Li-Po Batteries Explained - Part 2

I explained about Li-ion batteries (the predecessor of Li-Po) in my earlier post (Li-Po Batteries Explained - Part 1). Now let's see what these 'Lithium Polymers' are.

Lithium-Polymer Batteries - The real ones

Li-Po batteries - also known as Li-Poly, Pi-Pol, PLI and PLI - are a type of rechargeable batteries that uses same types of electrodes as the Li-ion batteries. The difference is with the electrolyte it uses. Instead of the liquid solvent used in Li-ions, Li-Po uses a polymer electrolyte. In the original design, which dates back to the 1970's, the electrolyte is a dry, plastic-like, thin film. It doesn't conduct electricity, but allows the exchange of ions between the electrodes.

Theoretically, with this design, batteries could be made very thin (less than 1 millimeter) and to a variety of form factors. And because of the electrolyte being dry, the manufacturing process can be simplified and the safety of the battery will be higher because of no risk in flammability. But unfortunately, the dry polymer design has some problems. Because of poor conductivity and higher internal resistance, the ion exchange through the polymer layer is very slow. This increases the time required for charging and reduces the burst current the battery can supply. The ion exchange can be brought to an acceptable level by heating the battery to above 60°C, but this is not practical in most situations.

So how can there be batteries named 'lithium polymer' in the consumer market that works on room temperature? The truth is, almost all the 'Li-Po' batteries currently in the consumer market are not 'true' lithium polymers, but actually Li-Po hybrids.




Li-Po Hybrid Batteries

Li-Po hybrid batteries solve the problem of poor ion exchange by adding a gelled electrolyte to the dry polymer layer. The correct name for these are 'Lithium Ion Polymer' batteries, but for marketing reasons they are simply called Li-Po.

Structure of a Lithium Ion Polymer Battery

The gelled electrolyte layer solves the ion exchange problem, but it introduces a few other small problems. Although not as dangerous as Li-ion, Li-Po Hybrids can still catch fire if overcharged, shorted or punctured. However, because of the following advantages, Li-Po hybrids have become popular with portable electronic devices and radio controlled stuff like RC planes, helicopters boats etc.

• 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. Doesn't require a hard metal casing.
• Improved safety - Less chance for electrolyte leakage than Li-ion.

Li-Po hybrids mostly come in the 'pouch cell' packaging.  The hard metal container of Li-ion cells has been replaced with a flexible, heat-sealed foil. The electrical contacts consist of conductive foil tabs that are welded to the electrode and sealed to the pouch material.

A Pouch Cell

Structure of a Pouch Cell

This pouch cell design utilizes the available space more efficiently than any other types of battery. And, because there is no metal casing, the weight is very low. Pouch cells come in many different sizes because there are no standardized sizes for them.

They do have some standard ratings for voltage, capacity and discharge rate. Let's look at these ratings in the next post.

One thing to notice is that scientists are still trying to improve the original 'true' Li-Po battery design to make it work in normal conditions. If anyone out there can find a solution, just think of the possibilities :)

References
http://batteryuniversity.com/learn/article/the_li_polymer_battery_substance_or_hype
http://en.wikipedia.org/wiki/Lithium-ion_polymer_battery
http://en.wikipedia.org/wiki/Lithium-ion_battery