Bis(fluorosulfonyl)imide　triethylamine　salt　(FSI–HNEt3)　is　a　crucial　intermediate　for　the　synthesis　of　next-generation　electrolyte　lithium　bis(fluorosulfonyl)imide　(LiFSI).　The　NH3–SO2F2–NEt3　route　represents　a　promising　approach　for　LiFSI　production　but　faces　challenges,　including　low　efficiency　and　obscure　reaction　mechanisms.　In　this　study,　we　optimize　the　synthesis　of　FSI–HNEt3　using　NH3–SO2F2–NEt3　route　and　propose　a　new　process.　By　capturing　the　sulfur–fluorine　exchange　(SuFEx)　reaction　intermediate　with　a　novel　molecular　sieve-based　method,　we　clarify　the　reaction　mechanism　with　direct　evidence.　The　primary　mechanism　involves　a　two-step　SuFEx　process,　and　we　establish　the　complete　pathways　for　both　main　and　side　reactions.　Our　study　further　reveals　that　hydrolysis　and　self-polymerization　reactions　during　synthesis　significantly　reduce　the　yield　of　FSI–HNEt3.　Density　functional　theory　(DFT)　calculations　demonstrate　how　triethylamine　(NEt3)　enhances　the　nucleophilicity　of　reactants,　confirming　the　two-step　SuFEx　mechanism　and　providing　insights　into　the　overall　reaction　system.　These　findings　offer　valuable　guidance　for　the　safe　and　efficient　synthesis　of　FSI–HNEt3.　©　2025　Elsevier　Ltd