Understanding Rate-Limiting Mechanisms in LiFePO4 Cathodes for Li-Ion Batteries

This work encompasses modeling and experimental work to improve understanding of transport and other resistances for LiFePO4 composite cathodes used in Li-ion batteries. Modeling LiFePO4 active material requires consideration of the carbon coating around the particle, phase-change behavior, and diffusion in only one of the lattice dimensions of the crystal. Physically realistic parameters for the full-cell sandwich model were measured directly in separate experiments where possible. Despite the complexity of solid-state diffusion in this system, we found that a constant diffusion coefficient was generally adequate. The full model was compared to experiments of LiFePO4 cathodes vs. lithium and good agreement was obtained for a range of electrode thicknesses and discharge rates. We found that a distribution of inter-particle contact resistances is needed to obtain this level of agreement if one wishes to keep all model parameters at physically realistic values. The model shows that resistances corresponding to bulk electronic conductivity and even more so, to local inter-particle contact are significant and in need of further optimization for the cells tested.