In a quest for safer lithium-ion batteries, a team of engineers at the University of Illinois (UI) has come up with a solid polymer-based electrolyte that can not only heal itself, but is recyclable without the need for high temperatures or strong acids. By using special cross-linked polymers, the new electrolyte gets stiffer under heat rather than breaking down.
Lithium-ion batteries are one of the great success stories of modern electrical technology. Without them, devices ranging from smartphones to electric cars would be impractical – but they are far from perfect. As they go through regular charging cycles, branch-like structures (dendrites) made of solid lithium metal form and grow through the battery's structure. This can result in reduced service life or electrical shorts. It can also, in extreme cases, damage the battery itself, resulting in fire and explosions.
Part of the reason for these explosive failures is that lithium-ion batteries use a liquid electrolyte – it can enter into a chemical reaction with the electrodes, if the battery is severely compromised. Solid polymer or ceramic electrolytes have been considered as a substitute, but according to Brian Jing – a materials science and engineering graduate student at UI – these tend to melt at the high temperatures generated inside the battery. They also tend to be brittle, making it difficult to maintain electrolyte-to-electrode contact.
One way to get around this problem is to use cross-linked polymer strands to produce a rubbery lithium conductor. This has a longer service life than the more rigid solid electrolytes, but it isn't self-healing and is difficult to recycle.
The UI team has developed a way to make the cross-links so they produce exchange reactions, and swap polymer strands between them. This means the polymer becomes stiffer when heated and is self-healing, resulting in less dendrite growth. In addition, the polymer can be broken down without the need for strong acids or high temperatures. Instead, it dissolves in water at room temperature. However, the technology is not yet practical.
"I think this work presents an interesting platform for others to test," says team leader Christopher Evans. “We used a very specific chemistry and a very specific dynamic bond in our polymer, but we think this platform can be reconfigured to be used with many other chemistries to tweak the conductivity and mechanical properties."
The research was published in the Journal of the American Chemical Society.
Source: University of Illinois