Pb(Lead acid) & LiIon Battery lifespan
This is due primarily to internal corrosion of the lead frames and the electrodes, short circuits cause the sulfation of the lead.
  This causes sulfation, that makes the PbSO4 crystals merge into ever larger networks.
  This reduces the electrochemical active surface of the PbSO4. Due to the now smaller surface, the PbS04 cannot be "come off" that good , as it could before.
  So it takes very long until a sufficiently high concentration of Pb2+ is present.
  In addition, the electrical conductivity of the sulfate is lower than that of lead. The resulting increased Internal resistance of the load cell leads to increased voltage drop. 
  Give that process enough time and your at the point where you wont have any cell voltage at all.

I hope this helped you.

P.S.: i didnt know a good word for "ablösen" in english, since unsolder is not the one i was looking for and peel of is not the right term. but i think youll still get it.