When it comes to battery storage - it's all about how many atoms you can cram into the space. Lithium is good because it's atomic number is just 3. The third lightest element in the universe. It's a solid at room temperature, it's a metal and it's chemically active. That makes it just about the perfect thing to make batteries out of.
There are two, even lighter, elements: helium is a gas at room temperature, but worse - it's chemically non-reactive (noble gas) - that makes it completely impossible to use for batteries. 
Hydrogen IS chemically active, and the lightest element of all. Trouble is -it's a gas at room temperature, and that's not ideal for batteries (though hydrogen-cells may win out in the end-  they aren't quite 'batteries' though). But:
Metallic hydrogen has the highest energy density of any substance possible.  We can't make it reliably yet, and it will be a nightmare to store.

But if we crack it. We'll build Tessla's with a hundred-thousand kilometer battery range.