Reconstruction-Simulation Approach Verifies Impedance-Derived Ion Transport Tortuosity of a Graphite Battery Electrode

The performance of a composite battery electrode depends strongly on the morphology of its liquid electrolyte-filled pore space, where ion transport takes place. Ion transport limitations within the pore space are quantified by the tortuosity, which can be determined from diffusion simulations in the reconstructed pore space or through electrochemical impedance spectroscopy (EIS) using, for example, the transmission line model. Although rarely directly compared, the tortuosities determined by the two approaches have so far been lacking in agreement, raising doubts about the validity of each method. In this study, we use a graphite composite electrode to demonstrate that the two methods deliver comparable tortuosity values under two conditions: (i) The pore space is reconstructed by focused ion-beam scanning electron microscopy using an osmium-based contrast agent for positive staining. (ii) The dimensions of the reconstructed volume are sufficient to represent the electrode microstructure and to prevent finite-size effects in the diffusion simulations. Fulfillment of these conditions is proven by a comprehensive morphological analysis, comprising porosity profiles, chord length distributions of solid phase and pore space, and the geometrical tortuosity. Our results confirm EIS as a reliable method for assessing the ion transport tortuosity of battery electrodes. (c) 2018 The Electrochemical Society.