Instead of having the batteries' anode and cathode on either side of a nonconducting separator, the group intertwine the components in a self-assembling, 3D gyroidal structure, with thousands of nanoscale pores filled with the elements necessary for energy storage and delivery.
"This 3D architecture basically eliminates all losses from dead volume in your device," Prof. Wiesner says. "More importantly, shrinking the dimensions of these interpenetrated domains down to the nanoscale, as we did, gives you orders of magnitude higher power density. In other words, you can access the energy in much shorter times than what's usually done with conventional battery architectures."
According to Prof. Wiesner, the nanoscale dimensions mean that,"by the time you put your cable into the socket, in seconds, perhaps even faster, the battery would be charged."
The architecture for this concept is based on block copolymer self-assembly, which the Wiesner group has employed for years in other devices, including a gyroidal solar cell and a gyroidal superconductor.
Joerg Werner, lead author, has experimented with self-assembling photonic devices previously, and wondered if the same principles could be applied to carbon materials for energy storage.
The gyroidal thin films of carbon - the battery's anode, generated by block copolymer self-assembly - featured thousands of periodic pores on the order of 40nm wide. These pores were then coated with a 10nm-thick, electronically insulating but ion-conducting separator through electropolymerization, which by the very nature of the process produced a pinhole-free separation layer.
According to the team, this is vital – defects like holes in the separator can lead to failure, giving rise to fires in devices such as cellphones and laptops.
The next step is the addition of the cathode material - in this case, sulfur - in an amount that doesn't quite fill the remainder of the pores. Since sulfur can accept electrons but doesn't conduct electricity, the final step is backfilling with an electronically conducting polymer - known as PEDOT (poly[3,4-ethylenedioxythiophene]).
While this architecture offers proof of concept, Prof. Wiesner says, it's not without challenges. Volume changes during discharging and charging the battery, gradually degrade the PEDOT charge collector, which doesn't experience the volume expansion that sulfur does.
"When the sulfur expands," Prof. Wiesner says, "you have these little bits of polymer that get ripped apart, and then it doesn't reconnect when it shrinks again. This means there are pieces of the 3D battery that you then cannot access."
The group says it is still perfecting the technique, but has applied for patent protection on the proof-of-concept work.
