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⁻¹vs. 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 °C) and 2.5–3 in the liquid phase (80 °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 °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 °C, the salt dissociation ratio can be up to 90% ± 10%.