In-situ XRD study of the succinonitrile–lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) phase diagram

The salt lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) when dissolved in plastic crystal succinonitrile has been demonstrated to have particularly good conductivity even at room temperature. In-situ X-ray diffraction (XRD) measurements have previously proved invaluable in interpreting the differential scanning calorimetry (DSC) behavior, but the practical lower temperature limit of approximately − 45 °C was higher than the − 100 °C starting temperature of the DSC measurements, and the important crystalline to plastic crystal transition of succinonitrile. An improved cryo-flow system capable of capillary sample temperatures down to − 192 °C without icing can now easily match the DSC conditions. The previously puzzling DSC behavior of the succinonitrile–LiTFSI phase diagram at low temperatures has now been explained, with a surprising formation of a TFSI-rich adduct on heating from − 100 °C even at concentrations as low as 2 mol%.