Influence of water and thermal history on ion transport in lithium salt-succinonitrile plastic crystalline electrolytes

Abstract

Important issues of water and thermal history affecting ion transport in a representative plastic crystalline lithium salt electrolyte: succinonitrile (SN)–lithium perchlorate (LiClO₄) are discussed here. Ionic conductivity of electrolytes with high lithium salt amounts (∼1 M) in SN at a particular temperature is known to be influenced both by the trans–gauche isomerism and ion association (solvation), the two most important intrinsic parameters of the plastic solvent. In the present study both water and thermal history influence SN and result in enhancement of ionic conductivity of 1 M LiClO₄–SN electrolyte. Systematic observations reveal that the presence of water in varying amounts promote ion-pair dissociation in the electrolyte. While trace amounts (≈1–15 ppm) do not affect the trans–gauche isomerism of SN, the presence of water in large amounts (≈5500 ppm) submerges the plasticity of SN. Subjugating the electrolyte to different thermal protocol resulted in enhancement of trans concentration only. This is an interesting observation as it demonstrates a simple and effective procedure involving utilization of an optimized set of external parameters to decouple solvation from trans–gauche isomerism. Observations from the ionic conductivity of various samples were accounted by changes in signature isomer and ion-association bands in the mid-IR regime and also from plastic to normal crystal transition temperature peak obtained from thermal studies.