Lithium, a metal with a name that has become synonymous with rechargeable batteries, has been instrumental in pushing technology into producing batteries with better life and better performance for a wide range of applications.
But lithium won’t last forever.
That’s why researchers are looking at alternatives. New evidence from the Georgia Institute of Technology suggests that sodium and potassium could potentially fill lithium’s big shoes. The two metals have been known to hold less energy than other alternatives, and also to decay and degrade quickly.
"But,” explained Matthew McDowell, an assistant professor in materials science and engineering, “we've found that's not always the case.”
As batteries charge and discharge, the ions on which they are based (whether lithium, sodium or potassium) are constantly reacting with and penetrating the battery’s electrodes. This causes large volume changes, often breaking the electrode particles into small pieces and causing degradation. Sodium and potassium are believed to be worse offenders because their ions are larger than lithium.
In experiments using iron sulfide — also known as pyrite or “fool’s gold” — in the role of a battery electrode, researchers were able to use an electron microscope to observe what happens during internal battery reactions. They found that iron sulfide was more stable during reaction with sodium and potassium — meaning that batteries based on either could have a much longer life than expected. In fact, the iron sulfide expanded like a balloon when exposed to sodium and potassium — but when exposed to lithium, it appeared to almost explode.
The study also casts doubt on the idea that volume changes occurring during electrochemical reactions are always a precursor to particle fracture.
It is possible, the researchers said, that different ions reacted in different ways because lithium is more likely to concentrate its reaction along the particle's sharp cube-like edges. By contrast, the reaction with sodium and potassium is more diffuse along all of iron sulfide’s surface.
The research findings could help scientists design battery systems that use alternate materials.
"Lithium batteries are still the most attractive right now because they have the most energy density; you can pack a lot of energy in that space," McDowell said. "Sodium and potassium batteries at this point don't have more density, but they are based on elements a thousand times more abundant in the earth's crust than lithium. So they could be much cheaper in the future, which is important for large-scale energy storage."
The research first appeared today in the journal Joule.
