Batteries based on lithium polymer (Li-polymer)
have been “the next big thing” in portable
power for the last 10 years. Li-polymer
batteries started appearing in small consumer
electronics applications, such as wireless headsets, several years
ago. But these cells are finally becoming mainstream, as they
are now designed into everything from laptop computers to
medical monitors.
Many of the initial objectives of Li-polymer researchers and
designers have not been met, while other advantages have been
more recently recognized and exploited. This evolution of Lipolymer
products has led to many misconceptions about the
advantages and design limits of this new type of cell.
BY THE NUMBERS
Li-polymer batteries are most easily thought of as a subset
of the more common Li-ion type of battery. Li-polymer cells
and Li-ion cells have similar performance characteristics, since
their fundamental materials are similar.
Li-polymer cells typically exhibit a nominal voltage of 3.6 V
and 500 duty cycles per lifetime. A less than 1C optimal load
current is common, and an average energy density of about
200 Wh/kg is typical. Li-polymer, like Li-ion, exhibits a low
self-discharge rate of less than 10% per month in storage.
The major difference is that the lithium-salt electrolyte is
not held in an organic solvent as in the Li-ion design. Instead,
it’s held in a solid polymer composite such as polyethylene
oxide or polyacrylonitrile. This allows a semi-rigid form factor
and very thin cells.
Li-polymer cells can be encased in aluminum foil laminate
pouches that are just 0.1 mm thick, rather than the 0.25- to
0.4-mm thick aluminum or steel cans traditionally used with
Li-ion cells. Also, Li-polymer cells are constructed by stacking
electrode and electrolyte materials in a flat sandwich, rather
than winding them in a jellyroll fashion like Li-ion cells.
BUSTING THE MYTHS
The misconceptions about Li-polymer start with its flexible
packaging. This flexibility is often misleading, as Li-polymer
cells should remain flat when installed in a device, not even
bending for installation in the battery system. Bending the cell
brings the anode and cathode materials closer together, which
can cause preferential plating and shorting. This results in
reduced cycle life and presents a potential safety hazard.
While many reports have been written about the safety
improvements in Li-polymer, its fundamental material set is
almost the same as Li-ion. So the safety hazards are similar, and
care must be taken to ensure that the packaging is not compromised
with the inclusion of some rigid enclosure or support in
battery pack design.
Also, swelling issues have plagued Li-polymer manufacturers
and concerned portable product designers, but they have
mostly been overcome. The expected swelling is now usually
about 6%, similar to Li-ion prismatic cells. And one potential
safety improvement afforded by the packaging is that excess
gas is not likely to build up to explosive pressures, as it is
allowed to escape in small amounts.
Early in its development, Li-polymer technology had problems
with internal resistance, leading to low maximum discharge
rates, and challenges included longer charge times compared
to more mature technologies. Yet Li-polymer batteries
have the potential to be made very thin, and many are based on
lithium-manganese-oxide (LiMn2O4) cathode materials.
This cathode material has a 3D structure, which lends itself
to good ionic conductivity, and short path lengths are achievable
with thin designs. As a result, the rate capability available
with Li-polymer batteries has the potential to equal or surpass
conventional Li-ion batteries.
The thin profile is the major advantage for Li-polymer
cells. These cells are being manufactured extremely thin. Lipolymer
cells are usually available in custom sizes. While some
are quite large, the most common applications are small, single
cells as thin as 2 mm.
BEST IS YET TO COME
Recent improvements in Li-polymer cells have expanded their
reach to other applications. The energy density is rising,
approaching—and possibly soon exceeding—that of other Liion
cells. Improvements in lot capacity uniformity have made
multiple cell configurations a possibility, so applications that
require high voltage can now be based on Li-polymer cells. The
choices available to designers of portable products continue to
expand, and Li-polymer batteries are another option in design
for a mobile world.