A class of polysulfide catholytes for lithium–sulfur batteries: energy density, cyclability, and voltage enhancement

Liquid-phase polysulfide catholytes are attracting much attention in lithium-sulfur (Li-S) batteries as they provide a facile dispersion and homogeneous distribution of the sulfur active material in the conductive matrix. However, the organic solvents used in lithium-polysulfide (Li-PS) batteries play an important role and have an impact on the physico-chemical characteristics of polysulfides. For instance, significantly higher voltages (∼2.7 V) of the S/S(n)(2-) (4 ≤n≤ 8) redox couple are observed in Li-PS batteries with dimethyl sulfoxide (DMSO) and N-methyl-2-pyrrolidone (NMP) solvents. Accordingly, high power Li-PS batteries are presented here with the catholyte prepared with NMP solvent and operated with the highly reversible sulfur/long-chain polysulfide redox couple. On the other hand, a remarkable cyclability enhancement of the Li-PS battery is observed with the long-chain, ether-based tetraglyme (TEGDME) solvent. The voltage enhancement and the cyclability enhancement of the Li-PS batteries are attributed to the solvation effect, viscosity, and volatility of the solvents. Finally, highly concentrated polysulfide catholytes are successfully synthesized, with which high energy density Li-PS batteries are demonstrated by employing a multi-walled carbon nanotube (MWCNT) fabric electrode.