A 3D battery model incorporating Si particle expansion induced electrolyte motion

Abstract

Silicon is frequently used as active material in the negative electrode of Lithium-ion batteries as it provides substantial improvements in the energy density compared to conventional graphite electrodes. Due to large volume changes during cycling, the Si content in state-of-the-art Si/graphite composite electrodes is often rather low. As significantly higher Si contents are desirable, the effects of structural changes that influence ion and electron transport and, thus, battery performance and degradation have to be analyzed. An aspect which is often overlooked is the displacement of electrolyte into the void regions of the cells. In our work, we developed a homogenized electrochemical model of Li-ion batteries including single phase flow through the porous electrode media to account for electrolyte motion. The considered Darcy flow is generated by the change in active material volume during battery operation. In our studies, we keep track of the amount of displaced electrolyte and the Li concentration therein. Considering different material compositions and cell designs highlights that the ratio of anode and cathode thickness and permeability are the dominant parameters determining the Li concentration in the displaced electrolyte volume. The difference between inner and displaced electrolyte Li concentration results in a concentration gradient in outflow direction. While the accumulation or depletion of local Li concentration favors degradation during prolonged cycling, our simulations show that this effect can be mitigated by tuning the permeability.