What is killing your batteries?

A number of teams working at several Department of Energy laboratories have published papers aimed at improving our understanding of how lithium-ion batteries degrade. Despite the sheer number of “lion” batteries on the market today, there’s still quite a bit about how these devices operate that remains a mystery. Particularly, while there are models to explain the gradual breakdown of the cathode and anode materials, and thus the gradual breakdown of the battery, none could previously explain why this breakdown occurred so quickly. Now, by looking at these reactions at an unprecedented level of detail, an explanation may finally have presented itself.

The secret has to do with nano-scale crystals that form on the electrodes during power transfer. These crystals build up in a pattern the researchers compare to rock salt; the tiny crystals can come to impede electron transfer and greatly diminish a battery’s power efficiency. The higher the voltage of the battery, the faster the crystals were formed. Even with a modest jump from 4.3 to 4.7 volts, the more powerful battery has a measurably shorter lifetime.

When crystallization begins, it does so only at imperfections in the electrode, via a process called nucleation. The uneven distribution of crystals, based on largely random distributions of imperfections in the surface of the electrodes, is one reason some lithium ion batteries last so much longer than others.

his is the same process behind bubble formation in hot water. If you put a perfectly smooth glass or ceramic cup full of distilled water into a microwave (no spinning plate!) it’s possible to super-heat water beyond its boiling point — without imperfections in the glass or water, and without any mechanical shocks from movement, no bubbles can form and so the water can only slowly steam and evaporate away. Just be careful reaching in; the instant you touch the cup, you’ll provide the shock necessary to begin nucleation, and if it’s hot enough to water could flash-boil and explode out of its vessel!

This research could help battery researchers design coatings and nano-scale powders to prevent crystal formation or at the very least promote it evenly over the whole surface of the electrode. Though lithium ion batteries are likely on their way out, they will probably still be on their way out in 5 years; it’s important to improve the technology too, rather than simply replacing it.