Temperature Stability of Interfaces in Lithium-Sulfur Batteries with Nominally Stable Electrolytes

Abstract

Li-S batteries are a promising alternative to conventional Li-ion batteries as Li-S batteries enable low-cost, lightweight, and high capacity cells. However, the polysulfide shuttle effect and Li reactivity with common organic solvents have limited commercialization of Li-S batteries. Highly concentrated electrolytes known as solvate electrolytes have been shown to limit polysulfide solubility in Li-S cells by reducing the ability of solvent molecules to solvate polysulfides through coordination to Li+.1 The challenges with using solvate electrolytes in electrochemical cells are associated with the high viscosity and low ionic conductivity of solvates. Addition of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) significantly decreases the solvate viscosity with slight improvement in ionic conductivity.1 TTE has been shown to suppress the polysulfide shuttle effect and is suggested to act as a nonsolvent for polysulfides. Although bulk electrolyte speciation is unchanged when the solvate is exposed to Li metal,2 the reactivity of the solvate electrolyte with and without TTE with Li metal has not been studied in detail. To study the reactivity, we evaluate the behavior of Li-S cells and Li metal at various temperatures in the solvate electrolyte with and without TTE. Increasing the temperature of the solvate electrolyte affects the equilibrium between free and coordinated MeCN and results in increased electrolyte decomposition and anode passivation. We demonstrate here that reactivity between the solvate electrolyte and the Li anode significantly impacts the cyclability and capacity retention in Li-S cells. The results indicate that TTE is necessary to stabilize the electrode/electrolyte interface and mitigate Li reactivity. Introducing a protecting layer onto the Li anode enables moderate cyclability but does not protect against decomposition over time. 1. Energy Environ. Sci. 2014, 7, 2697. 2. J. Am. Chem. Soc. 2014, 136, 13, 5039.