THERMAL MANAGEMENT
“If you try to force too many
lithium ions in at once and they
can’t move into the material fast
enough, that leads to lithium
dentrite forming”
Billy Wu, senior lecturer, Dyson School of Design Engineering at Imperial College
SAFETY IN SOLID STATE?
Solid-state lithium-ion cells appear to be
the most likely enabler of a generational
leap for electric vehicle batteries,
promising much higher-energy density
than liquid-electrolyte units and hopes
for faster charging, too. But its inherent
stability also offers safety benefits,
which could re-shape pack design.
“Solid-state solutions aren’t production
ready yet but do offer the ability to
eliminate internal short-circuiting
from dendrite growth and a nonflammable
interface between the current
collectors,” says Giles Muddell, chief
engineer of control systems at Prodrive.
“If they can be shown to meet the
energy density, power density, cost
and cycle life required of automotive
batteries, this would represent a
significant step in safety.”
Imperial College’s Wu also sees big
potential for the technology, including
removing the need for complex thermal
management systems, but he believes
consumer electronics will get it first.
“I think it’s more than 10 years away.
When we create a new chemistry it
drops into the existing manufacturing
systems that we have, whereas when we
manufacture solid-state batteries we’ve
got to change a lot of equipment and
processes. Engineering solutions,
like tweaking how you cool
the battery, are a lot faster
to implement.”
to lithium dendrite
shards of metallic
lithium forming.
“If that touches
between the anode
and cathode it causes a
short circuit, like if you put
a spanner across a lead acid
battery. When that happens, lots of current
fl ows and lots of heat is generated, which leads
to this feedback loop of thermal runaway, and
the battery catches fi re,” explains Wu.
“What you should do is measure the
temperature you’re charging at and,
if it’s very low, you gradually
ramp it up. You can also do
pulse charging where,
instead of constantly
applying current, you
periodically give the
battery time to rest.”
Professor Dr Andreas
Hintennach, senior
manager, battery research
at Daimler AG, also sees
eff ective management as a
vital component. “We pay a lot of
attention to the development of our
battery management systems, and we
program them in a way that dendrite
formation is very unlikely to happen,” he says.
The hard cell
But these incidents do come
with unique challenges, as Paul
Freedland, principal engineer at
Cosworth, points out: “The big
problem is that as the lithium-ion
cells start to burn, they provide both
the fuel source and the oxygen to
sustain combustion, which makes them very
di cult to extinguish, and keep extinguished.
The fi res also normally started due to internal
faults within the cells that can be very di cult
to detect prior to the onset of thermal runaway.”
Consumer expectations are also putting
additional stresses on batteries. New models
are off ering longer ranges, higher performance
and exponentially faster charging speeds. At
350kW, the Porsche Taycan is charging more
than 100 times faster than a domestic socket,
the typical method 10 years ago. This highvoltage
charging requires careful management
to avoid damaging the cells.
“Lithium-ion batteries work through what
we call a ‘rocking-chair mechanism’,” explains
Billy Wu, senior lecturer at the Dyson School
of Design Engineering at Imperial College.
“During charging, lithium ions move from the
cathode to the anode, which is normally made
of graphite. These materials are layered like
sheets of paper, and if you try to force too
many lithium ions in at once and they can’t
move into the material fast enough, that leads
3. Cell testing is a vital
element to battery
development and design
4. Battery electrodes as
seen under microscope
5. Battery management
systems are necessary
to prevent dentrite
formation occuring
www.electrichybridvehicletechnology.com // January 2020 // 53
3
4
5
/www.electrichybridvehicletechnology.com