N-Methyl-(n-butyl)pyrrolidinium bis(trifluoromethanesulfonyl)imide-LiTFSI–poly(ethylene glycol) dimethyl ether mixture as a Li/S cell electrolyte

We have characterized a ternary mixture of N-methyl-(n-butyl)pyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR14TFSI) + 0.5 M 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 S cm−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 + 0.5 M LiTFSI + y PEGDME (y = 2.0) can deliver about 1300 mAh g−1-sulfur at 0.054 mA cm−2 at ambient temperature at the first cycle. A better charge/discharge cyclability of the Li/S cell in PYR14TFSI + 0.5 M 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 0.054 mA cm−2 at 40 °C. Consequently the PYR14TFSI + LiTFSI + PEGDME mixture is a promising electrolyte for Li/S cells.