Today, lithium-ion
is one of the most successful and safe battery chemistries available. Two
billion cells are produced every year
Lithium-ion cells
with cobalt cathodes hold twice the energy of a nickel-based battery and
four-times that of lead acid. Lithium-ion is a low maintenance system, an
advantage that most other chemistries cannot claim. There is no memory, no
sulfation problem of lead acid that occurs when the battery is stored without
periodic topping charge. Lithium-ion has a low self-discharge and is
environmentally friendly. Disposal causes minimal harm.
Long battery runtime have always been the wish of many consumers. Battery
manufacturers responded by packing more active material into a cell and making
the electrodes and separator thinner. This enabled a doubling of energy density
since lithium-ion was introduced in 1991.
The high energy density comes at a price. Manufacturing
methods become more critical the denser the cells become. With a separator
thickness of only 20-25µm, any small intrusion of metallic dust particles can
have devastating consequences.
Recall of lithium-ion batteries
With the high
usage of lithium-ion in cell phones, digital cameras and laptops, there are
bound to be issues. The recall of defective battery (thermal problem) by Dell
and Apple bring many people to pay attention to the battery’s safety.
Sony Energy
Devices (Sony), the maker of the lithium-ion cells in question, says that on
rare occasions microscopic metal particles may come into contact with other
parts of the battery cell, leading to a short circuit within the cell. Although
battery manufacturers strive to minimize the presence of metallic particles,
complex assembly techniques make the elimination of all metallic dust nearly
impossible.
A mild short will
only cause an elevated self-discharge. Little heat is generated because the
discharging energy is very low. If, however, enough microscopic metal particles
converge on one spot, a major electrical short can develop and a sizable
current will flow between the positive and negative plates. This causes the
temperature to rise, leading to a thermal runaway, also referred to 'venting
with flame.'
Lithium-ion cells with cobalt cathodes (same as the recalled
laptop batteries) should never rise above 130°C (265°F). At 150°C (302°F) the
cell becomes thermally unstable, a condition that can lead to a thermal runaway
in which flaming gases are vented.
During a thermal runaway, the high heat of the failing cell
can propagate to the next cell, causing it to become thermally unstable as
well. In some cases, a chain reaction occurs in which each cell disintegrates
at its own timetable. A pack can get destroyed within a few short seconds or
linger on for several hours as each cell is consumed one-by-one. To increase
safety, packs are fitted with dividers to protect the failing cell from
spreading to neighboring cells.
Safety level of
lithium-ion systems
There are two basic types of lithium-ion chemistries: cobalt and
manganese (spinel). To achieve maximum runtime, cell phones, digital cameras
and laptops use cobalt-based lithium-ion. Manganese is the newer of the two
chemistries and offers superior thermal stability. It can sustain temperatures
of up to 250°C (482°F) before becoming unstable. In addition, manganese has a
very low internal resistance and can deliver high current on demand.
Increasingly, these batteries are used for power tools and medical devices.
Hybrid and electric vehicles will be next.
The drawback of spinel is lower energy density. Typically, a cell made of a
pure manganese cathode provides only about half the capacity of cobalt. Cell
phone and laptop users would not be happy if their batteries quit halfway
through the expected runtime. To find a workable compromise between high energy
density, operational safety and good current delivery, manufacturers of
lithium-ion batteries can mix the metals. Typical cathode materials are cobalt,
nickel, manganese and iron phosphate.
In fact, the lithium-ion batteries are safe and heat related failures
are rare. The battery manufacturers achieve this high reliability by adding
three layers of protection. They are: [1] limiting the amount of active
material to achieve a workable equilibrium of energy density and safety; [2]
inclusion of various safety mechanisms within the cell; and [3] the addition of
an electronic protection circuit in the battery pack.
These protection devices work in the following ways: The PTC device built into
the cell acts as a protection to inhibit high current surges; the circuit
interrupt device (CID) opens the electrical path if an excessively high charge
voltage raises the internal cell pressure to 10 Bar (150 psi); and the safety
vent allows a controlled release of gas in the event of a rapid increase in
cell pressure. In addition to the mechanical safeguards, the electronic
protection circuit external to the cells opens a solid-state switch if the
charge voltage of any cell reaches 4.30V. A fuse cuts the current flow if the
skin temperature of the cell approaches 90°C (194°F). To prevent the battery
from over-discharging, the control circuit cuts off the current path at about
2.50V/cell. In some applications, the higher inherent safety of the spinel
system permits the exclusion of the electric circuit. In such a case, the
battery relies wholly on the protection devices that are built into the cell.
operation comes from the outside, such as with an electrical
short or a faulty charger. Under normal circumstances, a lithium-ion battery
will simply power down when a short circuit occurs. If, however, a defect is
inherent to the electrochemical cell, such as in contamination caused by
microscopic metal particles, this anomaly will go undetected. Nor can the
safety circuit stop the disintegration once the cell is in thermal runaway
mode. Nothing can stop it once triggered.
What every battery user
should know
A major concern arises if static electricity or a faulty charger
has destroyed the battery's protection circuit. Such damage can permanently
fuse the solid-state switches in an ON position without the user knowing. A
battery with a faulty protection circuit may function normally but does not
provide protection against abuse. So it needs to consider second protection.
Another safety issue is cold temperature charging. Consumer grade lithium-ion
batteries cannot be charged below 0°C (32°F). Although the packs appear to be
charging normally, plating of metallic lithium occurs on the anode while on a
sub-freezing charge. The plating is permanent and cannot be removed. If done
repeatedly, such damage can compromise the safety of the pack. The battery will
become more vulnerable to failure if subjected to impact, crush or high rate
charging.
In summary, a good battery pack safety is related to many factors, such as:
1. Good quality cell selection,
2. Good battery pack structure design and assembling, strictly quality control
3. Set charge & discharge protection on device for first layer’s
protection.
4. Add protection circuit /BMS with the battery pack for second layer’s protection
5. Add additional protection like PTC, thermal fuse, 0V charge function not
allowed ... etc for third protection.
6. Set up proper charge method for the battery
7. User manual to the end-users for Precautions and Cautions!
P.S: Some information from Battery University.