New rubber-like coating can lead to better batteries

A novel rubber-like coating can help create longer-lasting, higher-capacity lithium rechargeable batteries, which are commonly used in consumer electronics.
Researchers have demonstrated that a coating that makes high capacity silicon electrodes more durable could lead to a replacement for lower-capacity graphite electrodes.
"Understanding how the coating works gives us an indication of the direction we need to move in to overcome the problems with silicon electrodes," said materials scientist Chongmin Wang of the US Department of Energy's Pacific Northwest National Laboratory.
Silicon has high electrical capacity potential and is currently used in lithium ion battery development.
Replacing the graphite electrode in rechargeable lithium batteries with silicon could increase the capacity ten-fold, making them last many hours longer before they run out of juice.

However, silicon electrodes aren't very durable after a few dozen recharges, they can no longer hold electricity.

Last year, materials scientist Chunmei Ban and her colleagues at the National Renewable Energy Laboratory in Golden, Colorado, and the University of Colorado, Boulder found they could cover silicon nanoparticles with a rubber-like coating made from aluminum glycerol.
The coated silicon particles lasted at least five times longer - uncoated particles died by 30 cycles, but the coated ones still carried a charge after 150 cycles.
Researchers did not know how this coating improved the performance of the silicon nanoparticles.
The nanoparticles naturally grow a hard shell of silicon oxide on their surface, much like stainless steel forms a protective layer of chromium oxide on its surface.

No one understood if the oxide layer interfered with electrode performance, and if so, how the rubbery coating improved it.
To better understand how the coating worked, PNNL's Wang and colleagues, including Ban, turned to expertise and a unique instrument at DOE's Environmental Molecular Sciences Laboratory.
Ban's group - which developed the coating for silicon electrodes, called alucone, and is currently the only group that can create alucone-coated silicon particles - took high magnification images of the particles in an electron microscope.
But Wang's team has a microscope that can view the particles in action, while they are being charged and discharged. So, Yang He from the University of Pittsburgh explored the coated silicon nanoparticles in action at EMSL.

The team discovered that, without the alucone coating, the oxide shell prevents silicon from expanding and limits how much lithium the particle can take in when a battery charges.
At the same time, they found that the alucone coating softens the particles, making it easier for them to expand and shrink with lithium.