In order to better understand and protect wild stocks of salmon, it's necessary to track their whereabouts using implanted acoustic tags. Needless to say, the longer that those tags are able to transmit a signal, the greater the amount of data that can be gathered. Scientists at Washington state's Pacific Northwest National Laboratory (PNNL) are helping make that happen, by developing batteries that have both a smaller size and higher energy density than conventional fish tag batteries.
Currently, acoustic tags are large enough that they can't be used on very young fish, and they must be surgically implanted. This not only stresses out the animals, but it's also time-consuming for the fisheries workers who are doing the tagging. Because of this, the US Army Corps of Engineers (which designs dams based around salmon migrations) tasked PNNL with creating a transmitter small enough that it could be used to make injectable tags. A smaller tag, of course, also needs a smaller battery.
To make those batteries, the PNNL team turned to the "jellyroll" technique, commonly used in the manufacture of larger household batteries. This involved first laminating together multiple flat layers of material, which included "a separating material sandwiched by a cathode made of carbon fluoride and an anode made of lithium."
Those layers were then rolled up like a jellyroll, and stuffed into a cylindrical housing. The technique maximizes the surface area of the battery's electrodes, without increasing its overall size. At the same time, it allows electrons to travel freely within the battery without impeding each others' movements – a performance-decreasing problem which is often encountered in other micro-batteries.
At 70 milligrams, the new batteries weigh a little over half as much as those currently used in acoustic tags, while offering over twice the energy density – 240 watt hours per kilogram, as opposed to around 100. This allows them to power a 744-microsecond signal sent every three seconds for around three weeks, or approximately every five seconds for a month.
They're also minuscule, measuring 6 x 3 mm, or about the size of a grain of rice.
So far, 700 tags incorporating the hand-made batteries have been implanted (not injected) into salmon in a field trial at Washington's Snake River. Not only have they let scientists start tracking fish earlier in their life cycle, but they've also allowed fish to be tracked over greater distances and they perform better in cold water, thanks to the use of the lithium and carbon fluoride.
The research was led by materials scientist Jie Xiao, and was recently described in a paper published in the journal Scientific Reports.