The lithium-ion capacitor (LIC) is a new 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, a 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 we use a conventional organic electrolyte because an electrode based on a 3-D current collector should be massive and hence too long ionic diffusion pathways. As a result, LIC using a 3-D current collector may deliver limited power. Here ionic liquids (ILs) that have a high carrier density would be useful as an electrolyte for LIC to maintain power. The purpose of this study is applying IL-based electrolytes to LICs with the porous 3-D current collector.

We assembled a three-electrode cell. A positive electrode using an aluminum 3-D current collector was composed of activated carbon, acetylene black (AB) and polyvinylidene di-fruoride (PVdF). A negative electrode using a copper 3-D current collector was composed of hard carbon, AB and PVdF. A lithium foil was used as a counter electrode as well as reference electrode. We used lithium bis(fluorosulfonyl)imide/1-ethyl-3-methylimidazorium bis(fluorosulfonyl)imide (LiFSI/EMImFSI) as an IL-based 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.

When we observed the potential profiles of the positive and negative electrodes during charge and discharge at the 2nd, 1000th and 3000th cycles, it was found that the LIC cell containing our FSI-based IL electrolyte can be charged and discharged reversibly as well as stably. Even though we applied long-term cycling such as 3000 cycles to the LIC cell, the capacity has not significantly degraded. We compared rate performance among the IL electrolyte and conventional organic electrolytes. Although the IL has a relatively high viscosity, it was shown that the FSI-based IL electrolyte has high rate performance comparable to that of a LiPF₆-based organic electrolyte. This may be due to a high carrier density of the IL. These results suggest that application of the FSI-based IL electrolyte to LIC electrodes with the porous 3-D current collector is promising to keep or enhance energy and power capability.