Power/Performance Bits: May 3

Nanowire batteries

University of California, Irvine researchers invented a nanowire-based battery material that can be recharged hundreds of thousands of times.

Nanowires have long been sought as a battery material. However, these filaments are extremely fragile and don’t hold up well to repeated discharging and recharging, or cycling. In a typical lithium-ion battery, they expand and grow brittle, which leads to cracking.

This problem was addressed by the researchers through coating a gold nanowire in a manganese dioxide shell and encasing the assembly in an electrolyte made of a Plexiglas-like gel. The combination is reliable and resistant to failure.

The testing electrode was cycled up to 200,000 times over three months without detecting any loss of capacity or power and without fracturing any nanowires.

“That was crazy,” said Reginald Penner, chair of UCI’s chemistry department, “because these things typically die in dramatic fashion after 5,000 or 6,000 or 7,000 cycles at most.”

“The coated electrode holds its shape much better, making it a more reliable option,” said UCI doctoral candidate Mya Le Thai and leader of the study. “This research proves that a nanowire-based battery electrode can have a long lifetime and that we can make these kinds of batteries a reality.”

Old batteries storing light

Smartphones have an average consumer lifetime of about three years. The lithium ion batteries that power them, however, can last for about five years — meaning that just about every discarded smartphone generates e-waste and squanders the battery’s twilight years. To cut down on the environmental waste and provide storage for rural communities, researchers at Kyung Hee University in Seoul have proposed a model for recycling unspent lithium ion batteries into energy storage units for solar-powered LED lamps.

The candle and kerosene lamps used to light the homes within many rural communities are harmful, inefficient and more expensive than a small solar home lighting system, provided the right approach is taken, said Boucar Diouf, a professor in the Department of Information Display at Kyung Hee. “Using the battery of mobile phones in small solar home systems becomes obvious in order to make access to electricity easier to those who live without.”

The prototype system consisting of a solar panel and 12V LED lamp wired to a battery pack containing three Samsung Galaxy Note 2 batteries. (Source: Diouf/Kyung Hee University)

Batteries are one of the more expensive components of a solar home system and contribute significantly to the cost barrier of decent lighting systems in rural communities. Old car batteries, which are lead-acid based, are commonly used storage units in improvised systems, but don’t have a very good lifespan.

A standard lithium ion phone battery of 1000 milliamp-hour capacity can power a one-watt LED lamp for about three hours, or a 0.5-watt lamp — bright enough for reading and writing — for about six hours. When wired to a small solar panel, this system can last maintenance-free for approximately three years.

The researchers also constructed a full 12-volt system made of three mobile phone batteries of 3100 millliamp-hour capacity each, with a 5-watt LED lamp and a small solar panel, for less than $25. These systems have the capability to light up a room for about five hours each day, and can last for approximately three years without any maintenance.

Investigating the shape of nanoparticles

A team of Stanford engineers obtained a first look inside phase-changing nanoparticles, and how their shape and crystallinity – the arrangement of atoms within the crystal – can have dramatic effects on their performance.

The work’s immediate applications are in the design of energy storage materials, but could eventually find its way into data storage, electronic switches and any device in which the phase transformation of a material regulates its performance.

For instance, in a lithium ion battery, the ability of the battery to store and release energy repeatedly relies on the electrode’s ability to sustain large deformations over several charge and discharge cycles without degrading. Recently, scientists have improved the efficiency of this process by nanosizing the electrodes. The nanoparticles allow for faster charging, increased energy storage and an extended lifetime, but it is unknown which nanoparticle shapes, sizes and crystallinities produce the best performance. Addressing this question served as inspiration for the present study.

In their experiment, the group examined how varying the shapes and crystallinity of palladium nanoparticles affected their ability to absorb and release hydrogen atoms – an analog to a lithium-ion battery discharging and charging. They prepared cubic, pyramidal and icosahedral nanoparticles and developed novel imaging techniques to look inside nanoparticles at various hydrogen pressures, determining where the hydrogen was located.

First insights into the structures of phase-changing nanoparticles shows that shape matters. Materials composed of cubes and pyramids (left and center) for instance might yield more efficient batteries than those made of icosahedra (right). Credit: Dionne Group

The technique relied on an environmental transmission electron microscope, allowing the engineers to discern exactly how the hydrogen was distributed within the nanoparticles and to do so with incredibly high – sub-2-nanometer – resolution. The microscope enables analysis of particles using several different techniques, such as direct imaging, diffraction and spectroscopy.

The researchers found that nanoparticle structure significantly influences performance. The icosahedral structures, for instance, show reduced energy storage capacity and more gradual hydrogen absorption than the single crystalline cubes and pyramids. High-resolution maps of the particles demonstrate that hydrogen is excluded from the center of the particle, thus lowering the overall capacity to incorporate hydrogen. Structural characterization shows that the gradual absorption of hydrogen occurs because different regions of the particle absorb hydrogen at different pressures, unlike what is observed in single crystals.