Batteries based on lithium polymer (Li-polymer) have been “the next big thing” in portable power for the last 10 years. Li-polymer batteries started appearing in small consumer electronics applications, such as wireless headsets, several years ago. But these cells are finally becoming mainstream, as they are now designed into everything from laptop computers to medical monitors.
Many of the initial objectives of Li-polymer researchers and designers have not been met, while other advantages have been more recently recognized and exploited. This evolution of Lipolymer products has led to many misconceptions about the advantages and design limits of this new type of cell.
BY THE NUMBERS
Li-polymer batteries are most easily thought of as a subset of the more common Li-ion type of battery. Li-polymer cells and Li-ion cells have similar performance characteristics, since their fundamental materials are similar.
Li-polymer cells typically exhibit a nominal voltage of 3.6 V and 500 duty cycles per lifetime. A less than 1C optimal load current is common, and an average energy density of about 200 Wh/kg is typical. Li-polymer, like Li-ion, exhibits a low self-discharge rate of less than 10% per month in storage.
The major difference is that the lithium-salt electrolyte is not held in an organic solvent as in the Li-ion design. Instead, it’s held in a solid polymer composite such as polyethylene oxide or polyacrylonitrile. This allows a semi-rigid form factor and very thin cells.
Li-polymer cells can be encased in aluminum foil laminate pouches that are just 0.1 mm thick, rather than the 0.25- to 0.4-mm thick aluminum or steel cans traditionally used with Li-ion cells. Also, Li-polymer cells are constructed by stacking electrode and electrolyte materials in a flat sandwich, rather than winding them in a jellyroll fashion like Li-ion cells.
BUSTING THE MYTHS
The misconceptions about Li-polymer start with its flexible packaging. This flexibility is often misleading, as Li-polymer cells should remain flat when installed in a device, not even bending for installation in the battery system. Bending the cell brings the anode and cathode materials closer together, which can cause preferential plating and shorting. This results in reduced cycle life and presents a potential safety hazard.
While many reports have been written about the safety improvements in Li-polymer, its fundamental material set is almost the same as Li-ion. So the safety hazards are similar, and care must be taken to ensure that the packaging is not compromised with the inclusion of some rigid enclosure or support in battery pack design.
Also, swelling issues have plagued Li-polymer manufacturers and concerned portable product designers, but they have mostly been overcome. The expected swelling is now usually about 6%, similar to Li-ion prismatic cells. And one potential safety improvement afforded by the packaging is that excess gas is not likely to build up to explosive pressures, as it is allowed to escape in small amounts.
Early in its development, Li-polymer technology had problems with internal resistance, leading to low maximum discharge rates, and challenges included longer charge times compared to more mature technologies. Yet Li-polymer batteries have the potential to be made very thin, and many are based on lithium-manganese-oxide (LiMn2O4) cathode materials.
This cathode material has a 3D structure, which lends itself to good ionic conductivity, and short path lengths are achievable with thin designs. As a result, the rate capability available with Li-polymer batteries has the potential to equal or surpass conventional Li-ion batteries.
The thin profile is the major advantage for Li-polymer cells. These cells are being manufactured extremely thin. Lipolymer cells are usually available in custom sizes. While some are quite large, the most common applications are small, single cells as thin as 2 mm.
BEST IS YET TO COME
Recent improvements in Li-polymer cells have expanded their reach to other applications. The energy density is rising, approaching—and possibly soon exceeding—that of other Liion cells. Improvements in lot capacity uniformity have made multiple cell configurations a possibility, so applications that require high voltage can now be based on Li-polymer cells. The choices available to designers of portable products continue to expand, and Li-polymer batteries are another option in design for a mobile world.