Molecular Structural Influence of Glymes on Co-Intercalation Behavior of Solvated Li+ in Graphite Electrodes

Solvated Li+ ions are intercalated into graphite in a dilute glyme-based solution and this reaction has attracted much attention for use in advanced lithium-ion batteries due to their high rate capability. In this study, Li+ co-intercalation behaviors in asymmetric glymes (CH3O(CH2CH2O)nC4H9, n = 2 or 3) were investigated and compared to those in symmetric glymes (CH3O(CH2CH2O)nCH3, n = 1–4). Despite the large alkyl sidechains of asymmetric glymes, co-intercalation reactions of solvated Li+ proceeded as well as those in symmetric glymes. In charge and discharge measurements, the reversible capacities in asymmetric glyme-based solutions were smaller than those in symmetric glymes except for tetraglyme, which showed the lowest reversible capacity. There was no clear difference in the sandwich distances of glyme-Li-GICs calculated from X-ray diffraction patterns of both symmetric and asymmetric glymes. Therefore, solvated Li+ ions with asymmetric glymes occupied more space in the graphite interlayer compared to symmetric glymes, which resulted in smaller reversible capacities for asymmetric glymes. These results demonstrate the influence of solvent-related structures of GICs on reversible capacities for co-intercalation systems.