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By: Isidor Buchmann The phrase "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 lithium-ion and lithium-ion polymer battery. We look at packaging techniques and evaluate the cost-to-energy ratio of these batteries. The lithium-polymer battery 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 electrolytes. The dry polymer design offers simplifications with respect to fabrication, ruggedness, safety and thin-profile. There is no danger of flammability because no liquid or gelled electrolyte is used. With a cell thickness measuring as little as 1mm (0.039in), design engineers are left to their own imagination in terms of form, shape and size. Theoretically, it is possible to create designs that 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 lithium-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 degrees C (140 degrees F) and higher increases the conductivity to acceptable levels. This requirement, however, is unsuitable for portable applications. Some dry solid lithium-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 lithium-polymer battery is high. Engineers are continuing to develop a dry solid lithium-polymer battery that performs at room temperature. A dry solid lithium-polymer version is anticipated by 2005. This battery should be very stable, would run 1,000 full cycles and would have higher energy densities than today's lithium-ion battery. How then is the current lithium-polymer battery made conductive at ambient temperatures? Most of the commercial lithium-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 "lithium-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 lithium-ion and lithium-ion polymer? Although the characteristics and performance of the two systems are very similar, the lithium-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 lithium-ion polymer battery. In addition, the promised superiority of the lithium-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 lithium-ion battery. For the present, there is no cost advantage in using the lithium-ion polymer battery. The major reason for switching to the lithium-ion polymer is form factor. It allows wafer-thin geometries, a style that is demanded by the highly competitive mobile phone industry. Table 1 summarizes the advantages and limitations of the lithium-ion polymer battery. Table 1. Advantages and limitations of lithium-ion polymer batteries. | 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 change for electrolyte leakage. | | Limitations: | Lower Energy Density and Decreased Cycle Count Compared to Lithium-Ion -- the potential for improvements exist.
Expensive to Manufacture -- once mass-produced, the lithium-ion polymer has the potential for lower cost. A reduced control circuit offsets higher manufacturing costs. |
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