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 alpha, beta and gamma existing at low, intermediate and high temperatures, respectively. While the occurrence of a polymorphic transition between the beta and gamma phases has been noted previously, the existence of the low temperature alpha-phase is reported for the First time. The phase behaviour is complex, with slow kinetics of the gamma -> beta transition allowing for super cooling of the gamma-phase, where it exhibits a glass-like transition at intermediate temperatures and a phase transition to the alpha-phase at around -100 degrees C. There is no alpha <->beta transition observed in this system and on heating the alpha-phase transforms directly to the gamma-phase. Electrical characterization by impedance spectroscopy shows that the ionic conductivity of the gamma-phase is higher than that of the room temperature stable beta-phase. Supercooling of the gamma-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.