Over time, Lithium Polymer battery packs tend to puff up full of gases and electrolyte fluids. This is caused by a number of issues, but in general most old Lithium Polymer batteries suffer from puffed cells. This contributes to mechanical stresses within LiPo batteries, which causes them to loose cell capacity because of the pressure put on the LiPo cells and the loss of electrolyte fluid within each cell.
After only a couple hundred discharge cycles, the puffed LiPo battery above needs to be replaced.
Since the cells are enclosed by no more than a few layers of plastic, wear and tear tend to exacerbate the puffed condition of the battery and thereby reduce capacity. Unfortunately, this means that LiPo batteries need to be replaced more often to maintain performance and adequate battery capacity. Additionally, the puffed pack is more prone to rupture and fluid leaking from cells as the plastic covering continues to expand.
Unlike Lithium Polymer cells, Lithium Ion cells do not have the same puff issues that contribute to lower battery life and reduced cell capacity. Lithium Ion cells are contained in a metal casing that protects the lithium metal inside the battery.
The metallic covering on Li Ion cells can prevent electrolyte fluid leakage in the case of an RC plane crash or other incident. The battery seen above was dropped on a sharp object but is still usable. If the same were to happen to a LiPo battery, the cells could become damaged and puff up. In the worst case, if a sharp object were to puncture the LiPo plastic wrap, the battery would burst into flames.
Generally, all lithium batteries contain a thin layer of lithium metal. Lithium polymer batteries have layers of lithium stacked on top of each other. This is why LiPo batteries are generally square, although since LiPo batteries don’t have a metal casing, the layers of lithium can be damaged easily.
The lithium metal plate inside a Li-Ion cell is wrapped around in a cylindrical pattern surrounded by electrolyte fluid. As a result, it’s the lithium metal, even if the cell surface has a dent.