Vanadium Stores Solar Power Efficiently

"The ease with which you can hand electrons to the vanadium and take them away - this is the basis of a very, very stable battery."

Vanadium "redox flow" batteries are indeed stable. They can be discharged and recharged 20,000 times without much loss of performance, and are thought to last decades (they have not been around long enough for this to have been demonstrated in practice).

http://www.bbc.com/news/magazine-27829874