Quantifying the effects of strains on the conductivity and porosity of LiFePO4 based Li-ion composite cathodes using a multi-scale approach

The effects of the lithium concentration-induced and external load-induced strains on the porosity and electrical conductivity of a LiFePO4 based Li-ion composite cathode are within the focus of this paper. A micro finite element analysis is first performed considering the arrangement and interaction of the particles in a representative volume element (RVE) of a LiFePO4-PVDF mixture. The aim is to capture the deformation behavior under different levels of local state of charge (SOC) in lithium concentration, and external loads. Subsequently, apparent conductivities and porosities as a function of SOC and apparent macroscopic volumetric strains are extracted. A larger macro-scale cathode sample is then analyzed, using macro finite element simulations and the extracted apparent properties. Estimated representative spatial SOC profiles under different external pressures are supplied as input. It is found that external assembly loads should not have a considerable influence on the electrochemical performance, since the changes in porosity and conductivity are negligible. Nevertheless, lithium concentrations could account for up to 5% alteration in porosity and conductivity. Even though relatively small, such levels could be meaningful in situations of high rate and poor cell heat dissipation, which is typical in electrified vehicles applications. These strain effects could be considered in a rigorous electrochemical-thermal framework by using porosities and conductivities as a function of local Li concentrations and apparent volumetric strains. (C) 2010 Elsevier B.V. All rights reserved.