Solid electrolyte may end the catastrophic failures of lithium batteries

April 2, 2013 - via Nature Materials

Lithium batteries have become a very popular technology, powering everything from cell phones to cars. But that doesn't mean the technology is without its problems; lithium batteries have been implicated in some critical technological snafus, from exploding laptops to grounded airplanes. Most of these problems can be traced back to the electrolyte, a liquid that helps ions carry charges within the battery. Liquid electrolytes can leak, burn, and distort the internal structure of the battery, swelling it in ways that can lead to a catastrophic failure.

The solution, of course, would be to get rid of the liquids. But ions don't tend to move as easily through solids, which creates another set of problems. Now, researchers have formulated a solid in which lithium ions can move about five times faster than any previously described substance. Better yet, the solid—a close chemical relative of styrofoam—helps provide structural stability to the battery. Don't expect to see a styrofoam battery in your next cellphone though, as the material needs to be heated to 60°C in order to work.

The problem with liquid electrolytes has to do with the fact that, during recharging, lithium ions end up forming deposits of metal inside the battery. These create risks of short circuits (the problem that grounded Boeing's Dreamliner 787) and can damage the battery's structure, causing leaks and a fire risk. Solid electrodes get around this because the lithium ions will only come out of the electrolyte at specific locations within the solid, and can't form the large metal deposits that cause all of the problems.

However, as noted above, solids don't allow lithium to move through them very easily. This creates a bit of a practical problem, in that the batteries typically need to be heated to around 80°C before charges start moving at all, but long-term performance problems are even worse. Because the ions move so slowly, a gradient of lithium gets built up across the battery during discharging, with ions piling up near the negative electrode. This slows the rate at which they can charge and discharge.

The key thing about the new material is that it avoids having a single, charged site where the lithium ions would be stored, since this strong attraction is what slows the ions down. Instead, each of the polystyrene groups has an added chemical that includes two sulfur atoms bridged by a nitrogen. Although the nitrogen technically carries a negative charge, it's shared with the two sulfurs, making it far more diffuse. As a result, it doesn't hang on to the lithium as tightly, meaning the lithium can move through the material much faster.

Almost everything about the material is an improvement over existing solid electrolytes: rather than needing temperatures of 80°C to operate, it works well at 60°C; the ions move about five times faster than they do in existing electrolytes; and the polystyrene-based electrolyte has a tensile strength ten times that of the existing material.

The researchers built a number of test batteries with this, and showed that the material was stable for dozens of charge/discharge cycles, and operated well over a wide variety of temperatures (though all of them a bit elevated). There were a few performance numbers but, given the unusual electrodes and battery material in this case, it's hard to make a comparison between them and the existing liquid electrolyte batteries.

In the end, the comparisons might not be especially relevant. Because these batteries require elevated temperatures just to operate, they're not going to find the same uses as our current gadget batteries. But there are a number of use cases—like in aircraft, for just one example—where keeping batteries at an elevated operating temperature wouldn't be an issue. And that also happens to be a use case where the things this battery does bring to the table, like mechanical strength and limited risk of fires or explosions, may make a somewhat lower performance worth putting up with.

Nature Materials

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Author:John Timmer