The Effects of Cycling on Ionic and Electronic Conductivities of 18650 Li-Ion Cells

Abstract

During cycling or other aging processes, the capacity of lithium-ion batteries decreases and it is also observed that cell impedance increases. In order to better understand these processes, we are investigating the electronic conductivity and ionic conductivity of electrodes harvested from 18650 cells that have been cycled by different amounts. Effective ionic conductivity of a porous electrode film is sometimes expressed in terms of tortuosity, which is correlated by a Bruggeman-type relationship [1]. A reasonably accurate method of measuring tortuosity is using the previously developed blocking electrolyte method [2]. As for effective electronic conductivity of electrodes, our group has developed experimental methods for accurately making this measurement as well [3]. In this work, the effects of cycling on both ionic and electronic conductivities are examined. We investigate different commercially made 18650 (cylindrical) cells, including those with iron phosphate and cobalt oxide cathodes. Each cell was charged and discharged for a different number of cycles, leading to increasing capacity losses. After certain number of cycles, each cell was cut open and the electrodes were harvested for testing. We will provide details on this experimental method as well as the conductivity results. [1] Zacharias et al, Electrochem. Soc. 160, A306 (2013). [2] Pouraghajan et al., Electrochem. Soc. 165, A2644, (2018). [3] Lanterman et al., Electrochem. Soc. 162, A2145, (2015). Figure 1