Solvation structure around the Li+ ion in succinonitrile-lithium salt plastic crystalline electrolytes

Herein, we discuss the study of solvation dynamics of lithium-succinonitrile (SN) plastic crystalline electrolytes by ultrafast vibrational spectroscopy. The infrared absorption spectra indicated that the CN stretch of the Li+ bound and unbound succinonitrile molecules in a same solution have distinct vibrational frequencies (2276 cm(-1) vs. 2253 cm(-1)). The frequency difference allowed us to measure the rotation decay times of solvent molecules bound and unbound to Li+ ion. The Li+ coordination number of the Li+-SN complex was found to be 2 in the plastic crystal phase (22 degrees C) and 2.5-3 in the liquid phase (80 degrees C), which is independent of the concentration (from 0.05 mol kg(-1) to 2 mol kg(-1)). The solvation structures along with DFT calculations of the Li+-SN complex have been discussed. In addition, the dissociation percentage of lithium salt was also determined. In 0.5 mol kg(-1) LiBF4-SN solutions at 80 degrees C, 60% +/- 10% of the salt dissociates into Li+, which is bound by 2 or 3 solvent molecules. In the 0.5 mol kg(-1) LiClO4-SN solutions at 80 degrees C, the salt dissociation ratio can be up to 90% +/- 10%.