(Invited) Ion Transport in Graphite Composite Electrode

Increasing concerns about environmental and energy problems accelerate the demands for high-energy-density batteries. Lithium-ion batteries (LIB) have large energy densities and have been well applied on not only portable devices such as smart phones but also electronic vehicles (EV). However, the energy densities of present LIBs for EV-use seem not to satisfy for a long cruising distance. Present LIBs for EV-use possess large power densities by the sacrifice of energy densities in order to shorten the charging time. In the practical LIBs for portable devices, dense and thick composite electrodes are used. In this case, the ion transport of electrolyte solution through the pores in the composite electrodes becomes a slow process. In order to enhance the energy densities with remaining high power densities, the ion transport rate should be increased for the dense and thick composite electrodes. Unfortunately, to develop dense and thick electrodes with high power densities, fundamental understanding of the ion transport in composite electrodes has not been clear. In this study, ion transportation behavior was investigated with ac impedance spectroscopy. In a conventional use of ac impedance spectroscopy, a composite electrode is used as a working electrode of the measurement cell. In this case, the result of impedance spectroscopy contains all resistances and capacitances accompanying the whole of the charge and discharge reaction including ion transfer through electrolyte and active materials, ion diffusion within active materials, etc. To focus on the impedance of ion transportation of electrolyte, it must be discriminated from that derived from other reaction steps. In order to obtain impedance spectra derived from only ion transportation, composite electrodes without a current collector were used. As a result, we obtained semi-circles, and this semi-circle was assigned to the ion transport process through the graphite composite electrode. As the potential went down, two semi-circles were observed. The characteristic frequency and the resistance of the semi-circle at higher frequency almost did not change. However, at lower frequency, the characteristic frequency and resistance changed. Further, we used a model electrode to understand the semi-circles. The detail analysis will be presented at the meeting.