In-situ EIS to Investigate Influence of Conductive Additive Addition Rate on Charge Transfer Resistance of Lithium Ion Batteries

Abstract

In order to develop the lithium ion batteries (LIBs) with higher capacity, it is important to select the suitable compositions of conductive additives for the reduction of the internal resistance of the positive and negative electrodes of LIBs1). In the present study, the influence of the conductive additive addition rate on the internal resistance of the positive electrode was investigated by in-situ EIS 2). This method enables us to measure impedance spectra of the positive electrode, negative electrode and cell of LIBs simultaneously without stopping the measurement of charge/discharge curve. The positive electrode was fabricated by screen printing. The mixture of the slurry containing LiCoO2 powder, carbon nanowire (CNW), polyvinylidene fluoride and N-methylpyrrolidone were screen-printed on the aluminum foil. In this case, the compositions of CNW were 8 to 30 wt % as the conductive additive. In order to investigate the effect of CNW on electrochemical properties of LIB, an acetylene black (AB) was used as the conductive additives instead of the CNW. The spherical natural graphite on cupper foil was used as the negative electrode and a lithium wire was used as the reference electrode (RE). All measurements were carried out by the three-electrode cell (Toyo system co. ltd.) at room temperature (25℃) in an argon atmosphere. In the impedance spectrum of the positive electrode measured by the in-situ EIS, three loops were observed on the Nyquist plane. Since the diameter of the loop in the low frequency range was decreased with the increase of the states of charge (SOC), it is considered that this loop is related to the time constant of charge transfer resistance R ct and electric double layer capacitance 2). The values of R ct of the positive electrode estimated from the diameter of the loop in the low frequency range were decreased with increasing with the addition rate of the conductive additive. The effect of the addition rate of the conductive additive of the positive electrode on the values of R ct at each SOC were discussed.