Effects of the lithium intercalation on the optical band edge of the layered dichalcogenide WS2 have been studied. Transmission spectra were made at low-temperature (T=5 K) on WS2 and LixWS2 single crystals. A red shift of the fundamental absorption edge and a screening of the excitonic transitions were observed upon Li intercalation. The modifications of the optical properties are attributed both to a weak charge transfer of electrons from intercalant species to the host matrix and to the destabilization of the electronic band structure due to the interaction between the 2s state of Li localized in the van der Waals gap with the (s,p) group of the host. Results show that the rigid-band model is clearly not applicable to this system.