07
Causes of ageing in lithium-ion cells
aLimhi
Somerville, Paul Jennings, Andrew McGordon, bChris Lyness
aUniversity
of Warwick, bJaguar Land Rover
Ageing in lithium-ion cells permanently degrades at least
one of two things…
All causes of ageing from literature can be broken down into 7 root causes!
How often we have to recharge
them (capacity)…
this information helps us mitigate correctly
Acceleration / top speed (power)…
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We have all bought a mobile phone that used to last for two days between charges and now we have to charge it twice a day, this is ageing. Although
annoying in a hand held device, in an electric vehicle where batteries cost £10,000 it is impractical to replace them every couple of years. Our aim is to
mitigate the causes / accelerants of ageing in lithium-ion cells used within the automotive industry. This will decrease ageing (capacity and power loss) and
feed into warranty provision for electric vehicles.
What does the literature report as causes of cell ageing?
Cycle number
Charge procedure
Time
Overpotential
Depth of discharge
Total coulombs
Compressive strength
Low discharge
Float charge
Material composition
Frequency
Pulse charge / discharge
Storage
Orientation
Vibration
?
Causes from lit. / Root causes State of charge Temperature
Cycle number
Charge procedure
Time
Overpotential
Depth of discharge
Total coulombs
Compressive strength
Low discharge
Float charge
Material composition
Frequency
Pulse charge / discharge
Storage
Orientation
Vibration
Percentage change in state of charge Current Vibration Pressure Orientation Beyond the scope
Test Dependent
Blue dots show how the causes
reported from literature can be
further broken down to the root
causes we have identified in our
work
N.B.
Although
temperature,
orientation, vibration and pressure
are not highlighted they will
affect all tests. However, a robust
test matrix would quantify them.
All of the root causes we have
identified cannot be further
broken down to any other
conditions. This is what defines a
root cause
What chemical mechanisms do these root causes excite?
Temperature
Percentage change in state of charge
Increased current rate
State of charge (SoC)
Work on ageing, as the name implies, is a long and arduous process. It often involves using millions of pounds worth of equipment for months or years at a
time. For this reason many studies use only a handful of cells, follow very different procedures for analysis or their duration is too short to be conclusive. It is
our hypothesis that many of the causes reported in literature are either not correct or are amalgamations of many of the real causes.
Orientation
Chemical mechanisms support our hypothesis
e.g. charge procedure is reported as increasing SEI growth…
Electrode (graphite)
Vibration
CONTRIBUTING FACTORS TOWARD
'CHARGE PROCEDURE' AND RELATIVE SEI
GROWTH
SEI
Solvent / electrolyte
Li+ ions
Solid electrolyte interphase (SEI) forms by reaction of electrolyte with
lithium-ions at the potential of the electrode, it is seen as the primary
cause of power fade in most lithium-ion cells
CURRENT
SoC vs Current
SEI growth
No available studies
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STATE OF CHARGE (%)
cURRENT (C-rate)
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sTATE OF CHARGE (%)
SoC vs Current
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2. Current
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SEI growth
These graphs shows that the SEI growth associated with charge procedure can be
further broken down to SoC and current
CATAPULT Conference
International Digital Laboratory
14th Nov. 2013
80
1. State of
charge
Charge
Procedure
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These flow charts show the root cause as it is associated with either the negative or positive electrodes then details the chemical reactions occurring and what effect that has on the consumer. Root causes of ageing like ‘increased temperature’
accelerate existing or instigate new chemical reactions which cause reductions in capacity and power. Knowing what these chemical reactions are and what truly causes them is the key factor in mitigation for improved cell life and better
warranty information.
These graphs shows that the SEI growth associated with charge procedure can be
further broken down to SoC and current
CONTRIBUTING FACTORS TOWARD
'CHARGE PROCEDURE' AND THE RELATIVE
SEI GROWTH
0
Pressure
3. Percentage
change in SoC
By removing or negating SoC, current rate or percentage
change in SoC we can reduce SEI growth. Therefore,
charge procedure is not the cause of ageing but rather a
mixture of current, state of charge and percentage change
in state of charge.
This work is a solid foundation for the next steps
1. Cycle / put into storage cells under each of these key root cause conditions
2. Produce a cell autopsy procedure in collaboration with Argonne National laboratories and
perform this on the cells conditioned in 1.
3. Collate this data and decide on the best course of action in mitigation of ageing
I. This will take the form of inputs or boundaries for the battery management system
II. Or better design of the module / pack / cell for automotive use
A cell being deconstructed. In the autopsy procedure
this will occur under inert conditions to ensure
further reactions do not occur