Modeling and simulation of a commercial graphite–LiFePO4 cell in a full range of C-rates

Abstract

A pseudo two-dimensional model (P2D) was presented to describe the electrochemical behaviour of a commercial 18650 cylindrical cell composed of graphite and LiFePO4 (LFP) electrodes. Simulations were conducted by COMSOL MULTIPHYSICS 5.2. The model validation was done with experimental data taken from Hydro-Quebec for a full range of C-rates (currents). A mosaic model based on a C-rate dependent particle radius in positive and negative electrodes was assumed. The reaction kinetics and diffusion in a solid phase were recognized as cell performance limiting factors in the flat area and in the steep area at the end of discharge of the cell voltage–capacity curve, respectively. Since the diffusion polarization in a solid phase played an important role in the steep area at the end of discharge of the cell voltage–capacity curve, a concentration dependent diffusion coefficient in LFP positive electrode was considered. Based on the fact that activation overpotential was a major polarization in the flat area of the cell voltage–capacity curve in addition to decreasing the particle radius at higher C-rates, a contact resistance between the surface of the particles and the solid matrix was predicted. This contact resistance on the surface of active materials in the positive electrode described the feature of low electronic conductivity in LFP. There was a good agreement between the simulated results with experimental discharge data in a full range of C-rates. Simulated (solid lines) and experimental (symbols) cell voltage-capacity curves for different C-rates.