Find out why lithium-polymer is so popular.
The polymer hype of the early 2000s is still going strong, but most users cannot distinguish between a regular Li-ion and one with polymer architecture. While many people identify the term “polymer” as a “plastic,” polymers range from synthetic plastics to natural biopolymers and proteins that are from a fundamental biological structure.
Lithium-polymer differs from other battery systems in the type of electrolyte used. The original polymer design dating back to the 1970s used 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 60°C (140°F) and higher to enable current flow. The much anticipated “true plastic battery” promised in the early 2000s did not materialize as the conductivity could not be attained at ambient temperature.
To make the modern Li-polymer battery conductive at room temperature, gelled electrolyte has been added. All Li-ion polymer cells today incorporate a micro porous separator with some moisture. Li-polymer can be built on many systems, such as Li-cobalt, NMC, Li-phosphate and Li-manganese, and is not considered unique battery chemistry. Most Li-polymer packs are for the consumer market and are based on Li-cobalt.
With gelled electrolyte added, what is the difference between a normal Li ion and Li ion polymer? As far as the user is concerned, lithium polymer is essentially the same as lithium-ion. Both systems use identical cathode and anode material and contain a similar amount of electrolyte. Li-polymer is unique in that a micro porous electrolyte replaces the traditional porous separator. Li-polymer offers slightly higher specific energy and can be made thinner than conventional Li-ion, but the manufacturing cost is higher by 10–30 percent.
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. Thin film technology liberates design as the battery can be made into any shape, fitting neatly into stylish mobile phones and laptops. Li-polymer can also be made very slim to resemble a credit card. [BU-301a: Types of Battery Cells]
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 prismatic and pouch cells 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.
Research with the solid electrolyte (SE) continues and attempts are made by using metallic lithium as anode material. Solid lithium has a higher energy density than in modified lithium-ion form, but lithium anodes have been tried before and battery manufacturers were forced to discontinue production because of safety issues. Lithium tends to form metal filaments, or dendrites, that cause short circuits. Scientists are trying to overcome this invasion by using specially designed separators and other remedies.
The key objectives for the so-called “solid state lithium ion battery” are achieving sufficient conductivity at room temperature and below and delivering a high enough cycle count, a weak point with most new battery designs. Prototypes of the solid state lithium ion only reach 100 cycles. Targeted applications are load leveling for renewable energy source and fulfilling the need for personal transportation in cars that are non-polluting, charge in minutes and do not prompt range anxiety. Commercialization can take 10 years or longer.
Last Updated 3/30/2015
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