Locally Concentrated Ionic Liquid Electrolytes for Lithium/Sulfurized Polyacrylonitrile Batteries
Authors: Liu Xu , Passerini Stefano 
Published: February 2024

Low-cost sulfurized polyacrylontrile (SPAN), which shows negligible polysulfide dissolution and stable cycling up to hundreds of cycles due to a solid-phase mechanism in carbonate-based electrolytes, is a valuable high-energy cathode material for long-lifespan lithium metal batteries (LMBs).1
-3 However, the conventional carbonate-based electrolytes for commercial lithium-ion batteries are incompatible with lithium metal anodes (LMAs).4 The unstable solid-electrolyte interphases (SEIs) result in lithium dendrite growth and low lithium stripping/plating CEs, which further cause safety concerns and limited lifespan of Li/SPAN cells with low negative to positive areal capacity (N/P) ratio. Therefore, the development of Li/sulfurized polyacrylonitrile (SPAN) batteries requires electrolytes that can form stable electrolyte/electrode interphases (EEIs) simultaneously on lithium metal anodes (LMAs) and SPAN cathodes. With the remarkable compatibility toward LMAs and low flammability,4–7 locally concentrated ionic liquid electrolytes (LCILEs) might be promising candidates for Li/SPAN cells.

Herein, a low-flammability locally concentrated ionic liquid electrolyte (LCILE) employing monofluorobenzene (mFBn) as the diluent is proposed for Li/SPAN cells.8 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 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, 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.

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