There are many examples in daily life where dissolving a salt in a liquid solvent is key to realizing some goal – such as adding table salt to water while cooking. The dissolution process (solvation) involves the solvent separating the salt crystal into its component ions (solutes). This transition of the ions, from the solid phase to the solvated phase, can occur through establishing stable solvation cages that keep the oppositely charged, attractive ions apart. Understanding ion solvation at the molecular level, including its energetics and structural characteristics, remains a great challenge, yet it is key to design and control processes related to several technical applications including drug discovery, ion-exchange membranes, and advanced energy storage systems.
A battery is a device that connects a positive and negative electrode via an ion-conducting electrolyte, which is often a liquid phase of dissolved ions. The ease with which ions can cross this electrolyte and be accommodated or released at the electrodes, where they provide or accept electronic charge, contributes to battery performance in terms of their stability and how quickly they can be charged or discharged. To this end, researchers have been struggling to determine and ultimately control the chemical structure of the solvent around dissolved ions.
Scientists from the Joint Center for Energy Storage Research have been working with experts at the Molecular Foundry to explore this problem of ion solvation in battery electrolytes using computer simulations. Their work was recently published in the Journal of Physical Chemistry Letters. By modeling the dynamics of electrolytes with atomic detail, they can reveal how the ions are surrounded by different solvents commonly used in advanced batteries. Furthermore, using advanced theoretical methods, they can ask key questions about why one particular molecular arrangement is favored over another, or test commonly held beliefs and expectations for how ions and solvents should interact. These extensive simulations are made possible using the supercomputing resources of the National Energy Research Scientific Computing Center (NERSC) at Berkeley Lab.
These researchers explored a range of common solvents – acetonitrile, tetrahydrofuran (THF) and diglyme – containing dissolved, positively-charged ions, like lithium, but also magnesium and zinc, as alternatives for future battery technologies. In a surprising discovery, they found that these simple ions may have multiple stable solvation configurations in contrast to the commonly accepted assumption of a single common molecular environment around each ion. For example, magnesium dissolved in THF may exist in one of two distinct populations, surrounded by 5 or 6 THF molecules. Furthermore, when these solvation structures break apart due to fluctuations in the liquid that remove a molecule from the solvation cage, that molecule is not guaranteed to be replaced (a transition from 6 to 5 THF) or may be replaced by more than one (a transition from 5 to 6 THF).
The relative stability of these different ion-solvent configurations defines both their relative populations – more stable being more common – and also the ease at which they can accept electrons from an electrode – the least stable being the first to accept an electron. This discovery implies that the minority species may play the dominant role in some electrochemical systems. The team has only just revealed this possibility within the electrolyte alone and is planning to extend their studies to the functional region where the electrolyte and electrode meet. The dynamical process by which the ions emerge into the electrolyte or are taken out of it may also reveal similar peculiarities. Overall, this work has a direct impact on the performance of complex electrolytes in batteries and potentially provides another avenue to control and engineer smart interfaces in batteries.