| 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 (Li-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.
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Advantages
and Limitations of Li-ion Polymer Batteries |
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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. |
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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. |
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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.
Summary
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.
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