The heterogeneity of the SOC is also clearly visible from Figure , where for both materials, the particles on the separator side (top) always show a higher SOC. At the end of the charge, the differences between the SOC of graphite and Si is of 0.81 and 0.37, respectively. Thus, we can clearly say that the overpotential also arises from the lithium concentration gradient in the liquid phase, which is shown in Figure (c). Indeed, the concentration difference can be as high as 700 mol/m 3 , which largely affects the lithium exchange current density as included in the Butler-Volmer equation, causing low reaction current densities in low lithium concentration zones. When using lower applied currents, the SOC distribution appears to be more homogeneous along the thickness of the model electrode, where a fully lithiated state is reached for both materials.