Recently, there has been renewed world interest in vanadium. It is used
in some high-capacity Lithium and Lithium-ion batteries, with applications in
electric and hybrid cars. More significantly, experimental "Vanadium redox"
batteries are showing promise to allow electricity generating stations to
finally solve the problem of storing the massive amounts of excess energy
which today are simply lost when demand drops below generating capacity. This
is an especially acute problem for wind, solar and other unpredictable energy
generators.

http://en.wikipedia.org/wiki/Vanadium_redox_battery