Modeling Solid-Electrolyte Interphase (SEI) Fracture: Coupled Mechanical/Chemical Degradation of the Lithium Ion Battery

In lithium ion batteries, intercalation and deintercalation of lithium may result in volume changes that induce stresses in the lithium-host electrode-material particles. At relatively high rates of charging or discharging, the host electrode particles may see large lithium concentration gradients which may result in fracture and pulverization due to large diffusion-induced stresses. Conversely, during low-rate charge/discharge operations, the lithium concentration gradients in the particle are minimal; in turn, the internal stresses to which the electrode particles are subjected are low. The electrode particle-cracking models fail to explain why cells exhibit higher coulombic capacity loss during low-rate cycling than during storage. The primary reason being that most of these models focus on understanding the host particle pulverization but fall short of recognizing the importance of possible mechanical degradation of the solid electrolyte interphase (SEI) layer. In this article, we develop a mathematical model to study stresses experienced by the SEI and demonstrate that stresses of large magnitude are exerted on the SEI layer during the expansion/contraction of an electrode particle which may fracture the SEI layer. With these stress calculations we also show that the larger the state-of-lithiation (SOL) change (or 'swing') during a lithiation event, the larger the possibility of SEI fracture. We propose that at lower discharge/charge rates of battery operation the SEI cracking and reforming, rather than the host particle fatigue, is a dominant mechanism of cell capacity loss. The capacity loss due to SEI cracking is shown to be proportional to the square of the SOL swing of the electrode particle during lithiation. An equation is derived to estimate battery capacity fade with SEI fracture and reformation as the main capacity-loss mechanism. The equation is used to estimate capacity fade during actual cell cycling experiments with satisfactory agreement between estimated and observed capacity fade with only one adjustable parameter.