Identification and quantification of ageing mechanisms in Li-ion batteries by Electrochemical impedance spectroscopy.

Transportation sector reported almost a quarter of global CO2 emissions. Thus, efforts to decarbonize this sector are essential for achieving net zero emission goals. Among the actions to mitigate the effects of climate change in favor to decarbonization, lithium-ion electric vehicle market has expanded over the past years because of both scientific advances and encouraging public policies. The analysis of ageing in lithium-ion batteries is essential to ensure optimal performance and determine the end of the useful life for that purpose. Lithium-ion batteries degradation is a complex multi-causal process. Ageing mechanisms could be grouped mainly into three degradation modes: Loss of Conductivity (CL), Loss of Active Material (LAM) and Loss of Lithium Inventory (LLI). Ageing battery process can be evaluated as a state of health (SoH) and tracked based on capacity and power. Although SoH quantifies the battery degradation through the decrease in capacity, its definition does not include an indication of the underlying deterioration mechanisms causing the degradation. Combined with electrochemical impedance spectroscopy (EIS), degradation modes can be identified and quantified non-destructively with the aim of finding a correlation between their evolutions with SoH. This paper proposes a method to identify and measure the ageing mechanisms in commercial 18650 NMC lithium-ion batteries over time using the EIS technique. The EIS spectra were fitted to a proper equivalent electrical circuit and the main mechanisms responsible for the degradation identified through the calculated parameters variations with time. Thus, the increase in Rohm was assigned to CL, Rsei and Rct with LLI and Rw with LAM. Correlation between degradation modes with SoH was also reported.