Simanaitis Says

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NANOTECH AND THE LITHIUM BATTERY

BATTERIES OF lithium-ion technology are the current state of the art, suppling power to everything from smart phones to Battery Electric Vehicles. Plenty of companies worldwide are striving to make these batteries smaller, lighter and more powerful. The application of nanotechnology in this is examined in a recent issue of Science, the weekly magazine of the American Association for the Advancement of Science. Here, I discuss several of the article’s aspects and share some fascinating Science graphics.

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“The Battery Builder” by Robert F. Service is a featured article in Science, May 27, 2016. It gives details of Stanford University scientist Yi Cui and Amprius, the company he founded six years ago. As the article notes, Cui is taking nanotechnology and using it to control chemistry.

What’s more, graphics in the Science article are exemplary in extending the basics of lithium-ion operation into the nanorealm.

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A basic lithium-ion battery. This and other graphics by V. Altounian/Science, May 27, 2016.

As noted in Science, “When a lithium-ion battery discharges its current to an external circuit, lithium ions (tan) give up electronics and move from the anode through a separator to the cathode. There, they meet up with the electrons that traveled through the circuit. When the battery is charged, the flow of electronics and lithium ions is reversed.”

These “electrons that traveled through the circuit” are also known by a more familiar term: electricity.

Limitations of today’s lithium-ion technology are inherent in its anode of graphite. Graphite is highly conductive, and this promotes quick discharge, i.e., the battery’s power. However, in recharging it takes six graphite atoms to grasp a single lithium ion, and this limits the energy a battery can store.

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Silicon is more efficient at lithium-grasping than carbon.

A silicon anode can do far better: Each silicon atom binds to four lithium atoms. “In principle,” Science notes, “that means a silicon-based anode can store up to 10 times as much energy as one made from graphite.”

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A silicon tradeoff: its eventual disintegration.

On the other hand, as batteries charge and discharge, silicon anodes expand and contract. These physical changes lead to eventual disintegration of the anode.

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An early fix: Nanowires of silicon.

Nanotech to the rescue! In an early fix, anodes fabricated of silicon nanowire gave room for the swelling and shrinking. However, these silicon nanowires proved more difficult to make and more expensive than bulk graphite.

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At left, egg-shell nanoparticles; at right, with protection.

As a second approach, the nanofiliments were replaced by tiny spheres. These were easier to fabricate, but again swelling/shrinkage precluded durability.

A subsequent fix encased each silicon atom “yolk” in a protective shell of carbon. In a 2012 paper in Nano Letters, the team reported that this yolk-shell composition retained 97 percent of its original capacity after 1000 cycles of charge/discharge. An abstract of the paper is freely available.

Science, though, is never over. “Earlier this year,” Science notes, “Cui and colleagues reported a solution that outdoes even their complex pomegranate assemblies.” A sandwich is formed of micrometer-scale silicon wrapped in carbon sheets of graphene.

Sure enough, these silicon particles, larger than nano-scale, fractured after a few charge/discharge cycles. However, their graphene wrapping prevented the electrolyte from adversely reaching the silicon.

Science notes, “What’s more, the team reported in Nature Energy, the larger silicon particles packed more mass—and thus more power—into a given volume, and they were far cheaper and easier to make than the pomegranates.” An abstract can be accessed at no charge.

I would have suspected that enjoying a sandwich is easier than savoring a pomegranate. And isn’t this a tasty bit of science? ds

© Dennis Simanaitis, SimanaitisSays.com, 2016

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