SoH evaluation of LiFePO4 cells using impedance and thermal measurements

LiFePO₄ is a promising cathode material used in Li-ion batteries. It offers better cyclability and is safer in terms of thermal runaway than previous LiCoO₂ or LiMnO₂ cathodes. With new applications as electric vehicles (EV) a reliable State-of-Charge (SoC) estimator becomes a must. Cell´s degradation or State-of-Health (SoH) is crucial when determining SoC. We aim to find a parameter related to SoH in order to have realiable SoC indicators at any time or condition. Impedance, dynamic resistance and thermodynamics of cells with different degradation levels have been studied. Correlation of SoH and impedance of those cells is not found at a glance. In contrast, dynamic resistance, temperature and entropy show good agreement with SoH evolution.