Magnesium-based batteries are being actively researched, with Mg being more abundant and having a higher theoretical energy density than Li. Consequently, suitable electrolytes for magnesium-ion batteries are being actively pursued. A recent publication introduced a novel and highly efficient electrolyte containing benzopyrrole and bromide as anions. In THF solution, the indolyl electrolyte is present in a Schlenk-type equilibrium as shown in Equation (1):
2 Mg(Ind)Br · 4THF ⇄ MgBr2 · 4THF + Mg(Ind)2 · 4THF (1)
To help support and understand the experimental findings, calculations were done at the Institute of Electrochemistry in Ulm. Considering the solvation using the conductor-like screening model (COSMO) as implemented in ADF, the solvation energies of the electrolyte species were calculated, and the favored side of the Schlenk-type equilibrium was predicted. Therefore, the solvation energy Esolv was determined in a two-step process. The first solvation shell (corresponds to 4 THF molecules) was included explicitly, while additional THF solvation was considered implicitly with COSMO [Equation (2)]:
Esolv = Eexplicit + Eimplicit (2)
The explicit fraction of THF solvation Eexplicit was calculated by subtracting the total energies of the electrolyte species without THF solvation Eelectrolyte,tot(vacuum) plus the THF reference ETHF,tot(vacuum) from the total energy of the electrolyte with the first solvation shell included Eelectrolyte·4THF,tot(vacuum) [Equation (3)]:
Eexplicit = Eelectrolyte·4THF,tot(vacuum) – ( Eelectrolyte,tot(vacuum) + 4 · ETHF,tot(vacuum) ) (3)
On the other hand, the implicit fraction of THF solvation was equal to the difference in total energies from a COSMO Eelectrolyte·4THF,tot(COSMO) and a vacuum calculation Eelectrolyte·4THF,tot(vacuum) according to Equation (4):
Eimplicit = Eelectrolyte·4THF,tot(COSMO) – Eelectrolyte·4THF,tot(vacuum) (4)
Further information, a detailed method description, and calculated solvation energies for the indolyl-based electrolyte can be found in the publication.
The calculations supported the experimental findings of a fast Schlenk-type equilibrium, where Mg(Ind)Br · 4THF is more favorable due to solvation energies. The new benzopyrrole-based electrolyte facilitates de- and intercalation in the cathode with very good long-term cycling behavior.
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S. Zaubitzer, S. Dongmo, P. Schüler, S. Krieck, F. Fiesinger, D. Gaissmaier, M. van den Borg, T. Jacob, M. Westerhausen, M. Wohlfahrt-Mehrens, M. Marinaro, A Novel and Highly Efficient Indolyl-Based Electrolyte for Mg Batteries, Energy Technol. 2022, 2200440Key conceptsADF batteries solvation