This paper details the failure analysis of swelling in prismatic lithium-ion batteries (LIBs) after undergoing several charge–recharge cycles subsequent to long-term storage. The methods of analysis are nondestructive and mainly involve the use of computed tomography (CT) and cell disassembly. Two situations are compared. The first cell is examined after long-term storage, in which case a void space, presumably a gas bubble, is observed around the Ni tap in the cell, and traces of side reactants are found around the center of the anode electrode. This cell is compared with a swollen cell examined after undergoing several cycles after long-term storage, in which, according to the CT results before and after gas removal, gas is detected inside the jelly roll and inside the space between the can body and the jelly roll. Moreover, side reactants are identified near the center of the anode electrode, causing the color of the anode electrode to become irregular. The results show the gas bubbles and side reactants generated as a result of long-term storage are determined to be responsible for the swelling in the cell due to prolonged cycling.