Page 2 (click here to go to page 1) The Pouch Cell The lithium-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 1 illustrates a typical pouch cell. 
Figure 1. 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. |
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. 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 every-day applications is not well documented, but is presently less than that of the lithium-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 lithium-ion or polymer cells do not generate gas if they are 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 provisions 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. Additionally, 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 of 2,000mAh. The larger 26650 cell has a diameter of 26mm and delivers 3,200mAh. The disadvantage of the cylindrical cell is its bulky size and the 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 18mm is required, the prismatic lithium-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 2,000mAh. 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 4mm, the best (and perhaps the only choice) is lithium-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 lithium-ion polymer offers little or no energy gain over conventional lithium-ion systems; neither do the slim-profile lithium-ion systems meet the cycle life of the rugged 18560 cell. The cost-to-energy ratio increases as the cell size decreases in thickness. Cost increases in multiples of three to four compared to the 18650 cell, and are common on exotic slim battery designs. If space permits, 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. This article contains excerpts from Batteries in a Portable World-A Handbook on Rechargeable Batteries for Non-Engineers. | 
