Redox Flow
Redox Flow

“Next Generation Batteries” for residential & commercial applications

The resource-saving vanadium redox flow technology is an important component of VoltStorage’s multi-technology approach. With the continuous development of this storage technology, VoltStorage is making important contributions to the development of pioneering stationary battery solutions.

Technological benefits

Hand mit kleiner Weltkugel


With the vanadium redox flow technology, we rely on a proven CO2-reducing battery technology. A study by the Technical University of Munich has shown that VRF batteries produced by VoltStorage result in up to 37% less CO2 emissions than the production of comparable lithium batteries.

Pflanze mit Sonne


The vanadium redox flow technology does not require any rare earths. The vanadium used in the storage medium is obtained as a by-product of the iron production – and thus without overexploitation of nature and the associated impact on our ecosystems.

Frau auf Wiese bei Sonnenuntergang

100% non-flammable

Operational safety plays a key role in energy storage. The VRF technology offers a decisive advantage in this respect: The vanadium-based electrolyte consists exclusively of non-combustible components, and is mostly pure water. VRF storage systems are therefore 100 percent non-flammable and offer more operational safety than other storage solutions.

operating principle


The storage medium is a vanadium-based liquid electrolyte. It is stored in two separate tanks takes on different oxidation states through a redox process. This process makes storing energy possible.


The electrolyte flows through the battery cells in two separate circuits. When the battery is charged, the electrolyte is reduced in the negative half cell and oxidized in the positive half cell. When discharging, this process is reversed again.


A special pump system flows the electrolyte into and through the battery cells. This makes it possible that not all the electrolyte has to be kept in the cells, enabling the decoupling of energy capacity and power. The pumping system is only activated on demand.

Vanadium-Redox-Flow Info Graphic

Positive electrolyte

Discharged, the positive electrolyte circuit contains vanadium ions with an oxidation state +4. In this state the electrolyte exhibits a blue colour. During charging, the vanadium ions oxidize and take on the oxidation state +5.

When the storage system is fully charged, the positive electrolyte circuit only contains vanadium +5. The electrolyte then has a yellow colour.

Negative electrolyte

Discharged, the negative electrolyte circuit contains vanadium ions with an oxidation state of +3. In this state the negative electrolyte exhibits a blue-green colour. During charging, the vanadium ions reduce and take on the oxidation state +2.

When the storage system is fully charged, the negative electrolyte circuit only contains vanadium +2. The electrolyte then has a purple colour.

Battery stack

The positive and negative electrolytes are pumped through special battery cells for the charging and discharging processes. Each battery cell consists of two half-cells separated by a selective membrane. The membrane is ion-permeable so that during charging and discharging, freed-up ions can migrate through the membrane into the other electrolyte circuit. However it is selective, so that the vanadium ions can not pass. Inside of each half cell, the electrolytes undergo redox reactions which ensure that electrical energy is converted into chemical energy and stored.

Pumping system

A pump system moves the positive and the negative electrolytes within each circuit, supplying the battery cells with fresh electrolyte from the storage tanks.

Redox-Flow Batteriezelle von VoltStorage

Bipolar plate

The bipolar plate electrically connects the cells of a redox-flow battery stack. It thus provides the electrical conductivity, which is increased from a material standpoint by using particularly conductive and corrosion-resistant materials, such as graphite.

Ion exchange membrane

The transparent and thin ion exchange membrane separates the positive and negative half cells. The membrane ensures selective ion exchange between the half-cells, preventing transfer of vanadium ions between half cells, which is necessary for efficient charging and discharging.

Graphite felt

The electroactive area is located in the centre and is completely filled with a graphite felt. The fine-meshed felt has a very large surface area and thus improves the electrochemical redox process while at the same time allowing the electrolyte to flow through. Direct contact with the ion exchange membrane results in a charge exchange between the differently charged half cells.

Cell frame with electrolyte channels

The cell frame of the redox flow battery cells from VoltStorage contain many electrolyte-guiding channels. These channels have been specially designed to ensure uniform supply of electrolyte to the electroactive area of the battery cells.

Vanadium Redox Flow Battery cell

The cells of a vanadium redox flow battery each consist of two half cells. Each half cell contains a frame with specially designed channels to ensure evenly distributed electrolyte supply to the entire electroactive area. The electroactive area is located in the centre of the half cell and is completely filled with graphite felt. The half cells are separated by an ion exchange membrane. Each battery cell is enclosed by two bipolar plates.

suitable applications

VoltStorage customer with battery

Residential batteries

Thanks to our multi-technology approach, we can develop pioneering battery storage solutions for all stationary applications, including private households. These systems are particularly characterized by long lifetimes without energy losses as well as a high level of operational safety.

Commercial applications

With the vanadium redox flow technology, we rely on a proven and tested technology to enable commercial customers to cover their energy needs during periods of low wind or low solar power generation.

track record

Since the market launch of our residential flow battery system in 2019 , we have constantly increased the number of installed systems. Meanwhile, we have the largest fleet of operating flow batteries in the world.


Technological outlook

Commercial flow battery development

Following the successful launch of our home storage solution based on the vanadium redox flow battery technology, we are already working on the next “Next Generation Battery”: A VoltStorage battery storage system for commercial applications – resource-saving, climate-friendly and safe.

We want to give commercial businesses and farms the opportunity to rely on resource-saving battery technologies. For 100% renewable energy around the clock.

Next generation: flow battery cell stack

With our patented flow battery cell design, we were the first company in the world to succeed in developing a battery cell stack for flow batteries that can be manufactured automatically and thus produced in high numbers. But we are not standing still and are already working on the next generation of flow battery cell stacks.

The experience of developing, building and operating our flow battery fleet and the analysis of more than 1,000 components and materials is enabling us to develop a new and improved flow battery cell design. This sets the course for the successful development of our “Next Generation Batteries”.