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

The transportation sector reported almost a quarter of global CO2 emissions. Thus, efforts to decarbonize this sector are essential to achieving net zero emission goals. Among the actions to mitigate the effects of climate change in favor of 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 useful life for that purpose. The degradation of lithium-ion batteries is a complex multi-causal process. The 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 or state of health was tracked based on capacity and power. Through the state of health (SoH) the degradation of the battery is quantified based on the decrease in capacity. However, the definition of SoH 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 find the correlation between their evolution with SoH. This paper proposes a method to identify and quantify the ageing mechanisms in commercial 18650 NMC lithium-ion batteries over time using the EIS technique. The EIS spectra were fitted to the equivalent electrical circuit. Through the variation of the calculated parameters with time, the main mechanisms responsible for the degradation are identified, associating the Rohm increases with CL, Rsei and Rct with LLI and Rw with LAM respectively. A correlation between degradation modes with SOH were reported.