There's no denying that building the world’s smallest battery is a notable achievement. But while they may lay the groundwork for future battery technologies, today such microbatteries are mostly laboratory curiosities.
Developing a battery that's no bigger than a grain of rice—and that's actually useful in the real world—is quite another kind of achievement. Researchers at Pacific Northwest National Laboratory (PNNL) have done just that, creating a battery based on graphene that has successfully been used in monitoring the movements of salmon through rivers.
The microbattery is being heralded as a breakthrough in biotelemetry and should give researchers never before insights into the movements and the early stages of life of the fish.
The battery is partly made from a fluorinated graphene that was described last year by the PNNL researchers in the Journal of Materials Chemistry A (“Tunable electrochemical properties of fluorinated graphene”). The development of the microbattery itself was recently described in Scientific Reports (“Micro-battery Development for Juvenile Salmon Acoustic Telemetry System Applications”).
“There's nothing like this available commercially, that can be injected,” said Z. Daniel Deng, a PNNL engineer in the press release. “Either the batteries are too big, or they don't last long enough to be useful. That's why we had to design our own." Developing a battery that could balance both size and capacity took Deng and his team three-years.
Deng's PNNL colleague Jie Xiao was responsible for the novel design of the graphene-based battery. The battery has what's called a “jelly-roll” structure, in which layer after layer are laid on top of another and then rolled up into a cylindrical shape, like a jelly roll. The three layers are a separating material sandwiched on either side by a fluorinated graphene cathode and a lithium-based anode.
This design enabled a significant increase in the surface area of the electrodes without increasing the overall size of the battery. Measurements indicate that the capacity of the material to store a charge is about double that of traditional microbatteries previously used in acoustic fish tags. The researchers claim that the battery can power a 744-microsecond signal sent every three seconds for about three weeks, or about every five seconds for a month. With these numbers, the researchers believe it’s the smallest battery with enough energy capacity to maintain that level of signaling.
While the microbattery is unparalleled for these research purposes, it would seem that the manufacturing process isn't exactly scalable. The PNNL researchers had to hand-craft each microbattery: they cut small pieces of the constituent materials, flattened them, laid the sheets on top of each other and then hand rolled them into a cylinder.
For the time being it would seem that microbatteries will need to continue their upstream swim, along with the salmon that they track.