New high-performance electrolyte for Mg-based batteries

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 E_solv 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)]:

E_solv = E_explicit + E_implicit (2)

The explicit fraction of THF solvation E_explicit was calculated by subtracting the total energies of the electrolyte species without THF solvation E_{electrolyte,tot(vacuum)} plus the THF reference E_{THF,tot(vacuum)} from the total energy of the electrolyte with the first solvation shell included E_{electrolyte·4THF,tot(vacuum)} [Equation (3)]:

E_explicit = E_{electrolyte·4THF,tot(vacuum)} – ( E_{electrolyte,tot(vacuum)} + 4 · E_{THF,tot(vacuum)} ) (3)

On the other hand, the implicit fraction of THF solvation was equal to the difference in total energies from a COSMO E_{electrolyte·4THF,tot(COSMO)} and a vacuum calculation E_{electrolyte·4THF,tot(vacuum)} according to Equation (4):

E_implicit = E_{electrolyte·4THF,tot(COSMO)} – E_{electrolyte·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.

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, 2200440