When designing a wearable device, engineers must consider how long a battery will run between recharging sessions. We write about new battery technologies often, most recently covering stretchable, flexible batteries that are durable, comfortable, and convenient when incorporated in clothing. Researchers and scientists from the University of Warwick Department of Physics recently published a report in the Journal of Power Sources that reveals a previously unsuspected strain on electrodes in batteries commonly used with wearables.
The Warwick physicists analyzed cylindrical lithium cobalt oxide (LCO) batteries, a type of lithium-ion cell frequently used with small devices. LCO technology offers the energy density and high voltage platform that appeal to wearable product designers who seek sufficient power and run times for their devices. Unfortunately, LCO batteries develop significant surface layer resistance that increases impedance and disrupts normal energy flow when charging and discharging. The result is a battery technology that has impressive power characteristics but becomes less efficient over time. That is the problem the researchers were studying.
According to Warwick researchers, physicists generally attribute decreased LCO battery efficiency to resistive growth on the surface of the battery cathode and anode interfaces. The growth is believed to build up over time as a result of reactions between the electrodes and the electrolyte. When the scientists disassembled and inspected LCO batteries after 500 charging and discharging cycles, they discovered asymmetrical delamination on the inside of the jellyroll-like cylindrical batteries. To study changes in the battery structure after multiple cycles, the team used electrochemical testing, X-ray photoelectron spectroscopy (XPS), X-ray computed tomography (XCT), and scanning electron microscopy SEM). Both sides of the batteries were under stress: tensile stress on the outer side of the roll, and compression stress on the inner side. The surprise was that compression stress had a more significant effect on battery performance degradation than tensile strain.
The Warwick research points to the need for additional study into the potential for LCO battery design changes, including differences in surface coatings dependent on the varied pressure on inner and outer layers.
We may not see the benefits of the Warwick finding for a few years. Still, improved LCO designs could lead to longer battery runtime per charge, shorter charging times, and extended power source lifetimes. This would be a significant improvement for wearable device designers and users.
