Lithium batteries a reality - why not sodium, pottasium batteries? Lithium batteries seem to be mainstream now, offering high energy density, high output current capability.

Stepping down the periodic table there are alkali metals with greater electronegativity than Lithium: sodium and potassium, etc. 

So why not sodium and potassium batteries. Can't these offer higher energy density, higher current output? Is the reactivity just so great that the chemistry can't be engineered to provide a practical battery? Or are the safety issues just too great?
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.