Li-Ion
Li-Ion batteries use a flammable solvent based organic liquid as the electrolyte. This electrolyte is responsible for the lithium ion exchange between the electrodes (anode and cathode) just like any type of battery. Li-Ion batteries are usually encased in a hard metal can to keep the electrodes wound up tight against the separator sheet adding weight and not allowing many different options as far as shape and size.
Li-Po
A true Li-Po battery does not use a liquid electrolyte but instead uses a dry electrolyte polymer separator sheet that resembles a thin plastic film. This separator is sandwiched (actually laminated) between the anode and cathode of the battery (lithium carbon coated aluminum & copper plates) allowing for the lithium ion exchange – thus the name lithium polymer. This method allows for a very thin and wide range of shapes and sizes of cells.
The problem with true Li-Po cell construction is the lithium ion exchange through the dry electrolyte polymer is slow and thus greatly reduces the discharge and charging rates. This problem can be somewhat overcome by heating up the battery to allow for a faster lithium ion exchange through the polymer between anode and cathode, but is not practical for most applications. If they could crack this problem, the safety risk of lithium batteries would be greatly reduced.
With the big push towards electric cars and energy storage, there is no doubt some pretty huge developments will be made in ultra light weight dry and safe Li-Po’s in the coming years. Seeing that theoretically this type of battery could be made flexible, almost like a fabric, just think of the possibilities.
Li-Po Hybrids
All RC Li-Po batteries out there at the time of this write up (July 2014) are actually a hybrid lithium polymer battery. The correct name for this type of battery is lithium-ion polymer, but the battery world of today simply calls them lithium polymer even though they are not a true dry type Li-Po battery.
By introducing a gelled organic/solvent based electrolyte to saturate the polymer separator, the lithium ion exchange rate between anode and cathode is improved immensely. Li-Po hybrids like Li-Ion can still burst and catch on fire if over charged, shorted, punctured, or incinerated.
When first introduced, Li-Po batteries were more expensive than Li-Ion because they are more labor intensive to manufacture. Fortunately prices have dropped substantially since they have become as, if not more popular than Li-Ion battery technology. This holds especially true for electric powered RC aircraft and the real driver behind Li-Po battery research –portable communication/entertainment devices.
Li-Po hybrids use the same flat cell structure as their dry counter parts meaning they have the same flexibility with sizes and shapes allowing for very specialized shaped battery packs perfect for use in our RC models.
Almost every RC Li-Po battery cell is packaged in a foil pouch coincidentally called a pouch cell.
Pouch cells are the perfect solution for building battery packs since the flat pouch cell can be stacked with no wasted air spaces like found within round celled battery packs. Of course since Li-Po’s use this light weight pouch instead of a metal can, less weight is the result making Li-Po’s the preferred choice over Li-Ion in a weight conscious application such as RC aircraft.
One interesting characteristic hybrid Li-Po batteries share to an extent with their dry counterparts is they do get more efficient at ion exchange once warmed up. If you have ever noticed your RC model seem to gain a little more power a minute or so after working the battery; what you are experiencing is the increase in ion exchange efficiency once the battery chemistry warms up.
This should have you thinking that if you fly your electric RC helicopter or plane in the winter time, you might want to keep your RC Li-Po battery packs in a warm place prior to the flight.
