Chapter 1
Lithium-Ion Battery Manufacturing for Electric Vehicles: A Contemporary Overview

Wayne Cai

Manufacturing Systems Research Laboratory, General Motors Global R&D Center, Warren, MI, USA

1.1 Introduction

During the last few decades, environmental concern about the petroleum-based transportation has led to renewed and stronger interest in electric vehicles (EV). In an EV, energy storage devices (such as batteries, supercapacitors) or conversion devices (such as fuel cells) are used to store or generate electricity to power the vehicle. The first highway-capable EV with mass production in the modern age was GM's EV1 [1], which used lead-acid-based batteries as onboard energy storage. With the advancement of newer generations of high-density energy storage batteries such as the metal-hydride batteries and most recently the lithium-ion (Li-ion) batteries, battery electric vehicles (BEVs) have seen tremendous growth in the past decade. Batteries used as the power and energy sources to drive BEVs are called traction batteries.

A BEV falls into one of the following four categories: hybrid electric vehicle (HEV), plug-in electric vehicle (PHEV), extended range electric vehicle (EREV), and pure BEV. An HEV is generally powered by an internal combustion engine and a battery pack. The internal combustion engine is the primary source of energy during medium or high-speed driving conditions with the batteries serving as the main power source in stop-and-go traffic as well as power assist in vehicle acceleration, where the batteries are also called power batteries. The battery pack is relatively small and recharged by the internal combustion engine and regenerative braking. An exemplary vehicle is Toyota's Prius (2015 model year), offering an EPA-estimated 50 mpg fuel economy using a small 4.3 kWh of Li-ion battery pack [2]. A PHEV operates under either the battery mode, the internal combustion engine mode, or a combination of the two modes. The battery pack, however, can be charged via an external electrical power grid. An exemplary vehicle is Toyota's Prius Plug-in [2]. Depending on the design intent and the size of the battery pack, the traction batteries in PHEV can be either power or energy batteries. An EREV differs from a PHEV in that the battery pack is relatively large and the vehicle operates primarily under the electric mode. The internal combustion engine in the vehicle is used exclusively or primarily to charge the traction batteries (although the internal combustion engine can also be used to assist the battery mode driving in special circumstances). An exemplary vehicle is GM's Chevrolet Volt [3]. A pure BEV is powered entirely electrically by an onboard battery pack through the traction motors. The battery pack is typically recharged via an external electrical power grid. Although many automakers are mass-producing BEVs in the marketplace, the most notable models are Tesla Model S [4], Nissan LEAF [5], and BMW i3 [6,7]. Figure 1.1 shows a complete landscape of major BEV manufacturers and their Li-ion battery cell suppliers. A comparison with reference [8] immediately finds significant industry evolution during the past 5 years. While every single global automotive manufacturer is now producing BEVs, a few BEV start-ups and many traction battery joint ventures have been reorganized or even gone out of business during the past few years. At the end of 2014, Panasonic, AESC, LG Chem, and BYD were the top four largest manufacturers of traction battery cell in the world, supplying batteries to Tesla Model S (pure BEV), Nissan LEAF (pure BEV), GM Chevrolet (EREV), and BYD (pure EV and PHEV), among others [9]. Table 1.1 lists some key technical data for several major BEVs in the marketplace.

Figure 1.1 Major BEV manufacturers and Li-ion battery suppliers (2015 landscape).

Table 1.1 Selected technical data for major BEVs

1.2 Li-Ion Battery Cells, Modules, and Packs

This section reviews different formats and structures of Li-ion battery cells, modules, and packs as seen in BEVs. The focus is on the characteristics relevant to the joining, assembly, and packaging rather than the battery chemistries, functions, and performances.

1.2.1 Formats of Li-Ion Battery Cells

A battery cell is the most basic and fully independent operating unit in a storage battery. It is primarily composed of positive and negative electrodes, separators, electrolytes, and a container. In the current marketplace, there exist primarily three different cell formats for a traction battery cell: cylindrical, prismatic, and pouch. Due to legacy reasons, the cylindrical format has been the mainstream ranging from alkaline (such as AA cells) to NiMH to Li-ion (such as 18650) cells. However, when rechargeable batteries such as NiMH or lithium-ion batteries have been considered for automotive battery applications, other formats of battery cells such as prismatic and pouch types have been developed to improve the volumetric efficiency [10], accommodate thermal management, and/or packaging requirement.

1.2.1.1 Cylindrical Cells

Figure 1.2a shows an 18650 cylindrical cell (i.e., 18 mm in diameter and 65 mm in height) as used in Tesla's EVs. Figure 1.2b is an anatomy of a representative cylindrical cell [11], although the exact structure varies for different manufacturers. The cylindrical cells are known to be less volumetric efficient than prismatic cells. However, the commercial production lines for many cylindrical cells allow for some advanced passive safety features such as positive temperature coefficient (PTC) and current interrupt device (CID) (Figure 1.2c and d) to be built in reference [12]. PTC is a type of material that demonstrates significantly high electrical resistivity at high temperatures so as to melt the PTC itself to break the circuit at higher electrical current [13]. CID is another passive device that breaks when the pressure inside a cell reaches high levels. Normally, when the cell is overheated, such as in a thermal runaway, the pressure increases to a level to break the CID and thereof the circuit [13]. Although manufacturers make cylindrical cells of many different dimensions, 18650 cells are the most produced cells due in part to Tesla's needs.

Figure 1.2 Cylindrical format Li-ion battery cells.

1.2.1.2 Prismatic Cells

For a prismatic Li-ion battery cell, the current collectors (after anode and cathode coatings) and separators are either wound, as shown in Figure 1.3 [14], or laminated (not shown), and then inserted into a prismatic shaped container and sealed all together after filling in the electrolyte. The container is normally made of steels or aluminum alloys (although can be plastics too) and offers rigidity for dimensioning, handling, and protecting the cell. No standard exists as to the size of prismatic cells.

Figure 1.3 An anatomy of a prismatic Li-ion battery cell. (Reproduced from Ref. [14] with permission from Cadex Electronics, Inc.)

1.2.1.3 Pouch Cells

A number of BEV manufactures use pouch cells for light-weighting, better volumetric energy density, and high spatial efficiency. Inside the cells (Figure 1.4) [15], multiple layers of precut positive/negative electrodes and separators are stacked with electrode leads (or tabs) and then welded. Then the edges of the aluminum-laminated films (i.e., the pouch materials) are heat sealed. Similar to prismatic cells, no standards exist as to the size of pouch cells.

Figure 1.4 Schematics of pouch-type cells. (Reproduced from Ref. [15] with permission from Automotive Energy Supply Corporation.)

1.2.2 Battery Modules and Pack

A module is a group of two or more battery cells joined together that can be replaced in maintenance and repair without impacting the rest of the battery pack. A module is also typically the minimum unit installed with safety components, power and heat management electronics. Modules can vary in their sizes, see Table 1.1. A...