Influence of Decreasing Electrode Thickness on Its Structure and Resistance for Lithium-Ion Batteries

Abstract

Introduction An internal resistance in an electrode for lithium ion batteries (LIB) is consisted of three resistances, which are an electric resistance, ionic resistance and charge-transfer resistance. Since a length of Li-ion conductive pathways decreases with decreasing electrode thickness, the internal resistance can be reduced by decreasing the electrode thickness [1]. Therefore, decreasing the electrode thickness is one of the effective methods to improve the power of LIB. On the other hand, making the thin electrodes by the same procedure as the thick electrodes probably causes a change in the electrode structure and increase of the internal resistance. However, it has not been clarified. In this study, the LIB electrodes with various thicknesses were examined to clarify the influence of decreasing the electrode thickness on its structure and the internal resistance. Half cell tests and scanning electron microscopy with focused ion beam (FIB-SEM) analyses were carried out to evaluate electrochemical and structural properties of the electrodes. Results and Discussion Fig. 1 shows the electrode thickness dependence on the three resistances: the ionic resistance (Rion), charge-transfer resistance (Rct), and total internal resistance (R).The resistances are normalized by the weight of active materials. At around 15 μm of the electrode thickness, R showed the minimum. At less than 15 μm of the electrode thickness, R increased due to the Rct increase. Fig. 2 shows 3D reconstructed images of active material phase, acetylene black (AB) and binder phase, and pore phase in the thin (9 μm) and thick (25 μm) electrodes . The porosity of the thin electrode was larger than that of the thick electrode. In addition, the ABs and binder in the thin electrode were dispersed less uniformly than those in the thick electrode. The large porosity and localization of the ABs and binder could produce a high contact resistance among active materials. As a result, the reduced reactive surface area increased the Rct in the electrode. In our presentation, we will discuss the influence of decreasing the electrode thickness on .the Rct more detail with other analysis results, and the method for improving Rct. Acknowledgment This work was supported by New Energy and Industrial Technology Development Organization (NEDO) of Japan. Reference [1] C. Fongy et al. , J. Electrochem. Soc , 157, A885 (2010). Figure 1