Depends on the model, but lithium polymer usually use cobalt oxide style lithium cells, the ones most likely to ignite if abused, but also the most energy-dense. They have the problem that lithium has a higher affinity for oxygen than cobalt, allowing a type of thermite reaction between Li and CoO2 to be triggered easily if the cell overheats. These cells are 3.7V nominally per cell.
The exact formulation will be proprietry, note, manufacturers don't easily give away their commercial secrets! For instance carbon nanotubes are often employed to increase the current handling (power density).