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Injectable microbattery boasts twofold increase in power

24 Feb 2014  | Paul Buckley

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A team of scientists at the US Department of Energy's Pacific Northwest National Laboratory have developed what they describe is an injectable microbattery that packs twice the energy of current microbatteries used to monitor the movements of salmon through rivers around the world.

The battery, which could help transform biotelemetry monitoring, is a cylinder about the size of a long grain of rice that stores enough energy to power acoustic fish tags. The battery is small enough to be injected into an organism and holds much more energy than of a similar size.

For scientists tracking the movements of salmon, the lighter battery translates to a smaller transmitter which can be inserted into younger, smaller fish. That would allow scientists to track their welfare earlier in the life cycle. The battery is able to power signals across longer distances, allowing researchers to track fish further from shore or from dams, or deeper in the water.

Injectable microbattery

"This battery essentially revolutionizes the biotelemetry world and opens up the study of earlier life stages of salmon in ways that have not been possible before," said M. Brad Eppard, a fisheries biologist with the Portland District of the U.S. Army Corps of Engineers. "For years the chief limiting factor to creating a smaller transmitter has been the battery size. That hurdle has now been overcome."

The Corps and other agencies use the information from tags to chart the welfare of endangered fish and to help determine the optimal manner to operate dams. Three years ago the Corps turned to Z. Daniel Deng, a PNNL engineer, to create a smaller transmitter, one small enough to be injected, instead of surgically implanted, into fish. Injection is much less invasive and stressful for the fish, and it's a faster and less costly process.

"This was a major challenge which really consumed us these last three years," said Deng. "There's nothing like this available commercially, that can be injected. 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."

Deng called in materials science expert Jie Xiao to create the battery design.

To pack more energy into a small area, Xiao's team improved upon the 'jellyroll' technique commonly used to make larger household cylindrical batteries. Xiao's team laid down layers of the battery materials one on top of the other in a process known as lamination, then rolled them up together, similar to how a jellyroll is created. The layers include a separating material sandwiched by a cathode made of carbon fluoride and an anode made of lithium.

The technique allowed her team to increase the area of the electrodes without increasing their thickness or the overall size of the battery. The increased area addresses one of the chief problems when making such a small battery keeping the impedance, which is a lot like resistance, from getting too high. High impedance occurs when so many electrons are packed into a small place that they don't flow easily or quickly along the routes required in a battery, instead getting in each other's way. The smaller the battery, the bigger the problem.

Using the 'jellyroll' technique allowed Xiao's team to create a larger area for the electrons to interact, reducing impedance so much that the capacity of the material is about double that of traditional microbatteries used in acoustic fish tags.

"It's a bit like flattening wads of Play-Doh, one layer at a time, and then rolling them up together, like a jelly roll," said Xiao. "This allows you to pack more of your active materials into a small space without increasing the resistance."

The new battery is a little more than half the weight of batteries currently used in acoustic fish tags, just 70mg, compared to about 135mg, and measures 6mm long by 3mm wide. The battery has an energy density of about 240Wh/kg, compared to around 100 for commercially available silver oxide button microbatteries.

The battery packs enough energy to send out an acoustic signal strong enough to be useful for fish-tracking studies even in noisy environments such as near large dams. The battery can power a 744ms signal sent every three seconds for about three weeks, or about every five seconds for a month.

A bonus is that the batteries also work better in cold water where salmon often live, sending clearer signals at low temperatures compared to current batteries. The performance benefit derives from the active ingredients being lithium and carbon fluoride, a chemistry that is promising for other applications but has not been common for microbatteries.

A PNNL team led by Deng surgically implanted 700 of the tags into salmon in a field trial in the Snake River in the summer of 2013. Preliminary results show that the tags performed well. Battelle, which operates PNNL, has applied for a patent on the technology.

- Paul Buckley
  EE Times Europe




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