Gaurav TATRARI1, Sourav BHOWMICK1, Andrei FILIPPOV11, Patrik JOHANSSON2, Faiz Ullah SHAH1
1Lulea technology University, Lulea, Lulea, Sweden
2Department of Physics, Chalmers University of Technology, SE-412 96 , Gothenburg, , Sweden
Conventional organic electrolytes have a number of drawbacks limiting their applicability in energy storage devices, mainly in terms of safety e.g. flammability, and performance e.g. electrochemical stability. Furthermore, conventional organic electrolytes contain lithium salts with fluorinated anions, e.g. LiPF6-, which hydrolyzes in the presence of moisture and produces both toxic and corrosive products. To resolve the safety issue, ionic liquid (IL) based electrolytes have been launched, but most of the commonly studied IL-based electrolytes still contain fluorinated anions. Therefore, fluorine-free IL-based electrolytes are highly desirable to achieve improved safety and performance of energy storage devices.
Here, a comparison of ion transport, physico-chemical and electrochemical properties of a set of new families of structurally flexible pyrrolidinium and morpholinium cation-based ionic liquids (ILs), all with oligoether phosphate-based anions, is presented. These ILs have high thermal stabilities, low glass transition temperatures, and wide electrochemical stability windows, but moderate ionic conductivities. The anions and cations of the pyrrolidinium-based ILs both diffuse faster than those of the morpholinium-based ILs, in agreement with the ionic conductivity data, and overall the former ILs offer faster ion transport and significantly more promise for applications as e.g. electrolytes in energy storage devices. In a supercapacitor, the pyrrolidinium-based IL sustains a wide potential window and shows high coulombic efficiency. Cyclic voltammetry, galvanostatic charge-discharge, and electrochemical impedance spectroscopy data depicted a decent electrochemical performance of the device.