As humanity moves towards renewable energy and a more sustainable economy, storage solutions with high energy density and low cost will be required. A recent scientific review found that rechargeable zinc-air batteries with neutral instead of alkaline electrolytes are one option that holds plenty of promise in building greener energy solutions in future.

The review¹, written by researchers from the University of Sydney and published in journal EnergyChem, looked at the evolution of the zinc-air battery from when it was first patented in 1878 until now, noting that while these batteries were first made with neutral electrolytes, today’s versions switched to an alkaline solution which provided higher energy densities but which caused “critical obstacles” in reaching a long battery life.

As well as reviewing the evolution of this critical battery technology, the researchers, guided by the University of Sydney’s Dr Zengxia Pei, also outlined the research topics they suggest should be examined in future studies in an effort to put rechargeable zinc-air batteries back into the mainstream.

“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 realising a long battery life. Thus, neutral electrolytes are attracting growing interest,” the researchers wrote.

A short primer on how batteries work

Typical batteries have a positively charged ‘anode’ and a negatively charged ‘cathode’ with an electrolyte solution sitting in-between. The anode reacts with the electrolyte to produce electrons while another reaction at the cathode allows it to accept electrons — a difference which allows for electron flow and the production of electricity.

In metal-air batteries, the metal acts as the anode while oxygen gas in the air is the cathode. During discharge, the metal is oxidised (meaning an atom of oxygen is added to the metal with a loss of electrons) while the oxygen gas in the air is ‘reduced’ into water or hydrogen peroxide depending on the electrolyte solution used.

“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. Among them, zinc-air batteries (ZABs) have attracted significant interest due to their inherent safety of using non-flammable aqueous electrolytes and non-toxic Zn, low cost and abundance of Zn metals, and high theoretical specific energy density,” the researchers write.

The first zinc-air battery was made in 1878 using a neutral NH4Cl solution as the electrolyte. While the batteries initially all used neutral electrolytes, zinc-air batteries with alkaline electrolytes eventually took over, as they came with a higher energy density (the amount of energy stored in a specific volume). In 1997, the first rechargeable zinc-air batteries with alkaline electrolytes entered the spotlight.

As a comparison, zinc-air batteries have a theoretical energy density that is four times higher than the commonplace lithium-ion batteries we know and use frequently.

Video on zinc-air batteries from New Horizons College of Engineering in Bengaluru, India

The problem with alkaline electrolytes

Talking with Lab Down Under, review co-author Zheng Zhou of the University of Sydney’s School of Chemical and Biomolecular Engineering said that zinc-air batteries with alkaline electrolytes came with various drawbacks including the build-up of spiky structures called dendrites and an eventually shortening of the battery life — issues which can be tackled by switching to neutral electrolytes.

“The zinc dendrite formation can be minimised in the neutral electrolyte, and some neutral electrolytes are also non-toxic and non-corrosive, safer for creating batteries for flexible and wearable electronics. So far, theoretical discussions and practical applications regarding zinc-air batteries with neutral electrolytes are still rare and elusive. Therefore, we believe it is necessary to deliver a review paper to systematically summarises the progress of neutral zinc-air batteries,” Zhou said.

During the chemical reactions within the battery’s alkaline solutions, various discharge products are formed. After repeated charging, these can build up, creating the dendrites. Severe growth in these dendrites can cause a short circuit between anode and cathode and cause battery failure.

Because zinc-air batteries are exposed to the air, having an alkaline electrolyte causes further problems as carbon dioxide from outside the battery can dissolve in the electrolyte and lower its conductivity.

“Although some CO₂ adsorbents have been used to mitigate this issue, additional storage space requirements for adsorbents and routine adsorbent replacements prevent their practical implementation. These issues have been critical obstacles to create practical [rechargeable zinc-air batteries] with a long cycling life,” the researchers write.

Promises and problems with zinc

Because of these issues, researchers have turned their attention to rechargeable zinc-air batteries with neutral electrolytes and have made significant progress in this area, Zhou told Lab Down Under.

“In my opinion, the biggest advances in rechargeable zinc-air batteries lie in two aspects. One, the fast development of efficient and robust O₂ electrocatalysts allows for the fabrication of high-performance air cathodes for rechargeable zinc-air batteries,” he said.

“Two, apart from improvements in the electrode materials, advanced battery configuration designs such as flow batteries and flexible batteries allow for the creation of rechargeable zinc-air batteries for various applications.”

However, further development of these batteries also faced a number of significant hurdles before they could be properly used in the mainstream, Zhou noted.

For instance even with a neutral solution, dendrite formation still developed upon miniscule protruding sections of the zinc surface created “mainly due to the inhomogeneous plating process,” the researchers wrote.

A reaction known as the hydrogen evolution reaction (HER) also stripped the anode of its zinc, reducing the reversibility of the charge and discharge cycle and lowering the capacity of the anode.

“Thus more efforts are encouraged to carry out on developing new zinc electrode materials and structural designs,” Zhou said.

Other chemical reactions such as the chlorine evolution reaction in neutral solutions such as zinc chloride and ammonium chloride also created issues for the large-scale production of rechargeable zinc-air batteries, he added.

A guide for future battery research

Zhou said he hoped that further research and discoveries would come out of the review, which discussed the various components of these batteries and highlighted the present challenges to be tackled.

“We hope our work can guide scientists in their field of expertise to address the current issues for each part of the rechargeable zinc-air batteries. Furthermore, most studies and applications of rechargeable zinc-air batteries at the current stages are merely carried out at the lab scale, yet the environment for practical large-scale applications is far more complicated,” he said.

“It is also important to consider the temperature and environmental compatibility of rechargeable zinc-air batteries, as practical applications may require rechargeable zinc-air batteries to function in a wide arrange of temperature and environmental conditions.”

According to the review, topics of further research include maintaining a stable pH in the electrolyte solution, optimising water-in-salt electrolyte solutions to avoid viscosity issues, improving ionic conductivity of gel polymer electrolytes, optimising battery configuration for practical applications, and implementing testing services using standardised conditions.

Faced with these unsolved problems, it is difficult to estimate exactly how long it will take until we see the widespread use of these batteries since further effort is required to enhance overall performance and improve temperature and environmental compatibility, Zhou noted.

“Fortunately, many researchers have realised the importance of these aspects and have carried out the corresponding works. For example, our group members studied the electrolytes under low-temperature conditions. We believe the widespread use of rechargeable zinc-air batteries will be achieved in the forthcoming future,” he said.

If you would like to find out more about Zhou’s research, please check out his ResearchGate and Google Scholar profiles.

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¹ Wang C, Li J, Zhou Z, Pan Y, Yu Z, Pei Z, Zhao S, Wei L, Chen Y. Rechargeable zinc-air batteries with neutral electrolytes: Recent advances, challenges, and prospects. EnergyChem, Volume 3, Issue 4, July 2021, 100055

Featured image: Batteries power electricity charger technology. Picture by Thor Deichmann. Used under the Pixabay licence.