In-depth evaluation of micro-resistance spot welding for connecting tab to 18,650 Li-ion cells for electric vehicle battery application

In an automotive battery pack, many Li-ion cells are connected to meet the energy and power requirement. The micro-resistance spot welding (micro-RSW) process is one of the commonly used joining techniques for the development of cylindrical cell-based battery packs, especially for low to medium volume applications. This paper is focused on identifying the effect of influencing parameters of the micro-RSW process and developing an optimized joining solution to connect a 0.2-mm-thin nickel tab to 18,650 Li-ion battery cells. The effect of welding parameters including weld current, weld time, squeeze time, pre-heat current, pre-heat time, dwell time and hold time were investigated to optimize joint strength. Firstly, the welding pilot runs were conducted between Ni connector and two different thicknesses (i.e. 0.3 mm and 0.4 mm) of Hilumin coupons, representative of negative and positive terminals of 18,650 cylindrical cells. Secondly, it was observed that the weld current had the most significant effect on the weld strength followed by weld time. Finally, at the optimum parameter combination, the live cell welding was conducted between the Ni tab and both positive and negative terminals of LG HG2 18,650 Li-ion cells, and the joints were relatively strong; no intermetallic compounds (IMCs) appeared. Weld microstructure studies provided insightful information on under-weld, good-weld and over-weld characterization and correlated with the joint strength. In addition, electrical contact resistance and temperature rise at the joint is equally important for electric vehicle battery applications. The joint performance was evaluated by analyzing the change in contact resistance and joint temperature rise when different amplitudes of current (i.e. 10 A, 20 A and 30 A) passed through the joints