Insight into the Solvation Structure of Tetraglyme-Based Electrolytes via First-Principles Molecular Dynamics Simulation.

Glyme-lithium salt equimolar mixtures, as solvate ionic liquid electrolytes for rechargeable lithium secondary batteries, are of great interest, due to the desirable properties such as high oxidative stability, low vapor pressure, and nonflammability. However, the fundamental understanding of the solvation shell structure in glyme electrolytes has not been clearly established. Herein, we employ first-principles molecular dynamics (FPMD) simulation to study the lithium bis(trifluoromethylsulfonyl)-amide (LiTFSA) and tetraglyme (G4) electrolyte system. For the case of equimolar ratio, a positive correlation between the total coordination number of Li+ ions and the phase stability is clearly established. At the ground state of equimolar LiTFSA-G4 electrolyte, most of the Li+ ions are coordinated to four O atoms of a curled G4 molecule and one O atom of a TFSA- anion, equivalent to the second most stable contact ion pair in gas-phase cluster calculations. By contrast, Li+ ions prefer to be coordinated by two G4 molecules and not in direct contact with TFSA- anions at a low concentration of Li salt. The significantly increased probability of pairing between the Li-G4 complexes and TFSA- anions at the equimolar ratio could be highly relevant to its ionic-liquid-like properties.