A review on dissimilar laser welding of steel-copper, steel-aluminum, aluminum-copper, and steel-nickel for electric vehicle battery manufacturing

Highlights

• Review of the main challenges and scientific contributions.

• The relationship between the process parameters and joint properties are explored.

• The effect of intermetallic compounds on the joint properties is assessed.

• The applicability of interlayers and coatings are discussed.

• Opportunities for further research in this field are highlighted.

Abstract

The electric vehicle (EV) battery systems are complex assemblies of dissimilar materials in which battery cells are connected using several thousand interconnect joints. Every single joint influences the functionality and efficiency of the whole battery system, making the joining process crucial. Laser welding is considered a desirable choice for EV battery manufacturing due to its non-contact nature, high energy density, precise control over the heat input, and ease of automation. However, incompatible thermos-physical properties of dissimilar materials used in battery tabs and interconnectors pose a significant challenge for achieving complete metallurgical bond. Furthermore, the formation of undesirable weld microstructures such as hard and brittle intermetallic compounds (IMCs) substantially undermines the structural, electrical, and thermal characteristics of battery joints. This paper reviews the fundamental difficulties and latest developments in dissimilar laser welding of steel-copper, steel-aluminum, aluminum-copper, and steel-nickel, some of the potential joint combinations in EV battery pack manufacturing. The weld microstructure and common metallurgical defects, as well as mechanical and electrical properties of joints are discussed. In addition, the effects of laser welding process parameters on the joint properties and the applicability of various interlayers and coatings in laser welding of battery materials are assessed.

Introduction

The transport sector accounts for 24% of global CO₂ emissions due to the combustion of fossil fuels [1]. It has been reported that internal combustion engine (ICE) vehicles are responsible for almost three-quarters of this amount [2]. Under this threatening situation, carbon emission legislations have been set out across the globe to mitigate the harmful effects of climate change [3], [4]. Such policies have prompted nations to modernize the automotive sector and develop electric vehicles (EVs) to decrease their carbon footprint [5]. The UK, for example, plans to ban the sale of new petrol and diesel cars from 2030 and bring all greenhouse gas emissions to net-zero by 2050 [6]. Similar targets have been set by other major industrialized countries such as China and the EU [7], [8].

Although EVs offer a promising alternative to conventional vehicles, they only accounted for 2.6% of global car sales and about 1% of the entire global car stock in 2019 [9], [10]. The fundamental barrier to the large-scale adoption of EVs is the limited driving range which combined with insufficient charging infrastructure can lead to “range anxiety” in EV drivers [11], [12], [13]; the fear of stranding with an empty battery [14]. Currently, most EVs can only go around 100–250 km on a single charge, much shorter than their ICE counterparts [15]. Using larger batteries is not a feasible solution owing to limited space in EVs, additional cost, higher weight, and the requirement of more rare-earth elements [16], [17]. Hence, there is a need to enhance the energy density of the existing battery system as the key component that determines the vehicle’s performance [18], [19].

Currently, lithium-ion solid-state batteries are the most commonly used source of power for many low to high-capacity applications, including portable electronics and EVs [20]. While in mobile devices such as cell phones and laptops only a handful of cells are required, up to several thousand cells are inter-connected in EV battery systems to deliver the necessary power. Thus, the cell-to-cell or module level joining is the most critical joining process in battery pack manufacturing which directly influences the battery capacity [21]. The overview of the EV battery pack consisting of cell, module, and pack structure is illustrated in Fig. 1.

EV batteries currently use three cell formats: cylindrical, prismatic, and pouch cells [[20], [22]]. Fig. 2 shows these three cell formats [20]. Welding occures in all of them. For example, in the pouch format, the cell tabs are usually made of aluminum or copper and are generally connected in parallel or series configurations using steel, aluminum, copper or nickel busbars [21]. Weld joints in EV battery pack involve low-thickness materials (typically 0.3 mm to 1 mm) and the welding process is normally performed in lap, fillet, or spot configuration [23], [24]. A typical joint between Al tab to Cu busbar in pouch cells is presented in Fig. 3 [25]. The differences in thermos-physical properties of dissimilar materials such as melting temperature and thermal conductivity make it difficult to obtain a complete metallurgical bond without considerable cracks and porosities [26]. Furthermore, the formation of hard and brittle intermetallic compounds (IMCs) at the weld interface greatly reduces the battery's electrical capacity and structural performance. A weak joint cannot withstand the harsh driving environments, dynamic loading, vibrations, and possible crash and might even result in fire due to short-circuiting [27], [28]. The presence of IMCs also intensifies the heat generated during charging and discharging cycles due to their low electrical conductivity thereby accelerating the degradation process of the battery. The possibility of corrosion due to the presence of IMCs which further deteriorates the joint performance should be considered as well [29]. Atmospheric, localized, crevice, pitting, and galvanic are the most known types of corrosion that can happen here [30]. Corrosion not only degrades the mechanical performance of joints but it can also increase the connection electrical resistance [31]. In summary, a proper joint in the EV battery system must satisfy the following requirements:

• Low electrical resistance

• Good strength

• High fatigue resistance

• Low corrosion risk [32], [33].

Wire bonding (WB), resistance spot welding (RSW), ultrasonic welding (UW), and laser welding (LW) are the most investigated joining techniques for EV battery manufacturing [21]. Each of these methods has its advantages and limitations and is used based on cell type, properties, and thickness of the materials involved [20]. Other joining processes such as soldering, friction stir welding, micro-TIG or pulsed arc welding, joining by forming, and adhesive bonding have also been proposed [34], [35], [36], [37], [38]. However, due to the lack of information at present, further research is needed to thoroughly assess their feasibility [21].

In ultrasonic welding, a high-frequency (typically 20 kHz or above) ultrasonic vibration is applied under pressure to join substrates [20]. Oxides and contamination on the surfaces are removed during the welding and the result is a metallurgical bond created without melting, based on diffusion and adhesion of the softened metals [39], [40]. UW can produce good welds (without porosity, hot-cracks, and bulk intermetallics) between highly conductive dissimilar metals, and has been considered particularly superior for pouch cells. However, it can damage the structural integrity of cylindrical and prismatic cells due to the high frequency of vibration. It is also restricted to lap joints [20]. Wire bonding can be defined as single-sided ultrasonic welding of a small diameter Ag, Cu, or Al wire (typically below 0.5 mm), first to one substrate and then to the second or more substrates sequentially [22]. It is a technique frequently used in semiconductor device technology [21], and regardless of no scientific literature on its application in EV battery manufacturing, wire bonding has been employed in Tesla Model-S to connect battery cells and busbars [20]. Resistance spot welding is another method that can be used for EV battery welding. When a high current passes through the interface, it creates localized heating and melting, resulting in fusion welding of substrates [41]. However, resistance spot welding of highly conductive materials like aluminum and copper remains challenging and currently, this process is only suitable for low-conductivity materials [20], [42]. Laser welding is a highly efficient fusion welding technique with the advantages of creating a narrow heat-affected zone and a small targeted deformation [43]. Compared to other main welding techniques for EV battery pack manufacturing (i.e., RSW and UW), the lowest electrical contact resistances and highest joint strengths have been reported with laser welding [44]. Laser welding has the potential to be used for all three types of lithium-ion cells [20]. However, A poor metallurgical affinity between dissimilar materials normally limits the laser welding process and leads to potential defects such as the formation of detrimental intermetallic phases and crack sensitivity. Studies so far have reported that the joint performance could be improved by optimizing the welding process parameters [45]. Furthermore, the applicability of different interlayers and coatings to improve joint properties has been the subject of some recent investigations [46]. There have also been advances in novel lasers (i.e., blue and green lasers) which allow higher energy absorption on highly reflective surfaces of metals such as Cu and Al [47], [48].

This paper presents a comprehensive review on the dissimilar laser welding of the most common joint combinations in EV battery system including steel-copper, steel-aluminum, aluminum-copper, and steel-nickel. The fundamental metallurgical and structural challenges are discussed and the latest developments in process optimization have been highlighted to provide a basis for further studies on this topic.