I have the Unu battery case for my iPhone 5S. The case is a little thicker to have room for the battery, but it is great to be able to flick a switch and charge my phone! The micro-USB connection charges the phone, the battery case, and is a pass-through for tethering to a PC (iTunes...etc).
They have a thin version for the iPhone 6, and a higher capacity version which is probably a bit thicker like my case.
iPhone 6 Battery Case, Chargers and Battery Packs | uNu
-Z-man.