Power/Performance Bits: Jan. 16

Lithium-iron-oxide battery
Scientists at Northwestern University and Argonne National Laboratory developed a rechargeable lithium-iron-oxide battery that can cycle more lithium ions than its common lithium-cobalt-oxide counterpart, leading to a much higher capacity.

For their battery, the team not only replaced cobalt with iron, but forced oxygen to participate in the reaction process as well. If the oxygen could also store and release electrical energy, the battery would have the higher capacity to store and use more lithium. Although other research groups have attempted this strategy in the past, few have made it work.

“The problem previously was that often, if you tried to get oxygen to participate in the reaction, the compound would become unstable,” said Zhenpeng Yao, a PhD student at Northwestern. “Oxygen would be released from the battery, making the reaction irreversible.”

The battery uses both oxygen and iron to store and release electrical energy. (Source: Zhenpeng Yao, Northwestern University)

Through computational calculations, the team discovered a formulation that works reversibly. First, they replaced cobalt with iron, which is advantageous because it’s among the cheapest elements on the periodic table. Second, by using computation, they discovered the right balance of lithium, iron, and oxygen ions to allow the oxygen and iron to simultaneously drive a reversible reaction without allowing oxygen gas to escape.

The fully rechargeable battery starts with four lithium ions, instead of one. The current reaction can reversibly exploit one of these lithium ions, increasing the capacity beyond today’s batteries, and there’s potential to cycle all four back and forth by using both iron and oxygen to drive the reaction.

“Not only does the battery have an interesting chemistry because we’re getting electrons from both the metal and oxygen, but we’re using iron,” said Christopher Wolverton, a professor of materials science and engineering at Northwestern. “That has the potential to make a better battery that is also cheap.”

A provisional patent for the battery has been filed. Next, the team plans to explore other compounds where this strategy could work.

Solar windows with quantum dots
Researchers at Los Alamos National Laboratory developed a new double-paned solar window, utilizing quantum dots to improve energy generation.

The key to this advance is “solar-spectrum splitting,” which allows one to process separately higher- and lower-energy solar photons. The higher-energy photons can generate a higher photovoltage, which could boost the overall power output. This approach also improves the photocurrent as the dots used in the front layer are virtually “reabsorption free.”

The team incorporated ions of manganese that serve as highly emissive impurities into the quantum dots. Light absorbed by the quantum dots activates these impurities. Following activation, the manganese ions emit light at energies below the quantum-dot absorption onset. This trick allows for almost complete elimination of losses due to self-absorption by the quantum dots.

A new window architecture which utilizes two different layers of low-cost quantum dots tuned to absorb different parts of the solar spectrum. (Source: Los Alamos National Laboratory)

For the window, the team deposited a layer of highly emissive manganese-doped quantum dots onto the surface of the front glass pane and a layer of copper indium selenide quantum dots onto the surface of the back pane. The front layer absorbs the blue and ultraviolet portions of the solar spectrum, while the rest of the spectrum is picked up by the bottom layer.

Following absorption, the dot re-emits a photon at a longer wavelength, and then the re-emitted light is guided by total internal reflection to the glass edges of the window. There, solar cells integrated into the window frame collect the light and convert it to electricity.

“The approach complements existing photovoltaic technology by adding high-efficiency sunlight collectors to existing solar panels or integrating them as semitransparent windows into a building’s architecture,” said lead researcher Victor Klimov.

Light-absorbing graphene
Researchers at the University of Central Florida developed a method to make graphene better at absorbing light, with more than 45% absorption of light in a single layer.

Graphene alone has very poor light absorption – less than 2%, and researchers have focused on ways to increase that percentage to make the material functional. Previous efforts have used metal particles on graphene sheets to show enhanced light absorption, but in those cases the majority of the light was absorbed by the metal, thus defeating the purpose.

Instead of modifying the properties of graphene, the team placed the graphene on a polymer substrate as a support layer and stamped it using a polymeric stamp to form a nanoscale pattern on the pristine graphene.  An optical cavity stopped light from escaping and repeatedly bounced light back to the patterned graphene layer and enhanced light absorption.

In addition, the process allows voltage driven change in electron energy and thus in the electron density, enabling absorption of different wavelengths of light, making the process dynamically tunable.

“Theoretical studies show further design optimization can lead to further enhanced absorption close to 90%,” said Michael N. Leuenberger, a researcher in the NanoScience Technology Center at UCF.

Applications of highly light-absorbent graphene include tunable infrared cameras that can operate at room temperature and could be used to develop multispectral night vision equipment, gas and chemical sensors.