Anisotropic Compositional Expansion and Chemical Potential of Lithiated SiO2 Electrodes: Multiscale Mechanical Analysis

The use of high-capacity electrode materials (i.e., Si) in Li-ion batteries is hindered by their mechanical degradation. Thus, oxides (i.e., SiO2) are commonly used to obtain high expected capacities and long-term cycle performances. Despite extensive studies of the electrochemical-mechanical behaviors of high-capacity energy storage materials, the mechanical behaviors of amorphous SiO2 during electrochemical reaction remain largely unknown. Here, we systematically investigate the stress evolution, electronic structure, and mechanical deformation of lithiated SiO2 through first-principles computation and the finite element method. The structural and thermodynamic role of O in the amorphous Li-O-Si system is reported and compared with that in Si. Strong Si-O bonds in SiO2 show high mechanical strength and brittle behavior, but as Li is inserted, the Li-rich SiO2 phases become mechanically softened. The relaxation kinetics of SiO2, inducing deviatoric inelastic strains under mechanical constraints, is also compared with that of Si. The finite element model including the kinetic model for anisotropic expansion demonstrates that the long-term cycling stability of core-shell Si-SiO2 nanoparticles mainly arises from the reaction kinetics and high mechanical strength of SiO2. These results provide fundamental insights into the chemomechanical behavior of SiO2 for practical use.