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Lithium Polymer Batteries

What are Lithium Ion Batteries?

History: The Li-polymer batteries differentiate themselves from other battery systems in the type of electrolyte used. The original design, dating back to the 1970s, uses a dry solid polymer electrolyte only. This electrolyte resembles a plastic-like film that does not conduct electricity but allows an 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 geometry.

Lithium Polymer Batteries - Custom built by PMBL

There is no danger of flammability because no liquid or gelled electrolyte is used.

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With a cell thickness measuring as little as one millimeter (0.039 inches), equipment designers are left to their own imagination in terms of form, shape and size. 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 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. 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 is unsuitable in commercial applications.

Research is continuing to develop a dry solid Li-polymer battery that performs at room temperature. A dry solid Li-polymer version is expected to be commercially available by 2005. It is expected to be very stable; would run 1000 full cycles and would have higher energy densities than today’s Li-ion battery.

In the meantime, some Li-polymers are used as standby batteries in hot climates. One manufacturer has added heating elements that keeps 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 affect the service life of this battery in the same way it does the VRLA, for example.

To make a small Li-polymer battery conductive, some gelled electrolyte has been added. Most of the commercial Li-polymer batteries used today for mobile phones are a hybrid and contain gelled electrolyte. The correct term for this system is ‘Lithium Ion Polymer’. For promotional reasons, most battery manufacturers mark the battery simply as Li-polymer. Since the hybrid lithium polymer is the only functioning polymer battery for portable use today, we will focus on this chemistry.

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 it uses a solid electrolyte, replacing 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. This postponement, as some critics argue, is due to ‘cashing in’ on the Li-ion battery. Manufacturers have invested heavily in research, development and equipment to mass-produce the Li-ion. Now businesses and shareholders want to see a return on their investment.

In addition, the promised superiority of the Li-ion polymer has not yet been realized. No improvements in capacity gains have been 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 thin profile has, however, compelled mobile phone manufacturers to use this promising technology for their new generation handsets.

One of the advantages of the Li-ion polymer, however, is simpler packaging because the electrodes can easily be stacked. Foil packaging, similar to that used in the food industry, is being used. No defined norm in cell size has been established by the industry.

Additionally, Lithium Polymer (or Li-poly) Battery Companies (such as PMBL) can shape the battery pack to almost any shape they please, which can be important to manufacturers of small or odd shaped electronic devices that often require smaller, thinner, and lighter devices. Some Lithium Polymer Battery Packs are becoming increasingly popular in radio controlled vehicles and aircraft where the benefits of lower weight and significant increases to power longevity justifies the higher price. 

There are however some potentially explosive issues with Lithium Polymer Batteries. For example, they need Battery Pack Chargers specific to Lithium Polymer Batteries to avoid igniting and exploding. Explosions can also occur if the Lithium Polymer Battery Pack is short-circuited, as a very high current passes through the battery pack almost instantaneously. 

It's therefore essential that precautions are taken to ensure that leads and connectors are manufactured and installed correctly and the appropriate insulation is used. Also, to prevent battery pack damage, controls must be included to halt any excessive discharge.

PMBL (a UK based Custom Battery Manufacturer) can supply Lithium Polymer batteries from Varta, Sanyo, Ultralife and many others.

Advantages and Limitations of Lithium Polymer Batteries

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. 

For more information about the Lithium Ion Batteries and how they can help you build your application or if you would like advice from one of our Battery Specialists, please get in touch. 

Call us on 01460 66366 or email solutions@pmbl.co.uk or Click Below. 

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