Extruded conductive hybrid-filled polymer films as current conductor foil for bipolar lithium-sulfur battery applications

Electrically conductive hybrid-filled polypropylene films (hf-PP) with multiwalled carbon nanotubes (MWCNT) and carbon black (CB) fillers were produced by continuous melt extrusion using flat dye. The application of these films as a current conductor foil in energy storage applications offers a cost-effective and scalable option for the next generation of lithium-sulfur bipolar batteries. Using combined fillers solves the critical drawback, the typically strong anisotropic conductivity of pure MWCNT fillers, by integrating CB. This second filler interconnects the MWCNT-based percolation network, resulting in enhanced through conductivity caused by its non-orientation during the extrusion process. The evaluation of production conditions, filler distribution, and resulting filler content ensures a scalable, continuous extrusion of hf-PP films below 50 μm thickness. Important for processing optimization are the direction-dependent mechanical film properties and how the mechanical load affects the electrical conductivity, whereas SEM and cAFM measurements unravel the near-surface percolation network and the localized conductivity. Core findings are suitable processing conditions to extrude hole-free hf-PP films with thicknesses down to 50 μm, well-distributed filler admixtures, a sufficient mechanical stability and an intact conductive percolation network that is robust against mechanical load. Electrical measurements show direction-dependent resistivity values down to about 1 Ωcm based on a well-pronounced percolation network. These findings enable the development of mechanically stable and isotropically conductive thin hf-PP films as current conductor foil for battery applications, demonstrated on a working lithium-sulfur pouch cell.