Synthesis and Characterization of Dicationic Ionic Liquids with the Difluorophosphate Anion

Abstract

Listen

Translate article

Dicationic ionic liquids (DILs) can be diversely designed by tuning spacer and cationic-center structure. Their features possibly superior to conventional ionic liquids include thermal stability, wide potential window, and high Li ion transference number, which make them good electrolyte candidates for secondary batteries. However, there are few reports of DILs and many unknown factors about the relationship between their structure and physical properties.Although Li[PO2F2] is known as an additive in lithium secondary batteries that improves the negative electrode performance, [1] PO2F2 - itself forms ionic liquids with a variety of onium cations. [2] In this study, we attempted syntheses of DILs with PO2F2 -, especially the ones with ether oxygen in the spacer and evaluated the physical properties of the resulting DILs.The synthesis of DILs was carried out in three steps. Initially, ethylene glycol was reacted with paraformaldehyde and trimethylsilyl chloride in dichloromethane at room temperature to prepare 1,2-bis(chloromethoxy)ethane. In the second step, dihalide reacted with two equivalents of amines at room temperature and converted to the corresponding chloride salts. Finally, the chloride salts and K[PO2F2] were reacted in dichloromethane at room temperature to exchange anions.According to the results of differential scanning calorimetry, the dication salts with ether spacer exhibit lower melting points than the corresponding dication salts without ether spacer owing to the flexibility around the ether group. Comparison of densities between pyrrolidinium- and linear alkylammonium-based DILs revealed the higher density of the former salt, which arises from the rotational freedom of the alkyl chain in the latter. The higher viscosity of the DIL with BF4 - than that of the DIL with PO2F2 - is due to the high dipole moment and thus polarity of PO2F2 - consisting of two oxygen and two fluorine atoms. High ionic conductivity is an important factor as electrolyte. The ion conductivity of the DIL with PO2F2 - is higher than that of the DIL with BF4 -, which implies the potential of PO2F2 - DILs in battery applications.