(550d) A Theoretical Study on the Solvation of Lithium Ion with Asymmetric Anions for the Development of Concentrated Electrolytes for Li-Ion Batteries

The structure of Li⁺ solvation in concentrated electrolytes has a significant impact on Li⁺ transport and is influential in the rate capability of rechargeable Li-ion batteries (LIBs). Ionic Liquids (ILs) present an extreme case of a concentrated electrolyte where the electrolyte is made entirely of discrete ions and lacks neutral solvent molecules. ILs have desirable characteristics for LIBs such as large electrochemical windows, negligible volatility, and lack of flammability. However, Li⁺ transport in ILs is complex and hindered by the high viscosity. The specific Li⁺-anion interactions lead to the formation of clusters which decreases the mobility of Li⁺ in the bulk liquid. The goal of this work is to understand the solvation strength of Li⁺ with IL anions as it relates to the vehicular and hopping transport mechanisms of Li⁺, and dissociation energies at the electrode-electrolyte interfaces. Specifically, density functional theory (DFT) geometric optimization calculations of anion-cation systems as well as the individual components are used to calculate dissociation energies that describe the energy associated with Li⁺ coordination in multicomponent IL mixtures. The dissociation energies are analyzed in terms of the number of coordinating anions, size and asymmetry of the anions. As a reference, three parent anions: bis(trifluoromethanesulfonyl)imide ([TFSI]), bis(fluorosulfonyl)imide ([FSI]), and dicyanamide ([DCA]) are studied as well as their mixtures. The asymmetric analogues of the mixtures including the cyano(trifluoromethanesulfonyl)imide ([CTFSI]) and (fluorosulfonyl)(trifluoromethanesulfonyl)imide ([FTFSI]) were compared to the symmetric anions and the mixtures. The results show that the asymmetric anion has lower dissociation energy with Li⁺, compared to the symmetric anions and their mixtures. The degree of asymmetry was further increased by the addition of fluorinated chain on one side of the [TFSI] anion. While the increased fluorination and the anion size do not have a significant impact on the dissociation energies calculated by DFT, the entropic effects are anticipated to influence Li⁺ mobility in experiments. This study shows that the Li⁺ solvation cage in IL electrolytes can be weakened by IL mixtures and further by asymmetric anions. This is expected to promote Li⁺ hopping transport mechanism within the electrolyte and have an effect on the rate capability of LIBs.