Localized High-Concentration Binary Salt Electrolytes with Suppressed Li2Sx Solubility to Achieve Stable Li–S Pouch Cells with High Sulfur-Loading Cathodes under Lean Electrolyte Conditions

Abstract

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[TFSA_0.8LiFSA_0.2] ([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[TFSA_0.8LiFSA_0.2]. 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 μL 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.