84 Chapter 7 prevent a failure event. In the ceramic-coated separators, the ceramics have a much higher temperature at which they will fail and so may enable higher cell temperatures before thermal runaway onset. Two other benefits of ceramic separators is that they will not shrink like the PP or PE separators do and the ceramic separator is also believed to help improve the ways the cells fail during nail penetration events. These safety features can be designed into many of the cell types, but the traditional 18650 cylindrical cell most often has the CID- and PTC-type safety feature.
In the larger cell sizes, these features are a bit more difficult to integrate and as noted above the PTC will limit the maximum voltage so is not appropriate for cells that are intended for high-voltage systems. Lithium-Ion Cell Types and Sizes There are essentially three main types of lithium-ion cell form factors: small cylindrical, large prismatic, and pouch (or polymer) cells. By far the highest volume lithium-ion cell format in production today is the 18650 cylindrical cell with nearly 660 million cells produced annually (TrendForce, 2013). The nomenclature 18650 means that the cell is 18 mm diameter by 65 mm in length.
When it comes to automotive applications, nearly all of the major auto manufacturers have identified “small” cylindrical cells as being appropriate mainly for u-HEV- and HEV-type power applications, with the exception of Tesla who is using exclusively high-volume 18650 cells. For plug-in hybrid electric vehicle (PHEV)- and battery electric vehicle (BEV)-type applications, the majority of the auto manufacturers are using either large rectangular or cylin- drical prismatic cells or flat “pouch”-type cells.
The reason for this is mainly due to the quantity of cells required to achieve the voltage and energy needed which from a reliability analysis means that there are many more connections with small cells than when using a larger cell and therefore many more potential areas of failures in assemblies of small cells. From the Original Equipment Manufacturer (OEM) perspective a high quantity of cells equates to a high potential for future warranty cost. They tend to look at this from a quality perspective: with over 7000 cells that means over 14,000 connections to those cells, compared with 576 welds for the volt pack.
So from a “Six-Sigma” quality perspective, there is a much higher percentage chance that one of the 14,000 connections will fail than in a pack with only 576 cells connections. As an example, the Tesla uses an 18650 style cell from Panasonic and in their Model-S Sedan, they require over 7000 cells in their 85-kWh-pack (they also offer a 60-kWh-“base” pack). While the General Motors uses only 288 pouch-type cells in their 16.5-kWh-Chevrolet Volt and Nissan only uses 192 pouch-type cells in their 24-kWh-LEAF. As we begin looking at physical types of cells, there are basically three cell types that are in mass production today: cylindrical, prismatic, and polymer.
As has been stated above, the 18650 is by far the highest volume cell in production today. However, there are many other small cylindrical cells being produced from a 32330 (32 mm diameter Ч 330 mm length) produced by A123 and the 18 mm by 36 mm by 65 mm (essentially the same size as two