One of the biggest problems with batteries is the time it takes to recharge them. Run out of juice and it’ll be several hours before you’re mobile again, a particular showstopper for electric vehicles.
Today, Ibrahim Abou Hamad at Mississippi State University and few buddies reveal an entirely new technique for charging lithium ion batteries that could lead to exponential improvements in charging time.
The business end of a lithium battery, the anode, consists of a graphite electrode, in other words a stack of graphene sheets, bathed in an electrolyte of ethylene carbonate and propylene carbonate molecules through which lithium and hexafluorophosphate ions diffuse. During charging, an electric field pushes the lithium ions towards and into the graphene sheets, where they have to cross a potential barrier to become embedded and stored, a process called intercalation.
The Mississippi team have studied the movement of these ions and molecules by creating a computer model of the forces acting on them. Their model consists of 160 carbon atoms arranged in 4 graphene sheets, 69 propylene carbonate and 87 ethylene carbonate molecules forming a liquid electrolyte and finally, two hexafluorophosphate ions and10 lithium ions. They then apply an electric field across this system and watch what happens.
It turns out that while the electric field pushes the lithium ions towards the graphene, the rate limiting step is the process of intercalation–the rate at which the lithium ions can cross the potential barrier into the graphene .
What Hamad and co have found is a relatively simple way to overcome this barrier. The trick is to superimpose an oscillating electric field onto the charging field. This has the effect of helping the lithium ions to hop over the barrier.
But get this: the team says there is an exponential relationship between the intercalation time and the oscillating field amplitude. So a small increase in amplitude of the field leads to a massive speed up of the process of intercalation.
“These simulations suggest a new charging method that has the potential to deliver much shorter charging times, as well as the possibility of providing higher power densities,” they say.
That’s a neat piece of work which should be relatively straightforward to test in a real battery.
That doesn’t mean that we’ll see a ten minute charging time for electric vehicles any time soon.
Battery performance is a complicated balance between huge numbers of competing factors. If this oscillating field does improve charging time in real batteries, manufacturers will then have to check its effect on other performance metrics such as the number of these charging cycles a battery can withstand and how long it holds its charge, to name just two.
Nevertheless, these Mississippi guys have come up with an interesting new approach that will have more than peaked the interest of battery makers around the globe.
Ref: arxiv.org/abs/1003.1678: A New Battery-Charging Method Suggested By Molecular Dynamics Simulations