The word ‘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 Li-ion and Li-ion polymer battery. We look at packaging techniques and evaluate the cost-to-energy ratio of these batteries.
The Li-polymer 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 electrolyte.
The dry polymer design offers simplifications with respect to fabrication, ruggedness, safety and thin-profile profile. There is no danger of flammability because no liquid or gelled electrolyte is used.
With a cell thickness measuring as little as one millimeter (0.039 inches), design engineers are left to their own imagination in terms of form, shape and size. Theoretically, it is possible to create designs which 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 Li-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°C (140°F) and higher increases the conductivity to acceptable levels. This requirement, however, is unsuitable for portable applications.
Some dry solid Li-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 Li-polymer battery is high.
Engineers are continuing to develop a dry solid Li-polymer battery that performs at room temperature. A dry solid Li-polymer version is anticipated by 2005. This battery should be very stable; would run 1000 full cycles and would have higher energy densities than today’s Li‑ion battery.
How then is the current Li-polymer battery made conductive at ambient temperatures? Most of the commercial Li-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 Li-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 (Li-ion polymer).
With gelled electrolyte added, what then is the difference between Li‑ion and Li‑ion polymer? Although the characteristics and performance of the two systems are very similar, the Li‑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 Li‑ion polymer battery. In addition, the promised superiority of the Li‑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 Li‑ion battery. For the present, there is no cost advantage in using the Li‑ion polymer battery. The major reason for switching to the Li-ion polymer is form factor. It allows wafer-thin geometries, a style that is demanded by the highly competitive mobile phone industry. Figure 1 summarizes the advantages and limitations of the Li-ion polymer battery.
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Advantages and Limitations of Li-ion Polymer Batteries |
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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 chance for electrolyte leakage. |
Limitations | 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. |
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Figure 1: Advantages and limitations of Li‑ion polymer batteries.
The pouch cell
The Li-ion polymer battery is almost exclusively packaged in the so-called ‘pouch cell’. This cell design made a profound advancement in 1995 when engineers succeeded in exchanging the hard shell with flexible, heat-sealable foils. The traditional metallic cylinder and glass-to-metal electrical feed-through has thus been replaced with an inexpensive foil packaging, similar to what is used in the food industry. The electrical contacts consist of conductive foil tabs that are welded to the electrode and sealed to the pouch material. Figure 2 illustrates a typical pouch cell.
The pouch cell concept makes the most efficient use of available space and achieves a packaging efficiency of 90 to 95 percent, the highest among battery packs. Because of the absence of a metal can, the pouch pack has a lower weight. No standardized pouch cells exist, but rather, each manufacturer builds to a special application.
| Figure 2: The pouch cell.
The pouch cell offers a simple, flexible and lightweight solution to battery design. This new concept has not yet fully matured and the manufacturing costs are still high.
© Cadex Electronics Inc. |
At the present time, the pouch cell is more expensive to manufacture than the cylindrical architecture and the reliability has not been fully proven. The energy density and load current are slightly lower than that of conventional cell designs. The cycle life in everyday applications is not well documented but is, at present, less than that of the Li‑ion system with cylindrical cell design.
A critical issue with the pouch cell is swelling, which occurs when gas is generated during charging or discharging. Battery manufacturers insist that Li‑ion or Polymer cells do not generate gas if properly formatted, are charged at the correct current and are kept within allotted voltage levels. When designing the protective housing for a pouch cell, some provision for swelling must be taken into account. To alleviate the swelling issue when using multiple cells, it is best not to stack pouch cells, but lay them flat side-by-side.
The pouch cell is highly sensitive to twisting. Point pressure must also be avoided. The protective housing must be designed to safeguard the cell from mechanical stress.
The cost of being slim
The slimmer the battery profile, the higher the cost–to-energy ratio becomes. By far the most economical lithium-based battery is the cylindrical 18650 cell. ‘Eighteen’ denotes the diameter in millimeters and ‘650’ describes the length in millimeters. The new 18650 cell has a capacity 2000mAh. The larger 26650 cell has a diameter of 26 mm and delivers 3200mAh.
The disadvantage of the cylindrical cell is bulky size and less than maximum use of space. When stacking, air cavities are formed. Because of fixed cell sizes, the battery pack must be designed around the available cell.
If a thinner profile than 18 mm is required, the prismatic Li‑ion cell is the best choice. The cell concept was developed in the early 1990s in response to consumer demand for slimmer pack sizes. The prismatic cell makes almost maximum use of space when stacking.
The disadvantage of the prismatic cell is slightly lower energy densities compared to the cylindrical equivalent. In addition, the prismatic cell is more expensive to manufacture and does not provide the same mechanical stability enjoyed by the cylindrical cell. To prevent bulging when pressure builds up, heavier gauge metal is used for the container. The manufacturer allows some degree of bulging when designing the battery pack.
The prismatic cell is offered in limited sizes and chemistries and the capacities run from about 400mAh to 2000mAh. Because of the very large quantities required for mobile phones, custom prismatic cells are built to fit certain models.
If the design requirements demand less than 4 mm, the best (and perhaps the only choice) is Li‑ion polymer. This is the most expensive option. The cost-to-energy ratio more than doubles. The benefit of this architecture is strictly slim geometry. There is little or no gain in energy density per weight and size over the 18650, even though the metal housing has been eliminated.
Summary
The Li-ion polymer offers little or no energy gain over conventional Li‑ion systems; neither do the slim profile Li-ion systems meet the cycle life of the rugged 18560 cell. The cost-to-energy ration increases as the cell size decreases in thickness. Cost increases in the multiple of three to four compared to the 18650 cell are common on exotic slim battery designs.
If space permitted, the 18650 cell offers the most economical choice, both in terms of energy per weight and longevity. Applications for this cell are mobile computing and video cameras. Slimming down means thinner batteries. This, in turn, will make the cost of the portable power more expensive.
Fuente: http://www.buchmann.ca/Article6-Page1.asp
