Effect of Electrode Density on Electrochemical Performances of Carbon-Electrode for Hybrid Capacitor

Recently, a substantial improvement in the energy density has been achieved through an asymmetric electrode design of utilizing a lithium-ion intercalating electrode as the negative electrode, instead of one of the activated carbon (AC) electrodes in a commercial electric double layer capacitor (EDLC) ; so-called hybrid capacitor such as lithium-ion capacitor (LIC) or nano hybrid capacitor^1-7). In these hybrid capacitor, during charge/discharge, lithium-ion intercalation/de-intercalation occurs within the bulk of the negative electrode, whereas, anion adsorption/desorption occurs on the surface of the AC positive electrode. Negative electrodes, in generally, are typically produced with increased density in order to minimize contact resistance of carbons and obtain the higher volumetric capacity in the electrode. However, C-rate of graphite according to literatures shows rapid reduction in C-rate after approximately 2C⁸⁾. In this study, we have investigated the manufacturing parameters to enhance the rate-capability of negative carbon-electrode by controlling the electrode density.

Natural graphite as a negative electrode material was commercially supplied by Aldrich Chemical Co. The negative electrodes were prepared by coating the mixture of natural graphite with particle size of 20μm, a carbon additive (Super P), and poly(vinylidene difluoride) (PVdF) as a binder by 96 : 1 : 3 mass ratio on a Cu foil. The negative electrodes with different electrode density were prepared by controlling the loading density. Unit cells composed by having Li foil for positive electrode and natural graphite electrode for negative electrode, and 1M LiPF6 in EC/DEC/EMC was used as an electrolyte.

In this study, the electrode densities were controlled in the range of 0.7 to 1.5 g/ml. It was observed, in the C-rate capabilities, that Rapid reduction in C-rate after approximately 2C was observed for natural electrode with 1.5 g/ml, which is similar behavior as shown in results of literature⁸⁾. However, it can be seen that there is rapid improvement in C-rate when the electrode density is varied. As the electrode density decreases from high density to low density, the enhanced C-rate performances were observed. In this result, it can be noted that 50% of relative discharge ultimate at 30C was obtained for the graphite electrode with 0.7 g/ml. High power performances in graphite electrode with lower electrode density can be discussed as follows. Electrode with high density presumably has wide contact surface between the carbons with narrow space in between the carbon particles, which would cause unsmooth supply of electrolytes. Consequently, this would cause relatively increased diffusion resistance since electrolytes need to be brought from area included in the separator that is present between the electrodes. However, there are many open voids in the electrode with lower density. These open voids acts as electrolyte reservoir, and ionic mobility would be fast between open voids and carbons. Therefore, these fast rocking chair behavior of ions causes to decrease the diffusion resistance.

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