A vanadium redox flow battery (VRFB), is reportedly very competitive to lithium-ion battery options when looking at grid-scale storage. Why? They are apparently more easily scaled-up, can release charge over a far-more extended period of time (10 - 18 hours) and can reportedly be charged and discharged an unlimited number of times with no capacity degradation - meaning they last decades. Oh - any they also don't catch on fire as easily as lithium-ion batteries either.