Lithium-ion micro-battery demos potential new path to high performance

  • 23-Apr-2013 04:56 EDT

Lithium ions cross the gap between the 3-D anode and cathode of a novel-design micro-battery.

A lithium-ion battery can have good energy density, which could, for example, give an EV extended range, but it can’t also provide high power density for, say, acceleration. The hard realities of electrochemistry mean that attempts to improve power capability tend to degrade energy performance. And worse, this classic trade-off typically extends as well to another key battery metric: recharging speed. To date, building a practical package that optimizes all three parameters has been problematic.

Recent research developments may indicate that the traditional obstacles to high-performance may be starting to fall to the way side. The innovation is a novel, three-dimensional battery design that simultaneously expands the cell’s reactive surface area while boosting its ion and electron transport capacity.

The new approach, which has been demonstrated at millimeter-scale in the lab by a team of researchers at the University of Illinois, Urbana, has shown high-performance capabilities. The device has a volume of only 0.03 m3 (1831 in3). Talks are under way to commercialize the micro-battery to power microelectronics systems.

The Illinois team, said Bill King, the Bliss Professor of mechanical science and engineering, has produced a miniature lithium-ion power source that can be placed on a silicon integrated chip to power microelectronic systems. The micro-batteries offer power densities that greatly exceed those of other micro-batteries and even compete with the best small supercapacitors, he claims.

With so much power, the small, highly optimized Li-ion batteries could enable sensors or radio signals that broadcast 30 times farther or devices 30 times smaller. The batteries are rechargeable and can charge 1000 times faster than competing technologies—imagine juicing up a credit-card-thin phone in less than a second. Applications in consumer electronics, medical devices, lasers, sensors, and other areas are envisioned.

New way for batteries

“This is a whole new way to think about batteries. It shows that a battery can deliver far more power than anybody ever thought,” said King. His efforts focused on the miniature battery market because “we knew we could deliver superior performance in a small package. We hope it’s the first step toward greater things.”

There has been a lot of work on improving materials for battery electrodes during the past few years, he said, but mostly using chemistry-based approaches, with fewer groups targeting the mechanical aspects of electrode function—mechanisms that could assist the basic electrochemical processes to proceed. Further, few of those who work on next-generation electrodes generally complete the “hard steps required to bring it to being a real battery,” he said.

On the one hand is the need to improve the ability of ions to flow easily, he said; on the other is ensuring that the ions and electrons can get in and out of the electrode easily. “A balance is needed,” said King, noting that “some people have tried various porous corrugations to expand surface area, while others made a sort of thin-film sandwich where a membrane is rolled up.” But most of these ended up limiting the rate at which ions could move through the electrode.

Building on their colleague Paul Braun’s earlier work, the scientists’ advance involves finding a new way to integrate the anode and cathode at the microscale. “The secret is that our electrodes are tiny little fingers that interconnect three-dimensionally to form a lung-like charge transfer mechanism,” King explained. “The battery electrodes have small interdigitated, or intertwined, fingers that reach into each other.”

High area, high mobility

“That does a couple of things,” said King. “It allows us to make the battery have a very high surface area even though the overall battery volume is extremely small. And it gets the two halves of the battery very close together so the ions and electrons do not have far to flow.”

The battery is built on layers of electrodeposited nickel that serve as the structure scaffold and current collector for other active materials that are electrodeposited onto it. This fabrication method ensures precision integration of the electrodes on a single substrate. Electrodeposited nickel-tin alloy is used for the anode, whereas lithiated manganese oxide forms the cathode. The metal deposition falls on tiny, packed-down polystyrene pellets that serve as microscopic forming mandrels for creating interconnected foam-lattice structures. The spheres are etched away to leave a 3-D metal scaffold.

Li-ions and electrolyte solution were next added, resulting in a series of alternating metal foam stripes, each about 30 mm (1.2 in) wide.

Technical challenges

Right now, the lab battery can’t hold a charge after withstanding many charging and discharging cycles, which may be characteristic of the design but which may just be a teething problem. Any product must also be robust.

King said that his group has been making small numbers of these devices, but considerable work will be required to scale up the process to larger areas. Scale-up could be difficult because the fabrication process needs to be extremely efficient (high yield) and cheap to compete on the market.

In addition, they must be safe. The tiny Li-ion batteries still confront the same safety issues as conventional Li-ion batteries with liquid electrolytes, although there is little risk in the first chip-scale applications. But larger, similar devices could face the familiar thermal runaway problems. King said the team plans to address the issue by switching to a safer polymer-based electrolyte soon. He hopes to have the technology ready to be tested as a power source for electronic equipment before the end of 2013.

A similar 3-D battery design is being developed at Prieto Battery of Fort Collins, Co. Amy Prieto, an assistant chemistry professor at Colorado State University, hopes to exploit her expertise in electrodeposition to make a 3-D battery in which the electrodes and electrolyte are brought together on an intimate scale. Her design posits a porous foam electrode conformally coated by an ultrathin polymer electrolyte and then surrounded by a cathode matrix. There has been no word of late from Prieto on progress toward a marketable product as yet.

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