In-Situ Microscale Compression Testing of Li-Si Micropillars

Abstract

Silicon is considered as an ideal anode material for high-capacity Li-ion batteries (LIB) thanks to its high theoretical capacity, but the commercial viability is severely hampered by considerable volume expansion of Si upon lithiation that can lead to mechanical failure (e.g., cracks and delamination). Hence, understanding the mechanical behaviors of the Li-Si system is of high interest, but the experimental studies have been limited to thin film electrodes by means of wafer curvature measurements or nanoindentation. In this work, we for the first time present in-situ microscale compression testing of Li-Si micropillars and extract the key mechanical properties (e.g., Young’s modulus, yield strength, and fracture energy) from the resulting stress-strain curves. Prior to testing, Si micropillars of various sizes/shapes and oriented at different crystal directions were lithiated/delithiated at room temperature. We then performed the microscale compression testing on these lithiated Si micropillars using a flat-head indenter in environmental scanning electron microscope (ESEM) while obtaining live video images for data-image correlation. The results suggest that the measured elastic moduli of the Li-Si system agree with the values obtained from the standard nanoindentation experiments but with much smaller variance. The precise nature of these experiments allows us to observe the mechanical properties dependent on the crystal orientation of Si and lithiation rate. In addition, various failure behaviors of Li-Si under compressive stresses will be presented, which is difficult to be obtained through the existing experimental techniques.