Graphene batteries could give EV sales some serious thurst

A new battery under development promises to store twice as much and power an electric vehicle for more than 300 miles

6/3/14 5:20 am chumakdenis 2

A research team from the Lawrence Berkeley National Laboratory has a 300-mile electric vehicle battery range in its sights, thanks to a unique combination of different electrochemical technologies including a new material called sulfur-graphene oxide (S-GO). CleanTechnica is one of the world’s leading fans of graphene so naturally we are  totally excited by that. So, here goes.

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How it was developed

S-GO was developed in-house by Berkeley Lab, for use in next generation EV batteries based on lithium-sulfur technology.

Sulfur has some key advantages over conventional lithium-ion battery technology in terms of storage capacity (far better), toxicity (none), cost (far less), and weight (ditto), but it is also very brittle.

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The gist of the problem is that sulfur tends to be soluble in the organic solvents used in conventional batteries. That process forms polysulfide ions - I know, right? - which can get to the lithium anode and re-solidify as precipitates, forming a barrier that interferes with storage capacity.

The result is that typical lithium-sulfur prototypes can’t last more than a dozen or so charge-recharge cycles without losing it, “it” being their ability to store a charge.

The Berkeley solution was to develop a nanomaterial composed of small particles of graphene flakes coated with sulfur, namely S-GO. As described by Berkeley writer Allan Chen, S-GO is characterized by a large, cavity-speckled surface area, which allows for more “intimate electronic contact” with sulfur while minimizing loss of contact with the current collector of the electrode.

When used as a cathode material in a lithium-sulfur battery, S-GO binds with lithium during discharge and releases it back to the anode during recharge.

Meanwhile, S-GO resolves some other key issues, including the massive bloating that bedevils lithium-sulfur technology. The graphene lends an element of flexibility that enables S-GO to accommodate the volume increase of up to 76 percent that sulfur suffers through as it is converted to lithium sulfide during discharge.

How the S-GO cathode works together with other electrochemical technologies to extend  EV battery range in a lithium-sulfur battery

Aside from the vastly improved cathode performance, the new battery sports such goodies as an enhanced binder (elastomeric styrene butadiene rubber combined with a thickener) that increases power density.

To deal with the polysulfide issue, the team used a coating of cetyltrimethyl ammonium bromide (a surfactant commonly used in drug delivery systems) on the sulfur electrode.

Also helping out with the polysulfides thing was a new electrolyte based on an ionic liquid, developed in-house at Berkeley (ionic liquids are non-volatile and  non-flammable).

The new ionic liquid also provides a huge boost in the rate of battery operation, while increasing the speed of charging and the delivery of power during discharge.

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Here’s the result as reported by Chen:

“The battery initially showed an estimated cell-specific energy of more than 500 Wh/kg and it maintained it at >300 Wh/kg after 1,000 cycles—much higher than that of currently available lithium-ion cells, which currently average about 200 Wh/kg”.

That puts the new battery’s potential well within sight of a 300-mile EV battery range:

“For electric vehicles to have a 300-mile range, the battery should provide a cell-level specific energy of 350 to 400 Watt-hours/kilogram (Wh/kg). This would require almost double the specific energy (about 200 Wh/kg) of current lithium-ion batteries. The batteries would also need to have at least 1,000, and preferably 1,500 charge-discharge cycles without showing a noticeable power or energy storage capacity loss”.

The next steps include increasing the use of sulfur, maintaining performance in extreme conditions, and of course, scaling up to size.

How can they be used in electric vehicles?

Well, not only people from Berkeley Lab work under this and  team of South Korean scientists has developed a new graphene supercapacitor that can store almost as much energy as a lithium-ion battery, but charge in only 16 seconds.This makes it an ideal material to store braking energy and could be exactly what the electric car industry needs.

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Scientists from Gwangju Institute of Science and Technology in South Korea made the breakthrough by creating an especially porous form of graphene.

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Incredibly, a single gram of this specialized graphene has the same amount of surface area as a basketball. This greatly increased surface area enables the supercapacitor to store far more energy than previous versions of the material, which had been keeping graphene supercapacitors out of the running as an alternative to lithium-ion batteries for electric cars. Because they don't use chemicals, graphene supercapacitors also have a much longer life than lithium ion batteries. Oh, and did I mention how fast it charges? That's right; I did. 16 seconds.

Finding success in the lab and bringing a product to market are two different things, so it's unclear how quickly we might see this impressive technology at work in electric cars. The South Koreans say that these "supercapacitor energy storage devices… can be scaled up for manufacturing in the near future for electric vehicle applications."

So that's something. But only time will tell whether these supercapacitors take over before Tesla manages to build  its famously ambitious lithium-ion factory.
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