High-performance lithium-ion capacitor composed of electrodes with porous three-dimensional current collector and bis(fluorosulfonyl)imide-based ionic liquid electrolyte

Ionic liquid (IL) electrolytes have been applied to lithium ion capacitors (LICs) composed of porous three-dimensional (3D) current collector. LIC cells containing IL electrolytes showed reversible charge-discharge potential profiles and their capacity degradation was hardly observed during 3000 cycles. In particular, a cell with 1-ethyl-3-methyl imidazolium bis(fluorosulfonyl)imide (EMImFSI) containing lithium FSI (LiFSI) as Li salt kept over 90% of its initial capacity even after 3000 cycles. The EMImFSI-based IL electrolyte system (LiFSI/EMImFSI) also provided better rate performance than that of a conventional LiPF6-based organic electrolyte system. Considering high-temperature (60 °C) characteristics in the IL system and LiPF6-based system, charge-discharge operation in LiFSI/EMImFSI was stable compared to that in the LiPF6-based solvent system. Moreover, the LIC cell with LiFSI/EMImFSI was stably cycled even at 0 °C and its discharge capacity was superior to that of the LiPF6-based solvent electrolyte at 0 °C.