Area-Perimeter Relation for Si-Based Anode Materials Using Cross-Sectional SEM Analysis

Abstract

Silicon is one of the promising anode materials for next generation high energy density batteries due to its high theoretical capacity. However, its large volume expansion during lithiation hinder its practical application. Here, we report a relation of cross-sectional area and particle perimeter of lithiated silicon using cross-sectional SEM analysis in order to discuss volume expansions and surface degradations of Si-based materials.The silicon electrodes were prepared by a mixture of Si-based materials, conductive materials and binders in a weight ratio of 80:10:10. After gently fabricating cross-sectional electrodes with an ion-milling, cross-sectional SEM images before and after lithiation were obtained. A software, ImageJ was utilized to process the SEM images and calculate length of X-axis and Y-axis, cross-sectional areas and particle perimeters.Figure 1 shows SEM images of a Si-based electrode before and after lithiation. Si expanded uniformly with X-axis and Y-axis expansion ratio of 128%. The cross-sectional area was the expansion ratio of 165%. Since volume expansion is uniform, volume should be calculated by setting to 3 power of X-axis (or Y-axis) or 3/2 power of cross-sectional area. According to the above data, we achieved similar volume expansion rates those were 210% (calculated by X-axis) and 214% (calculated by cross-sectional area). With this method, expansion rate of graphite was shown to be 112%, which is similar to previous Schweidler’s report (113%) [1]. Figure 2 shows processed images of high resolution SEM before and after cycling in order to calculate precise perimeter of Si-based material. The perimeter after the initial lithiation increases to 1.2 times larger than the pristine electrode, which is equivalent to the volume expansion. However, a perimeter after cycling is 3 times larger and it could be caused by a surface erosion. It suggests that the perimeter relates the surface erosion with SEI formation, not only the volume expansion.We will demonstrate the detailed results of cross-sectional SEM analyses on several other Si-based materials.