Ions in molten salts can go 'against the flow'

January 27, 2021

In a new article published in the scientific journal Communications Chemistry, a research group at Uppsala University show, using computer simulations, that ions do not always behave as expected. In their research on molten salts, they were able to see that, in some cases, the ions in the salt mixture they were studying affect one another so much that they may even move in the "wrong" direction - that is, towards an electrode with the same charge.

Research on the next-generation batteries is under way in numerous academic disciplines. Researchers at the Department of Cell and Molecular Biology, Uppsala University have developed and studied a model for alkali halides, of which ordinary table salt (sodium chloride) is the best-known example. If these substances are heated to several hundred degrees Celsius, they become electrically conductive liquids known as "molten salts". Molten salts are already used in energy contexts: for concentrated solar power in the Sahara desert and as electrolytes in molten-salt batteries that can be used for large-scale storage of electricity.

Despite their wide-spread use, some of the molten salt's basic properties are not yet fully understood. When it comes to batteries, optimising conductivity is a frequent goal. To produce a battery that is as efficient as possible, knowing what happens to individual ions is vital. This is what the Uppsala researchers are now investigating with their simulations.

"In the long run, the purpose of this research is to develop physical models for biological molecules. But these salts are relatively simple and make a good test bed," says Professor David van der Spoel, the group leader for the modelling project.

However, the researchers' simulations show that the salts are not as simple as they may seem at the first glance, and that they have some interesting properties, especially if various alkali halides are mixed together.

In a simplified theory, ions that move in an electric field (for example in a battery) do not interact with each other and are affected solely by the electric field. In their newly published study, the researchers were able to demonstrate that this is not always true. The study shows how, in a mixture of lithium ions with ions of fluoride, chloride and iodide, the lighter anions, fluoride and chloride, move towards the negative cathode along with the lithium ions in a (simulated) battery electrolyte.

"The negative ions are attracted both by the lithium ions and by the positive anode, and the net effect of these forces makes the lighter anions move slowly towards the cathode, since the positive lithium ions are also moving in that direction," says the first author of the study, Marie-Madeleine Walz.

In their continued research, the group will develop a water model to study the interaction of water molecules with ions. Their investigation will include, for example, how the properties of ions are affected by an electric field when there is water in the mixture.
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Uppsala University

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