Lithium batteries a reality - why not sodium, pottasium batteries?
The sodium–sulfur battery is already in use for power leveling and other stationary applications, such as a 1.2 MWh installation in WV, USA. The Ford Ecostar did use a Na-S battery, but it did not have sufficient energy for distant travel (and some of the experimental batteries caught fire, but so do Li-ion cells on occasion. Oops!).

Sodium-ion batteries are also in development. In the JACS, the article Na-ion batteries get closer to replacing Li-ion batteries discusses some improvements in maximum current capacity and total energy storage.

That said, for mobile use, lithium has a higher energy/weight ratio than heavier metals, but for other use, sodium is less expensive and is readily available.