Safer, Longer-Lasting Batteries

Thin-film technology is still expensive, but it could soon run remote sensors and medical implants–and one day electric vehicles.

While recent massive recalls have highlighted safety concerns of lithium-ion batteries, today’s battery technologies actually have a number of weaknesses. If damaged, overcharged, or overheated, batteries can explode (see “Safer Lithium-Ion Batteries”). But they also leak energy and lose power and longevity if used in extreme temperatures, say, on a winter day in Iowa or a heat wave in Arizona.

Long-lasting thin-film batteries could power medical implants and remote sensors for decades, and eventually be used in electric vehicles.

A new type of rechargeable battery will soon be available commercially that overcomes these problems. But at a cost.

These new batteries replace the liquid or gel electrolyte with thin layers of solid glass-like or polymer materials, which are more stable. “Nothing can leak, nothing can freeze, nothing can boil, rupture, or explode,” says Tim Bradow, vice president of business development at Infinite Power Solutions of Golden, CO, a leading developer of thin-film batteries.

In a battery, the electrolyte allows positive ions to move from one electrode to the other, while forcing electrons to travel through an external circuit, providing power. Bradow’s company and a handful of others are using a solid glassy electrolyte, which they deposit as one of a series of flat layers that make up the battery.

In addition to being safer, this solid material allows developers to use electrodes of pure lithium metal, which has the potential to significantly increase storage capacity. The batteries can survive extremes of cold and heat, which means, for example, they could be built into rubber tires to power air pressure sensors, says John Bates, chief technical officer at Oak Ridge Micro-Energy in Tennessee.

Thin-film cells also can be stored for decades and retain almost all their charge, developers say–and deliver a powerful burst of energy when finally needed. And, in many applications, they can be actively used for decades, since they can be charged and discharged tens of thousands of times.

These characteristics make thin-film batteries ideal for some new technologies. Remote sensors that scavenge tiny amounts of energy from vibrations, radio transmissions, or light, require batteries that can store this micro-supply of energy without leaking it away over time. And remote sensors need the high-power bursts many of these cells can deliver, to send data via radio signals to a central station.

The ability to power radio transmission is also important for future medical implants that will deliver drugs or measure glucose levels. And these applications will also benefit from the batteries’ long lifetimes; they can be recharged and discharged over many years, eliminating the need for surgery to replace them. “It’s the perfect kind of battery for powering any RF device, because it’s pulse power–instant-on and then it goes into sleep mode,” says Bradow. “That’s what our battery loves and other batteries hate.” His company plans to start mass-producing its batteries next year.

Nonetheless, thin-film batteries may not be the next-generation choice for most laptops. That’s because the processes used to make them, such as physical vapor deposition, are still too expensive for producing large batteries. Also, these batteries, which can be a mere one-tenth of a millimeter thick, each hold only micro-amounts of energy–as little as one-thousandth the amount in today’s laptop batteries. While they could be stacked to provide adequate storage capacity, the layers of packaging separating the active materials in each battery would cancel out their capacity advantages. That is, they’d likely cost more, but not necessarily be smaller.

The first applications, such as in industrial sensor packages in high-temperature equipment or oil wells, will be ones in which buyers are willing to pay $100 apiece for batteries that meet their needs. Bradow says their batteries could be made for much less in high volumes, however, eventually making them practical for distributed sensor networks.

In spite of the current drawbacks to thin-film batteries, Donald Sadoway, professor of materials chemistry at MIT, says some versions of them will power laptops–and electric vehicles–in the future. To his thinking, their key advantage, in addition to safety, is that they allow the use of pure lithium in one of the electrodes, which isn’t possible using liquid electrolytes: “If you can switch to lithium, you’ve achieved the ultimate in anode capacity,” he says.

In contrast to the glass-like electrolyte used by Infinite Power Solutions and others, Sadoway has developed a solid-polymer electrolyte (today’s lithium-ion polymer batteries use a gel) for use in thin-film batteries. This electrolyte, he says, could be processed in rolls like newspaper, or some other high-throughput process. Such a process for thin-film batteries, although not now being developed by industry, could bring down costs, he says, while innovative ways of packaging electrodes could reduce size. “We’ve made batteries in the laboratory that are 300 watt-hours per kilogram,” he says. “That’s two times the best lithium-ion [battery] on the market today.”

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