Features of cycling of lithium-sulfur cells with electrolytes based on sulfolane solutions of LiPF6 and LiBF4

In lithium-sulfur batteries, cell design, specifically electrolyte design, has a key impact on the battery performance. Development of sulfolane based electrolytes may grant the use of more available lithium salts such as LiClO 4 , LiPF 6 and LiBF 4 , which are typically not preferred in lithium-sulfur batteries due to their reactive anions. Herein, in order to assess the reactivity of lithium polysulfides with various dimensions (with different lengths of the polysulfide chain) towards the anions of lithium electrolyte salts, the cycling of lithium-sulfur cells with 1 M solutions of LiPF 6 , LiBF 4 , and LiClO 4 in sulfolane is studied in a wide (1.5–2.8 V) and narrow (limited) voltage ranges, corresponding to the high-voltage (2.1–2.8 V) and low-voltage (2.3–1.5 V) plateaus on the discharge curves. It is shown that the electrochemical characteristics of lithium-sulfur cells with sulfolane based electrolytes of lithium perchlorate and lithium hexafluorophosphate differ significantly from those for sulfolane electrolyte based on lithium tetrafluoroborate. We believe that these differences are due to the direct chemical interaction of lithium polysulfides with the BF 4 − anion, which is confirmed by chronopotentiometry, 7 Li and 11 B NMR spectra, UV–Vis and electrochemical impedance spectroscopy of sulfolane solutions of the studied lithium salts, saturated with lithium polysulfides. • PF 6 − does not interact with lithium polysulfides. • BF 4 − interacts with lithium polysulfides and forms lithium boron polysulfides. • Lithium boron polysulfides (LiBF x (S n Li) 4-x ) have electrochemical activity. • LiBF x (S n Li) 4-x are highly reactive towards metallic lithium.