RECTANGULAR LITHIUM ION SECONDARY BATTERY

A rectangular lithium ion secondary battery disclosed here includes an electrode body, an electrolyte, and a rectangular battery case housing the electrode body and the electrolyte. The battery case includes an outer case and a lid. The battery case includes a welded portion in which the outer case and the lid are joined by welding. The outer case includes a rectangular bottom wall facing the lid, a pair of long walls, and a pair of short walls. The electrode body has a flat portion in which a positive electrode and a negative electrode are stacked. At least one of the pair of long walls of the outer case includes a groove located between the lid and an end of the flat portion of the electrode body toward the lid in a direction perpendicular to the lid.