Absolute appearance energies of field-desorbed Li+ ions were obtained from mass-to-charge resolved retarding potential analyses of Li+ emitted from the first and second Li layer on W(111). Activation energies for Li+ field desorption were derived from desorption rate measurements. The field-independent binding energy of Li adatoms has been found from field-dependent Li+ appearance and activation energy values, indicating a negligible field-induced charge transfer in the applied field range. We use the cluster embedded in jellium model, based on density-functional theory, to interpret the data by calculating local field enhancements, surface potentials, and activation energies for Li field desorption as a function of field strength and surface coverage as well as geometry.