Vanadium redox battery

The vanadium redox battery (VRB), also known as the vanadium flow battery (VFB) or vanadium redox flow battery (VRFB), is a type of rechargeable flow battery that employs vanadium ions in different oxidation states to store chemical potential energy. The vanadium redox battery exploits the ability of vanadium to exist in solution in four different oxidation states, and uses this property to make a battery that has just one electroactive element instead of two. For several reasons, including their relative bulkiness, most vanadium batteries are currently used for grid energy storage, i.e., attached to power plants or electrical grids.
The possibility of creating a vanadium flow battery was explored by Pissoort in the 1930s, NASA researchers in the 1970s, and Pellegri and Spaziante in the 1970s, but none of them were successful in demonstrating the technology. The first successful demonstration of the all-vanadium redox flow battery which employed vanadium in a solution of sulfuric acid in each half was by Maria Skyllas-Kazacos at the University of New South Wales in the 1980s. Her design used sulfuric acid electrolytes, and was patented by the University of New South Wales in Australia in 1986.
Numerous companies and organizations are involved in funding and developing vanadium redox batteries.
Advantages over other battery types.
The main advantages of the vanadium redox battery are that it can offer almost unlimited energy capacity simply by using larger electrolyte storage tanks; it can be left completely discharged for long periods with no ill effects; if the electrolytes are accidentally mixed, the battery suffers no permanent damage; a single state of charge between the two electrolytes avoids the capacity degradation due to a single cell in non-flow batteries; the electrolyte is aqueous and inherently safe and non-flammable; and the generation 3 formulation using a mixed acid solution developed by the Pacific Northwest National Laboratory operates over a wider temperature range allowing for passive cooling. 
VRFBs can be used at depth of discharge (DOD) around 90% and more, i.e. deeper DODs than solid-state batteries (e.g. lithium-based and sodium-based batteries, which are usually specified with DOD=80%). In addition, VRFBs exhibit very long cycle lives: most producers specify cycle durability in excess of 15,000-20,000 charge/discharge cycles. These values are far beyond the cycle lives of solid-state batteries, which is usually in the order of 4,000-5,000 charge/discharge cycles. Consequently, the levelized cost of energy (LCOE, i.e. the system cost divided by the usable energy, the cycle life, and round-trip efficiency) of present VRFB systems is typically in the order of a few tens of $ cents or € cents, much lower than the LCOEs of equivalent solid-state batteries and close to the targets of $0.05 and €0.05, stated by the US Department of Energy and the European Commission Strategic Energy Technology (SET) Plan, respectively.
Disadvantages from other battery types.
The main disadvantages of vanadium redox technology are a relatively poor energy-to-volume ratio compared to standard storage batteries, and relatively poor round trip efficiency. Furthermore, the aqueous electrolyte makes the battery heavy and therefore only useful for stationary applications. Another disadvantage is the relatively high toxicity of oxides of vanadium (see ).
Operation.
A vanadium redox battery consists of an assembly of power cells in which the two electrolytes are separated by a proton exchange membrane. The electrodes in a VRB cell are carbon based; the most common types being carbon felt, carbon paper, carbon cloth, and graphite felt. Recently, carbon nanotube based electrodes have gained marked interest from the scientific community. Both electrolytes are vanadium-based, the electrolyte in the positive half-cells contains VO2+ and VO2+ ions, the electrolyte in the negative half-cells, V3+ and V2+ ions. The electrolytes may be prepared by any of several processes, including electrolytically dissolving vanadium pentoxide (V2O5) in sulfuric acid (H2SO4). The solution remains strongly acidic in use.
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.
When the vanadium battery is being charged, the VO2+ ions in the positive half-cell are converted to VO2+ ions when electrons are removed from the positive terminal of the battery. Similarly in the negative half-cell, electrons are introduced converting the V3+ ions into V2+. During discharge this process is reversed and results in a typical open-circuit voltage of 1.41 V at 25 °C.
The positive electrode (cathode) half-reaction is
The negative electrode (anode) half-reaction is
The full reaction (left to right: discharging, right to left: charging) being
Note that a proton must be transferred across the cell membrane when an electron is transferred between the electrodes in order to maintain charge neutrality.
Other useful properties of vanadium flow batteries are their very fast response to changing loads and their extremely large overload capacities. Studies by the University of New South Wales have shown that they can achieve a response time of under half a millisecond for a 100% load change, and allowed overloads of as much as 400% for 10 seconds. The response time is mostly limited by the electrical equipment. Unless specifically designed for colder or warmer climates, most sulfuric acid-based vanadium batteries work only between about 10 and 40 °C. Below that temperature range, the ion-infused sulfuric acid crystallizes. Round trip efficiency in practical applications is around 65–75 %.
Proposed improvements.
Second generation vanadium redox batteries (vanadium/bromine) may approximately double the energy density and increase the temperature range in which the battery can operate. The vanadium/bromine and other vanadium based systems also reduce the cost of vanadium redox batteries by replacing the vanadium at the positive or negative electrolyte by cheaper alternatives such as cerium.
Specific energy and energy density.
Current production vanadium redox batteries achieve a specific energy of about 20 Wh/kg (72 kJ/kg) of electrolyte. 
More recent research at UNSW indicates that the use of precipitation inhibitors can increase the density to about 35 Wh/kg (126 kJ/kg), with even higher densities made possible by controlling the electrolyte temperature. This specific energy is quite low compared to other rechargeable battery types (e.g., lead–acid, 30–40 Wh/kg (108–144 kJ/kg); and lithium ion, 80–200 Wh/kg (288–720 kJ/kg)).
Applications.
The extremely large capacities possible from vanadium redox batteries make them well suited to use in large power storage applications such as helping to average out the production of highly variable generation sources such as wind or solar power, helping generators cope with large surges in demand or leveling out supply/demand at a transmission constrained region.
The limited self-discharge characteristics of vanadium redox batteries make them useful in applications where the batteries must be stored for long periods of time with little maintenance while maintaining a ready state. This has led to their adoption in some military electronics, such as the sensor components of the GATOR mine system. Their ability to fully cycle and stay at 0% state of charge makes them suitable for solar + storage applications where the battery must start each day empty and fill up depending upon the load and weather. Lithium ion batteries, for example, are typically damaged when they are allowed to discharge below 20% state of charge, so they typically only operate between about 20% and 100%, meaning they are only using 80% of their nameplate capacity.
Their extremely rapid response times also make them superbly well suited to uninterruptible power supply (UPS) type applications, where they can be used to replace lead–acid batteries and even diesel generators. Also the fast response time makes them well-suited for frequency regulation. Also, these capabilities make vanadium redox batteries an effective "all-in-one" solution for microgrids that depend on reliable operations, frequency regulation and have a need for load shifting (from either high renewable penetration, a highly variable load or desire to optimize generator efficiency through time-shifting dispatch).
Largest vanadium grid batteries.
A 200 MW, 800 MWh (4 hours) vanadium redox battery is under construction in China; it was expected to be completed by 2018 and its 250 kW/ 1MWh first stage was in operation in late 2018
Companies funding or developing vanadium redox batteries.
Companies include UniEnergy Technologies, StorEn Technologies, and Ashlawn Energy in the United States; Renewable Energy Dynamics Technology and VoltStorage in Europe; Prudent Energy in China; Australian Vanadium in Australia.