Discharge properties of more than 40 Li–LiCoO2 solid-state thin-film batteries with cathode film thickness from 50nm to 4μm have been examined with respect to the lithium diffusion in the cathode. For all but the thinnest cathodes or very low current densities, the discharge capacity from 4.2 to 3.0V is limited by the lithium diffusivity in Li x CoO2 and the ultimate formation of a resistive layer at the interface as x approaches 1.0. Measurements of the equilibrium open circuit potential and ac impedance upon deep discharge support this model. Promoting a higher diffusion for the 0.96<x<1.0 phase would greatly enhance the energy achievable by the thin-film batteries. The lithium diffusion rate in the cathode films is observed to be sensitive to subtle variations in the synthesis conditions when crystallized at temperatures ≤700°C.