Bis(fluorosulfonyl)imide (HFSI) is a strong superacid and key intermediate for high-performance electrolyte salts. Its lithium salt (LiFSI) is widely used in lithium-ion batteries, offering high ionic conductivity, excellent thermal stability, and wide electrochemical window, which enhance battery cycle life and low-temperature performance. In organic synthesis, HFSI serves as a catalyst for Friedel-Crafts alkylation, acylation, and rearrangement reactions, enabling high yields under mild conditions. It is also a precursor for synthesizing ionic liquids with high thermal and electrochemical stability, used in electrochemistry and green chemistry. Additionally, HFSI is applied in fuel cell membranes to improve proton conductivity under low-humidity conditions. Its strong electron-withdrawing property makes it useful in preparing advanced materials and as a research tool in superacid chemistry.

GHS classification: H314 (Causes severe skin burns and eye damage), H331 (Toxic if inhaled). It is a corrosive superacid; direct contact causes severe burns to skin, eyes, and mucous membranes. Inhalation of vapor or mist irritates the respiratory tract, leading to coughing, shortness of breath, or pulmonary edema. No evidence of carcinogenicity or mutagenicity in standard tests, but prolonged exposure may cause tissue damage. Environmental hazard: Harmful to aquatic life; avoid release into water or soil. Handle in a fume hood with chemical-resistant gloves, safety goggles, and a respirator. Store sealed at 2–8 °C, away from moisture, strong bases, and oxidants. Spills should be neutralized with weak base (e.g., NaHCO?) and absorbed with inert material. Dispose of waste per local hazardous waste regulations.