Direct evidences for bis(fluorosulfonyl)imide anion hydrolysis in industrial production: Pathways based on thermodynamics analysis and theoretical simulation

LiFSI (lithium bis(fluorosulfonyl)imide) is a promising lithium salt for electrolytes in Li-ion batteries. However, the accumulation of harmful gases and heat during LiFSI hydrolysis could lead to serious safety accidents. Here we systematically investigate LiFSI hydrolysis processes under comprehensive conditions: higher temperature/acidity/basicity and lower water content can accelerate the hydrolysis, whereas the presence of DEC (diethyl carbonate) solvent, and other alkali metals (Na+, K+) can stabilize FSI−. Unexpectedly, under alkaline conditions, temperature/water content could not affect the hydrolysis greatly. By monitoring the hydrolysis intermediates and products using time-dependent ion chromatography, infrared spectra, and nuclear magnetic resonance, the hydrolysis routes are proposed and validated by accelerating rate calorimetry, differential scanning calorimetry measurements, and theoretical calculations. Under neutral/acidic conditions, electrophilic attack on the S–N bond generates FSO2NH2 and FSO3−, while nucleophilic attack on the S–F bond produces FSO2NSO32− and SO3NHSO32− under alkaline conditions. As indicated by DFT calculation, the weaker S–N bond and larger S–N–S angle facilitate the electrophilic attack under acid conditions. Furthermore, very unstable intermediates (FSO2NH2 and CH3CH2OSO3H) are determined for the first time. Based on these hydrolysis mechanisms, strategies for inhibiting LiFSI hydrolysis are provided, which is significant for the high-efficiency production and safe storage/transportation of LiFSI.