Effect of propylene carbonate-Li+ solvation structures on graphite exfoliation and its application in Li-ion batteries

The effect of propylene carbonate (PC)-Li+ solvation structures on graphite exfoliation was investigated over a range of concentrations of PC-based electrolytes. At low concentrations of lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) in PC (1.3 M and 2.1 M), the graphite anode was exfoliated. However, the graphite exfoliation could be effectively suppressed when the concentrations of dissolved LiTFSI were increased to 2.5 M and 3.3 M. The results of spectroscopic analyses and density functional theory (DFT) calculations revealed that electrochemical exfoliation of the graphite anode is closely associated with a special spatial configuration of Li+-(PC)n (1 ≤ n ≤ 4) solvation structures at various Li-salt concentrations and corresponding solid electrolyte interface (SEI) film formation mechanisms. When the concentration of LiTFSI increased from 1.3 to 3.3 M, the spatial configuration of Li+-(PC)n (1 ≤ n ≤ 4) solvation gradually changed from a tetrahedron (occupied space of 10.19 Å) to planar (occupied space of 3.05 Å), which reduced the structure change for co-intercalation into the graphite interlayers of Li+-(PC)n (1 ≤ n ≤ 4) solvates. Meanwhile, the affinity between Li+-(PC)n (1 ≤ n ≤ 4) solvation cations and TFSI− anions was increased, leading to the significant contribution of TFSI− anions to SEI formation on the surface of graphite. Additionally, Al corrosion was not of concern in concentrated LiTFSI electrolyte. The 3.3 M LiTFSI/PC concentrated electrolyte exhibits promising electrochemical performance in graphite||LiNi1/3Co1/3Mn1/3O2 full cells.