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News

Self-healing stretchy polymer heals battery electrode cracks

Stanford University : 02 December, 2013  (Technical Article)
A battery electrode that heals itself open a potentially commercially viable path for making the next generation of lithium ion batteries for electric cars, cell phones and other devices. A stretchy polymer coats the electrode, binds it together and spontaneously heals tiny cracks that develop during battery operation.
Researchers worldwide are racing to find ways to store more energy in the negative electrodes of lithium ion batteries to achieve higher performance while reducing weight. One of the most promising electrode materials is silicon; it has a high capacity during charging and discharge, but silicon electrodes swell to three times normal size and shrink back down during each cycle. The brittle material soon cracks and falls apart, degrading battery performance. This is a problem for all electrodes in high-capacity batteries.
 
A number of ways have been explored to keep silicon electrodes intact and improve their performance. Some are being explored for commercial uses, but many involve exotic materials and fabrication techniques that are challenging to scale up for production.
 
The self-healing electrode, which is made from silicon microparticles that are widely used in the semiconductor and solar cell industries, is the first solution that seems to offer a practical road forward. To make a self-healing coating, scientists at a Stanford laboratory deliberately weakened some of the chemical bonds within polymers – long, chain-like molecules with many identical units. The resulting material breaks easily, but the broken ends are chemically drawn to each other and quickly link up again, mimicking the process that allows biological molecules such as DNA to assemble, rearrange and break down.
 
The self-healing polymer has been developed in a research group which has been working on flexible electronic skin for use in robots, sensors, prosthetic limbs and other applications. For the battery project were added tiny nanoparticles of carbon to the polymer so it would conduct electricity.
 
“Self-healing is very important for the survival and long lifetimes of animals and plants,” said Chao Wang, a postdoctoral researcher at Stanford. “We want to incorporate this feature into lithium ion batteries so they will have a long lifetime as well. We found that silicon electrodes lasted 10 times longer when coated with the self-healing polymer, which repaired any cracks within just a few hours,”.
 
“Their capacity for storing energy is in the practical range now, but we would certainly like to push that,” said Yi Cui, an associate professor at SLAC and Stanford. "The electrodes worked for about 100 charge-discharge cycles without significantly losing their energy storage capacity. “That’s still quite a way from the goal of about 500 cycles for cell phones and 3000 cycles for an electric vehicle, but the promise is there."
 
The researchers said they think this approach could work for other electrode materials as well, and they will continue to refine the technique to improve the silicon electrode’s performance and longevity.
 
This prototype lithium ion battery, made in a Stanford lab, contains a silicon electrode protected with a coating of self-healing polymer. The cables and clips in the background are part of an apparatus for testing the performance of batteries during multiple charge-discharge cycles. (Brad Plummer/SLAC)
 
Self-healing chemistry enables the stable operation of silicon microparticle anodes for high-energy lithium-ion batteries
Chao Wang, Hui Wu, Zheng Chen, Matthew T. McDowell, Yi Cui & Zhenan Bao
Nature Chemistry 5, 1042–1048 (2013) doi:10.1038/nchem.1802

 

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