Flexible zinc-air batteries for wearable electronics

Why zinc-air batteries?

Flexible zinc-air batteries consist of a flexible zinc metal anode and a flexible catalyst cathode. For discharge and to supply the energy, the zinc is oxidized, grabbing an oxygen atom to create zinc oxide, and the oxygen is reduced by adding a hydrogen atom from the air to create a negatively charged ion or “anion” called hydroxide (OH-) at the battery’s cathode. The OH- migrates to the battery’s zinc anode releasing electrons that travel to the cathode generating energy to power devices.

To charge the battery, the zinc oxide is reduced to zinc and the OH- is oxidized to oxygen, releasing hydrogen back into the air. The advantages of these batteries are that they have a high energy density, they are low-cost, lightweight, and environmentally friendly thanks to the abundance of zinc and the element’s recyclability. They are also safer than other batteries such as lithium-based batteries which carry a relatively small risk of short circuiting and causing a fire, and they even been known to release toxic gases when they overheat.

This is because the material used as a medium for OH- is a water-based and is thus therefore non-toxic and not flammable.

As helpful as flexible zinc-air batteries are, however, there are still significant problems hampering their widespread adoption, including how many times they can be charged and discharged.

A new paper published in the journal Advanced Materials presents a novel hydrogel component for batteries that gives flexible zinc-air batteries an improved life cycle.

A solution in three dimensions

A hydrogel is a three-dimensional (3D) mixture of porous, permeable solids with an interstitial fluid in the form of a network of hydrophilic polymers. These polymers can swell and hold a large amount of water and salt while still maintaining their structure thanks to the physical cross-linking of individual polymer chains that comprise them.

Hydrogels are important to zinc-air batteries as they can form the medium through which the OH- is transferred between the anode and cathode. Because hydrogels can easily lose water, however, they can lead to significant problems in zinc-air batteries. These include their poor life cycle — the limited number of times they can be charged and discharged — and a slow cycling capacity, a measure of how fully they can charge and discharge each time they are used.

The team used a non-volatile and ionic liquid that absorbs moisture from the air, a “hygroscopic” substance, as an additive to prepare their novel hydrogel that can hang on to water longer.

“The ionic liquid remains in the hydrogel during the preparation, leading to a porous hydrogel. The porous hydrogel enables high water uptake and fast ion transfer,” paper author and associate professor at Tsinghua-Berkeley Shenzhen Institute (TBSI), Guangmin Zhou, said. “The hygroscopic ionic liquid suppresses the evaporation of water, leading to the hydrogel with high water retention.”

The researcher added that the team’s hydrogel has high ionic conductivity, water uptake, and water retention meaning that the zinc-air battery has a long cycle life.

“Based on the hydrogel, flexible zinc-air batteries can have a cycle life of more than 300 hours,” Zhou continued. “Also, the batteries can work at a wide range of temperatures from a low of minus 40 degrees Celsius to a high of 60 degrees Celsius, indicating flexible zinc-air batteries can work in the hot summer and cold winter.”

Zhou added that the batteries with the novel hydrogel the team created can be scaled up for wide-scale manufacture but that isn’t going to stop the team from aiming to improve their material. He concluded by adding these improvements will include an increased cycling capacity and a cycle life in excess of 1000 hours.