Composition-Dependent Structure and Dynamics of Lithium–Sulfur Battery Electrolytes Revealed by Molecular Dynamics Simulations

Molecular dynamics simulations were employed to investigate the structure and dynamics of [LiTFSA]:[FEC]:[HFE] (TFSA, bis(trifluoromethanesulfonyl)amide; FEC, fluoroethylene carbonate; HFE, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether), which acts as an optimal electrolyte for lithium–sulfur batteries; however, it exhibits high performance (ratio of 1:6:3) or low performance (ratio of 1:4:5) depending on the ratio of the constituents. Simulations revealed that both electrolyte systems spontaneously liquid–liquid phase-separate into two distinct domains, namely, a large LiTFSA-rich microphase and an HFE-rich phase; however, the microphase properties significantly vary with the constituent composition. For the high-performance electrolyte, the flexible microphase facilitates the formation of small Li-ion-containing clusters in the HFE-rich phase, promoting enhanced Li-ion diffusion. Furthermore, the Li ions diffuse via different mechanisms on the 0.001, 0.1, 10, and 100 ns time scales. In contrast, the low-performance electrolyte exhibited a more rigid microphase with fewer small clusters, restricting Li-ion transport mainly to the microphase. These variations alter the pathways for ionic conduction, implying that even when identical chemical species are used, changes in the constituent composition can result in minor modifications in the physical properties of the microphase, significantly altering the performance. This insight underscores a key consideration for electrolyte design, not only for lithium–sulfur batteries but also for other battery systems.