Beneficial properties of solvents and ions for lithium ion and post-lithium ion batteries: Implications from charge transfer models

Abstract

We study the solvation properties of standard lithium salts in various aprotic apolar solvents using conceptual density functional theory. The electronegativity and the chemical hardness of the species are introduced as reliable criteria for identifying suitable solvent-ion combinations. All salt solvation energies reveal positive values as a consequence of the ion association reaction which implies a tendecy for increased ion pair formation. Our findings reveal that frequently used solvents like propylene carbonate, ethylene carbonate, γ-butyrolactone and dimethoxyethane mainly favor the solvation of the lithium ion due to their nucleophilic and thus Lewis-basic properties. The solvation of standard anions like BF4−, PF6− and BOB− is of minor importance in terms of their contribution to the total solvation energy. Based on the values for the electronegativity and the chemical hardness, we show that the increased tendency towards ion pair formation can be avoided by using more acidic and softer anions in combination with more basic and softer solvents. A variation of the cation does not significantly lower the amount of ion pairs. Our results are in qualitative agreement with recent empirical assumptions and shed more light on the underlying interaction patterns.