Symmetric Vs Asymmetric Imide Anion Decomposition Pathways And Their Influence On Solid Electrolyte Interphase Stability For Si Anodes

A sturdy solid electrolyte interphase (SEI) is imperative for extending the calendar-life of Si anodes in lithium-ion batteries (LIBs). However, carbonate electrolytes form unstable interphases, hindering their practical implementation. Alternatively, fluorinated sulfonylimide (FSI⁻/TFSI⁻) based ionic liquid (IL) electrolytes, coupled with Li-imide salts, enable anion-derived SEI formation, thereby enhancing capacity retention. However, the functional role of these anions in directing SEI formation and evolution within IL-based systems is poorly understood. Moreover, the mechanistic interplay between the decomposition pathways of symmetric and asymmetric anions in governing interfacial chemistry remains elusive. Herein, we investigate the chemistry and morphology of SEIs formed using various imidazolium-based ILs containing both symmetric and asymmetric anions. We reveal that the synergistic interactions between symmetrical bis(fluorinated sulfonyl)imide anions and imidazolium cations facilitate an inorganic-rich (LiF/LiOH) inner and a Li₂SO₄/polymeric outer layer SEI, conformally coating the 3D Si interface. The complementary effects of inorganic-rich and polymer components, featuring key Li₂SO₄ species, reinforce mechanical integrity and flexibility, suppressing pulverization and enabling reversible capacity of 2489 mAh/g at 1C over 250 cycles. Correlating electrochemical performance with surface analysis provides critical insights into the impact of fluorinated sulfonylimide on passivation behavior and battery performance, guiding future design of ionic liquid electrolytes for LIBs.