3D-printed Li-Ion microbatteries

3D printing can now be used to print lithium-ion microbatteries the size of a grain of sand. These batteries could supply electricity 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 with enough energy stored 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.

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 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 ultrathin design, these solid-state microbatteries do not pack sufficient energy to power tomorrow’s miniaturised devices. The scientists realised they could pack more energy if they could create stacks of tightly interlaced, ultrathin electrodes that were built out of plane.

For this they turned to 3D printing. 3D printers follow instructions from three-dimensional 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. The technique is used in a range of fields, from producing crowns in dental labs to rapid prototyping of aerospace, automotive and consumer goods.

To print 3D electrodes, the researchers first created and tested several specialised inks. Unlike the ink in an office inkjet 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 that are as narrow as those produced by thin-film manufacturing methods. 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.

Dr Jennifer Lewis, the Hansjörg Wyss Professor of Biologically Inspired Engineering at the Harvard School of Engineering and Applied Sciences (SEAS) and a Core Faculty Member of the Wyss Institute for Biologically Inspired Engineering at Harvard University, is the senior author of the study. Dr Lewis led the project in her prior position at the University of Illinois at Urbana-Champaign in collaboration with co-author Shen Dillon, an Assistant Professor of Materials Science and Engineering.