Mechanical Stability of the Heterogenous Bilayer Solid Electrolyte Interphase in the Electrodes of Lithium–Ion Batteries

Mechanical stability of the solid electrolyte interphase (SEI) is crucial to mitigate the capacity fade of lithium–ion batteries because the rupture of the SEI layer results in further consumption of lithium ions in newly generated SEI layers. The SEI is known as a heterogeneous bilayer and consists of an inner inorganic layer connecting the particle and an outer organic layer facing the electrolyte. The growth of the bilayer SEI over cycles alters the stress generation and failure possibility of both the organic and inorganic layers. To investigate the probability of mechanical failure of the bilayer SEI, we developed the electrochemical-mechanical coupled model with the core–double-shell particle/SEI layer model. The growth of the bilayer SEI is considered over cycles. Our results show that during charging, the stress of the particle changes from tensile to compressive as the thickness of bilayer SEI increases. On the other hand, in the SEI layers, large compressive radial and tensile tangential stress are generated. During discharging, the compressive radial stress of the bilayer SEI transforms into tensile radial stress. The tensile tangential and radial stresses are responsible for the fracture and debonding of the bilayer SEI, respectively. As the thickness ratio of the inorganic to organic layers increases, the fracture probability of the inorganic layer increases, while that of the organic layer decreases. However, the debonding probability of both layers is decreased. In addition, the SEI covering large particles is more vulnerable to fracture, while that covering small particles is more susceptible to debonding. Therefore, tailoring the thickness ratio of the inorganic to organic layers and particle size is important to reduce the fracture and debonding of the heterogeneous bilayer SEI.