Bending the rechargeable battery rules

The future of rechargeable batteries and the technologies they will power just got a lot more flexible, thanks to a recent breakthrough in screen-printable, high-capacity silver-zinc batteries by a research team from University of California, San Diego and ZPower, a California-based company formed in 1996 to build a better battery.

Typical flexible batteries have a limited life cycle and low capacity, which limits their use to low-power, disposable electronics.

These limitations have been overcome by an innovative battery design made possible by a proprietary cathode technology from ZPower combined with special inks and characterization techniques developed by UC San Diego nanoengineers.

"Our batteries can be designed around electronics, instead of electronics needed to be designed around batteries," said Lu Yin, a Ph.D. student in the research group led by UC San Diego's nanoengineering Professor Joseph Wang.

The yet-to-be-named battery is expected to be a contender as a next-generation power source for consumer electronics, including internet of things devices. Because this rechargeable battery is biocompatible, flexible, and has a high power and energy density, it is also an ideal fit for the present and future wearables market.

"As the 5G and internet of things market grows rapidly, this battery that outperforms commercial products in high current wireless devices will likely be a main contender as the next-generation power source for consumer electronics," said Jonathan Scharf, a Ph.D. candidate in the research group led by UC San Diego's nanoengineering Professor Ying Shirley Meng.

According to the team, the silver-zinc battery they have developed has a higher capacity than any of the flexible batteries currently available on the market. This is due to having much lower impedance – the resistance of an electric circuit device to alternative current. The lower the impedance, the better the battery performance against high current discharge.

Silver oxide-zinc is traditionally considered unstable and therefore not viable to battery production, but ZPower's silver oxide cathode material relies on a proprietary lead oxide coating to improve the battery's electrochemical stability and conductivity.

As an added benefit, the battery's printed current collectors – a specially designed part of present-day batteries to help regulate current between electrodes and external circuits – has excellent conductivity, which also helps achieve lower impedance.

The areal capacity for this innovative battery is 50 milliamps per square centimeter at room temperature, which is 10 to 20 times greater than the areal capacity of a typical lithium-ion battery. So, for the same surface area, this battery is capable of providing five to 10 times more power.

In tests powering a flexible display system equipped with a microcontroller and Bluetooth modules, the silver oxide-zinc battery outperformed commercially available lithium-ion coin cells.

After being recharged for more than 80 cycles, the flexible battery showed no major signs of capacity loss. The cells also remained functional in an included stress test of repeated bending and twisting.

The team at UC San Diego, by testing various solvents and binders, were able to find an ink formulation that makes it now viable for printing, which is what initially brought ZPower to UC San Diego for collaboration.

Once the ink is prepared, this battery can be printed in only a few seconds and after just a few minutes drying time is ready to be used. As a result, the battery developed by the UC San Diego and ZPower research team is much easier to manufacture than most flexible batteries, which need to be produced in sterilized conditions under vacuum.

Combine the quick and easy production with the abundance of silver and zinc, sibling minerals that are almost always mined together, supply of these metals would be able to easily keep up with production – even if ZPower achieves its founding purpose of proliferating the world with a better battery technology.

Ideally, the battery could be printed in a roll-to-roll process, which would increase the speed and make manufacturing scalable, which is exactly what the US San Diego nanoengineers and ZPower are now working on – a cheaper, faster charging battery with the flexibility to meet future demands.