Next-generation batteries based on Na and K, which are alkali metals common in the earth's crust are considered promising alternatives to lithium-ion batteries in future energy storage and storage systems.

Sulfur-containing compounds, such as sulfolane and its derivatives, are currently considered as promising solvents for use in lithium and post-lithium-ion batteries. However, the properties of alkali metal salts solutions in sulfolane are poorly understood.

The aim of our work was to study the physicochemical properties of solutions of alkali metal perchlorates in sulfolane and intermolecular interactions in solutions (cation-solvent molecules, cation-anion) using the molecular dynamics method.

In our work we used the OPLSaa non-polarizable force field. The molar ratio of salt and solvent was 1:20 (~0.5 M) in solutions of ClO₄^- salts of Li, Na, K in sulfolane (SL). The calculations were carried out in the same way as in the work ¹.

The results of calculating the physicochemical properties of MeClO₄ solutions in SL are summarized in Table. The calculated density values of LiClO₄/SL and NaClO₄/SL systems differ from those measured experimentally ^2,3 by less than 0.6% (Table). It can be assumed that the calculated value of the density of the KClO₄/SL solution is also close to the measured value. The calculated values of the specific electrical conductivity of LiClO₄/SL and NaClO₄/SL exceed the experimentally measured values by 2.5 and 1.3 times, correspondently. The overestimated calculated values can be explained by a strong simplification of the Nernst–Einstein relation which takes into account only cations and anions motion, and doesn’t take into account the motion of ion associates.

The calculated transfer numbers of cations in MeClO₄/SL are about 0.4.

There are two peaks on the RDF curves of SL oxygen atoms around Li⁺, Na⁺, K⁺ (g(O(SL)-Me)): the first – clearly narrow and the second – broadened (Figure). The difference between the positions of the peak maxima (0.23 nm) is comparable to the distance between oxygen atoms in the SL (0.256 nm). Thus, the second broadened peak in the g(O(SL)–Me) curves is due to the second oxygen atom of the same SL molecule. The probability of Me⁺ solvation by the first or second oxygen atom of SL is the same. This is confirmed by the coincidence of the integral curves N(O(SL)-Me) of SL oxygen atoms around Me⁺ (Figure).

There is one narrow clear peak on the RDF curves of the sulfur atom of SL around the cation g(S(SL)-Me). The maximum position is shifted relative to the maximum position of the RDF Me-O by 0.14 nm. It is comparable to S-O bond length (0.13 nm)) (Figure). Thus, the cation interacts with one oxygen atom of SL while the second oxygen atom doesn’t participate in the cation solvation and is turned in the opposite direction from it.

There are two peaks on the RDF curves of the oxygen atoms of the ClO₄^- around the cation g(O(ClO₄^-)-Me): the first - clearly narrow and the second - broadened. The location of the first peak relative to the cation corresponds to the first solvation shell of the cations (Figure 1). The height of the first peak is significantly lower than the height of the peak on g(O(SL)-Me) curve. This indicates a low probability of the ClO₄^- entering the first solvation shell of cations under the studied conditions (MeClO₄/SL = 1:20, 303 K, 1 atm). Preferably, the ClO₄^- is located in the outer coordination sphere of the solvate complex.

Coordination numbers (CN) of metal cations were calculated based on the integral curves of RDF. The sum of the CN for the oxygen atoms of SL corresponds to the calculated CN of cations for the sulfur atom of SL. This indicates that SL is coordinated with the Li⁺, Na⁺, K⁺ by one oxygen atom.

The coordination numbers of the Li⁺ < Na⁺ < K⁺ cations increase in the series 4.5 < 5.4 < 5.6 for SL and 0 < 0.1 = 0.1 for the ClO₄^-, correspondently.

The work was carried out within the framework of the theme of state assignment No. 21111900148-3.

Reference

1. Yamaguchi T., Yamada H., Fujiwara T. and Hori K. 2020 Mol. Liq. 312 113288
2. Dokko K., Watanabe D., Ugata Y., Thomas M. L., Tsuzuki S., Shinoda W., Hashimoto K., Ueno K., Umebayashi Y. and Watanabe M. 2018 Phys. Chem. B. 122 47
3. Sheina L.V., Karaseva E.V. and Kolosnitsyn V.S. 2021 J. Phys. Chem. A. 95 5