It is a common observation that when ionic liquids are added to electrolytes the performances of lithium ion cells become poor, while the thermal safeties of the electrolytes might be improved. In this study, this behavior is investigated based on the kinetics of ionic diffusion. As a model ionic liquid, we chose butyldimethylimidazolium hexafluorophosphate (BDMIPF6). The common solvent was propylene carbonate (PC), and lithium hexafluorophosphate (LiPF6) was selected as the lithium conducting salt. Ionic diffusion coefficients are estimated by using a pulsed field gradient NMR technique. From a basic study on the model electrolytes (BDMIPF6 in PC, UPF6 in PC, and BDMIPF6 + LiPF6 in PC), it was found that the BDMI+ from BDMIPF6 shows larger diffusion coefficients than the Li+ from LiPF6. However, the anionic (PF6-) diffusion coefficients present little difference between the model electrolytes. The higher diffusion coefficient of BDMI+ than that of Li+ suggests that the poor C-rate performance of lithium ion cells containing ionic liquids as an electrolyte component can be attributed to the two-cation competition between Li+ and BDMI+.