Power/Performance Bits: Aug. 16

Dissolving batteries

Researchers at Iowa State University developed a self-destructing lithium-ion battery capable of delivering 2.5 volts and dissolving or dissipating in 30 minutes when dropped in water. The battery can power a desktop calculator for about 15 minutes.

Making such devices possible is the goal of a relatively new field of study called “transient electronics.” These transient devices could perform a variety of functions – until exposure to light, heat or liquid triggers their destruction.

According to Reza Montazami, an ISU assistant professor of mechanical engineering, said it’s the first transient battery to demonstrate the power, stability and shelf life for practical use.

“Unlike conventional electronics that are designed to last for extensive periods of time, a key and unique attribute of transient electronics is to operate over a typically short and well-defined period, and undergo fast and, ideally, complete self-deconstruction and vanish when transiency is triggered,” the scientists wrote in their paper.

Illustration of the dissolving battery. (Source: Ashley Christopherson)

“Any device without a transient power source isn’t really transient,” Montazami said. “This is a battery with all the working components. It’s much more complex than our previous work with transient electronics.”

The transient battery is made up of eight layers, including an anode, a cathode and the electrolyte separator, all wrapped up in two layers of a polyvinyl alcohol-based polymer.

The battery itself is tiny – about 1 millimeter thick, 5 millimeters long and 6 millimeters wide. Montazami said the battery components, structure and electrochemical reactions are all very close to commercially developed battery technology.

But, when you drop it in water, the polymer casing swells, breaks apart the electrodes and dissolves away. Montazami is quick to say the battery doesn’t completely disappear. The battery contains nanoparticles that don’t degrade, but they do disperse as the battery’s casing breaks the electrodes apart.

And what about applications that require a longer-lasting charge? Larger batteries with higher capacities could provide more power, but they also take longer to self-destruct. The team suggested applications requiring higher power levels could be connected to several smaller batteries.

Vitamin B for batteries

A team of University of Toronto chemists created a battery that uses flavin from vitamin B2 as the cathode.

While bio-derived battery parts have been created previously, this is the first one that uses bio-derived polymers – long-chain molecules – for one of the electrodes, essentially allowing battery energy to be stored in a vitamin-created plastic, instead of costlier, harder to process, and more environmentally-harmful metals such as cobalt.

The team created the material from vitamin B2 that originates in genetically-modified fungi using a semi-synthetic process to prepare the polymer by linking two flavin units to a long-chain molecule backbone.

This allows for a green battery with high capacity and high voltage, something increasingly important to the Internet of Things.

B2’s ability to be reduced and oxidized makes its well-suited for a lithium ion battery. “B2 can accept up to two electrons at a time,” said Dwight Seferos, an associate professor in U of T’s Department of Chemistry. “This makes it easy to take multiple charges and have a high capacity compared to a lot of other available molecules.”

While the current prototype is on the scale of a hearing aid battery, the team hopes the work could lay the groundwork for powerful, thin, flexible, and even transparent metal-free batteries.

Sideways solar

University of Wisconsin-Madison engineers created high-performance, micro-scale solar cells that could power myriad personal devices.

Large, rooftop photovoltaic arrays generate electricity from charges moving vertically. The new, small cells capture current from charges moving side-to-side, or laterally. And they generate significantly more energy than other sideways solar systems.

New-generation lateral solar cells show promise for compact devices because arranging electrodes horizontally allows engineers to sidestep a traditional solar cell fabrication process: the task of perfectly aligning multiple layers of the cell’s material atop one another.

“From a fabrication point of view, it is always going to be easier to make side-by-side structures,” says Hongrui Jiang, a UW-Madison professor of electrical and computer engineering. “Top-down structures need to be made in multiple steps and then aligned, which is very challenging at small scales.” Lateral solar cells also offer greater flexibility in materials selection.

Top-down photovoltaic cells are made up of two electrodes surrounding a semiconducting material. When light hits the top electrode, charge travels through the center to the bottom layer and creates electric current. In the top-down arrangement, one layer needs to do two jobs: It must let in light and transmit charge. Therefore, the material for one electrode in a typical solar cell must be not only highly transparent, but also electrically conductive. Very few substances perform both tasks well.

Instead, the group created a densely packed, side-by-side array of miniature electrodes on top of transparent glass. The resulting structure separates light-harvesting and charge-conducting functions between the two components.

Existing top-of the-line lateral solar cells convert 1.8% of incoming light into useful electricity. The team was able to improve on that, achieving up to 5.2% efficiency.

“In other structures, a lot of volume goes wasted because there are no electrodes or the electrodes are mismatched,” said Jiang. “The technology we developed allows us to make very compact lateral structures that take advantage of the full volume.”

The team is working to make their solar cells even smaller and more efficient by exploring materials that further optimize transparency and conductivity. Ultimately they plan to develop a small-scale, flexible solar cell that could provide power to an electrically tunable contact lens.