Allocacoc PowerBank Slim Aluminum 5000mAh Silver
Probably the thinnest you can find, this power bank from Allocacoc then also makes a great portable option.
It has a capacity of 5000mAh, which means you can fully charge your smartphone up to 1.5 times.
Check how much charge is left by shaking the PowerBank. The battery level is indicated by three different colours. Charge the PowerBank with a microUSB or Lightning.