Development of lithium–sulfur (Li–S) batteries has suffered from insufficient capacity and poor cycle-life. One of the reasons for these drawbacks is loss of active material, which is associated with the rapid diffusion of highly soluble lithium polysulfides formed as intermediates of discharge products in organic electrolytes, resulting in internal shuttling of lithium polysulfides. The diffusion of lithium polysulfides is determined largely by the physicochemical properties of electrolytes. Therefore, design of the physicochemical properties of the electrolyte to restrain the internal shuttling is vital to promote high performance for Li–S batteries. Here we present a newly designed room temperature ionic liquid (RTIL)-based organic electrolyte for Li–S battery. Our electrolyte provides a trade-off between solubility and diffusion rate of lithium polysulfides by mixing very different physicochemical properties of two solvents: high lithium polysulfide solubility of 1,2-dimethoxyethane (DME), and high viscosity of N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)imide (PP13-TFSI). An adequate composition ratio of mixed PP13-TFSI/DME afforded large capacity, high Coulombic efficiency, improved capacity retention, and suppressed internal shuttling.