Solvation Environment and Interface Dynamics of LiBH and LiBF Electrolytes Uncovered by Theory and Operando Optical and FTIR Spectroelectrochemistry.

In this work, we evaluated two closo-borate salts (LiBH and LiBF) in propylene carbonate from theoretical and experimental perspectives to understand how the coordination environment influences their spectroscopic and electrochemical properties. The coordination environments of the closo-borate salts were modeled via density functional theory (DFT) and molecular dynamics (MD). Vibrational spectra calculated from the predicted coordination environments are in agreement with experimentally measured steady-state FTIR data. This theoretical investigation also suggested that LiBF would possess a higher ionic conductivity than LiBH, which was corroborated experimentally. Additionally, an electrochemical cell was designed and fabricated that enabled operando optical and FTIR spectroelectrochemical (OP-IR-SEC) measurements. This allowed for the simultaneous measurement of the relative changes of species at a lithium electrode-liquid electrolyte interface and the visualization of lithium plating at the electrode surface. This technique could provide new chemical insights and potentially link optical changes at the electrode-electrolyte interface to specific chemical species in similar electrochemical systems. The LiBF electrolyte was found to have a higher thermal stability, which may find utility in applications for batteries that are subject to high-temperature conditions.