Hi All
Lithium-ion batteries (sometimes abbreviated Li-ion batteries) are a type of
rechargeable battery
commonly used in
consumer electronics.
They are currently one of the most popular types of battery for
portable electronics,
with one of the best
energy-to-weight ratios,
no
memory effect,
and a slow
loss of charge
when not in use. They can be dangerous if mistreated, however, and unless
care is taken their
lifespan
may be reduced. Although originally intended for consumer electronics,
Lithium-ion batteries are growing in popularity with the defense and
aerospace industries
because of their high energy density.
A more advanced lithium-ion battery design is the
lithium polymer cell.
History
Lithium-ion batteries, first proposed in the 1960s, came into reality once
Bell Labs
developed a workable graphite anode
to provide an alternative to lithium metal, the
lithium battery.
Following groundbreaking cathode research by a team led by
John Goodenough
Oxford University,
now at the
University of Texas,
Austin), the first commercial lithium ion battery was released by
Sony
in 1991. Used in numerous commercial applications these batteries
revolutionized consumer electronics.
One of the latest uses is in
hybrid electric cars
and eventually
electric vehicles,
as
commodity cells.
Tesla,
Reva
and
Kewet
are all releasing new lithium ion battery electric car models in 2007.
Advantages and disadvantages
Advantages
Lithium-ion batteries can be formed into a wide variety of shapes and sizes
so as to efficiently fill available space in the devices they power.
Li-ion batteries are lighter than other equivalent
secondary batteries
-often much lighter. The energy is stored in these batteries through the
movement of
lithium
ions.
Lithium is the third lightest element, giving a substantial saving in weight
compared to batteries using much heavier metals. However, the bulk of the
electrodes
are effectively "housing" for the ions and add weight, and in addition "dead
weight" from the electrolyte, current collectors, casing, electronics and
conductivity additives reduce the charge per unit mass to little more than
that of other rechargeable batteries. A key advantage of using Li-ion
chemistry
is the high
open circuit voltage
that can be obtained in comparison to
aqueous batteries
(such as
lead acid,
nickel metal hydride
and
nickel cadmium).
Li-ion batteries do not suffer from the
memory effect.
They also have a low self-discharge rate of approximately 5% per month,
compared with over 30% per month in
nickel metal hydride
batteries and 10% per month in
nickel cadmium
batteries.
According to one manufacturer, Li-ion cells (and, accordingly, "
dumb"
Li-ion batteries) do not have any
self-discharge
in the usual meaning of this word.
What looks like a self-discharge in these batteries is a permanent loss of
capacity, described in more detail below. On the other hand, "smart" Li-ion
batteries
do self-discharge, due to the small constant drain of the built-in voltage
monitoring circuit. This drain is the most important source of
self-discharge
in these batteries.
Disadvantages
A unique drawback of the Li-ion battery is that its life span is dependent
upon aging from time of manufacturing (shelf life) regardless of whether it
was
charged, and not just on the number of charge/discharge cycles. So an older
battery will not last as long as a new battery due solely to its age, unlike
other batteries. This drawback is not widely publicised.
At a 100% charge level, a typical Li-ion
laptop
battery that is full most of the time at 25 degrees
Celsius
or 77 degrees
Fahrenheit
will irreversibly lose approximately 20% capacity per year. However, a
battery stored inside a poorly ventilated laptop may be subject to a
prolonged exposure
to much higher temperatures than 25 °C, which will significantly shorten its
life. The capacity loss begins from the time the battery was manufactured,
and occurs even when the battery is unused. Different storage temperatures
produce different loss results: 6% loss at 0 °C(32 °F), 20% at 25 °C(77 °F),
and 35% at 40 °C(104 °F). When stored at 40% charge level, these figures are
reduced to 2%, 4%, 15% at 0, 25 and 40 degrees Celsius respectively.
Li-ion batteries can even go into a state that is known as deep discharge.
At this point, the battery may take a very long time to recharge. For
example,
a laptop battery that normally charges fully in 3 hours may take up to 42
hours to recharge. Or the deep discharge state may be so severe that the
battery
will never come back to life. Deep discharging only takes place when
products with rechargeable batteries are left unused for extended periods of
time
(often 2 or more years) or when they are recharged so often that they can no
longer hold a charge. This makes Li-ion batteries unsuitable for back-up
applications
compared to lead-acid batteries, or even to
nickel metal hydride
batteries.
Because the maximum power that can be continuously drawn from the battery
depends on its capacity, in high-powered (relative to C, the battery
capacity
in A·h) applications, like portable computers and video cameras, rather than
showing a gradual shortening of the running time of the equipment, Li-ion
batteries may often just abruptly fail.
Low-powered cyclical applications, like mobile phones, can get a much longer
lifetime out of a Li-ion battery.
A stand-alone Li-ion cell must never be discharged below a certain voltage
to avoid irreversible damage. Therefore all Li-ion battery systems are
equipped
with a circuit that shuts down the system when the battery is discharged
below the predefined threshold.
It should thus be impossible to "deep discharge" the battery in a properly
designed system during normal use. This is also one of the reasons Li-ion
cells
are rarely sold as such to consumers, but only as finished batteries
designed to fit a particular system.
When the voltage monitoring circuit is built inside the battery (a so-called
"smart" battery) rather than the equipment, it continuously draws a small
current
from the battery even when the battery is not in use; furthermore, the
battery must not be stored fully discharged for prolonged periods of time,
to avoid
damage due to deep discharge.
Li-ion batteries are not as durable as
nickel metal hydride
or
nickel-cadmium
designs and can be extremely dangerous if mistreated. They are usually more
expensive.
Li-ion chemistry is not safe as such, and a Li-ion cell requires several
mandatory safety devices to be built in before it can be considered safe for
use
outside of a laboratory. These are: shut-down separator (for
overtemperature), tear-away tab (for internal pressure), vent (pressure
relief), and thermal
interrupt (overcurrent/overcharging).
The devices take away useful space inside the cells, and add an additional
layer of unreliability. Typically, their action is to permanently and
irreversibly
disable the cell.
Li-ion batteries are the subject of frequent recalls (see
#Controversy).
The number of safety features can be compared with that of a
nickel metal hydride
cell, which only has a hydrogen/oxygen recombination device (preventing
damage due to mild overcharging) and a back-up pressure valve.
There is ongoing research to develop alternative Li-ion chemistries that
would be safe with fewer or no safety devices, such as
Valence Technology.
A lithium-ion battery from a mobile phone
A lithium-ion battery from a mobile phone
Specifications and design
. Specific energy density: 150 to 200
W·h/
kg (540 to 720
kJ/
kg)
. Volumetric energy density: 250 to 530 W·h/L (900 to 1900 J/cm3)
. Specific power density: 300 to 1500 W/kg (@ 20 seconds
[10]
and 285 W·h/L)
Lithium-ion batteries have a nominal
open-circuit voltage
of 3.6
V
and a typical charging voltage of 4.2 V. The charging procedure is done at
constant voltage with current limiting circuitry. This means charging with
constant
current until a voltage of 4.2 V is reached by the cell and continuing with
a constant voltage applied until the current drops close to zero. Typically
the charge is terminated at 7% of the initial charge current. In the past,
lithium-ion batteries could not be fast-charged and typically needed at
least
two hours to fully charge. Current generation cells can be fully charged in
45 minutes or less; some reach 90% in as little as 10 minutes.
Electrochemistry
The
anode
of a conventional Li-ion cell is made from
carbon,
the
cathode
is a metal
oxide,
and the
electrolyte
is a
lithium
salt
in an
organic
solvent.
The underlying chemical reaction that allows Li-ion cells to provide
electricity is:
\mathrm{Li}_{1-x} \mathrm{Co} \mathrm{O}_2 + \mathrm{Li}_{x}\mathrm{C}_6
\leftrightarrows \mathrm{C}_6 + \mathrm{Li}\mathrm{Co}\mathrm{O}_2
It is important to note that lithium ions themselves are not being oxidized;
rather, in a lithium-ion battery the lithium ions are transported to and
from
the cathode or anode, with the transition metal,
Co,
in LixCoO2 being oxidized from Co3+ to Co4+ during charging, and reduced
from Co4+ to Co3+ during discharge.
edit]
Solid electrolyte interphase
A particularly important element for activating Li-ion
batteries
is the solid electrolyte interphase (SEI).
Liquid
electrolytes
in Li-ion
batteries
consist of solid
lithium-
salt
electrolytes,
such as
LiPF6,
LiBF4,
or
LiClO4,
and
organic
solvents,
such as
ether.
A liquid
electrolyte
conducts Li ions, which act as a carrier between the
cathode
and the
anode
when a
battery
passes an electric current through an external circuit. However, solid
electrolytes and organic solvents are easily decomposed on
anodes
during charging, thus preventing battery activation. Nevertheless, when
appropriate
organic solvents
are used for electrolytes, the electrolytes are decomposed and form a solid
electrolyte interface at first charge that is electrically insulating and
high
Li-ion conducting. The interface prevents decomposition of the electrolyte
after the second charge. For example,
ethylene carbonate
is decomposed at a relatively high voltage, 0.7 V vs. Li, and forms a dense
and stable interface.
See
uranium trioxide
for some details of how the cathode works. While uranium oxides are not used
in commercially made batteries, the way in which uranium oxides can
reversibly
insert cations is the same as the way in which the cathode in many
lithium-ion cells work.
edit]
Guidelines for prolonging Li-ion battery life
. Unlike
Ni-Cd batteries,
lithium-ion batteries should be charged early and often. However, if they
are not used for a longer time, they should be brought to a charge level of
around
40%. Lithium-ion batteries should never be "deep-cycled" like Ni-Cd
batteries.
. Lithium-ion batteries should never be
depleted
to below their minimum voltage, 2.4v to 3.0v.
. Li-ion batteries should be kept cool. Ideally they are stored in a
refrigerator. Aging will take its toll much faster at high temperatures. The
high temperatures
found in cars cause lithium-ion batteries to degrade rapidly.
. According to one book,
lithium-ion batteries should not be frozen (should not be stored below -40
°C), because most lithium-ion battery electrolytes freeze at
approximately -40
°C (this is much colder than the lowest temperature reached by household
freezers, however).
. Li-ion batteries should be bought only when needed, because the aging
process begins as soon as the battery is manufactured.
. When using a notebook computer running from fixed line power over extended
periods, the battery should be removed and stored in a cool place so that it
is not affected by the heat produced by the computer.
Storage temperature and charge
Storing a Li-ion battery at the correct temperature and charge makes all the
difference in maintaining its storage capacity. The following table shows
the
amount of permanent capacity loss that will occur after storage at a given
charge level and temperature.
table Caption: Permanent Capacity Loss versus Storage Conditions
Storage Temperature
40% Charge
100% Charge
0 °C (32 °F)
2% loss after 1 year
6% loss after 1 year
25 °C (77 °F)
4% loss after 1 year
20% loss after 1 year
40 °C (104 °F)
15% loss after 1 year
35% loss after 1 year
60 °C (140 °F)
25% loss after 1 year
40% loss after 3 months
Source: BatteryUniversity.com
table end
It is significantly beneficial to avoid storing a lithium-ion battery at
full charge. A Li-ion battery stored at 40% charge will last many times
longer
than one stored at 100% charge, particularly at higher temperatures.
If a Li-ion battery is stored with too low a charge, there is a risk of
allowing the charge to drop below the battery's low-voltage threshold,
resulting
in an unrecoverably dead battery. Once the charge has dropped to this level,
recharging it can be dangerous. An internal safety circuit will therefore
open to prevent charging, and the battery will be for all practical purposes
dead.
In circumstances where a second Li-ion battery is available for a given
device, it is recommended that the unused battery be discharged to 40% and
placed
in the refrigerator to prolong its shelf life. While the battery can be used
or charged immediately, some Li-ion batteries will provide more energy when
brought to room temperature.
Lithium-ion batteries can easily rupture, ignite, or explode when exposed to
high temperatures,
They should not be stored in a car during hot weather. Short-circuiting a
Li-ion battery can cause it to ignite or explode. Never open a Li-ion
battery's
casing. Li-ion batteries contain safety devices that protect the cells
inside from abuse. If damaged, these can also cause the battery to ignite or
explode.
Contaminants inside the cells can defeat these safety devices. The mid-2006
recall of 10 million Sony batteries used in
Dell,
Sony,
Apple,
Lenovo/
IBM,
Panasonic,
Toshiba,
Hitachi,
Fujitsu
and
Sharp
laptops was stated to be as a consequence of internal contamination with
metal particles. Under some circumstances, these can pierce the separator,
rapidly
converting all of the energy in the cell to heat.
However, there are problems that go beyond this and this explanation is not
complete.
The mid-2006 Sony laptop battery
recall
wasn't the first of its kind, but it was the largest. During the past decade
there have been numerous recalls of lithium-ion batteries in cellular phones
and laptops owing to overheating problems. Last December, Dell pulled about
22,000 batteries from the U.S. market. In 2004, Kyocera Wireless recalled
about
1 million batteries used in phones.
In March 2007, Lenovo recalled another 205,000 9-cell lithium ion batteries
because of an explosion risk.
"It is possible to replace the
lithium cobalt oxide
cathode material in li-ion batteries with
lithiated metal phosphate
cathodes that don't explode and even have a longer
shelf life.
But for the moment these safer li-ion batteries seem mainly destined for
electric cars
and other
large-capacity battery
applications, where the safety issues are more critical... The fact is that
lithiated metal phosphate batteries hold only about 75 percent as much power
[sic]..."
Another option is to use
manganese oxide
or
iron phosphate
cathode
