Enhancing the Reversibility of Li Deposition/Dissolution in Sulfur Batteries Using High-Concentration Electrolytes to Develop Anode-Less Batteries with Lithium Sulfide Cathode

The reversibility of Li deposition/dissolution onto/from a copper substrate is analyzed herein for fabricating anodeless Li2S-cathode-based batteries using sulfolane (SL)-based sparingly solvating electrolytes to prevent lithium-polysulfide dissolution. The composition of electrolytes—binary lithium bis(trifluoromethanesulfonyl)amide/lithium bis(fluorosulfonyl)amide (LiTFSA/LiFSA; 9:1 molar ratio) in SL solvent—considerably affects the reversibility; a high-concentration electrolyte (HCE) and a localized high-concentration electrolyte (LHCE: HCE diluted by non-coordinating hydrofluoroether) exhibit superior reversibility than that of 1 M solution. Solid electrolyte interphases (SEI) with high F compositions and relatively low O concentrations are formed on the HCE- and LHCE-deposited Li surfaces. SEI formation via FSA decomposition is promoted in the HCE and LHCE, whereas SL decomposition is prohibited by the fluorine-rich SEI. The Li-deposit morphology is significantly affected by the nucleation overpotential at the initial deposition, which is influenced by the nature of the formed SEIs. Small-surface-area deposits with a low nucleation overpotential suppress the electrolyte decomposition and the formation of dead Li, resulting in high reversibility. Optimal reversibility is achieved by performing low-current-density initial deposition at 60 °C with the LHCE. The charge/discharge characteristics of anode-less Cu||Li2S batteries are determined under the reversibility-optimized conditions; 75 charge/discharge cycles and a 50-cycle capacity retention rate of 47.5% are achieved.