Manufacturing technologies for Lithium-Ion battery cells on hybrid vehicels

Abstract.

Battery packs for electric vehicles (EV),hybrid electric (HEV) and plug-in hybrid electric vehicles (PHEVs) usually consist of a large number of battery cells. These cells must be assembled together with robust mechanical and electrical joints. Joining the battery cells presents several challenges, such as high welding conductive and different materials, joining several sheets, and variable combinations of material thickness. Furthermore, different cell types and packet configurations have implications for battery joining methods. Details advantages and disadvantages of joining technologies as related to the manufacture of batteries, including resistance welding, laser welding, ultrasonic welding and mechanical joining and discusses appropriate manufacturing issues. Joining processes for electrode-to-tab, tab-to-tab (tab-to-bus bar) and the module-by-module assembly are discussed on the cell package types and configuration.

Keywords: Battery Manufacturing, Sheet-metal joining, Ultrasonic Metal Welding, Laser Welding, Mechanical Fastening.

1. Introduction

Re-chargeable batteries are everywhere in our life today and they are considered a key technology for renewable, sustainable, and portable energy applications. Portable electronic devices, such as mobile telephones, notebooks, etc., rely on batteries for their power supply. New development in high energy / power density batteries is a key enabler for auto manufacturers in developing fuel efficient vehicles.

The performance of battery electric vehicles (BEV) depends on the power and energy capacity of battery packs. Of different battery options, the lithium-ion batteries in particular have received great attention since they provide the highest energy density of all available systems [1], and are one critical technology that may determine the auto industry’s future in the next several decades. In current automotive lithium-ion battery manufacturing, different sizes and shapes of cells are being manufactured and are subsequently assembled into packs of different configurations. An automotive battery pack typically consists of a large number of battery cells, sometimes several hundreds, even thousands, to meet desired power and capacity needs. Several cells are usually joined together to form a module. There are tens of modules in a battery pack. I would a result, a significant amount of joining such as welding is needed to deliver electricity in a battery pack. It is not easy to join such a large number of battery cells because of the difficulty with welding multiple layers of thin, highly conductive and dissimilar materials, with 100% reliability. Into the addition, automobile battery is exposed to the harsh driving environment such as vibration, severe temperature, and possible crash - which can affect the battery performance and safety. Furthermore, hundreds of thousands of battery packs will be produced annually for automotive volume production. As such, batteries must be assembled using robust joining processes and the development of effective joining technologies for battery manufacturing is becoming an essential condition for auto manufacturers.

A significant amount of research has been conducted on the lithium-ion battery cells in terms of cell material, design, safety, and performance. But, very limited literature is available on lithium-ion battery manufacturing. As the size of the battery pack increases for BEVs, new manufacturing challenges are being presented as stated above. Currently, most auto manufacturers are entering the BEV market, but they are having difficulties in realizing battery manufacturing due to the lack of experience or precedent technologies, especially battery joining methods. Hence, this paper provides a comprehensive review of the state of the art joining technologies for battery manufacturing, pack architectures and their implications to joining processes.

2. Cutting-Edge Of Technologies For Cell Manufacturing

This section reviews joining technologies that are currently used in battery manufacturing (i.e., resistance welding, laser welding, ultrasonic welding and mechanical joining), and discusses manufacturing issues with regard to the joining of lithium-ion cells.

2.1 Resistance Welding

Resistance welding relies on the electrical resistance at the metal interface to cause a localized heating and fusion of materials. The pressure exerted by the electrodes, through which the electrical current flows, holds the workpieces to be welded in close contact before, during, and after the welding cycle[2]. Resistance welding process is very fast and can be easily automated. It has a wide application in the automotive, electronics, and heavy industries. However, resistance welding has many challenges when applied to battery welding. First, lithium-ion batteries use highly conductive materials such as aluminum and copper for electrodes and tabs, which are not best suited to resistance welding. TABLE 1 is a simple weldability summary for aluminum, copper, and nickel, the most commonly used materials for battery tab. Aluminum and copper (i.e., main materials for battery tab) are difficult to be resistance welded due to their high thermal and electrical conductivities. Second, generally in fusion welding (e.g., resistance welding or laser welding), dissimilar materials are difficult to weld due to different melting temperatures. Lastly, it is difficult to produce large-sized weld nuggets in for battery tab joining due to material thickness, but large size welds are desirable to reduce electrical resistance of the joints and heat generation when large amount of current flows through the cells.