The construction of Hitachi Maxell's 2.8-mm Li-ion cell can varies from the method typically used in Li-ion cells. A special can forming technique produces a very shallow can (picture a very small shoebox with the lid removed), into which the wound prismatic cell is placed. Then, the lid is placed over the can and sealed with a laser weld that follows the device's footprint.
By this sealing process, a "rib" is created around the can's perimeter. This makes it easier to fit the canned cell into a battery pack, where it will probably go unless destined for an embedded application. The rib also provides a convenient space to put the cell protection module in, without affecting the overall size of the cell.
For traditional Li-ion cells, the cell winding is slid lengthwise into a preformed can and laser-welded just at the top, along the width and thickness axes of the cell. But that type of assembly becomes very difficult to perform with a can that's less than 3-mm thick.