Changes in samples of Li1-xCoO2 were measured by X-ray diffractometry (XRD) after thermal aging treatments that cause capacity losses in electrochemical cells. Changes in lattice parameters were used to identify lithium re-intercalation into Li1-xCoO2 when it was aged in the presence of LiClO4, LiPF6, and LiAsF6 in propylene carbonate (PC). Li+ re-intercalation could account for the reversible capacity loss. Thermal aging at 75 degrees C in pure PC or pure argon gas resulted in other changes that are attributed to the formation of spinel phase. The rate of the lithium re-intercalation increases in the following sequence: LiPF6 < LiClO4 < LiAsF6. (C) 2004 The Electrochemical Society.