Combination of a solvent–salt complex [acetonitrile(ACN)2–LiTFSI] with a hydrofluoroether (HFE) co-solvent unveil a new class of Li–S battery electrolytes. They possess stability against Li metal and viscosities which approach that of conventional ethers, but they have the benefit of low volatility and minimal solubility for lithium polysulphides while exhibiting an uncharacteristic sloping voltage profile. In the optimal system, cells can be discharged to full theoretical capacity under quasi-equilibrium conditions while sustaining high reversible capacities (1300–1400 mA h g−1) at moderate rates, and capacities of 1000 mA h g−1 with almost no capacity fade at fast discharge rates under selected cycling protocols. A combination of operando X-ray absorption spectroscopy at the S K-edge, and electrochemical studies demonstrate that lithium polysulphides are indeed formed in these ACN-complexed systems. Their limited dissolution and mobility in the electrolyte strongly affect the speciation and polysulphide equilibria, leading to controlled precipitation of Li2S.
