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Ryan Uyehara
9739241279
Writ 340
May 7, 2013
The Dangers of Lithium-Ion Batteries
Abstract
The two recent fires aboard Boeing’s new 787 Dreamliner have brought the safety
of lithium-ion batteries into question. By being often made of materials that are
thermally unstable, batteries can enter a state called thermal runaway in which the
battery continually gets hotter. However, with the implementation of alternate
cathode compounds and other safety measures, and through the refining the
manufacturing process, lithium-ion batteries have been and will continue to become
extremely safe to use.
Introduction
Lithium-ion batteries are in many of the portable electronics we own; yet in
some applications, they can kill us. Spontaneously combusting lithium-ion batteries
aboard two of Boeing’s new 787 Dreamliners have sparked an international
investigation into the battery design. Furthermore, with fires breaking out on the
planes, the entire Dreamliner fleet has been grounded, costing Boeing and airlines
millions of dollars.
However, despite the risk of spontaneously combusting, these batteries have
valuable benefits including the ability to store the most energy for its size and
weight and the ability to charge and discharge for many cycles. With these benefits,
lithium-ion batteries have become a common source of energy in countless devices
we have, and are already starting to be used in entirely different applications, such
as the Boeing 787. In an effort to combat the rising price of jet fuel, the Dreamliner
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relies heavily on electricity from these batteries to power the plane instead of
relying on jet fuel.
Figure 1: Energy densities of various battery types
Data source: C. Zu and H. Li, “Thermodynamic analysis on energy densities of
batteries,” Energy and Environmental Science, 2011.
Even though the large lithium-ion batteries aboard the 787 caught fire, small
lithium-ion batteries commonly found in electronics have had an almost entirely
explosive-free history and are extremely safe. Therefore, these larger batteries need
to be further studied and made safer before being applied to other large
applications.
How Lithium-Ion Batteries Work
Lithium-ion batteries are able to store and release electricity by moving
lithium ions back and forth between the two sides of the battery. Much like other
batteries, the lithium-ion battery is composed of a positive electrode (cathode), a
membrane that prevents the positive terminal from touching the negative terminal
(separator), a solution (electrolyte), and a negative electrode (anode). During the
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charging process, these components are used to store electrons and energy in the
battery. First, electrons are inserted into the battery’s cathode by the charging
device, which is usually an outlet on the wall. Once in the cathode, the electrons
draw out the lithium ions that exist in the lithium-based compound in the cathode.
The voltage difference across the battery then pushes the lithium ions and electrons
through pores in the separator membrane to the other side of the battery. After
arriving at the anode, the lithium ions and electrons attach to the carbon-based
material in the anode.
Figure 2: The charging process of a lithium-ion battery
Modified From: J. Yuan, X. Liu, and H. Zhang, Lithium-Ion Batteries, Boca Raton, FL:
CRC Press, 2012.
During the discharging process, this process is reversed. The device that the
battery is powering creates a voltage difference across the battery. This creates a
force that causes the lithium ions and electrons to detach from the anode and get
pushed back to the cathode. Once in the cathode, the electrons leave the battery to
power the connected device while the lithium ions recombine with the compound in
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the cathode (J. Yuan et al.). In other words, the two electrodes share the lithium ions.
When the battery is charging, the anode gets to have the lithium ions and the
electrons, but when the battery is discharging, the cathode gets to have the lithium
ions.
Lithium-Ion Battery Potential Failures
Abusing a lithium-ion battery will cause the battery to be more likely to
breakdown. By storing the battery at high temperatures, charging the battery past
normal levels, physically crushing the battery, having the cathode and anode touch
(creating a short circuit), or by abusing it in other ways, the battery can be placed in
situations that exceed the normal range of conditions it is designed for. In turn, the
materials in the battery can start to degrade and trigger chemical reactions that
release heat. This increase in heat can cause more chemical reactions that release
more heat and in turn cause the temperature of the battery to exponentially
increase through a process called thermal runaway (Balakrishnan, Ramesh, Kumar).
This thermal runaway can not only completely destroy the battery, but also be a
hazard to anything around it should the rapid increase in temperature cause it to
catch fire.
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Figure 3: Boeing 787 battery after undergoing thermal runaway
Source: M. Ahlers. Dreamliner battery type requires safeguards, safety advocate says
[Online] Available: http://www.cnn.com/2013/02/06/travel/lithium-ion-batteries
While there are countless types of lithium-ion batteries including lithium
manganese oxide and lithium iron phosphate, the most commonly used, and the
type found in the Dreamliner, is lithium cobalt dioxide. Lithium cobalt dioxide is a
particularly hazardous cathode compound because it decomposes and releases
oxygen at high temperatures. This oxygen reacts with the electrolyte to speed up the
chemical reactions that trigger thermal runaway. Furthermore, the release of
oxygen causes the battery to provide fuel for its own fire should a fire start, making
it similar to a can of gasoline catching on fire since the gasoline would only further
increase the fire.
With this increased hazard, batteries that are made of lithium cobalt dioxide
or other cathode compounds that release oxygen can create a major problem for
airplanes that utilize this technology such as the Boeing 787. These batteries that
undergo thermal runaway can spark fires that are more difficult to extinguish than
traditional fires. This is because there is no way to stop thermal runaway once it
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starts since thermal runaway is a chain reaction process that will continue until all
of the reactants are gone. Chairman of the National Transportation Safety Board,
Deborah Hersman, pointed out “They (aircraft) don't have the opportunity to pull
over when there's a fire. They don't always have accessible compartments. They
don't have fire suppression in some of those areas” (M. Ahlers). Without a
substantial fire suppression system, these battery fires can be catastrophic to
airplanes. Therefore, since there are currently no materials that offer both a high
energy density and safety, Boeing’s choice to use a lithium cobalt dioxide battery
maximized the energy density of the battery, but increased the risk of the plane
becoming a flying fireball.
With the investigation into the cause of the battery fires still ongoing, the
exact cause of the fires is sill unknown. The most likely cause is an error in the thin
film separator between the cathode and the anode. With a battery that is
considerably larger than those found in electronics, the separator needs to be
relatively large (about 35,000 square centimeters) in order to prevent the cathode
and anode from touching and causing the battery to short circuit and potentially
enter thermal runaway. Therefore, the Boeing battery has an increased probability
that an error could occur in the manufacturing process or through normal wear and
tear. A small, singular error in the form of a hole in the 35,000 square centimeters of
material that separates the terminals can lead to a short circuit, which can in turn,
generate the heat needed to spark thermal runaway and a fire (J. Paur).
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Solutions
In order to prevent errors in the separator from initiating thermal runaway
in such large energy storage devices, current research is developing a separator
coating that ensures that the separator possesses the capability to shut down the
battery. The separator membrane contains small pores that are designed to close if
the battery enters thermal runaway in order to shut down the battery. However,
errors in the manufacturing process and normal wear and tear can cause holes to
form in the separator. Should a hole form, the separator will not be able to shut
down the battery because it will not be able to close the hole, allowing the battery to
potentially catch on fire. In order to prevent holes from causing thermal runaway,
current research at the Department of Energy’s Energy Frontier Research Center is
developing coatings for the separator that melt at certain temperatures in order to
plug both the pores and holes in the separator and shut down the battery.
Researchers have discovered that by coating each side of the separator with
extremely small, heat-sensitive capsules, the separator will be able to operate
normally at room temperature, but will be able to be completely sealed off as the
temperature increases and the capsules melt to seal all of the pores and holes in the
separator (P. Glynn). This coating is essentially a sensor in that it shuts down the
device if conditions exceed a threshold level. In doing so, this coating can completely
isolate the two electrodes by preventing ions from moving between the electrodes
and in turn, shut down the battery and prevent thermal runaway.
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Figure 4: The melting of small, heat-sensitive capsules as temperature
increases
Modified From: P. Glynn. Preventing Laptop Fires and “Thermal Runaway” [Online]
Available: http://science.energy.gov/discovery-andinnovation/stories/2012/127035/
As battery technology continues to improve, more safeguards are being
developed, such as alternative cathode compounds, which prevent the battery from
releasing oxygen in thermal runaway. One such alternative for large lithium-ion
batteries is lithium iron phosphate (LiFePO4), which is much safer than lithium
cobalt dioxide. Even though the compound contains oxygen, the oxygen is not
released, even at high temperatures, because the chemical bonds between iron,
phosphate, and oxygen are much stronger than the bonds between cobalt and
oxygen atoms (J. Voelcker). With stronger bonds, lithium iron phosphate is less
likely to enter thermal runaway.
Even though lithium iron phosphate has a lower energy density and capacity
than lithium cobalt dioxide, lithium iron phosphate can increase its energy density
through nanotechnology. By using nanotechnology to make the compound more
conductive, lithium iron phosphate will be able to generate more power and thus,
become more energy dense (J. Yuan et al.). With this technology still in the testing
phase, researchers are still working to further improve the energy density of these
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batteries. Even with this technology, the energy density of this compound will still
be slightly below the output levels of lithium cobalt dioxide batteries. However,
nanotechnology at least allows the energy density of lithium iron phosphate
batteries to be comparable to other commonly used battery types, such as those
found in the current Boeing 787 design. Therefore, by switching from traditional
lithium cobalt dioxide to lithium iron phosphate compounds, large lithium-ion
batteries such as those found in the Dreamliner can reduce the risk of thermal
runaway and fires, while only slightly reducing conductivity and power output.
Figure 5: Comparison of different cathode materials
Data source: J. Yuan, X. Liu, and H. Zhang, Lithium-Ion Batteries, Boca Raton, FL: CRC
Press, 2012.
Future
Despite the recent large lithium-ion battery fires on the Boeing 787, the small
lithium-ion batteries found in electronics are extremely safe. As the manufacturing
process for small batteries has been refined, the failure rates of these batteries have
dropped to almost zero with computer batteries having a failure rate of one in 10
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million (M. Ahlers). Therefore, as the manufacturing processes for larger lithium-ion
batteries, such as those found in the Dreamliner, become more refined and alternate
safeguards are implemented, these too will become extremely safe, probably within
the next decade.
With these larger batteries having lower failure rates, they will not only be
able to be used in aircrafts, but also in electric utility applications as well.
Renewable energy generation facilities such as wind and solar farms are currently
only able to provide power to the grid when the wind is blowing or the sun is out.
However, by placing large lithium-ion batteries at every intermittent generation
facility, these farms can store any excess energy from the wind or sun and feed the
stored energy into the grid even when it is cloudy and there is no wind. In doing so,
we will be able to decrease our dependence on fossil fuels and make the world a
better place.
Author Biography
Ryan Uyehara is a junior at the University of Southern California and is pursuing a
bachelors of science degree in electrical engineering. Ryan’s interests primarily
relate to the power field.
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Multimedia Applications
Java application showing electrons and lithium ions moving through the battery as it
charges and discharges.
Links to Related Sites
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http://www.thedailybeast.com/newsweek/2013/01/13/dreamliner-snightmare.html
S. Megahed and B. Scrosati, “Lithium-ion rechargeable batteries,” J. of Power
Sources, vol. 51, pp. 79-104, Aug.-Sept. 1994.
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Bibliography
C. Zu and H. Li, “Thermodynamic analysis on energy densities of batteries,” Energy
and Environmental Science, 2011.
J. Paur. Boeing Dreamliner Investigation Focuses on Combusting Batteries [Online]
Available: http://www.wired.com/autopia/2013/01/boeing-787-investigationbatteries/
J. Voelcker, “Lithium Batteries Take to the Road,” IEEE Spectrum, Sept. 2007.
J. Yuan, X. Liu, and H. Zhang, Lithium-Ion Batteries, Boca Raton, FL: CRC Press, 2012.
M. Ahlers. Dreamliner battery type requires safeguards, safety advocate says [Online]
Available: http://www.cnn.com/2013/02/06/travel/lithium-ion-batteries
P.G. Balakrishnan, R. Ramesh, T. Prem Kumar, “Safety mechanisms in lithium-ion
batteries,” J. of Power Sources, vol. 155, pp. 401-414, Apr. 2006.
P. Glynn. Preventing Laptop Fires and “Thermal Runaway” [Online] Available:
http://science.energy.gov/discovery-and-innovation/stories/2012/127035/