Dimers and solvent layering determine electrochemically relevant species at electrolyte interfaces in Ca/BH₄/THF

The increasing demand of large-scale energy storage with improved performance, safety and sustainability for renewable variable energy sources and automotive applications drives the search for post-Li-ion batteries based on multivalent cation chemistries (e.g., Mg, Zn and Ca). However, strong solvation shells limit their electrochemical activity, resulting in large overpotentials.

We study Ca(BH4)2 in THF as a promising candidate, spurred on by recent studies of bulk electrolyte and electrode interface. Here we use metadynamics to explore the three-dimensional free energy landscape of a Ca/BH4/THF electrolyte at a neutral and negatively charged inert interface. Specifically, we show that dimers, dominant at bulk, provide the most likely route to ionic species through disproportionation. A combined approach that leverages focused Umbrella Sampling for specific local minima and unsupervised data mining of the trajectories reveals molecular details of populations of distinct coordination environments; e.g., the rich isomerism of dimer structures. We find that the interfacial ionic species seem to respect the underlying solvent structure, resulting in a discretization of permitted interfacial dipole orientations and a modified disproportionation pathway. Furthermore, present the key finding that thermodynamic shifts and favors the monocation at the negative electrode but with a reversal of polarity.