An Alternative Ionic Conductivity Mechanism for Plastic Crystalline Salt–Lithium Salt Electrolyte Mixtures

Abstract The phase behavior and ionic conductivity of tetraethylammonium bis(trifluoromethane‐sulfonyl)imide (Et 4 NTFSI) salt mixtures with LiTFSI have been examined. In addition, the phase behavior and crystal structure of neat LiTFSI is also reported. Two (1‐ x ) Et 4 NTFSI‐( x ) LiTFSI ( x = 0.50 and 0.67, where x is the mol fraction) mixed‐salt crystalline phases form. Large variations in ionic conductivity are observed; these are attributed to solid‐solid phase transitions of the neat Et 4 NTFSI salt creating disordered plastic crystalline phases and the formation of a low‐melting eutectic composition between the neat Et 4 NTFSI salt and the 1/1 Et 4 NTFSI/LiTFSI ( x = 0.50) phase. Although Et 4 NTFSI and LiTFSI melt at 102 and 234 °C, respectively, the two salts form a eutectic system with a melting temperature of 32 °C. Based upon the findings reported, a new conductivity mechanism is proposed for plastic crystalline salt‐lithium salt electrolytes which is not ascribed to solid‐state diffusion/conduction.