Spectroscopic measurements of ionic association in common lithium salts and carbonate electrolytes
FTIR and NMR spectroscopy were used to investigate the structure and composition of lithium ion solvation spheres of single salt electrolyte solutions composed of common lithium salts (LiTFSI, LiPF6, LiBF4, and LiClO4) dissolved in aprotic polar linear and cyclic carbonate solvents (propylene carbonate (PC), dimethyl carbonate (DMC)). The carbonyl oxygen of the solvents coordination to the lithium ion is observed by FTIR spectroscopy. Determination of relative percent coordination of PC, DMC, and a binary blend was used to calculate solvent coordination numbers in a range from 2 to 7. Solvent coordination was also observed with 13C NMR spectroscopy and agreement found when comparing the two spectroscopic methods allowed for increased confidence in the results. Solvent coordination is dependent upon the magnitude of salt dissociation. LiTFSI, LiPF6, and LIClO4 dissociated to approximately the same degree, while LiBF4 dissociated less. Dissociation trends between salts were approximate under all solvent systems. The primary solvation sphere was determined to form contact ion pairs at low solvent to salt ratios (up to 4:1) and solvent separated ion pairs in dilute solutions up to (30:1). The free energy was approximated to determine preferential solvation and none was found, which is contrary to expectations that PC with a significantly higher dielectric constant would predominate in coordination over DMC. This investigation was conducted in order to develop a fundamental understanding about the solution structure and aid as a tool for the ionic association evaluation of commonly used LIB electrolytes.^
"Spectroscopic measurements of ionic association in common lithium salts and carbonate electrolytes"
Dissertations and Master's Theses (Campus Access).