Polymorphism in LiN(CF3SO2)2

Abstract The structural, thermodynamic and electrical properties of lithium bis(trifluoromethylosulfone)imide, LiTFSI, are presented. The results obtained by differential scanning calorimetry and X-ray diffraction indicate that at temperatures below the melting point, the salt exhibits three different crystalline structures, denoted as α, β and γ existing at low, intermediate and high temperatures, respectively. While the occurrence of a polymorphic transition between the β and γ phases has been noted previously, the existence of the low temperature α-phase is reported for the first time. The phase behaviour is complex, with slow kinetics of the γ → β transition allowing for super cooling of the γ-phase, where it exhibits a glass-like transition at intermediate temperatures and a phase transition to the α-phase at around −100 °C. There is no α ↔ β transition observed in this system and on heating the α-phase transforms directly to the γ-phase. Electrical characterization by impedance spectroscopy shows that the ionic conductivity of the γ-phase is higher than that of the room temperature stable β-phase. Supercooling of the γ-phase could be important for development of concentrated “polymer in salt” systems, in which the electrical properties of the electrolyte are determined by processes which occur in clusters of the salt.