Lithium Ion, also known as 18650 Lithium Battery cell, is an older generation battery which is still the most commonly used batteries in lower capacity power banks. This is mainly due to their low price.

Lithium Polymer, is a more advanced battery with higher specs and provides higher level of safety features, they are therefore more expensive than Lithium Ion batteries.   Despite the higher price, It is advisable to go for Lithium Polymer Power Banks especially when it comes to higher capacity power banks.