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Safety First –
The Story of why all Lithium Batteries are not the same
Lithium ion batteries have no memory effect and significantly better energy
density than other types of batteries - but are they safe?
By V. Evan House, PhD; and Fayth Ross, MS, Altairnano Inc.
May 1, 2007
http://www.powermanagementdesignline.com/howto/showArticle.jhtml;jsessi
onid=3JRMJ1X5LSAOAQSNDLRCKHSCJUNN2JVN?articleID=199202938
We’ve all seen pictures of the infamous exploding laptop, and heard about or been affected by
the massive, unprecedented recall of lithium-ion batteries. In August of 2006, Dell recalled
4.1 million notebook lithium-ion batteries, and Apple Computer recalled 1.8 million batteries.
A month later, Panasonic recalled 6,000 batteries. As we’ve seen with this substantial recall,
current lithium ion packs have one significant drawback: safety.
Widely used in consumer electronics, you’ll find lithium-ion (Li-ion) batteries everywhere.
From cell phones, to laptops, Li-ion batteries have one of the best energy-to-weight ratios, no
memory effect, and a slow loss of charge when not in use. Li-ion batteries have two to three
times the energy density of nickel-cadmium and nickel-metal hydride batteries, and four
times the energy density of lead-acid batteries (higher energy density means lighter and
more compact batteries). But are they safe?
The safety of Li-ion batteries has been in question since before 2001, when the U.S.
Department of Transportation’s Research and Special Programs Administration implemented
increased safety checks, including puncture and crush tests, to ensure that the batteries could
be safely transported. According to the results, 120,000 lithium-ion cells caught fire during
the tests – strong evidence that the safety of the battery was in question.
Battery Basics
All batteries produce energy from electrochemical reactions. Batteries are comprised of
several components, but primarily consist of the following:
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A positive electrode and a negative electrode
An ionic electrolyte: a solution that contains and aids the movement of ions (charged
particles) back and forth between the two electrodes
A porous separator (ensures the two electrodes do not touch but allows ions to travel
back and forth between the electrodes)
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When a lithium-ion battery is charged, lithium ions travel from the positive electrode to the
negative electrode. On discharge, these ions return to the positive electrode releasing energy
in the process.
What caused the laptop blowup?
The cause of the battery blowup originated within the manufacturing of the battery cell itself.
Batteries contain several metal parts, which can sometimes result in undesirable metal
impurities within the battery. These impurities are typically sharp metal shards from the
battery casing or electrode material. In the case of the exploding batteries, the metal
impurities rested between an electrode and the separator. Through battery cycling and
changes in size in the negative electrode (due to cycling), the metal shard eventually
punctured the separator, causing the positive and negative electrode to bridge, resulting in a
short circuit. The short circuit produced high heat which ultimately resulted in fire.
High heat levels, fire, and/or explosion are all results of thermal runaway: a condition
whereby a battery can enter into an uncontrollable reaction. Thermal runaway is a process
where the internal temperature of a battery reaches a point (approximately 120°C) that
begins an irreversible reaction that is highly exothermic and causes fires.
Besides manufacturing defects, additional causes of thermal runaway include physical abuse
(such as banging a laptop around) and punctures. The nature of a traditional Li-ion battery is
that any type of short circuit will result in thermal runaway.
In response to overheating problems, the Japan Electronics and Informational Technology
Industries Association (JEITA) and the Battery Association of Japan released new safety
guidelines on April 20, 2007. Their guidelines stressed the importance of avoiding high-speed
charging using higher than rated voltages. According to JEITA, problems in the Li-ion
batteries occur at high charge voltages and when foreign substances are present in battery
cells.
JEITA was formed one week after Sony Corp. initiated a recall of batteries for its Vaio
notebook computers. JEITA is comprised of all PC vendors that used the trouble batteries
(excluding Apple Computer Inc.) and focuses on safely using Li-ion batteries in laptops.
Li-Ion in Cell Phones
Lithium-ion batteries are the battery choice for phone batteries, due principally to their
compactness and lightness. Despite the benefits, however, Li-ion batteries in cell phones still
have problems.
In cell phones, Li-ion batteries can overheat because of a short circuit or improper control. If
the temperature rises slowly, the battery can melt. If the temperature rises rapidly, enough
pressure can create a small explosion. Consumers have experienced severe burns as a result
of these failures. When not controlled, the chemical reaction that produces energy in a Li-ion
battery can be quite violent.
Sometimes deformed electrode plates are installed in batteries. If batteries are accidentally
subjected to strong external impact, which results in a surface dent or similar depression, the
deformed plates could pierce the batteries’ internal insulation – resulting in an electrical short
during, or right after, charging.
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Li-Ion in Electric Vehicles
The United States imports nearly 60% of the roughly 20 million barrels per day of total oil
consumed, which comes at a cost of nearly $260 billion dollars a year. In 2002, consumption
of oil for transportation alone was nearly double the level of domestic production, and in
2005, the Energy Information Administration reported that consumption outstripped domestic
production by nearly 4:1. According to the Department of Energy’s National Energy Policy
Group, transportation accounted for an estimated 66% of all oil consumed in 2001, or roughly
13 million barrels of oil a day.
A shift toward alternative transportation, such as battery or electric vehicles, is essential for
reducing oil dependency. An expected 6 million, or about 10%, of all cars sold globally will be
hybrid by 2010.
Popularity of electric vehicles (EV) is a step in the right direction toward a more efficient fuelbased economy, and major automotive manufacturers have embraced this technology.
Today, the Hybrid Electric Vehicle (HEV) market primarily uses nickel metal hydride (NiMH)
batteries. NiMH batteries, however, are expensive and self-discharge at a fast rate and are
generally agreed upon to not be the ideal solution. NiMH batteries must also be closely
controlled as they too can enter into thermal runaway conditions.
Traditionally used in consumer electronics and notebooks which require a limited amount of
energy, Li-ion use in electric vehicles has been under scrutiny. Li-ion advocates site Li-ion’s
high specific energy and low weight as ideal for EV adoption. Skeptics site cost, intolerance of
temperature extremes, and safety – safety being the single most significant hurdle in
adopting Li-ion batteries for use in electric vehicles and the sole reason that traditional Li-ion
batteries are not used in any EV today.
Battery electric vehicles typically have their batteries arranged in large battery packs of
varying voltage and capacity to supply the required energy to drive the vehicle. The drawback
to using current Li-ion batteries is the significant engineering required to both heat and cool,
as well as monitor the safety of the pack. Thermal runaway is a significant concern for
automotive applications because of the large number of batteries used in a battery pack. Any
scale of market adoption in the battery electric vehicle market will not occur until problems
with the thermal runaway are resolved.
On March 13, 2007, at the Geneva Motor Show in Switzerland, General Motors Corporation
Vice-Chairman Bob Lutz confirmed 2010 as the target year for production of the all-electric
Chevrolet Volt. While a prototype is expected by the end of 2007, Lutz cautioned that
uncertainty remains whether Li-ion batteries can be developed soon enough to power the Volt
both economically and safely.
Success in the transportation market for EVs, HEVs, and PHEVs is dependant upon a
significant improvement in battery technology. For all battery types, the inherent safety and
cost of the design is determined by the types of electrodes and electrolytes used, as is the
overall energy density capacity, power density capacity, and cycle life of the cell.
The SEI Layer in Common Li-Ion Batteries
Li-ion batteries generally use metal oxides for the positive electrode, carbon/graphite for the
negative electrode, and a lithium salt in an organic solvent for the electrolyte. The energy is
released in these batteries through the movement of lithium ions and electron processes at
the electrodes.
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When a traditional Li-ion battery is first assembled, it typically goes through a processing step
called “Formation”. Upon assembly, a traditional Li-ion battery is in an uncharged state and
the first step in the formation process is to charge the battery. The first charge results in the
formation of an electrolyte decomposition layer on the surface of the negative electrode due
to the high reactivity of graphite and lithium ions in the presence of the electrolyte. This layer
is called the Solid Electrolyte Interphase (SEI). The SEI layer is a crucial element of
traditional Li-ion batteries and acts as a safety feature by maintaining a protective barrier
between the reactive negative electrode and the electrolyte. The SEI is just porous enough to
allow the passage of lithium ions for low to moderate rate charge and discharges.
Safety Problems in Low Temperatures
On the downside, the SEI layer limits the discharge rates and seriously renders the battery
un-chargeable at cold temperatures and cannot be charged below -5°C. At cold temperatures
the pores in the SEI are effectively closed. If charging a cell at severely low temperatures is
attempted, lithium metal will plate on the surface of the SEI, resulting in two dangerous
conditions, either of which can cause the battery to enter thermal runaway.
Safety Problems in High Temperatures
If the temperature of the battery rises above 120°C the SEI layer dissolves and the negative
electrode can chemically react vigorously with the electrolyte. The SEI breakdown
temperature can be relatively easily achieved through aggressive charging or manufacturing
defects that result in internal shorting. Either reaction also causes the battery to enter
thermal runaway.
Safety First: The nLTO-Based Li-Ion Battery
One battery manufacturing company, Altairnano, has created a new Li-Ion battery with
multiple benefits, including the elimination of safety issues. Altairnano’s technology is based
on a nano-size lithium titanate oxide (nLTO) battery electrode material where nLTO is
substituted for graphite, the standard negative electrode material employed in common Li-ion
rechargeable batteries.
Unlike a traditional Li-ion battery, the nLTO-based battery undergoes no formation during the
first charge cycle. This is because the nLTO material is not reactive to the electrolyte in the
presence of lithium ions. Therefore, no SEI layer is formed. As discussed previously, the SEI
in a traditional Li-ion battery is a necessary component for the battery to operate safely.
However, this safety can be compromised quite easily as the SEI is meta-stable.
Safety in Low Temperatures
An nLTO-based battery can be charged to about 90% of its room temperature capacity in 30
minutes at -30°C. Other battery types including lead acid, NiMH, and NiCd show virtually no
charge acceptance at such low a temperature. Due to the lack of a SEI, the large surface
area, and the ease of lithium atom migration, charge kinetics achieved at normal
temperatures are retained at very cold temperatures in an nLTO-based battery.
Safety in High Temperatures
Within an nLTO-based battery, the nLTO does not react with the electrolyte. Therefore, an
nLTO-based battery operates safely at temperatures as high as 75°C. In fact, an nLTO
battery will perform better at elevated temperatures in terms of charge and discharge rates
than at room temperature, while maintaining its inherent safety. As an nLTO battery
operates at a lower voltage than a traditional Li-ion battery (2.3 vs 3.6), there is no danger of
encountering the failure modes associated with lithium metal deposition.
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Figure 1: High temperature testing results; Left – Traditional Li-Ion battery; Right – nLTO-based battery
(note the nLTO battery did not explode)
The discharge voltage limit of a traditional Li-ion battery must be tightly controlled as copper
used as the negative electrode current collector will dissolve in the electrolyte below about
2V. This can lead to shorting and thermal runaway. A traditional Li-ion battery must be
highly controlled in terms of its temperature and voltage states and has a limited window of
safety operation with only six regions of temperature and voltage safety stages. An nLTObased battery demonstrates a greater range of safety – anywhere from -40°C to +260°C.
Figure 2: Traditional Li-ion battery safe temperature ranges and voltage states
are confined to a relatively small window
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Figure 3: nLTO-based battery safe temperature ranges and voltage states
indicate a greater margin of safety (from -40°C to +260°C)
Safety in All Tests
All Li-ion batteries should undergo thermal runaway tests including deliberate shorting, via
nail penetration tests and thorough super heating the battery to initiate thermal runaway at
high temperatures. nLTO-based batteries exhibit pass marks with no smoke or flames in short
circuit, forced discharge, over charge, over discharge, nail puncture, crush, over temperature,
and drop tests. Furthermore, nLTO-based battery packs do not need significant cooling or
temperature monitoring, thereby reducing costs and eliminating safety concerns.
About the authors
V. Evan House, PhD is the Director of Advanced Materials & Power Systems at Altairnano email: [email protected]; and Fayth Ross, MS is the Marketing Manager at Altairnano
- email: [email protected].
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