The vanadium redox battery (or redox flow battery) exploits the ability of vanadium to exist in 4 different oxidation states, and uses this to make a battery that has just one chemical electrolyte instead of two.
The main advantages of the vanadium redox battery is that it can offer almost unlimited capacity simply by using larger and larger storage tanks, it can be left completely discharged for long periods with no ill effects, it can be recharged simply by replacing the electrolyte if no power source is available to charge it, and if the electrolytes are accidentally mixed the battery suffers no permanent damage. The main disadvantages with Vanadium redox technology are a relatively poor energy-to-volume ratio, and the difficulty in storing and handling large volumes of the (somewhat corrosive) liquid electrolytes.
A Vanadium redox battery consists of a power cell in which two electrolytes are kept separated by an ion exchange membrane, somewhat confusingly, both electrolytes are vanadium based. The electrolyte in the positive half-cell contains VO2+ and VO2+ ions, the electrolyte in the negative half-cell, V3+ and V2+ ions. This electrolyte is typically prepared by electrolytically dissolving vanadium pentoxide (V2O5) in sulphuric acid (H2SO4) and the solution remains strongly acidic in use.