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Li-polymer Battery: Substance or Hype?
The polymer hype of the early 2000s is still going strong, however, most users cannot distinguish between a regular Li-ion and one with polymer architecture. Lithium-polymer differs from other battery systems in the type of electrolyte used. The original polymer design dating back to the 1970s uses a solid (dry) polymer electrolyte that resembles a plastic-like film. This insulator allows the exchange of ions (electrically charged atoms) and replaces the traditional porous separator that is soaked with electrolyte. A solid polymer has a poor conductivity at room temperature and the battery must be heated to 50–60°C (122–140°F) to enable current flow.The much anticipated “true plastic battery” promised in the early 2000s did not materialize; the conductivity could not be attained at ambient temperature.To make the modern Li-polymer battery conductive at room temperature, gelled electrolyte is added. All Li-ion polymer cells today incorporate a micro porous separator with moisture. The correct term is “Lithium-ion polymer” (Li-ion polymer or Li-polymer for short). Li-polymer can be built on many systems, such as Li-cobalt, NMC, Li-phosphate and Li-manganese. For this reason, Li-polymer is not considered a unique battery chemistry. Most Li-polymer packs for the consumer market are based on Li-cobalt.
With gelled electrolyte added, what then is the difference between a normal Li ion and Li ion polymer? As far as the user is concerned, the lithium polymer is essentially the same as the lithium-ion battery. Both use identical cathode and anode material and contain a similar amount of electrolyte. Although the characteristics and performance of the two systems are alike, the Li polymer is unique in that a micro porous electrolyte replaces the traditional porous separator. The gelled electrolyte becomes the catalyst that enhances the electrical conductivity. Li-polymer offers slightly higher specific energy and can be made thinner than conventional Li-ion, but the manufacturing cost increases by 10–30 percent. Despite the cost disadvantage, the market share of Li-polymer is growing.
Li-polymer cells also come in a flexible foil-type case (polymer laminate or pouch cell) that resembles a food package. While a standard Li-ion needs a rigid case to press the electrodes together, Li-polymer uses laminated sheets that do not need compression. A foil-type enclosure reduces the weight by more than 20 percent over the classic hard shell. Furthermore, thin film technology liberates the format design and the battery can be made into any shape, fitting neatly into stylish cell phones and laptops to make them smaller, thinner and lighter. Li-polymer can be made very slim to resemble a credit card. Read about the Pouch Cell.
Charge and discharge characteristics of Li-polymer are identical to other Li-ion systems and do not require a special charger. Safety issues are also similar in that protection circuits are needed. Gas buildup during charge can cause some Li-polymer in a foil package to swell, and equipment manufacturers must make allowances for expansion. Li-polymer in a foil package may be less durable than Li-ion in the cylindrical package. Li-polymer is not limited to a foil package and can also be made into a cylindrical design.
