Investigation into the Solvation of Lithium Anions and the Search for a Connection with Electrode Passivation Behavior
Recently published studies have shown that there is a complex relationship between a lithium ion and its solvent molecules in two-solvent electrolyte systems. Cyclic carbonates like ethylene carbonate are stable in the Li+ solvation sheath while linear carbonates like dimethyl carbonate move in and out frequently, reflecting a difference in coordinating stability with Li+. Because of this bias in Li+ coordinating ability, a correlation can be drawn between the composition of the Li+solvation sheath and the resulting composition of the passivating solid electrolyte interphase that forms on the graphitic anode of a Li-ion battery during the first charge.
We are investigating the solvation condition of the anion in the Li-ion electrolyte system with the goal of identifying a potential correlation between the solvation condition of the anion (including PF6-, BF4-, ClO4-, and TFSI-) and the formation and composition of a cathode passivation layer.
Our previous work indicated that although typical electrolyte anions are poorly coordinated, both PF6- and BF4- showed a preference for the low-permittivity linear carbonate ethyl methyl carbonate over the high-permittivity solvent ethylene carbonate. The PF6-results are summarized in Fig. 1. This result spurred further investigation into the solvation condition of the anion, accomplished using electrospray mass spectrometry, linear voltammetry, x-ray photoelectron spectroscopy, and density functional theory simulations of anion-solvent cluster stability.