Power/Performance Bits: April 14

Elastic energy harvesting

Researchers from the Korea Advanced Institute of Science and Technology (KAIST) and Seoul National University collaborated to develop a hyper-stretchable elastic-composite energy harvesting device.

Their stretchable piezoelectric generator can harvest mechanical energy to produce a ~4V power output with around 250% elasticity and a durability over 104 cycles. These results were achieved thanks ­­to the non-destructive stress-relaxation ability of the very long silver nanowire-based stretchable electrodes as well as the good piezoelectricity of the device components.

Top row shows schematics of hyper-stretchable elastic-composite generator (SEG) enabled by very long silver nanowire-based stretchable electrodes. The bottom row shows the SEG energy harvester stretched by human hands over 200% strain. (Source: KAIST)

The new device could be applied to a wide-variety of wearable energy-harvesters to transduce biomechanical-stretching energy from the body (or machines) to electrical energy.

“This exciting approach introduces an ultra-stretchable piezoelectric generator,” said Keon Jae Lee, professor at KAIST. “It can open avenues for power supplies in universal wearable and biomedical applications as well as self-powered ultra-stretchable electronics.”

Perovskite nanolasers

Perovskites are a focus of attention for high-efficiency solar cells, but a team from the University of Wisconsin-Madison found another area in which they excel.

“While most researchers make these perovskite compounds into thin films for the fabrication of solar cells, we have developed an extremely simple method to grow them into elongated crystals that make extremely promising lasers,” says Song Jin, professor of chemistry at UW-Madison. The tiny rectangular crystals grown in Jin’s lab are about 10 to 100 micrometers long by about 400 nanometers across.

Jin says the nanowires grow in about 20 hours once a glass plate coated with a solid reactant is submerged in a solution of the second reactant. “There’s no heat, no vacuum, no special equipment needed,” says Jin. “They grow in a beaker on the lab bench.”

Unsorted nanowire crystals immediately after production. (Source: Song Jin/UW-Madison)

Nanowire lasers have the potential to enhance efficiency and miniaturize devices, and could be used in devices that merge optical and electronic technology for computing, communication and sensors.

“These are simply the best nanowire lasers by all performance criteria,” says Jin, “even when compared to materials grown in high temperature and high vacuum. Perovskites are intrinsically good materials for lasing, but when they are grown into high-quality crystals with the proper size and shape, they really shine.”

Plus, simply tweaking the recipe for growing the nanowires could create a series of lasers that emit a specific wavelength of light in many areas of the visible spectrum.

Before these nanowire lasers can be used in practical applications, Jin says their chemical stability must be improved. Also important is finding a way to stimulate the laser with electricity rather than light, which was just demonstrated.

Seeing the whole battery

Recharging lithium batteries is not without problems, some of them leading to the premature death of the battery. An eruption of lithium at the tip of a battery’s electrode, cracks in the electrode’s body, and a coat forming on the electrode’s surface are a few of them. Using a powerful microscope to watch multiple cycles of charging and discharging under real battery conditions, researchers at the Pacific Northwest National Laboratory have gained insight into the chemistry that clogs rechargeable lithium batteries.

The problem in the past has been similar to Heisenberg’s uncertainty principle in that observations caused the environment to change. Unlike other views of the inner workings of batteries at high magnification, most of which use only part of a battery or have to study them under pressures not typically used in batteries, the team created a complete functioning battery cell within a transmission electron microscope operating under normal operating conditions.

“This work is the first visual evidence of what leads to the formation of lithium dendrites, nanoparticles and fibers commonly found in rechargeable lithium batteries that build up over time and lead to battery failure,” said Nigel Browning, a physicist at PNNL.

Solving these problems could also make electric vehicles and renewable energy more attractive. Using metals such as magnesium or aluminum in place of lithium could improve batteries life and cost, but research and development into non-lithium rechargeables lags far behind the common commercial lithium ion ones.

Although these experiments taught them about lithium behavior, Browning said he’s more excited to apply the technology to study other metal anodes, metals such as magnesium, copper and others that might lead to a new generation of battery systems.