Electrolyte Motion Induced Salt Inhomogeneity in Prismatic Lithium-Ion Cells and the Influence of the Mechanical Constraint

We have recently described a novel aging mechanism, namely the electrolyte motion induced salt inhomogeneity (EMSI), which was first observed in cylindrical lithium-ion cells. In the present study, large (>100 Ah) prismatic cells are cycled under different external mechanical pressures to investigate whether the EMSI mechanism also occurs in the prismatic format. It is the main finding of this study that the EMSI mechanism can indeed occur in prismatic cells, but the mechanical boundary condition plays an important role: the EMSI mechanism only occurs if the mechanical constraint prevents reversible cell swelling during charge/discharge and therefore initiates electrolyte motion inside the cell. If, in contrast, reversible cell swelling is allowed by the mechanical constraint, no (or less) electrolyte motion and therefore no (or a weaker) EMSI effect take place. Furthermore, the prismatic cells used in this study contain a reservoir of free electrolyte outside of the jelly roll. This electrolyte reservoir was found to inject LiPF₆ into the jelly roll which delays the negative impact of the EMSI effect on cell performance and aging.