Effect of Conductive Materials and Binder Distribution on Mass Transport by Multi-Scale Simulation Method

Abstract

Recently, from the viewpoint of the effective utilization of energy and the positive application of renewable energy, the high performance and large-scale secondary batteries are expected as the energy storage devices, the basic technology for energy security and the power storage of automotive. Especially, in order to increase the high-rate performance of lithium ion secondary batteries, the kinetics and transport phenomena of ion and electron have to be understood in detail more and more. This knowledge is very important for the innovative design of electrode structure. In the case with large-scale cell, the electrode layer has to be evaluated in the micro-scale and macro-scale because of ion and electron transfer mechanisms consist of various paths and phenomena, such as interface contact, tortuosity, volume ratio and heterogeneous structure. However, it is very difficult to understand these complex conditions by only direct experimental measurements and observations because actual electrode is heterogeneous structure. In our previous study, we focused on the carbon black (CB) conductive materials. Various CB structures were simulated and characterized by numerical analysis. And the effect of primary aggregation structure on the effective electron conductivity was evaluated. In this study, actual porous electrode structure was obtained by FIB-SEM. By using this structure, the effect of CB primary and secondary aggregation and the contact interface between active materials and CB on the conductivity were evaluated by simulation. Moreover, the effect of CB surface properties on discharge performance was evaluated by experiment. LiCoO2 positive electrode layer was used. The weight ratios of active material, conductive material (Acetylene black) and binder (PVdF) were 92.6, 4.6 and 2.8 wt%. The numerical 3D reconstruction image of this electrode was obtained by FIB-SEM (slice pitch: 500 nm, 160 images) and image processing. CB primary aggregate was simulated by setting particle diameter distribution and particle numbers, and these structures were compared by experimental results, such as TEM image and primary aggregate size distribution. This CB was distributed in 3D reconstruction image by using adhesion to active material model and CB agglomeration model. Effective CB conductivity was simulated by random-walk method and simple conductivity obtained by direct measurement. As a result, it was found that the effective electrical conductivity and effective active material utilization were strongly affected by CB structure and CB agglomeration. And tortuosity of electron path in actual electrode is much larger than that of packed bed model. Furthermore, in the case of high dispersed CB by controlling surface property, the effective electrical conductivity and discharge performance were increased. In addition, we examined the effect of fiber structure conductive material on micro and macro electron transfer.