(Invited) Beneficial Effect of Fsi-Based Electrolytes on Capacitor Performance

Abstract

A lithium ion capacitor (LIC) is an electric energy storage device that combines an electric double-layer capacitor (EDLC) with a lithium ion battery (LIB). Although LIC features an excellent power density like EDLC, the energy density of LIC is lower than that of LIB. Therefore, improvement of energy density is required for LIC. In the present study, to achieve high energy density of LIC, porous 3-dimensinal (3-D) current collector is applied to LIC electrodes. This 3-D current collector can increase the packing density of active material. It is, however, expected that diffusion of ionic carriers is limited when using conventional organic electrolytes because electrodes based on the 3-D current collector should be massive. As a result, LIC using the 3-D current collector may deliver limited power. Here bis(fluorosulfonyl)imide (FSI)-based ionic liquids (ILs) that have high carrier density would be useful as an electrolyte for LIC to maintain power. The purpose of this study is applying an FSI-based IL electrolyte to LICs with porous 3-D current collector. We assembled a three-electrode cell. A positive electrode using aluminum 3-D current collector was composed of activated carbon, acetylene black (AB) and polyvinylidene di-fruoride (PVdF). A negative electrode using copper 3-D current collector was composed of hard carbon, AB and PVdF. A lithium foil was used as a counter electrode and reference electrode. We used LiFSI/EMImFSI as an IL electrolyte. The cell was charged and discharged for 3000 cycles at 1.0 C-rate in a voltage range of 2.0 – 3.8 V. We also evaluated rate performance of the LIC cells by rapid charging and discharging test up to 30 C-rate. Potential profiles of positive and negative electrodes were observed during charge and discharge at the 2nd, 1000th and 3000th cycles. It turns out that the LIC cell containing the IL electrolyte can be charged and discharged reversibly as well as stably. Even though a test was long-term cycling such as 3000 cycles, the capacity has not significantly decreased. We compared rate performance among the IL electrolyte and two organic electrolytes. Although ILs have high viscosity, it is shown that the FSI-based IL electrolyte has high rate performance comparable to that of a LiPF6-based organic electrolyte. This may be ascribed to high carrier density of the IL. These results suggest that applying the IL electrolyte to a LIC electrode with the porous 3-D current collector is promising to keep or enhance energy and power capability. We will also report the effect of FSI-based electrolytes including IL systems and solvent-based ones on the charge-dicharge performance of EDLCs.