Effect of Lithiation-Dependent Porosity Variation on the Mechanical Integrity of Silicon Composite Electrode

Abstract

Attributing to the noteworthy volume change of silicon active particles upon cycling, the porosity of the coated silicon composite electrode can vary significantly and therefore be expected to affect the apparent mechanical response of the composite electrode. However, direct experimental evidence is still lacking. By stripping the active layer from the current collector and performing quasi-static stretching tests, this work shows a direct correlation between the variation of tensile properties and related coating porosity of the silicon composite electrode during lithiation. Although silicon particles soften when lithiated, it is found that the increased particle volume can significantly lower the porosity of the coating, resulting in the densification of the silicon composite electrode and thus reducing the toughness of the silicon composite electrode and making the electrode more prone to lose its mechanical integrity under small strain in service. Based on finite element simulation and experimental data analysis, analytical expressions of equivalent modulus and strength of the porous silicon composite electrode were also constructed and are in good agreement with the experimental values. Moreover, the maximum tensile stress of the electrode was found to be amplified by at least 1.8 times when the coating-dependent porosity is considered, indicating the necessity in the design of electrode structural integrity and optimization in service. The results of work are expected to provide important experimental data and model basis for the mechanical design of silicon composite electrodes upon usage.