Dissimilar welding of thin-gauge metals is extensively used in a variety of industrial applications within the automotive, medical, and electronics industries. Due to increased requirements for emission control, electric vehicles (EVs) will become an increasingly important automotive product. With a view to obtaining a longer operating range for EVs, at least several hundred battery cells are required to be built into the battery packs to meet the desired power and capacity. Therefore, a robust joining process for battery cells is tremendously important since the failure of the battery represents the failure of the EV. Furthermore, the high energy demand from large-size EVs will require higher currents and/or power outputs by parallel connections of multiple cells.
Hence, it would be useful to have a robust joining process for multiple dissimilar battery tabs configured in parallel.
A variety of joining technologies have been developed for thin-gauge metals in the battery industry. Lee, et al. [1] reviewed those techniques, such as resistance welding, laser welding, ultrasonic welding and mechanical joining, and compared their advantages and disadvantages. Among these technologies, resistance and laser welding processes suffered from low joint strength due to the formation of brittle intermetallic compounds (IMC) between aluminum and copper [2] . Jarwitz, et al. [3] showed a laser dissimilar welding process with spatial beam oscillation that resulted in a low melting depth minimizing the growth of the Al-Cu IMC. However, this made the joining process sensitive and difficult to control for mass production. Friction stir welding
Dissimilar welding of thin-gauge metals is extensively used in a variety of industrial applications within the automotive, medical, and electronics industries. Due to increased requirements for emission control, electric vehicles (EVs) will become an increasingly important automotive product. With a view to obtaining a longer operating range for EVs, at least several hundred battery cells are required to be built into the battery packs to meet the desired power and capacity. Therefore, a robust joining process for battery cells is tremendously important since the failure of the battery represents the failure of the EV. Furthermore, the high energy demand from large-size EVs will require higher currents and/or power outputs by parallel connections of multiple cells.
Hence, it would be useful to have a robust joining process for multiple dissimilar battery tabs configured in parallel.
A variety of joining technologies have been developed for thin-gauge metals in the battery industry. Lee, et al. [1] reviewed those techniques, such as resistance welding, laser welding, ultrasonic welding and mechanical joining, and compared their advantages and disadvantages. Among these technologies, resistance and laser welding processes suffered from low joint strength due to the formation of brittle intermetallic compounds (IMC) between aluminum and copper [2] . Jarwitz, et al. [3] showed a laser dissimilar welding process with spatial beam oscillation that resulted in a low melting depth minimizing the growth of the Al-Cu IMC. However, this made the joining process sensitive and difficult to control for mass production. Friction stir welding
Dissimilar welding of thin-gauge metals is extensively used in a variety of industrial applications within the automotive, medical, and electronics industries. Due to increased requirements for emission control, electric vehicles (EVs) will become an increasingly important automotive product. With a view to obtaining a longer operating range for EVs, at least several hundred battery cells are required to be built into the battery packs to meet the desired power and capacity. Therefore, a robust joining process for battery cells is tremendously important since the failure of the battery represents the failure of the EV. Furthermore, the high energy demand from large-size EVs will require higher currents and/or power outputs by parallel connections of multiple cells.
Hence, it would be useful to have a robust joining process for multiple dissimilar battery tabs configured in parallel.
A variety of joining technologies have been developed for thin-gauge metals in the battery industry. Lee, et al. [1] reviewed those techniques, such as resistance welding, laser welding, ultrasonic welding and mechanical joining, and compared their advantages and disadvantages. Among these technologies, resistance and laser welding processes suffered from low joint strength due to the formation of brittle intermetallic compounds (IMC) between aluminum and copper [2] . Jarwitz, et al. [3] showed a laser dissimilar welding process with spatial beam oscillation that resulted in a low melting depth minimizing the growth of the Al-Cu IMC. However, this made the joining process sensitive and difficult to control for mass production. Friction stir welding
Dissimilar welding of thin-gauge metals is extensively used in a variety of industrial applications within the automotive, medical, and electronics industries. Due to increased requirements for emission control, electric vehicles (EVs) will become an increasingly important automotive product. With a view to obtaining a longer operating range for EVs, at least several hundred battery cells are required to be built into the battery packs to meet the desired power and capacity. Therefore, a robust joining process for battery cells is tremendously important since the failure of the battery represents the failure of the EV. Furthermore, the high energy demand from large-size EVs will require higher currents and/or power outputs by parallel connections of multiple cells.
Hence, it would be useful to have a robust joining process for multiple dissimilar battery tabs configured in parallel.
A variety of joining technologies have been developed for thin-gauge metals in the battery industry. Lee, et al. [1] reviewed those techniques, such as resistance welding, laser welding, ultrasonic welding and mechanical joining, and compared their advantages and disadvantages. Among these technologies, resistance and laser welding processes suffered from low joint strength due to the formation of brittle intermetallic compounds (IMC) between aluminum and copper [2] . Jarwitz, et al. [3] showed a laser dissimilar welding process with spatial beam oscillation that resulted in a low melting depth minimizing the growth of the Al-Cu IMC. However, this made the joining process sensitive and difficult to control for mass production. Friction stir welding is able to reduce the IMC formation since it is a solid-state joining process, however, aluminum and copper have a high affinity for each other at a temperature higher than 120ºC and the welding process requires heavy tooling that limits the application to bulk parts [4] [5] . Ultrasonic welding is another solid-state welding process, which is currently the most commonly used in battery manufacturing, however, it suffers from the limitations of maximum joint thickness and the number of the metal tabs in the stack-up [1] [2] .
In this study, a novel process, dubbed resistive joining (RJ), was used to join dissimilar aluminum and copper battery tabs. The joint geometry was analyzed using optical microscopy and scanning electron microscopy (SEM), and compositional analysis was performed using energydispersive x-ray spectroscopy (EDS). Joint strength was determined through tensile tests of lap-shear specimens to fracture.
As-received battery tabs, with dimensions of 45mm by 43mm and a thickness of 0.2mm, were composed of electrolytic-tough-pitch copper C11000 or aluminum A1050 alloy, respectively. The copper tabs exhibited double-sided nickel coating at a thickness of approximately 2.5µm. Battery pack bus bars were made of electrolytic-touch-pitch copper C11000 at a thickness of 0.5mm and sheared into the same dimension as those battery tabs.
A pedestal-style resistance spot welder was used as the energy source for the RJ process, and the force was applied via a pneumatic cylinder that provided a constant load of 150 lbs. The welder used water-cooled electrodes that were composed of Class 10 copper tungsten alloy with press-fit caps. In order to concentrate the heat, an electrically resistive coating of nickel-phosphorus (Ni-P) with ~7 wt% phosphorus was electroless plated on the both sizes of aluminum tabs prior to the RJ process, with coating thickness of approximately 7µm. Weld current was in-situ measured with a data acquisition system, and the weld force was preset through use of a weld probe force gauge. The joint geometry was measured via laser scanning microscopy and the strength was determined by tensile testing to fracture. Figure 1 shows a schematic of the resistive joining process for multi-layer thin-gauge metals. The metallic tabs are coated with electrically resistive layers on both sides except the two tabs in contact with the electrodes at the top and bottom. Consequently, heat is concentrated at the tab interfaces due to the presence of these resistive coatings, and the local heat is accumulated at the joint from the energy input per Eq. (1):
where E, I, R, and t represent energy, electrical current, electrical resistance, and time, respectively. With the correct selection of the coating material based on the metals to be joined, the RJ process is capable of welding similar and/or dissimilar metal stack-ups with minimum energy input as the relatively conductive base metals will only be slightly heated by the application of electric current. As a result, the number of metal sheets in the stack-up is not as crucial as that for ultrasonic and/or laser welding processes. The melting temperature of the resistive coating is preferred to be lower than that of all of the base metals to simplify the resistance welding process. However, in some certain cases, the melting temperature of the resistive coating must be higher than at least one of the base metals, so that the base metal is partially or fully melted at the interface due to the heat conduction from the coating, and the liquid metal dissolves the solid coating and finally forms a metallurgical bond after it solidifies. 
The capability of joining multiple dissimilar metal sheets is one key advantage of the RJ process. For instance, aluminum and copper are the two metals used for the cathode and anode in automotive battery cells, which are required to be robustly joined. It is challenging to join aluminum to copper due to the formation of brittle IMC which can reduce the joint performance in terms of mechanical strength and fatigue. Therefore, selection of the resistive coating material is important for the RJ process, and three key factors need to be considered: (1) the coating material maintains a barrier between dissimilar tabs to prevent the formation of Al-Cu IMC; (2) the material must be electrically resistive; (3) the material can be simply and inexpensively pre-placed on the parts prior to the RJ process to meet mass production requirements.
There are several materials can be used for joining aluminum and copper. Aluminum-and copper-rich alloys, e.g. Al-Si, Cu-Ag-P, etc., are commonly used for brazing [6] [7] . However, those materials are not easily pre-placed on these battery tabs. Cold metal transfer [8] was developed as a fast deposition method for pre-placing a braze material on the workpiece. However, it would not be applicable for use with thin sheet metals due to the high processing temperature. Since neither aluminum nor copper alloys are suitable for this application, more resistive materials have been investigated. Wolverton, [9] assessed the formation enthalpies of aluminum-rich equilibrium compounds both numerically and experimentally, and showed scandium, titanium, cobalt and nickel were the potential metals which formed IMC with aluminum with high chemical bonding energies. Among these candidates, nickel was readily available and Ni3Al was proven as a strong IMC that could be used in many industrial applications [10] . There are many methods to deposit nickel, and electroplating is one of the most inexpensive processes with large-area coating capabilities at a high deposition rate. Lin, et al. [11] studied the interfacial structure in a tin-silver solder on nickel-coated copper plate during aging, and demonstrated that a thin layer of tin-nickel-copper IMC was formed at the interface with the presence of nickel coating, which was able to prevent the formation of brittle tin-copper IMC. Therefore, nickel is a good candidate to build the connecting bridge between copper and aluminum with strong IMC layers at both interfaces. Table 1 shows the electrical and thermal properties of nickel, aluminum and copper. In comparison to copper and aluminum, the melting temperature of nickel (1445ºC) is much higher and the electrical resistivity is not high enough to concentrate heat during the RJ process. With the intention of enabling RJ weldability, electroless plating of nickel (with P) was investigated. Parkinson, [12] evaluated the material properties of the electroless nickel coatings, and found the electrical resistivity was linearly proportional to the amount of phosphorus and the melting temperature was inversely proportional to the phosphorus content when it was less than 11wt%. For example, electroless nickel with approximately 7 wt% phosphorus is one of the plating industrial standards. Even though the melting temperature of the Ni-P coating is 1070ºC, the electrical resistivity can reach 70μΩ·cm which is 26 and 41 times that of aluminum and copper, respectively. In addition, as electroless plating is a chemical process with no gross current flow involved, the rate of nickel deposition on all areas should be equal, and provides relatively uniform coatings. Also, electroless nickel can also be used as a corrosion protection layer due to its unique anti-corrosive material properties. As a consequence, electroless nickel coatings are suitable for use in the RJ process as applied to aluminum and copper battery tabs. 
In this study, the architecture of first-generation lithiumion battery packs was selected as an example to prove the concept of the RJ process for dissimilar battery tab joining as shown in Fig. 2a . Copper and aluminum tabs representing the anodes and cathodes of the battery cells were joined to a copper bus bar. As-received copper tabs were 0.2mm thick with a pure nickel coating on both sides, with a thickness of approximately 2.5µm (Fig. 2b) . As-received aluminum tabs were 0.2mm thick without any coating. Electroless nickel (Ni-P) was applied on both sides of aluminum tabs at a thickness of approximately 7µm (Fig. 2c ) and the phosphorus content was measured to be ~7 wt% by EDS (not shown). The copper bus bar was 0.5mm thick. In this stack-up, both faying interfaces of the parts to be joined contained a resistive layer of Ni-P. Spherical-faced electrodes were used for the RJ process, and the press-fit caps were composed of Class 10 copper-tungsten alloy for the purpose of reducing the sticking behavior between the electrodes and copper workpieces. As those welds were dominantly carrying shear loads in the battery packs, the joining strength was measured through the lap shear test to fracture. The two 0.2mm-think Al and Cu tabs were clamped at one side of the jaw at the tensile tester, and the 0.5mm-think Cu bus was held on the other side (Fig.  2d ). Figure 3 shows the profile of the electric current schedule for a typical resistive joining process. Current is applied in a pulsing schedule with ramped peak values from 1kA to 16kA. This is helpful to reduce the sticking behavior between the electrodes and workpieces due to fast heating and cooling cycles which expand and shrink the opposing components at the contact surface. In addition, during the weld schedule, the cooling time between impulses allows the copper to conduct heat away from the joint, reducing the bulk temperature and, consequently, the bulk resistance. This cooling time also interrupts the thermal runoff, which makes the following impulse encounter a greater fraction of contact resistance as opposed to the bulk, and so focuses the heat at the joint interface [7, 13] . As a result, the resistive coating at the joint reaches a temperature that is high enough to partially or fully melt the aluminum tab and/or the coating itself for producing a sound final joint. Figure 4a shows optical microscopy images of the RJwelded sample with the weld schedule shown in Fig. 3 . The aluminum tab in the middle was partially melted and squeezed to the lateral direction under a constant load of 150 lbs (from the electrodes). This action led to a thickness reduction from 200µm to approximately165µm at the center of the joint. The thickness of the Ni-P layers at both sides of the aluminum tab were measured to be ~5.7µm (Fig. 4b and 4c ), and the missing 1.3µm-thick coating material (deducted from the original thickness of 7µm) was dissolved by the molten aluminum, resulting in the formation of Ni-Al eutectic ( Fig.  4e and 4f) . Based on the Ni-Al binary phase diagram (not shown), the secondary phase in the eutectic could be inferred as Al3Ni IMC. There was not any physical phase change of the remaining Ni-P coating nor of the pure Ni coating on the copper tab ( Fig. 4d and 4g) , which indicated the melting only took place at the aluminum tab near the Ni-P/Al interfaces during the RJ process because the melting temperatures of the Ni-P coating (1070ºC) and copper (1084ºC) were much higher than that of the aluminum (660ºC). Therefore, the Ni-P coating and Al tab formed a metallurgical bond. However, there was no obvious evidence to show how the Ni-P coating bonded with either the copper bus bar or the Ni-coated copper tab. A further study of the EDS scan at the interface between the Ni-P coating and copper tab was carried out and shown in Fig. 5 . Aluminum was not detected in the remaining Ni-P coating, which indicates the coating successfully prevented the formation of Al-Cu IMC. The changes of the chemical composition distribution at the both interfaces of Cu/Ni and Ni/Ni-P (i.e. the presence of Cu in both nickel coated layers) were observed. Therefore, these solid-state bonds were formed at both interfaces by material interdiffusion under a high temperature and pressure during the RJ process. In steady state, the diffusion rate can be estimated by Fick's first law [14] : (2) where J is the diffusion flux, C is the concentration, x is the position, and D is the diffusion coefficient or diffusivity could be calculated by the Ahrrenius equation [15] :
where D0 is a temperature independent pre-exponential factor determined by the materials being joined, Q is the activation energy for diffusion, and K is Boltzmann's constant.
Sabetghadam [16] showed the diffusion joining of stainless steel and copper with a nickel interlayer at an elevated temperature of 800-950ºC and a pressure of 12MPa for 60 minutes, and demonstrated the IMCs of Fe-Ni, Fe-Cr-Ni and Cu-Ni were obtained in the diffusion interlayers. The same phenomenon would be applicable to the RJ process where the Ni-P coating could be considered as the interlayer. The pressure can be calculated to be approximately 23.6MPa with the electrode contact area of 6mm in diameter. Due to the high thermal conductivity of copper and aluminum, the temperature at the faying interfaces should be between the melting temperatures of aluminum (660ºC) and the Ni-P coating (1070ºC). In parallel, diffusion takes place at the interfaces of Ni-P/Ni and Ni/Cu during this dynamic welding process at a high temperature and pressure (Fig. 5) . 
To assess the RJ joints and the necessity of the Ni-P coating, tensile tests in a lap-shear configuration were carried out for both uncoated and Ni-P coated Al tabs. Since the stackup contained three layers, Al and Cu tabs were clamped together at one side of the sample grips and the copper bus bar was held at the other side. Both stackups were welded under the same schedule shown in Fig. 3 , and the relationships between lap shear load and displacement were revealed in Fig. 6 . The joint of the stackup with uncoated Al tab failed at 270N and the sharp peak of the curve indicated brittle fracture. The contact resistance between Cu and Al concentrated the heat during the application of the electric current and the Al melted. Cu was dissolved by the liquid Al at the interfaces and Al-Cu IMC was formed at the joint. Due to the brittleness of the Al-Cu IMC, the lap shear test failed at the joint interface as revealed by a three-dimensional laser scanning profile in Fig. 7a . In contrast, ductile fracture was observed for the RJ welded stackup with the Ni-P coated Al tab, and the weld button was pulled out shown in Fig. 7b . A fracture load of 390N and a displacement of 3.8mm were measured during the tensile test (Fig. 6) , which indicated that the Ni-P coating successfully blocked the formation of Al-Cu IMC and Al-Ni/Cu-Ni IMCs were stronger than the base metal. Since Al and Cu tabs were clamped together during the test, the loading curve reflected that the failure of the Al tab took place at the peak load (390N) and subsequently the Cu tab started to fail at the load of ~350N (displacement of 1mm). The elongation of the displacement was due to the failure of the copper at the periphery of the weld, where the button pull-out fracture was observed. A parametric study on the RJ joints of these two stackups was summarized through a series of tensile tests in a lap-shear configuration as shown in Fig. 8 . The initial peak values of the applied current impulses were set to 1kA. The final values were 8 to 16 kA for the stackups with uncoated Al tabs, and 14 to 22kA for the ones with Ni-P coated Al tabs, respectively. An increase of 0.5kA was applied between each impulse. All the uncoated samples failed at the joint in the manner shown in Fig. 7a with a maximum load of 325N at 12kA. In contrast, all the Ni-P coated samples fractured at the base metal (Fig. 7b ) when the final impulse was higher than 14kA. The lap-shear loads showed a linear trend with an increase in the applied current. However, the sticking force between the electrodes and workpiece was unacceptable for final current impulses of more than 22 kA. Therefore, when an electroless nickel coating is present, resistive joining is demonstrated to be a robust welding process for dissimilar (copper and aluminum) battery tabs. 
A novel joining process, resistive joining, has been developed for robust joining of aluminum and copper battery tabs. The electroless nickel coating is proven to be electrically resistive enough to concentrate the heat at the faying interfaces that minimizes the energy input. With a high melting temperature of the coating, the aluminum tab is partially melted while applying the electric current and finally a continuous metallurgical bond is formed. The EDS line scan profiles at the interfaces between the Ni-P coatings and the copper tab and bus bar reveal that the presence of Ni-P coating prevents the formation of brittle Al-Cu intermetallic compounds. In addition, material interdiffusion is detected at those interfaces which signifies solid-state bonds are generated via diffusion under the high temperature and pressure during the RJ process. The joints display strengths greater than that of the base metal, which indicates that resistive joining is a viable and promising method for creating robust joints for dissimilar thin sheet metals.