Researchers at the University of California, Berkeley, have developed an innovative flexible battery designed to power soft robots and wearable technology. This new battery offers the unique ability to endure bending, twisting, and even cutting, all while being housed in a non-toxic, jelly-like material. Traditional lithium-ion batteries are typically encased in rigid, sealed structures that protect them from stress and keep harmful electrolytes away from the air, but these designs are not suitable for applications that require flexibility.
While past efforts have produced flexible batteries utilizing hydrogel electrolytes, they were significantly limited in operating time. “All such batteries could [only] operate [for] a short time, sometimes a few hours, sometimes a few days,” explained Liwei Lin, a mechanical engineering professor at UC Berkeley and the senior author of the study. The newly developed battery, however, has proven capable of enduring up to 500 complete charge cycles, comparable to the longevity expected from conventional smartphone batteries.
A Battery That Works in Water
Lin stated, “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.” Flexible materials often allow air or water to penetrate, which can react with traditional electrolytes, leading to heat generation and potential fires or explosions. This inspired researchers to explore quasi-solid-state hydrogel electrolytes beginning in 2017.
The developed hydrogels comprised a network of polymers that defined their structure, held together by crosslinkers such as borax or hydrogen bonds, and included a liquid phase of water along with salt or other additives that supplied ions for charging and discharging.
However, these hydrogels posed their own challenges, primarily due to a limited electrochemical stability window—an essential voltage range where the batteries can operate safely. As Peisheng He, a researcher at the UC Berkeley Sensor and Actuator Center and the study’s lead author, highlighted, “This really limits how much voltage your battery can output.” Most batteries currently operate around 3.3 volts, necessitating a stability window of at least four volts. The hydrogel’s water base typically breaks down into hydrogen and oxygen when subjected to voltages around 1.2 volts, leading researchers to utilize highly concentrated salt water combined with fluorinated lithium salts to prevent breakdown. However, this approach introduces safety concerns due to the toxicity of fluorinated lithium salts to humans.
