Chlorine Substitution in Disordered Rock Salt Li-Rich Transition Metal Sulfides

Li-rich sulfides are promising alternatives to Li-rich oxides as intercalation materials, the latter suffering from limited cycling reversibility and copious voltage fade, all associated with the redox activity of oxygen ligands upon cycling. Although moving from oxygen to sulfur ligands alleviates some of these drawbacks, sulfides suffer from their lower redox potential, which limits the energy density. Here, we partially replace divalent sulfur ligands with monovalent chlorine and synthesize transition metal sulfochlorides Li2M1–xMnxS2Cl (with M = Ti4+ and Nb5+) crystallizing in a cation-disordered rock salt (DRX) structure. Owing to the greater electronegativity of chlorine compared to sulfur, we demonstrate an increase in the average redox potential for DRX sulfochlorides. Combining ex situ X-ray absorption spectroscopy measurements at various edges and density functional theory calculations, we demonstrate that chlorine ligands preferentially form Mn–Cl and Li–Cl bonds, while sulfur ligands preferentially coordinate the high valence d0 metals. While sulfur ligands are redox active throughout the charge (and discharge), Mn2+ redox activity depends on the chemical composition, with Li2Ti0.5Mn0.5S2Cl showing cationic redox activity only at the end of the charge and beginning of the discharge. More dramatically, our experimental and computational results demonstrate that the S–S bond formation induces sizable changes in local coordination and partial material dissolution into the electrolyte, triggering cell corrosion and/or short formation as early as the second cycle. Through an electrolyte engineering approach, combining a Cl-scavenger molecule with a cathode electrolyte interphase former, we demonstrate that corrosion and/or shorts formation can be suppressed, and intrinsic cycling properties of sulfochloride DRX materials are investigated. Our work extends the chemical space for designing better intercalation materials, showing the unique opportunities brought by mixed anion compounds.