Temperature-Dependent Hysteresis Behavior Modeling of the Open Circuit Potential and Impedance of LiFePO4 Batteries

Lithium Iron Phosphate (LFP) batteries present many challenges for accurate behavioral modeling during charge and discharge processes, as the two-phase interactions between cathode particles create multiple stable, degenerate distributions of lithium across the electrode. These degenerate distributions can cause the battery’s equilibrium open circuit potential (VOC) and equivalent series resistance (ESR) to vary at a given temperature and state of charge (SOC) depending on recent history, such as if the battery was most recently charged or discharged, a phenomenon known as hysteresis. A behavioral two-capacitor model was developed that incorporates temperature-dependent hysteresis between charge and discharge (affecting both VOC and ESR) and mimics temperature-dependent and SOC-dependent kinetics and impedance. The model includes a coupled finite volume thermal transport model with heat generation calculated due to a combination of Ohmic heating and entropic heating. The model was validated against high C-rate cyclic charge and discharge, as well as intermittent discharge and charge profiles at temperatures ranging from zero to 50°C. It is seen that for temperatures near 0°C, new physics come into play during recharge and that VOC hysteresis between charge and discharge increases as temperature decreases.