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                <p>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. </p>
<p>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.</p>
<p>The business end of a lithium battery such as <a rel="nofollow"
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battery</a>, 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. </p>
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<p>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. </p>
<p>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 .</p>
<p>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. </p>
<p>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.</p>
<p>"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.</p>
<p>That's a neat piece of work which should be relatively straightforward
to test in a real battery.</p>
<p>That doesn't mean that we'll see a ten minute charging time for
electric vehicles any time soon.</p>
<p>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.</p>
<p>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.</p>                <!--INFOLINKS_OFF-->
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posted:10/19/2011
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