Fast Charging of Li-Ion Cells: Part II. Nonlinear Contributions to Cell and Electrode Polarization

In this series, Li/Cu microprobes are used to monitor potentials of individual electrodes in situ during high-rate charging of Li-ion cells. Here we focus on capacity-limited charging of these cells to 6C, and present a general treatment of polarization that allows for data reduction and accurate interpolation/extrapolation over a wide range of charge rates. We show that the anode impedance as measured both using this new treatment and the more established pulsed-current techniques is not significantly changed after high-rate aging of the cell, including the conditions under which Li plating has occurred. Our measurements suggest that the changes in cell and electrode polarization to a large extent occur through nonlinear effects that involve time-delayed processes. An electrochemical model that includes phase dynamics in lithiated graphite is shown to capture some but not all of the observed trends suggesting that important facets of the high-rate behavior need to be included in such models.