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The Li-Polymer battery: Substance or hype?(B)

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 availablespace and achieves a packaging efficiency of 90 to 95 percent, thehighest among battery packs. Because of the absence of a metal can,the pouch pack has a lower weight. No standardized pouch cellsexist, but rather, each manufacturer builds to a specialapplication.


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.

Atthe present time, the pouch cell is more expensive to manufacturethan the cylindrical architecture and the reliability has not beenfully proven. The energy density and load current are slightlylower than that of conventional cell designs. The cycle life ineveryday applications is not well documented but is, at present,less than that of the Li‑ion system with cylindrical celldesign.
A critical issue with the pouch cell is swelling, which occurs whengas is generated during charging or discharging. Batterymanufacturers insist that Li‑ion or Polymer cells do not generategas if properly formatted, are charged at the correct current andare kept within allotted voltage levels. When designing theprotective housing for a pouch cell, some provision for swellingmust be taken into account. To alleviate the swelling issue whenusing multiple cells, it is best not to stack pouch cells, but laythem flat side-by-side.
The pouch cell is highly sensitive to twisting. Point pressure mustalso be avoided. The protective housing must be designed tosafeguard the cell from mechanical stress.
The cost of beingslim
The slimmerthe battery profile, the higher the cost–to-energy ratio becomes.By far the most economical lithium-based battery is the cylindrical18650 cell. ‘Eighteen’ denotes the diameter in millimeters and‘650’ describes the length in millimeters. The new 18650 cell has acapacity 2000mAh. The larger 26650 cell has a diameter of 26 mm anddelivers 3200mAh.
The disadvantage of the cylindrical cell is bulky size and lessthan maximum use of space. When stacking, air cavities are formed.Because of fixed cell sizes, the battery pack must be designedaround the available cell.
If a thinner profile than 18 mm is required, the prismatic Li‑ioncell is the best choice. The cell concept was developed in theearly 1990s in response to consumer demand for slimmer pack sizes.The prismatic cell makes almost maximum use of space whenstacking.
The disadvantage of the prismatic cell is slightly lower energydensities compared to the cylindrical equivalent. In addition, theprismatic cell is more expensive to manufacture and does notprovide the same mechanical stability enjoyed by the cylindricalcell. To prevent bulging when pressure builds up, heavier gaugemetal is used for the container. The manufacturer allows somedegree of bulging when designing the battery pack.
The prismatic cell is offered in limited sizes and chemistries andthe capacities run from about 400mAh to 2000mAh. Because of thevery large quantities required for mobile phones, custom prismaticcells are built to fit certain models.
If the design requirements demand less than 4 mm, the best (andperhaps the only choice) is Li‑ion polymer. This is the mostexpensive option.  The cost-to-energy ratio more than doubles.The benefit of this architecture is strictly slim geometry. Thereis little or no gain in energy density per weight and size over the18650, even though the metal housing has been eliminated.
The Li-ionpolymer offers little or no energy gain over conventional Li‑ionsystems; neither do the slim profile Li-ion systems meet the cyclelife of the rugged 18560 cell. The cost-to-energy ration increasesas the cell size decreases in thickness. Cost increases in themultiple of three to four compared to the 18650 cell are common onexotic slim battery designs.
If space permitted, the 18650 cell offers the most economicalchoice, 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 thecost of the portable power more expensive.

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