Lithium Polymer Vs. Ion Battery
Lithium ion batteries were developed in the 1970s and became commercially available at the end of the 1990s and quickly became ubiquitous in many mobile electronic devices. Their popularity grew because of their smaller size and the fact that they had no memory effect, unlike other rechargeables of the time, most notably nickel metal hydrides. Lithium polymer batteries, though not having the same capacity as lithium ions, were developed in the 1990s and are even smaller in size than lithium ion batteries.
Lithium Ion Chemistry
Lithium ion batteries use an anode of carbon, a cathode made of lithium cobalt oxide and an electrolyte gel. When charging, lithium ions move from the cathode to the anode. When discharging, they move in the opposite direction.
Lithium Polymer Chemistry
Lithium polymer batteries have the same basic chemistry as lithium ion batteries. However, the polymer cells use a porous separator that, when exposed to the electrolyte, turns to a gel. Because the gel isn't flammable, lithium polymer batteries have a different architecture, with the anode and cathode developed as a plate and stacked on top of each other. Lithium polymer batteries do not need a metal shell the way that lithium ion batteries do. In fact, the shell of lithium polymer batteries is often plastic.
Dangers
Lithium ion batteries do have issues, mainly due to their potential to overheat. Because of this, lithium ion batteries have an active protection circuit--essentially an onboard computer--that prevents the battery from overheating and potentially bursting into flame. Lithium polymer batteries do not need the active protection circuit, which is why it is possible to manufacturer them in sizes as small as a credit card.
Degradation
Lithium ion batteries start to degrade from the moment they are made, and, in fact, will become inoperable after two to three years even if they aren't used.
Uses
Lithium ion batteries have a greater energy capacity than lithium polymer batteries, so in devices that have higher current requirements, lithium ion batteries are preferred. In applications where size is paramount, lithium polymer is the choice. Lithium ion batteries are also cheaper to manufacture than lithium polymer batteries, so when cost is a factor, lithium ion is the choice.