Why would my iPhone 6 be stuck in a dead battery boot loop?
In such cases I, myself, do a boot into DFU mode which actually uses way less energy mainly because the display stays off. 
Then I wait for about a half an hour and then I reboot the iDevice in order to have it properly booted. 

P.S. Follow this procedure in order to enter an iDevice into DFU mode:


Make sure your iDevice is plugged into a power source via it's USB cable; 
Hold down both Home and Power buttons on the iDevice for about 10 seconds until the screen goes off; 
Keep holding the Home button and release the Power button; 
Open  iTunes to check if the iDevice is connected. A message saying that an iDevice in a Recovery mode is connected should show up.