20th June 2013

Printing tiny batteries

Novel application of 3D printing could enable the development of miniaturised medical implants, compact electronics, tiny robots and more.

3D printing can now be used to print lithium-ion microbatteries the size of a grain of sand. The printed microbatteries could supply power to tiny devices in fields from medicine to communications, including many that have lingered on lab benches for lack of a battery small enough to fit the device, yet provide enough stored energy to power them.

To make the microbatteries, a team based at Harvard University and the University of Illinois at Urbana-Champaign printed precisely interlaced stacks of tiny battery electrodes – each less than the width of a human hair.

"Not only did we demonstrate for the first time that we can 3D-print a battery; we demonstrated it in the most rigorous way," said Professor Jennifer Lewis, senior author of the study.

In recent years, engineers have invented many miniaturised devices, including medical implants, flying insect-like robots, and tiny cameras and microphones that fit on a pair of glasses. But often, the batteries that power them are as large as or larger than the devices themselves, which defeats the purpose of building small.

To get around this problem, manufacturers have traditionally deposited thin films of solid materials to build the electrodes. However, due to their ultra-thin design, these solid-state micro-batteries do not pack sufficient energy to power tomorrow's miniaturised devices.

The scientists realised they could pack more energy by creating stacks of tightly interlaced, ultrathin electrodes that were built out of plane. For this, they turned to 3D printing. 3D printers follow instructions from 3D computer drawings, depositing successive layers of material – inks – to build a physical object from the ground up, much like stacking a deck of cards one at a time. This technique is used in a range of fields, from producing crowns in dental labs to rapid prototyping of aerospace, automotive, and consumer goods.

Lewis’ group has greatly expanded the capabilities of 3D printing. They have designed a broad range of functional inks – each with useful chemical and electrical properties. And they have used those inks with their custom-built 3D printers to create precise structures with the electronic, optical, mechanical, or biologically relevant properties they want.

To print 3D electrodes, Lewis' group first created and tested several specialised inks. Unlike the ink found in a typical office printer, which comes out as droplets of liquid that wet the page, the inks developed for extrusion-based 3D printing must fulfil two difficult requirements. They must exit fine nozzles like toothpaste from a tube, and they must immediately harden into their final form.

In this case, the inks also had to function as electrochemically active materials to create working anodes and cathodes, and they had to harden into layers as narrow as those produced by thin-film manufacturing. To accomplish these goals, the researchers created an ink for the anode with nanoparticles of one lithium metal oxide compound, and an ink for the cathode from nanoparticles of another. The printer deposited the inks onto the teeth of two gold combs, creating a tightly interlaced stack of anodes and cathodes. Then the researchers packaged the electrodes into a tiny container and filled it with an electrolyte solution to complete the battery.

Next, they measured how much energy could be packed into the tiny batteries, how much power they could deliver and how long they held a charge. “The electrochemical performance is comparable to commercial batteries in terms of charge and discharge rate, cycle life and energy densities – just on a much smaller scale,” said co-author Shen Dillon, Assistant Professor of Materials Science and Engineering.

“Jennifer’s innovative microbattery ink designs dramatically expand the practical uses of 3D printing, and simultaneously open up entirely new possibilities for miniaturisation of devices, both medical and non-medical. It’s tremendously exciting,” said Wyss Founding Director Donald Ingber.

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