This is a great idea!
I wonder about their specs, though. It has the shape of a 16500 cylindrical Li-ion cell, which is typically 3.7V nominal at 2800 mAH. However, by the time the power goes through the boost converter to make 5.1V for the USB jack, it's no longer 2800 mAH, but more like 2000 mAH. After all, boosting voltage requires more input current than output current.
To learn more about boost converters and charging phones, look up AdaFruit's MintyBoost, which is the original version that uses AA alkaline cells and fits in an Altoids tin.