What are the disadvantages of Lithium Ion batteries compared with other rechargeable batteries?
Lithium-ion batteries are more expensive than similar capacity NiMH or NiCd batteries. This is because they are much more complex to manufacture. Li-ion batteries actually include special circuitry to protect the battery from damage due to overcharging or undercharging. They are also more expensive because they are manufactured in much smaller numbers than NiMH or NiCd batteries. Li-ion batteries are becoming less expensive and over time we should see their price decrease significantly.
Lithium ion batteries are not available in standard cells sizes (AA, C and D) like NiMH and NiCd batteries.
UPDATE: I just discovered an overseas manufacturer that sells down regulated li-ion cells in several standard cell configurations, AA, AAA, etc. that outputs 1.5 VOLT S - whaatttt?? How long have they been on the market? Are they reliable? Are they safe? Kentli is their name and I need to get some samples and test the heck out of them ASAP. Lithium ion is considered a "volatile" cell chemistry so I am a bit skeptical but finally a manufacturer has gone and ventured into that territory. This could be both disruptive and awesome. More to come. ;-)
Lithium-ion batteries also require sophisticated chargers that can carefully monitor the charge process. And because of their different shapes and sizes each type of Li-ion battery requires a charger designed to accommodate its particular size. This means lithium ion battery chargers are more expensive and more difficult to find than NiMH and NiCd battery chargers.