The effects of C2-methylation of imidazolium-based ionic liquid electrolytes on the lithium-ion transport

In salt-in-ionic liquid electrolytes (SILEs), the wrong drifting direction of lithium ion (Li+) with its coordinating anions raises the internal resistance and limits the applications of SILEs in lithium (ion) batteries. By applying molecular dynamics simulations to study two imidazolium-based SILEs, both with 0.25 molar fraction of lithium salt, we found that the trapped Li+ by counter ions is more easily released by a methyl substitution of the acidic H2-atom in 1-butyl-3-methyl-imidazolium (BMIM+) cation to 1butyl-2,3-dimethyl-imidazolium (BMMI+), namely, C2-methylation. Upon C2-methylation, the smeared charge distribution and steric hinderance of the methyl group make BMMI+ reduce the amount of high-coordinated Li+ and depress the formation of Li+-containing agglomerates (AGGs), while the presence of the H2-atom makes BMIM+ stabilize the high coordination of Li+ and enhances the formation of AGGs. Consequently, more non-AGG Li+ ions exist in the SILE consisting of BMMI+, which results in a desired change of the transference number of Li+ and demonstrates that a tiny difference in ionic structure can exert promising effect on the transport properties of SILEs. (c) 2022 Elsevier B.V. All rights reserved.

Automated summary

In salt-in-ionic liquid electrolytes (SILEs), the coordination of lithium ion (Li+) with its anions affects the internal resistance and the formation of Li+-containing agglomerates (AGGs). Molecular dynamics simulations showed that a methyl substitution in the imidazolium cation can release trapped Li+ ions and reduce the formation of AGGs. This structural modification alters the transport properties of SILEs and improves the transference number of Li+.