Deciphering the degradation mechanism of thick graphite anodes in high-energy-density Li-ion batteries by electrochemical impedance spectroscopy

The utilization of thick electrodes represents a promising strategy for high energy density batteries, but practical application is hindered by the observed challenges of low cyclic stability and rate performance. To address these issues, we employed electrochemical impedance spectroscopy (EIS) as a non-destructive method to establish a diagnostic model capable of identifying the causes of the instability of thick electrodes. While EIS models for thick electrodes have previously been discussed, the connection between these models and the degradation mechanism has yet to be fully understood. Our investigation revealed that resistances of the current collector, solid electrolyte interphase, or electrolyte, increase with the increment of electrode thickness and further increases following the cycling test with a similar degree, indicating that the degradation of thick electrodes is not governed by those resistances. Rather, a new resistance component emerged in the thick electrode after the cycling test, indicating the emerged resistance plays as the predominant factor driving degradation. The new resistance component on the impedance spectra is linked to Li dendrite formation, due to impeded Li-ion transfer. The hindered Li-ion movement is probably due to the migration of low-weight molecules in the drying process and/or the extended distance Li-ions must transverse.