Molecular Dynamics Study of Fracture Behavior of LixMn2O4 in Li-Ion Batteries

Abstract

Fracture represents one of key degradation mechanisms in Li-ion batteries over repeating cycles, because it causes the loss of electric contact and increases the surface areas exposed to the electrolyte. However, there remains insufficient understanding of the quantitative correlation between mechanical damage and capacity fade in the electrode material. To address this issue, considerable efforts have been made to develop electrochemical and mechanical coupled models. For fracture analysis using the theoretical model, fracture toughness is an important mechanical property. However, the fracture toughness of lithium manganese oxides (LMOs) (widely used for commercial electrodes) has not yet been reported. In this study, the fracture toughness of LixMn2O4 was calculated by molecular dynamics (MD) simulations. For these calculations, the stress response of LMO materials was obtained upon tensile loading. The MD simulations showed that the fracture stress decreased and plastic behavior increased at the atomic scale as Li concentrations decreased. The decomposition of the stress response revealed that the Mn4+-O2- pair interaction dominated the decline in the fracture stress. In addition, hopping of Li ions to empty sites at low Li concentrations initiated amorphization, enhancing the plastic behavior. From the simulations, the fracture toughness of LiMn2O4 was calculated as about 1.4 MPam(1/2) and that of Li0.5Mn2O4 as about 1.6 MPam(1/2). The values calculated for fracture toughness can serve as useful input for the continuum model of fracture analysis in Li-ion batteries to guide operating conditions and engineering design of electrode materials.