New insight into liquid electrolyte of rechargeable lithium/sulfur battery

Abstract

The lithium/sulfur (Li/S) cell is a liquid electrochemical system, in which the dissolution of lithium polysulfide (PS, the series of the sulfur reduction intermediates) plays an essential role in the cell's performance. In solution, the PS undergo a series of complicated disproportionations, as described by a general equation of Li2Sn→m/8S8+Li2Sn−m, to form the less soluble sulfur and low-order PS, which become inactive once precipitated out of the liquid electrolyte or deposited into the pores of separator. Based on the hard and soft acids and bases (HSAB) theory, quaternary ammonium cations are expected to stabilize the PS anions through a chemical interaction between the soft acid and soft base. In this paper, we report a significant improvement in the capacity retention of a Li/S cell by introducing a tetrabutylammonium triflate (NBu4SO3CF3) or an N-methyl-N-butylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR14TFSI) into the liquid electrolyte. For example, by using an electrolyte of 0.25molkg−1 LiNO3–0.25molkg−1 PYR14TFSI dissolved in a 1:1 (wt.) mixture of dimethyl ether (DME) and 1,3-dioxolane (DOL), a Li/S cell with a cathode containing 77% sulfur and a 2mgcm−2 sulfur loading exhibited an initial capacity of 1227mAhg−1 and retained a capacity of 875mAhg−1 after 40 cycles when cycled at 0.2mAcm−2 between 1.7V and 2.8V. We contribute the improved performance to the stabilized PS anions by the added quaternary ammonium cations.