Localized High-Concentration Binary Salt Electrolytes with Suppressed Li2S x Solubility to Achieve Stable Li-S Pouch Cells with High Sulfur-Loading Cathodes under Lean Electrolyte Conditions

The endurance of lithium-sulfur (Li-S) cells depends on the stability of lithium (Li) metal anodes and their consistent efficiency during extended Li dissolution and deposition cycles. Electrolytes containing Li[N(SO2F)2] (Li[FSA]) have shown potential in enhancing Li anode reversibility by promoting the formation of a favorable inorganic-rich solid-electrolyte interphase (SEI) on the Li metal electrode. However, the use of Li[FSA] as the primary electrolyte salt in Li-S batteries is hindered by the spontaneous side reactions of [FSA]- anions with soluble lithium-polysulfides (Li2S x , 2 <= x <= 8). To overcome this challenge, we have developed a localized high-concentration electrolyte (LHCE) with reduced Li2S x solubility, composed of Li[TFSA0.8LiFSA0.2] ([TFSA]: [N(SO2CF3)2]) binary salts dissolved in sulfolane (SL) and diluted by 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (HFE). This LHCE solution demonstrates superior stability of [FSA]- anions, due to the restricted dissolution of Li2S x within the LHCE. We experimentally evaluated the critical factors affecting reversibility of Li dissolution/deposition in electrolytes containing Li[TFSA0.8LiFSA0.2]. Increased salt concentration, combined with HFE dilution, widens the reduction potential gap between the anion and Li+, which thermodynamically promotes anion reduction, controls SEI composition, and improves Li reversibility. We demonstrate the operation of a Li-S pouch cell under practical conditions with a high sulfur loading of 5.5 mg(S) cm-2 and an extremely low electrolyte/sulfur (E/S) ratio of 3.0 mu L mg(S) -1. The battery delivers a high energy density of 280 Wh kg-1. Our findings provide insights into the critical factors for achieving prolonged Li dissolution/deposition reversibility, particularly under practical Li-S pouch cell conditions, through electrolyte formulation design.