Structure and Transport Properties of Lithium-Doped Aprotic and Protic Ionic Liquid Electrolytes: Insights from Molecular Dynamics Simulations.

Protic ionic liquids (PILs) have been recently the subject of experiments for lithium-ion batteries (LIBs) applications, showing promising properties, in some cases better performance than aprotic ionic liquids (AILs). With the aim of a direct comparison between the performance of PIL and AIL electrolytes in LIBs, we conducted molecular dynamics simulations of corresponding lithium-salt-doped electrolytes. Our PIL and AIL electrolytes are triethylammonium bis(trifluoromethanesulfonyl)imide ([N222][TFSI]) and triethylmethylammonium bis(trifluoromethanesulfonyl)imide ([N1222][TFSI]), respectively, each doped with LiTFSI. Three fundamental structural changes were observed upon the addition of Li salt into IL solutions: (1) formation of rigid Li-TFSI complexes, (2) cation-anion coordination became larger, and (3) formation of Li aggregates. Moreover, we observed that the density and viscosity of the electrolytes increased with increasing lithium salt mole fraction ( xLi), and correspondingly, the self-diffusivity of ions, ionic conductivity, and ionicity became lower. The Li conductivity exhibits a maximum at xLi = 0.2 due to the competition between increasing Li salt concentration and viscosity. More importantly, the PIL electrolytes display higher Li conductivity and better transport properties over their AIL counterpart.