Why do batteries have a short lifetime despite having no moving parts?
It is due to the solid state chemistry. In the most simplistic of descriptions, the surfaces of the anode and cathode will have imperfections. Over time, this will result in some of the ion channels becoming restricted as the ions react with the anode/cathode surface, deforming the structure. As the ions cannot move as freely, capacity will suffer.

http://www.extremetech.com/extreme/183338-scientists-discover-the-reason-that-batteries-lose-capacity-over-time-nanocrystals