Ultrasonic Welding of Nanostructured Carbon Conductors to Reduce Contact Resistance with Metallic Foils

Electrochemical devices such as batteries, fuel cells, and capacitors which employ nanostructured carbon conductor electrodes require a low contact resistance connection with metallic foils for termination. Techniques such as soldering and crimping have limited utility in bonding planar electrodes with foils, whereas ultrasonic welding has emerged as a viable technique for creating robust nanostructured carbon metallic connections via solid-state welding. Emerging nanostructured carbon materials include carbon nanotubes (CNTs) and graphene sheet materials due to their mechanical strength, and thermal and electrical conductivity. Characterization of the interface between nanostructured carbon materials and metallic foils is an emerging field of interest where optimization to improve device performance through modification of the electrical and physical connection is critical.<br/>The current work investigates two simultaneous strategies to enhance the electrical contact via chemical doping of CNTs with potassium tetrabromoaurate (KAuBr₄) and ultrasonic welding to Cu foil. The specific contact resistivity was measured using the transfer length method (TLM) to evaluate laser cut CNT rectangular ribbons with geometrically sequenced tabs (RGT) having ultrasonically welded Cu contacts with and without selective doping isolated to the contact region. KAuBr₄ doped CNT-Cu samples had a specific contact resistivity that was more than 50% lower than the purified CNT-Cu samples. Doped CNT-Cu ultrasonic welds reduced the extent of Joule heating beyond the contact region when applying increasing current to failure in a 2-terminal test structure. The advantage of chemical doping has been further validated on test articles that were scaled up from a rectangular ribbon geometry used in TLM characterization to an area of 6 cm x 8 cm, which is consistent with battery electrode form factors. Selective doping (confirmed using emissivity measurements to spatially profile dopant location) of the CNTs prior to ultrasonic welding showed a 1.5x reduction in peak temperature of the contact for each form factor analyzed.<br/>Additionally, ultrasonic welding of graphene sheet conductors to Cu foil was also demonstrated. A series of displacement amplitudes with constant applied energy and pressure values were evaluated to determine optimal welding conditions. A purified and KAuBr₄ doped CNT interlayer was introduced between the graphene sheet and Cu foil to determine the impact on weld properties. Mechanical testing of various ultrasonic weld conditions demonstrated that a CNT interlayer improved the tensile strength of welded samples. Electrical characterization of graphene-CNT/Cu foil samples with and without KAuBr₄ doping show a consistent reduction in specific contact resistivity when KAuBr₄ doping is introduced. Morphological characterization at the weld interface was performed for graphene-CNT/Cu foil conductors via scanning electron microscopy and elemental mapping. These results show the spatial influence of the chemical dopants within the CNT interlayer and how the nature of the material interactions affect the ultrasonic weld interface between nanostructured carbon conductors to metallic foils.