The Li-Polymer battery: substance or hype?
Isidor Buchmann, President
Cadex Electronics Inc.
The word ‘Lithium Polymer’ has become synonymous with advanced
battery technology. But what is the relationship between ‘polymer’
and the classic Lithium Ion battery? In this article we examine
the basic differences between the Li-ion and Li-ion polymer
battery. We look at packaging techniques and evaluate the
cost-to-energy ratio of these batteries.
The Li-polymer differs from other battery systems in the
type of electrolyte used. The original design, which dates
back to the 1970s, uses a polymer electrolyte. This electrolyte
resembles a plastic-like film that does not conduct electricity
but allows the 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 profile.
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), design engineers are left to their own imagination
in terms of form, shape and size. Theoretically, 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 a 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.
The 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, however,
is unsuitable for portable applications.
Some dry solid Li-polymers are currently used in hot climates
as standby batteries for stationary applications. One manufacturer
has added heating elements in the cells that keep 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 degrade the service life of this battery
in the same way as it does with the VRLA type. Although longer
lasting, the cost of the Li-polymer battery is high.
Engineers are continuing to develop a dry solid Li-polymer
battery that performs at room temperature. A dry solid Li-polymer
version is anticipated by 2005. This battery should be very
stable; would run 1000 full cycles and would have higher energy
densities than today’s Li‑ion battery.
How then is the current Li-polymer battery made conductive
at ambient temperatures? Most of the commercial Li-polymer
batteries or mobile phones are a hybrid. Some gelled electrolyte
has been added to the dry polymer. The correct term for this
system is Lithium Ion Polymer. For marketing reasons,
most battery manufacturers call it simply Li-polymer.
Since the hybrid lithium polymer is the only functioning polymer
battery for portable use today, we will focus on this chemistry
variation but use the correct term of lithium ion polymer
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 the solid
electrolyte replaces 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. In addition, the promised superiority of the Li‑ion
polymer has not yet been realized. No improvements in capacity
gains are 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 major reason for switching to the Li-ion polymer
is form factor. It allows wafer-thin geometries, a style that
is demanded by the highly competitive mobile phone industry.
Figure 1 summarizes the advantages and limitations of the
Li-ion polymer battery.
Advantages and Limitations of Li-ion
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.
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 1: Advantages and limitations
of Li‑ion polymer batteries.
The pouch cell
The Li-ion polymer battery is almost exclusively packaged
in the so-called ‘pouch cell’. This cell design made a profound
advancement in 1995 when engineers succeeded in exchanging
the hard shell with flexible, heat-sealable foils. The traditional
metallic cylinder and glass-to-metal electrical feed-through
has thus been replaced with an inexpensive foil packaging,
similar to what is used in the food industry. The electrical
contacts consist of conductive foil tabs that are welded to
the electrode and sealed to the pouch material. Figure 2 illustrates
a typical pouch cell.
The pouch cell concept makes the most efficient use of available
space and achieves a packaging efficiency of 90 to 95 percent,
the highest among battery packs. Because of the absence of
a metal can, the pouch pack has a lower weight. No standardized
pouch cells exist, but rather, each manufacturer builds to
a special application.
Figure 2: The pouch cell.
The pouch cell offers
a simple, flexible and lightweight solution to battery
design. This new concept has not yet fully matured and
the manufacturing costs are still high.
© Cadex Electronics Inc.
At the present time, the pouch cell is more expensive to
manufacture than the cylindrical architecture and the reliability
has not been fully proven. The energy density and load current
are slightly lower than that of conventional cell designs.
The cycle life in everyday applications is not well documented
but is, at present, less than that of the Li‑ion system
with cylindrical cell design.
A critical issue with the pouch cell is swelling, which occurs
when gas is generated during charging or discharging. Battery
manufacturers insist that Li‑ion or Polymer cells do
not generate gas if properly formatted, are charged at the
correct current and are kept within allotted voltage levels.
When designing the protective housing for a pouch cell, some
provision for swelling must be taken into account. To alleviate
the swelling issue when using multiple cells, it is best not
to stack pouch cells, but lay them flat side-by-side.
The pouch cell is highly sensitive to twisting. Point pressure
must also be avoided. The protective housing must be designed
to safeguard the cell from mechanical stress.
The cost of being slim
The slimmer the battery profile, the higher the cost–to-energy
ratio becomes. By far the most economical lithium-based battery
is the cylindrical 18650 cell. ‘Eighteen’ denotes the diameter
in millimeters and ‘650’ describes the length in millimeters.
The new 18650 cell has a capacity 2000mAh. The larger 26650
cell has a diameter of 26 mm and delivers 3200mAh.
The disadvantage of the cylindrical cell is bulky size and
less than maximum use of space. When stacking, air cavities
are formed. Because of fixed cell sizes, the battery pack
must be designed around the available cell.
If a thinner profile than 18 mm is required, the prismatic
Li‑ion cell is the best choice. The cell concept was
developed in the early 1990s in response to consumer demand
for slimmer pack sizes. The prismatic cell makes almost maximum
use of space when stacking.
The disadvantage of the prismatic cell is slightly lower
energy densities compared to the cylindrical equivalent. In
addition, the prismatic cell is more expensive to manufacture
and does not provide the same mechanical stability enjoyed
by the cylindrical cell. To prevent bulging when pressure
builds up, heavier gauge metal is used for the container.
The manufacturer allows some degree of bulging when designing
the battery pack.
The prismatic cell is offered in limited sizes and chemistries
and the capacities run from about 400mAh to 2000mAh. Because
of the very large quantities required for mobile phones, custom
prismatic cells are built to fit certain models.
If the design requirements demand less than 4 mm, the
best (and perhaps the only choice) is Li‑ion polymer.
This is the most expensive option. The cost-to-energy ratio
more than doubles. The benefit of this architecture is strictly
slim geometry. There is little or no gain in energy density
per weight and size over the 18650, even though the metal
housing has been eliminated.
The Li-ion polymer offers little or no energy gain over conventional
Li‑ion systems; neither do the slim profile Li-ion systems
meet the cycle life of the rugged 18560 cell. The cost-to-energy
ration increases as the cell size decreases in thickness.
Cost increases in the multiple of three to four compared to
the 18650 cell are common on exotic slim battery designs.
If space permitted, the 18650 cell offers the most economical
choice, both in terms of energy per weight and longevity.
Applications for this cell are mobile computing and video
cameras. Slimming down means thinner batteries. This, in turn,
will make the cost of the portable power more expensive.
This article contains excerpts from the second edition book
entitled Batteries in a Portable World — A Handbook on Rechargeable
Batteries for Non-Engineers. In the book, Mr. Buchmann evaluates
the battery in everyday use and explains their strengths and
weaknesses in laymen’s terms. The 300-page book is available
from Cadex Electronics Inc. through email@example.com,
tel. 604-231-7777 or most bookstores. For additional information
on battery technology visit www.buchmann.ca.
About the Author
Isidor Buchmann is the founder and CEO of Cadex Electronics
Inc., in Richmond (Vancouver) British Columbia, Canada. Mr.
Buchmann has a background in radio communications and has
studied the behavior of rechargeable batteries in practical,
everyday applications for two decades. The author of many
articles and books on battery maintenance technology, Mr.
Buchmann is a well-known speaker who has delivered technical
papers and presentations at seminars and conferences around
About the Company
Cadex Electronics Inc. is a world leader in the design
and manufacture of advanced battery analyzers and chargers.
Their award-winning products are used to prolong battery life
in wireless communications, emergency services, mobile computing,
avionics, biomedical, broadcasting and defense. Cadex products
are sold in over 100 countries.