Structure–Property Relationships to Guide the Selection of Fluorinated Ethers for Li–S Batteries

A variety of fluorinated ethers have been used to improve key performance metrics in lithium-sulfur batteries, such as specific energy and cycle life. However, most previous articles employed only one fluorinated ether, and there is currently a lack of criteria or consensus about which fluorinated ether provides the largest performance improvement. This stems from a lack of fundamental understanding of the correlation between the chemical formula of the fluorinated ether and its effect on lithium-sulfur battery reactions. In this work, we systematically investigated nine fluorinated ethers and tested their effects in lithium-sulfur batteries representative of commercially relevant conditions. Electrochemical measurements were complemented by ex situ and operando NMR characterization of the lithium metal anode reactions, ex situ SEM and EDX imaging of cycled lithium and sulfur electrodes, and ab initio calculations and molecular dynamics simulations of the electrolytes, thus revealing the key motifs in the chemical structure of the fluorinated ethers that lead to performance improvements. The best-performing fluorinated ethers were those that interact with lithium ions with a strength similar to that of the other solvent(s) in the electrolyte, and as a result, fast solvent exchange reactions and high lithium-ion mobility are promoted. This benefited both the lithium and the sulfur electrode reactions, with the suppression of lithium dendrites, the formation of a more protective lithium SEI, and the promotion of polysulfide reactions that avoid the passivation of the carbon in the sulfur electrode. The best results were obtained with TFMP (1,1,2,2-tetrafluoro-3-methoxypropane), which had not been used before in lithium-sulfur batteries.