A micrograph of silicon-graphene. Credit: XG Sciences.
When the U.S. Dept. of Energy (DoE) first put together their Electric Vehicle Roadmap
, speculating as to what the next generation of lithium-ion batteries might look like, they suggested silicon anodes. It has been well known for some time that silicon can store a great deal of lithium, an attribute that could provide a lithium battery with unprecedented storage capacity.
The problem was that no one knew how to make one of those that wouldn't self-destruct after a few dozen cycles. Technically speaking, the term is self-pulverization, which comes about as the result of the silicon swelling in size when loading up with lithium, and then shrinking back down when unloaded.
Not long after, a team of researchers at Northwestern University, led by Professor Harold H. Kung
, published a paper in the journal Advanced Energy Materials
, which said that by supporting silicon in a nano-composite graphene matrix, a high-performance battery could be produced using a silicon anode that could theoretically achieve a dramatic improvement in capacity as well as in charging time. They predicted that this technology could be expected to hit the market in three to five years.
They were wrong. Eighteen months later, XG Sciences
of Lansing, Mich., a company that spun off from Michigan State University, made an announcement. They had been producing graphene platelets for a variety of applications since 2010. Graphene is the basic building block of graphite which consists of a single layer of atoms. It has incredible properties. Besides being the strongest material known to man
, it is also quite flexible and highly conductive both thermally and electrically. No surprise then, that the two scientists, Andre Geim and Konstantin Novoselov, from the University of Manchester who first extracted it, in tiny quantities, using adhesive tape, won the 2010 Nobel Prize for doing so.
The production floor at XG Sciences, where graphene is manufactured into a powder. Credit: XG Sciences.
XG Sciences developed an efficient and economically viable production process, capable of producing 80 tons of graphene per year, the world's largest output. It's been used for everything from structurally reinforcing plastic materials, to thermally conductive paper used as lightweight heat sinks for electronics (XG Leaf), to anti-wear additives for lubricants, to additives used to improve electrical conductivity in batteries. Most of their product is distributed in powder form to over 600 customers in 32 countries, who then incorporate it into their various products. They have, by now, licensed the manufacturing process to two manufacturers: Cabot
. XG then brought in Rob Privette to expand their line of energy-related applications for the material
I spoke with Rob about their latest announcement, which appears to fulfill the expectation of that DoE roadmap for electric vehicles as well as some of the claims made by the Northwestern team.
Starting with their existing production process, and taking advantage of the unique combination of properties that graphene has to offer -- namely its strength, flexibility and electrical conductivity -- XGS was able to produce a nano-composite anode that involves silicon particles, wrapped with graphene platelets. The silicon still swells in size when it absorbs lithium, only with this super-strong and flexible structure holding it together, it does not break up as it did before. Instead, it exhibits impressively durable cycle life.
The result is an active battery material with a specific storage capacity that is four times greater than the current state of the art, based on ordinary graphite. The powdered material is moistened to form a slurry, which is then used to coat a foil electrode. According to the company announcement:
"Our new Silicon-graphene anode material, when used in combination with our existing xGnP® graphene products as conductive additives, provides significantly higher energy storage than conventional battery materials. This is great news for applications like smartphones, tablet computers, stationary power, and vehicle electrification that use rechargeable lithium-ion batteries. We are working with battery makers to translate this exciting new material into batteries with longer run-time, faster charging and smaller sizes than today's batteries."
The exact performance achieved will depend on the specific formulations used by the battery makers. Batteries made using this anode exhibit several characteristics that have long been considered "holy grails" of battery makers.
- high capacity - 1500 mA-hr/g as opposed to 372 for graphite.
- stable performance over its cycle life,
- 85 percent first cycle efficiency.
The cycle efficiency, which grows to 99.9 percent after several cycles, is not an exceptional number for batteries in general, though it is exceptional for silicon. More important though, it's good enough to get in the game as a serious commercial contender, which is something that has been a struggle for developers of silicon anodes until now.
"The key," said Privette, "is being able to use the silicon by virtue of the electrically conductive graphene mechanical support structure."
Privette does not, however, make the same claims about fast recharging times that Northwestern's Kung was suggesting. Perhaps that will come along in some future development. If it did, that could prove critical in the battle between batteries and hydrogen-powered fuel cells
for dominance in the electrical vehicle market. The ability to fill up a car with hydrogen in a few minutes, as one does today with gasoline, might be hydrogen's last significant advantage over a battery that can go for hundreds of miles on a charge, but still requires several hours to fully charge up.
As to where these materials might show up first, Privette said, "We expect initial adoption in the highly-competitive consumer electronics markets that are dominated by Asian battery makers. But we also have research and development partners that are focused on hybrid and electric vehicles, grid storage, military, and specialty industrial applications. Over time, we anticipate formulating custom nano-engineered anode materials with specific properties for each of these major markets."
XG Sciences' CEO Mike Knox made sure to add that, "in keeping with our focus on developing cutting-edge science for commercial applications, we engineered the nano-materials and processes to produce anode materials that are available today at commercially attractive prices."
There are actually two different ways to produce graphene. One is a "bottoms up" approach where the layers are built up using a vapor deposition process. The other, is a "tops down" approach, which starts with graphite and exfoliates it, as Geim and Noloselov did in their seminal work. This is the approach that XG Sciences uses, though in a highly automated, volume production manner rather than by hand using Scotch™ tape. The platelets, which are so thin as to be considered two-dimensional materials, range in size from 5 to 25 microns in diameter.
Two of XG's strategic partners, POSCO and Hanwha Chemical
, already manufacture electrode materials for lithium-ion batteries. And licensee Cabot Corp. recently introduced their first battery additive based on the technology. So it should not be long before batteries containing these new materials appear on the market.