Recent Advances in Electrolytes for Lithium–Sulfur Batteries

The rapidly increasing demand for electrical and hybrid vehicles and stationary energy storage requires the development of “beyond Li-ion batteries” with high energy densities that exceed those of state-of-the-art Li-ion batteries. Li–S batteries, which have very high theoretical capacities and energy densities, are believed to be one of the most promising candidates. The sulfur-based electrochemical reaction requires novel electrolytes to replace the classical carbonate-based electrolyte systems inherited from Li-ion batteries because carbonates are incompatible with the intermediate polysulfides in Li–S batteries. In addition, the theoretical specific capacities and projected energy densities of Li–S batteries are difficult to achieve experimentally, mainly because of the electronically insulating nature of sulfur and lithium sulfide cathodes, and the shuttle effect; this is a serious issue associated with the dissolution and diffusion of soluble polysulfides in most potential electrolytes and causes rapid capacity fading. It is therefore highly desirable to explore, modify, and/or optimize electrolytes for Li–S batteries to address these issues and improve their capacities, cycling stabilities, rate performances, and energy densities. An overview of recent developments in electrolytes for Li–S batteries is provided with a focus on the chemistry of polysulfides in different electrolyte media, including polysulfide solubility and its relevance to battery performance.