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Polymer coating enables Li-ion batteries to self-heal

21 Nov 2013

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"That's still quite a way from the goal of about 500 cycles for cell phones and 3,000 cycles for an electric vehicle," Cui said, "but the promise is there, and from all our data, it looks like it's working."

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 for soaking up lithium ions from the battery fluid during charging and then releasing them when the battery is put to work.

But this high capacity comes at a price: Silicon electrodes swell to three times their normal size and shrink back down again each time the battery charges and discharges. The brittle material soon cracks and falls apart, degrading battery performance. This is a problem for all electrodes in high-capacity batteries, said Hui Wu, a former Stanford postdoc who is now a faculty member at Tsinghua University in Beijing, and the other principal author of the paper.

To make the self-healing coating, scientists 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.

Researchers in Cui's lab and elsewhere have tested a number of ways 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 industry, is the first solution that seems to offer a practical road forward, Cui said.

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.


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