Lithium Ion batteries pack a lot of power by weight compared to other types. There are 2 things that need to be handled differently than nicad on NiMH:
1. They cannot be used as a direct substitute (even if they look like other AA’s) since they run at about 3.6 (or so) volts.
2. They cannot be charged in the same way as nicad or NiMH.
Sooner or later you are gonna need a Li Ion charger in one of your projects, so it’s better to be prepared.
DIY Lithium Ion Battery Charger: [Link]