Simulation-Based Analysis of Aging Phenomena in a Commercial Graphite/LiFePO4 Cell

Capacity loss of a commercial graphite/LiFePO 4 cell during either open-circuit-potential storage or under cycling conditions at 25 and 45°C during one year is analyzed with the aid of postmortem analyses and simulations of the cell performance decay over the course of aging. An in-depth understanding of capacity-loss mechanisms under both storage and cycling conditions is gained by refining some parameters of a single-particle model of the cell at different extents of aging. The simulation-based analysis of the aging data reveals that the capacity fade during cell storage only results from the loss of cyclable lithium because of side reactions whereas the loss of graphite active material is an additional source of aging for the cells under cycling conditions. A simple kinetic analysis of electrode/electrolyte interactions is provided for the cells under storage conditions. Moreover, the growth of solid-electrolyte interphase (SEI) at the graphite electrode under storage conditions is simulated in order to refine the solvent-reduction kinetic parameters and solvent diffusion coefficient in the SEI layer. From the analysis, it is shown that the SEI growth during storage is under mixed kinetic/diffusion control.