Under a bench at Sang-Young Lee’s lab is an ordinary, somewhat beat-up ink-jet printer he has modified so that it spits out electronic circuits and a type of energy storage device called a supercapacitor. To make it work, Lee empties the ink cartridges and refills them with specially formulated battery materials and conductive inks. When loaded with treated paper, his hacked printers make flexible, durable supercapacitors and simple circuit components in the form of a high-resolution map of the Republic of Korea, a flower, a logo, or any other desired design.
Lee, a battery chemist at the Ulsan National Institute of Science and Technology (UNIST) in South Korea, has been working on flexible printed batteries for the past five years. “The architecture of the battery hasn’t changed since the birth of the lithium-ion battery,” he says. Energy-storing materials are cast onto metal foil and packaged with a liquid electrolyte into a few basic shapes—pouches, coins, cylinders, and rectangular prismatic cells. For example, the design of wearable health monitors, whether they’re in textiles or worn in a wristband, is constrained by the need for a battery box or pouch. Instead, Lee wants to make flexible batteries that disappear into a design and can be made using simple equipment, such as an ink-jet printer.
To make it work, he had to tailor all the materials in the recipe. If the inks smear or run in the paper, the supercapacitor won’t work. So the first layer to be printed is a cellulose primer that absorbs inks and won’t run. That’s followed by carbon nanotubes, which replace the foil current collector in a battery, and silver nanowire electrodes, followed by an electrolyte ink. Each ink had to be formulated so that it would flow through the print head and not clump up while sitting in the cartridge.
The key to Lee’s system was developing an electrolyte compatible with ink-jet printing. The electrolyte, the medium that conducts ions and electrons, is typically a liquid. Lee is the first to make a fully ink-jet-compatible set of materials that includes the electrolyte. Other research projects, he says, require a liquid electrolyte to be added after the other parts are printed. The need to hold in that liquid constrains the design of the printed battery. There are solid-state electrolyte materials, but they’re not compatible with ink-jet printing and may not be flexible.
Lee’s prototype, described in the journal Energy & Environmental Science, shows how the battery and the circuit alike disappear into printed designs. On one printed sheet, the word “BATTERY” powers the words “Printed Circuit,” which carry electricity into an LED. In a design for a coffee-mug wrap, a supercapacitor powers a sensor to turn on a blue light by the printed word “COLD” or a red light by the word “HOT” depending on the beverage’s temperature.
“The goal for the Internet of things and ubiquitous computing is to have technology go into the background so we can interact with the world in ways that feel natural,” says Inna Lobel, a mechanical engineer and industrial designer at the design firm Frog in New York City. These printed supercapacitors suggest what such technologies and materials might look like, she says.
Still, the technology is a work in progress. In a testament to the novelty of the field, Lee had to build the equipment to test his flexible batteries. A custom-made machine measures electrical performance while the batteries are being twisted and otherwise deformed. The next step is to continue to improve the total energy storage of the printed devices, he says, and try printing on different materials besides paper.