At a typical 100% charge level (notebook battery, full most of the time) at 25 degrees
Celsius, Li-ion batteries irreversibly lose approximately 20% capacity per year from the time they are manufactured, even when unused. (6% at 0 °C, 20% at 25 °C, 35% at 40 °C). When stored at 40% charge level, these figures are reduced to 2%, 4%, 15% at 0, 25 and 40 degrees Celsius respectively.) Every (deep) discharge cycle decreases their capacity. The degradation is sloped such that 100 cycles leave the battery with about 75% to 85% of the original. When used in notebook computers or cellular phones, this rate of deterioration means that after three to five years the battery will have capacities too low to be still usable.
A unique drawback of the Li-ion battery is that its life cycle is dependent upon aging from time of manufacturing (shelf life) regardless of whether it was charged, and not on the number of charge/discharge cycles. This drawback is not widely publicized.