Equivalent Circuit-based Analysis of EIS Data for Commercial Cylindrical Lithium-ion Cells

Sammanfattning

Battery health assessment is essential for safe and reliable operation of lithium-ion bat-tery systems. This thesis investigates battery degradation characterization through electro-chemical impedance spectroscopy (EIS) and equivalent circuit modeling across multipleoperating conditions.Multiple battery cells spanning 22 battery models from various manufacturers wereanalyzed, including different chemistries, size (18650 and 21700), and ageing states rangingfrom new cells to aged cells. EIS measurements were conducted at multiple state-of-chargelevels and temperatures under different application profiles.The 3RC-Warburg-RL equivalent circuit model was created for parameter extractionbased on comprehensive model comparison. This model demonstrates good fitting perfor-mance with R2 values exceeding 0.99 across all test conditions. Ten model parameters wereextracted to quantify impedance variations.Results show systematic parameter evolution with ageing. The largest time constantτ1 exhibits the most dramatic increases (656-7760%). Ohmic resistance R0 increases con-sistently as battery cell ages, though the magnitude varies between cell models. The War-burg coefficient σ increases for all cell types, indicating progressive deterioration of dif-fusion processes. Temperature analysis reveals strong parameter sensitivity to operatingconditions. Impedance increases 3-10 times at -20°C compared to 25°C for most parame-ters. Silicon anode cells show exceptional temperature sensitivity, with impedance valuessignificantly exceeding other cell types at low temperatures. SOC analysis demonstratescell-dependent parameter variations, with aged cells showing more pronounced SOC de-pendencies than new cells.The thesis establishes that EIS-based parameter extraction effectively quantifies batterydegradation across multiple dimensions. The 3RC-Warburg-RL model provides a goodframework for characterizing impedance variations due to ageing, temperature, and SOCchanges. These findings enable development of improved battery health assessment frame-works that account for operating condition dependencies.