Cylindrical Cell
A cylindrical cell is a battery cell sealed in a rigid, round metal can. It usually contains long positive- and negative-electrode sheets with separators wound into a spiral assembly, often called a jelly roll. The format describes the enclosure and geometry, not the chemistry, capacity, quality, or a particular size. Pacific Northwest National Laboratory: Materials Review of Current/Near-Term Lithium Batteries
What the format changes
The circular wall provides substantial mechanical support for the electrode roll and resists internal pressure efficiently. When round cells are arranged in a rectangular battery pack, gaps remain between neighboring cells. Designers may use that space for cooling channels, holders, insulation, adhesive, or propagation barriers; whether the layout works well depends on the complete pack.
Heat may leave through the curved wall, the base, or the terminal end. The result depends on cell diameter and height, current collection, electrode design, chemistry, and the cooling system. Smaller cells provide more external surface area relative to stored energy but require more cells and connections. Larger cells reduce the cell count but store more energy per cell and create longer heat and current paths. Neither approach is automatically better.
Cylindrical is one of the common physical formats used for lithium-ion cells. For detailed construction, cooling, size, and pack trade-offs, see Cell Formats.