From Wikipedia!
A drawback of the Li-ion battery is that like other secondary cells, its service life is dependent upon aging (shelf life), and with some chemistries this aging is more rapid than with others. From time of manufacturing, regardless of whether it was charged or the number of charge/discharge cycles, the battery will decline slowly and predictably in "capacity". This means the charge in an older battery will not last as long as in a new battery due solely to its age, unlike other
batteries. This is due to an increase in internal resistance, which affects its ability to deliver current, thus the problem is more pronounced in high-current applications than low