Materials for automobile batteries

For this reason, automakers today are developing and deploying a variety of electric-vehicle technologies to achieve greater fuel efficiency. In addition to these developments, many countries and regions are indicating plans to regulate internal-combustion engines (ICEs), the key components of gasoline- and diesel-powered vehicles, giving all the more reason to expect an increasingly prominent role for electric vehicles in the coming years.

The world’s strictest vehicle regulations are found in the European Union, which has announced policy revisions that would effectively ban production of ICEs after 2035. This ban would include hybrid and plug-in hybrid electric vehicles (HEVs and PHEVs), which incorporate both ICEs and electric motors. Consequently, European automakers are racing to develop battery electric vehicles (BEVs) as quickly as possible.

Elsewhere in the world, a recent executive order in the U.S. calls for BEVs, PHEVs, and fuel-cell electric vehicles (FCEVs) to account for 50% of all new vehicle sales by 2030. In China, the China Society of Automotive Engineers has announced that, by the year 2035, it would be desirable if 50% of vehicles were new-energy vehicles (NEVs, a category encompassing BEVs, PHEVs, and FCEVs), with the remaining 50% represented by vehicles with lower fuel efficiency (HEVs). Finally, Japan has established that, by 2035, electric vehicles (HEVs, PHEVs, BEVs, and FCEVs) will account for 100% of new vehicle purchases, including Japan’s “K-car” light vehicles (information current as of February 2022).

Three critical components of electric vehicles are the motor, the battery, and the inverter.  

In conventional ICEs, gasoline from a fuel tank is supplied to an engine, which generates a rotational force (torque) that is modified by the vehicle’s transmission and transmitted to the tires to induce motion. In BEVs, in contrast, the engine and fuel tank are replaced respectively by a motor and a battery, and electrical energy supplied by the battery is mediated by the inverter—which controls the current, voltage, and frequency—and used to drive the motor or various electrical components.
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HEVs and PHEVs incorporate both ICEs and battery-powered electric motors as mechanisms to induce motion; the difference between the two is that PHEVs, unlike HEVs, allow the battery to be charged from an external source. The failure of HEVs to allow external charging means that these vehicles are primarily powered by the engine, with the motor only a secondary driving source (although some vehicles are able to exploit other mechanisms such as energy regeneration through braking). In PHEVs, which allow external battery charging, this relationship can be reversed: the motor may furnish the primary source of power, with the engine playing a secondary role.

In general, ICE-powered vehicles in motion tend to emit significant quantities of CO2, while CO2 emissions from BEVs in motion are minimal. However, it is important to consider each vehicle’s total environmental impact, including factors such as CO2 emissions during the production of vehicle batteries and, beyond the vehicles themselves, the distribution of energy sources responsible for powering the electrical grid in various nations and regions.

This perspective has led to growing adoption of an environmental-impact analysis technique known as life-cycle assessment (LCA). LCA is a comprehensive approach that seeks to quantify the environmental impact of automobiles at every stage in their life cycle, from resource identification and extraction through vehicle discarding and recycling. As the LCA mindset grows in acceptance, increasing attention will be paid to the ease and efficiency of reusing resources—and to frameworks for facilitating this reuse—not only at the manufacturing stage but also when vehicles are discarded and recycled. For batteries, in particular, a variety of research initiatives and practical tests are currently underway to explore manufacturing-related CO2 emissions and effective utilization of precious resources.

Let us now take a closer look at one of the most important components of electric vehicles: the battery.

Although batteries come in many varieties, the most common type of secondary battery in widespread use today is the lithium-ion battery, which has been extensively studied and adopted in electric vehicles for the two primary advantages it offers: high energy density and high storage capacity, allowing battery sizes to be reduced while still supplying copious quantities of energy. On the other hand, the high energy density for lithium-ion batteries also ensures that safety precautions are essential for working with these devices, and a variety of strict safety regulations and protocols governing their use have been established and put into effect.  

For lithium-ion batteries, the smallest structural element is known as the cell. Cells come in three shapes: cylindrical, prismatic, or laminate (also known as pouch).

• Cylindrical cells
  As their name suggests, these cells have the same circular-cylinder shapes as common disposable batteries. These batteries are formed by stacking three layers of material—a positive electrode, a negative electrode, and a separator—then rolling into a cylinder and inserting into a cylindrical metal case together with an electrolyte fluid. In comparison to cells of other shapes, cylindrical cells are particularly suitable for mass production and are considered relatively inexpensive. On the other hand, the shape of cylindrical cells ensures that these cells cannot be packed together in large numbers without significant gaps arising between cells, resulting in inefficient use of space.
• Prismatic cells
  These cells involve positive electrode, negative electrode, and separator layers rolled together, and packaged with electrolyte fluid into a rectangular metal case, whose shape is responsible for the main advantages of these cells: high mechanical strength and high energy density per unit volume.
• Laminate (pouch) cells
  In these cells, the metallic cases used for cylindrical and prismatic cells are replaced by laminate-film packages. The internal electrolyte may be a liquid electrolyte, as in typical metal-case cells, or a polymer-like substance consisting of electrolyte fluid enclosed in a gel. Manufacturing and quality control are challenging for these cells, but they offer the advantages of reduced spatial footprint and considerable shape flexibility.

As these descriptions indicate, each type of cell has its own distinct characteristics, and the choice of cell type for a given application must take into account design guidelines, manufacturing methods, and the purpose for which the cell will be used, among other considerations.

In particular, batteries intended to serve as large energy sources for applications such as electric vehicles must be realized in the form of modules, consisting of multiple cells packaged together into a frame or tray to constitute a single large battery. In this case, the electrodes of multiple cells must be interconnected; for cylindrical or prismatic cells this is achieved using a metallic slab known as a bus bar.

For increased capacity, multiple modules may be joined together by a battery management system (BMS)— equipped with sensors for monitoring voltage, current, and temperature —and packaged with a cooling system (involving cold air blowers or pipes circulating chilled water), with additional components such as wire-harness connectors to interconnect various subsystems, to form a complete battery system known as a pack. Such packs are assembled from a variety of materials and in a variety of shapes to meet the specifications of automakers (regarding mass, capacity, shape, etc.) with added protection to insulate battery units from damage due to shock or vibration. Only after careful system design to satisfy safety requirements and other performance criteria are battery packs ready to be installed in electric vehicles.