Locally Concentrated Ionic Liquid Electrolyte with Partially Solvating Diluent for Lithium/Sulfurized Polyacrylonitrile Batteries

Abstract The development of Li/sulfurized polyacrylonitrile (SPAN) batteries requires electrolytes that can form stable electrolyte/electrode interphases simultaneously on lithium‐metal anodes (LMAs) and SPAN cathodes. Herein, a low‐flammability locally concentrated ionic liquid electrolyte (LCILE) employing monofluorobenzene (mFBn) as the diluent is proposed for Li/SPAN cells. Unlike non‐solvating diluents in other LCILEs, mFBn partially solvates Li⁺, decreasing the coordination between Li⁺ and bis(fluorosulfonyl)imide (FSI⁻). In turn, this triggers a more substantial decomposition of FSI⁻ and consequently results in the formation of a solid electrolyte interphase (SEI) rich in inorganic compounds, which enables a remarkable Coulombic efficiency (99.72%) of LMAs. Meanwhile, a protective cathode electrolyte interphase (CEI), derived mainly from FSI⁻ and organic cations, is generated on the SPAN cathodes, preventing the dissolution of polysulfides. Benefiting from the robust interphases simultaneously formed on both the electrodes, a highly stable cycling of Li/SPAN cells for 250 cycles with a capacity retention of 71% is achieved employing the LCILE and only 80% lithium‐metal excess.