Power/Performance Bits: Dec. 29

Safer Li-ion batteries
Scientists from Stanford University and the Department of Energy’s SLAC National Accelerator Laboratory propose a way to make lithium-ion batteries lighter, more efficient, and fire resistant.

One of the heaviest components of lithium-ion batteries are the copper or aluminum sheets that act as current collectors.

“The current collector has always been considered dead weight, and until now it hasn’t been successfully exploited to increase battery performance,” said Yi Cui, a professor at SLAC and Stanford and investigator with the Stanford Institute for Materials and Energy Sciences (SIMES).

“But in our study, making the collector 80% lighter increased the energy density of lithium-ion batteries — how much energy they can store in a given weight — by 16-26%. That’s a big jump compared with the average 3% increase achieved in recent years.”

Lithium-ion batteries have two current collectors, one for each electrode. They distribute current flowing in or out of the electrode, and account for 15% to as much as 50% of the weight of some high-power or ultrathin batteries.

The team used a lightweight polymer called polyimide, which resists fire and stands up to the high temperatures created by fast battery charging. The fire retardant triphenyl phosphate (TPP) was embedded in the polymer, which was then coated on both surfaces with an ultrathin layer of copper. As well as distributing current, the copper would also protect the polymer and its fire retardant.

When exposed to open flame, lithium-ion pouch batteries made with today’s commercial current collectors (top row) caught fire and burned vigorously until all the electrolyte burned away. Batteries with the new flame-retardant collectors (bottom row) produced weak flames that went out within a few seconds, and did not flare up again even when the scientists tried to relight them. (Credit: Yusheng Ye/Stanford University)

When exposed to an open flame from a lighter, pouch batteries made with today’s commercial current collectors caught fire and burned vigorously until all the electrolyte burned away, said Yusheng Ye, a postdoctoral researcher at Stanford. But in batteries with the new flame-retardant collectors, the fire never really got going, producing very weak flames that went out within a few seconds, and did not flare up again even when the researchers tried to relight it.

Cui said that the new collector should be easy to manufacture with a lower cost, as the polymer is cheaper than the copper. The researchers have applied for a patent and plan to engage with battery manufacturers.

Sensors dropped by moths
Researchers at the University of Washington developed a sensor system small and light enough to be carried by a moth. The 98 milligram sensor system is small enough to ride along with a small drone or insect until it reaches a target destination. It can then drop fall up to 72 feet, or from about the sixth floor of a building, and land without breaking. Once on the ground, the sensor can collect data, such as temperature or humidity, for almost three years.

“We have seen examples of how the military drops food and essential supplies from helicopters in disaster zones. We were inspired by this and asked the question: Can we use a similar method to map out conditions in regions that are too small or too dangerous for a person to go to?” said senior author Shyam Gollakota, a UW associate professor in the Paul G. Allen School of Computer Science & Engineering. “This is the first time anyone has shown that sensors can be released from tiny drones or insects such as moths, which can traverse through narrow spaces better than any drone and sustain much longer flights.”

The sensor is held on the drone or insect using a magnetic pin surrounded by a thin coil of wire. To release the sensor, a researcher on the ground sends a wireless command that creates a current through the coil to generate a magnetic field. The magnetic field makes the magnetic pin pop out of place, releasing the sensor package.

The sensor was designed with its battery, the heaviest part, in one corner. As the sensor falls, it begins rotating around the corner with the battery, generating additional drag force and slowing its descent. That, combined with the sensor’s low weight, keeps its maximum fall speed at around 11 miles per hour, allowing the sensor to hit the ground safely.

The researchers see potential for using this system to create a sensor network over an area of interest. They are working on ways to recover the sensor packages ones the batteries have died and are also looking to replacing the battery with a solar cell and automating sensor deployment in industrial settings.