Mechanical properties of heterogeneous, porous LiFePO4 cathodes obtained using statistical nanoindentation and micromechanical simulations

Among cathode materials for Li-ion batteries, LiFePO4 (LFP) is one of the most relevant for commercial batteries in electric vehicle applications. In this work, the Young's modulus and the indentation hardness of a commercial composite LiFePO(4 )cathode are measured using statistical nanoindentation. The influence of the surface roughness, a substrate effect, as well as particle size effect and densification on the measurement are discussed in detail. A focus lies on the analysis of the mechanical properties of the porous secondary LiFePO4 particles and the porosity dependence of the Young's modulus. The measurements are validated via finite element simulations on reconstructed real microstructures. It was found that the mechanical properties of the secondary LiFePO4 particles are significantly lower than those of bulk LiFePO4 and a porosity dependence of the Young's modulus was established. The experimental Young's modulus of the porous LiFePO4 is 47.3 GPa and the indentation hardness is 2.5 GPa. For porous ceramics such as these, this results in an estimation of the yield strength of 1.5 GPa.