Welding Journal Research Supplement

Abstract

In this work, a novel ultrasonic-assisted soldering (UAS) process was demonstrated for joining large stacks (64 films) of aluminum metallized polymer current collectors (MPCCs) to a tab for lithium-ion batteries (LIBs). When integrated into cells, MPCCs can act as a passive safety mechanism for thermal runaway and increase the gravimetric energy density. Joining MPCCs is difficult due to their composite nature, and traditional processes may be insufficient to enable integration of MPCCs in LIBs at scale. This experimental study investigated the impact of three key process parameters of through-hole UAS (welder cylinder pressure, ultrasonic vibration time, and vibration amplitude) on the electrical resistance and mechanical strength of the resulting joints. Varying the cylinder pressure had an insignificant effect on joint quality, except that at high cylinder pressures, film damage occurred at the joint area. Inconsistent joint quality was observed with short ultrasonic vibration times. For long ultrasonic vibration times, over-soldering occurred where the solder penetrated through the tab. The amplitude of vibration significantly impacted the mechanical strength and electrical resistance of the joints. The best performing joints were prepared within the low to-middle range of vibrational amplitude tested (7.8 14.4 µm). A more thorough layer-by-layer resistance analysis was also performed on joints produced with near-optimum conditions. These joints had an average layer resistance of 0.0045 ohms. Optical microscopy and SEM imaging revealed that joints made with near-optimum conditions had deeper solder penetration between MPCC films, compared to joints made with a lower amplitude.