Benzimidazole and imidazole lithium salts for battery electrolytes

The intrinsic anion oxidation potential (ΔEv) and lithium ion pair dissociation energy (ΔEd) are two important properties for predicting the potential use of new lithium salts for battery electrolytes. In this work several cyano substituted fluoroalkylated benzimidazole and imidazole anions have been investigated computationally to obtain ΔEv and ΔEd. Varying the number and position of cyano substituents results in large effects on the electrochemical stability of the anion and on the possible lithium ion pair configurations. The lengthening of the fluoroalkyl group introduces several new stable ion pair configurations and a small increase in anion oxidation stability. The most promising fluoroalkylated anions in the present work are the 4,5,6,7-tetracyano-2-fluoroalkylated benzimidazolides (TTB and PTB), with oxidation potentials suitable for high voltage Li-ion battery applications (<4.2 V) and much improved ΔEd compared to PF6−—a benchmark for commercially available anions. Further improvements in ΔEd, with maintained stability towards oxidation, are obtainable by replacing the fluoroalkyl group by an additional cyano group, but possibly demanding increased synthesis efforts.