Lithium-sulfur batteries (LSBs) are regarded as the most promising next-generation energy system due to their high theoretical energy density. However, LSBs suffer the “shuttle effect” if undergoing the solid–liquid–solid sulfur conversion process during cycling. Herein, we design a solvent-in-salt (SIS) electrolyte with co-solvent vinylene carbonate (VC) to synthesize an in situ dense cathode electrolyte interface (CEI) and successfully change sulfur conversion into a solid–solid way to avoid shuttle effect by separating the contact of sulfur and ether solvent. Dense CEI is formed at the beginning of first discharge by the combined action of SIS electrolyte and filmogen VC. Experiments and simulations show that SIS electrolyte controls the initial formed lithium polysulfides (LiPSs) to stay very closely on the cathode surface, and then converts them into a dense CEI film. As a result, Coulombic efficiency (above 99%) and cycling performance of LSBs are improved. Furthermore, the in situ dense CEI can nearly stop the self-discharge of LSBs, and enable the LSBs to work under a pretty lean electrolyte condition.