Rechargeability looms for zinc batteries

For years, researchers have sought a viable method for making the common zinc alkaline battery rechargeable. Now, a team of international researchers has developed a technique for doing just that.

Prolonged research led the scientists to develop a new electrode design that is set to enable the rechargeability of alkaline zinc, one of the most common types of non-rechargeable batteries used in our daily lives. The technique also sheds light on a potentially wider application of rechargeable batteries.

Batteries are increasingly important in the age of smart cities, electric vehicles, and global digitalization. Compared with other types of primary batteries, alkaline zinc batteries are cheap, safe, and store large amounts of energy relative to their size. They are used in many household items, such as flashlights and remote controls.

Yet most of these batteries on the market are not rechargeable. Instead, they are tossed in the trash bin after a single use, a practice that environmental activists say poses a serious threat to the environment.

Zinc metal, the first-ever battery anode in Alessandro Volta's voltaic pile, invented in 1799, never ceases to attract research scientists' attention to its unfulfilled potential in a rechargeable battery.

Being one of the most abundant metals on earth, zinc releases two electrons upon oxidation and is able to work in an aqueous electrolyte, eliminating fire hazard and lowering the cost. It can also be dropped into commercial alkaline nickel cells to replace the expensive metal hydrides or be paired with an air cathode to afford practical, specific energy as high as 400 watt-hours per kilogram.

By comparison, the batteries that Tesla uses in its electric cars deliver about 254 Wh/kg.

However, neither zinc alkaline nor zinc air batteries last for long under the condition necessary to compete against lithium-ion batteries.

Still, given their low-cost and safety advantages, researchers worldwide have continued to seek the path to making zinc alkaline batteries rechargeable.

Research, however, continues to fall short of the goal because the battery reaction of zinc is not reversible. When the battery is discharged, zinc particles in the zinc electrode are covered with a thick and non-uniform layer of insulating zinc oxide, obscuring the metal surface and electric conductivity, which are both necessary for the electrode to be recharged.

Progress in Hong Kong

An international research team led by Professor Qing Chen, Ph. D., at the Hong Kong University of Science and Technology, recently developed a nanoporous zinc metal electrode that is capable of stabilizing the electrochemical transition between zinc and zinc oxide, successfully turning an alkaline zinc-air coin cell, a type of primary battery usually found in hearing aids, into a rechargeable battery stable for over 80 hours.

The team shaped zinc into curvy filaments hundreds of nanometers wide, nested in a freestanding solid with numerous, similarly narrow pores. When the battery is discharged, a thin layer of zinc oxide forms nuclei on the zinc filaments, preserving the metallic network and enabling the zinc electrode to return to its initial structure.

The team also tested the nanoporous zinc electrode in alkaline nickel-zinc batteries, a kind of uncommon secondary zinc battery that normally offers 50-80 times of discharging and charging under a condition that is competitive against state-of-the-art lithium-ion batteries. The result demonstrated a multi-fold increase to over 200 times.

"The needs for batteries are diverse and difficult to be met by a single technology. Zinc batteries are finding their niche. We just need to make sure that the microstructure of the zinc electrodes can withstand hundreds, and hopefully thousands, of times of discharging and charging when getting the most energy out of the batteries," said Chen. "Our work achieves so by understanding and then designing how atoms organize themselves at the liquid-solid interface that is manifested by the nanoporous structure, which has been applied to address a range of technological challenges."

While a few hundred times of discharging and charging may not seem like a lot, alkaline zinc batteries have an edge in safety and low cost, which are ideal for industrial applications such as golf carts and forklifts, Chen said. They also suit emerging applications like the backup power for data centers, which do not demand many times of discharging and charging but do require batteries to be extremely safe, he added.

The Hong Kong-led team has been working with industrial partners since the beginning of their research in 2018 and will continue engaging them for the commercialization of the promising technologies.

"Nature Communications" published an article describing the team's breakthrough in its May 24, 2022, edition.

HKUST postdoctoral fellow Li Liangyu, Ph. D., co-authored the article with Chen. Other members of the team are former research assistant Anson Tsang Yung-Chak, Ph. D., of the George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, Ga.; HKUST student XIAO Diwen, and former postdoctoral fellow ZHU Guoyin, Ph. D., School of Chemistry and Materials Science, Institute of Advanced Materials and Flexible Electronics, Nanjing University of Information Science and Technology, Nanjing, China, as well as Prof. ZHI Chunyi, Ph. D. at the City University of Hong Kong.

Titled "Phase-transition tailored nanoporous zinc metal electrodes for rechargeable alkaline zinc-nickel oxide hydroxide and zinc-air batteries," the article outlines the team's approach.

Funding for the research was provided by Hong Kong's Innovation and Technology Fund and Research Grants Council, University Grants Committee, and China's National Foundation of Natural Science.

Three of the researchers have filed U.S. provisional and Chinese patent applications on the process and its use in zinc batteries.

A new design

By tweaking the building materials of zinc-air batteries, another group led by researchers in Germany have created a prototype that can be recharged hundreds of times.

Such long-lasting devices, described in the Jan. 1, 2022, edition of "Science," could one day power electric cars or other electronics.

Wei Sun, a materials scientist at the University of Münster in Germany, China and the United States, observed that rechargeable alkaline zinc-air batteries, which promise high energy density and safety, suffer from a sluggish four electron-oxygen chemistry that requires participation of water, as well as from the electrochemical irreversibility originating from parasitic reactions caused by caustic electrolytes and atmospheric carbon dioxide.

"The problem is, this reaction is not very reversible," says Sun. "And that makes it difficult to recharge the battery. The caustic electrolyte in conventional zinc-air batteries also can degrade the cathode and anode."

By contrast, the zinc peroxide chemistry developed by Sun and his team proceeds through a two-electron-oxygen process in non-alkaline aqueous electrolytes that enables highly reversible redox reactions in zinc-air batteries.

In the article titled "A rechargeable zinc-air battery based on zinc peroxide chemistry," the team described how they built the zinc-air battery using a new electrolyte that contains water-repellant ions. Those ions stick to the cathode, preventing water in the electrolyte from reacting with incoming oxygen at the cathode surface.

As a result, zinc ions from the anode can travel to the cathode and react directly with oxygen from the air. This relatively simple reaction is easy to run backward to recharge the battery.

"The nonalkaline zinc-air battery thus constructed not only tolerates stable operations in ambient air but also exhibits substantially better reversibility than its alkaline counterpart," Sun wrote.

The new electrolyte doesn't degrade the battery's electrodes, which helps the battery last longer. In lab experiments, Sun and his colleagues were able to drain and recharge a new zinc-air battery cell 320 times over 160 hours.