Sub-zero temperature electrolytes for lithium-sulfur batteries: Functional mechanisms, challenges and perspectives

• The challenges of LSBs were described at low temperatures. • Strategies to improve low-temperature electrolytes performance were proposed. • Future research directions for LSBs in terms of low-temperature electrolyte design were proposed. The currently used lithium-ion batteries are facing two challenges of insufficient energy density for recharge mileage requirement of electric vehicles and low performance at sub-zero temperatures. Lithium-sulfur batteries (LSBs) with high theoretical energy density may be the next generation of lithium-based batteries. However, LSBs still have their challenges at low temperatures, including accumulation of lithium polysulfide, nucleation of lithium sulfide, lithium deposition, and formation of solid electrolyte interphase film, solvation degree and so on. Electrolytes of LSBs are partially responsible for these issues. To facilitate further research and development of LSBs particularly operated at sub-zero temperatures, this paper reviews the electrolytes of LSBs in terms of electrolyte materials, characterization, functional mechanisms, and performance optimization. The recent advances in electrolyte design strategies, including solvent optimization, lithium salt modification, additive introduction, and solid-state electrolytes of LSBs are emphasized. The challenges of both liquid and solid electrolytes at low temperatures are analyzed and the research directions for overcoming the challenges are also proposed in this paper for further research and development toward practical application.