There are a couple of problems with Lithium Ion batteries:
1. The life of the battery decreases over time regardless of usage (as
much as 50% reduction of battery life per year)
2. Lithium Ion batteries do not handle high current as well as NiMH or
NiCd. NiCd handles high current loads the best between the 3 battery
3. Lithium Ion batteries do not charge quickly.
4. Lithium Ion batteries are not very durable.
5. Lithium Ion batteries have MAJOR problems with overcharging (i.e.
you will severely damage your batteries if you overcharge them).
6. Lithium Ion batteries typically only survive 300-500 recharge
There are advantages to Lithium Ion batteries:
1. No memory effect (although it IS possible to recondition a NiCd and
NiMH to remove memory effect, but it requires dismantling the
2. Highest ratio of energy density vs weight (i.e. they are light)
In the situation of power tools, I would say Lithium Ion batteries are
a BAD choice. But don't take my word for it. There are some good
articles on www.wikipedia.com (just do a search on each type of
battery) and www.batteryuniversity.com. I would wait until LiIon or
Lithium Polymer battery technology/chemistry further matures.