The Effects of Aging on the Tortuosity of Li-Ion Battery Electrodes

Abstract

Li-ion battery performance is greatly affected by how well ions travel through the electrodes, which in turn depends on the microstructure of the porous pathways through the electrodes. During cell formation and cycling, changes in the microstructure therefore lead to changes in ionic transport. Currently what causes these changes and their magnitude is poorly understood. Changes in effective ionic conductivity and diffusivity can measured in terms of tortuosity, a dimensionless geometric factor. This report focuses on the effect of cell assembly and repeated cycling on tortuosity. Using a method previously developed by our group, namely the polarization-interrupt method [1], as well as AC impedance techniques, we were able to determine electrode tortuosity for various electrode films. There is clear evidence of changes in tortuosity during cell formation and cycling steps. These experiments also suggest that electrolyte wetting is a time-dependent process that changes the measured tortuosity. Three different stages of electrodes were compared: (1) pristine electrodes which had never been in contact with electrolyte, (2) electrodes harvested from newly assembled batteries, and (3) electrodes harvested from cycled batteries. This was done for both anodes and cathodes. In the case of the harvested electrodes, a careful washing procedure is needed to remove Li salt. To validate our results we used two different experimental methods, namely the polarization-interrupt method and a blocking-electrolyte method recently discussed by Gasteiger and coworkers [2]. This research was funded by the BMR program of the US Department of Energy. [1] Zacharias et al. , J. Electrochem. Soc. 160, A306 (2013). [2] Landesfeind et al., J. Electrochem. Soc. 163, A1373, (2016). Fig. 1 SEM/FIB cross section of Li-ion anode showing tortuous paths created by the pores in between active material. Figure 1