Environmental Engineering Reference
In-Depth Information
the high cost of cobalt and to the complex protective circuitry required by
every battery.
Although cost and overcharging are issues for Li-ion batteries, the most
prevalent problem seems to be overheating, especially in larger applica-
tions.
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Overheating can cause significant decreases in performance ratings
and even catastrophic failure. If Li-ion batteries are to be used more effi-
ciently and for larger scale applications, the heat issues must be resolved.
The primary concerns regarding high temperatures in Li-ion batteries are
(1) increase in the probability of thermal runaway, (2) loss in capacity, (3)
increased undesirable side reactions.
Thermal Runaway
The potential for thermal runaway is the most important concern with high-
temperature operation. If a battery consistently generates more heat than it
can dissipate, a condition known as thermal runaway can occur.
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It can
eventually lead to leaks, venting of gas, and possible explosion or fire. Recent
recalls of laptop batteries due to isolated thermal runaway events increased
this concern. To avoid thermal runaway, temperatures in a cell must be kept
below 105 to 145°C, depending on the state of charge (SOC).
Thermal runaway, or the extreme overheating of a battery, can arise for a
number of reasons. Safety issues generally occur if a cell exceeds the critical
temperature above which the increase of temperature is irreversible due to
the heat produced by the cathode, anode, and electrolyte and their interac-
tions above the critical temperature.
Another cause of thermal runaway may be excessive ambient tempera-
tures. Computers, cell phones, and other portable devices are often exposed
to high ambient temperatures that can be caused by nothing more than the
sun. Portable electronic devices stored in a car on a hot day or left next to
a heater will be exposed to temperatures well above the level necessary to
trigger thermal runaway. Excessive ambient temperatures will lead to elec-
trolyte heating, thus increasing the exothermic chemical reactions.
A third source may be overcharging, leading to build-up of lithium depos-
its that eventually penetrate the separating film and short the two electrodes.
All Li-ion batteries are equipped with protective circuitry to avoid overcharg-
ing, but if the protective circuitry fails, Li ions will build up on the graphite
anode, forming a lithium dendrite. If the charging continues, the dendrite
will grow until it penetrates the separator, creating a short circuit by con-
necting with the cobalt oxide cathode.
Regardless of the cause of thermal runaway, the result is the same and in
many cases very dangerous. If Li-ion batteries are to be used on a larger scale
and work more safely, they must be unaffected by higher temperatures or
cooled more effectively to maintain lower temperatures. Many theories for
solving this issue have been proposed but they are too costly or significantly
alter the weight, size, and durability advantages of Li-ion batteries.
