Scientists Create Stretchable Lithium Battery That’s Safe to Bend, Cut, and Puncture

A group of scientists from the University of California at Berkeley (UC Berkeley) has made progress toward creating a safe lithium battery. Current lithium batteries are at risk of catching fire when damaged, a problem that the new development does not have. Moreover, the battery created by the researchers can stretch, bend, and even be partially destroyed without consequences – doing everything that is necessary for the development of robotics.

Image source: UC Berkeley

Previous developments in this area could not boast of a long service life and a large number of charge cycles. The battery “from Berkeley” overcame these and other limitations, demonstrating the ability to withstand at least 500 charge cycles in a prototype. Modern mass-produced lithium-ion batteries promise to withstand approximately the same number of cycles.

In the process of creating a battery that is resistant to rough physical impacts, the researchers solved two main problems. First, the battery should not contain toxic materials. Second, the electrolyte structure should independently retain its shape – if this definition is appropriate for an electrolyte in the form of “jelly”. Scientists coped with both problems, although not everything turned out perfectly.

«Current batteries require a rigid shell because the electrolyte used in them is explosive. We wanted to create a battery that could be safely used without rigid packaging, the scientists explain. “Unfortunately, flexible packaging made of polymers or other elastic materials can easily let air or water through, which react with standard electrolytes, releasing a lot of heat and potentially leading to fires and explosions. That’s why in 2017 we began experimenting with quasi-solid hydrogel electrolytes.”

Since there was no ready-made solution, the researchers tried many compounds before they managed to form reliable molecular bonds in the electrolyte while maintaining acceptable ionic conductivity. In particular, they managed to raise the operating voltage of the battery to 3 V and even slightly higher, whereas previously quasi-solid hydrogel electrolytes limited this indicator to a level of about 1.2 V, which is insufficient for practical use.

The new electrolyte is based on so-called zwitterionic polymers, a class of macromolecules that contain both positively and negatively charged groups in the main or side chains. These charges are located close to each other and often neutralize each other, forming molecules with overall electrical neutrality. In batteries, such a polymer uses its “positive sides” to establish bonds with water molecules, and its negative charge to attract lithium ions.

Experiments showed that the hydrogel battery in a soft shell absorbed only 19% of the moisture from the air at 50% humidity. This allowed the battery to operate at a voltage of 3.1 V. Two notable drawbacks of the new battery were a faster loss of capacity – up to 60% of the initial level after 500 charge cycles (while modern batteries lose no more than 20%), as well as a low density of stored energy, which is only about 10% of the level of modern batteries.

But the new soft battery in flexible polymer packaging could be bent, twisted, punctured and even cut without consequences. It even recovered from cuts – although this required baking it in an oven. Scientists are confident that the newly acquired properties will help in creating safer electronics – from robotics to wearable devices. Moreover, the battery characteristics can still be improved. It is only a matter of time and further scientific research.