To all, from news.computeroemonline.com
11/06/98 Overview of Li-ion Polymer
Rechargeable Batteries
Despite a slow start, polymer
batteries will gain a strong
position in the portable
computing market.
By Ian Irving, Executive Vice
President of Operations, Battery
Engineering Inc.
Rapid growth in the
portable computing
market, along with
increased complexity and
higher power
requirements, have led
manufacturers toward the
development of better
rechargeable battery
technologies. The first step in this
development effort was nickel
metal hydride (NiMH) cells that
generally out performed the more
traditional nickel cadmium (NiCd)
technology. The next step in
rechargeable technology was
lithium ion (Li-ion) technology,
which is currently being employed
in laptop computers, cellular
phones, and expanding into other
portable applications.
Containing no lithium metal, Li-ion
technology simply uses the ions of
lithium and shuttles them back
and forth on the charge and
discharge cycles. This technology
can be generally described as a
metal encased cylindrical cell
containing a lithiated cobalt oxide
and a liquid electrolyte. These
cylindrical cells generally feature
a 120 Wh/kg energy density. If the
cobalt oxide is replaced with
Maganese oxide, these cells can
achieve a 110 Wh/kg energy
density.
Li-ion batteries are now becoming
a main stream technology in
today's wireless and portable
computing industries. As a result,
designers are looking for new
methods and technologies to
reduce size, decrease weight, and
increase power in lithium
batteries. To meet these
requirements, many companies
are focusing their efforts on Li-ion
polymer batteries.
Currently, Li-ion polymer cells use
either a lithiated Maganese oxide
(Mn2O4) or a lithiated cobalt oxide
and feature an electrolyte that
usually does not contain free
flowing liquid. Polymer cells can
be made very thin and flat and
have some potential to be flexible
and conformable. This tends to
give these cells distinct
advantages in the area of form
factor particularly over cylindrical
cells.
The form factor of polymer cells
provides for a higher energy
density-both volumetric and
gravimetric-due to the smaller
ratio of packaging to active
materials. In fact, Li-ion polymer
batteries are expected to offer an
energy density between 90 and
125 Wh/Kg.
Polymer cells have been
discussed for a number of years.
Despite this, there are no viable
polymer products currently
available in the commercial
market. A number of
manufacturers, however, have
started developing Li-ion polymer
batteries for wireless and mobile
computing applications. These
manufacturers are at different
stages in the development
process.
Bellcore Technology
The Bellcore license is the most
widely used Li-ion polymer
technology today. Bellcore calls
their technology a "plastic"
battery and deliberately avoids
calling it a polymer cell. The
Bellcore process does use an
added liquid electrolyte, but it is
believed to be contained within
the polymer structure.
Currently, Valence, Varta, Duracell,
Gould, Ultralife, and unnamed
Japanese companies are using
Bellcore's technology. Batteries
that employ this technology,
however, are still not commercially
available.
Other Approaches
Bellcore is not the only
organization that has developed a
Li-ion polymer technology.
Recently, Johns Hopkins
University announced a polymer
cell. In addition, 3M has been
working with Hydro Quebec on
developing Li-ion polymer
technology for electric vehicle
(EV) applications.
Battery Engineering has also
developed its own patented,
polymer technology, using cobalt
oxide and a proprietary polymer
technology. Battery Engineering's
solid polymer electrolyte consists
of a proprietary mix of three
monomers, a carbonate-based
plasticizer solution, and lithium
salt formed on a synthetic fabric.
The solid polymer is formed in
place, which adds to the strength
of the cell and improves its
handling characteristics and
manufacturability. Due to the total
absence of liquid electrolyte,
bipolar stacks can be easily
assembled for thin, high-voltage,
batteries.
The Battery Engineering
proprietary cell is charged to +4.2
VDC and has an average discharge
voltage of +3.6 VDC. Continuous
cycling has yielded greater than
80% of initial capacity after as
many as 1,000 charge and
discharge cycles at the 0.4°C rate.
Energy density, although
somewhat dependent on cell size,
is generally about 125 Wh/kg while
packaged volumetric energy
density ranges from 174 to 255
Wh/l.
Battery Engineering's polymer
cells will self discharge as little as
approximately 5% per month.
Recent tests have shown the
irrecoverable capacity losses can
be as low as 8% over a 6-month
storage time in the fully charged
state at ambient temperatures.
Battery Engineering has developed
a credit card-sized Li-ion polymer
cell with a dimension of 86 x 54 x
0.5 mm and a capacity of 100 mAh.
The company has also employed
this polymer technology in a Z fold
cell structure that measures 86 x
54 x 5 mm and having a capacity
of 1,000 mAh.
Where The Market's Going
Development of lithium batteries
started in the 1980s, with first
commercial shipments beginning
in the early 90s and first
significant volumes being shipped
in 1994. By 1997, the small
rechargeable battery market had
reached about 2 billion cells and is
variously projected to reach 3 to 4
billion cells by the year 2001. In
1996, Li-ion accounted for about
100 million cells and is projected
to reach 700 million by 2001 and
as much as 1.1 billion by 2005.
Is it reasonable to expect this kind
of growth projection to be
realistic? Yes, it probably is. Li-ion
and polymer cells, because of their
higher voltage difference, are
expected to replace nickel
cadmium and nickel metal hydride
at the rate of 1 to 3 cells. These
figures show that lithium-based
batteries have a distinct design
advantage over other
rechargeable battery
technologies.
There is some indication that
current growth has been limited,
to some extent, by product
availability. As availability
increases, there will be more
portable computing applications
employing lithium technology. This
will increase competition between
liquid electrolyte and polymer
technologies. Despite the
competition, there will be many
new applications to keep polymer
and liquid electrolyte systems
growing.
Safety issues have also slowed
the acceptance of lithium battery
technology in wireless
applications. Liquid electrolyte
Li-ion cells generally need very
precise charge controls,
protecting the cells or battery
pack from overcharge and
overdischarge. Li-ion cells need to
be controlled to prevent
overcharging and overdischarging.
If overcharging or overdischarging
occurs, the liquid electrolyte Li-ion
battery may vent and cause a fire
or possibly even an explosion.
Li-ion developers are addressing
these safety concerns.
Manufacturers are equipping Li-ion
cells with sophisticated electronic
controls to prevent fires and
explosions. In addition,
manufacturers are switching to
less combustible electrolytes.
Finally, manufacturers are turning
to polymer structures. Polymer
cells are generally considered to
be safer than liquid electrolyte
cells because they do not have a
free flowing electrolyte. A gel type
polymer cell will not leak
electrolyte and is unlikely to
create excess gas, making them
safer than today's liquid
electrolyte cells.
Irreversible capacity loss is yet
another problem that seems to
exist with some cylindrical and
polymer technologies. A cell
stored in the charged condition
will lose capacity and not all of
that lost capacity can be
recovered on subsequent charge
cycles. The irreversible losses are
higher on initial cycles and then
gradually decrease. Developments
will continue on this problem and it
can reasonably be expected to be
resolved.
An Infant Technology
Lithium technology is still in its
infancy and we can expect
continued development of this
technology. It has already been
projected that there will be a
change from cobalt oxide probably
to a manganese oxide or even a
tin oxide as one manufacturer is
currently using. Changing from
cobalt oxide could help with
overall cost reductions. Although
this change might have an adverse
short term effect on energy
density, with development, the
energy density of these cells is
still expected to increase to as
much as 140 Wh/kg by the year
2000.
Ian Irving, Executive Vice President
of Operations, Battery Engineering,
100 Energy Drive, Canton, MA 02021.
Phone: 781-830-5806; Fax:
781-575-1545.
Edited by Robert Keenan and
Bruce A. Bennett
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