Lithium-sulfur　(Li-S)　batteries　are　considered　the　most　promising　alternative　for　energy　storage,　however,　their　practical　applications　are　still　limited　by　lithium　dendrites　growth,　slow　lithium　polysulfides　(LiPSs)　conversion　kinetics,　shuttle　effect,　and　deposition　of　＂dead　sulfur＂　at　Li　anode　surface.　Herein,　a　novel　ionic　liquid　tetrabutylammonium　triiodide　(TBAI(3))　is　adopted　as　a　multi-effect　electrolyte　additive　to　solve　low　coulombic　efficiency　and　short　life　issues　of　Li-S　batteries.　A　series　of　in　situ　characterization　technologies,　theoretical　calculations,　potentiostatic　Li2S　deposition　experiments,　and　different　kinds　of　symmetric　and　asymmetric　cells　are　conducted　to　reveal　the　multifunctional　electrochemical　work　mechanism.　It　is　found　that　the　TBA(+)　cations　can　coordinate　with　solvent　molecules,　reduce　desolvation　barrier,　and　accelerate　Li+　transport　kinetics;　they　can　also　form　a　dynamic　electrostatic　shielding　layer　at　the　lithium　protrusions　and　induce　uniform　lithium　deposition.　The　I-3(-)/I-　redox　pairs　continuously　eliminate　＂dead　sulfur＂　by　transforming　Li2S　deposits　into　soluble　LiPSs　and　release　active　substances　during　cycling,　while　the　reduzate　I-　can　be　electrochemically　rejuvenated　into　I-3(-)　when　charged　to　2.89　V.　Therefore,　Li-S　batteries　with　TBAI(3)　additives　exhibit　a　ultra-long　cycle　performance　of　503　mAh　g(-1)　at　2　C　after　1000　cycles　with　an　average　coulombic　efficiency　of　99.99%.