Electrolytes that possess both interfacial stability toward lithium metal anodes and compatibility with high voltage cathodes are required for high energy density batteries. Here a high concentration LiTFSI salt firstly dissolves in an ionic liquid Pyr1,3FSI. This electrolyte with synergetic effect of TFSI and FSI anions, which not only effectively suppresses lithium dendrite growth because of the highly concentrated solid electrolyte interphase film, but also beneficially improve the cycling performance of lithium batteries. Lithium plating/stripping tests reveal that the concentrated electrolyte has superior compatibility with a lithium metal anode than an electrolyte with low salt concentration. Furthermore, a lithium metal battery containing the electrolyte and a lithium cobalt oxide cathode achieves high coulombic efficiency and good cycling stability at 4.4 V and high temperature. These results indicate that the highly concentrated ionic liquid electrolyte is promising for use in lithium metal batteries.