This was my great fear when I owned a large iPod collection (I no longer own any BTW). But most iPods can be easily damaged when opened, removing the batteries would be quite the undertaking depending on the size of your collection... Any luck so far?
So far I have correctly disposed of all the batteries, I have not done any damage except for some aesthetic marks on the edge of some.
However, if you want change battery on ipod nano 4 and 5 gen you have to perform a very delicate and complex process that leads to damage very easily and finding a 4th or 5th generation nano with a working battery today is a big challenge.
I still have 2 ipod nano 5gen with the battery inside, but it's practically exhausted and I'll have to get rid of it sooner or later.
Also the ipod touch still has battery, but sooner or later i'll have to open it to do that but i don't like working with glued devices.