Battery design for powering tiny devices

Though many people have never heard of them, the emerging realm of micro-scale devices - called microelectromechanical systems, or MEMS - could change the medical, automotive and aerospace industries, except for one thing.

No battery yet exists that will provide long-lasting power and still fit inside devices smaller than the width of a human hair.

Bruce Dunn, a materials science professor from the UCLA Henry Samueli School of Engineering and Applied Science, believes a radical new design for a lightweight, rechargeable battery - a design based on three dimensional geometry - will provide power to a host of devices so small that traditional batteries simply cannot be used.

"Our team of engineers and chemists is establishing the enabling science for a new battery that represents a real paradigm shift," Dunn said.

Much larger consumer electronic devices such as laptop computers and mobile phones currently use traditional, two-dimensional batteries, each with positive and negative electrodes stacked on one another like sheets of paper.

To give the battery more power, more layers of electrodes are added, making the battery bigger and heavier.

While this may work for laptops, says Dunn, when one attempts to shrink these types of batteries down to the size required to power a MEMS device, it lacks the energy to do the job.

The UCLA-led team proposes changing from two-dimensional sheets of electrodes to rods arranged in a three-dimensional array in which hundreds of rods are stacked next to each other like tubes on a flat-bed truck.

Each rod is only a thousandth of a centimetre in size.

This design keeps the battery compact and the distance the ions have to travel short, which is important.

"A more efficient path for the movement of ions means less power loss and a longer-lasting battery," Dunn said.

The group is designing a battery roughly five millimetres in size, which presents significant design challenges.

"We're going to use fairly well-known lithium battery terminals," Dunn said. "The hard part is fabricating it into a structure. That's where the real engineering emphasis will be."

UCLA professor CJ Kim, from the department of mechanical and aerospace engineering, is an expert in micromachining techniques.

He and his students are creating silicon chips to be used as moulds.

The electrode materials are placed in the moulds, left to harden, and finally the silicon mould is etched away, leaving behind the three-dimensional battery electrode structure.

Batteries this small are necessary to run MEMS devices used in the medical, automotive and aerospace industries.

For example, doctors could use implantable devices that deliver drugs or protect transplanted cells.

Other devices could be used to automate blood, tissue and cellular analysis at much lower costs than conventional techniques.

Though the research team is focused on finding a way to provide power to MEMS devices, there could be far-reaching implications for other more common electronic products.

The need for a lightweight battery that will not sacrifice energy for small size is only going to grow as mobile phones and video cameras shrink in size.

Already, up to 35% of a laptop's total weight comes from its battery, and electronic product manufacturers are busily searching for more lightweight power alternatives.

Dunn believes it will be a while before 3D battery designs make it into the consumer market.

"The portable power market is so vast that if we are very successful, I am sure our concepts and designs will be used to try to make 3D power supplies. But probably not for at least another five years."

The group is in the first year of a five-year collaborative effort funded by a $4 million grant from the Office of Naval Research.

It includes researchers from the University of Florida, the University of Utah and other UCLA faculty members, including chemistry professors Fred Wudl and Sarah Tolbert, who are leading the effort toward 3D processing of the electrode materials in the battery.

Dunn says there is a number of unknowns associated with 3D battery design.

"The field is wide open," he said. "It's exciting. We have the opportunity to take electrochemical materials and designs in a new direction."