I know there is a huge effort regarding throwing away /recycle/re-use.
My iPhone 6 used to belong to DD , it got gifted to me . I am happy enough , it does the job, but the battery life is now rubbish. (It used to be brilliant when I got it)
For example - fully charged , it was down to 4% by lunch time , I hadn't used it , had to plug it into the portable charger to make a call.
Then , from full charge it went to 52% just from clearing some emails 
So, it isn't safe if I rely on it . I need to carry my charger pack and cable ( like carrying a spare phone size-wise)
The phone has the minimal functions open (I don't use wifi , internet , don't text much or take many photos. I do have HandsFree)
Its the battery , no point buying a new one , I don't really want a new phone . I can keep charging though its a pain in the arse
But this is going to be a problem for 1000s of people, practically everyone has an iPhone now . What will happen to them all when the batteries go?