'Stretchable' toothpaste-like battery offers flexible future for gadgets

Rigid, bulky batteries could one day be replaced by soft, flexible ones, a new paper argues.

Scientists at a Swedish university have created a new form of a soft, fluid-based battery that can be shaped to meet any form, according to findings published Friday in Science Advances.

“The texture is a bit like toothpaste," co-author Aiman Rahmanudin said in a statement. 

That flexible quality means the ability to 3D-print the battery in any form — opening up the way for "a new type of technology,” Rahmanudin added.

In its current form, the battery is far from ready for industrial use. It can store just under 1 volt — less than 8 percent of the voltage of a standard car battery.

But Rahmanudin argued it has demonstrated a breakthrough in flexibility — and that increasing the voltage can be done by adding commonly available metals, like zinc or manganese.

Consumer product forecasters see a pressing need for such solutions. Some industry estimates suggest that nearly 40 billion devices worldwide will be connected to the internet by 2033 — twice the number as in 2023.

That forecast demand for consumer electronics, wearable medical devices or soft robotics has driven research into new kinds of flexible batteries beyond the current metallic boxes and cylinders that now define the market. 

New technologies require not just long storage life but “power sources that can bend and flex without compromising performance or durability,” according to a December survey in Science Bulletin.

That made it essential to design battery materials “that can endure repetitive folding, twisting, and stretching,” the scientists in that December study noted.

In the race to provide such materials, scientists are experimenting with flexible batteries using both familiar chemistries — lithium-ion and sodium-ion — as well as more novel ones, like zinc-ion, or a combination of zinc and magnesium-oxygen.

Friday’s study details an attempt to solve what lead researcher Rahmanudin described as the core paradox: bigger-capacity batteries require thicker electrodes, the conductive material that carries the charge from positive to negative poles. 

That property means more energy storage tends to come at the price of more rigidity.

The Swedish researchers say they have solved this problem by making liquid electrodes — based on a combination of flexible, conductive plastics and lignin, a polymer found in wood and bark.

Lignin, which the scientists extracted from paper pulp, is a raw material plentiful in any country with a big paper industry — which includes Sweden, but also the U.S. and battery-producing giant China.

That material offers a valuable upcycling of a waste product, said Mohsen Mohammadi, one of the lead authors on the paper.

“By repurposing a byproduct like lignin into a high value commodity such as a battery material we contribute to a more circular model,” he said.