system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What's so special about lithium that makes it good for batteries, and why is it impossible to substitute a different element—sodium, say, or potassium?
gpt
The energy of batteries is due to the change in the oxidation state of Cobalt when the cathode transforms between LiCoO2 and CoO2.  So the Lithium has to be physically removed from the cathode crystal structure and transported to the anode via an ionic conductive electrolyte during charging.

Lithium is small enough so that it can enter and exit the cathode with relative ease.  Larger ionic atoms like Sodium or Potassium would deform the cathode too severely, leading to a much faster capacity degredation.