Electrochemical Properties of Surface-Treated Hard Carbon Electrode

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Introduction Li ion battery technology, since its birth in early 1990s, has been powering the rapid digitization of our daily life and finally applying to EVs and HEVs, resulting from its superior energy density compared to conventional rechargeable batteries. However, some improvements of this amazing energy storage system should be made in the future, in particularly, the enhancement of rate capabilities. The rate of lithium-ion batteries is influenced by several factors such as ion diffusion in the composite electrodes, charge transfer resistances, diffusion in the active materials, etc. Among them, we have paid much attention on the charge transfer resistances. According to the Arrhenius equation, the charge transfer resistances are inverse proportion to the frequency factor and exp(-Ea/RT), where Ea, R, and T denote to the activation energy, gas constant, and absolute temperature. Therefore, the decrease of the charge transfer resistances can be made by the increase of the frequency factors and the decrease of Ea. As for the negative electrodes of commercial lithium-ion batteries, graphite electrodes have been mainly used. Hard carbon electrodes have been also used in HEVs since the rate performance is superior to that of graphite. This should be principally resulted from the large frequency factors. Then, we have focused on the surface treatment of hard carbon electrodes to decrease the activation energy of Ea. Here, we report the electrochemical properties of surface-treated hard carbon electrodes by Li3PO4. Experimental Glassy carbon electrodes were used as model electrodes in this study. In advance, the glassy carbon electrodes have been heat-treated at 773 K in the air for 1 h. Surface treatment of the glassy carbon electrodes were made by the sputtering of Li3PO4 in several conditions. Three-electrode cell was fabricated by using surface-treated glassy carbon electrode as a working electrode and lithium metal for counter and reference electrodes. Electrolyte solution was 1 mol dm-3 LiClO4/ EC + DEC (1:1). Cyclic voltammograms were conducted with a potential rage of 0 – 3 V at a scan rate of 0.1 mV/s. Ac impedance spectra of the glass carbon electrodes were measured at given potentials and temperatures. Results and discussion Cyclic voltammograms (Fig. 1) of the surface-treated glassy carbon showed the large reductive and oxidative currents which corresponded to the insertion and extraction of lithium-ion at the electrode, respectively. In addition, it was found that the surface-treatment enhanced the glassy carbon electrode properties by comparing the pristine one. In the Nyquist plots at different potentials, various semi-circles were seen. Assignment of each semi-circle will be discussed in the conference. Figure 1