Molecular Dynamics Simulations of Lithium-Doped Ionic-Liquid Electrolytes

Lithium bis(trifluoromethanesulfonyl)imide (LiNTf2) doped ionic liquids (ILs) are investigated herein, as potential electrolytes for lithium-ion batteries, via scaled-charge molecular dynamics simulations. Four model ILs based on the [NTf2]−anion and heterocyclic ammonium cations were studied with varying concentrations, ranging from 0 to 1 M solutions, of the dissolved lithium salt. The pyrrolidinium ([pyrHH]+), piperidinium ([pipHH]+), N-butyl-pyrrolidinium ([pyrH4]+), and N-butyl-N-methyl-pyrrolidinium ([pyr14]+) cations were considered to evaluate the combined effects of increased ring size, as well as the introduction of apolar groups on the nitrogen atom of the cations, on the liquid structure properties of the electrolytes. Among the investigated ILs, [pyr14][NTf2] is the only aprotic IL allowing for a comparison of protic and aprotic ILs. The lithium coordination shell is seen to be quite different in the various IL-based systems; networks of lithium ions bridged by [NTf2]−ions have interesting consequences on the solvation shells and coordination numbers. Aggregate existence and velocity autocorrelation functions are finally evaluated in order to characterize the caging effect of [NTf2]−ions around lithium ions. In conclusion, we find that the lithium mobility and transport are directly proportional to the strength of the interionic interactions within the liquids, whereas the ease of solvation shows opposite trends.