Changes in samples of Li_(1-x)CoO_2 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 Li_(1-x)CoO_2 when it was aged in the presence of LiClO_4, LiPF_6, and LiAsFg in propylene carbonate (PC). Li~+ re-intercalation could account for the reversible capacity loss. Thermal aging at 75 deg 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: LiPF_6 < LiClO_4 < LiAsF_6.
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