Environmental Engineering Reference
In-Depth Information
Capacity Fading
Capacity fading is a problem at lower temperatures. Capacity fading is a
reduction in battery capacity after repeated cycles. Li-ion batteries show bet-
ter performance than other secondary batteries applicable to EVs, but still
show capacity fading after a high number of cycles. Elevated temperatures
accelerate the rate at which capacity fading occurs with most lithium interca-
lation compounds. The number of cycles when the batteries are operated at
50°C or above are well below the EV requirement of >5000 cycles.
17
It is known
that most capacity loss at these high temperatures is due to degradation of
the electrode materials, particularly after a great number of cycles. Capacity
fading can also occur when a battery is stored at a high temperature. Storage
at 60°C for 60 days resulted in a 21% reduction in Li-ion battery capacity.
Loss of High-Rate Discharge Capacity
Operating temperatures of batteries vary from about -40 to +150°C. Li-ion
batteries typically fall in the range of 1 to 35°C. Unlike other types of bat-
teries, Li-ion battery performance can be affected significantly by operating
temperatures outside this range.
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The increased temperature will cause an
exponential increase in the rate at which undesirable chemical processes
occur.
These chemical reactions cause higher internal impedance that shortens
the life of a battery and in some cases leads to the decomposition of the bat-
tery. Even a slightly elevated operating temperature of 40°C can diminish
battery performance up to 35%. The batteries of high-power EVs must be able
to discharge at high rates. In some cases, the increase in storage temperature
(even to 60°C) reduces the high-rate discharge capacity by over 90%. Due to
the requirement of high-rate discharge for EVs, increased temperatures must
be avoided.
To make Li-ion batteries the industry standard for large applications, over-
heating and thermal runaway are not the only issues to consider. Costly
production is another issue. As stated earlier, the increased expense can be
attributed mostly to the costly metal cobalt used in the cathodes. Despite its
stability and high-rate capability, LiCoO
2
is plagued by toxicity and the high
cost of Co. If Co use could be eliminated, the cost of production would dras-
tically be reduced. In the past few years, a large research effort has focused
on replacing LiCoO
2
cathodes. As a result, Co has been partially and fully
replaced with Ni and/or Mn.
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One viable LiCoO
2
replacement material is the
Li(Ni
1/3
Mn
1/3
Co
1/3
)O
2
layered structure.
Other researchers believe they have found the cost solution in a cathode
made from a lithium iron phosphate (LiFePO
4
). This cathode involves inex-
pensive raw materials, but differs significantly from the lithium manganese
titanium oxide cathodes now in production.
20
The lithium iron phosphate
must undergo a much more rigorous and costly production.
