Electrochemical Impedance Spectroscopy response study of a commercial graphite-based negative electrode for Li-ion batteries as function of the cell state of charge and ageing

The successful development of electrified vehicles is a key factor in the transition to a more environmentally friendly transportation sector. Li-ion batteries, which are today's choice to power electrified vehicles, have to fulfill more stringent requirements in terms of ageing and need advanced tools to study the interfaces evolution upon cycling. This work is thus focused on understanding the impedance behavior of a commercial graphite-based negative electrode, which is used in a Li-ion battery designed for such vehicles. 3-electrode pouch cells were assembled with such negative electrode, a LMO-layered oxide-based positive electrode, a Celgard® type separator soaked with a carbonate solvents-LiPF6 mixture electrolyte and a LTO-based electrode as reference. Electrochemical Impedance Spectroscopy measurements were performed at different cell states of charge and ageing times. The impedance of the graphite-based anode is analyzed for first time with de Levie's equation for porous electrodes. The analysis is supported by designed SEI layer formation experiments with vinylene carbonate and vinylene ethyl carbonate additives. The high frequency domain of the interfacial kinetic loop reflects porosity effects and the graphite particles-composite matrix electric tranfer. The SEI layer and charge transfer phenomena are reflected in the medium and medium to low frequency domains respectively, and their impedance contributions depend on the Li content of the graphite particles. Upon ageing, the interfacial impedance of the graphite-based electrode should increase due to SEI layer growing. However, from 100% to 80% of battery capacity retention, the impedance decreases. Our analysis backed by post-mortem characterizations allows to assign this unexpected behavior to porosity rise and slight Mn-contamination of the SEI layer.