Inhomogeneous Carbon Binder Distribution: Implications for Tortuosity Determination from Electrochemical Impedance Spectroscopy

Abstract

A key factor determining battery performance at high currents is the electrode tortuosity. Most commonly, this parameter is obtained from fitting a transmission line model to an electrochemical impedance spectroscopy in blocking conditions. However, this approach does not consider the effects of a nonuniformly distributed double-layer capacitance, coming from an inhomogeneous carbon-binder distribution. Here, we scrutinize the validity of this disregard and critically assess the impact of inhomogeneous distributions. First, we analyze different inhomogeneous distributions of double-layer capacitance and local ionic resistance with a transmission line model, demonstrating opposing effects on the overall apparent ionic resistance. Subsequently, we simulate microstructures with different carbon-binder gradients to investigate the combined effects on electrochemical impedance spectra. Applying the classical tortuosity determination algorithms to the resulting impedance spectra leads to an underestimation of the actual tortuosity. Finally, our pseudo-2-dimensional simulations demonstrate the tortuosity offset leading to a substantial overestimation of the rate capability.