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.
It should be noted that National Power in the UK have also developed a redox flow battery, but one that is based on the use of two very different solutions, one of sodium sulphide and one of sodium bromide. While this system appears to give a higher power density it does carry additional problems as cross-contamination of the electrolytes is destructive to the battery.
In vanadium flow batteries, both half-cells are additionally connected to storage tanks and pumps so that very large volumes of the electrolytes can be circulated through the cell. This circulation of liquid electrolytes is somewhat cumbersome and does restrict the use of vanadium flow batteries in mobile applications, effectively confining them to large fixed installations, although there is one company focusing on electric vehicle applications, where rapid replacement of electrolyte is used to refuel the battery.