The researchers made the printed batteries flexible and stretchable by incorporating a hyper-elastic polymer material made from isoprene, one of the main ingredients in rubber, and polystyrene, a resin-like component. The substance, known as SIS, allows the batteries to stretch to twice their size, in any direction, without suffering damage.
The ink used to print the batteries is made of zinc silver oxide mixed with SIS. While zinc batteries have been in use for a long time, they are typically non-rechargeable. The researchers added bismuth oxide to the batteries to make them rechargeable.
"This is a significant step toward self-powered stretchable electronics," said Joseph Wang, one of the paper's senior authors and a nanoengineering professor at the Jacobs School of Engineering, where he directs UC San Diego’s Center for Wearable Sensors.
"We expect this technology to pave the way to enhance other forms of energy storage and printable, stretchable electronics,” Wang added. “Not just for zinc-based batteries but also for lithium-ion batteries, as well as supercapacitors and photovoltaic cells."
Researchers used standard screen printing techniques to make the batteries--a method that dramatically drives down the costs of the technology. Typical materials for one battery cost only $0.50 USD.
A comparable commercially available rechargeable battery costs $5.00 Batteries can be printed directly on fabric or on materials that allow wearables to adhere to the skin. They also can be printed as a strip, to power a device that needs more energy. They are stable and can be worn for a long period of time.
Recharging Flexible Batteries
The key ingredient that makes the batteries rechargeable is a molecule called bismuth oxide which, when mixed into the batteries' zinc electrodes, prolongs the life of devices and allows them to recharge. Adding bismuth oxide to zinc batteries is standard practice in industry to improve performance, but until recently, there hasn't been a thorough scientific explanation for why.
Last year, UC San Diego nanoengineers led by Professor Y. Shirley Meng published a detailed molecular study addressing this question. When zinc batteries discharge, their electrodes react with the liquid electrolyte inside the battery, producing zinc salts that dissolve into a solution. This eventually short circuits the battery. Adding bismuth oxide keeps the electrode from losing zinc to the electrolyte. This ensures that the batteries continue to work and can be recharged.
The work shows that it is possible to use small amounts of additives, such as bismuth oxide, to change the properties of materials. "Understanding the scientific mechanism to do this will allow us to turn non-rechargeable batteries into rechargeable batteries--not just zinc batteries but also for other electro-chemistries, such as Lithium-oxygen," said Meng.
Bringing Flexible Batteries to Market
Rajan Kumar, a co-first author on the paper published in Advanced Energy Materials is leading a team along with Wang that’s focused on commercializing aspects of this work. Kumar is excited at the prospect of taking advantage of all that the IGE Technology Accelerator has to offer.
"For us, it's strategically perfect," said Kumar, referring to the $50,000 funding for prototype improvements, the focus on prototype testing with a strategic partner, and the entrepreneurship mentoring.
"It's now about making sure our energies are focused in the right direction," said Kumar.
Through these programs, Kumar is focused on leading the team through a series of milestones in order to best position their innovations to refine "both what to build and who to build it for," he said.
For more flexible electronics news, learn about Generating Electricity with a Flexible Glass Thinner than a Business Card.