FIB/SEM-based calculation of tortuosity in a porous LiCoO2 cathode for a Li-ion battery

We present a new method to quantify tortuosity in the porous, LiCoO2 cathode of a Li-ion battery. The starting point is a previously published 3D reconstruction from FIB/SEM images with three phases, the active material domain, carbon-binder domain and pore space. Based on this geometrical configuration, we compute effective diffusivities, from which we in turn derive tortuosity values for the pore space ranging between 5 and 11.6 for the three spatial directions. In a next step, we compare our approach to an imaging method that employs back-filling material. These methods do not differentiate between the carbon-binder domain and the pore space. Thus we remove the carbon-binder domain from our 3D reconstruction and add its volume to the pore space. As a result of this procedure, the tortuosity is greatly reduced to values between 1.5 and 1.9. Experiments suggest that both results for tortuosity are inaccurate and that the real values lie somewhere between these parameter sets. Hence, based on experimental data, we propose a nanoporous carbon-binder domain and derive intermediate tortuosity values between 4.2 and 6.1. These values are consistent with experimental values for similar Li-ion cathodes reported previously. (C) 2012 Elsevier B.V. All rights reserved.