| Abstract Scope |
Metallized polymer current collectors (MPCC’s) are a composite material created by vapor depositing or sputtering a thin aluminum or copper layer (1 micron) on both sides of a polymer substrate (6 micron). When integrated into batteries, MPCC’s can electrically isolate internal short circuits and therefore act as a passive safety mechanism for lithium-ion batteries. Additionally, cells that utilize MPCC’s have increased gravimetric energy density due to the lightweight polymer substrate. Unfortunately, MPCC’s are difficult to join with conventional processes. The MPCC metallized layer is very thin making it difficult to form a weld, and the polymer layer acts as a barrier to electrical current. For an MPCC joint to be conductive in the thickness direction, some polymer must be displaced, or a connection must be formed along the edges of the MPCC’s. Ultrasonic metal welding (UMW) can create MPCC to tab joints, however, the process is inconsistent, and the joints have poor mechanical and electrical properties. In order to integrate MPCC’s into batteries on a manufacturing scale, a joining method must be devised that can join MPCC’s to tabs with high consistency and acceptable mechanical/electrical properties.In phase 1 of this research project, we explored potential joining processes for MPCC’s using literature review and laboratory experiments. The joining processes and methods that were explored included: ultrasonic metal welding, resistance spot welding, laser welding, mechanical joining, soldering, electrically conductive adhesive and various hybrid processes. Soldering provides the unique ability to flow conductive material into desired areas on an MPCC or between layers in an MPCC stack. However, soldering of aluminum requires a corrosive flux that would not be desirable in battery manufacturing. Therefore, we explored ultrasonic assisted soldering, which is a fluxless soldering method. Ultrasonic assisted soldering (UAS) is the introduction of high-frequency (10—70 kHz), low amplitude (10 µm—250 µm) mechanical vibrations into molten solder, which facilitates joining through removal of oxides and enhanced wetting. We developed a UAS process, dubbed “through-hole UAS”, which is able to join MPCC’s to tabs with high process consistency, and improved mechanical/electrical properties (over UMW). To compare the through-hole UAS joints, electrical and mechanical testing was performed on through-hole UAS joints and UMW joints which consisted of a stack of MPCC’s (32 or 64 films) joined to a 0.1 mm aluminum tab. Electrical testing was done by performing the four-point probe resistance measurement technique on each film within a joint. Mechanical testing was done by performing pull and peel tests on the stacks of MPCC’s against the tab. Creating the UMW joints demonstrated the limitations of this process, as UMW was only able to join a maximum of 32 films to a tab, while through-hole UAS was able to join 64 films to a tab, with a high likelihood that even more than 64 films could be used. In UMW of MPCC’s the UMW horn must pierce through each MPCC film in the stack to create a conduction pathway in the thickness direction. Even at maximum weld pressure, UMW was unable to pierce through 64 MPCC films, although a specially designed horn could help with this issue. Additionally, through-hole UAS had a process success rate of 100% compared to 60% for UMW. The failed UMW samples demonstrated no bond between the stack of films and the tab, following welding. We have hypothesized that in UMW of MPCC’s, the bond between the MPCC’s and the tab is due to mechanical interlocking and melted polymer substrate, rather than any metallurgical bond. In the layer-by-layer resistance testing of the through-hole UAS joints, no single layer had a resistance above 0.1 ohms and the total average layer resistance was 0.04 ohms. In contrast, the UMW joints had a total average layer resistance of 0.11 ohms. Mechanical testing revealed that through-hole UAS joints have superior mechanical performance over UMW joints. When the joints were pulled in tension or peel, (full stack of films against the tab), the UAS joints consistently failed in the tab, while the UMW joints consistently failed at the interface between the tab and films. When MPCC’s are integrated into lithium-ion batteries they make the cell more resistant to thermal runaway. Additionally, cells manufactured with MPCC’s have a higher gravimetric specific energy due to the lightweight polymer substrate within MPCC’s. However, welding or joining of MPCC’s to each other and a tab is difficult due to the thin metal layer of MPCC’s and the polymer substrate, which acts as an insulator in the thickness direction and can melt/degrade at elevated temperatures. In this research project, various potential methods were analyzed for joining of MPCC’s. Ultrasonic assisted soldering proved to be a promising method to join MPCC’s without the use of flux. The through-hole UAS method was used to join MPCC’s to themselves and tabs, resulting in much higher process consistency and superior mechanical/electrical properties than joints made with UMW. Future work is necessary to devise a prototype manufacturing platform for through-hole UAS and to qualify its process window. Additional future work is necessary to explore the fundamentals of UAS and how various ultrasonic induced phenomena impact joining.
Keywords: Battery, current collector, thermal-runaway, ultrasonics, soldering |