Mechanisms of impedance rise in high-power, lithium-ion cells

Cells were life-cycled cells using profiles with a 3, 6, or 9% change in state of charge (ΔSOC) at 40, 50, 60, and 70 °C. From the voltage response of the cells to the life-cycle profile at each temperature, we separated the overall impedance rise into two simpler terms, R o (ohmic) and R p (polarization), using an equivalent circuit model. The R o data tend to follow the expected trends (40>50>60>70 °C). Although the R p data trends show that R p can either decrease or increase asymptotically with time, the overall temperature-dependent behavior is similar to that of R o. We illustrate the types of processes that can occur in one lithium-ion cell chemistry. Based on the initial rates, the processes are complex. The R o term dominates the observable cell impedance, but R p adds a non-trivial contribution.