Quantifying Tortuosity of Porous Li-Ion Battery Electrodes: Comparing Polarization-Interrupt and AC Impedance (Blocking-Electrolyte) Methods

Abstract

Tortuosity is a geometric parameter of porous electrodes that quantifies the tortuous path ions take to meet electrons in order for the chemical reaction to take place on the surface of the active material. Ionic resistance in the electrodes, which is directly related to the tortuosity, is a key factor that influences battery performance, and must be accurately represented in battery models. This work is intended to help battery researchers understand and be able to reliably determine the tortuosity of different electrodes. The polarization–interrupt method previously developed in our research group is an effective way to directly measure electrodes tortuosity [1]. This method determines tortuosity based on effective diffusivity in the sample by solving diffusional equations along with the polarization-interrupt experiment. It requires that an electrode film be delaminated from its current collector. The blocking electrolyte method is another technique used for measuring tortuosity that was recently introduced by Gasteiger and coworkers [2]. This method is based on an AC-impedance measurement of the electrode sample using a non-intercalating or blocking electrolyte. In this method the tortuosity is determined based on an effective conductivity fit with a transmission-line model. In this work the two methods were used to determine the tortuosity of the several Li-ion cathodes and anodes. The results from each method are compared and the advantages, disadvantages, and validity of each method are discussed. Furthermore, for the blocking electrolyte method, we investigated the effects of different salts and solvents on the obtained tortuosity and discuss the implications on choosing an electrolyte for this method. We also added extra terms to the transmission-line model to account for contact impedance in the active material film and current collector interface in both the electronic and ionic path to help the model to better capture the experimental data. This work was supported by the U.S. Department of Energy through the BMR program. -------------------------------------------------------------------------------------------------------- [1] Thorat et. al., Journal of Power Sources, vol. 188, p. 592–600, 2009. [2] Landesfeind et. al., Journal of The Electrochemical Society, vol. 163, p. A1373-A1387, 2016. Figure 1: Example results for the two techniques: a) cell potential during a polarization-interrupt experiment and b) Nyquist impedance plot of blocking-electrolyte experiment. Figure 1