2, General Motors Global R&D Center, Warren, Michigan, United States
Silicon has received much attention as a promising negative electrode material, however, it undergoes extremely large volume changes during lithiation/delithiation. This leads to substantial stresses inside of particle-based electrodes, which are believed to cause poor cycling performance. Composite electrodes that also incorporate oxidized silicon are a cost-effective way to accommodate these stresses and extend cycle life. To obtain fundamental information about chemomechanical phenomena in these composite structures, several different types of materials are being investigated: (1) Si nanoparticles with oxide shells, (2) Si thin films with oxidized surface layers, and (3) composite SiOx particles. The evolution of internal stresses in all of these structures was monitored with precise in-situ curvature in conjunction with parallel electrochemical measurements. Ex situ characterization with electron microscopy, x-ray diffraction, and XPS provide important complementary information about changes in the materials. The different types of materials used for this work make it possible to systematically investigate key length scales, by independently varying oxide layer thicknesses and particle sizes. Analysis of these results requires assessments and models of both the chemical and mechanical effects oxide surface layers and silicon encapsulation. The implications for optimizing these composite electrode structures will also be presented.