Lithium-Ion and Other Cell Chemistries 85 18650 cells side by side) “Swing” and “Sonata” cells produced by Boston-Power. Saft produces a series of large cylindrical cells that are in wide use in aviation, aerospace, and military applications. The benefit of the cylindrical cell is that it uses a steel, nickel-coated steel, or aluminum can which offers a high-strength packaging requiring a lot of energy to damage it and it provides “stack” pressure on the jellyroll inside the can. There are also a significant number of cell assembly and manufacturing equipment suppliers that provide “off-the-shelf” manufacturing solutions for this product today.
One of the challenges involved in using smaller format cells, aside from size, is in the lid assem- bly. Many manufacturers have transitioned to laser welding the lids onto the cells, while some manufacturers continue to use a crimping process to attach the lid. The concern over crimping was exemplified in 2006 with the $250 million dollar recall issued by Sony when cells started to fail when metallic particles began to make their way inside the cells (Mook, 2006). Subsequent research found that it was during the crimping process some of the nickel that was used to plate the steel cells was flaking off and causing the internal short circuits and fires in consumer applications.
Another disadvantage, yet a relatively small one, is that the cylindrical cells tend to have much higher initial impedance than a comparative prismatic- or polymer-type cell. This means that it will have a higher rate of heat generation and in this format the best cooling solution is to air-cool the pack. However, this is also the least effective active cooling method- ology. Yet, because the cells are small, the heat generation is easily manageable. Prismatic-type cells, a hard steel, plastic or aluminum can in a rectangular shape, is quickly gaining a lot of attention from major auto manufacturers.
One of the reasons for this is that the Verband der Automobilindustrie (VDA), an auto industry association made of European auto makers, has brought forth one of the first sets of proposed “standards” around cell sizing (see Table 6 below). The benefit of the larger prismatic cell is that it requires less “pack Table 6: Verband der Automobilindustrie (VDA) standard prismatic cell formats EV, electric vehicle; HEV, hybrid electric vehicle; PHEV, plug-in hybrid electric vehicle.
Standard VDA Cell Sizes HEV PHEV1 PHEV2 PHEV2+ EV1 EV2 Form factor Prismatic can Capacity 5.5–7 20–25 25–32 35–37 40–50 60 + (Ah) Length (mm) 120 173 148 148 173 173 Height (mm) 85 85 91 125 115 115 Thickness 12.5 21 26.5 26.5 32 45 (mm) Volume (L) 0.128 0.309 0.357 0.490 0.637 0.895 Ah/L 55 65–81 62–76 65–76 63 67 Wh/L 155–200 234–292 255–275 260–275 226 241