Lithium is expensive, scarce and geopolitically contested. But the energy transition requires cheap, long-term storage in large quantities. Chinese researchers have now presented an iron flow battery that can withstand 6,000 charging cycles without loss of capacity and is said to be up to 40 percent cheaper than lithium-ion storage. We explain how the technology works and where it reaches its limits.
A research team from the Institute of Metal Research of the Chinese Academy of Sciences (CAS) in Shenyang has developed a novel iron flow battery. The system should be able to withstand more than 6,000 complete charging and discharging processes without any measurable loss of capacity.
According to the scientists, this corresponds to over 16 years of daily use. If there is no wind and sun, the power grid can use exactly such long-term storage.
Iron dissolved in water, an abundant and non-flammable raw material, serves as the active material. The results were published by the team led by first author Wei Wei and co-leaders Tang Ao and Li Ying in the journal Advanced Energy Materials.
Molecular tank solves problem of iron flow batteries
Until now, such batteries have often failed due to the so-called cross-over effect. Because: The active material gradually decomposed, migrated through the separating membrane and thus drastically shortened the lifespan of the cells. According to the study, the Chinese research team solved this problem with a newly developed iron complex. Its rigid structure physically keeps aggressive hydroxide ions at bay like an armor.
In addition, the high negative charge of the complex creates a strong electrostatic barrier via the so-called Donnan effect. This reduces the disruptive membrane penetration by a hundredfold. According to the study, no harmful byproducts or deposits occurred in the battery across all 6,000 cycles.
0.16 US dollars per kilowatt hour: This is how cheap the iron battery is
According to the Interesting Engineering portal, a prototype of the battery achieved a very high Coulomb efficiency of 99.4 percent. Even under a heavy load of 150 milliamperes per square centimeter, the energy efficiency remained high at 78.5 percent. The peak power density in the experiments was 392.1 milliwatts per square centimeter.
At 30 to 40 watt hours per liter, the battery stores significantly less energy than conventional lithium-ion batteries. However, according to the experts, the pure volume is of secondary importance for stationary network storage. What is much more important is the so-called levelized cost of storage, i.e. the cost per kilowatt hour stored over the entire service life. According to calculations, the costs for ten-hour systems amount to around 0.16 US dollars per kilowatt hour, which is up to 40 percent below the level of lithium-ion storage.
Race with the USA: Who will build the first large-scale iron battery?
Internationally, the race for iron-based flow batteries is currently gaining momentum. In the USA, the company ESS Tech from Oregon is developing acid iron flow systems and is cooperating with Google. Delivery is planned as part of a joint project by the end of 2027. However, these US designs would have to contend with needle-shaped crystals called dendrites, which could trigger dangerous short circuits.
The alkaline chemistry of the new Chinese variant avoids the problem of this crystal formation. However, the researchers still face some technical challenges before a widespread market launch. Until they reach mass production, the membranes would have to be further optimized for large systems on a megawatt-hour scale. According to the report, the pumps used and the system integration also required intensive development work.
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