|
The Li-polymer differentiates itself from other battery systems
in the type of electrolyte used. The original design, dating
back to the 1970s, uses a dry solid polymer electrolyte only.
This electrolyte resembles a plastic-like film that does not
conduct electricity but allows an exchange of ions (electrically
charged atoms or groups of atoms). The polymer electrolyte
replaces the traditional porous separator, which is soaked
with electrolyte.
The dry polymer design offers simplifications with respect
to fabrication, ruggedness, safety and thin-profile geometry.
There is no danger of flammability because no liquid or gelled
electrolyte is used.
With a cell thickness measuring as little as one millimeter
(0.039 inches), equipment designers are left to their
own imagination in terms of form, shape and size. It is possible
to create designs which form part of a protective housing,
are in the shape of a mat that can be rolled up, or are even
embedded into a carrying case or piece of clothing. Such innovative
batteries are still a few years away, especially for the commercial
market.
Unfortunately, the dry Li-polymer suffers from poor conductivity.
Internal resistance is too high and cannot deliver the current
bursts needed for modern communication devices and spinning
up the hard drives of mobile computing equipment. Although
heating the cell to 60°C (140°F) and higher increases the
conductivity to acceptable levels, this requirement is unsuitable
in commercial applications.
Research is continuing to develop a dry solid Li-polymer
battery that performs at room temperature. A dry solid Li-polymer
version is expected to be commercially available by 2005.
It is expected to be very stable; would run 1000 full
cycles and would have higher energy densities than today’s
Li-ion battery.
In the meantime, some Li-polymers are used as standby batteries
in hot climates. One manufacturer has added heating elements
that keeps the battery in the conductive temperature range
at all times. Such a battery performs well for the application
intended because high ambient temperatures do not affect the
service life of this battery in the same way it does the VRLA,
for example.
To make a small Li-polymer battery conductive, some gelled
electrolyte has been added. Most of the commercial Li-polymer
batteries used today for mobile phones are a hybrid and contain
gelled electrolyte. The correct term for this system is ‘Lithium
Ion Polymer’. For promotional reasons, most battery manufacturers
mark the battery simply as Li-polymer. Since the hybrid lithium
polymer is the only functioning polymer battery for portable
use today, we will focus on this chemistry.
With gelled electrolyte added, what then is the difference
between Li-ion and Li-ion polymer? Although the
characteristics and performance of the two systems are very
similar, the Li-ion polymer is unique in that it uses
a solid electrolyte, replacing the porous separator. The gelled
electrolyte is simply added to enhance ion conductivity.
Technical difficulties and delays in volume manufacturing
have deferred the introduction of the Li-ion polymer
battery. This postponement, as some critics argue, is due
to ‘cashing in’ on the Li-ion battery. Manufacturers
have invested heavily in research, development and equipment
to mass-produce the Li-ion. Now businesses and shareholders
want to see a return on their investment.
In addition, the promised superiority of the Li-ion
polymer has not yet been realized. No improvements in capacity
gains have been achieved — in fact, the capacity is slightly
less than that of the standard Li-ion battery. For the
present, there is no cost advantage in using the Li-ion
polymer battery. The thin profile has, however, compelled
mobile phone manufacturers to use this promising technology
for their new generation handsets.
One of the advantages of the Li-ion polymer, however,
is simpler packaging because the electrodes can easily be
stacked. Foil packaging, similar to that used in the food
industry, is being used. No defined norm in cell size has
been established by the industry.
|
|
|
Advantages
and Limitations of Li-ion Polymer Batteries
|
|
|
|
Advantages
|
Very low profile — batteries that resemble the profile
of a credit card are feasible.
Flexible form factor — manufacturers are not bound
by standard cell formats. With high volume, any reasonable
size can be produced economically.
Light weight – gelled rather than liquid electrolytes
enable simplified packaging, in some cases eliminating
the metal shell.
Improved safety — more resistant to overcharge; less
chance for electrolyte leakage.
|
|
Limitations
|
Lower energy density and decreased cycle count compared
to Li-ion — potential for improvements exist.
Expensive to manufacture — once mass-produced, the
Li-ion polymer has the potential for lower cost. Reduced
control circuit offsets higher manufacturing costs.
|
|
|
Figure 2-8: Advantages
and limitations of Li-ion polymer batteries.
|
