10/29/1998
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Overview of Li-ion Polymer Rechargeable Batteries
Despite a slow start, polymer batteries will gain a strong position in the wireless market.
By Ian Irving, Executive Vice President of Operations, Battery Engineering Inc.
<Picture>Rapid growth in the wireless market, along with increased complexity and higher power requirements, has 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 wireless 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 industry. 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 not any viable polymer products currently available in the commercial wireless 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 become 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 Headed
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 wireless 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.
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Edited by Robert Keenan |
