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

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.