By measuring nanoscale forces, researchers learn to make lithium-ion batteries that pull themselves together.
Researchers at MIT have designed a rechargeable lithium-ion battery that assembles itself out of microscopic materials. This could lead to ultrasmall power sources for sensors and micromachines the size of the head of a pin. It could also make it possible to pack battery materials in unused space inside electronic devices.
Yet-Ming Chiang, a professor of materials science at MIT, and his colleagues selected electrode and electrolyte materials that, when combined, organize themselves into the structure of a working battery. The researchers had been looking for ways to exploit short-range forces between micro- and nanoscale particles. After measuring such forces between materials using ultraprecise atomic-force microscope probes, they were able to select materials with just the right combination of attractive and repulsive forces. As a result, similar materials clustered together to form opposite electrodes, while a gap necessary for the battery to function was maintained between the electrodes. The work is the cover story in the current issue of Advanced Functional Materials.
Self-assembly is attractive because it could potentially reduce manufacturing costs and allow molecular-level control of the structure of the batteries, leading to materials and devices not easy to make using conventional manufacturing methods. Self-assembly has already been used to create a number of materials and a handful of simple devices, including half a battery. (See “Powerful Batteries That Assemble Themselves.”) “Ultimately, the goal is just to chuck a bunch of stuff into a bucket and have it self-assemble into a battery,” says Jeff Dahn, professor of chemistry and physics at Dalhousie University, in Canada. Chiang’s work creating a prototype self-assembling battery is “really nice science,” Dahn says. “Just the fact that you can do it is pretty cool.”
The researchers faced a number of challenges in designing the self-assembling batteries. They are limited to materials with the electrochemical properties necessary for battery electrodes. And within each electrode, the particles need to pack together tightly, which can be accomplished if they are attracted to each other. The particles must also be attracted to materials that conduct electrons to and from the electrodes. Most important, the battery’s two electrodes need to be kept separate–a challenge because they are oppositely charged and therefore tend to attract each other.
By relying on their new understanding of short-range forces, Chiang and his colleagues were able to select two electrode materials that, at very short distances on the order of a couple dozen nanometers, had surface repulsive forces greater than their attractive forces. As a result, there is always a space left between the electrodes.
The researchers used lithium cobalt oxide and microbeads of graphite for the electrodes–materials commonly used in lithium-ion batteries–pairing them with a carefully selected liquid electrolyte. The electrolyte serves as an insulator, allowing ions to shuttle between the electrodes but forcing electrons to move through an external circuit, where they can be used to power a device.
In the researchers’ prototype battery, the graphite microbeads pack together to form one electrode and connect to a platinum current collector, all the while staying clear of the lithium cobalt oxide that forms the other electrode. The researchers tested the battery and showed that it could be both discharged and recharged multiple times.
The extent to which such batteries will find commercial applications is unclear. Dahn points out that in manufacturing today’s batteries, the electrode materials are compressed under enormous pressures to ensure as great as possible energy storage. Such forces could not be applied to a self-assembled battery, so Dahn says it will be “very tough” to compete with conventional batteries in terms of energy capacity and maybe even in terms of cost. Dahn also notes that challenges still remain before such batteries can be commercialized. For example, it is still necessary to find a way to package the self-assembled materials to protect them once they have formed a battery.
One potential application is in very small devices. “It should be relatively easy to make a very small footprint device, rice-grain-size and smaller–the size of the head of a pin,” Chiang says. He adds that self-assembly could allow more-efficient use of space than conventional batteries can. That’s in part because it’s possible for the electrode particles to pack into irregular shapes within a device or follow its outside contours.
As the researchers move toward such applications, which could include use in distributed sensors for the military, their next step is to replace the liquid electrolyte with a solid polymer to make the battery more rugged. The better understanding of the relevant short-range forces could also be used to select different materials for applications in transistors or certain types of solar cells.
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