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Ionic liquids study could lead to safer, more eco-friendly batteries

09 June 2013

A new study into the behaviour of charged molecules in ionic liquids may lead to the creation of cleaner, more sustainable, and non-toxic batteries.

Illustration: Matthew Gebbie (see foot of article for explanation)

The research was carried out by a team at UC Santa Barbara led by Jacob Israelachvili, a professor in the Departments of Chemical Engineering and Materials. According to co-researcher, graduate student Matthew Gebbie, the framework could provide a nice strategy to begin discussions toward batteries utilising ionic liquids.

An ionic liquid is a salt - like rock salt but in the liquid state - usually one that can melt at temperatures from ambient to 100 degrees Celsius, so the liquid is composed entirely of homogeneous molecules with positive and negative ions.

"You'd expect that at room temperature, with ionic liquids that are made entirely of positive and negative charges, that the ions should be mobile," says Israelachvili.

But, despite the abundance of ions and a free-flowing environment, ionic liquids have never lived up to their promise of delivering the same kind of energy as commonly used electrolytes, such as sulphuric acid. Their conductivity is just not as high, say the scientists.

Using a surface forces apparatus, a device developed in the Israelachvili lab that can measure forces between surfaces to the sub-nano scale, the researchers analysed the interactions of the charges in an ionic liquid, the effective voltage of the liquid and the ions' interactions with each other, as well as with the electrodes that are meant to pick up or discharge, and thereby conduct their charges.

They found that the ions in the ionic liquids are 'stickier' than previously thought.

"They're bound to each other, and it's related to a complex property of any liquid or material, called the dielectric constant, which is the measure of how much you would expect charges to be free," Israelachvili explains.

In fact, the somewhat overlooked dielectric constant, which is a measure of how well charged particles stick to each other in a liquid, plays a larger role in the conductivity of ionic liquids than was previously assumed. Instead of the estimates of 50 percent separation that have been made, the experiments with the surface forces apparatus yielded a less than 0.02 percent separation between ions for typical ionic liquids.

"The connection that nobody had made before that emerged from our work was that it's not enough just to know how sticky the ions are to each other in a vacuum; you need to account for all the other billions of ions that surround any two ions in the liquid state," said Gebbie.

With that parameter taken into account along with the materials' dielectric constant, it became possible to come up with a simple equation that quantitatively predicts the number of free (effectively separated) ions that are present in ionic liquids.

"It's so simple. It really captures the physics of what's going on, but it's also simple enough to be used for predictive purposes," said Gebbie, adding that the group is now in active discussions with potential collaborators to refine and improve the equation.

The research has wide implications. With the formula, it would be possible to design an ionic liquid with particular desired properties, instead of performing countless trial-and-error tests or experiments.

To date, over a million combinations of positive and negative ions have been identified that can be mixed together to form an ionic liquid, according to the researchers. To further blend these liquids to find, change, or add properties, the number of possible combinations is a phenomenal trillion trillion.

Not only could efficient charge-conducting ionic liquids be found in a shorter amount of time, but other properties could also be incorporated via molecular fine-tuning, such as less toxicity, reduced corrosiveness, or increased biodegradability.

"An electric vehicle has to have a very large battery. So if that very large battery is based on something that's acid, then you have a large compartment of acid. In an accident, if you had a nonflammable, nontoxic ionic liquid, then at least you could take some of that risk out of the equation," Gebbie concludes.

The schematic above depicts how the ionic liquid molecules arrange in electrically charged interfaces. The green shading represents the 99.98 percent of the molecules that exist in a neutral state, the blue shapes represent positive ions and the red shapes represent negative ions. The reaction at the top of the illustration shows that the molecules can exist in two states: a neutral ionic liquid molecule (99.98 percent of the molecules) and separated positive (blue) and negative (red) ions (0.02 percent of the molecules).

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