The Batcave is not only a sanctum-sanctorum for the Batman. It& also a laboratory. Is it any wonder, therefore, that The Dark Knight would celebrate one of the more violently reactive alkali metals in his theme song?
Mixing the alkali metals (The left-most column on the periodic table) with water has increasingly interesting effects as you work your way down the periodic table to larger elements. Lithium, the Period 2 alkali metal, isnt super interesting.