The Li-ion batteries that power everything from smartphones to electric cars are usually packed in rigid, sealed enclosures that prevent stresses from damaging their components and keep air from coming into contact with their flammable and toxic electrolytes. It’s hard to use batteries like this in soft robots or wearables, so a team of scientists at the University California, Berkeley built a flexible, non-toxic, jelly-like battery that could survive bending, twisting, and even cutting with a razor.
While flexible batteries using hydrogel electrolytes have been achieved before, they came with significant drawbacks. “All such batteries could [only] operate [for] a short time, sometimes a few hours, sometimes a few days,” says Liwei Lin, a mechanical engineering professor at UC Berkeley and senior author of the study. The battery built by his team endured 500 complete charge cycles—about as many as the batteries in most smartphones are designed for.
Power in water
“Current-day batteries require a rigid package because the electrolyte they use is explosive, and one of the things we wanted to make was a battery that would be safe to operate without this rigid package,” Lin told Ars. Unfortunately, flexible packaging made of polymers or other stretchable materials can be easily penetrated by air or water, which will react with standard electrolytes, generating lots of heat, potentially resulting in fires and explosions. This is why, back in 2017, scientists started to experiment with quasi-solid-state hydrogel electrolytes.
These hydrogels were made of a polymer net that gave them their shape, crosslinkers like borax or hydrogen bonds that held this net together, a liquid phase made of water, and salt or other electrolyte additives providing ions that moved through the watery gel as the battery charged or discharged.
But hydrogels like that had their own fair share of issues. The first was a fairly narrow electrochemical stability window—a safe zone of voltage the battery can be exposed to. “This really limits how much voltage your battery can output,” says Peisheng He, a researcher at UC Berkeley Sensor and Actuator Center and lead author of the study. “Nowadays, batteries usually operate at 3.3 volts, so their stability window must be higher than that, probably four volts, something like that.” Water, which was the basis of these hydrogel electrolytes, typically broke down into hydrogen and oxygen when exposed to around 1.2 volts. That problem was solved by using highly concentrated salt water loaded with highly fluorinated lithium salts, which made it less likely to break down. But this led the researchers straight into safety issues, as fluorinated lithium salts are highly toxic to humans.
