Capacity fading of advanced lithium-ion batteries after elevated temperature storage was investigated by three-electrode measurements. Capacity fading of a battery increases by increasing the state-of-charge (SOC) during storage, especially at elevated temperatures. The reversible capacity of a battery (SOC = 100%) at 60° C decreases from 820 to 650 mAh (79.3% capacity retention) after 60 days. At room temperature, a battery SOC influences the capacity fading only slightly; after 65 days of storage, the reversible capacity decreases from 820 to 805 mAh (98.2% capacity retention). Individual effects by the anode, cathode, and electrolyte on capacity fading are analyzed with three-electrode electrochemical ac impedance. The major contribution, from X-ray photoelectron spectroscopy (XPS) and energy-dispersive spectroscopy results, comes from cathode degradation as a result of cobalt dissolution at the LiCoO2 surface layer. A minor contribution comes from the continuous reactions between lithiated mesocarbon microbead (MCMB) electrode and electrolyte components, which in turn thicken the SEI film and consume available lithium ions. From X-ray diffraction and XPS results, high-temperature storage influences only the surface properties of MCMB and LiCoO2 electrodes; bulk properties remain unchanged. © 2005 The Electrochemical Society.