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Lithium-ion cells with 5-Ah capacity were fabricated using a spinel Li 1.1 ͑Ni 0.025 Ti 0.025 Mg 0.02 ͒Mn 1.83 O 4 as a cathode active material, graphitized carbon as an anode active material, and 1 M LiPF 6 /ethylene carbonate + diethyl carbonate + dimethyl carbonate as an electrolyte. In order to improve the calendar life of the cell, we investigated the degradation mechanism by measuring the thickness of the solid electrolyte interphase ͑SEI͒ on anode active material. The SEI thickness was measured by focused ion beam, scanning electron microscope, and X-ray photoelectron spectroscopy. The thickness of the SEI was initially 0.04 m, and after storage for 392 days at 25 and 40°C, the thickness was 0.15 and 0.45 m, respectively. The capacity decreased with increase in the thickness of SEI, because Li in the cell is consumed by forming SEI. The amount of Li consumption was estimated theoretically assuming that SEI is formed by a reaction between intercalated Li and the electrolyte in SEI on the negative carbon surface, and a diffusion of the electrolyte in the SEI is the rate-determining step of the reaction. The theoretical equation showed a good agreement with experimental capacity fade at 25, 40, and 60°C for the storing days up to 380 days. A voltage decrease of the cell after 1-s at 20 A of discharge current was measured to estimate roughly the increase of the cell internal resistance during storage. The increase of SEI resistance was estimated by the theoretical equation and compared with the experimental voltage drop data after 1-s discharge. However, the theoretical data was not in a good agreement with the experimental data. The reason is that the charge-transfer resistance on the anode also increases during storage. Another reason is the resistance change of the cathode during the storage.

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