BellcoreRed Bank, N.J.

Flexible rechargeable plastic battery

Picture a battery. what do you see? Perhaps a small box such as a car battery, or a cylinder like the one powering a bunny across your TV screen, or a rectangular device for smoke alarms, or a button shape for a watch. Now imagine a lightweight sheet of flexible plastic about five feet wide and a tenth of an inch thick zooming out of a manufacturing line like newspapers or yard goods. Imagine you can cut this plastic sheet into any shape, fold it, twist it, poke holes in it, cram it into any space you wish, hook two wires to it -- and have it power anything from a laptop to an electric vehicle to devices we don't even know about yet. Now consider that this plastic sheet contains no toxic, heavy, or reactive metals such as lead, cadmium, mercury, or lithium, as found in other battery technologies, nor does it leak or contain any dangerous liquids, such as sulfuric acid, as in your car battery. Stop imagining and meet the new plastic rechargeable battery developed by Bellcore, Red Bank, N.J., the research arm of the Baby Bells. Based on lithium-ion technology, though containing no metallic lithium, this thin-film plastic battery can deliver the same power at half the weight of nickel cadmium batteries (NiCAD, general purpose rechargeables) and at one third the weight of lead/acid batteries (car batteries). It loses just 5% of its charge in a month's storage (3% per month thereafter) compared with 20% for NiCAD and 60% for nickel metal-hydride, a new nickel-based technology. Prototypes have been recharged 3,000 times (still recovering 70% of initial power) compared with typical cycle lives of about 1,000 recharges for NiCAD and 200 for lead/acid. Also, the Bellcore battery experiences no memory effect, or loss of the power not used if the battery is not discharged completely, a characteristic of NiCADs. To control the output of a Bellcore battery system just stack layers and connect them in series for more voltage, or increase the surface area via a bigger sheet to increase current. As impressive as these credentials are, the real breakthrough features of the battery are its thin-film, flexible form and its ability to be manufactured in existing plastic-film processing equipment. "The form and flexibility of our battery will free designers from shape limitations -- give them degrees of freedom they never had before," says Vassillis Keramidas, executive director of Bellcore's energy storage group. "If I were to design a portable telephone, I would have to accommodate a rectangular space for the present batteries, which are bulky. Now, with the total flexibility that our battery provides, you can have a power source that fits the back of the telephone from top to bottom. Or if you want to roll it or fold it and stick it somewhere else, you can." Today's cellular phones, for instance, are about 50% battery by weight. The Bellcore battery will allow new designs while also reducing the overall weight by one-quarter for the same performance or, at the same weight, doubling the time-in-use between recharges. This lightweight flexible battery can be exploited in unique designs of today's portable electronic devices, such as laptops, pagers, games, personal digital assistants, and flat-panel displays. It could also drive development of completely new devices capitalizing on its form and manufacturability. "Our battery opens up a whole new world of electronic equipment," says battery-team member Frough Shokoohi. Some new applications already suggested include:

  • Bar-code-type labels that broadcast information, that could be recharged and reprogrammed for their next use.
  • Telephone credit cards that carry programmed units of "currency" for paying for calls. When all the units are spent, the system is recharged and reprogrammed with more exchange units.
  • Card-like devices worn by employees to monitor exposure to dangerous gases or environments, including electronics and alarm systems.
  • Battery "clothing," such as a vest, as an alternative to carrying a heavy battery pack for powering medical devices, or military communications, or night-vision gear. The battery could also be implanted in the body and charged externally by induction.
  • Electric vehicles with the battery "hidden" throughout the car in the roof, doors, and fenders.
  • Sophisticated toys.
The basic simplicity of the battery will allow manufacturers to piggyback on current plastic-processing techniques. "Think of the battery as a strip of polymer with particles dispersed close to both surfaces [the electrodes], then impregnated with liquid [the lithium-ion electrolyte]," says battery-team member Paul Warren. In actually making the battery, three strips are fused together: top and bottom plastic strips -- the particle-containing electrodes -- each surfaced with a mesh-like metal current collector, and a center strip of pure plastic sandwiched in-between. This tri-layer package could be assembled at high speed in continuous fashion while exposed to heat sufficient to fuse the layers into one so there are no interfaces. After fusing, the strip is immersed in a bath of electrolyte, which, by a proprietary process, entraps the liquid in the molecular structure of the plastic polymer, filling the center area and surrounding the electrode particles in the outer areas. "Our battery can be made like any other multilayer plastic laminate films, such as those used in food packaging," says Dr. Warren. "Only at the end does it get fussy about being a battery. It's like making textile yard goods, then the battery manufacturer activates it [impregnates it with electrolyte]." Processing the film in 60-inch widths is not unreasonable, says Dr. Keramidas. In between lamination and electrolyte impregnation, the film can be rolled up and stored indefinitely. All of the materials in the battery are readily available, and only the electrolyte makes a truly expensive contribution. "Isolating the cost of materials only, not considering our simple manufacturing process, we beat all battery technologies except lead/acid," says Dr. Keramidas. The impetus for the lithium-ion battery can be traced to 1990 when Bellcore researchers tackled the challenge of new backup power sources for current telephone and future network distribution systems. At present, batteries at central switching offices provide backup power for public telephone networks. The batteries kick in any time there is a power glitch, so telephone service is maintained. To meet the new consumer demand for broadband services (telephone, multichannel TV, data, video), telecommunications companies are considering connecting customers with optical-fiber distribution systems with local fiber-to-home, fiber-to-curb, or fiber-coaxial systems. These optical-fiber based systems "will require multiple powering points in a distributed system, not a central powering system," says battery-team member Tony Gozdz. In the special case where optical fiber is routed to the curb, transmission from the curb to the home could be wireless. Battery power in the devices in the home will be required to back up utility power in case of a power outage. Lead/acid has been the traditional battery technology for central-station backup power, but "lead/acid batteries are unreliable in some applications because they do not work well in high-temperature environments," continues Dr. Gozdz. "In areas like Arizona, Texas, Florida, and Louisiana, lead/acid batteries used for backup power have very short life spans unless they are in environmentally controlled vaults." The need for backup power in optical systems at nodes on the network and at the customer location magnifies the need for a more reliable system, one that doesn't require a controlled environment to function. "The stimulus for this whole program was for us to come up with a better high-temperature operating battery for telecommunications backup," says Dr. Keramidas. To meet the challenge, a team of researchers, each with a particular scientific expertise, was assembled and let loose on the project. The team included a polymer chemist, a plastics-processing expert, a physical chemist, and an electrochemist, all led by a solid-state chemist, who reports to an applied physicist. The multidisciplinary approach yielded rapid results. In 1992 the team's first rechargeable lithium-ion battery was released for licensing. This system capitalizes on the high energy density, good recharge cycle life, and other features of lithium-ion electrochemistry, while providing a reliable operating range up to 60 C (140 F). In this technology, however, the electrolyte is still a free liquid, not entrapped in a polymer. Thus the battery had to be "packaged" to a rigid geometry like other batteries with liquid electrolytes, such as lead/acid, NiCAD, and nickel metal-hydride. The contribution of this initial technology compared with a competing liquid-electrolyte lithium-ion battery was in the electrode, where cobalt in the cathode was replaced with manganese for reasons of cost, availability, toxicity, and problems with battery deterioration on overcharge. The last hurdle for the team was to overcome the disadvantages of a liquid electrolyte and remove the constraints of the package. Then came the breakthroughs: the discovery of a readily available polymer that could carry both the electrode particles and the liquid electrolyte, and a process to entrap the liquid electrolyte in the polymer matrix. The result was a battery that could be cut, bent, and punched with holes, without leaking, yet retained all the benefits intrinsic to the liquid lithium-ion technology. "Lithium ion is considered the last frontier in battery technology," says Dr. Keramidas. "We've taken that technology and made it into a flexible solid. This in a nutshell has been our achievement." The only real limitation to the technology relates to the lithium ion itself, the species that shuttles back and forth allowing the battery to be rechargeable. In lead/acid, NiCAD, and nickel metal-hydride battery technologies, the ion moving in the battery is the hydrogen ion, a proton, the smallest positively charged ion that exists. The lithium ion is "six times heavier and two and a half times larger than the proton, a tennis ball compared to a ping-pong ball," says team member Caroline Schmutz. As such, the lithium ion is not as nimble, "so batteries based on lithium-ion technology don't have a high power rate, or capacity for high-speed discharge," says Dr. Schmutz. Thus they cannot generate the power surge required of some power tools, such as a power screwdriver, nor the quick pulse to start a car. The rest of the field, however, is wide open. To reap the benefits of its development, Bellcore has chosen to license its technology. Interestingly, even companies outside the battery business are investigating licenses. "Because of its ease of fabrication, it is the first battery to open the door to nonbattery manufacturers," says Dr. Keramidas. "Even traditional battery manufacturers have to change their mindset." Of potential licensees, he notes, "we have visionaries who say, 'This is what we've been waiting for.' We have others driven by specific applications, such as electric-vehicle or military applications, who see the opportunity and say, 'I'll make it myself.' Then there are the companies already doing sheet plastics. From enthusiast to cynic, everyone who has seen the batteries has left impressed. "My role is to project the achievement with proper technical appreciation so it will have maximum impact," continues Dr. Keramidas. "So far we have one licensee signed and by the end of the year we could have three more. There are about a dozen industrial entities in different stages of serious negotiations, and this has only been since March, when the technology was introduced."
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