The influence of anomalous diffusion on the impedance response of LiCoO2|C batteries

A process model for impedance analysis was developed in the context of reactions and transport processes that were hypothesized to govern the performance of the Li-ion battery. At the carbon electrode, lithium ions and solvent were considered to react to form a solid-electrolyte interphase (SEI). In addition, lithium ions were assumed to diffuse through the SEI to intercalate into the graphene layers. As these two processes involve an addition of currents, the corresponding impedances were considered to be in parallel. Anomalous diffusion of lithium ions was invoked at the LiCoO2 electrode to account for the low-frequency line that had a slope steeper than that predicted by ordinary diffusion. This model was fit to all impedance data collected. The model provided a good description for impedance of batteries under normal operating potentials and temperatures. The contribution of anomalous diffusion was diminished at elevated temperatures, suggesting that the free-energy well associated with anomalous or sticky diffusion was shallow.