Power/Performance Bits: Oct. 22

Flexible battery; harvesting microwaves; metamaterials for photonics.


Flexible battery
Researchers at ETH Zurich developed a flexible thin-film battery that can be bent, stretched, and twisted without interrupting the supply of power.

Key to the battery is a new electrolyte and entirely flexible components. “To date, no one has employed exclusively flexible components as systematically as we have in creating a lithium-ion battery,” said Markus Niederberger, Professor for Multifunctional Materials at ETH Zurich.

The current collectors for the anode and cathode are made of a bendable polymer composite containing electrically conductive carbon, which also serves as the outer shell. On the interior surface of the composite, the researchers applied a thin layer of micron-sized silver flakes. The silver flakes overlap like roof tiles, so they don’t lose contact when the elastomer is stretched. This guarantees the conductivity of the current collector even if it is subjected to extensive stretching. Even if the silver flakes do lose contact with each other, the electrical current can still flow through the carbon-containing composite, albeit more weakly.

The lithium manganese oxide cathode and vanadium oxide anode were sprayed onto defined areas of the silver layer. The two current collectors with electrodes were then stacked with a barrier layer containing the electrolyte gel in the middle.

The electrolyte gel is less toxic and flammable than standard commercial electrolytes, according to the team. It contains water with a high concentration of a lithium salt, which not only facilitates the flow of lithium ions between cathode and anode while the battery is charging or discharging, but also keeps the water from electrochemical decomposition. It also means that if the battery leaks, the liquid won’t cause damage.

Currently, the battery is joined together with adhesive. “If we want to market the battery commercially, we’ll have to find another process that will keep it sealed tight for a longer period of time,” said Niederberger.

He notes that more research is necessary to optimize the flexible battery before they consider commercializing it, including increasing the amount of electrode material it can hold.

Harvesting microwaves
Researchers from the Japan Science and Technology Agency (JST), Fujitsu, and Tokyo Metropolitan University developed a highly sensitive rectifying element in the form of a nanowire backward diode capable of converting low-power microwaves into electricity.

Energy can be harvested from low-power radio waves in the ambient environment, such as those emitted by mobile phone base stations. Such a system could use an antenna to collect radio waves and a rectifying element to power battery-free sensors.

However, common rectifying elements aren’t sensitive enough to convert low-power microwaves weaker than microwatts into electricity. To create a diode with higher sensitivity, the team shrunk the capacity of and miniaturized a backward diode that is capable of steep rectification operations with zero bias.

The conventional process of fabricating backwards diodes and fragility of materials makes it difficult to process and operate them at a submicron size. Instead, the team was able to grow nanocrystals with a diameter of 150nm comprised of n-type indium arsenide (n-InAs) and p-type gallium arsenide antimonide (p-GaAsSb) for a tunnel junction structure necessary for the characteristics of the backward diode. The resultant nanowire backward diode showed over 10 times the sensitivity of conventional Schottky barrier diodes.

In testing the nanowire backward diode in the microwave frequency of 2.4GHz, which is currently used in 4G LTE and Wi-Fi, the sensitivity was 700kV/W, roughly 11 times that of the conventional Schottky barrier diode (with a sensitivity of 60KV/W). The technology can efficiently convert 100nW-class low-power radio waves into electricity, enabling the conversion of microwaves emitted into the environment from mobile phone base stations in an area that is over 10 times greater than was previously possible. This corresponds to 10% of the area in which mobile phone communications are possible, the team said.

The researchers expect that the system can be used as a source of power for sensors, and plan to optimize the design of the diode and the radio wave-collecting antenna while adding power control for constant voltage.

Photonics with metalenses
Researchers at the University of Delaware designed an integrated photonics platform utilizing a one-dimensional metalens along with metasurfaces that limit the loss of information.

The metalens, which focuses light in a specific way, was fabricated on a silicon-based chip programmed with hundreds of tiny air slots, enabling parallel optical signal processing all within the chip.

Thanks to the metasurfaces, high signal transmission was possible with less than one decibel loss over a 200nm bandwidth. When they layered three of their metasurfaces together, they demonstrated functionalities of Fourier transformation and differentiation.

“This is the first paper to use low-loss metasurfaces on the integrated photonics platform,” said Tingyi Gu, assistant professor of electrical and computer engineering at UD. “Our structure is broadband and low loss, which is critical for energy efficient optical communications.”

The new device is lighter and smaller than conventional devices of its type, the team said, and it doesn’t require manual alignment of lenses. “It’s just much faster than conventional structures,” said Gu. “There are still a lot of technical challenges when you try to actively control them, but this is a new platform we are starting with and working on.”

The team sees potential applications in imaging, sensing and quantum information processing, such as on-chip transformation optics, mathematical operations, and spectrometers, as well as neural networks.

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