Crystal structures and quantitative structure–property relationships of spirobipyrrolidinium and the oxygen-containing derivatives

Crystal structures of a popular electrolyte for EDLCs, 1,1′-spirobipyrrolidinium and its oxygen-containing derivatives including BF3C2F5 anion are reported. The properties of the BF4 salts, such as free energy change of fusion and dielectric constants were analyzed using molecular volume information. We used volume-based thermodynamics method to explain the reason for highest melting point in least-polar and most loosely packed crystal. The dielectric constants estimated from volumetric change during the phase changes are consistent with the polarity of the salts. We expect further optimization of these parameters (using known RTILs) to enhance the accuracy of quantitative analysis and prediction.