Life Simulation of a Graphite/LiFePO₄Cell under Cycling and Storage

An aging model of a commercial graphite/LiFePO 4 cell is developed that takes into account side-reaction kinetics and solvent-diffusion limitations across growing passive films at both electrodes. Additionally, a progressive loss of active material at the anode is implemented as an empirical function of current density, temperature and lithium content in the anode particles. The model is calibrated using 1-year aging experiments under eight different conditions, consisting either of cycling or storage. Side-reaction and passive-film parameters are adjusted by means of the aging data of OCP-stored cells, and cycling conditions are simulated based on that set of parameters. A good agreement is found between cycling experiments and simulations if the loss of anode active material is dependent on the lithium content in the particles, with more loss experienced for the Li-rich particles. Still, simulations progressively depart from experiments for the most severe conditions (cycling at 45°C), which suggests the existence of an additional aging phenomenon that is not accounted for in the present model. Once the model is validated, simulations are readily performed at longer time than experiments in order to determine the battery lifetime under various aging conditions, providing they remain in the range of applicability of the model.