If you can, avoid lithium poly batteries !!!!
The only applications I would use them for are high current draw projects like r/c motors used outdoors.
They are very dangerous should you miss handle them in just about any way, lithium reacts violently with oxygen. The cells can be damaged easily from mischarging, discharging, temperatures, and so on. When a cell is damaged it will produce gas, usually this is contained by it coverings, but if that fails it may explosively pop followed by very hot burning chemical fire.
If your just powering an arduino switch to something more traditional or it's safer cousin lithium iron. Less voltage per cell, still has the same if it leaks risks but considered more stable, less likely to fail from miss handling.