Ionic liquid electrolytes based on sulfonium cation for lithium rechargeable batteries

Abstract

We report the preparation and characterization of ionic liquid electrolytes composed of the ionic liquid triethyl sulfonium bis(trifluoromethylsulfonyl imide) ([S222TFSI]) and four different lithium salts, namely lithium bis(trifluoromethylsulfonyl imide) (LiTFSI), lithium perchlorate (LiClO4), lithium hexafluorophosphate (LiPF6) and lithium trifluoromethanesulfonate or lithium triflate (LiFSI). Among the electrolytes we studied, [S222TFSI] – LiTFSI has the lowest viscosity. DSC analysis showed that all these ionic liquid electrolytes have their melting peak below −5 °C, confirming that they are in liquid state over the temperature range at which batteries are usually required to operate. The ionic conductivity at room temperature is in the order of 10−3 S cm−1 and increases to 10−2 S cm−1 at higher temperatures. The [S222TFSI] – LiTFSI electrolyte system is electrochemically stable up to 5 V against lithium metal which indicates the feasibility of using it with high voltage electrode materials. Reversible redox process of lithium stripping/deposition in this electrolyte was observed in cyclic voltammetry by the appearance of redox peaks in both forward and reverse scans within the voltage range, −0.4 V–2.5 V. In addition to correlating the viscosity of the prepared electrolytes with their ionic conductivity as a function of temperature, as a proof of concept, we demonstrate the electrochemical performance of the electrolyte in a coin cell configuration with lithium iron phosphate (LiFePO4) cathode. We observed voltage plateaus around 3.5 V and 3.28 V during the first galvanostatic charging and discharging steps, respectively.