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Sponge-like Si bests graphite as Li-ion electrode

Posted: 10 Jul 2014 ?? ?Print Version ?Bookmark and Share

Keywords:battery? silicon? electrode? sponge?

A team of researchers from the Pacific Northwest National Laboratory (PNNL) recently discovered that a sponge-like silicon material could help Li-ion batteries store more energy and run longer on a single charge for laptops and electric vehicles.

Researchers developed the porous material to replace the graphite traditionally used in one of the battery's electrodes, as silicon has more than 10 times the energy storage capacity of graphite. A paper describing the material's performance as a Li-ion battery electrode was published in Nature Communications.

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Figure 1: The sponge-like silicon can give the batteries' electrodes the space they need to expand without breaking. Source: PNNL

"Silicon has long been sought as a way to improve the performance of Li-ion batteries, but silicon swells so much when it is charged that it can break apart, making a silicon electrode inoperable," said Pacific Northwest National Laboratory Fellow Ji-Guang "Jason" Zhang. "The porous, sponge-like material we've developed gives silicon the room it needs to expand without breaking."

Room for improvement

Rechargeable Li-ion batteries have two electrodes: one that's positively charged and made of lithium and another that's negative and typically consists of graphite. Electricity is generated when electrons flow through a wire that connects the two. To control the electrons, positively charged lithium atomswhich scientists call ionsshuffle from one electrode to the other through another path: the electrolyte solution in which the electrodes sit.

The chemistry of Li-ion batteries limits how much energy they can store. To increase the battery's energy capacity, researchers are looking at new materials such as silicon. A Li-ion battery with a silicon electrode could last about 30 per cent longer than one with a graphite electrode. Today's average electric vehicle could drive about 130 miles on a single charge if it used a Li-ion battery with PNNL's silicon electrode.

Unfortunately, silicon expands as much as three times in size when it charges, creating pressure within the material that causes it to break. Many scientists have attempted to make tiny, nano-sized battery components with the idea that the smaller size would give silicon enough room to expand, but these efforts haven't produced market-ready technologies.

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