You aren’t so far away from the truth!

To make a battery you need to have something that holds negative electrical charge and something with a positive electrical charge and both need to be able to change to a different state when you use it or reverse that change when you charge it.

Lithium is the lightest and smallest metal, meaning for the same size and electrical charge, your battery will weigh less.

Then you just need to find ways to make two kind of lithium compounds which have different electrical charge and can be changed between two states.

And if it doesn’t explode when a child throws their battery powered bear on the ground, that would also be a good characteristic.