Northwestern University engineers show in the journal Advanced Energy Materials they have developed technology that could hugely improve lithium batteries.  The new anode technology suggests a cellphone battery might recharge in 15 minutes and last ten times longer.

Graphene Sheets Sandwich Silicon. An artist representation of the graphene sheet with its holes, silicon and lithium ions.

The scientists combined two chemical engineering approaches to address two major battery limitations — energy capacity and charge rate — in one design. In addition to better batteries for cellphones and iPods, the technology could pave the way for more efficient, smaller batteries for electric cars.

Lithium-ion chemical batteries charge through a reaction in which lithium ions are sent between the anode and the cathode ends of the battery. As energy in the battery is used, the lithium ions travel from the anode through the electrolyte to the cathode.  As the battery is recharged, they travel in the reverse direction.

Lithium battery performance is set in two ways. Its energy capacity — how long a battery can maintain its charge voltage — is limited by the charge density, or how many lithium ions can be packed into the anode or cathode. Meanwhile, a battery’s charge rate — the speed at which it recharges — is limited by another factor: the speed at which the lithium ions can make their way from the electrolyte into the anode.

Harold H Kung, lead author of the paper says, “We have found a way to extend a new lithium-ion battery’s charge life by 10 times. Even after 150 charges, which would be one year or more of operation, the battery is still five times more effective than lithium-ion batteries on the market today.”

The better current rechargeable batteries’ anode is made of layer upon layer of carbon-based graphene sheets that can only accommodate one lithium atom for every six carbon atoms. To increase energy capacity, scientists have previously experimented with replacing the carbon with silicon, as silicon can accommodate much more lithium: four lithium atoms for every silicon atom.  But silicon expands and contracts dramatically in the charging process, which fragments the electrode destroying its charge capacity rapidly.

Kung’s research team’s techniques solve the problem with sandwiched clusters of silicon between the graphene sheets.  This stabilizes the silicon allowing a greater number of lithium atoms in the electrode while utilizing the flexibility of graphene sheets to accommodate the volume changes of silicon during use.

Current battery charge rate speed is a result of the shape of the graphene sheets: they are extremely thin — just one carbon atom thick — but by comparison, very long. During the charging process, a lithium ion must travel all the way to the outer edges of the graphene sheet before entering and coming to rest between the sheets. And because it takes so long for lithium to travel to the middle of the graphene sheet, a sort of ionic traffic jam occurs around the edges of the material.

Kung’s team uses a chemical oxidation process to create miniscule holes (10 to 20 nanometers) in the graphene sheets — termed “in-plane defects” — so the lithium ions would have a “shortcut” into the anode and be stored there by reaction with the silicon. This reduced the time it takes the battery to recharge by up to 10 times.

Kung says, “Now we almost have the best of both worlds. We have much higher energy density because of the silicon, and the sandwiching reduces the capacity loss caused by the silicon expanding and contracting. Even if the silicon clusters break up, the silicon won’t be lost.”

That’s half the challenge – The Northwestern team will begin studying changes in the cathode that could further increase effectiveness of the batteries. They also will look into developing an electrolyte system that will allow the battery to automatically and reversibly shut off at high temperatures — a safety mechanism that could prove vital in electric car applications.

This looks like a very strong improvement that might only require a processing step for making the holes and adding silicon, which isn’t expensive.  The main question is still the total recharges to full voltage – a serious matter when the application is a major investment such as an electric vehicle.

For now though, the team’s paper is sure to get intense study by the lithium battery industry – it’s a big improvement.


1 Comment so far

  1. Matt Musson on November 16, 2011 3:20 PM

    Unfortuately Lithium Brine ponds are horrendous for the planet. And, this means those ponds are going to be more and more prevalent in China and Peru.

    On the bright side – the sole use lithium producer just built a new plant in North Carolina that does not use brine ponds for separation.

    I wish people (especially the greenies) would realize how much better it is for the environment for things to be produced in the USA!

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