Electrolyte motion induced salt inhomogeneity – a novel aging mechanism in large-format lithium-ion cells

Abstract

The electrification of the transport sector places ever-increasing demands on the energy density, fast-charging performance, and lifetime of lithium-ion cells. In this study, we investigate fast-charging of high energy density (∼ 800 W h L⁻¹) prototype cylindrical 4695 lithium-ion cells with two different (“low” and “high”) electrolyte amounts. Using pore volume calculations, computer tomography and moment of inertia measurements, we find that the volume change of the active material causes electrolyte motion into and out of the jelly roll upon cycling in the high electrolyte cells, while no electrolyte motion occurs in the low electrolyte cells. At the same time, the high electrolyte cells show a significantly worse capacity retention during fast charge cycling compared to the low electrolyte cells (24% vs. 5% capacity loss after 130 cycles). We demonstrate that a coupling of in-plane electrolyte motion with the through-plane LiPF₆ concentration gradient rapidly causes a strong LiPF₆ concentration gradient along the jelly roll height (= in-plane) on a cm-scale: after only 18 cycles, LiPF₆ concentrations at the jelly roll edges and center (as determined by ion-chromatography) deviate by more than ± 50% in comparison to the initial average value. We term this hitherto unknown effect “electrolyte motion induced salt inhomogeneity” (EMSI). This long-scale salt concentration gradient causes a loss of cell capacity, an increase in resistance, and eventually highly localized lithium plating. Finally, we discuss the far-reaching implications of the EMSI effect on cell design and testing, not only for cylindrical cells but any large-format lithium-ion cell under high compression.