I trust that most batteries from iPhones are currently recycled through proper means either by Apple or third party firms.

I don't know how most people will dispose of user replacement batteries, but I suspect the recycle rates will be lower. If you want to ensure higher rates you also need to do something they do in the USA for car lead acid batteries. Charge a deposit fee on the new battery that is returned only when the battery is turned into a valid recycling entity.