21 December 2010—Printing promises to make electronics cheap, flexible, and scalable. Printed solar cells, sensors, and displays are already close to market, and makers of energy-storage devices are now starting to catch on. Two printable battery start-ups, with different technologies targeting different applications, described their devices at the Printed Electronics conference in Santa Clara, Calif., on 2 December.
Planar Energy Devices, an Orlando, Fla.–based spinoff of the U.S. Department of Energy’s National Renewable Energy Laboratory, has developed a technique to print solid-state lithium batteries for electric vehicles. Its printed batteries store more energy, last longer, and are safer than their commercial counterparts, according to the company. Troy, N.Y.–based Paper Battery Co., meanwhile, is making flexible 100-micrometer-thick energy-storage sheets that could be molded onto electronics and medical devices or laminated beneath flexible solar panels. Both companies claim they should be able to print meters of batteries at a low cost.
Planar Energy replaces the liquid electrolyte found in conventional lithium-ion batteries with a ceramic electrolyte. Liquid electrolytes degrade the battery’s cathode in a few thousand cycles and require extra materials, such as binders, that add bulk and cost. Solid electrolyte batteries are safer, can hold more energy by weight, and can last for tens of thousands of recharge cycles, says Planar’s CEO, Scott Faris. The company’s batteries store 400 watt-hours of energy per kilogram. "Our energy density is a factor of two or three higher than lithium ion, and we can do that at about a third of the cost," he says.
Infinite Power Solutions, in Littleton, Colo., and Cymbet, in Elk River, Minn., already sell thin-film batteries that contain glassy electrolytes. Their batteries have been used mainly for wireless sensors, but the complex vacuum-based deposition methods have been too costly for mass-producing large batteries.
Faris says Planar has made two key breakthroughs. One is the ceramic electrolyte that works as well as a liquid electrolyte. The other is a manufacturing technique in which the company lays down the electrode and electrolyte layers sequentially; each layer’s chemical precursors are sprayed on a surface, where they interact to form uniform micrometers-thick films. "We spray the chemical constituents of the desired film, but the film assembles on its own," Faris says.
Paper Battery’s power sheets offer advantages in cost, scalability, and longevity over traditional batteries, in addition to flexibility, says the company’s CEO, Shreefal Mehta. But unlike Planar, the company is not making a true battery. Instead, it’s constructing ultracapacitors—devices that store charge on their electrode surfaces (batteries store and release charge through chemical reactions). Conventional ultracapacitor cells consist of aluminum current collectors sandwiching an electrolyte and coal-like activated carbon electrodes that store charge; a porous plastic separator keeps the charge from moving between the electrodes. Multiple cells are pressed together and packaged into battery-like devices.
Mehta says that the company’s design uses nanostructured cellulose, the main ingredient in paper, as the separator. The original concept, developed at Rensselaer Polytechnic Institute, consists of cellulose sheets embedded with carbon nanotubes and an ionic liquid electrolyte, which are then sandwiched between aluminum films.
The new design eliminates rigid battery-like packaging, instead producing millimeter-thick multilayered sheets of energy-storing material that hold 10 to 15 watt-hours of energy per square meter. "If you are able to contact the full surface, you can get a very large amount of power," Mehta says. "The sheets can stand alone, but we see this as being integrated into another structure." The company is codeveloping a product with the R&D spinoff of a large medical diagnostic device maker and hopes to integrate it into thin-film solar panels.
Paper Battery and Planar are the first companies to print rechargeable batteries, says Raghu Das, CEO of printed electronics research and analysis firm IDTechEx, in the United Kingdom. Comparatively weak, nonrechargeable printed batteries have been available for 10 years from Israel-based Power Paper and others. These are used in cosmetic and therapeutic skin patches but are too expensive and don’t last long enough to be used in electronics, says Das.
Printed rechargeable batteries could be cheap and lead to new products, says Das. And Paper Battery’s form factor and Planar’s high-energy density are novel, he adds. But, Das cautions, "it’s still fairly early days." The cost advantage of printing will come only at high-volume manufacturing. To reach that point, he says, the companies will "really have to find new markets…creatively coming up with new products leveraging the flexibility [and longevity] of the batteries."
About the Author
Prachi Patel is a contributing editor at IEEE Spectrum and a freelance journalist based in Pittsburgh. In October 2010, she reported on the outlook for jobs in renewable energy.