a gas

Device configurations of ZABs with neutral electrolytes

There are currently several device configurations for R-ZABs with neutral electrolytes, similar to those of ZABs with alkaline electrolytes. Fig. 19a shows a conventional device configuration with three parts: a Zn anode, an air cathode with O2 electrocatalysts supported on a porous gas diffusion layer, and a reservoir of liquid electrolytes. Liquid electrolytes enable ion transfer and keep two electrodes apart. Depending on the distance between the two electrodes, an additional porous

Challenges and prospects

We have summarized recent research advances and discuss relevant issues related to developing R-ZABs with neutral electrolytes. Although ZABs have been studied for many years, the research of R-ZABs with neutral electrolytes is an emerging new area, and many challenges remain to be addressed. In our view, the following research topics are critical for future studies:

  • 1)

    For neutral/near-neutral aqueous electrolytes based on inorganic or organic salt solutions, maintaining a stable pH in

Acknowledgments

This work was supported by the Australian Research Council under the Future Fellowships scheme (FT160100107).

Author contributions

Cheng Wang, Jing Li, Zheng Zhou: Conceptualization, Writing - original draft, Writing - review & editing. Yuqi Pan, Zixun Yu, Zengxia Pei, Shenlong Zhao, Li Wei: Writing - review & editing. Yuan Chen: Conceptualization, Supervision, Writing - review & editing.

Conflict of Interest

The authors declare no conflict of interest.

Cheng Wang received his B.S. degree from Beijing University of Chemical Technology, and he is currently a Ph.D. student at The University of Sydney. His-research interests mainly focus on zinc-based energy storage devices.

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    Cheng Wang received his B.S. degree from Beijing University of Chemical Technology, and he is currently a Ph.D. student at The University of Sydney. His-research interests mainly focus on zinc-based energy storage devices.

    Jing Li is currently a Ph.D. candidate at the University of Sydney. She received her M.S. degree from Tsinghua University in 2019. Her research interests are zinc metal batteries and lithium metal batteries.

    Zheng Zhou received his Ph.D. degree from the School of Chemical & Biomolecule Engineering, the University of Sydney, Australia, in 2019. His-research focuses on catalyst design and energy conversion electrolysis, including metal-air batteries, transition metal-based electrocatalysts for hydrogen evolution, oxygen reduction, and oxygen evolution reactions.

    Yuan Chen received a bachelor's degree from Tsinghua University and a Ph.D. from Yale University. He is a professor at The University of Sydney. His-research focuses on carbon materials and their sustainable energy and environmental applications, including batteries, supercapacitors, electrocatalysts, membranes, and antibacterial coatings. He is a fellow of the Royal society of chemistry and the institution of chemical engineers. He is currently serving as an editor for Carbon and Journal of Alloys and Compounds.

    1

    These authors contributed equally to this work.

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