Published on March 19th, 2015 | by Tina Casey3
From Pottery Class To Next-Gen Lithium-Sulfur EV Batteries
March 19th, 2015 by Tina Casey
We’ve been hearing a lot about ceramics lately, but this is not something you scrape off your father’s kickwheel. We’re talking about a family of nano-layered ceramics called MAX phase, discovered about 20 years ago at Drexel University, and all that hard work is about to pay off. A team of Drexel researchers has found that they can nudge a nanoscale sheet from a MAX phase material which could go to work in a new generation of lithium-sulfur batteries for electric vehicles among other applications.
Until now, lithium-ion batteries have been the gold standard for EV batteries, but they are still fairly heavy and costly affairs, and their limitations are the key reason why EVs are still beyond the reach of many auto buyers (even if it’s their benefits that have brought electric vehicles within the reach of millions). Lithium-sulfur batteries offer a potentially cheaper, more energy-dense alternative that could really bust the EV market wide open.
What’s The Matter With Lithium-Sulfur EV Batteries?
If you really want to know all the dirt on lithium-sulfur EV batteries, our friends over at Lawrence Berkeley National Laboratory have a good rundown of the benefits and challenges, but for those of you on the go, aside from potential performance and cost benefits, part of the attraction is the use of a non-toxic material, namely sulfur.
Although the current crop of lithium-ion batteries has established a good safety record, lithium-sulfur EV batteries would offer additional improvements in terms of avoiding overheating.
The problem is, conventional lithium-sulfur batteries don’t last very long. They start to degrade when they discharge, so their performance drops after just a few charge/discharge cycles.
That’s a form of chemical degradation, and there is also a mechanical degradation that needs to be addressed. Part of the battery swells and shrinks significantly during the charge/discharge cycle. The result is that particles of sulfur drift around without an electrical function, cutting down on efficiency and contributing to the delinquency problem.
What Is This MAX Phase Of Which You Speak?
The Drexel University solution is based on a two-dimensional sheet, or nanolaminate, which they peeled from a MAX phase material, which they’re calling Ti2SC MAX phase (Ti for titanium, S for sulfur, C for carbon).
We never heard of MAX phase materials before so we looked it up, and so can you. Check out Drexel’s Max phase page for many more fascinating details.
For those of you on the go, MAX phase refers to a class of around 70 layered carbides (carbide is fancyspeak for a compound of carbon and other stuff). The figure of 70 refers to those known so far, and there may be more.
MAX phase materials are frisky little devils that sometimes behave like ceramics and sometimes not. Here’s a snippet from Drexel:
As a consequence of their layered structure, these materials kink and delaminate during deformation and also exhibit an unusual, and sometimes unique, combination of properties; they are not sure whether they want to be metals or ceramics.
While they conduct heat and electricity like metals, they are elastically stiff, strong, brittle, and heat-tolerant like ceramics. They are resistant to chemical attack, readily machinable, and thermal shock, damage tolerant, and sometimes fatigue, creep, and oxidation resistant.
Wow. Not only that, but MAX phase materials are very showy when it comes to nanoscale imaging. The one pictured at the top of this article caught our eye because it highlights the complex layering, and you can see more pics on Drexel’s MAX/MAXene page. For sheer artistry, this award-winning “MAX Dragon” image is our favorite:
The Key To A Better Lithium-Sulfur Battery
So, now that you have this a Ti2SC MAX phase nanolaminate, now what?
The Drexel researchers teamed up with some players at Aix-Marseille University in France to develop a highly stable cathode material for lithium-sulfur batteries based on their new nanolaminate.
Here’s a schematic illustrating their new lithium-sulfur battery:
You can read all about it in the journal Angewandte Chemie, but for those of you on the go, here’s the money quote from Drexel:
The researchers found that carbon/sulfur nanolaminates have covalent bonding between carbon and sulfur and an extremely uniform distribution of sulfur between the atomically thin carbon layers. This structure is key to their potential for being used as electrode materials for lithium-sulfur batteries.
Since we’ve been on a group hug kick lately, we’re not going to ask you for another group hug, but all you US taxpayers out there can go ahead and give yourselves a pat on the back, because the new research was supported by the Army Research Office and the National Science Foundation.
Drexel’s research into MAX phase materials is also supported by the Department of Energy’s Office of Basic Energy Science.
As for the probability of lithium-sulfur batteries in our future, Oak Ridge National Laboratory is among other research institutions hot on the trail of a solution to the degradation problem.