Lobato,Killian Paulo KiernanRodrigues,Diogo Vilardell Gomes2026-01-062026-01-062025http://hdl.handle.net/10400.5/116487Tese de Mestrado, Engenharia da Energia e do Ambiente, 2025, Universidade de Lisboa, Faculdade de CiênciasThis dissertation contributes to the broader context of energy transition, addressing the urgent need for efficient energy storage systems to support renewable energy integration and electric mobility. Lithium iron phosphate (LFP) batteries, a type of lithium-ion battery, are particularly relevant due to their high energy density, long cycle life, and proven performance. The objective of this study is to investigate the degradation mechanisms of commercial LFP batteries under cycling, using Electrochemical Impedance Spectroscopy (EIS) and capacity analysis. The experimental methodology involved accelerated cycling protocols (1C charge / 3C discharge) followed by EIS measurements at various states of charge and temperatures, with data fitted to equivalent electrical circuits. Capacity fade was tracked over 1080 fast cycles, and the impact of temperature on impedance spectra was also examined through controlled thermal tests. The results indicate a relatively low capacity loss (5–8%) after extensive cycling, confirming the long-term stability of LFP batteries. EIS spectra revealed significant sensitivity to temperature, with increased charge transfer resistance and Warburg impedance at lower temperatures. While the mild degradation observed limited the ability to isolate specific degradation mechanisms, the study validates EIS as a non-destructive diagnostic tool for tracking performance evolution in LFP cells. Future work should consider more aggressive ageing protocols and tighter temperature control to enhance degradation characterisation.application/pdfengLithium-ion batteriesElectrochemical Impedance SpectroscopyEquivalent Circuit ModellingTemperature EffectsCharge Transfer Resistancemaster thesis204176000