Hybrid lithium-ion capacitors based on novel 1-butyl-3-methylimidazolium bis(nonafluorobutanesulfonyl imide) (BMImBNFSI) ionic liquid electrolytes: a detailed investigation of electrochemical and cycling behaviors

Recently, electrochemical energy storage devices and hybrid lithium-ion capacitors (HLICs), in particular have received intensive interest because of their ability to combine the performance of high-energy lithium-ion batteries and high-power supercapacitors. In the present investigation, a new bulky fluorinated ionic liquid, 1-butyl-3-methylimidazolium bis(nonafluorobutanesulfonyl imide), was synthesized via an ion-exchange method and tested for its applications in HLICs. The prepared ionic liquid was used to prepare nonaqueous electrolytes in combination with the lithium salt lithium bis(nonafluorobutane sulfonyl imide) and the solvent propylene carbonate. The electrochemical properties of the resultant ionic liquid electrolytes were analyzed and evaluated through linear sweep and cyclic voltammetry analyses. The maximum ionic conductivity of the prepared electrolytes was on the order of 10−3 S cm−1 at room temperature. HLIC cells fabricated using the prepared ionic liquid electrolytes and activated carbon electrodes delivered a maximum specific capacitance of 102.1 F g−1 at a current density of 1 A g−1. Similarly, the prepared HLIC cell with LiCoO2 exhibited a maximum discharge capacity of 128.25 mA h g−1 at ambient temperature. The results suggest that the ionic liquid electrolyte has potential applications as an active separator in future HLIC devices.