The phase behavior and ionic conductivity of tetraethylammonium bis(trifluoromethane-sulfonyl)imide (Et4NTFSI) salt mixtures with LiTFSI have been examined. In addition, the phase behavior and crystal structure of neat LiTFSI is also reported. Two (1-x) Et4NTFSI-(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 Et4NTFSI salt creating disordered plastic crystalline phases and the formation of a low-melting eutectic composition between the neat Et4NTFSI salt and the 1/1 Et4NTFSI/LiTFSI (x = 0.50) phase. Although Et4NTFSI 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.