We have characterized a ternary mixture of N-methyl-(n-butyl)pyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR14TFSI)+0.5M LiTFSI+ y poly(ethylene glycol) dimethyl ether (PEGDME) (y =kg PEGDME/kg PYR14TFSI) as an electrolyte in Li metal/S cells. The presence of PYR14TFSI in the mixture resulted in a significant improvement of the thermal stability and the ionic conductivity (σ) of the mixture with increasing PEGDME contents (for example, σ =4.2×10−3 Scm−1 at 29°C for y =2.0). These improvements are most significant at low temperatures, which is probably due to a lowering of the viscosity of the mixture with higher amounts of PEGDME. It is found that the mixture has good compatibility with respect to Li metal as demonstrated by time-dependent interfacial impedance and galvanostatic Li stripping/deposition measurements. We found that a Li/S cell with PYR14TFSI+0.5M LiTFSI+ y PEGDME (y =2.0) can deliver about 1300mAhg−1-sulfur at 0.054mAcm−2 at ambient temperature at the first cycle. A better charge/discharge cyclability of the Li/S cell in PYR14TFSI+0.5M LiTFSI+ y PEGDME was found at higher PEGDME contents, and a Li/S cell with the mixture having y =2.0 exhibited a capacity fading rate of 0.42% per cycle over 100 cycles at 0.054mAcm−2 at 40°C. Consequently the PYR14TFSI+LiTFSI+PEGDME mixture is a promising electrolyte for Li/S cells.