A very helpful tool to understand the principles underlying material properties is the descriptor concept, which can be regarded as the relation between desired or undesired features such as the ion mobility with basic material properties. Once such a descriptor has been successfully established, the search for materials with desired functional properties can be significantly facilitated as there is no longer the need to perform numerous timeconsuming experiments or numerically challenging quantum chemical calculations. However, identifying descriptors can not been fully separated from the fundamental issues discussed above as the very existence of a descriptor already reflects the presence of a fundamental principle underlying the existence of this descriptor.