Evaluation of Electrochemical Stability and Li-ion Interactions in Ether Functionalized Pyrrolidinium and Phospholanium Ionic Liquids

Abstract

Potential electrolytes for lithium ion batteries (LIB) depicting high electrochemical stability and ionic conductivity still represents a great challenge. Herein, we investigate the reduction stability and Li+ cation interactions with substituted cyclic phospholanium (CylP5+) and pyrrolidinium (Pyr+) ionic liquids (ILs) with bis(trifluoromethanesulfonyl)imide (TFSI−) anion. The cations CylP5+ and Pyr+ are functionalized with varying chain lengths of alkyl and alkoxy substituents and their reduction potentials evaluated with respect to Li+/Li. CylP5+ cations, in general have better stability than Pyr+ based ILs, while the alkoxy substitution is found to lower the reduction stability compared to alkyl substituents in both ILs. Furthermore, the interaction energies between the substituent modified-cations (C), Li+ cation and the anion (A) were evaluated. The ion-pair (C-A) interaction energies on addition of Li+ cation is shown to decrease for both Pyr+ and CylP5+ cations with shorter alkoxy chain lengths, thereby implying faster diffusion of ions due to weaker interactions. Similarly, among the two cations, lowest ΔE were noted between alkoxy substituted [Li(CylP5)]2+ cation and TFSI−, signifying faster diffusion of ions in CylP5+ based electrolytes than that of Pyr+ based ILs. These results govern the synthesis of novel ILs with promising base cation and functionalization for LIB.