Whatever happened to the liquid metal battery
Large-scale storage systems are becoming more and more important in order to guarantee grid stability and security of supply with increasing proportions of renewable energies in the German energy system. The specific performance and energy-related costs as well as the long-term performance of the storage systems play an important role in the decision for or against a certain technology.
Liquid metal batteries, i.e. batteries in which both electrodes and the electrolyte are in the liquid state, represent a promising concept. The use of cheap, readily available raw materials as active materials in large battery cells leads to their cost reduction. Liquid metal batteries are therefore a competitive option for large storage systems.
With their completely liquid inventory, liquid metal batteries have a number of advantages over conventional cells: With the appropriate selection of the liquid density, a stable layering results - the cell builds itself up almost by itself. Processes at liquid-liquid boundary layers show high kinetics. This enables high charging and discharging currents without major losses in efficiency. Since the electrode structure of the liquids always returns to its original state, there are no aging effects, so that an almost infinite number of cycles can be achieved.
The high current densities and the desired high cell cross-sections can result in very high currents per cell. This is where electromagnetic fields and fluid mechanics - i.e. magnetohydrodynamics - come into play. Lorentz forces, caused by the interaction of the high cell current with its own magnetic field, can stimulate the Tayler instability (TI), as described by Seilmayer et al. (2012) was demonstrated.
The rotating flow caused by the TI can displace the electrolyte (molten salt) between the electrodes. This short circuit would cause the battery to fail. Depending on the aspect ratios of the cells, other instabilities can also disrupt the operation of liquid metal batteries, for example surface waves similar to the sloshing in aluminum reduction cells.
We investigate these instabilities experimentally and numerically with OpenFOAM and develop countermeasures. The development of TI can be prevented by applying an additional magnetic field to the cell, which is superimposed on the original field. The overall field can then be designed in such a way that the cell is no longer susceptible to TI.
One possible source for the additional magnetic field can be found in the current that is conducted to or from the cell. By passing this current through the vertical cell axis in isolation, for example, the cell will stabilize even at high currents.
The strong coupling of electrodynamics, fluid dynamics and electrochemistry can lead to the occurrence of complex phenomena in the simulation of the cells, which is associated with increased computing times in the simulation. Large-scale experiments are planned in DRESDYN, where the necessary infrastructure for the safe handling of liquid metals and the necessary flow measurement technology are provided.
Research group liquid metal batteries
In our battery laboratory we are able to examine liquid electrodes and electrolytic molten salts electrochemically as well as to manufacture and test small cells. For the scaling of the cells, to improve the efficiency and to guarantee the long-term stability, tests with different materials for the housings and insulators of the cells are necessary. Research activities on cell operation, scale-up and integration into the energy system are carried out as part of the joint Energy System 2050 initiative, a network of eight Helmholtz centers with the aim of developing usable system-technical knowledge and technological solutions that politics and business can take up .
In 2017 we organized the first workshop on the fluid dynamics of liquid metal batteries (LMBFD 2017) and held it on May 16 and 17 in Dresden. The focus of the workshop was on the fluid dynamics of liquid metal batteries and similar systems (e.g. aluminum reduction cells).
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