Ambitious Outsiders

An article from carl 03|2025

by Frank Frick

The lithium-ion battery is currently the benchmark for all other types of battery storage. But in labs across the globe, alternatives are currently in the starting blocks for certain applications. We take a look at German research into a special type of battery: redox flow batteries.

Lithium-ion batteries are big business in China. Six of the ten biggest battery manufacturers for electric cars are Chinese. Yet away from European attention, China is also working on expanding its leading position into another battery technology: redox flow systems, which are primarily promising as large, stationary batteries for wind and solar power.

The Chinese port city of Dalian is home to the world’s largest redox flow battery, with an output of 100 megawatts and a storage capacity of 400 megawatt hours. This battery could supply around 60,000 German two-person homes with energy for a whole day. The plant, constructed by the company Dalian Rongke Power, has been on the grid since 2022. In China, more than 30 further such batteries with around 100 megawatt output and a total capacity of 18 gigawatt hours are planned. In addition, Chinese companies have published tenders for even bigger plants with an output of one gigawatt.

Redox flow batteries are made up of a control unit, two pumps, at least two tanks and a stack. A membrane separates two circuits in the stack electronically, but allows ions to pass through. The electrolytes – solutions of substances that can absorb or release electrons – are located in the tanks. To store energy, the electrolyte is pumped into the two separate circuits through the stack. Redox reactions, which consume energy, then take place in parallel at the electrodes in the stack. When the energy is needed later, the solutions are pumped into the stack again via the same circuits, where the redox-active substances are converted back again and, in the process, release the stored energy.

The energy density of redox flow batteries is significantly lower than that of lithium-ion batteries, for example [1], but redox flow batteries have a special feature: the storage capacity and output can be increased separately from each other. That’s because you can select the volume of the tank independently of the size of the reaction stack and the electrode surfaces. Thanks to this property, redox flow batteries are especially suited for home energy storage or other stationary use cases that require bridging extended periods of low electricity supply. 

Technologically mature: vanadium flow batterie

Redox flow batteries can be traced back to an idea by the American chemical engineer Lawrence Thaller from the year 1975. But somebody else was responsible for the practical implementation of this idea: Maria Skyllas-Kazacos, an Australian chemical engineer. She used a solution containing only vanadium compounds in both tanks. The tanks in Thaller’s experimental batteries, on the other hand, were filled with different solutions – one contained iron ions, the other contained chrome ions. Thaller’s system never worked permanently, partly because the solutions penetrated the separating membrane and mixed together. Skyllas-Kazacos patented her battery in 1988. The patent expired in 2006 and the technology became freely available. China is almost certainly promoting the use of vanadium flow batteries partly because it supplies almost two-thirds of the vanadium being traded on the world markets and has access to huge resources.

The technology of the vanadium flow battery is considered to be fully developed. That’s why, in 2023, the Fraunhofer Institute for Systems and Innovation Research (ISI) in Karlsruhe, rated the level of development, on an internationally accepted scale of one (description of the operating principle) to nine (qualified system with evidence of successful deployment), as nine. Nonetheless, and despite the unique advantage that the output and the storage capacity can be adjusted to the application independently of each other, vanadium-redox flow batteries have lost out in European grids compared to large lithium-ion batteries.
 

One reason is the high costs of the vanadium solutions. What this means specifically is shown by the major, publicly-funded RedoxWind project: between 2012 and 2017, researchers from the Fraunhofer Institute for Chemical Technology ICT in Pfinztal, together with industrial partners, developed stacks and power electronics for a large vanadium flow battery and connected it to a wind power plant. To achieve the planned storage capacity of 20 megawatt hours, EUR 3.5 million was required for 850,000 litres of vanadium-containing liquid. The price had quadrupled in the time between project planning and implementation. As a result, the public funding was not sufficient and a redox flow battery was created with half of the original capacity. Today, the Fraunhofer ICT and its customers use the redox wind turbine for stack tests and the development of operating management systems.

Long depreciation periods of around 20 years for the high investments are making the market launch of vanadium flow batteries in Europe more difficult. In addition, the power losses during charging and discharging are greater than with lithium-ion batteries – which is reflected in the operating costs.
 

In search of more sustainable alternative

The high price of vanadium, China’s market position, environmental problems with ore mining, the corrosive effect of vanadium-containing electrolytes, which are harmful to the environment when released into the soil or groundwater: there are plenty of reasons for researchers to search for alternatives. One team that has set itself this task is based at the Helmholtz Institute Münster (HI MS), a satellite station of the Jülich research centre. The solutions for redox flow batteries that the team is looking into are free from vanadium or other metals. Instead, they only contain organic substances as the redox active material. “These can be produced from plant-based raw materials, such as cellulose, or common base chemicals with a petrochemical origin,” said team leader Mariano Grünebaum. “This means significant independence from individual suppliers or countries.”

Organic substances open up a lot of scope for the scientists, as they are almost infinitely diverse in terms of structure. In fact, researchers worldwide have shown dozens of times that redox flow batteries can be developed with solutions of a wide range of different organic substances [2]. Grünebaum attributes the fact that none of these have made it onto the market to one problem in particular: the systems do not work for long enough, i.e. not for years or decades.

Avoiding radicals

The battery researcher from Münster is certain that, with cetacides, he has stumbled across a substance class in the large set of organic substances that could cope with this difficulty. Usually organic redox pairs only transfer one electron. “This produces radicals, i.e. bonds with a free electron, which are highly reactive and therefore initiate degradation processes,” explains Grünebaum. With cetacides, on the other hand, two electrons are transferred almost simultaneously. This is possible because the spatial arrangement of the atoms in the cetacide molecules changes during the transfer of the first electron in such a way that the transfer of the second electron is made significantly easier. This means no radicals are formed. In the lab, this improved the durability of small redox flow test batteries.

The researchers must adapt the design and structure of these batteries when using new redox active solutions. “If the viscosity, density or polarity of the solutions changes, a change to the battery design may be necessary to ensure a good flow to the electrodes,” explains Grünebaum. This can result in performance losses from the two-digit percentage range to total failure. The researchers use modern 3D printing techniques to produce the test systems [3]. This means that, within a few hours, they can determine whether the newly designed battery will work as desired in practice. Conventional prototype production in the institute’s workshops would slow the researchers down for weeks.

Grünebaum indicates that it is challenging to engage the industry with redox flow concepts other than the vanadium battery. While sustainability and raw material benefits generate interest, companies feel that the road to a marketable product is still very long. The team from Münster is sticking with it and is currently preparing several research projects together with industry partners.

[1] J. Winsberg et al., 2017, Angew. Chemie, 129, 702, doi.org/10.1002/ange.201604925
[2] L. Kortekaas et al., 2023, Batteries, 9, 4, doi.org/10.3390/batteries9010004
[3] L. Kortekaas et al., 2023, Batter. Supercaps, 6, doi.org/10.1002/batt.202300045

Image credits: Mohn, Helmholtz-Institut Münster / Tatjana Pospiech, Carl ROTH / UniEnergy Technologies