The Li-Polymer battery: Substance or hype?(A)
The word ‘Lithium Polymer’ has become synonymous with advancedbattery technology. But what is the relationship between ‘polymer’and the classic Lithium Ion battery? In this article we examine thebasic differences between the Li-ion and Li-ion polymer battery. Welook at packaging techniques and evaluate the cost-to-energy ratioof these batteries.
The Li-polymer differs from other battery systems in the type ofelectrolyte used. The original design, which dates back to the1970s, uses a polymer electrolyte. This electrolyte resembles aplastic-like film that does not conduct electricity but allows theexchange 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 tofabrication, ruggedness, safety and thin-profile profile. There isno danger of flammability because no liquid or gelled electrolyteis used.
With a cell thickness measuring as little as one millimeter (0.039inches), design engineers are left to their own imagination interms of form, shape and size. Theoretically, it is possible tocreate designs which form part of a protective housing, are in theshape of a mat that can be rolled up, or are even embedded into acarrying case or a piece of clothing. Such innovative batteries arestill 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 currentbursts needed for modern communication devices and spinning up thehard drives of mobile computing equipment. Although heating thecell to 60°C (140°F) and higher increases the conductivity toacceptable levels. This requirement, however, is unsuitable forportable applications.
Some dry solid Li-polymers are currently used in hot climates asstandby batteries for stationary applications. One manufacturer hasadded heating elements in the cells that keep the battery in theconductive temperature range at all times. Such a battery performswell for the application intended because high ambient temperaturesdo not degrade the service life of this battery in the same way asit does with the VRLA type. Although longer lasting, the cost ofthe Li-polymer battery is high.
Engineers are continuing to develop a dry solid Li-polymer batterythat performs at room temperature. A dry solid Li-polymer versionis anticipated by 2005. This battery should be very stable; wouldrun 1000 full cycles and would have higher energy densities thantoday’s Li‑ion battery.
How then is the current Li-polymer battery made conductive atambient temperatures? Most of the commercial Li-polymerbatteries or mobile phones are a hybrid. Some gelled electrolytehas been added to the dry polymer. The correct term for this systemis Lithium Ion Polymer. For marketing reasons, most batterymanufacturers call it simply Li-polymer. Since the hybrid lithiumpolymer is the only functioning polymer battery for portable usetoday, we will focus on this chemistry variation but use thecorrect term of lithium ion polymer (Li-ion polymer).
With gelled electrolyte added, what then is the difference betweenLi‑ion and Li‑ion polymer? Although the characteristics andperformance of the two systems are very similar, the Li‑ion polymeris unique in that the solid electrolyte replaces the porousseparator. The gelled electrolyte is simply added to enhance ionconductivity.
Technical difficulties and delays in volume manufacturing havedeferred the introduction of the Li‑ion polymer battery. Inaddition, the promised superiority of the Li‑ion polymer has notyet been realized. No improvements in capacity gains are achieved —in fact, the capacity is slightly less than that of the standardLi‑ion battery. For the present, there is no cost advantage inusing the Li‑ion polymer battery. The major reason for switching tothe Li-ion polymer is form factor. It allows wafer-thin geometries,a style that is demanded by the highly competitive mobile phoneindustry. Figure 1 summarizes the advantages and limitations of theLi-ion polymer battery.
Advantages and Limitations of Li-ion Polymer Batteries
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.