Thanks for your question and interest in being responsible for your old batteries! iFixit prefers that our customers take their old batteries to their local battery recycling facility for proper dispoal. You can find one by doing a quick internet search for "battery recycling facility" and including your zip/postal code or city name in the search.
Alternatively, there is a handy website that we recommend folks use to find a battery drop-off location if you live far away from an actual recycling facility. Check out http://call2recycle.org and plug in your zip/postal code. This will show you all of the registered drop-off locations near you.
Whatever you choose to do, we sincerely hope that you do not dispose of your old batteries in the trash. We try to do our part to keep electronics, especially batteries, from being added to the landfills across the planet.
I hope this helps!