Evaluation of Low Frequency Characteristics of Li-ion Cells using Laplace Transformation

Lithium Ion (Li-ion) cells are efficient energy storage devices with high energy density and long life, but their performance changes over time due to aging mechanisms sensitive to conditions like temperature, current, and state-of-charge range. Each cell ages uniquely, requiring methods to identify the underlying causes. This work presents an automated approach to assess changes in diffusive resistance from voltage relaxation curves. Using finite length Warburg elements, Laplace inversion, and particle swarm optimization, diffusion modelling elements are fit to time domain data. The approach was demonstrated using a 9-month ageing study of Nickel-Manganese-Cobalt/Gr cells at 45°C. Analysis results show early life and gradual increase in late life. Diffusion resistances showed localized trends due to the movement of electrode phase changes relative to cell state-of-charge. Diffusion time constants elongate consistently over life. This shows control systems must adapt for complex resistance parameter changes and significant dynamic effects during lifetime.