A Brief Guide to the Structure of High-Temperature Molten Salts and Key Aspects Making Them Different from Their Low-Temperature Relatives, the Ionic Liquids.

Shobha Sharma,Claudio J Margulis,Alexander S Ivanov

doi:10.1021/acs.jpcb.1c01065

Shobha Sharma, Claudio J Margulis + Show 1 more

Open Access

https://doi.org/10.1021/acs.jpcb.1c01065

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Abstract

High-temperature molten salt research is undergoing somewhat of a renaissance these days due to the apparent advantage of these systems in areas related to clean and sustainable energy harvesting and transfer. In many ways, this is a mature field with decades if not already a century of outstanding work devoted to it. Yet, much of this work was done with pioneering experimental and computational setups that lack the current day capabilities of synchrotrons and high-performance-computing systems resulting in deeply entrenched results in the literature that when carefully inspected may require revision. Yet, in other cases, access to isotopically substituted ions make those pioneering studies very unique and prohibitively expensive to carry out nowadays. There are many review articles on molten salts, some of them cited in this perspective, that are simply outstanding and we dare not try to outdo those. Instead, having worked for almost a couple of decades already on their low-temperature relatives, the ionic liquids, this is the perspective article that some of the authors would have wanted to read when embarking on their research journey on high-temperature molten salts. We hope that this will serve as a simple guide to those expanding from research on ionic liquids to molten salts and vice versa, particularly, when looking into their bulk structural features. The article does not aim at being comprehensive but instead focuses on selected topics such as short- and intermediate-range order, the constraints on force field requirements, and other details that make the high- and low-temperature ionic melts in some ways similar but in others diametrically opposite.

Highlights

There has been a constant stream of articles1−69 including salts in their molten or glass states since the early 1900s; see for example, the works of Zachariasen70 and Rosenhain.71 Yet, through the years, molten salts continue to be rediscovered for applications in energy technologies including nuclear energy,72−76 solar energy harvesting,77−79 batteries,80−82 and separations83 to mention just a few
What we mean by this is that whereas it is easy to get transport properties wrong for ionic liquids (ILs), it is hard for a reasonable force field to not show the three typical characteristics of an IL: which are polar−apolar alternation [the so-called prepeak or first sharp diffraction peak (FSDP)], charge alternation, and adjacency correlations.[116−126] Each of these features appears in a specific q-range, as shown in Figure 5.119,127−134 Instead, structural properties of molten salts including coordination numbers and intermediate range order appear to be quite sensitive to the model and, as opposed to the case for ILs, it is very easy to get these wrong
In the case of the most common ILs, this feature is due to polarity alternation; this is the pattern of apolar domains separated by charge networks or that of charge networks intercalated by apolar domains

Summary

■ INTRODUCTION

There has been a constant stream of articles− including salts in their molten or glass states since the early 1900s; see for example, the works of Zachariasen and Rosenhain. Yet, through the years, molten salts continue to be rediscovered for applications in energy technologies including nuclear energy,− solar energy harvesting,− batteries,− and separations to mention just a few. The 1970s and early 1980s brought a series of interesting spectroscopic measurements that could be directly linked to the 3D structure of salts in the molten phase, in particular via Raman spectroscopy,− and soon after, pioneering X-ray and neutron scattering results− revealed more information about short and intermediate range order. These results followed or were followed by pioneering theory developments and early simulation work.−−−− Multiple force fields to simulate salts have been developed, but modern point-polarizable force fields with quantum mechanical accuracy− are the current go-to models for most simulations of divalent and multivalent cationic systems coupled with polarizable anions. For the purpose of this perspective article, when comparisons are made between molten salts and ILs (see Figure 1), these are made with the most common modern ILs in mind, which are based on organic cations and organic or inorganic anions instead of the early chloroaluminates

■ RESULTS AND DISCUSSION

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES