Application of Ionic Liquid Electrolyte to Lithium Ion Capacitor Based on Electrodes with Porous Three-Dimensional Current Collector

Abstract

A lithium ion capacitor (LIC) is a new storage device which combines an electric double-layer capacitor (EDLC) with a lithium ion battery (LIB). Namely, LIC consists of an activated carbon as positive electrode and lithium-ion-intercalating carbon material such as hard carbon as negative electrode. LIC also contains functionalities derived from both EDLC and LIB. During charge and discharge of LIC, ion adsorption/desorption occurs on the surface of the positive electrode, while lithium ion intercalation/de-intercalation occurs at the negative electrode. 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 (3D) current collector is applied to LIC electrodes. It is possible to increase the packing density of active material and make a LIC cell lighter. It is, however, expected that diffusion of carriers is limited with conventional organic electrolytes because electrodes using porous 3D current collector become massive; the typical thickness is ca. 1 mm. As a result, LIC using porous 3D current collector may deliver limited power. Thus, ionic liquids which have high carrier density would be useful as electrolyte of LIC to maintain power. The purpose of this study is applying an ionic liquid electrolyte to LICs with porous 3D current collector and investigating the basic operating characteristics.We assembled a three-electrode cell. A positive electrode using aluminum porous 3D current collector was made from activated carbon (AC, 87 wt.%), acetylene black (AB, 3 wt.%) and polyvinylidene di-fruoride (PVdF, 10 wt.%). A negative electrode using copper porous 3D current collector was made from hard carbon (HC, 87 wt.%), AB (8 wt.%) and PVdF (5 wt.%). A lithium foil was used as a counter electrode and reference electrode. The electrolyte was 1.5 mol dm−3 LiFSI/EMImFSI. The cell was galvanostatically charged and discharged for 3000 cycles at 1.0 C-rate in a voltage range of 2.0 – 3.8 V after a pre-doping process under predetermined conditions. We also evaluated rate performances of LIC cells with various electrolytes by rapid charging and discharging test. 1.5 mol dm−3 LiFSI/EMImFSI was used as standard ionic liquid electrolyte. 1.0 mol dm−3 LiPF6 / EC : DMC = 1 : 1 (v/v) and 1.0 mol dm−3 LiBF4 / EC : DMC = 1 : 1 (v/v) were used as organic electrolytes for comparison. C-rates for the power performance test were varied from 0.1 C to 30 C every 5 cycles after 100 pre-cycles.Fig. 1 shows potential profiles of positive and negative electrodes during charge and discharge at 2, 1000, 3000 cycles. It turns out that the LIC cell containing ionic liquid electrolytes is possible to charge and discharge reversibly as well as stably. Even though it was long-term cycling such as 3000 cycles, its capacity has not significantly decreased. Therefore, long-term cycle stability is high. Fig. 2 compares rete performances with the ionic liquid electrolyte and two organic electrolytes. Although ionic liquids have high viscosity [1], it is shown that FSI-based ionic liquid electrolyte has high rete performances comparable to a LiPF6-based organic electrolyte. This may be ascribed to high carrier density of ionic liquids. Rate performances of the FSI-based ionic liquid electrolyte and LiPF6-based organic electrolyte are superior to that of the LiBF4-based organic electrolyte. This is attributed to lower ionic conductivity of LiBF4. These results suggest that applying the ionic liquid electrolyte to a LIC electrode with porous 3D current collector is promising. FSI-based ionic liquid electrolytes may replace conventional organic electrolytes as electrolyte of LIC because of their resulting high cycle stability and high energy density without power loss.