Although Li-rich layered oxides (Li1+xNiyCozMn1-x-y-zO2 > 250 mAh g(-1)) are attractive electrode materials providing energy densities more than 15% higher than today's commercial Li-ion cells, they suffer from voltage decay on cycling. To elucidate the origin of this phenomenon, we employ chemical substitution in structurally related Li2RuO3 compounds. Li-rich layered Li2Ru1-yTiyO3 phases with capacities of ~240 mAh g(-1) exhibit the characteristic voltage decay on cycling. A combination of transmission electron microscopy and X-ray photoelectron spectroscopy studies reveals that the migration of cations between metal layers and Li layers is an intrinsic feature of the charge-discharge process that increases the trapping of metal ions in interstitial tetrahedral sites. A correlation between these trapped ions and the voltage decay is established by expanding the study to both Li2Ru1-ySnyO3 and Li2RuO3; the slowest decay occurs for the cations with the largest ionic radii. This effect is robust, and the finding provides insights into new chemistry to be explored for developing high-capacity layered electrodes that evade voltage decay.