Lithium-Polymer batteries are often used in cellphones, and other applications where weight is key. Instead of suspending the electrolite in a solvent like LiIo batteries, they are mixed with a polymer, pressed into sheets and laminated onto each other. Because they are constructed of laminated layers, a metal casing isn't needed, so they are much lighter and can be constructed in a variety of odd shapes and sizes. They are also non-flammable.