An electrochemical impedance spectroscopy (EIS) has been applied to the investigation of the electrochemical properties of LIB because the time constant related to the electrode/solution interface can be discriminated in impedance spectrum. In the present study, the effect of particle size of the cathode active material on the electrochemical properties of LIB was investigated by in-situ EIS²⁾. The in-situ EIS²⁾ allows for the impedance measurement of the positive electrode, negative electrode, and cell of LIB simultaneously without stopping the measurement of the charge/discharge curve.

A three-electrode cell was used for the impedance measurement. An active material of positive electrode was LiCoO₂ (LCO) with particle size of 5 μm, 10 μm and 20 μm. The conductive additive and binder of positive electrode were acetylene black (AB) and polyvinylidene fluoride (PVDF), respectively. The positive electrode was fabricated by hand-screen printing. An Al sheet was used as the current collector. In the present study, the theoretical capacity of the positive electrode was calculated from the capacity density of LCO, whose value was defined as 1C-rate. The C-rate denotes the charge/discharge rate, and the 1 C indicates the charge/discharge rate that can fill/empty the total capacity of a battery in an hour. An active material and a current collector of negative electrode were natural spherical graphite and Cu sheet, respectively. A lithium ring was used as the reference electrode (RE), and a mixture of an ethylene carbonate (EC) and an ethyl methyl carbonate (EMC) (3:7 by volume) containing 1 M LiPF₆ were used as the electrolyte. After the charge/discharge curve measurement using potentio-galvanostat (sp-50, Bio-Logic), the impedance measurement of the positive electrode during charge/discharge was measured at 1 C by in-situ EIS. The impedance measurement was carried out in the frequency range of 100 mHz to 100 kHz at 5 frequencies per decade with AC amplitude of 10 mV. An electrochemical measurement system (HZ-7000, Hokuto Denko) was used for the measurement.

After the charge/discharge curve measurement at 0.3 C, the charge transfer resistance R_ct was estimated from the curve fitting of the impedance spectra of the positive electrode during charge/discharge at 1 C using an equivalent circuit. It was confirmed that the R_ct value decreased with increasing SOC at each particle size. Comparing the R_ct values at each SOC, the R_ct values at discharge were larger than that at the charge. This indicates that R_ct is varied depending on the direction of current. Furthermore, the R_ct values at each particle size during charge and discharge were decreased with decreasing the particle size, demonstrating the increase of the reaction surface area on the cathode active material. On the basis of these results, the diffusion distance of Li ions in the solid phase during charge/discharge was discussed.

References:

[1] D. Aurbach, Y. Ein-Eli, B. Markovsky, A. Zaban, S. Luski, Y. Carmeli, H. Yamin, J. Electrochem. Soc., 142, 2882 (1995).

[2] M. Itagaki, K. Honda, Y. Hoshi, I. Shitanda, J. Electroanal. Chem., 737, 78-84 (2015).