The production of lithium-ion batteries requires several different joining techniques to contact the individual cells. In addition to mechanical strength, low contact resistance between the contact points, durability and safety are crucial. Focusing the mostly used pouch cells, at cell level individual foils need to be conductively joined. Resistance spot, ultrasonic or laser beam welding are mainly used to join battery cells in the production of large battery assemblies and are state of the art. Each of these welding techniques has its own characteristics, depending on the material properties and contact geometry. However, access to the joint must be ensured, often on both sides, which can have a negative effect on the joint geometry or the required installation space. In addition, different types of materials must be joined, which poses major problems from a metallurgical and production engineering point of view. Moreover, the welding processes always involve heat input, which should be locally limited in order not to damage the battery cells. In contrast to these joining processes, one solution is contacting using electrically conductive adhesives. This joining technique allows other design principles and enable more form flexibility or layer quantity. For this reason, a new and innovative contacting technique are developed to establish electrical contact between the individual cells.
In this work epoxy resins filled with silver particles or graphite and carbon black are investigated as alternative joining system for electrical contacting of current collector foils in lithium-ion battery (LIB) cells. As a reference, the current foils are joined by ultrasonic welding. Then, parameters like processing conditions, curing temperature, the appropriate amount of adhesive per layer, different filling quantities of graphite and carbon black are investigated. Subsequently, the samples generated by parameter optimized ultrasonic welding and bonding by means of conductive adhesives are compared in destructive lap shear tests regarding their mechanical strength using the same sample geometry. To characterize the contact resistances between the two joining methods, the four-point resistance method is used. The obtained results show that the resistance of ultrasonically welded samples is smaller to that of joined specimens with conductive adhesives. However, the lap shear strength of the joined samples with the electrically conductive adhesive is higher compared to all parameter combinations of the series of ultrasonic welding tests (bonded samples: 2.5 ± 1.2 MPa and welded samples: 1.2 ± 0.3 MPa). Steps of optimization of the curing temperature, reduction of adhesives amount per layer and a better incorporation of graphite and carbon black in the epoxy resin are necessary to increase the conductivity of the bonded samples. The focus of this work is to identify adhesives systems and parameters and to evaluate whether conductive adhesives have the potential as an alternative joining system for cell contacts.
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