A ‘tonic’ that helps zinc-air batteries live longer

If you were asked to make a list of everyday tasks that involved the use of a battery, the list would undeniably run quite long. A sizeable subset of these tasks such as the working on a laptop, using a cellphone, wearing a smartwatch or driving an electric vehicle would have a rechargeable battery in the picture. Metal-air batteries have begun carving a niche for themselves in the move towards creating more efficient systems of energy storage. Pallavi Thakur, a graduate student at T. N. Narayanan’s lab, TIFR Hyderabad, has successfully led a research effort aimed at extending the life cycle of one such metal-air battery, the zinc-air variant, thus, addressing a major drawback associated with the commercial variety of rechargeable zinc-air batteries.

In a zinc-air battery, the anode is zinc and cathode is porous interface where oxygen gas interact with the electrolyte. Typically, zinc-air batteries use an alkaline electrolyte such as potassium hydroxide dissolved in water. To ensure that the battery is able to convert the chemical energy to electricity, the electrolyte needs to have access to the zinc metal. With usage, these zinc-air batteries witness a deposit of zinc oxide on the zinc anode. This is a major disadvantage which diminishes the capacity of a zinc-air battery to generate current.

Pallavi Thakur explains a crucial reaction that goes on inside the battery, “The potassium hydroxide electrolyte interacts with the zinc to form potassium zincate ions. When the electrolyte gets saturated with zincate ions, they break down into zinc oxide. The zinc oxide ends up forming a deposit on the zinc anode forming a barrier between the anode and electrolyte.” The authors of this study captured images of this deposit using very powerful microscopes. It appeared to form a uniform coating on the zinc, leaving no gaps for the electrolyte to access any remaining zinc metal on the electrode. Apart from this issue, another challenge was to recharge the battery without the need for a huge external potential.

In a bid to extend the life of the zinc-air battery, Pallavi explored different compositions of the electrolyte. She tried to see if any alteration in the composition of the electrolyte would delay the deposition of zinc oxide on the anode. The candidate she zeroed in upon was a hydroxide of a metal belonging to the same family as potassium: lithium. In her experiments, she used a combination of lithium hydroxide and potassium hydroxide as the electrolyte.

Introducing lithium hydroxide into the electrolyte proved to be successful, ushering in two major changes. First, the lithium created a porous deposit of zinc oxide, thus allowing the electrolyte mix to access the anode for a longer duration. Second, it surprisingly came in very handy in helping recharge the battery easily. But how? At this juncture, a team of researchers from Harish-Chandra Research Institute came in to understand how the reactions are altered inside the battery upon changing the composition of the electrolyte.

Now how does one recharge this battery? The situation inside the battery is slowly becoming chaotic. There are zincate ions in the electrolyte. On reaching a saturation point, some zincate ions end up as zinc oxide. In order to recharge this battery, the stock of zinc has to be replenished. During the process of recharging, the zincate ions roaming in the electrolyte would split in order to produce zinc again. Khorsed Alam, a graduate student who was a part of this collaborative effort says, “In our theoretical calculations, we found out that the dissociation of lithium zincate to zinc is easier than that of potassium zincate.” Introducing lithium ions in the picture helps split the zincate ions faster, thus, reducing the requirement of a high potential to recharge the battery.

Now that the lithium ions have breathed life into these batteries, how does the improved working prototype of the zinc-air battery compare with the market variety? Interestingly, the team observed that the battery life has increased to 55 hours from the usual 32 hours.

Prof. M. Deepa (IIT Hyderabad), who specialises in applied electrochemistry and was not associated with this study, remarks, “This work showcases how by using an extremely simple strategy, significant improvements can be achieved in the battery response, thus, opening up opportunities for adapting this method to other metal-air battery analogues as well.”

Zinc-air batteries are being considered as a safer and cheaper alternative to existing commercial lithium ion batteries, especially in electric vehicles traversing long distances. For a given weight of the battery pack, a zinc-air battery can provide more energy than a lithium ion battery. Perhaps, the new and improved version of the zinc-air battery may soon replace the bulky lithium ion batteries, making our journey into a sustainable future a smoother one.

Publication: Thakur, P., Alam, K., Roy, A., Downing, C., Nicolosi, V., Sen, P., & Narayanan, T. N. (2021). Extending the Cyclability of Alkaline Zinc–Air Batteries: Synergistic Roles of Li+ and K+ Ions in Electrodics. ACS Applied Materials & Interfaces, 13(28), 33112-33122.

The author thanks Pallavi Thakur for discussions while writing the article.