How do lithium-ion batteries work,
and how does this contribute to their potential dangers?
Last week, Risk Bites looked at the hazards and risks of these batteries. This week, we
dive deeper into the science of how they work.
Lithium-ion batteries are a great technology,
they allow large amounts of energy to be squeezed into a very small volume.
And they allow that energy to be extracted very fast.
This is good news for users of cell phones, computers, and digital cameras.
It's even better news if you have an electric car and want to drive it further
than the corner store. And it's fantastic news for airlines,
where lighter planes reduce environmental impact and increase profits.
But these benefits are not risk-free.
Lithium-ion batteries store energy chemically and release it electrically.
Lithium-rich material, such as lithium cobalt oxide (the battery's positive
electrode) is separated from a lithium-storing material such as carbon
(the negative electrode).
The stuff that separates the electrodes - the electrolyte - allows
positively charged lithium ions to move from one material to the other.
This electrolyte is usually based on an organic solvent.
When the battery is charged, lithium ions are pushed up a chemical energy-slope
from the positive electrode into into the negative one.
Now we have an electrode stuffed with lithium that wants nothing more than to
slip back down that energy slope to where it came from.
But it can't do this without some help.
In moving the lithium from one electrode to the other, a second energy-slope has
been created - an electrical one.
Each lithium ion that moves to the carbon is associated with an electron
that would love to slip back down to the positive electrode.
But it can't unless an electrical connection is made.
Making this connection allows the electrons to flow, and for each
electron that makes the journey, a lithium ion is kicked out of the carbon
and slides down the corresponding chemical energy slope.
Electrons flow, ions move, the battery discharges and work is done. So far,
so good.
The battery can store a lot of energy because you can squeeze in a lot of
lithium ions - which are relatively small - inside the carbon electrode.
And because the ions can move easily between the electrodes, this energy can be
extracted fast - which means lots of power.
So where are the hazards? And what turns these into risks?
The primary hazard - the thing that leads to the most safety problems with lithium-
ion batteries -
is the electrolyte.
In many cases this is flammable and corrosive - fine if it stays where it's
supposed to,
but not so good if it gets out.
As last week's Risk Bites mentioned, the two things that you do not want to do with
a lithium-ion battery are to mechanically compromise it, so that the
electrolyte is released, or to allow it to get too hot.
And the thing that you really don't want to do is to allow both to happen at
the same time!
The problem is,
the properties that make lithium-ion batteries so useful also make them
potentially dangerous.
As those lithium ions roll down the chemical energy slope they
release energy in the form of heat -
the more electrical current that's drawn, the more ions roll down the slope
and the hotter the battery gets.
Usually, great care is taken to limit the current that can be drawn.
But if the battery short circuits, things can get very hot, very fast.
This is not a problem if the battery remains intact.
But if there's a mechanical breach, the subsequent release of electrolyte into a
hot environment could be a fire and corrosion hazard.
Because of this,
an internal short circuit is the last thing you want.
Lithium-ion batteries have an insulating membrane between the electrodes to
prevent this from happening,
but if this is compromised, electrons flow , ions move,
and we have a problem.
To make matters worse, the hotter the battery gets, the more readily the lithium ions
roll down the chemical energy slope and the faster things heat up.
In the worst case, this positive feedback loop leads to thermal runaway and a
battery that thinks it's an incendiary device.
Internal short circuits can be caused by manhandling.
But they can also come about in other ways.
As the negative electrode becomes stuffed with lithium atoms, it expands.
Too much lithium and this expansion can mechanically stress the
battery and compromise the internal insulation.
Overcharging can also lead in some cases to electron-conducting metallic
deposits between the electrodes -
good news for electrons,
not so good for anyone using the battery.
well-designed lithium ion batteries that are used sensibly
are relatively low risk, as the inherent hazards are well contained.
But as with many complex technologies, a great deal of diligence is needed to
ensure the risks remain low, irrespective of hazards.
For more information on lithium-ion batteries,
make sure you check out the links below. And stay safe.