The multi-material interface is where two extruded filaments are mixed upon cooling down. This joint is of particular importance, considering the mechanical properties of the final printout as well as the leak-tightness of the electrochemical cell. On the other hand, uncontrolled diffusion of the carbon black filler from the CB-PLA into the insulating PLA at the joint may form excessive conductive paths and increase electrode ESA. To verify this effect, we performed XRM studies, as presented in Fig. . Pores appear naturally even with 100% infill, both at the multi-material interface and between the printed layers, and may lead to small geometric surface and ESA variations between 3DP samples. These are visualised with colours in Fig. and the porosity volume distribution in Fig. , testifying that the pore thickness does not exceed 150 m and the pore volume is 5.32%. Notably, one must consider that this value is highly dependent on the 3DP conditions and the printer model. The analysis of the particle distribution within the 3DP sample, depicted in Fig. , reveals the filler with a sphere diameter in the range of between 10 and 120 m. A higher volume equivalent sphere diameter is observed for the insulating PLA sample compared to the CB-PLA, most likely related to the pigment and other fillers used by the manufacturer. The CB filler, which falls in the nanometre range, could not be observed. This analysis further confirms the lack of CB aggregates within the material that may occur in the filament during its processing, significantly deteriorating the electric and electrochemical characteristics .