Effect of Salt Concentration on the Interfacial Solvation Structure and Early Stage of Solid-Electrolyte Interphase Formation in Ca(BH4)2/THF for Ca Batteries

The Ca2+ solvation structure at the electrolyte/electrodeinterface is of central importance to understand electroreductionstability and solid-electrolyte interphase (SEI) formationfor the novel multivalent Ca battery systems. Using an exemplar electrolyte,the concentration-dependent solvation structure of Ca-(BH4)(2)-tetrahydrofuran on a gold model electrode has beeninvestigated with various electrolyte concentrations via electrochemicalquartz crystal microbalance with dissipation (EQCM-D) and X-ray photoelectronspectroscopy (XPS). For the first time, in situ EQCM-D results provethat the prevalent species adsorbed at the interface is CaBH4 (+) across all concentrations. As the salt concentrationincreases, the number of BH4 (-) anionsassociated with Ca2+ increases, and much larger solvatedcomplexes such as CaBH4 (+)center dot 4THF or Ca-(BH4)(3) (-)center dot 4THF form at the interfaceat high concentrations prior to Ca plating. Different interfacialchemistries lead to the formation of SEIs with different componentsdemonstrated by XPS. High electrolyte concentrations reduce the solventdecomposition and promote the formation of thick, uniform, and inorganic-rich(i.e., CaO) SEI layers, which contribute to improved Ca plating efficiencyand current density in electrochemical measurements.

Automated summary

The solvation structure of Ca2+ at the electrolyte/electrode interface is crucial for understanding electroreduction stability and SEI formation in Ca battery systems. Using EQCM-D and XPS, the solvation structure of Ca-(BH4)2-tetrahydrofuran on a gold electrode was investigated at various concentrations. In situ EQCM-D results showed that the prevalent species at the interface is CaBH4 (+), and higher concentrations resulted in the formation of larger solvated complexes before Ca plating. XPS analysis revealed that high electrolyte concentrations promote the formation of thick, uniform, and inorganic-rich SEI layers, leading to improved Ca plating efficiency and current density.