OPINION
40 // January 2020 // www.electrichybridvehicletechnology.com
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The redesign of a lith ium-ion batter y cel l t o m ake
it easier to cool could lead to more aff ordable EVs
Greg Offer
Cost – it is one of the ultimate factors influencing the
buying decision of an electric vehicle. Questions over
range anxiety and charging infrastructure are still important
but are frankly secondary as pre-order figures show there
are already more than enough people who want to buy an
electric car.
For example, over 400,000 people put a deposit down
on a Tesla Model 3 without even sitting in one. There has
probably never been a model launch in the history of the
automotive industry with this much excitement and
demand. The only thing holding the rest of us back from
buying a car now is quite simply cost. Like the majority of
prospective new EV owners, I’m just waiting another year
or two for a bit more choice, and for the prices to drop down
a little, and then along with millions of others I will be
making the switch.
The key to making this happen faster is battery
technology. Previously the
automotive industry has been
focused on the energy density of
the lithium ion batteries, as this was
necessary to develop vehicles with
enough range. Cell manufacturers
have worked hard and have
focused on squeezing as much
energy in as possible. Some of the
compromises when increasing
energy are well understood – you
Most lithium-ion cells
are signifi cantly
sub-optimized for
thermal management
can’t have both high power and
energy, and some of the new materials such as silicon
in the anode and high nickel cathodes can increase
degradation and shorten lifetime. The compromises
can be acceptable if you understand the trade-o s.
However, some of the unintended consequences
are not as well understood. In a recent Elon Musk
interview, he talked about how dangerous it is for
engineers to fall into the sub-system optimization
trap. I recently came to the conclusion that the
battery industry has fallen into this trap for their
cells, at the expense of the battery pack.
The latest research in our group supports this hypothesis,
as we have demonstrated that most lithium-ion cells are
significantly sub-optimized for thermal management.
Essentially, they are very di icult to keep cool, and the only
way to prevent them overheating compromises their
performance and increases degradation. We believe this has
happened because there is no easy way to compare how
easy it is to cool one cell compared to another. Instead, the
industry has competed on energy
density, minimizing any material that
is not contributing energy.
Our models show that by
redesigning a cell to be easier to
cool would decrease cell energy
density by a few percent, but could
increase energy density at the
system level by up to 20%. This
could either increase range or
reduce cost.
In response, we have proposed
a new metric – the cell cooling coe icient – which is an
empirically measured standard. Comparison drives
competition, hence if adopted, manufacturers will begin
to compete to design better cells. In the same way, pack
designers will have a metric to pick the best cells, enabling
them to design better battery packs. The end result of
cheaper electric vehicles will benefit everyone.
Dr Gregory Offer is a Reader in mechanical engineering at
Imperial College London. His research focuses on battery,
fuel cell and supercapacitor technologies, mainly in transport
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